Karlstad University 65188 Karlstad

Tfn 054-7001 00 00 Fax 054-700 14 60

www.kau.se information@kau.se

Faculty of Technology and Science

Department of Chemical Engineering

Muhammad Adeel Shahzad

Effect of temperature and time on acid sulfite

cooking for dissolving pulp

Degree Project of 30 credit Points

Master of Science in Engineering

Degree Program in Chemical Engineering

Date: 2012-11-19

Supervisor: Ulf Germgärd

Assistant supervisor: Niklas Kvarnlöf

Examiner: Lars Järnström

2

Contents Abstract ...................................................................................................................................... 3

Executive Summary ................................................................................................................... 4

Abbreviations: ............................................................................................................................ 5

Objective .................................................................................................................................... 6

1. Introduction ............................................................................................................................ 6

1.1 Wood ................................................................................................................................ 6

1.2 Pulp ................................................................................................................................... 7

1.3 Softwood and Hardwood .................................................................................................. 7

1.4 Cellulose ........................................................................................................................... 8

1.5 Hemicellulose ................................................................................................................... 8

1.6 Lignin ............................................................................................................................... 8

1.7 Extractives ........................................................................................................................ 9

1.8 Dissolving Pulp ................................................................................................................ 9

1.9 Sulfite Process ................................................................................................................ 11

2 Material and Method ............................................................................................................. 14

2.1 Screening of Chips: ........................................................................................................ 14

2.2 Preparation and analysis of cooking liquor: ................................................................... 14

2.3 Acid Sulfite cooking: ..................................................................................................... 14

3. The Process flow diagram of experiment ............................................................................. 15

4. Results & Discussion ........................................................................................................... 16

4.1 Effect of cooking time and temperature on spent liquor pH .......................................... 17

4.2 Effect of cooking time and temperature on total residual SO2 ....................................... 18

4.3 Effect of cooking time and temperature on total yield ................................................... 18

4.4 Effect of cooking time and temperature on kappa number ............................................ 19

4.5 Effect of cooking time and temperature on limiting viscosity ....................................... 20

4.6 Effect of cooking time and temperature on R18 .............................................................. 20

4.7 Effect of cooking time and temperature on reject .......................................................... 21

4.8 Black cooks at different cooking temperatures and times .............................................. 22

5. Conclusion ............................................................................................................................ 23

6. Recommendation for future work ........................................................................................ 23

7. Acknowledgements .............................................................................................................. 23

8. References: ........................................................................................................................... 24

9. Appendix: ............................................................................................................................. 26

9.1 Equations: ....................................................................................................................... 26

9.2 Summary of Results ....................................................................................................... 27

3

Abstract

The aim of this study was to investigate the effect of cooking temperature and cooking time

on spruce sulfite dissolving pulps. Dissolving pulp is used for the manufacturing of cellulose

derivative products like viscose fiber, rayon and synthetic material. The sulfite (magnesium

base) pulping process was used liquor to wood ratio was 4:1 and the total SO2 charge was

24%. Three different temperatures 140 ⁰C, 150 ⁰C and 160 ⁰C were used and various pulp

properties were measured at different time intervals until the black cook was obtained. After

sulfite, pH and total residual SO2 of spent liquor were measured. The yield, reject, kappa

number, limiting viscosity and R18 of the cooked pulp were analyzed according to ISO

standards. When the cooking time was increased at a specific temperature pH, total residual

SO2, reject, total yield, kappa number and limiting viscosity decreased and R18 value

increased. It was noted that when pulp was over cooked, kappa number and reject increased

due to lignin condensation which forms black cook.

Black cook was obtained after 17 hours, 5 hours and 3 hours at 140 ⁰C, 150⁰C and 160 ⁰C

respectively.

Keywords: Cellulose, Hemicellulose, Lignin, Sulfite pulp, Dissolving pulp, Cellulose derivatives

4

Executive Summary

The demands for dissolving pulps have substantially increased in the last decade because of

its high cellulose purity. Currently, dissolving pulps are produced by the acid sulfite and the

vapor-phase prehydrolysis kraft processes. Acid sulfite pulping renders a good basis for the

realization of biorefinery concept in which cellulose, hemicellulose and lignin may be

recovered to optimize the process. Viscose fiber market has boosted up the demand for the

production of dissolving pulps. Therefore, the industries and the academia are researching on

the processes available for the production of dissolving pulp to optimize the process. In view

of this aspect, an effort was made towards the production of dissolving pulp.

