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MANUALMONITOR AND CONTROL THE PRODUCTION

OF CHEMICAL PULP

This Monitor & Control manual must be used in conjunction

with the SOP / Operating Instructions manual

Unit Standard 256280NQF Level 4Credits: 15

Compiled by:Reneiloe Matlou

Rev.1 – January 09

Moderated by:

Learner Name:

Learner Number:

Sparrow Consulting © January 2009 Rev.1 – Jan 09

Table of Contents

UNIT 1: INTRODUCTION TO THE CHEMICAL PULPING PROCESS............91.1 Instructions.........................................................................................................9

1.2 Introduction......................................................................................................12

1.3 Chemical pulping: general principles..............................................................12

1.3.1 Pulp..................................................................................................................12

1.3.2 Pulping.............................................................................................................13

1.3.3 Basic chemical pulping process.......................................................................13

1.4 Chemical pulping equipment...........................................................................16

1.4.1 Digesters..........................................................................................................16

1.4.2 Blow tank.........................................................................................................26

1.5 Kraft pulping process.......................................................................................29

1.5.1 Principles of the Kraft pulping process............................................................29

1.5.2 Process flow diagram......................................................................................36

1.6 Acid sulphite pulping process..........................................................................36

1.6.1 Principles of the acid sulphite pulping process................................................37

1.6.2 Acid sulphite pulping in industry......................................................................38

1.6.3 Process flow diagram......................................................................................39

1.6.4 Advantages and disadvantages.......................................................................39

1.7 NSSC pulping process.....................................................................................40

1.7.1 Cooking liquor preparation...............................................................................41

1.7.2 Sulphur storage...............................................................................................41

1.7.3 Melting basin....................................................................................................41

1.7.4 Sulphur burner.................................................................................................41

1.7.5 Quench tower..................................................................................................42

1.7.6 Absorption tower..............................................................................................42

1.8 Soda pulping process......................................................................................43

1.8.1 Process description.........................................................................................43

UNIT 2: MONITOR AND CONTROL RELEVANT ANCILLARY SYSTEMS AND UTILITIES.........................................................................49

2.1 Instructions.......................................................................................................49

2.2 Introduction......................................................................................................53

2.3 Mechanical equipment.....................................................................................53

2.3.1 Conveying equipment......................................................................................53

2.3.2 Weighing equipment........................................................................................59

2.3.3 Storage equipment..........................................................................................60

Manual 2 US 256280


2.3.4 Solids storage methods...................................................................................60

2.3.5 Packaging equipment......................................................................................68

2.4 Electrical equipment........................................................................................68

2.4.1 Isolation of electrical, hydraulic and air driven machines or equipment..........69

2.5 Instrumentation................................................................................................69

2.5.1 Key functions of instrumentation......................................................................70

2.5.2 Methods of display...........................................................................................71

2.5.3 How to read a gauge.......................................................................................72

2.6 Utilities.............................................................................................................73

2.6.1 Compressed air supply....................................................................................74

2.6.2 Steam systems................................................................................................76

2.6.3 Steam generation............................................................................................76

2.6.4 Steam applications..........................................................................................77

2.7 Mill cooling water.............................................................................................78

2.8 Electrical power...............................................................................................79

2.9 Monitor the ancillary systems..........................................................................80

2.9.1 What is a parameter?......................................................................................80

2.9.2 What is a variable?..........................................................................................81

2.9.3 What is a deviation?........................................................................................81

UNIT 3: MONITOR AND CONTROL THE QUALITY OF PROCESS MATERIALS......................................................................83

3.1 Instructions.......................................................................................................83

3.2 Introduction......................................................................................................87

3.3 What is a Quality Management System?.........................................................87

3.3.1 Quality control..................................................................................................87

3.3.2 Quality assurance............................................................................................89

3.3.3 Quality management........................................................................................89

3.4 The quality standards of process materials.....................................................89

3.4.1 Quality of raw materials and ingredients..........................................................90

3.4.2 Quantities of raw materials and ingredients.....................................................90

3.4.3 Contamination..................................................................................................90

3.5 Wood quality requirements..............................................................................90

3.5.1 Wood related variables....................................................................................91

3.5.2 Wood decay.....................................................................................................95

3.6 Bagasse quality requirements.........................................................................97

3.6.1 Chemical and physical properties of bagasse.................................................97

3.7 Cooking liquor quality requirements................................................................97

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3.7.1 Kraft cooking liquors........................................................................................97

3.7.2 Raw material problems....................................................................................99

3.7.3 Kappa number...............................................................................................100

3.7.4 Consistency of pulp product..........................................................................100

3.7.5 Freeness........................................................................................................100

3.7.6 Pulp viscosity.................................................................................................100

3.7.7 Shive content.................................................................................................101

3.7.8 Dirt count........................................................................................................101

3.7.9 Brightness of the pulp....................................................................................101

3.8 Disposal strategies for unacceptable materials.............................................101

3.9 Consequences of poor quality control...........................................................101

3.9.1 Consequences for the customers..................................................................101

3.9.2 Consequences for the organisation...............................................................103

3.9.3 Consequences for the individual and colleagues..........................................103

3.9.4 Consequences for the community.................................................................104

UNIT 4: MONITOR AND CONTROL THE CHEMICAL PULPING PROCESS.....................................................................................105

4.1 Instructions.....................................................................................................105

4.2 Introduction....................................................................................................109

4.3 Normal operating conditions..........................................................................109

4.4 Maintaining process variables.......................................................................109

4.4.1 H-factor..........................................................................................................110

4.4.2 Pressure in the cooking vessel......................................................................110

4.4.3 Addition of liquor to the cooking vessel.........................................................110

4.4.4 Temperature in the cooking vessel................................................................111

4.4.5 Liquor to wood ratio.......................................................................................111

4.5 Deviations from normal conditions.................................................................111

4.5.1 Detecting problems in a process...................................................................112

4.6 Chemical pulping process equipment problems............................................113

4.6.1 Raw material feed variability..........................................................................113

4.6.2 Poor circulation..............................................................................................113

4.6.3 Poor packing of wood chips in the cooking vessel........................................113

4.6.4 Poor temperature control...............................................................................113

4.7 Abnormal conditions......................................................................................113

4.8 A problem solving strategy............................................................................114

ANNEXURE 1: RESOURCES............................................................116

ANNEXURE 2: US 256290...............................................................................117

Manual 4 US 256280


Figures and tablesFigure 1: Schematic of a chemical pulping process..........................................................14

Figure 2: Vertical batch digester and its auxiliary equipment............................................17

Figure 3: Vertical continuous digester and its auxiliary equipment....................................19

Figure 4: Horizontal continuous digester...........................................................................24

Figure 5: Blow tank............................................................................................................27

Figure 6: Pyramid digester loading...................................................................................30

Figure 7: Steam packing...................................................................................................31

Figure 8: Pre-steaming of chips........................................................................................32

Figure 9: Cooking liquor charging.....................................................................................33

Figure 10: Cooking configuration......................................................................................34

Figure 11: Turpentine recovery system.............................................................................35

Figure 12: Kraft pulping process flow diagram..................................................................36

Figure 13: Acid sulphite pulping process..........................................................................39

Figure 14: Sulphite semi chemical pulping process..........................................................40

Figure 15: Sulphur burner.................................................................................................42

Figure 16: Bagasse soda pulping process........................................................................44

Figure 17: Bagasse digester.............................................................................................48

Figure 18: Example of a belt conveyor..............................................................................54

Figure 19: A basic belt conveyor transporting solid material.............................................54

Figure 20: An inclined belt conveyor.................................................................................55

Figure 21: Pneumatic conveyor systems..........................................................................56

Figure 22: A basic positive pressure pneumatic conveying system..................................57

Figure 23: Basic negative pressure pneumatic conveying system...................................57

Figure 24: Positive and negative pressure pneumatic conveying systems.......................58

Figure 25: A fluidised system............................................................................................59

Figure 26: Chip conveyor with load cell............................................................................60

Figure 27: Storage bunker................................................................................................61

Figure 28: Silos.................................................................................................................61

Figure 29: A silo and its basic components.......................................................................62

Figure 30: Silos operating in series sharing a common filter............................................63

Figure 31: Storage hopper................................................................................................63

Figure 32: Self dumping steel hopper...............................................................................64

Figure 33: Hopper and storage bin...................................................................................65

Manual 5 US 256280


Figure 34: Cylindrical storage bins....................................................................................66

Figure 35: A horizontal storage bin with V-shaped bottom...............................................67

Figure 36: Bulk bags.........................................................................................................67

Figure 37: Types of instruments........................................................................................69

Figure 38: Digital and analogue displays..........................................................................71

Figure 39: Your eyes should always be level with a gauge..............................................72

Figure 40: Analogue display with X100 on the display......................................................73

Figure 41: Compressed air networks................................................................................75

Figure 42: A steam system...............................................................................................77

Figure 43: Cooling water system.......................................................................................79

Figure 44: Electricity distribution.......................................................................................80

Figure 45: Relationship between the different levels of quality.........................................87

Figure 46: Air pollution caused by the pulp and paper industry......................................104

Table 1: Specific densities for various wood species........................................................92

Table 2: Reaction-wood and normal wood properties.......................................................95

Manual 6 US 256280


Module overviewPlease note:

This manual deals only with the monitoring and controlling of the process and has to be

used in conjunction with the SOP / Operating Instructions manual which addresses the

pre-start up, start-up, shut-down and emergency shutdown of the equipment as well as

the process.

Introduction

In South Africa, hardwoods, softwoods and bagasse are the main sources of the

vegetable fibre used in the production of pulp and paper. Chemical pulping is the process

used to reduce these vegetable fibres to its component parts, cellulose and hemicellulose,

in the form of individual fibres.

The main purpose of chemical pulping is to remove lignin, the glue-like substance that

binds fibrous materials (fibres) together. This part of the process is referred to as

digestion (or cooking) and takes place in a digester.

Unit 1 will introduce you to the principles of the chemical pulping process and the four

different chemical pulping processes. Unit 2 deals with the ancillary systems related to

the chemical pulping processes. In Unit 3 you will learn about the quality principles

applicable to process materials. In Unit 4 you will learn about the procedures needed to

monitor and control the chemical pulping processes.

Learning outcomes

After working through this module, you will be able to:

Explain the purpose of the chemical pulping process in terms of the final product manufactured.

Explain the principles of the chemical pulping process by making use of a generic flow diagram.

Explain the chemical pulping process in relation to its supplier’s- and customer’s processes.

Trace the flow of material through the debarking section and identify all equipment using standard industry terminology.

Explain the purpose and functioning of each piece of equipment used in the debarking section in terms of its role in the overall process.

Explain the functions of all chemicals and additives used within the process in terms of their chemical and physical properties.

Identify and describe mechanical equipment used in the chemical pulping process in terms of purpose and application.

Manual 7 US 256280


Identify and describe electrical equipment used in the chemical pulping process in terms of purpose and application.

Identify and describe instrumentation used in the chemical pulping process in terms of purpose and application.

Identify and describe utilities used in the chemical pulping process in terms of purpose and application.

Discuss typical ancillary equipment problems within the chemical pulping process and offer solutions in accordance with workplace procedures.

Monitor ancillary systems and correct any deviations from operating parameters in accordance with operating procedures.

Explain the properties of process materials in terms of key characteristics.

Explain the purpose of process material quality control procedures as well as the consequences of not adhering to these procedures with regards to the impact thereof on the final product produced.

Explain the quality requirements of raw materials, process water, chemicals and additives or other materials according to general and workplace specifications.

Discuss typical raw material problems and its impact on the final debarked log properties and costs.

Discuss corrective action to be taken in the case of non-conforming raw materials in accordance with workplace procedures.

Evaluate product variations and take corrective action in accordance with workplace procedures.

Monitor the chemical pulping process and record parameters in accordance with workplace procedures.

Explain the impact of process variables on the product properties in terms of final products and costs.

Discuss typical equipment problems within the chemical pulping process and offer solutions in accordance to workplace procedures.

Evaluate variations in the product and take corrective action in accordance with workplace procedures.

US specific outcomes

The following specific outcomes are covered in this module:

SO1: Explain the fundamental principles applicable to the chemical pulping process.

SO2: Monitor and control the different ancillary systems interacting with the chemical

pulping process.

SO3: Monitor and control the quality standards of process materials in the chemical

pulping process.

SO4: Monitor and control the chemical pulping process

Manual 8 US 256280


UNIT 1: INTRODUCTION TO THE CHEMICAL PULPING PROCESS

Learning outcomes

After working through this unit, you will be able to:

Explain the purpose of the chemical pulping process in terms of the final product manufactured.

Explain the principles of the chemical pulping process by making use of a generic flow diagram.

Explain the chemical pulping process in relation to its supplier’s- and customer’s processes.

Trace the flow of material through the chemical pulping section and identify all equipment using standard industry terminology.

Explain the purpose and functioning of each piece of equipment used in the chemical pulping section in terms of its role in the overall process.

Explain the functions of all chemicals and additives used within the process in terms of their chemical and physical properties.

1.1 Instructions

Ref. No Resources Learning Methodology Workbook Assess Time

CCFO3-8 Learning materials

Workplace and other relevant procedures

It is assumed that you are competent in terms of the following outcomes or areas of learning before starting with this module:

Mathematics and literacy at NQF Level 3 or equivalent.

Act. 1

N/a N/a N/a

Manual 9 US 256280



SO 1, AC 1-6

CCFO 3-5, 7, 8

Learning materials

Read through Unit 1 of the learning materials. Make notes of things you do not understand and/ or need more information on and discuss it with your facilitator.

00:00

SO1 AC1-6

CCFO 2, 3, 5, 7, 8

Lecture room

Facilitator

Multimedia

Relevant flow diagrams

Attend a lecture and/ or facilitated discussion on:

Relevant industry specific terminology with regards to the chemical pulping process

The purpose of the chemical pulping process

The role of the chemical pulping process in the overall functioning of the mill

The chemical pulping process in relation to its suppliers and customers

The operating principles of the chemical pulping process

The flow of material through the chemical pulping system

The purpose and functioning of equipment used in the chemical pulping process such as chip conveyors, chip storage bins, chip feeders, chip monitors, cooking vessels, heating equipment, discharge valves, pulp receiving vessels and steam flash vessels

Functions of chemical used within the chemical pulping process such as sodium sulphide and sodium hydroxide

Ex. 1

Questions and flow diagrams

Ass. 1

Questions and

diagrams

00:00

Act. 2

Manual 10 US 256280



All SO’s and AC’s

CCFO 1-8

Lecture room

Facilitator

White/ black board

Brainstorm the critical safety requirements as applicable to the chemical recovery process.

Act. 3

00:00

SO1 AC1-6

CCFO 2, 3, 5, 7, 8

Learning materials and workbook

PoE

Facilitator/ SME

Revise the work that you have done up to this point. Make sure that you have completed the CCFO checklist and obtained the required evidence for your PoE. If there is anything that you do not understand, ask your facilitator. Act. 4

CCFOs CCFOs N/a

Total time allocated for this unit (00h00) 05:00

Manual 11 US 256280


1.2 IntroductionIn South Africa, hardwoods, softwoods and bagasse are the main sources of the vegetable

fibre used in the production of pulp and paper. Chemical pulping is the process used to

reduce these vegetable fibres to its component parts, cellulose and hemicellulose, in the

form of individual fibres.

The main purpose of chemical pulping is to remove lignin, the glue-like substance that binds

fibrous materials’ fibres together. This part of the process is referred to as digestion (or

cooking) and takes place in a digester.

There are four ways in which pulp can be produced namely chemical, semi-chemical, chemi-

mechanical and mechanical pulping. These methods differ in the way in which the

separation of fibres is achieved. Mechanical pulping uses physical action, while chemical

pulping uses chemicals to separate the fibres.

Chemical pulping can further be divided into four methods each using different chemicals to

effect the separation of fibres. The methods are:

Alkaline Process (sulphate or Kraft)

Acid Sulphite Process

Sulphite (Semi Chemical) Process

Soda Process (caustic) bagasse or wood pulp (hardwood) for bleaching

In the sections that follow, the general principles of chemical pulping will be discussed. This

will be followed by a discussion of each of the chemical pulping methods listed above.

1.3 Chemical pulping: general principlesIn order to understand the principles of chemical pulping, some fundamental principles must

be known.

1.3.1 PulpPulp can be described as wood that has been chemically or physically broken down so as to

liberate fibres that can be dispersed in water and reformed into a web. It is from this pulp

that paper is made. As mentioned before, pulp can be produced by a variety of processes.

The focus of this manual is chemical pulping.

Manual 12 US 256280


1.3.2 PulpingThe main purpose of chemical pulping is to remove lignin, the glue-like substance that binds

fibrous materials’ fibres together. This part of the process is referred to as digestion (or

cooking) and takes place in a digester. This is the most important part of the pulping process

and is called de-lignification. In this process, the chemical structure of lignin contained in

wood is broken down, making the lignin soluble in water.

Digestion basically consists of cooking the fibrous raw materials together with appropriate

cooking liquors at elevated temperature and pressure for a period of time. The cooking

liquor is a mixture of white liquor (i.e. regenerated cooking liquor) and spent black liquor from

a preceding cook. During the process, lignin is degraded and dissolved into the residual

liquor, leaving the cellulose and hemi-cellulose components for pulp and papermaking.

1.3.3 Basic chemical pulping processAlthough chemical pulping can refer to any of the four chemical pulping methods, the basic

principles of operation are rather similar. Here, the basic operation of chemical pulping

processes is discussed in general and later each of the methods will be discussed

individually. The figure below is a general schematic showing the main elements of the

chemical pulping process.

1.3.3.1 Chip screening

The raw materials of the pulping process are wood chips from the woodyard. These need to

be screened to eliminate fine material and over-sized chips. After screening, the acceptable

chips are the conveyed to the digester.

Manual 13 US 256280


Figure 1: Schematic of a chemical pulping process

1.3.3.2 Cooking (digesting)

The chips are fed to a pressure vessel, the digester. The chips are steamed with direct

steam to eliminate as much of the air in the chips as possible. The digester is then filled with

warm (80°C-100°C) cooking liquor to submerge the chips. The cooking liquor is a mixture of

white liquor (i.e. regenerated cooking liquor) and spent black liquor from a preceding cook.

The ratio of total liquor in the digester to the amount of dry chips, called the liquor-to-wood

ratio, is in the order of 3.5 – 5.0 ton of liquor per ton of wood.

The digester contents are heated to 160°C – 170°C, either by direct steam or by indirect

heating in a steam/ liquor heat exchanger by extracting liquor from the digester, circulating it

through a heat exchanger to heat it and reintroducing it to the digester. The cooking

temperature is maintained until the desired degree of de-lignification is reached, after which

the blow valve is opened and the digester contents is blown into the blow tank by the

digester pressure.