Purpose of this work was to investigate the effect of cooking temperature and time on acid

sulfite for dissolving pulp production. Acid sulfite cooking was done at 140 ⁰C, 150 ⁰C and

160 ⁰C with different time intervals, varying from 1.5 hours to 17 hours for different cooking

temperatures. After sulfite cooking, pH and total residual SO2 were analyzed from spent

liquor. The total yield, reject, kappa number, limiting viscosity and the R18 values of the

produced dissolving pulp were analyzed according to international standards.

Experimental work for this study was performed at Karlstad University, Sweden. Spruce

wood chips were collected from pulp mill StoraEnso Skoghall, Sweden dried and screened in

the lab of the company named Metso Paper Karlstad, Sweden. Overlarge and over thick chips

were separated. Only chips good in quality that passed through 8mm screening slots and

retained at 7mm round whole tray were used for the pulping process. 2.5 l stainless steel

autoclaves were used during cooking. The cooking liquor was prepared by mixing 5g of MgO

in deionized water to make the solution of 800g. The solution was stirred until 50g of SO2

was added to the solution. Total SO2 and combined SO2 were measured by titration against

KIO3 and NaOH respectively.

200 g of 93 % dried chips were added to the autoclave and steamed for 30 minutes at 2.5 bar

pressure and amount of water (steam) added to the wood chips was calculated. Calculated

amount of already prepared cooking liquor was added to the autoclave. Wood to liquor ratio

was kept 1:4 for the cooking process. Autoclaves were placed in PEG bath at 90 ⁰C as its

initial temperature and the PEG bath temperature was ramped up at the rate of 2.5 ⁰C /min

upto 115 ⁰C for 30 minutes for sulfonation reaction. PEG bath temperature was ramped up at

the rate of 0.41, 0.58 and 0.75 ⁰C /min upto 140 ⁰C, 150⁰C and 160 ⁰C respectively. Cooking

times for 140 ⁰C were 2,3,5,8 and 17 hours, cooking times for 150 ⁰C were 1.5, 2,3,4,5 and 6

hours and cooking times for 160 ⁰C were 2,3, and 4 hours. After cooking, spent liquors were

collected from autoclaves for measuring pH and total residual SO2.

Pulp was washed with deionized water, disintegrated and screened. The reject was collected

and dried overnight in the oven at 105 ⁰C. The screened samples were centrifuged for 10

minutes and dried at room temperature. After wards, total yield, kappa number, limiting

viscosity and R18 of the produced pulp were measured.

In this study, focus was to determine the effect of cooking time and cooking temperature with

a constant total SO2 charge i.e 24%. As the cooking time was increased, pH, total residual

SO2, reject, total yield, kappa number and limiting viscosity decreased and R18 value

increased with the increasing time at a specific temperature. When the pulp was overcooked,

kappa number and reject increased drastically due to lignin condensation which forms a black

cook. Black cook was obtained after 17 hours, 5 hours and 3 hours at 140 ⁰C, 150 ⁰C and 160

⁰C respectively.

5

Abbreviations:

SO2 sulfur dioxide

MgO Magnesium oxide

KI Potassium iodine

KIO3 Potassium iodate

Na2S2O3 Sodium thiosulfate

NaOH Sodium hydroxide

HSO3-

hydrogen sulfite ions

PEG polyethylene glycol

CED Copper-ethylene-di-amine

DP Degree of polymerization

ºC Degree centrigrade

6

Objective

This thesis focuses on the acid sulfite pulping process of spruce softwood for the production

of dissolving pulp, mainly because dissolving pulp production growth rate has significantly

increased due to various cellulose derivatives products production. Purpose of this work is to

investigate how the acid sulfite pulping process influences the pulp properties because under

acidic conditions lignin solubilizes through the addition of hydrophilic sulfonate groups and

its effects were studied. In the manufacturing of dissolving pulp it is most important to

remove the hemicelluloses, lignin and extractives to have cellulose as pure as possible.

1. Introduction

1.1 Wood

Wood is a natural product and a sustainable resource when used responsibly that need not

result in damage to the environment. It is one of the most important raw materials for human

beings. It is significant not only for the use in hundreds of products, but is also a renewable

natural resource because it is a cellular material of biological origin [1]

. Wood is produced by

the seed bearing plants and has hierarchic structure that is responsible for mechanical and

physical properties of all its products including pulp [2]

. Wood is a complex material with

different properties. It is a hygroscopic (ability to attract moisture from air) and anisotropic

(wood structure and properties vary in different directions) material of biological origin.

Biological origin of wood indicates the diversity and variation among different species of

trees. Wood properties and behavior depend fundamentally on the structure of wood from

molecular to cellular or anatomical level.

Figure 1: Elemental composition of dry wood

Wood is an organic material which consists of mainly carbon, hydrogen and oxygen.

Elementary composition of dry wood substance is about 50% carbon, 6% hydrogen and 44%

oxygen, with small variations in species like softwoods and hardwoods. These elements form

macromolecules at higher level called polymers which represent the main cell wall

compounds of cellulose, hemicelluloses and lignin and are the main constituents of wood. [1]

Wood is a raw material in the forest industry for the production of pulp and paper. But

nowadays, new technologies are emerging and the wood is also being used as the production

of other valuable products like cellulose derivative products. In Sweden, forests consist of

different types of trees but the main types are Spruce, Pine and Birch. However, Spruce is

available abundantly followed by Pine. Forest industry of Sweden used for different pulping

process is shown in figure 2.

7

Figure 2: Distribution of wood tree species in Sweden

Wood is composed of highly ordered axial and radial cell systems .These cells vary in both

size and shape. Wood is mainly classified into two major groups: softwoods and hardwoods.

[2]

1.2 Pulp

The term pulping indicates the process by which wood or other lignocellulosic material is

reduced to a fibrous mass known as pulp. Pulps are produced from different methods and

have different properties that make them suited to particular products. Normally pulps are

produced by chemical pulping process globally [1]

. The wood consists of cellulose,

hemicellulose and lignin. Fibers are separated by the dissolution of the lignin and for this

purpose; chemical and mechanical processes are employed [3]

. In chemical pulping process,

wood chips are cooked with chemicals which dissolve the lignin and separate the fibers [4]

.

Main chemical wood cooking techniques are kraft or sulfate process and sulfite process. For

dissolving pulp production, sulfite process is a dominating technology because lignin and

hemicelluloses are removed in the same step. [5]

1.3 Softwood and Hardwood

Softwoods and hardwoods are distinguished by the structure of their wood and cell elements.

Softwoods show a simpler structure than hardwoods. Softwoods are basically composed of

tracheids which are mainly oriented in the longitudinal direction while a few amounts of

tracheids are radially oriented within the rays. In hardwoods, vascular and vasicentric

tracheids are associated with vessels and fluid conducts through these vessels. Hardwoods

have much variation in rays width and height as compared to softwoods. Softwoods have

longer fiber length than the hardwoods and hardwood fibers have thicker cell walls, smaller

cell lumina. Softwoods and hardwoods have different composition of wood cell wall layers.[2]

Most softwoods like pines and larches are less suitable for sulfite pulping due to the presence

of certain part of extractives of phenolic character, which gives rise to the condensation

reactions with reactive lignin in the presence of acid sulfite cooking solutions.[1]

Hemicellulose in softwoods have a higher proportion of mannose and galactose than

hardwoods[6]

. As it is known that wood is no uniform substance but consists of many

chemical components that are different in quantity with different species. These components

are classified according to the available analytical methods, where an originally analytical

term has been applied to a chemical compound. These important terms are cellulose,

hemicelluloses and lignin.

8

1.4 Cellulose

Cellulose is the main component in cell wall of the plant [2]

. It is the most abundant compound

in the plants and wood [7].

It is a long homo saccharide of sufficient chain length which is

insoluble in water [3]

. Cellulose is a biopolymer and renewable raw material and this property

makes it important and fascinating. Many cellulosic materials consist of crystalline (ordered)

and amorphous regions (disordered) in different proportions [8]

and these crystalline and

amorphous regions affect the accessibility and chemical reactivity of the fibers [9]

. The degree

of polymerization of cellulose is very high; value of 15000 residues in single chain unit makes

cellulose to the longest of all known polysaccharides. Cellulose chains are not completely

straight and it may have extended helix [2]

.