Released heat is recovered in a blow heat recovery system. Volatile compounds formed

during heating and cooking are continuously purged from the digester to control cooking

pressure. The gases go to a condenser system for recovery of volatile wood compounds

such as turpentine.

Manual 14 US 256280


1.3.3.3 Blowing

On completion of the cooking cycle the digester pressure is first partially relieved through a

vent line, before the blow valve is opened at the bottom of the digester and the contents

blown into the “blow tank”, a cylindrical vessel with a cone shaped bottom.

Fibres are effectively separated by the expansion that occurs as steam is released within the

chips in the blow tank. The blow tank also serves as a temporary storage vessel and

provides reserve stock and storage space during short shutdowns of equipment upstream or

downstream from it.

Throughout all these steps the chips maintain their wood structure despite the loss of most of

their lignin content and about half of their total solid mass. However, the structure becomes

so weak that it breaks down to individual fibres when the digester is blown.

1.3.3.4 Washing

The pulp from the blow tank is then washed in a counter-current washing system. Since the

spent liquor is washed out with the wash water, the volume of wash water should be kept to

a minimum in order to be able to concentrate the spent liquor efficiently.

1.3.3.5 Screening

After washing, the product is screened once more. The screening removes incompletely

delignified residues of wood which are not broken down to fibres during the blow. Biological

knots and undefibrated chips are usually separated out of the suspension in knotters before

pulp washing, and are reintroduced to cooking for redelignification. Other impurities such as

bark and shives are removed in screening and cleaning systems, mechanically defibrated

and reintroduced to the fibre flow or used for other purposes.

1.3.3.6 Storage

The unbleached pulp is rethickened and stored at elevated consistency (concentration) for

further processing. This could be bleaching or the manufacture of unbleached paper and

board products.

Each of these steps, along with their equivalents in continuous digesters, affects pulp quality

and uniformity.

Manual 15 US 256280


1.4 Chemical pulping equipmentThe heart of the chemical pulping process is the digester. Several types of digesters will be

discussed in this section. Also, other types of equipment involved in the pulping process

such as chip conveyors chip feeders, chip storage bins, heating equipment, discharge valves

etc will be discussed. Let’s start by discussing the “heart “of the pulping process: the

digester.

1.4.1 DigestersA pulp digester is a large metal container with associated equipment to facilitate the following

operations (irrespective of the type of chemical pulping process used):

Raw material loading

Cooking liquor filling

Cooking liquor heating and circulation

Pressure control

Pulp removal

The three types of digesters that will be covered in the following sections are:

Vertical batch digesters

Vertical continuous digesters

Horizontal continuous digesters

1.4.1.1 Vertical batch digesters

A batch digester cooks one load of material at a time. The figure below illustrates a typical

Kraft batch digester with the main auxiliary equipment usually associated with it.

The size of batch digesters range from 70 to 350 m3, producing between 5 to 25 tons of pulp

per blow. Such a digester requires a charge of wood of between 11 and 54 tons and

operates (on an oven-dry basis) at a yield of 46%.

The following peripheral equipment is associated with a vertical batch digester:

Liquor filling system

Blow tank

Turpentine recovery system

Manual 16 US 256280


Figure 2: Vertical batch digester and its auxiliary equipment

Typically, the following sequence of events occurs during the batch digestion process:

The digester is loaded with chips, white liquor and some black liquor from the previous

cook.

The liquor is circulated within the digester for even distribution, following which;

additional chips are added to the digester.

The digester is closed and heated with steam to the cooking temperature.

Once the cooking temperature is reached, it is maintained for a set period of time.

Gases are vented off to the chemicals recovery section in order to keep the pressure at

a desired point.

Once cooking is complete, the contents of the digester are discharged to the blow tank

where the fibres are separated.

The digester is once again charged with chips and the process repeated.

Mills usually consist of a bank of six to eight batch digesters. This is so that when one

digester is being loaded, another is blowing or receiving repairs. This helps to keep

production up even when some digesters are down.

Manual 17 US 256280


1.4.1.2 Vertical continuous digesters

Vertical continuous digesters are cone-shaped and constructed of mild steel. The digesters

are sometimes lined with stainless steel on the inside if corrosive cooking liquors are being

used. These digesters vary in size depending on the production output required of the unit.

Liquor distribution inside the digester is achieved via compound pipes – a compound pipe is

made up of a pipe within a pipe within a pipe. Each of the pipes contains slots at different

points to facilitate liquor distribution.

Continuous digesters are also equipped with bottom scrapers, situated inside the cone of the

digesters

There are two different types of vertical continuous digesters, namely:

Hydraulic digesters

Steam-liquid phase digesters

We will only deal with the more common hydraulic continuous digester used in South Africa.

The figure below shows a schematic of a Kamyr, single-vessel, hydraulic digester with a

two-stage diffuser. In this type of digester, the chips enter at the top and leave at the bottom

of the digester. As the chips continue along the length of the digester, they are exposed to

various areas where the following occur:

Liquor impregnation

Heating to cooking temperature

Cooking

Washing

These processes are carried out in designated zones within the digester. In a typical

continuous process the following occurs:

Pre-steaming: Chips from the chip bin are sent to a pre-steaming chamber where they

are pre-heated. This also helps to drive off any air inside the chips so that liquor

impregnation an be carried out more efficiently.

Digester loading: Chips are then passed through a high pressure feeder. Cooking

liquor (a mixture of white liquor and black liquor) carriers the chips into the digester.

Liquor impregnation: Chips and cooking liquor move down through the impregnation

zone, where it typically remains for 45 minutes at 105 to 130 ºC.

.

Manual 18 US 256280


Figure 3: Vertical continuous digester and its auxiliary equipment

Manual 19 US 256280


Heating zone: Here the temperature is raised in two steps by two cooking circulation

systems equipped for indirect heating

Cooking zone: After heating, the chips and liquor pass through the cooking zone

with a controlled retention time of 1.5 to 2 hours. In conventional cooking the liquor

temperature in the cooking zone will be approximately 170° C

Washing: At the end of the cooking zone of the digester, extracted wash liquor is

circulated through the chips to quench the cooking reaction. The chips continue

downwards into the counter current washing zone. The washing time generally lasts

between 1.5 and 4 hours. Wash liquor from a subsequent filtrate tank, or fresh hot

water, is pumped into the digester bottom and flows upward, counter-current to the

chip flow. Elevated temperatures of 125 to 135ºC are controlled by an auxiliary,

wash liquor circulation and heater system.

The following paragraphs give a short description of the equipment associated with a

vertical continuous digester (as shown above):

Airlock

Cleaned, screened chips enter the system via a metering valve called an airlock. The

airlock prevents steam from the chip bin escaping to atmosphere during the feeding of

steam accumulating in this part of the system is directed to an exhaust system.

Chip Bin

The chips are steamed in the chip bin for 10 to 15 minutes at a temperature of 90ºC. The

steam for the chip bin is drawn from Flash Tank No. 2.

Bin activator

The chips are discharged by the bin activator into the chip meter.

Chip meter

Setting the speed of the chip meter, determines the production rate of the digester.

Low pressure feeder

The low-pressure feeder is a rotary feeder that feeds the chips into the steaming vessel.

Steaming vessel

The chips are retained in the steaming vessel for 2 to 3 minutes at a pressure of 103 to

124 kPa. The steam is generally obtained from the flash tank from the digester or

alternatively fresh low-pressure steam can be used. Air, terpines and some steam are

retrieved from the vessel and sent to a condenser.

Manual 20 US 256280


Tramp material separator

After steaming, the chips may then be fed through an optional “tramp material separator”.

This is often a large electro-magnet for removing ferrous metal (magnetic) pieces. If the

chip handling system (before the digester) is equipped with one, this may not be

necessary. Metal objects will obviously damage pumps and other process equipment.

The removed tramp material is dumped.

Chip chute

From the steaming vessel (or tramp material separator) the chips are routed into a chip

chute leading to the high-pressure feeder

Chip chute circulation pump

The chip chute circulation pump pumps liquor into the front end of the feeder to flush chips

form the chute into the feeder.

High-pressure feeder

The high-pressure feeder is a rotary pump that transports both liquid and solids. As the

rotor turns, chips and liquor are combined and high-pressure impregnation begins. Within

one minute, chips and liquor are conveyed from the high-pressure feeder to the top of the

digester, by means of the top circulation system.

Digester and top circulation system

The liquor required for the top circulation circuit is extracted through a cylindrical screen in

the top separator. The chips are moved evenly down the digester by a screw mechanism

which also keeps the top separator screen clean. Three strain gauge type level sensors

mounted on the digester wall detect the chip level in the digester.

High pressure pump

A high-pressure pump introduces liquor to the digester as well as removes excess liquor

from the chip chute. A magnetic flow meter and control valve controls the liquor flow. A

positive suction head is provided for this pump by a level tank.

Cold blow pump

The digester pressure is controlled by the cold blow pump delivering at a pressure of 1100

kPa to the heating zone. This ensures positive penetration in the impregnation zone,

absence of flashing at pump inlets and a hydraulically filled digester at all times.

Impregnation zone

Manual 21 US 256280


Chips and cooking liquor move down through the impregnation zone, where it typically

remains for 45 minutes at 105 to 130 ºC.

Heating zone

Here the temperature is raised in two steps by two cooking circulation systems equipped

for indirect heating.

Heating systems

Each system includes extraction strainers, pumps, heat exchangers and central circulation

ducts. Temperature controllers for the liquor leaving the heaters establish the precise

retention temperatures.

Cooking zone

After heating, the chips and liquor pass through the cooking zone with a controlled

retention time of 1.5 to 2 hours. In conventional cooking the liquor temperature in the

cooking zone will be approximately 170 0 C

Counter current in-digester washing

At the end of the cooking zone of the digester, extracted wash liquor is circulated through

the chips to quench the cooking reaction.

The chips continue downwards into the counter current washing zone. The washing time

generally lasts between 1.5 and 4 hours. Wash liquor from a subsequent filtrate tank, or

fresh hot water, is pumped into the digester bottom and flows upward, counter-current to

the chip flow. Elevated temperatures of 125 to 135ºC are controlled by an auxiliary, wash

liquor circulation and heater system.

Multiple-stage diffuser washer

The principle of diffuser washing has been used between bleaching stages for many years

and is also used for brown stock washing. In a diffuser washer the lignin rich liquor is

displaced by a wash liquid in a vessel with a number of concentric screens through which

the liquor is forced. (Full description is given in the kraft pulping section).

1.4.1.3 Horizontal continuous digesters

Horizontal continuous digesters are designed for the pulping of wood chips, sawdust,

bagasse and other types of (less common) raw materials. Bagasse, raw material and of

relevance to this course is where caustic soda (NaOH) plus steam is used for cooking.

The caustic soda and live steam are fed directly into the top of the digester.

Manual 22 US 256280


The digester consists of two or three horizontal tubes depending on the size of the

process.

The equipment used in a horizontal continuous digester normally consists of the following:

Pin Drum Feeder

Screw Feeder

Tee Pipe with Blow Back Damper

Digester Tubes

Internal Screw Conveyors

Intermediate Pipe

Screw Discharger

Blow Valve

Each of these components will be described briefly in the following paragraphs. A

diagram of a horizontal continuous digester is shown below

Manual 23 US 256280


Figure 4: Horizontal continuous digester

Each of these components is briefly described in the following paragraphs.

Pin drum feeder

A conveyor to an inlet chute conveys bagasse to where it can feed under gravity into the

pin drum feeder (the overall plant production rate is determined by the pin drum feeder).

Manual 24 US 256280


The pin drum feeder is mounted underneath the inlet chute and consists of a steel

housing, inside of which two horizontal steel cylinders are mounted one above the other.

The cylinders are provided with numerous steel pins. The one cylinder is driven by a

variable speed control whilst the other is directly driven by the first by means of spur

gears. When the bagasse is discharged from the pin drum feeder it falls down the

bagasse inlet into the receiving flights of a screw feeder.

Screw feeder

The screw feeder consists of a screw (within a housing) with a gradually decreasing pitch

and a tapered end. The feeder compresses the bagasse and forces it through a "plug"

pipe. The plug of bagasse formed, acts as a forward seal against the pressure and

temperature in the rest of the system. As long as bagasse is moving into the screw feeder

the "plug" is continuously formed and the seal effectively maintained.

As soon as the bagasse leaves the "plug" pipe, it comes in contact with the steam which

disintegrates the plug and enters the Tee-pipe.

Tee-pipe

The Tee-pipe is a vertical, cylindrical pressure chamber, which forms a link between the

screw feeder and the digester tubes. The bagasse, steam and liquor (NaOH), in their

proper ratios, are introduced at this point. The Tee-pipe has a blow back damper, which

is operated by an air cylinder that is controlled by a single solenoid valve. The blow back

damper can either be operated by remote manual control or automatic control.

Digester tubes

From the Tee-pipe the pulp, steam and liquor drop down the first intermediate pipe into

the first digester tube. The tubes are constructed of mild steel and the diameter of the

horizontal tubes depends on the capacity of the mill. Each of the digester tubes contains

a screw conveyor.

The internal screw conveyors

These are constructed of stainless steel to withstand corrosion. Controlling the speed of

the internal screws regulates retention time within the horizontal tubes. Pulping of the

fibrous material continues due to intimate mixing of chemicals and steam as the raw

material is conveyed through the tubes.

Intermediate pipes

Manual 25 US 256280


The intermediate pipes are simply connecting pieces between the digester tubes. The

lowest intermediate pipe may be provided with a flanged pipe connection for cold blow

liquor.

Screw discharger

The screw discharger is a housing constructed of carbon steel, and equipped with a

paddle screw on the inside. The neck of the housing is provided with pipe couplings for a

measuring device and a sampling point. The screw discharger discharges the cooked

bagasse into the blow valve.

Blow valve

The blow valve is mounted on the end flange of the screw discharger and is constructed

of stainless steel. It is also equipped with replaceable nickel hardeners (which is a hard

type of steel). The orifice size of the blow valve can be adjusted by means of a remote

operated air cylinder or by means of any other type of mechanical device. The blow valve

allows the cooked bagasse to be discharged from the digester for further processing.

1.4.2 Blow tankBlow tanks are large cylindrical vessels that digesters discharge pulp into. On completion

of the cooking cycle the digester pressure is first partially relieved through a vent line,

before the blow valve is opened at the bottom of the digester and the contents blown into

the “blow tank”, a cylindrical vessel with a cone shaped bottom.

In the case of well-delignified chips, as in batch digester operations, the fibres are

effectively separated by the expansion that occurs as steam is released within the chips in

the blow tank. The blow tank also serves as a temporary storage vessel and provides

reserve stock and storage space during short shutdowns of equipment upstream or

downstream from it.

The blow line normally enters the dome section of the blow tank where impact plating is

welded into the inside of the dome to protect the vessel from the high velocity stock

entering the blow tank from the blow line.

Steam and vapour is blown off to the atmosphere through the separator and top outlet of

the blow tank. The pulp in the blow tank is withdrawn from the cone section by pumps.

The blow section consists of the following equipment:

Blow Tank Shell

Separator

Manual 26 US 256280


Agitator

Dilution Line

Manhole

Stock Pumps

Associated Instruments

Drain Pipe

The blow tank shell is constructed of mild steel plate of considerable thickness to

withstand the pressure, heat and velocity of the stock from the blow line. Attached to the

top of the blow tank is a separator and an outlet to atmosphere.

The blow tank is welded to columns securely fastened to vibration resistant foundations.

The whole construction is designed for combined stresses due to dead and live loads and

further amplified by vibration and blow line jet reaction.

Figure 5: Blow tank

The separator situated at the top of the digester is a cylindrical vessel equipped with an

outlet damper to atmosphere and a downward pipe entering the digester dome. The

separator separates the particles of pulp and liquid liquor from the steam and vapour. The

Manual 27 US 256280


vapour and steam escapes into the atmosphere and the liquor and pulp runs down the

down pipe back into the blow tank.

The agitator, which is a rotating device, is situated in the cone section of the blow tank

and its purpose is to "stir" the pulp and liquor (stock) in the blow tank. Agitators can either

be of the blade type or the propeller type. The agitator therefore keeps the stock in a

continuous motion to make it easier for extraction. If the stock is not kept in motion it

would settle to the bottom of the blow tank making it extremely difficult to extract at the

end of the cycle.

The pulp to liquor ratio (consistency) must be kept within certain parameters in the blow

tank. If the consistency becomes too high, the torque on the mixer may increase up to a

point where it will either trip or damage the motor.

Liquor is therefore added to the blow tank through the dilution line and the consistency

controller to maintain the correct consistency within the blow tank.

The manhole, which is normally a hole cut into the blow tank and covered by a steel plate,

is used to gain access to the inside of the blow tank. This is required for inspection and

cleaning purposes.

If excessive dilution liquor is added (low consistency) the volume of material becomes

excessive leading to high storage and energy requirements.

There are stock outlets (pipes equipped with manual or automatic valves) situated in the

cone section and connected to centrifugal pumps, which extract the stock from the blow

tank and pumps it to the washing section. Some installations provide spare outlets and

pumps as standby equipment.

There are normally only two instruments fitted to the blow tank, namely a level indicator

and a consistency controller. The purpose of the level indicator is to give an accurate

indication of the amount of stock in the blow tank by recording the level on a chart. The

consistency controller is set at a set point where an automatic valve controls the flow of

liquor to the blow tank thus controlling the torque on the agitator and also controlling the

consistency at the same time.

The drainpipe is situated in the bottom of the cone section of the blow tank and can either

have a manual or automatic valve or blank flange attached to it. The purpose of this drain

line is to drain the contents of the blow tank if required.

Blow pits are not as commonly used as blow tanks are. Blow pits are sunken oblong

"vessels" and serve the same purpose as blow tanks, i.e. to receive the discharged pulp

from the digesters and to provide storage space for pulp until such time it is required.

Manual 28 US 256280


Blow pits are normally brick or concrete structures lined with chemical resistant tiles on

the inside. This is required because of the corrosiveness of certain pulping liquors.

Perforated stainless steel screens are located in the bottom of the blow pit to allow for

drainage of liquor. The blow pits normally have top coverings with openings for access

and vent pipes for the release of steam and vapour. In certain processes the blow pits

also act as a "pulp washing stage" by introducing wash waster in the top through a spray

system.

The washed pulp is withdrawn from the bottom of the blow pit by centrifugal pumps.

1.5 Kraft pulping processThe “Kraft” process (meaning “power” or “strong” in German) was developed in Germany

about 100 years ago. The process involves cooking wood chips in a solution of sodium

hydroxide (NaOH) and sodium sulphide (Na2S). The sodium hydroxide (an alkali – the

opposite of an acid) attacks and fragments the lignin molecules into smaller pieces.

These smaller parts are soluble in the alkali and are separated from the wood, leaving

only the pulp behind. The liquor and dissolved lignin are burned and the valuable alkaline

chemicals are recovered for reuse.

1.5.1 Principles of the Kraft pulping processThe Kraft pulping process consists of various steps from loading the digester with wood

chips to the collection and storage of the final product. Each of the steps will be

discussed in the sections that follow.

1.5.1.1 Digester loading

Wood chips are usually fed into the digester either via a chip hopper or directly from a

swing conveyer into the open capping valve. The chip conveyers often pass under strong

electromagnets to remove ferrous scrap metal before it enters the digester. The scrap

metal has to be cleared from the magnets on a regular basis.

The quantity of chips loaded is very important, as the ratio of the mass of the dry wood to

mass of the liquid in the digester must be accurately determined. The “liquor to wood

ratio” can be defined as is the mass of liquid in the digester to the mass of oven dry wood

expressed as a ratio.

The most commonly used ratio is 4:1 i.e. 4 kg of liquid to every 1 kg of oven dry wood. It

is important to note that the liquid component of the ratio, not only includes the cooking

liquor, but also the moisture in the chips prior to cooking.

Manual 29 US 256280


The quantity from the chip hopper can either be calculated from the volume of chips or the

hopper of chips has a load cell to measure the mass of chips inside it. The chip quantity

loaded directly from a conveyer is calculated by having a section of the conveyer mounted

on load cells. A computer then calculates the quantity of chips from the mass of the chips

crossing that section of the conveyer, taking the speed of the conveyer into account.

1.5.1.2 Steam packing

The chips, whilst being loaded, tend to form a pyramid as they fall into the digester as can

be seen below.

Figure 6: Pyramid digester loading

Low-pressure steam is therefore introduced at the top of the digester at a tangent to the

walls to blow the chips in a swirling action. This not only prevents the chips from falling

into a cone, but also ensures that the chips are packed evenly without cavities forming.

This arrangement can be seen below. Steam packing can increase the amount of dry

wood per digester volume by 30% to 45%

Manual 30 US 256280


Uniform chip packing is important to ensure that the cooking liquor is evenly distributed

throughout the entire bulk of the chip load. This ensures a uniform cook throughout the

digester thus results in better yields and quality of the pulp.

Figure 7: Steam packing

1.5.1.3 Pre-steaming

Wood that has been felled, debarked, made into wood chips and stored still retains a

certain amount of natural moisture in the form of sap.

Transporting very "green" wood shortly after felling, results in a very high remaining

moisture content which makes transportation costs high. Wood is therefore often stored

in the forest to allow for a drying out period.

Wood is made up of individual cells, which contain hollow tubes. The cooking liquor will

penetrate the wood chip much quicker and more effectively if the cells in the chip already

contain liquid in other words, by diffusion. Pre-steaming is a means of ensuring that

maximum liquid is present in the chips prior to cooking.

As soon as the chips have been introduced into the digester the capping valve is closed.

Manual 31 US 256280


Figure 8: Pre-steaming of chips

Low-pressure steam is then introduced for a short period. The steam condenses in the

chip, increasing the liquid content and temperature of the chips. At the end of the

pre-steaming cycle the digester is depressurised.

Not all mills practice pre-steaming as it is time consuming and costly in terms of steam

consumption. The figure above shows the process of introducing steam to the chips

loaded in the digester.

1.5.1.4 Cooking liquor charging

The Kraft cooking process employs a mix of fresh white liquor and spent black liquor in a

cook to ensure the correct liquor charge. White liquor is treated black liquor from the

chemicals recovery process. The black liquor is spent cooking liquor obtained from the

previous cook. The black liquor being added is used as a diluting agent to obtain the

correct liquor (chemical) to wood ratio.

The total liquor to wood ratio is very important – if the liquor to wood ratio is too low, the

contents of the digester will be too solid to blow out (the consistency will be too high, a

ratio of 4:1 equates to a consistency of ~12%).

If the liquor to wood ratio is too high it indicates that the maximum volume of wood chips

has not been loaded, thus wasting production capacity. It is therefore very important that

this ratio is maintained at a set value: usually 4:1. The figure below shows the cooking

liquor charging process.

The volume of the cooking liquors added in from the holding tanks to the digester is

determined by:

Manual 32 US 256280


the weight of the wood in the digester,

the concentration of the active ingredients in the cooking liquor.

Figure 9: Cooking liquor charging

1.5.1.5 Ramping

Once the cooking liquor is added, all valves, other than the circulation valves are closed.

The circulation pump is started and low-pressure steam is introduced into the liquor heat

exchanger to raise the temperature of the contents of the digester – this is called

"ramping". In directly heated digesters, live steam is introduced into the digester to raise

the temperature. When the temperature reaches a certain pre-determined level, the low-

pressure steam is switched over to medium pressure steam.

Kraft cooking normally operates at 170 ºC and at a pressure of 8 bar.

1.5.1.6 Cooking

Once the target cooking temperature and pressure is reached, the steam flow is reduced

so as to only maintain that temperature while the circulation pump continues circulating

the liquor. Gasses are continuously released from the digester to maintain a constant

pressure.

Manual 33 US 256280


In Kraft cooking the relief gases are collected since they have a foul smell. In most

modern mills turpentine is extracted from the gas while the remaining gasses are

incinerated to reduce odour.

Figure 10: Cooking configuration

1.5.1.7 Blowing

On completion of the cooking cycle the digester pressure is first partially relieved through

a vent line, before the blow valve is opened at the bottom of the digester and the contents

blown into the “blow tank”, a cylindrical vessel with a cone shaped bottom.

In the case of well-delignified chips, as in batch digester operations, the fibres are

effectively separated by the expansion that occurs as steam is released within the chips in

the blow tank. The blow tank also serves as a temporary storage vessel and provides

reserve stock and storage space during short shutdowns of equipment upstream or

downstream from it.

The blow line normally enters the dome section of the blow tank where impact plating is

welded into the inside of the dome to protect the vessel from the high velocity stock

entering the blow tank from the blow line.

Manual 34 US 256280


Steam and vapour is blown off to the atmosphere through the separator and top outlet of

the blow tank. The pulp in the blow tank is withdrawn from the cone section by pumps.

1.5.1.8 Turpentine collection

A typical turpentine recovery plant can consist of the following equipment:

Decanter Tank

Surge Tank

Stripper Tank

Condensers

Separators

Figure 11: Turpentine recovery system

The decanter tank is a vertical cylindrical tank used to separate the water from the

turpentine. Water and turpentine do not mix with turpentine floating on the top.

The surge tank (storage tank) is a cylindrical horizontal tank used for collecting the clean

turpentine from the decanter before being pumped to storage.

Manual 35 US 256280


The stripper tank is an upright cylindrical vessel that condenses the vapours from the

condenser system to recover the turpentine still present in the vapour.

The turpentine condensers are large/diameter horizontal "pipes" equipped with tubes

containing cooling water on the inside. Each system consists of a number of condensers

of various sizes and is used for cooling and condensing the various steam vapours for

turpentine recovery.

A recovery plant can consist of a number of separators, which are upright cylindrical

vessels with conical bottoms. These separators are mostly used for separating the

turpentine and condensate.

A plant can also contain a number of storage tanks - some are being used as settling

tanks (to allow time for the turpentine and condensate to separate) and others are used

for storing the clean/clear turpentine before it is despatched.

1.5.2 Process flow diagramA flow diagram of the Kraft pulping process is shown in the figure below.

.

Figure 12: Kraft pulping process flow diagram

1.6 Acid sulphite pulping processThe Acid Sulphite Pulping process was developed by Benjamin Tilghman, who used

sulphurous acid (H2SO3) to soften and defibre wood chips. He noted that the sulphurous

Manual 36 US 256280


acid alone produced a "burnt" or blackened pulp. Later he determined that the presence

of a cationic base such as calcium bisulphite (Ca(HSO3)2) prevented the discoloration.

Since the 1950s, the utilisation of bases other than calcium has been a major

development. The more soluble bases such as magnesium, sodium and ammonium are

more conducive cooking conditions that require a higher pH for delignification

The acid sulphite pulping process therefore involves cooking wood chips in a mixture of

calcium or magnesium bisulphite and an excess of sulphurous acid. The calcium based

cooking liquor is not recovered due to the fact that calcium scales up the process

equipment to an extent that recovery is not practical. The spent liquor is treated for lignin

recovery.

Only one pulp mill in South Africa, Sappi Saiccor, utilises the magnesium and calcium

based processes. The produced pulp is used for dissolving pulp (or viscose pulp) which is

the raw material to produce synthetic fibres such as rayon.

1.6.1 Principles of the acid sulphite pulping processOne molecule of calcium bisulphite is chemically equivalent to one molecule sulphurous

acid (referred to as "free SO2") plus one molecule of calcium sulphite (referred to as

"combined SO2").

Ca(HSO3)2 → H2SO3 + CaSO3

The percentage "free SO2" can be determined by a titration with NaOH according to

TAPPI methods.

The traditional Ca acid sulphite cook must be carried out at a low pH of 1,2 – 1,5 because

of the relative insolubility of Ca at higher pH. A higher pH would also cause scaling in the

digester and further along the pulping and recovery processes. Typically, 80% or more of

the SO2 is in the form of "free SO2".

The use of a soluble base like Mg permits a greater proportion of "combined SO2". True

bisulphite pulping is characterised by equal amounts of "free SO2” and "combined SO2"

and is carried out at a pH of 4,0 to 5,0.

With bisulphite pulping a more rapid heating cycle can be used than in acid-sulphite

pulping. The lower acidity of the bisulphite cook necessitates a higher temperature for

cooking. However, at the bisulphite's higher pH levels, degradation of the cellulose is

much less than at the lower acid-sulphite pH levels.

In spite of the higher cooking temperatures with bisulphite pulping, the total digester

pressure can be held at about the same level as with acid sulphite pulping. Steam

Manual 37 US 256280


requirement is not significantly increased since most of the heat in the digester is

transferred to the incoming liquor for a subsequent cook by means of the blowdown relief.

Bisulphite cooking liquor can be introduced into the digester at much higher temperatures

than acid sulphite liquor. For this reason it is possible to use a shorter cycle with

bisulphite pulping than with acid sulphite.

1.6.2 Acid sulphite pulping in industryIn South Africa both the Ca(HSO3)2 and Mg(HSO3)2 pulping processes are used at Sappi's

Saiccor pulp mill.

The introduction of bases other than calcium, such as magnesium as the cooking

chemical, produced pulps of higher yields with a wide range of properties. Wood species

that are unsuitable for calcium bisulphite pulping can now also be pulped with magnesium

bisulphite.

The relatively high cost of magnesium and sodium base chemicals has encouraged the

development of efficient recovery systems, which have since its development also

become vital for environmental control.

Recovery of cooking chemicals for the calcium base system has never been practiced.

The sulphite waste liquor is normally discharged into the nearest receiving water since it is

not feasible to recover usable chemicals from the calcium sulphate ash. The make-up

chemicals, limestone and sulphur are inexpensive and readily available. The calcium

present can also cause very serious scaling problem, especially in the evaporator section

of the mill.

Manual 38 US 256280


1.6.3 Process flow diagram

Figure 13: Acid sulphite pulping process

1.6.4 Advantages and disadvantagesSome of the main reasons for the success of the sulphite pulping process are:

relatively high yield,

lower cost of cooking chemicals compared to the soda process,

higher brightness of unbleached pulps, (thus requiring less subsequent bleaching),

the resulting pulp is more easy bleached and with relatively simple bleaching agents.

The sulphite pulping process, however, also suffers some distinct disadvantages:

the pulps produced were weaker than those produced by the Kraft process,

special equipment is needed to withstand the harsh acidic conditions and

a limited number of wood species could be pulped.

In making a decision about a new chemical pulping plant, not only should the product to

be manufactured be considered, but also the advantages and disadvantages of a process

in the context of continued new developments and improvements in the acid sulphite

process.

Manual 39 US 256280


1.7 NSSC pulping processSemi-chemical pulping combines chemical and mechanical methods. Essentially, the

wood chips are partially softened or digested with chemicals and the remainder of the

pulping action is done mechanically.

The number and types of semi-chemical pulping processes have increased in recent

years. These processes may be classified as “semi-chemical” processes, because they

use a combination of chemical pre-treatment followed by mechanical defibreing. The

following is an example of one of the many semi-chemical processes – the neutral

sulphite semi-chemical process (NSSC).

Figure 14: Sulphite semi chemical pulping process

This process uses neutral pH liquor containing sodium sulphite (acid) and sodium

carbonate (alkali). Hardwoods are the predominant feedstock in this process.

The only local NSSC (neutral sulphite semi-chemical) process in operation is the one at

Sappi Tugela on the North Coast. The equipment for cooking liquor preparation of each

system will be dealt with separately.

Manual 40 US 256280


1.7.1 Cooking liquor preparationIn the NSSC process the recovered spent liquor is not used for the preparation of cooking

liquor for this process, but, as stated in the previous section, this liquor is evaporated and

burnt in a Copeland reactor, from where the end product is sold to various industries.

Therefore, the preparation of cooking liquor for the NSSC process is totally a process on

its own. Raw sulphur and carbonate (soda ash) has to be purchased to produce the

cooking liquor for the digestion of the chips.

The cooking liquor preparation plant, the “Celleco” plant, consists of the following

equipment:

Sulphur storage

Melting basin

Rotary sulphur burner

Quench tower

Absorption tower

Cooking liquor storage

1.7.2 Sulphur storageSulphur arrives in a granulated form and is stored in a sheltered area with a capacity of a

number of tons. The sheltered area normally has a roof over it and an open side for the

addition and removal of sulphur with a front-end loader.

1.7.3 Melting basinA number of sulphur melting basins of various sizes and shapes can be used depending

on the mill. The basins are lined with heat and corrosive resistant material and each basin

is equipped with a number of steam heating coils, temperature controllers and a number

of sulphur pumps. The raw sulphur is conveyed to the basins where the heating coils heat

up the sulphur to its melting point in preparation for the burner.

All pumps and lines used in this cooking liquor preparation process are steam jacketed to

prevent the sulphur from cooling down.

1.7.4 Sulphur burnerThe sulphur burner is an air-cooled refractory lined (normally fire bricks) combustion

chamber consisting of a steel plated, cylindrical shell with conical ends – see the following

figure. These sulphur burners can vary in size depending on the process.

Manual 41 US 256280


Figure 15: Sulphur burner

Fine molten sulphur mixed with atmospheric air is sprayed into the burner to permit

complete combustion. A sulphur gun is fitted to one end of the burner, while a gas outlet

pipe to the quench tower is found at the other end. The burner is also fitted with

combustion and cooling fans.

1.7.5 Quench towerThe quench tower is an upright vessel constructed of stainless steel and lined with lead to

prevent corrosion of the construction metal. Direct cooling of the gases takes place inside

by a number of water sprays, which is circulated through a plate heat exchanger (water

cooled) where "heat transfer" takes place. The "cooled" gases are then piped to the

absorption tower.

1.7.6 Absorption towerIt is in this tower where the actual cooking liquor is produced. The absorption tower is

also an upright vessel constructed of stainless steel. The tower is divided into four

sections, the top section is the gas outlet, the next two sections are filled with plastic

saddles designed to slow the downward flow of carbonate. The "cooking liquor" is then

pumped to the storage.

A number of cylindrical mild steel storage tanks of various sizes are used to store the

different chemicals. The storage tanks for carbonate storage are fitted with steam heating

Manual 42 US 256280


coils, while all carbonate lines and pumps are also steam jacketed to keep the carbonate

from hardening in the system.

1.8 Soda pulping processBagasse can be pulped by any of the conventional processes, but only the soda process

will be dealt with since it produces a bleachable grade. Caustic soda is the chemical used

in this process as the cooking agent.

Bagasse requires far less cooking time (~15 minutes in total) than wood chips, partly

because it is shredded to the consistency of coarse grass cuttings. An added advantage

of bagasse is that there is no smell from this process.

Pith is the central non-fibrous material of sugar cane. As can be seen from the flow

diagram, the pith must be removed from the bagasse before it can be pulped. The reason

for this is that the fine structure of the pith will absorb and react with the caustic soda,

thereby wasting a considerable quantity of valuable chemicals. It also slows down paper

machine drainage, thus affecting washing and production rates.

Since bagasse is a by-product of the sugar industry, it is available in large quantities from

the sugar mills, giving it a unique position among the non-wood fibres for pulp

manufacture. A distinct advantage of bagasse over other non-wood plant fibres is that the

cost of collecting, crushing and cleaning is borne by the sugar mills. However, adequate

quantities of bagasse must be collected and stored during the seven month crushing

season to ensure continuous pulp mill operation until the next season.

The specifics of the soda process used to pulp bagasse as well as the type and amount of

cooking chemical depends to some extent on the desired end product. The specific focus

in this module will be the Soda-process as used at the Sappi-Stanger mill.

The composition, physical and chemical properties as well as pre-treatment of bagasse

prior to pulping will also be discussed in the following sections.

1.8.1 Process descriptionThe figure below gives an outline of the process:

Manual 43 US 256280


Figure 16: Bagasse soda pulping process

Each of the process steps will be described shortly in the following paragraphs.

1.8.1.1 Moist or humid depithing

Depithing is an important step in upgrading bagasse for the production of high -grade

pulps. Crude bagasse or partially depithed bagasse may be used for low grade

corrugating medium insulating boards and similar varieties. Properly depithed bagasse

requires less chemical charge in both the cooking and bleaching steps to still produce a

high quality pulp. The two mills operating in South Africa both only use depithed bagasse

to produce pulp.

The first moist or humid depithing step is normally done at the sugar mill since the pith can

be used as fuel in the sugar mill boilers to reduce the overall fuel cost.

Bagasse containing pith, grit, residual sugar and bacterial breakdown products like

organic acids and alcohols pass to a depithing plant where the bagasse is fed into a

hammer mill or another similar process. These machines are designed to break open the

fibre bundles and dislodge the embedded pith by mechanical rubbing and a mild

Manual 44 US 256280


disintegration action. Although a variety of machines are used for this function, they all

utilise some variation of the same basic principle.

This is followed by a rough separation step in which some two thirds of the pith is normally

removed for use in the sugar mill boilers. The separation consists of the bagasse first

being vigorously slurried at 2-3% consistency in a hydropulper to loosen any pith and grit.

From the hydropulper the bagasse is then transferred to a cyclone where the pith, heavy

fibre and grit are separated from the useful bagasse.

1.8.1.2 Bagasse storage

From the cyclone the moist, partially depithed bagasse is transported to the pulp mill by a

conveyor or road transport and deposited into a launder with “Ritter water”. Ritter water

(or Ritter liquor) is a "bacterial soup" which inoculates the bagasse with bacteria which

consume the sugars to produce VFA's (Volatile fatty acids which produce a sour smell)

and loosen the remaining pith.

From here it is deposited on a storage slab to allow for the necessary chemical reactions

to take place. The quality of the bagasse is improved by storage and is at the same time

a necessity to keep a supply in the off-crop season. The bagasse is kept on the storage

slab for about 6 – 7 weeks during the cutting season and up to six months in the off-

season.

The heap of bagasse is kept wet with process water or Ritter water to keep out any air.

However, the Ritter water slowly drains through the heaps of bagasse and the top

sections tend to dry out. Where the heap dries, aerobic bacteria grow and destroy the

bagasse. This activity is detected by a temperature rise in the heap and when the internal

temperature exceeds 40oC, irrigation with water or recycled Ritter water is employed.

Rain can also cool the heap down and keep it moist.

Some mills do not use the Ritter system, but recirculate the water used to soak the heap

after transfer. As this is acid, due to the growth of bacteria the effect is the same. After at

least 3 weeks storage the bagasse is recovered from the heap and fed via a pin drum

feeder to a launder channel irrigated by paper machine backwater.

1.8.1.3 Wet depithing

A conveyor (or front-loader) loads the bagasse into a pin drum feeder that feeds the

bagasse at a predetermined daily production rate to a hydropulper. Here the mass is

thoroughly wetted and partially disintegrated (thereby opening the fibre bundles) by

Manual 45 US 256280


vigorous mixing. The suspended slurry is then pumped to the depithing machine either

directly or by way of a settling tank and a magnetic separator.

Swing hammers in the depither breaks the remaining mass down to a pulp which then

completes the defibrating operation. The pith passes through a perforated screen after

which it is thickened by a dewatering press or a filter. The screened pithy liquor is clarified

for reuse, and the pith goes to landfill or can be used as fertilizer.

Counter current washing of the pulp over a number of vibrating screens removes any grit

remaining in the pulp, while the gritty liquor also goes to the clarifier. The separated fibre

is discharged to a conveyor system that delivers the depithed bagasse to the pulping unit.

1.8.1.4 Cooking liquor preparation

In this process there is no cooking liquor preparation, as crude caustic soda (at a pH of

13) is used as the cooking agent in the bagasse soda process. The caustic soda (about

50% solids) is delivered to the mill by either rail or road transport where it is off-loaded into

storage tanks. Before the caustic soda is used in the cooking process it has to be diluted

to the correct concentration. When this has been done it is then pumped directly into the

digester with the steam to cook the bagasse.

1.8.1.5 Bagasse digestion

Any design of digester may be used in the soda process, but the horizontal continuous

type is the most common. Of the several types of horizontal continuous digesters, the

most widely used consists of one or more horizontal tubes stacked one above the other.

Two or three tube units are more common than four tube units.

These two and three tube units are widely used in the cooking of bagasse, where the

bagasse, cooking liquor and steam is fed into the top tube. However, the digester can

consist of a two, three or four tube system; depending on the process (see Error:

Reference source not found). A typical operating pressure for a digester is ~800 kPa.

An important feature of bagasse is its springy texture; it has to be compressed before

entering the digester. From here the mixture is moved by revolving screws from one end

of each tube to the other. Live steam is introduced into the bottom of each horizontal tube

to maintain the pressure and the cooking temperature between 165- 175°C. Cooking time

in the digester varies from 10-20 minutes, depending on the process.

The cooked bagasse is blown out of the bottom of the digester, where "cold blow" is

introduced to cool down the bagasse – direct contact with the atmosphere will damage the

fibres. The bagasse is than blown into an atmospheric blow tank. From the blow tank

Manual 46 US 256280


bagasse pulp is put through a series of screens and cleaners to remove any rejects before

being passed on to the washing section.

The washing section consist of a number of washing stages incorporating a number of

thickening and dilution stages before it goes to the paper section or for further processing

in the bleaching section.

Manual 47 US 256280


Figure 17: Bagasse digester

Manual 48 US 256280


UNIT 2: MONITOR AND CONTROL RELEVANT ANCILLARY SYSTEMS AND UTILITIES

Learning outcomes


Identify and describe mechanical equipment used in the chemical pulping process in terms of purpose and application.

Identify and describe electrical equipment used in the chemical pulping process in terms of purpose and application.

Identify and describe instrumentation used in the chemical pulping process in terms of purpose and application.

Identify and describe utilities used in the chemical pulping process in terms of purpose and application.

Discuss typical ancillary equipment problems within the chemical pulping process and offer solutions in accordance with workplace procedures.

Monitor ancillary systems and correct any deviations from operating parameters in accordance with operating procedures.

2.1 Instructions


SO1 AC5

SO2 AC1-6

CCFO 3, 4, 5, 7, 8

Learning materials


Act. 5

N/a N/a 00:00

Manual 49 US 256280



SO1 AC5

SO2 AC1-4

CCFO 3, 4, 5, 7, 8

Lecture room

Facilitator

Multimedia

SOPs, workplace and other relevant procedures

Attend a lecture and/ or facilitated discussion on the various ancillary equipment (i.e. mechanical, electrical, instrumentation) and utilities relevant to the chemical pulping process. This includes:

Mechanical equipment e.g. bulk handling, conveying, weighing, storage, transport, packaging equipment

Electrical equipment, e.g. electrical motors, switchgear, drive equipment

Instrumentation e.g. process indicators, control valves, controllers

Utilities e.g. air, steam, electricity and cooling water

Function/ purpose

Application

Operating principles

Ex. 2

Questions

Ass. 1

Questions and

diagrams

00:00

Act. 6

Manual 50 US 256280


Ref. No Resources Learning Methodology Workbook Assess Tim

e

SO2 AC5, 6

CCFO 1, 3, 4, 5, 7, 8

Lecture room

Facilitator

Multimedia


Attend a lecture and/ or facilitate discussion on:

Ancillary equipment problems and solutions to these problems

Process parameters

Deviations from operating parameters and solutions to these deviations Ex. 3

Questions

Ass. 1

Questions and

diagrams

00:00

Act. 7

SO2 AC1-6

CCFO2, 4-8

Lecture room

Facilitator

In a group, discuss the interaction between the various ancillary systems, utilities and the chemical pulping process and the effect of non-conformities on the final product properties. Act. 8

00:00

SO2 AC1-6

CCFO 1, 3, 4, 5, 7, 8

On-site

Facilitator/ SME

PPE/ PPC

Notebook and pen


On-site, identify the main ancillary equipment and utilities relevant to the chemical pulping process and observe how these processes are monitored and controlled.

Take notes of the step-by-step procedures. Discuss your notes with a SME and draw up a checklist(s). Verify the checklist(s) for correctness.

Notes

Ass.2

Monitor & control

00:00

Act. 9Ex. 4

Checklist

Manual 51 US 256280



SO2 AC1-6

CCFO 1, 3, 4, 5, 7, 8

On-site

SME

PPE/ PPC


Under supervision monitor and control the ancillary and utility equipment and processes per SOPs and/ or other relevant workplace procedures

Act. 10

Ex. 5

Practical

Ass.2

Monitor & control

00:00

SO1 AC5

SO2 AC1-6

CCFO 3, 4, 5, 7, 8


PoE

Facilitator/ SME

Revise the work that you have done up to this point. Make sure that you have completed the CCFO checklist and obtained the required evidence for your PoE. If there is anything that you do not understand, ask your facilitator.

CCFOs CCFOs 00:00


Manual 52 US 256280


2.2 IntroductionYou should now have a very good idea of the chemical pulping process and the

equipment used in the process. However, you also need to be familiar with a whole range

of ancillary systems related to the chemical pulping process. In this unit we are going to

discuss the mechanical and electrical equipment, instrumentation and utilities that are

relevant to the chemical pulping process.

2.3 Mechanical equipmentThe mechanical equipment relevant to the chemical pulping process includes, for

example, bulk handling equipment, conveying equipment, storage equipment, transport

equipment and packaging equipment.

2.3.1 Conveying equipment Conveyor mechanisms are used as components in automated distribution and

warehousing. This allows for more efficient transportation of raw materials such as logs,

woodchips, minerals, powders, catalyst, etc. as well as final products such as fertiliser,

paint, etc. to the warehouse. It is a labour saving system that allows large volumes to

move rapidly through a process. Conveying systems are used to transport material

upwards, downwards and horizontally over a long or short distance. There are different

types of conveyors such as belt conveyors, pneumatic conveyors and screw conveyors.

2.3.1.1 Belt conveyors

The belt conveyor is the most well-know type of conveyor and is often found in a range of

different workplaces.

A belt conveyor is a machine that is used to move large tonnages of solid materials from one place to another over paths beyond the range of any other mechanical conveyor.

Manual 53 US 256280


Figure 18: Example of a belt conveyor

A conveyor belt or belt conveyor consists of two end pulleys, with a continuous loop of

material (belt) that rotates about them. The pulleys are powered, moving the belt and the

material on the belt on an inclined surface, either down, up against gravity or horizontally.

The following figure shows the basic principles of a belt conveyor.

Figure 19: A basic belt conveyor transporting solid material

In the above figure, the belt runs between two cylindrical drums of which one (anyone) is

driven by a motor.

Applications of belt conveyors

Manual 54 US 256280


In the pulp and paper industry conveyors are usually designed for a specific process. In

the chemical pulping process conveyors are used to convey wood chips to the digester,

especially in continuous systems. The figure below shows an inclined belt conveyor.

Figure 20: An inclined belt conveyor

2.3.1.2 Pneumatic conveyors

A pneumatic conveyor uses air to move bulk materials, either from storage facilities to a

process unit, or between process units.

This type of bulk solid transport equipment moves material that is suspended in a stream

of air. This movement can either be vertically or horizontally over a short or long distance.

Different systems of pneumatic conveyors are found in the manufacturing and processing

industry. The main ones are classified as follows:

Pressure system

Vacuum system

Pressure-vacuum system

Fluidising system

The first three types are defined by the type of pressure, namely gauge pressure that is

used to move the material. Gauge pressure is the pressure that can be seen on

measuring instruments. These instruments are called pressure gauges.

Manual 55 US 256280


Figure 21: Pneumatic conveyor systems

Pressure system

This type of system, which is also known as a positive pressure system, uses air that is at

high pressure above atmospheric pressure to deliver the material. Fans or compressors

are used to blow the air that is used to transport the material. A blower delivers air at high

pressure into the pipeline that is used for conveying material. Material is fed into this

pipeline by a feeder that is situated underneath the storage medium. The bulk material is

suspended in the air by the high velocity of the stream of air until it reaches the receiving

vessel. An air filter or cyclone separator that is situated above the receiving vessel is

used to separate the material from the air.

Manual 56 US 256280


Figure 22: A basic positive pressure pneumatic conveying system

Vacuum system

This type of system is characterised by an air stream that has pressure less than

atmospheric pressure. In the following figure, a blower is used to suck air from the filter.

The suction continues to the conveying line. The feeder that is connected to the storage

medium feeds material into the line that is under suction. This material is pulled to the

filter where it is separated from the air.

Initially the air that is fed into the conveying line is filtered before entering into the pipeline.

Figure 23: Basic negative pressure pneumatic conveying system

Manual 57 US 256280


Pressure-vacuum system

This is a combination of the pressure and the vacuum system. The material is sucked into

the conveying pipe by the vacuum and is moved a short distance to a cyclone separator

which separates the material that is conveyed from air. Air is passed through a filter and

then into the suction side of a positive displacement blower while the solids move into an

intermediate storage tank. In this tank, material is stored temporarily. The positive

pressure is caused by the blower that is connected to the separator.

Figure 24: Positive and negative pressure pneumatic conveying systems

The non-return valve that is situated after the blower ensures that the air does not move

back to the cyclone separator.

Material is fed into the pipeline by the feeder that is situated below the storage medium

and is blown by the air that comes from the bower. Material is then moved to the

receiving hoppers.

The configuration that is shown is not the only one that is used in industry. Other

complicated configurations are also used.

Fluidising system

Fluidisation is when material is suspended (floating) on air. The material is suspended by

the air that is moving upwards. Fluidised material tends to display properties of liquids like

the ability to flow from a lower to higher level. For example, a powder that was fluidised in

a vessel would flow from a hole that is situated on the sides of that vessel through a pipe

Manual 58 US 256280


that is fitted in the hole of the vessel. This will happen, provided the pipe is not so long

that the material is completely defluidised.

The fluidising system uses the principle of fluidisation. A blower is used to blow air into

the pipeline just below the storage structure. Material that comes from the storage

medium is suspended in the air by air from the blower. Material is then moved along the

pipeline still in a suspended form to the discharge.

The fluidising system conveys material that is pre-fluidised over a short distance such as

from storage vessels, bins or transportation vehicles to the entrance of a main conveying

system.

Figure 25: A fluidised system

2.3.2 Weighing equipmentChips must be weighed before being loaded so that the right amount of liquor can be

added to keep the liquor to wood ratio at the optimum value. Wood chips are usually fed

into the digester either via a chip hopper or directly from a swing conveyer into the open

capping valve. The quantity from the chip hopper can either be calculated from the

volume of chips or the hopper of chips has a load cell to measure the mass of chips inside

it. The chip quantity loaded directly from a conveyer is calculated by having a section of

the conveyer mounted on load cells. A computer then calculates the quantity of chips

Manual 59 US 256280


from the mass of the chips crossing that section of the conveyer, taking the speed of the

conveyer into account.

A schematic of a conveyor with a load cell is shown below.

Figure 26: Chip conveyor with load cell

2.3.3 Storage equipment

2.3.4 Solids storage methodsOnce the production process is complete, the end products – usually solids – have to be

conveyed, and sometimes temporarily stored. Here not only dosing, flow measurement

and fill-level measurement, but explosion protection and plant safety also have to be taken

into account.

Bulk storage containers, such as bunkers, silos, bins and hoppers, and their ancillary bulk

handling equipment, are important to operations in a range of industries including, petro-

chemical, chemical, pulp and paper, agriculture, etc. Bulk containers are used in many

industries to store a range of substances such as cement dust, paper pulp, plastic pellets,

farm products and fertilizer

2.3.4.1 Bunkers

A bunker is a cost effective, high volume storage system for various types of solids such

as fertilisers and pulp and paper raw materials such as wood chips.

Manual 60 US 256280


Figure 27: Storage bunker

2.3.4.2 Silos

Silos are structures for storing bulk materials for the chemical, pulp and paper, mineral,

food, grain and plastics industries and can be constructed from steel, concrete, wood or

plastic.

Silos provide the ultimate in product protection during storage and their vertical design

allows maximum product storage in minimum space.

Figure 28: Silos

Silos are usually 4 to 8 meters in diameter and 10 to 25 meters in height.

Silos can be constructed with hopper bottom, flat bottom or composite bottom, which is a

combination of both the hopper and flat bottoms.

Manual 61 US 256280


Figure 29: A silo and its basic components

For smooth operation in conveying lines that take material from a silo, the flow from the

silo itself must be smooth and continuous.

All silos have access throughout their height so that maintenance can be done on them.

Most of the designers provide a series of vertical ladders whereas some prefer spiral

stairways. At the silo roof level, handrails and toe plates are provided for safety.

For access to the inside of the silo manholes are made at / or just above the cone (the

bottom section of a silo). This is done for inspection and maintenance of the lower parts

of the silo. One must make sure that these manholes are closed properly when the silo is

in operation to prevent material from escaping.

Silos can also be operated parallel to each other. The following figure shows silos that

are connected to each other by vents. In this way they use the same dust filter that is

located at one of the silos. In other operations each silo has its own dust filter.

Manual 62 US 256280


Figure 30: Silos operating in series sharing a common filter

Silos can be hazardous with people dying every year in the process of filling and

maintaining the silos or by falling from the ladders or work platforms.

There have also been cases of silos exploding. If the air inside becomes laden with finely

granulated particles, such as grain dust, a spark can trigger an explosion powerful enough

to blow a concrete silo apart.

Industrial silos are used to store a great variety of materials, from food products to

concrete mixture to roofing chemicals. Depending on the products being stored, silos

need to be cleaned out periodically.

2.3.4.3 Hoppers

A hopper is a wide bin-/ funnel like discharge feeder - used for holding various solids

materials - that is open on top and tapers to the bottom where it is thinner to feed into

another process system, such as a silo or conveyor system.

Figure 31: Storage hopper

Manual 63 US 256280


Hoppers are manufactured from various materials. Aluminium or steel hoppers are used

for heavy duty applications whereas plastic made hoppers are used in light duty

applications.

Hoppers are available in different shapes and sizes such as wedge, pyramidal or conical

shape as well as in bottom or tilt type. In bottom type hoppers, the materials are released

from the hopper bottom. The bottom type hopper may use augers or screws to discharge

the materials. In tilt type hoppers, materials are dumped just by tilting the hopper.

Storage hoppers can be carbon steel or stainless and round, square or rectangular in

shape, with a conical or flat bottom. Their design is such that the material can be easily

discharged from the bottom in such a way that the flow does not affect other operations

negatively.

Commercial hopper tanks are typically used in applications where routine clean-out is

required, or simply to reduce the energy and labour cost of material handling.

Hoppers are available in a variety of configurations, capacities and drive methods, such

as:

Self dumping hoppers: This type of hopper is useful in in-plant housekeeping

applications. Self dumping hoppers are used to handle heavy loads. Self dumping

hoppers can be used for forklift or roller applications.

Figure 32: Self dumping steel hopper

Conical shaped hoppers: Conical shaped hopper is a funnel shaped hopper that

flow materials. Conical shaped hoppers are available in stainless steel or aluminium.

They are more efficient than pyramidal shaped hoppers.

Wedge shaped hoppers: Wedge shaped hoppers are most often used in

mechanical engineering and chemical engineering.

Vibratory bulk hoppers

Manual 64 US 256280


Hoist hoppers using the standard vibratory hopper drive unit, but with the advantage

of floor level loading. This makes a large capacity hopper available, even at an

above normal height, while eliminating overhead lifting.

Horizontal belt hoppers are very similar to vibratory units but featuring a conveyor

belt in place of the vibratory tray. This is an ideal solution for either heavy parts that

would overload a vibratory tray, or fragile parts that could be damaged by vibratory

motion.

Bottom hoppers discharge material from the bottom. Bottom hoppers are used

extensively in solids handling operations and are classified into core flow, mass flow,

and composite hoppers

Hopper tanks for “dry” chemical storage are available with custom slope hoppers

designed for the desired discharge pattern

With materials that have the tendency to be cohesive (such as those having high moisture

content or fine particles) there is a coincidence of the bulk material acquiring strength to

obstruct flow. These can happen by the material “bridging “across the hopper opening.

Figure 33: Hopper and storage bin

2.3.4.4 Bins

A bin is a storage vessel used for storing and transporting both raw materials and finished

product.

Manual 65 US 256280


Cylindrical storage bins

Cylindrical storage bins that have conical bottoms and gravity discharge outlets are

commonly used in pneumatic system applications. However, material tends to stick or

flood when handled by this type of a bin.

Cylindrical storage bins are found in a range of standard sizes (diameters and heights)

that suit any locations. They can either be made free-standing or be set on columns to

make space for other equipment.

The cylindrical storage bins have the following attachments and accessories:

Manholes for maintenance, cleaning and inspection of equipment.

Ladders and platforms to provide a way to manholes and various parts of the bin.

Removable bars for safety in the manholes.

Small doors for observation of levels or level indicators and conditions inside the bin

near these small doors.

Figure 34: Cylindrical storage bins

Horizontal indoor bins

The horizontal indoor bin is rectangular with v-shaped bottom and is also used in the

pneumatic system operations. The figure following shows a horizontal indoor bin.

Manual 66 US 256280


Figure 35: A horizontal storage bin with V-shaped bottom

2.3.4.5 Bags

Bulk bags can be manufactured from paper, plastic, flat woven or circular woven fabrics

and can be uncoated or coated depending on the application and are used in just about

every industry, but most often associated with food and chemical packaging.

A bag is typically a flexible container that may be used for holding, storing or carrying

materials.

Figure 36: Bulk bags

In the pulp and paper industry, bags are for example used to store granular products such

as nickel sulphate crystals and powders such as sodium thiosulphate.

Manual 67 US 256280


The main advantages of storing the materials in bags are that the materials can easily be

loaded onto flat-bed trucks and it flows easily from the bags.

2.3.5 Packaging equipmentPackaging is not common in the production of chemical pulp since the pulp remains inside

a closed system. It may sometimes happen that the pulp is left inside tanks for a while

before further processing, but the pulp is never removed from the system and packaged.

2.4 Electrical equipment During the chemical pulping process you will work with a range of electrical equipment, for

example electric motors, switchgear and drive equipment.

When working with any electrical equipment it is essential that you practise safe work

practices. Electrical current can be the direct cause of injury to a person if it passes

through any part of the body. It can also be the indirect cause, due to its heating and

burning effects, which are manifested by electric shock, explosion, fire, and arc eye.

Electric shock can cause muscular contraction, thus increasing the period of contact. If

the current passes through the heart, it upsets its pumping action and in this case death is

almost certain. A less serious shock may cause a reaction, which results in loss of

balance and a subsequent fall, which could have serious results. In addition to causing an

electrical shock, the accidental contact with live terminals may create a flash-over

between them and the consequent arc produced will dissipate considerable energy in the

form of intense heat which can cause extensive and serious burns, possibly contaminated

and vaporised metal. High frequency currents, if allowed to pass through the body, can

cause internal burns although little sensation of shock is experienced at the time.

The following guidelines will help you to work safely with any electrical equipment:

Always assume that electrical wires, switches, conductors, etc. are live and

dangerous.

Never touch any electrical equipment with wet hands or while standing in wet areas

Visually inspect all electrical equipment before use: especially check for exposed

wires and worn electrical cables and take any defective equipment out of service.

Ensure that power supply systems, electrical circuits and electrical equipment have

been grounded.

Manual 68 US 256280


2.4.1 Isolation of electrical, hydraulic and air driven machines or equipment

No person is allowed to work on any machine or equipment, unless it is properly

isolated.

Only a qualified electrician or millwright or a person who has been authorised to do

so in writing can do isolation of electrical driven machines and equipment.

Air-driven and hydraulic machines (equipment) can be isolated by qualified

millwrights, fitters or a person who has been authorised to do so in writing.

Lockout and/or permit to work procedures should be adhered to.

2.5 InstrumentationThere are four basic instrument types which are frequently used in process plants,

namely:

pressure instruments

temperature instruments

flow instruments

level instruments

Figure 37: Types of instruments

In the chemical pulping process you also have to be familiar with the working of key

process indicators, control valves and a range of different controllers that are used to

monitor and control the process.

Manual 69 US 256280


Apart from these instrument types, there are also a number of other instruments that

measure variables on a pulp and paper mill; some of which use very complicated

instruments.

Examples of these variables that need to be measured are:

Concentration - the amount of a substance in a specific volume of liquid or gas.

Humidity - the amount of moisture in the air.

pH - a measure of the acidity of a substance.

Viscosity - a measure of how easily or difficult a fluid flows.

2.5.1 Key functions of instrumentationWe can illustrate the function of instrumentation by using the following example: John

wants to take a bath. Before her opens the tap, he decides that he only wants the bath to

be half full. Thus he marks the half-full point. Then he opens the tap and looks at the

water as the level in the bath rises. When the water reaches the ½-full mark, he closes

the tap.

This example illustrates the connection between the role instruments play in a process

and the way a person uses his or her senses in daily circumstances. The following points

illustrate this connection:

Instruments measure the value of a process variable (e.g. pressure, temperature,

flow or level) that we want to know. John measures the level of the water by looking

at it.

The measured values are indicated on a gauge and stored or recorded on paper so

that we can use it when required. In the same way, John can record (remember or

even write down) the level of the bath.

The measured value can then be compared to the value that we aim to reach. The

value that we aim for is called the set point. In John’s case, the set point (his aim)

may be a half-full bath.

If we compare the measured value with the set point, it will tell us whether we should

take action, for example change something or leave it as it is. In John’s example, he

will compare the bath level with his set point of half-full. While the bath is still below

the half-full mark, he will not take any action, but as soon as it reaches the half-full

mark, he will close the tap. Adjusting a process based on the comparison between

the set point and the actual value is called process control.

Manual 70 US 256280


Our example shows the four key functions of instrumentation:

Measurement

Indication, storage or recording of the measurement

Comparison of the measured value with the set point

Applying process control

2.5.2 Methods of displayWe want to identify the value that is measured by an instrument. For this reason, there is

something called a display which shows the value in such a way that we can interpret and

understand the reading.

Displays might be digital, or analogue.

2.5.2.1 Digital display

A digital display is a numeric display, or in other words, a series of numbers, like the

distance indicator on a car. This means that it shows a number that is equal to the value

of the measured variable at the moment. A digital display only shows the value of the

measurement at certain times. These times will have a fixed gap in between. The display

therefore changes every second (depending of what the time interval is). The following

figure shows some examples of digital displays.

Figure 38: Digital and analogue displays

Manual 71 US 256280


2.5.2.2 Analogue display

An analogue display is a display that indicates the value of the variable with a pointer or

needle on a scale that may be horizontal, vertical or circular. It shows the value of the

measurement at every instant in time (continuously). The analogue display will therefore

change smoothly; it does not jump through the values as a numerical display.

2.5.3 How to read a gaugeWhen taking any readings off the display, it is important to do it using the right technique.

A gauge reading must always be read face on or at the same height as the display, and

not at an angle.

With some gauges (especially analogue displays) the pointer or mark is not level with the

numbering on the display. When reading this at an angle, the wrong value ends up being

aligned with the mark. Exactly the same happens when reading a liquid level in

glassware. Because the level rises slightly against the side of the glass, it is important to

have your eyes level with the liquid level.

Figure 39: Your eyes should always be level with a gauge

Some machinery with mounted gauges vibrates. These vibrations can cause the needle

to jump around. To protect the gauge and make gauge readings easier the gauge should

be fitted with a damping adaptor or the gauge can be filled with oil. If the needle still

jumps around too much, it is best to take an average reading. This means that we take a

reading in the middle of the values that the needle vibrates between. When the needle

jumps between zero and a reading, like with a pulsating pump, the bigger value must be

reported.

Manual 72 US 256280


The following figure shows an analogue pressure gauge. The X100 in the middle of the

gauge means that any value read from the gauge should be multiplied by 100. This is

done to get the reading in the units indicated on the gauge (kPa). If, for example, the

needle points a value of 6.6, the actual value is 660 kPa.

Figure 40: Analogue display with X100 on the display

When recording the reading (writing it down or filling it in on a sheet), one must never

report more significant numbers (the numbers that show the value of the reading) than

necessary. If a gauge is accurate to only 1 unit, it doesn’t make sense to write down a

reading with a value of 4.542 for example, because the gauge cannot accurately measure

a value of 4.542. This reading will just be reported as 5 (rounding the value up). The

amount of significant numbers that the gauge is calibrated in (marked in) shows how

many numbers should be reported.

It is your duty to become familiar with all the instruments that you have to work with so

that you are confident that you can read them accurately.

2.6 UtilitiesThe processing industry requires utilities (services) like air, steam, electricity and water to

be able to convert raw materials into a final product.

In this section, we will be looking at the types of energy and services required in the pulp

and paper industry.

Manual 73 US 256280


2.6.1 Compressed air supplyCompressed air has a wide range of industrial applications, each with specific

requirements concerning the quality of the air. For example, instrument and control

systems need air at a relatively low pressure but free from water, oil and dirt. If an air

consuming unit is to achieve optimum performance and maximum working life, the

compressed air must be properly prepared.

The properties of the air are normally measured in terms of:

Pressure

Dryness

Purity

Lubricant content

The proper maintenance of the distribution network is essential if the system is to be

efficient and reliable. All piping, valves, hoses and connections must be in good condition

to ensure undisturbed air delivery. The following are key elements of an efficient and

reliable system:

Correct air pressure at points of consumption

Minimum air leakage

Adequate flow rate

Correct air quality

2.6.1.1 System components

A compressed air system consists of two main items – the compressor centre and the

distribution networks. There are normally two independent networks, one for

instrument air and the other for process operating air.

Manual 74 US 256280


Figure 41: Compressed air networks

The compressor centre consists of the following:

Oil free compressor

Desiccant dryers

Refrigeration air dryer

Air receivers

Dust filters

Instrument air is used for the remote opening and closing of valves and controlling a

range of other equipment in the plant. Without air to this system, it is impossible to control

the plant from the control room.

The supply of instrument air is always given priority and controlled by a regulator. Should

there be a drop in air system pressure, the system will first cut back on the supply of

process air in order to maintain control of the plant instruments with the remaining

instrument air.

Process air is the air physically used in the process itself – for example the air used in

chemical reactions.

The process and operating air is dried in a refrigeration dryer and led to an air receiver before being distributed to the points of consumption. The instrument air is led via a

desiccant dryer and dust filter to another air receiver.

Manual 75 US 256280


When compressed air is cooled, its ability to retain water is reduced. In the refrigeration

dryer, the air is cooled to about 2C at which temperature most of the water condenses.

The temperature at which water begins to condense is called the dew point of the

compressed air. The condensed moisture is collected and removed.

Desiccant dryers eliminate moisture in the instruments and control valves. This drying is

done by absorption. Water vapour in the compressed air is attracted to the surface of

solid absorbent materials (desiccants). The desiccant used is usually silica gel. This

method of drying produces the compressed air with the lowest dew point. Absorption

dryers handle only water vapour. Silica gel is reactivated by oven drying to drive off the

absorbed moisture.

Silica gel used as a desiccant is treated with a dye that is blue when the silica gel is dry

and pink when it needs regenerating.

2.6.2 Steam systemsSteam is used extensively in two main areas of pulp and paper manufacturing:

To provide the heat necessary for pulping wood chips

Drying of pulp and paper

2.6.3 Steam generationIn most cases steam is produced by burning coal, oil or gas but is sometimes produced

electrically. Steam is produced in two ways which are unique to the pulp and paper

industry. They are:

The burning of waste liquors

Burning of bark and wood waste

A boiler system provides the means for converting fuel energy (chemical energy) into

steam (heat energy). Steam is a very efficient form of energy to perform work and

transfer heat. It can also be converted into electrical energy by means of a turbo-

generator (electrical energy).

The following figure shows a typical flow distribution for steam in an integrated pulp and

paper mill.

Manual 76 US 256280


Figure 42: A steam system

2.6.3.1 Waste boiler

The boiler itself consists of a series of drums and tubes which hold and transport water

and steam under pressure. A boiler system includes a furnace (where the fuel is burned).

Forced draught and induced fans are used for controlling combustion air.

Boilers are manufactured in many sizes depending on the required pressure, temperature

and quantity of steam to be produced. There are two types of boilers, the water tube and

the fire tube.

Most boilers are of the water tube design where water circulates within tubes and heat

transfer occurs from the hot gases on the outside of the tube to water inside the tube.

Water is heated in the tube nearest the furnace bed and then rises to the steam drum

where vapour is separated from the water. The water travels down connecting tubes to a

lower drum, called a mud drum. Here sludge formed from the concentration of mineral

impurities is removed by "blowdown".

A super heater is used to raise the temperature of the steam to some specified level

above the normal boiling point of water (100C). This “super heated” steam is used by

turbine generators.

2.6.4 Steam applicationsSteam is utilised in the pulp and paper industry in the following ways:

Paper, board and pulp drying.

Manual 77 US 256280


Heating chips in TMP refiners, heating chemical pulp digesters.

Electrical power generation.

2.7 Mill cooling waterCooling water in a pulp, board and paper mill serves many purposes. As the name

implies it is a cooling medium. This water is used in ventilating systems, cooling of pump

glands, condensing flash steam and turbine condensers.

In a cooling water system, the product or process being cooled is the source of heat and

the cooling water the receiver. Cooling water does not usually have direct contact with the

source. The two materials are normally separated by a barrier that is a good conductor of

heat, usually metal. The barrier that allows the heat to pass from the source to the

receiver is called the heat transfer surface. The barrier is a containment vessel and is

called a heat exchanger. Most sources and receivers in heat exchangers are liquids. If

the source is steam or another vapour that is liquefied the heat exchanger is called a

condenser. If the source is a liquid that is vaporised the exchanger is called an

evaporator.

Once the receiver has done its job in having cooled the source, it contains heat. This heat

now needs to be removed again and the water cooled for reuse. In the pulp and paper

mills an open recirculation system is used. In this system cooling towers or evaporation

ponds are used to dissipate (remove) the heat it has absorbed from the source. In this

process, water from the cooling tower basin passes through the process equipment that

needs cooling, then returns the warm water through the evaporation unit (cooling tower)

where the water is cooled. A certain amount of water evaporates. To replace this loss,

fresh water is used as a make up to the cooling tower basin or pond.

Cooling towers are designed to evaporate water by bringing the water into contact with air.

Cooling towers are classified by the method of induced air flow, either by counter flow or

cross flow, depending on the flow of the water. Induced draft cooling towers are either

counter or cross flow with fans on top pulling cooling air up through or horizontally across

the falling water.

The following figure shows a flow diagram of a typical water system for an integrated pulp

and paper mill.

Manual 78 US 256280


Figure 43: Cooling water system

2.8 Electrical powerElectrical power in pulp and paper mill operations may be a combination of purchased

power and self generated power. However, small operating mills often depend only on

power supplied by the national electricity grid.

The following figure shows how electricity is distributed in an integrated pulp and paper

mill. Electricity is brought into the mill from the national electricity grid, and/or it can be

produced in the mill from the steam from power boilers. Steam is fed to turbo-generators

that produce electricity for the processes.

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Figure 44: Electricity distribution

Electrical energy is a very expensive commodity and all operations endeavour to use the

electrical energy produced, or purchased as efficiently as possible.

2.9 Monitor the ancillary systemsTo monitor and control the ancillary systems you need to understand the following

terminology:

Parameters,

Variables

Deviations

2.9.1 What is a parameter?A parameter is a value for a variable that is kept constant to measure a process. When

a process is monitored over time, the variables are adjusted and the parameters are kept

constant.

A parameter can also be described as conditions, guidelines, settings, measurable characteristics, features, fixed boundaries or limits.

Manual 80 US 256280


2.9.1.1 Why should parameters be monitored?

Quality in the process industry means that the finished product meets the requirements that are set out for it at all times. In every phase of the production process, quality and

costs need to be controlled from the moment the raw product enters the production

process until the finished product is sent to the customer. Defective products are one of

the main factors that increase cost.

Every pulp and paper mill aims to produce quality products at the lowest possible cost.

Product parameters should thus be monitored to keep quality consistently high and to

keep costs low.

2.9.2 What is a variable?A variable is a measurable characteristic that can change during a process. Examples of

variables are temperature, pressure, flow, input materials and environmental conditions.

A process variable is the current value of a variable in a controlled process, for example;

the temperature of a furnace. Variables thus need to be controlled to ensure that the

process produces quality products.

Variables can also determine how smoothly the process will run. An increase in

environmental temperatures might cause a cooling process to happen with more difficultly

as the process has to work harder to compensate for the rise in temperature.

2.9.3 What is a deviation?A deviation is the difference between an observed value and the expected value of a

variable or function. Deviation can be either positive or negative, this means that the

deviation is either larger (positive) or smaller (negative) than what is should be. If there is

too much deviation, the quality of the product will suffer.

2.9.3.1 What can cause deviations?

Even though a process normally consists of a specific series of operations, the following

factors can influence it.

People. Different operators might perform specific operating tasks in different ways.

These differences, however small, can cause variations in the process.

Material. If the raw material differs slightly from what is normally used, there might

be certain variations in the process.

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Equipment can influence a process. If machinery doesn’t work properly, it can

cause deviations.

Environment. Factors such as temperature can influence the process. On a very

hot day, the cooling capacity of certain steps in the process might be much lower

than usual.

The requirements of the end product. If the requirements are more restrictive, it

may be more difficult to produce a product that meets all the specifications.

Manual 82 US 256280


UNIT 3: MONITOR AND CONTROL THE QUALITY OF PROCESS MATERIALS

Learning outcomes


Explain the properties of process materials in terms of key characteristics.

Explain the purpose of process material quality control procedures as well as the consequences of not adhering to these procedures with regards to the impact thereof on the final product produced.

Explain the quality requirements of raw materials, process water, chemicals and additives or other materials according to general and workplace specifications.

Discuss typical raw material problems and its impact on the final pulp properties and costs.

Discuss corrective action to be taken in the case of non-conforming raw materials in accordance with workplace procedures.

Evaluate product variations and take corrective action in accordance with workplace procedures.

3.1 Instructions


SO3 AC1-6

SO4 AC2, 4

CCFO 1, 2, 4-8

Learning materials


Act. 11

N/a N/a 00:00

Manual 83 US 256280



SO3 AC1-6

SO4 AC2, 4

CCFO 1, 2, 4-8

Lecture room

Facilitator

Multimedia

Quality policies/ procedures

Attend a lecture and/ or facilitated discussion on:

Industry terminology such as specifications, variations and deviations, properties, characteristics, etc.

Types of process materials, i.e. process raw materials, process water, products, chemicals and any additives or other materials forming part of the chemical pulping process

The purpose of quality control procedures as well as consequences of not adhering to these procedures i.t.o. impact on final products.

Ex. 6

Questions

Ass. 1

Questions, sketches

and diagrams

00:00

Act. 12

SO3 AC1-6

SO4 AC2, 4

CCFO 1, 2, 4-8

Lecture room

Facilitator

Multimedia

Specifications

SOPs and other relevant workplace procedures

Attend a lecture/ facilitated discussion on raw materials:

Raw material properties in terms of key characteristics

Quality requirements of raw materials

Specifications defined by machine or mill operating instructions. Specifications may include, but are not limited to fibrous raw material type, ash content, dirt content and condition, cooking liquor chemistry, strength and clarity.

Raw material problems refer to the problems such as under or oversized fibrous raw material, variability of cooking liquor strength and clarity

Corrective action to be taken in the case of non-conforming raw materials

Ex. 6

QuestionsAss. 1

Questions, sketches

and diagrams

00:00

Act. 13

Manual 84 US 256280



SO1 AC6

SO3 AC6

CCFO 1-8

Lecture room

Facilitator

Multimedia

Specifications


Attend a lecture and/ or facilitated discussion on chemicals, additives and other materials relevant to the chemical pulping process

Functions

Properties in terms of key characteristics

Quality requirements of chemicals, additives and other materials

Ex. 7

Questions

00:00

Act. 14

SO3 AC5

SO4 AC4

CCFO 1-8

Lecture room

Facilitator

Multimedia

Specifications


Attend a lecture and/ or facilitated discussion on final product:

Product properties include but are not limited to Kappa number, consistency, freeness, viscosity, shive count, dirt count, brightness and pH.

Quality requirements of final product

Product variations include but are not limited to the Kappa number, consistency, freeness, viscosity, shive count, dirt count, brightness. pH, chemical residual and the strength and quality of the final paper, tissue or board product

Corrective action to be taken in the case of non-conforming final product

Ex. 8

QuestionsAss. 1

Questions and

diagrams

00:00

Act. 15

Manual 85 US 256280



SO4 AC2

CCFO 1-8

Lecture room

Facilitator

Multimedia

In groups, discuss the impact of process variables on the product properties in terms of final product and cost. Process variables include but are not limited to H-factor or S-factor, pressure, cooking liquor addition, temperature and/ or liquor to wood ratio. Act. 16 Notes

00:00

SO3 AC1-6

SO4 AC2, 4

CCFO 1, 2, 4-8

On-site

SME

PPE/ PPC


On-site, observe the various raw materials, chemicals, additives and products applicable to the process. Make a list of possible problems and corrective actions.

Also make sure that you know where you can find the various specifications and quality standards.

Notes

Ass.2

Monitor & control

00:00

Act. 17Ex. 9

Checklist

SO3 AC3-6

SO4 AC2, 4

CCFO 1-8

On-site

SME

PPE/ PPC


Under supervision, monitor the material and product specifications and identify and correct problems as per SOPs and/ or other relevant workplace procedures

Act. 18Ex. 10

Practical

00:00

SO3 AC1-6

SO4 AC2, 4

CCFO 1-8


PoE


CCFOs CCFOs 00:00

Manual 86 US 256280



Facilitator/ SME


Manual 87 US 256280


3.2 IntroductionBefore we discuss the quality requirements of the process materials entering the chemical

pulping process, we should first discuss the general principles of a quality management

system so that you can understand how important it is.

3.3 What is a Quality Management System?

Most companies implement quality management systems

that meet international standards. The international

standards are generally referred to as ISO standards. The

ISO standards were developed from the system

implemented by the International Organisation for Standardisation and this is also what the acronym refers to.

When a company meets the ISO standard for quality

management systems, it means that they have a system in

place to control quality that will assure that the required standards are achieved all the

time.

A quality management (QM) system includes quality control (QC) and quality assurance

(QA), as shown in the figure below.

Figure 45: Relationship between the different levels of quality

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3.3.1 Quality controlQuality control is used during the production of products. It includes control measures

and various techniques and activities to measure product quality against specific

standards. The aim of quality control is to provide quality that is satisfactory, adequate,

dependable, and economic.

Quality control means that a product is inspected, tested or analysed to see how it

measures up to the quality standards or specifications. Products that do not conform to

the desired product standard or specifications are rejected as non-conforming products.

This means that the quality of a product is determined at a specific stage in manufacturing

by taking samples of the product to determine whether the batch conforms to

specifications. If the sample does not meet the required standards, the batch is rejected

and corrective actions are taken to ensure that a good quality end product is produced.

The corrective actions might include a re-work of the product, i.e. the product might go

through the system again to see if the problem can be fixed. Sometimes the problem

cannot be corrected and this can lead to a total reject resulting in the product being

discarded (thrown out). In essence, quality control asks the question “Are we doing

everything correctly during all the production stages?”

3.3.1.1 Setting of standards

Setting of standards (as part of quality control) is one of the first steps of managing

quality. Setting of standards can become quite a complex matter in the pulp and paper

manufacturing industry.

The following factors all play a role in setting the quality standards:

Products must meet standards specified by law.

Products must meet standards set by the South African Bureau of Standards (SABS

Codes)

Products must meet standards set by various international bodies

Products must satisfy the market requirements or the standards set by the consumer

Setting of standards alone however will not ensure that we achieve the quality we aim for.

We need to do more than this. A business must also develop a system to manage quality.

This system must focus on all the different factors that ensure that we achieve the quality

standards we aim for. It is also not acceptable if the desired quality standard is achieved

some of the time. We must do better than this and achieve quality standards all the

Manual 89 US 256280


time. Such a quality management system will be made up out of many controls and

policy and procedure documents that will ensure that we meet the desired quality

standards.

3.3.2 Quality assuranceThe term “assurance” can be explained as a promise or reassurance that something is

right.

Quality assurance is a management function which includes quality control measures.

Quality assurance can be described as the management processes that provide us with

confirmation or assurance that our quality control measures are in fact working. The

answers we get through quality assurance must tell us that we are producing products to

the required standard that meet the need of the customer. Quality is more than only doing

things right, it also asks if we are in fact doing the right things, the things the customers

expect from us. Quality assurance provides us with these answers.

Quality shows us whether an item or process measures up to the expected requirements.

Quality assurance is those actions that measure whether quality was in fact achieved.

Quality assurance is made up out of various management controls; checks and tests that

prove to us that our quality control measures are working, and in fact ensuring that the

products we produce meet the required standard.

It can be said that quality assurance prevents mistakes, whereas quality control detects and corrects mistakes.

3.3.3 Quality managementQuality management is the overall management function that ensures that Quality Control

and Quality Assurance together with all the other organisational policies and procedures

are implemented in the workplace. Quality Management can also be described as the

general management function which lays down the quality policy, aims and

responsibilities. Quality Management also includes the planning and control of quality and

ensuring and improving the quality.

Manual 90 US 256280


3.4 The quality standards of process materials

It is important to remember that the term “process materials” refers to all the raw

materials, process water, products, chemicals and any additives or other materials

forming part of the chemical pulping process.

The quality of the raw materials and ingredients used in the manufacturing process has a

major influence on the quality of the final product. If we are using poor quality raw

materials or ingredients we will never manufacture a good quality final product.

Remember the saying:

3.4.1 Quality of raw materials and ingredientsThe raw materials and ingredients must always conform to quality specifications. Raw

material and ingredients should always be tested and inspected at receiving in

accordance with quality control procedures. It is also common practice to insist on a

quality certificate from the supplier as proof that it is of good quality.

3.4.2 Quantities of raw materials and ingredientsWhen manufacturing a product the correct quantities of raw materials and ingredients

should be used as indicated by the product specifications. Quantities of raw materials

used will influence the composition (make up) and the effectiveness of the final product

being manufactured. If the correct quantities are not used the product will not conform

and be of poor quality.

3.4.3 ContaminationPoor quality products will be the end result if the raw materials or final products are

contaminated. Contamination is when your product comes into contact with something

that spoils the quality of the product, foreign matter like metal shavings in a liquid product.

Chemicals used as raw materials or ingredients must also be pure. Therefore there

should be strict control measures during the manufacturing process to prevent

contamination. You don’t want a white paint to be contaminated with a red dye which will

spoil it.

Manual 91 US 256280


Make sure that you understand the quality control measures and procedures that are

followed by your workplace to ensure quality raw materials and ingredients at all times.

3.5 Wood quality requirementsThe quality of the chips used for pulping is a very important factor in the operation of the

pulp mill and in final pulp quality. Therefore, it is important to know what variables will

affect the chip quality and the effect they have on the pulping operation and pulp quality.

Chip quality variables can be divided into two sections: Wood-related variables and

process-related variables:

Wood-related variables are concerned with properties of the wood itself, such as

selection of species, variation within species, wood deterioration during chip storage, and

wood decay.

Process quality variables relate to the chipping operation, chip size distribution, and chip

bulk density include blade sharpness, chipper design, screen type and operation.

3.5.1 Wood related variables

3.5.1.1 Wood species

Generally, softwood chips produce a stronger pulp than hardwood chips. This is because

the softwood fibres are longer and more flexible than hardwood fibres.

Softwoods normally give a slightly lower yield than hardwoods when pulped under the

same conditions. This occurs because softwood hemicellulose is more soluble than

hardwood hemicellulose, and softwood generally contains more lignin than hardwood.

Hardwood pulps produce a paper with good printing qualities. The hardwood fibres form a

smooth surface because of their small size.

3.5.1.2 Wood density

Wood density is a very important economic factor in pulping. With a denser wood, one

can pack more weight into a given digester volume and thus increase pulp production,

either per batch cook or per time unit in a continuous digester

Wood density varies with wood species. Hardwoods tend to have higher densities than

softwoods. Some examples of densities for different species can be seen in the table

below:

Species Specific Density (kg/m3)

Manual 92 US 256280


Eucalyptus

Grandis (Saligna gum) 500

Maculata (Spotted gum) 600-800

Acacia

Mollissima (Black Wattle) 600-800

Pinus

Patula 450-550

Ellottii 450-550

Table 1: Specific densities for various wood species

Specific density varies within species and even within the same tree. This variation

depends on cell wall thickness and cell size. Wood with thicker cell walls has more wood

substance and less void space per unit volume than wood with thin cell walls. Therefore,

it has a higher density.

Pulp quality and later operations including washing, refining, and papermaking, are

affected by the density of the wood source. Thick walled fibres (high density wood)

produce a coarse pulp with stiff fibres and a high water drainage rate. Thin-walled fibres

(low density wood) produce a pulp with very flexible fibres that give a high strength and

high-paper density.

3.5.1.3 Species used for bleached pulps

Bleached kraft hardwood pulps are commonly used for fine paper and high quality

publication grades where the main requirements are good sheet formation and a good

printing surface. High strength is not the primary concern. The most commonly used

species are Gum and Wattle.

Bleached kraft softwood pulps are used extensively for packaging papers where high

strength is critical. The most commonly used species is pine.

Softwood pulps are often mixed with hardwood pulps to give the stock higher strength,

primarily to withstand the tensions existing in high speed printing presses.

Manual 93 US 256280


3.5.1.4 Species used for linerboard

Linerboard is used as the outer plies of corrugated box stock and as wrapping paper. It

consists of two layers called the top and the base liner. The top liner is cooked to a lower

yield to provide a smoother surface for printing.

Linerboard must be tough and strong with a high level of stiffness and burst resistance.

The top liner also must have good printability. Because of the strength requirements,

linerboard is primarily produced from softwood pulp. However, a quantity of hardwood

pulp can be used for the top liner to improve surface smoothness and printability.

3.5.1.5 Variation within species

Growth location

Wood quality is affected by the environment where the tree grew. Climate, soil fertility and

elevation are the main factors.

For softwood, the properties altered most noticeably by the environment are ring width,

early-wood / latewood ratio (more early-wood in areas with warm weather and sufficient

moisture), fibre wall thickness and fibre size. As a general rule, warm temperature and

adequate moisture combine to produce wood of a higher specific gravity.

Juvenile and mature wood

A tree does not produce exactly the same type of cells over its whole life. Therefore,

there is a difference between juvenile wood produced during the first 5 to 20 years of a

tree's lifetime and the mature wood produced thereafter. The differences are more

marked in softwoods but are still noticeable in hardwoods. Generally, compared to

mature wood, juvenile wood has these characteristics.

shorter and narrower fibres and vessel elements

thinner cell walls

high early-wood / latewood ratio

lower specific gravity

lower cellulose content

higher hemicellulose content

higher lignin content

wider growth rings

higher fraction “reaction wood”

Manual 94 US 256280


Juvenile: Young

Reaction wood refers to the adapted fibres formed by a tree in order to strengthen deviations such as branches or leaning stems

Because of the different properties of juvenile and mature wood, chips from young trees

will not have the same pulping properties as chips from a fully grown tree. A big

disadvantage of juvenile wood kraft pulping is a lower yield and an increase in alkali

consumption. The reason for this is the lower cellulose content.

Early-wood and latewood

The growth rings in a tree stem consist of two sections: a lighter, wider section that is

formed during the early growing season, and a darker, narrower section that is formed

during the late growing season.

The wood produced during the early part is called early-wood (or springwood). It consists

of thin-walled fibres of large diameter. The wood produced during the later part of the

growing season is called latewood (or summerwood). It consists of more thick-walled

fibres with a narrower radial diameter.

The ratio of early-wood to latewood is an important factor in pulping. Early-wood has a

lower specific gravity than latewood and the thin-walled early-wood fibres collapse more

easily to form flat ribbons that provide a large area for fibre bonding during papermaking.

Thick-walled latewood fibres retain their tubular form and, therefore, do not bond as well.

A decreased early-wood to latewood ratio means a higher specific gravity and decreased

inter-fibre bonding (a weaker paper sheet) unless process changes are made, for

example, increased refining before papermaking.

Reaction-wood

Reaction-wood is wood formed in a stem that is leaning or where it is branched. Its

function is to help the tree maintain a vertical stem and keep the branches oriented in a

preferred direction.

Reaction-wood in conifers (such as pine) is called compression wood. It is located on the

underside of a leaning stem and under a tree branch. Reaction-wood in hardwoods is

called tension wood. It is located on the upper side of leaning stems and tree branches.

Compression wood has a darker colour than normal wood. Its fibres are also shorter and

more thick-walled. Compression wood also has a higher specific gravity, higher lignin

content, and lower cellulose content than normal wood. Because of these factors,

Manual 95 US 256280


compression wood gives lower yields than normal wood. Also, the pulp is darker and has

lower strength properties.

Tension wood has a lighter colour than normal wood. Fibre length is not affected, but the

fibre walls are thicker than normal. The specific gravity is higher, cellulose content higher

and lignin content lower than for normal wood. Yields and brightness from tension wood

are somewhat higher because of the higher fraction cellulose. Strength properties are

lower than for normal wood since the thick-walled cells result in lower fibre bonding.

Softwood Hardwood

Properties compared to normal wood

Designation Compression wood Tension wood

Position found Under branch Upper side of branch

Colour Darker Lighter

Brightness Lower Higher

Fibre length Shorter Same

Cell walls Thicker Thicker

Specific Gravity Higher Higher

Lignin Higher Lower

Cellulose Lower Higher

Production yield Lower Higher

Strength Weaker Weaker

Table 2: Reaction-wood and normal wood properties

Sapwood and heartwood

The wood in young trees and the outer wood in older trees contain living cells and can

transport sap. It is, therefore, called sapwood. The wood in the middle of an old tree

consists of dead cells and is not involved in transporting sap. It is called heartwood.

Normally, sapwood is light in colour while heartwood has a darker colour, especially in

hardwoods. When heartwood develops, extractives fill cell voids and impregnate the fibre

walls. Therefore, heartwood is not very easily penetrated by liquids.

In pulping, heartwood can cause problems. It does not chip as easily as sapwood

because of its lower moisture content and the higher fraction of extractives. Also, the

cooking liquor does not penetrate as easily. This can cause large amounts of knotter and

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screen rejects. The colour of heartwood can also cause a problem since the coloured

extractives must be removed to increase the pulp brightness.

3.5.2 Wood decayWood decay is caused by micro-organisms - mainly fungi, but also bacteria, yeasts, and

moulds.

Decay occurs both in standing, living trees and in piles of stored chips or pulpwood logs.

These two decay situations should be looked upon separately because they are caused

by different organisms. Wood decay will always occur to some degree during chip

storage. Distinct from this is the wood decay that takes place in some living trees.

3.5.2.1 White and brown rots

White and brown rots are both groups of fungi that attack standing trees. Brown rots are

more common in softwoods, but do attack hardwoods as well. In advanced stages of

decay, the wood turns brown and can be easily broken by hand. All brown rots prefer

carbohydrates over lignin and extractives. The fungi attack the cellulose and

hemicellulose by random cleavage of the molecular chains. This means the degree of

polymerisation (that is, the chain length) decreases quickly. Therefore, the loss in pulp

strength is big even at early stages of decay.

White rots generally attack both lignin and carbohydrates, leaving a whitish wood colour

behind. The white rot fungi attack the cellulose by “peeling” the glucose unit at the end of

the chain. The chain length is, therefore, reduced rather gradually and as a result, white

rot do not reduce pulp strength as rapidly as brown rots.

3.5.2.2 Effects of decay on pulping

Although decayed wood can be processed and pulped in the kraft process, there are

many factors that reduce profitability.

First, chipping and chip handling of decayed wood is more difficult. More fines and

over- and undersize chips are produced.

Decayed wood has a lower density, which means less pulp can be processed per

batch cook or time unit in continuous digesting.

Also, the weight percent yield decreases because of the degraded cellulose in the

decayed wood. Yield reductions up to 25% have been reported.

The decayed wood also causes the alkali consumption to increase. Decayed wood

contains a higher percentage of lignin and carbohydrates which are more susceptible

Manual 97 US 256280


to chemical breakdown by the pulping chemicals than sound wood, and therefore

consume more chemicals. Increases in alkali consumption of up to 25% per ton pulp

have been recorded.

The degradation of cellulose also leads to a reduction in pulp strength, as mentioned

earlier.

The lower yield and increased chemical consumption mean a higher load on the recovery

boiler. Since the capacity of the recovery boiler is often the production bottleneck in kraft

pulp mills, the mill productivity will be lower when pulping decayed rather than under

normal circumstances.

3.6 Bagasse quality requirementsBagasse is the only vegetable or non-wood fibre used in South Africa and is the shredded

fibrous debris from sugar cane after the sugar mill has removed the sugar juices.

Bagasse consists of pith, vessels and rind – in order to produce a good quality pulp the

pith have to be removed. Pith is made up of fine particles that contribute nothing to bond

strength; in fact it is detrimental to bond strength. The organic silica in the pith also reacts

preferentially with the caustic soda (the most expensive raw material) to produce a slimy

substance, silicic acid (SiO2.nH2O), which causes bad drainage and thus poor washing

and slow machine speeds.

The quality of bagasse is dependent on the variety of cane, age at cutting, agronomic and

soil conditions and the extent of crushing and milling operations carried out on the cane in

the sugar mill.

3.6.1 Chemical and physical properties of bagasseDry fresh bagasse is bright and varies in colour from brownish to light green depending on

the variety and age of the cane. Stored bagasse ranges in colour from yellowish – brown

to dark grey. The pith is relatively free and loose and becomes separated during the

storage and fermentation processes. Microorganisms grow on the pith cells during

storage because of the residual sugar content. Separation of the pith from the fibre is

easier if the bagasse has been stored for a while.

Bagasse pulps are very diverse in character, (vary in fibre length) and the average fibre

length depends largely on the depithing method, degree of depithing used in fibre

separation as well as the pulping process employed. The fibres are in some cases

comparable to that of softwood fibres, and therefore bagasse pulp can be used for the

manufacture of a wide range of high-quality papers.

Manual 98 US 256280


3.7 Cooking liquor quality requirementsCooking liquor composition requirements differ according to the type of chemical pulping

method used. This is the reason why the cooking liquors for each process will be

discussed separately.

3.7.1 Kraft cooking liquorsKraft cooking liquor is a mixture of the recovered white liquor and some black liquor from

the previous cook. The composition of these two liquors is discussed below.

3.7.1.1 White liquor

The aqueous solution of sodium hydroxide and sodium sulphide used as the cooking

liquor in kraft pulping is called “white liquor”. The approximate concentrations are 1.0

molar in sodium hydroxide and 0.2 molar in Na2S. The pH of this relatively colourless

liquor is approximately 13.5 - 14. In contrast with laboratory liquors that are made with

relatively pure caustic and sulphide, mill liquors contain appreciable amounts of other

inorganic ions. Inefficiencies in the recovery and causticising operations are the main

reasons for these impurities. The following table lists the main components of typical mill

white liquors.

The following are the usual results:

Sodium sulphate results mainly from incomplete reduction in the furnace,

Sodium carbonate from incomplete causticising, and

Sodium thiosulphate from air oxidation of the sulphide.

In addition, sodium chloride enters as an impurity in makeup chemical or from the

raw wood.

Potassium normally enters as a raw wood component, but may also be present in

small amounts in the makeup chemicals.

While sodium chloride and potassium salts have little direct effect on pulping, they can

cause serious corrosion problems in the chemical recovery area. The other minor

components, such as iron, magnesium, calcium, and silicate, likely originate in the wood

used to make the pulp. Some of the calcium also results from carryover from the

causticising operation. The term "dead load" is used to refer to all components besides

sodium sulphide and sodium hydroxide.

Manual 99 US 256280


The active components of the white liquor, the hydroxyl ion and the hydrosulphide ion

originate from sodium sulphide and sodium hydroxide. Their concentration has a major

effect on the rate of the pulping operation (this will be discussed later). The dead load

chemicals have no direct effect on pulping. However, because they help determine the

ionic strength of the liquor and thus the activity coefficients of the respective ions, rate

effects do occur. An increase in dead load can result in a decrease in lignin removal in

pulping. Therefore, an efficient pulping operation requires high recovery efficiencies.

3.7.1.2 Black liquor

The liquor that exits the digester with the cooked chips at the end of the kraft cook is very

dark in colour and is called "black liquor”. In addition to the inorganic material that entered

with the white liquor, black liquor contains both organic and inorganic material removed

from the wood during the cooking process. The composition of a typical black liquor from

the cooking of a southern softwood is given in the following table. By comparison to the

white liquor listed in Table 2-1, it can be seen that significant portions of the sulphur

compounds have been oxidised to sulphate and thiosulphate. In addition, the drop in pH

that occurs on cooking is shown as the large drop in hydroxide ion concentration.

In addition to these major ions, trace metals from the wood have increased. Calcium ion

is of special concern, as it can lead to problems with scale in subsequent concentrations

of black liquor. Also, silicate ion concentration has increased significantly. This can also

give problems with scaling on heat transfer surfaces.

The major difference between while and black liquor is the amount of organic components

found in the dissolved solids in black liquor. These are present mainly as organic acids.

Also, a significant amount of dissolved alcohols such as methanol are present. Some

organics may be recovered in the chemical recovery process as the mixture of resin and

fatty acids known as tall oil or as turpentine recovered in a liquid separation sequence.

However, most of the organics carry through to the recovery furnace, where they are

burned, generating heat, carbonate, and carbon dioxide.

3.7.2 Raw material problemsYou may encounter problems with the chemical pulping process that are directly related to

the quality of the raw materials fed to the process. In such a case you must ensure that

you are very familiar with the properties of acceptable raw materials (wood chips or

bagasse). You must also make certain that you use the correct workplace procedures to

reject the raw materials.

Manual 100 US 256280


In the same way you must ensure that you are familiar with the quality standards that the

raw materials must meet.

Typical raw material problems will results in variations in the product quality. The product

quality can be assessed by regarding its properties such as:

Kappa number

Consistency

Freeness

Viscosity

Shive count

Dirt count

Brightness

pH

Each of these factors are discussed briefly in the following paragraphs

3.7.3 Kappa numberThe Kappa number is a measure of the amount of lignin contained in the pulp. A higher

Kappa number means that the pulp has a high lignin content and vice versa. This variable

is used to monitor the cooking process because it is an indication of the amount of

delignification of chemical pulps. It is important that this factor is monitored so that pulp of

a desired quality can be produced. The Kappa number is determined by a specialised test

which will not be discussed in this module.

3.7.4 Consistency of pulp productThe consistency of the pulp is an indication of the solids content of the pulp. The

consistency is measured using the following formula

Consistency = (dry solids mass/total pulp mass) x 100

The consistency of the final product is important because the pulp should contain too

much liquid or too much solids. The consistency is especially important in the blow tank.

It must be kept within certain parameters in the blow tank. If the consistency becomes too

high, the torque on the mixer may increase up to a point where it will either trip or damage

the motor. To keep the consistency within these parameters, liquor may be added if the

pulp if the pulp gets too dry.



3.7.5 FreenessThe fibre liberation point is the point at which enough lignin has been removed from the

wood chips that they become soft enough to separate into fibres with little mechanical

action. Most chips reach this point after being cooked at 130°C-180°C. It is important to

keep the chips within the digester long enough for the chips to reach this point. This will

ensure adequate separation of fibres during blowing.

3.7.6 Pulp viscosityThe viscosity of the pulp is an indication of the average chain length of cellulose otherwise

termed the degree of polymerisation (DP). Higher viscosity means that the cellulose has a

higher average chain length and will therefore produce stronger pulp and paper. A

decrease in viscosity during pulping is inevitable but care must be taken to prevent further

decreases in viscosity by adequate control of process parameters which might otherwise

decrease the viscosity.

3.7.7 Shive contentShives are tiny bundles of fibres that have not been separated properly into individual

fibres during cooking. These shives affect the quality of the final product if left in the pulp.

Screening of pulp after the pulping process is necessary for the removal of such shives,

dirt and any other debris present in the pulp.

3.7.8 Dirt countDirt in pulp will obviously produce poor quality product. Like the shives, the dirt is removed

during pulp screening.

3.7.9 Brightness of the pulpBrightness refers to the whiteness of pulp or paper. This is measured on a scale of 0%,

which represents absolute black, to 100%. This scale is relative to Magnesium oxide

standard (MgO) which has an absolute brightness of 96%. Pulp produced by chemical

pulping process increase in brightness as more lignin is removed.

3.8 Disposal strategies for unacceptable materials

In the woodyard, a number of products undesirable for paper manufacture such as dirt,

stones, and sawdust, bark and log butts are removed from the main processing stream.

All the organic materials removed from the process (apart from the chips) are generally



used to generate steam in refuse boilers. In some cases facilities to process these

organic materials do not exist and these materials are disposed of through other methods

– mostly landfill or in some cases it is applied as source of heating energy in local

communities.

3.9 Consequences of poor quality control

3.9.1 Consequences for the customersQuality control always seeks to keep the customer happy. We have to be very clear on

who our customers are. Customers can be divided into two groups:

Internal customers

External customers

3.9.1.1 Internal customer

Every section or department in a production line produces products or renders services

used by the other departments or sections as inputs into their production activities. These

departments or sections rely on the quality of your products or services. These sections

or departments are your internal clients. If you do not meet their standards, they will not

be able to do their work to the quality standard expected from them. This will also result in

poor production. Production targets can only be achieved if every section in the

production chain performs to the expected standard. Non-conformance to quality

standards in one section has an impact on the ability of the rest of the production chain to

meet their targets.

In the same manner we all have internal suppliers. Those colleagues on whom we rely for

inputs in order for us to do our work. We are their internal customers.

Will you be able to do our work to the standard expected from you if the colleagues you depend on do not meet their standards? Does the same hold true for the colleagues that depend on you?

3.9.1.2 External customers

We produce products for a specific market and specific customers who purchase our

products. We must have a clear understanding of who these customers are and what

their expectations are.



External customers will expect products and services from us that meet standards such

as:

Products that meet technical specifications

Product packaging that meets specifications

Delivery on time

Delivery of their complete order

Delivery of the volumes they ordered

Delivery at the price they were quoted

In many instances, external customers depend on the quality of our products or services

for the success of their own business ventures. In the same way we also depend on our

suppliers for the success of our business. We are seen as suppliers by these customers

and we must meet their quality specifications. The quality control function in a

manufacturing business will establish clear communication channels with the customers.

Any quality problems or questions experienced by the customer will be channelled to the

quality control function and corrective measures will be taken to correct the situation to the

satisfaction of the customer.

What do you think will happen if we do not satisfy the needs of our external customers? Will you continue to support a specific product or business if your needs are not satisfied?

3.9.2 Consequences for the organisationIf the quality of products and services rendered by a workplace are not properly controlled,

the general efficiency of, and the ability of the organisation to stay in business can be

seriously affected.

Quality control measures that are poorly executed or managed can result in:

High costs to correct errors during manufacturing e.g. by re-working or rejection of

the product.

Lower productivity as the staff are re-working products and not manufacturing new

products.

Lower sales and less profit as their external customers take their business elsewhere.

An unhappy external customer can do a lot of damage to a company by “word of

mouth”. (Telling other potential customers of the poor quality products sold).



Contamination or pollution of the natural environment resulting in serious

consequences and expenses to rectify the situation.

3.9.3 Consequences for the individual and colleaguesWorkplace where quality problems are regularly experienced will find that the general

morale of the work teams and of individuals are low. This means that the employees will

not be motivated to work hard and do their best.

One member in a work team that does not comply with the expected quality standards can

mean failure for the whole team.

If a company has experienced financial losses due to poor quality control the people

working at the company may not have job security which will lead to low morale amongst

all employees.

3.9.4 Consequences for the community

Figure 46: Air pollution caused by the pulp and paper industry

If quality control in a company is poor there will be many defects and non-conformances

during manufacturing. One way to correct non-conformances is to reject the product

which will lead to a great deal of unnecessary expense and a loss of profits.

Pulp and paper mills all produce effluent and waste that has to be treated before the

material is released into the natural environment or discarded of. The effluent and waste

material pose specific hazards to the environment and can result in serious pollution



problems if not managed. Specific quality control measures should be applied by the

workplace to control effluent and waste treatment to ensure that the operation does not

have a detrimental impact on the environment and the community directly affected by the

pulp and paper mill.



UNIT 4: MONITOR AND CONTROL THE CHEMICAL PULPING PROCESS

Learning outcomes


Monitor the chemical pulping process and record parameters in accordance with workplace procedures.

Explain the impact of process variables on the product properties in terms of final products and costs.

Discuss typical equipment problems within the chemical pulping process and offer solutions in accordance to workplace procedures.

Evaluate variations in the product and take corrective action in accordance with workplace procedures.

4.1 Instructions


SO1 AC5

SO2 AC1-6

SO4 AC3

CCFO 3, 4, 5, 7, 8

Learning materials


Act. 19

N/a N/a 00:00




SO1 AC5

CCFO 3, 4, 5, 7, 8

Lecture room

Facilitator

Multimedia


Attend a lecture and/ or facilitated discussion on the main equipment relevant to the chemical pulping process. This includes:

Types and components

Function/ purpose

Application

Operating principles

Ex. 11

Questions

Ass. 1

Questions and

diagrams

00:00

Act. 20

SO4 AC1, 3

CCFO 1, 3, 4, 5, 7, 8

Lecture room

Facilitator

Multimedia


Attend a lecture and/ or facilitate discussion on:

Equipment problems e.g. fibrous material variability, poor circulation of cooking liquors, poor heat exchange efficiencies, poor fibrous raw material packing within the digester, and poor temperature control within the vessel.

Process parameters and variables include but are not limited to H-factor, S-factor, pressure, cooking liquor addition, temperature and liquor to wood ratio.

Deviations from operating parameters and solutions to these deviations

Recording process parameters

Ex. 12

Questions, Ass. 1

Questions and

diagrams

00:00

Act. 21

SO4 AC2

CCFO 1-8

Lecture room

Facilitator

Multimedia

In groups, discuss the impact of process variables on the product properties in terms of final product and cost.

Act. 22 Notes

00:00




SO4 AC1, 3, 4

CCFO 1, 3, 4, 5, 7, 8

On-site

Facilitator/ SME

PPE/ PPC

Notebook and pen


On-site, identify the main chemical pulping process equipment and observe how the process is monitored and controlled.

Also observe how to evaluate variations in the product and take corrective action.

Take notes of the step-by-step procedures. Discuss your notes with a SME and draw up a checklist(s). Verify the checklist(s) for correctness.

Notes

Ass.2

Monitor & control

00:00

Act. 23Ex. 13

Checklist

SO4 AC1, 3, 4

CCFO 1, 3, 4, 5, 7, 8

On-site

SME

PPE/ PPC


Under supervision, monitor and control the chemical pulping process as per SOPs and/ or other relevant workplace procedures

Act. 24

Ex. 14

Practical

Ass.2

Monitor & control

00:00

SO1 AC5

SO2 AC1-6

SO4 AC3

CCFO 3, 4, 5, 7, 8


PoE

Facilitator/ SME


CCFOs CCFOs N/a

Total time allocated for this activity (00h00) 30:00



Final practical – Monitor and control the production of chemical pulp


All SOs, ACs and CCFOs

On-site

SME

PPE/ PPC


Work under an experienced worker (SME) and apply all the procedures you have learned. When you feel competent that you have acquired the expected knowledge and skills, you can apply for the summative assessment. You will be assessed on your ability of:

Monitoring and controlling the various ancillary systems

Monitoring and controlling the quality standards of process materials

Monitoring and controlling the production of chemical pulp

Recording parameters in accordance to workplace procedures

NOTE: Throughout the above procedures you should comply with relevant health, safety and environmental standards

Act. 25

N/a Ass.2

Monitor & control

00:00

Total time allocated for this activity (00h00) 55:00



4.2 IntroductionThere are no hard and fast rules that apply to how you monitor and control the chemical

pulping process. Having said that, however, you should remember that there are a whole

range of workplace instructions and procedures that you have to adhere to while doing

your job.

In this unit we will discuss some of the factors that will help you work more efficiently.

4.3 Normal operating conditionsNormal operating conditions can be defined as the range of operating conditions within

which a device is designed to operate and for which operating limits are stated.

The main responsibility of an equipment operator is to make sure that the equipment

performs according to specified requirements, ensuring normal operating conditions.

Although this basic responsibility has not changed in recent years, factors such as

technological advancements and government regulations have resulted in significant

changes in facility requirements. Processes, equipment and policies have also changed

to the point where an operator's actions can have an increasingly significant effect on the

total mill operations.

By understanding and following standard operating procedures, as well as detecting and

correcting abnormal operating conditions, you can help to prevent problems that could

result in personal injury or equipment damage. Remember, you as an operator, perform

an important function to ensure the success of the whole operation. In most respects,

operators are the eyes and ears of process facilities.

4.4 Maintaining process variablesSome of an operator's responsibilities, such as maintaining process variables within

narrow ranges and maintaining the production of chemical pulp, apply to both batch and

continuous process operations. In some processes, it is essential to maintain certain

process variables, such as pressure or temperature, within close limits. If process

variables are outside these limits during system operation, the product could be of low

quality or completely ruined.

In the chemical pulping process the following process variables can have an impact on the

product properties:

H-factor



S-factor

Pressure in the cooking vessel

Addition of liquor to the cooking vessel

Temperature in the cooking vessel

Liquor to wood ratio

Each of these factors and their impact on the chemical puling process will be discussed

further in the following sections

4.4.1 H-factorThe H- factor is a factor used to represent the effects that temperature and residence time

have on chemical pulping processes. To obtain a normal Kraft pulp, cooking will have to

be done at 170 for one hour. The same result is achieved when cooking is done at 180 for

a period of 0.75 hours. Taking these facts into account, the H-factor was deduced in order

to combine the effects of these two variables into one number. An H factor of 1 represents

the pulping effect of one hour at 100 °C. At 171°C, the H-factor would be 1000.

The H-factor is one of the variables which should be kept at a desired value in order for

the product to satisfy quality requirements.

4.4.2 Pressure in the cooking vesselDuring the cooking process, pressure changes occur. In Kraft and NSSC pulping, the

pressure is initially atmospheric. For acid sulphite cooking, the pressure starts at values

higher than atmospheric. The pressure increases as the temperature is increased. When

the maximum allowable pressure is reached, some of the gases are vented off to reduce

the pressure to the desired value.

Pressure has a direct influence on the rate of cooking because it has an effect on the

cooking liquor composition. In sulphite cooking, a high pressure results in more rapid

cooking by maintaining a high sulphur dioxide concentration.

4.4.3 Addition of liquor to the cooking vesselAddition of cooking liquor must be done so as to maintain the liquor to wood ratio

discussed in Section 4.4.5. Care must be taken to provide adequate circulation so that

there is uniform distribution of liquor within the cooking vessel. This is important for

homogeneous cooking of all chips.



4.4.4 Temperature in the cooking vesselThe temperature in the cooking vessel is a very important variable. In order to produce

good quality pulp, the correct temperature must be maintained for the correct period of

time. It is important that the correct balance between the two be achieved. The

temperature and residence time is determined by the type of process used. Each of these

have been mentioned in the specific pulping process type.

4.4.5 Liquor to wood ratioThe liquor to wood ratio is the ratio in which cooking liquor and wood chips are fed to the

digester. The quantity of chips loaded is very important, as the ratio of the mass of the dry

wood to mass of the liquid in the digester must be accurately determined. The “ liquor to

wood ratio” can be defined as the mass of liquid in the digester to the mass of oven dry

wood expressed as a ratio.

The most commonly used ratio is 4:1 i.e. 4 kg of liquid to every 1 kg of oven dry wood. It

is important to note that the liquid component of the ratio, not only includes the cooking

liquor, but also the moisture in the chips prior to cooking.

The total liquor to wood ratio is very important – if the liquor to wood ratio is too low, the

contents of the digester will be too solid to blow out (the consistency will be too high, a

ratio of 4:1 equates to a consistency of approximately 12%).

If the liquor to wood ratio is too high it indicates that the maximum volume of wood chips

has not been loaded, thus wasting production capacity. It is therefore very important that

this ratio is maintained at a set value: usually 4:1. Figure 9 shows the cooking liquor

charging process

4.5 Deviations from normal conditionsTo make sure that a process produces quality products, control limits are set and various

controls used to show an operator when a process comes too close to these limits as well

as the corrective action that he/ she needs to take before any problems occur.

Control limits: calculated statistical points which form boundaries within which a process should be managed.

To reduce deviation in a process, you have to keep the process stable. When a process

is stable, the products will be produced to the same specifications most of the time. The

reason why the term “most of the time” is used, is because it is almost impossible to



produce a product that is exactly the same all the time. Factors like temperatures, the

specific operator’s actions and small changes in the raw material can affect the process.

These small changes are called “common cause deviations”. So it is normal for a process

to be constantly changing.

However, stable processes are much more valuable as it helps to reduce the total

deviation in the process and make the process more predictable.

After achieving stability in a process, the next step is to make sure that the deviations that

occur are safely inside the specified limits, making it thereby a capable process.

4.5.1 Detecting problems in a processA little deviation in a process is normal, but as soon as it exceeds certain limits, the

deviation becomes harmful to the company or customer. This is called an abnormal

deviation which is another way of saying that a problem has occurred in the process.

Abnormal deviations can have disastrous effects on the quality of your product, and have

to be corrected as quickly as possible.

Product deviations that could occur in the chemical pulping process are:

Kappa number

Consistency of pulp product

Freeness of the pulp

Pulp viscosity

Shive content

Dirt count

Brightness f the pulp

pH

Chemical residual

Each of these product variations has been discussed in Unit three and will not be

discussed further. Refer to Unit 3 for the discussions of these factors.



4.6 Chemical pulping process equipment problems

4.6.1 Raw material feed variabilityIt is absolutely impossible for the same amount of raw material (wood chips or bagasse

and cooking liquor) to be fed into the digester every time. These variations in feed

conditions will obviously have an effect on the final product properties. As mentioned

before, it is important to keep such process variables as constant as possible, but other

times it becomes impossible to due to circumstances beyond the operators’ control.

Malfunctioning of feeding equipment can cause such raw material feed variations. This

problem is best avoided by proper regular inspection and maintenance of equipment.

4.6.2 Poor circulationCirculation within the digester is important because it promotes even distribution of

cooking liquor and therefore more uniform cooking. Liquor circulation also helps to even

out temperature and chemical concentration gradients within the digester to make de-

lignification more homogeneous.

4.6.3 Poor packing of wood chips in the cooking vesselUniform chip packing is important to ensure that the cooking liquor is evenly distributed

throughout the entire bulk of the chip load. This ensures a uniform cook throughout the

digester thus results in better yields and quality of the pulp.

4.6.4 Poor temperature controlAs mentioned many times before, the cooking temperature is an important variable in the

chemical pulping process. It goes without saying that there should be adequate control of

temperature so that the desired temperature can be maintained most of the time. Poor

temperature control will result in either cooking the raw material at too high a temperature

or too low a temperature. Both these scenarios will produce a poor quality product.

4.7 Abnormal conditionsAn abnormal operating condition is a condition that consists outside the normal range of

operations. However, to determine whether something is normal or abnormal, you need

to know the normal ranges for the instruments associated with the system as well as

recognise the sounds that each component makes when it is operating properly. Only



after you have become familiar with these conditions, will you be able to observe

abnormal conditions quite easily.

Other abnormal conditions, such as an insufficient supply of lubricating oil or grease, and

physical damage, such as broken gauges or pipes, are also obvious enough to be

detected during a basic inspection.

Most process variables have critical operating parameters. If the value of a process

variable is outside of the established parameters, the safety of the process could be in

jeopardy. As such an abnormal condition can occur at any time, you must be ready and

able to respond correctly by knowing your unit’s standard operating procedures (SOP's)

as well as participating in emergency response drills and fire fighting drills.

When abnormal conditions occur, you must know what actions to take to protect

personnel health and safety and to prevent or minimise equipment damage. It may also

require that you make sure that everyone in the unit is accounted for.

When abnormal conditions occur, your primary responsibility is to operate the unit in a

way that will minimise danger to personnel. You can do this by following the specific

operating procedures for any piece of equipment, for example, you may need to perform

an emergency shutdown.

Any upset in your workarea increases the potential for injury and lost profits. Therefore

the combined efforts of all personnel are often needed to correct these abnormal

conditions and to resume normal operations.

You should also report abnormal conditions or potential problems immediately. Finding

and solving potential problems reduce downtime and production loss.

4.8 A problem solving strategyBy now you may realise that operating the machine is plain sailing as long as everything

works well. However, it is when things start going wrong that you need to fall back on

your training to solve the problem. Understanding and being able to implement a problem

solving strategy is of utmost importance to help you perform your tasks more efficiently.

The haphazard problem solving methods followed by some of us at home are not

necessarily the best way to address problems at work.

Following are the basic problem solving steps you have to follow to solve and address

problems in your workarea:

Identify all the symptoms



Define all the possible causes

Minimise the number of possible causes through elimination

Identify the most probable cause

Define a number of possible solutions

Select an optimum solution

Implement the solution

Verify the success of the solution



ANNEXURE 1: RESOURCES

Primary Resources PCY 201-P Study Guide

PTL 201-P Study Guide

PCY 301-P Study Guide

Birmann, CJ (1996) Handbook of pulping and papermaking, Academic Express

Rydholm SA (1965) Pulping processes, Interscience.

This manual was developed by

Sparrow Research and Industrial Consultants CC

e-mail: [email protected]

Tel: 012 – 460 9755



ANNEXURE 2: US 256290

SOUTH AFRICAN QUALIFICATIONS AUTHORITY

REGISTERED UNIT STANDARD:

Monitor and control the production of chemical pulp.

SAQA US ID UNIT STANDARD TITLE

256280 Monitor and control the production of chemical pulp.

ORIGINATOR REGISTERING PROVIDER

SGB Pulp and Paper

FIELD SUBFIELD

Field 06 - Manufacturing, Engineering and Technology

Engineering and related design

ABET BAND UNIT STANDARD TYPE NQF LEVEL CREDITS

Undefined Regular Level 4 15

REGISTRATION STATUS

REGISTRATION START DATE

REGISTRATION END DATE

SAQA DECISION NUMBER

Registered 2008-02-06 2011-02-06 SAQA 0875/ 08

LAST DATE FOR ENROLMENT LAST DATE FOR ACHIEVEMENT

2012-02-06 2015-02-06

This unit standard replaces:

US ID Unit Standard Title

NQF Level

Credits Replacement Status

114241Produce chemical pulp from wood chips using a batch digester

4 37 Complete

246717 Produce chemical pulp 4 15 This is only acceptable as a replacement if 246717 has the following statuses: Pending, Never Was Offered, Inactive. Please



check that this is the case, and inform the Data Quality Coordinator if not.

PURPOSE OF THE UNIT STANDARD Learners who demonstrate competence as described in the outcomes of this unit standard will be able to monitor and control the production of chemical pulp using any of the locally available digester configurations. The process ranges from the wood chip or washed bagasse feeder to either the pulp dump tank or pulp storage chest.

The qualifying learner is able to:

Explain the fundamental principles applicable to the chemical pulping process.

Monitor and control the different ancillary systems interacting with the chemical pulping process.

Monitor and control the quality standards of process materials in the chemical pulping process.

Monitor and control the chemical pulping process.

Note:

The impact of the chemical pulping process is measured at various stages later in the process and feedback is required for necessary control.

Learning assumed to be in place and recognition of prior learning Learners accessing this unit standard will have demonstrated competence against mathematics and literacy at NQF Level 3 or equivalent.

UNIT STANDARD RANGE The typical context of this unit standard covers the production of chemical pulp using any of the locally available digester configurations. The process ranges from the wood chip or washed bagasse feeder to either the pulp dump tank or pulp storage chest.

Range statements, which are applicable to the unit standard titles, specific outcomes and assessment criteria are found beneath the applicable assessment criteria.

Specific Outcomes and Assessment Criteria: SPECIFIC OUTCOME 1 Explain the fundamental principles applicable to the chemical pulping process.

ASSESSMENT CRITERIA ASSESSMENT CRITERION 1 The purpose of the chemical pulping process is explained in terms of the final product manufactured.

ASSESSMENT CRITERION 2 The principles of the chemical pulping process are explained by making use of a generic flow diagram.



ASSESSMENT CRITERION 3 The chemical pulping process is explained in relation to its supplier`s- and customer`s processes.

ASSESSMENT CRITERION 4 The flow of material through the chemical pulping section is traced and all equipment is identified using standard industry terminology.

ASSESSMENT CRITERION 5 The purpose and functioning of each piece of equipment used in the chemical pulping section is explained in terms of its role in the overall process.

ASSESSMENT CRITERION RANGE Equipment include but is not limited to chip conveyors, chip storage bins, chip feeders, chip monitors, cooking vessel, heating equipment, discharge valves, pulp receiving vessels, steam flash vessels.

ASSESSMENT CRITERION 6 The functions of all chemicals used within the chemical pulping process are explained in terms of their chemical and physical properties.

ASSESSMENT CRITERION RANGE Chemicals include but are not limited to cooking chemicals either sodium sulphide and sodium hydroxide or sodium hydroxide or acid bi sulphite or neutral sulphite liquors.

SPECIFIC OUTCOME 2 Monitor and control the different ancillary systems interacting with the chemical pulping process.

OUTCOME RANGE Ancillary systems refer to the interface between mechanical equipment, electrical equipment, instrumentation and utilities and the chemical pulping process. It only includes those parts of each ancillary system which interact directly with the chemical pulping process and not the full ancillary system.

ASSESSMENT CRITERIA

ASSESSMENT CRITERION 1 Mechanical equipment used in the chemical pulping process is identified and described in terms of purpose and application.

ASSESSMENT CRITERION RANGE Mechanical equipment may include bulk handling equipment, conveying equipment, weighing equipment, storage equipment, transport equipment and packaging equipment.

ASSESSMENT CRITERION 2 Electrical equipment used in the chemical pulping process is identified and described in terms of purpose and application.

ASSESSMENT CRITERION RANGE Electrical equipment may include electrical motors, switchgear and drive equipment.



ASSESSMENT CRITERION 3 Instrumentation used in the chemical pulping process is identified and described in terms of purpose and application.

ASSESSMENT CRITERION RANGE Instrumentation includes the key process indicators, control valves and controllers used to monitor and control the process.

ASSESSMENT CRITERION 4 Utilities used in the chemical pulping process are identified and described in terms of purpose and application.

ASSESSMENT CRITERION RANGE Utilities may include air, steam, electricity and cooling water.

ASSESSMENT CRITERION 5 Typical ancillary equipment problems within the chemical pulping process are discussed and solutions offered in accordance with workplace procedures.

ASSESSMENT CRITERION 6 Ancillary systems are monitored and any deviations from operating parameters are corrected in accordance with operating procedures.

SPECIFIC OUTCOME 3 Monitor and control the quality standards of process materials in the chemical pulping process.

OUTCOME RANGE Process materials include all raw materials, products, chemicals and any additives forming part of the chemical pulping process.

ASSESSMENT CRITERIA ASSESSMENT CRITERION 1 The properties of process materials are explained in terms of key characteristics.

ASSESSMENT CRITERION 2 The purpose of process material quality control procedures as well as the consequences of not adhering to these procedures are explained with regards to the impact thereof on the final product produced.

ASSESSMENT CRITERION 3 The quality requirements of raw materials, chemicals and additives are explained according to general and workplace specifications.

ASSESSMENT CRITERION RANGE Specifications are defined by machine or mill operating instructions. Specifications may include, but are not limited to, fibrous raw material (wood chips or bagasse) type, ash content, dirt content and condition as well as cooking liquor chemistry, strength and clarity.

ASSESSMENT CRITERION 4



Typical raw material problems and its impact on the final pulp properties and costs are discussed in terms of the purpose of the process.

ASSESSMENT CRITERION RANGE Raw material problems include but are not limited to under/over size of fibrous raw material, variability of cooking liquor strengths and clarity.

Final product properties include but are not limited to Kappa number, consistency, freeness, viscosity, shive count, dirt count, brightness and pH as identified by the workplace quality control system.

ASSESSMENT CRITERION 5 Corrective action to be taken in the case of non-conforming raw materials is discussed in accordance with workplace procedures.

ASSESSMENT CRITERION 6 Product variations are evaluated and corrective action taken in accordance with workplace procedures.

ASSESSMENT CRITERION RANGE "Product" includes pulp as well as the final paper, board or tissue product.

"Variations" include but are not limited to Kappa number, consistency, freeness, viscosity, shives, dirt count, brightness, pH, chemical residual and the strength and quality properties of the final paper, board or tissue product.

SPECIFIC OUTCOME 4 Monitor and control the chemical pulping process.

OUTCOME NOTES The monitoring and controlling of the chemical pulping process should be performed according to operational requirements and standard operating procedures.

ASSESSMENT CRITERIA ASSESSMENT CRITERION 1 The chemical pulping process is monitored and parameters recorded in accordance with workplace procedures.

ASSESSMENT CRITERION 2 The impact of process variables on the product properties is explained in terms of final products and costs.

ASSESSMENT CRITERION RANGE Process variables include but are not limited to H-factor or S-factor, pressure, cooking liquor addition, temperature and/or liquor to wood ratio.

ASSESSMENT CRITERION 3 Typical equipment problems within the chemical pulping process are discussed and solutions offered in accordance to workplace procedures.

ASSESSMENT CRITERION RANGE Equipment problems include but are not limited to fibrous raw material feed variability, poor circulation of cooking liquors, poor heat exchanging efficiencies, poor fibrous raw material packing within the cooking vessel, poor temperature control within the cooking vessel.



ASSESSMENT CRITERION 4 Variations in the product are evaluated and corrective action taken in accordance with workplace procedures.

ASSESSMENT CRITERION RANGE Product deviations include but are not limited to Kappa number, consistency, freeness, viscosity, shives, dirt count, brightness, pH and chemical residual.

UNIT STANDARD ACCREDITATION AND MODERATION OPTIONS An assessor, accredited with a relevant NQF Level 4 or higher qualification, will

assess the learner`s competency.

Only an Assessor with considerable first hand experience in process operations will assess the learner`s competency.

Anyone assessing a learner or moderating the assessment of a learner against this Unit Standard must be registered as an Assessor with the relevant ETQA.

Direct observation in simulated or actual work conditions is required.

UNIT STANDARD ESSENTIAL EMBEDDED KNOWLEDGE Qualifying learners understand and can:

Explain the names, functions and locations of:

All items of installed equipment.

Raw material/s.

Finished product/s.

Describe the properties and characteristics of:

Raw material/s.

Product/s.

Process equipment.

Auxiliary equipment.

Process system/s.

Explain the purpose of the:

Process, in terms of product/s, efficiencies and quality.

Explain the causes, effects and implications of:

Process variables.

Raw material variables (suppliers).

Product variables (customers).

Not complying with quality standards.

Not complying with standard operating procedures.

Demonstrate procedures and techniques of:

Monitoring and controlling the process.

Recording and reporting data.

Explain the regulations, legislation, agreements and policies related to:

Standard Operating procedures.



Quality specifications.

UNIT STANDARD DEVELOPMENTAL OUTCOME N/A

UNIT STANDARD LINKAGES N/A

Critical Cross-field Outcomes (CCFO): 1. UNIT STANDARD CCFO IDENTIFYING The learner is able to identify and solve problems in which responses display that responsible decisions, using critical and creative thinking, have been made by:

Identifying and addressing variations in ancillary system operation.

Identifying and addressing variations in material quality.

Identifying and addressing variations in the chemical pulping process.

Bringing all deviations into control.

Refer to the following Specific Outcome(s):



2. UNIT STANDARD CCFO WORKING Work effectively with others as a member of a team, group, organisation or community by:

Maintaining sound relations with co-workers.




3. UNIT STANDARD CCFO ORGANISING The learner is able to organise and manage himself and his activities responsibly and effectively by:

Working to achieve consistent results required by the process/customers.




4. UNIT STANDARD CCFO COLLECTING Collect, analyse, organise and critically evaluate information by:



Using various sources of information pertaining to the chemical pulping process, including basic scientific and engineering theory.

Monitoring and recording process, product and equipment variables.

Carrying out physical quality checks and tests.

Explaining the impact of non-conforming materials in terms of final products and costs.

Describing the relationship of the process to suppliers, customers and the overall chemical pulping process.

Refer to all Specific Outcomes.

5. UNIT STANDARD CCFO COMMUNICATING Communicate effectively by using mathematical and/or language skills in the modes of oral and/or written presentations during:

Recording of variables and actions taken to rectify.

Responding to questions and requests for additional information.

Liaising with relevant operational support units.

Drawing and interpreting diagrams and sketches.

Completing relevant documentation in accordance with workplace requirements.


6. UNIT STANDARD CCFO SCIENCE Use science and technology effectively and critically, showing responsibility towards the environment and health of others by:

Monitoring the process.

Bringing process, product and equipment variables into control.




7. UNIT STANDARD CCFO DEMONSTRATING Demonstrate an understanding of the world as a set of related systems by:

Explaining the purpose of the process as well as the relationship to the supplier and customer`s processes.


8. UNIT STANDARD CCFO CONTRIBUTING Contribute to the full personal development of each learner and the social and economic development of the society at large by:

Understanding the role of monitoring plant and process in a processing environment and the effect it has on the growth and development of the organisation, its customers and employees.




Icons

Icon Description

Know

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Skill

Develop a Checklist

Observe a Demonstration X X

Observe an Experiment X X

Participate in an Experiment

Participate in Group work/ discussions, role-play, etc. X

Attend a Lecture X X

Conduct a Presentation

Multimedia X

Use Multimedia as learning tool

Practical

Self-study / Individual activity

Site visit X

Theoretical – all types of questions X

Make sketches, write reports, case studies, etc.


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