1.5 Hemicellulose

Generally, hemicellulose occurs as heteropolysaccharides and it is the second most abundant

polysaccharide group in plants [10]

. Wood hemicelluloses are short in chain length. Therefore,

it has a lower degree of polymerization: up to 200 [2]

and lower molecular weight than the

cellulose. They are often branched rather than the linear polymer like cellulose. Hemicellulose

DP and composition depends on wood species. Hemicelluloses are embedded within the cell

wall and associated with cellulose and other components. Hemicelluloses are non-crystalline

because of the heterogeneity which makes it easier to hydrolyze [11]

. Chemical and thermal

stability of hemicelluloses are lower than the cellulose. Hemicelluloses are also found in the

matrix between cellulose fibrils in the cell wall and it may serve as an interface between the

cellulose and lignin [2]

. Hemicelluloses are assigned to those carbohydrates which degrade by

acid hydrolysis more rapidly than cellulose. Hemicelluloses are amorphous in structure.

Therefore, most chemical agents reach the hemicelluloses much more easily than the cellulose [12]

.

1.6 Lignin

Lignin is the third main component of wood. Wood is actually defined by the presence of

lignin in the cell wall structure. Infact the stiff woody appearance is due to the lignin [12]

.

Lignin is the hydrophobic polymer which is in between the cellulose microfibrils and

hemicelluloses, fixating them towards each other, resulting woody properties. Lignin has

complex structure and it has amorphous region. It is neither a polysaccharide nor a nucleotide.

Lignin is not a linear polymer as cellulose or a branched polymer as the hemicelluloses, but it

has three dimensional web structures [2]

. Lignin is a highly complex polymer consisting of

phenolic compounds. Lignin distribution within the cell wall and lignin contents of different

parts of a tree are different [1]

.

9

1.7 Extractives

Major structural components of wood include cellulose, hemicelluloses and lignin. Other

wood components contain exceedingly various types of high and low molecular weight

(organic) compounds which are called extractives [1]

. Extractives are extractable from wood

with hot water or organic solvents [12]

. Extractives amount varies in certain parts of the wood,

it is not constant throughout the wood [1]

.

1.8 Dissolving Pulp

For the production of dissolving pulp, chemistry of cellulose and hemicelluloses have the

most significance to optimize the production of high purity dissolving pulp, since the

hemicellulose and the cellulose reactivity with chemicals differ. Hundreds of cellulose

molecules aggregate to form microfibrils because of strong tendency of hydrogen bonds to

form intra and inter molecular hydrogen bonds. The microfibrils layering give the crystallinity

and non crystallinity regions (amorphous) in the cellulose chain [13]

.

Normally, softwoods are used for the production of dissolving pulp but sometimes hardwoods

are also used. Dissolving pulp is also being produced from cotton linters (soda pulping) and

wood via prehydrolysis kraft or acid sulfite processes. However, paper grade pulp is also

converting into dissolving pulp with selective removal of hemicellulose while ensuring the

high pulp reactivity [14]

. During dissolving pulp, manufacturing controlled variable is the

degree of polymerization (DP) of the cellulose because it gives an indication of the average

length of the polymer chains.

High grade cellulose pulp is known as dissolving pulp because it contains low amount of

hemicelluloses, lignin and resin. Dissolving pulp is a chemically refined bleached pulp,

consisting more than 90% alpha cellulose content. Quality of the dissolving pulp depends on

both the properties of raw material wood and pulping process [15].

Dissolving pulp production

is mainly done by acid sulfite and prehydrolysis kraft process. Sulfite pulp produces relatively

pure and uniform molecular weight distribution. Among these processes, acid sulfite process

is the most common and beneficial technique, including high recovery rates of the inorganic

cooking chemicals and the totally chlorine free bleaching. There is only one disadvantage that

acid sulfite process results in pulps with a broad molecular weight distribution of cellulose [7]

.

Different cellulose sources are being used for the production of dissolving pulp. About 85%

of the total volume of the wood (hardwoods and softwoods) uses wood derived celluloses.

The morphology of these two types of wood is different. The appearance of the fiber from its

natural state in wood and also how the pulping process has affected the fiber is of utmost

importance for the end user of dissolving pulp. Fiber properties and fiber morphology like

fiber length, fiber length distribution, roughness and fiber shape are vital to the physical

properties of many cellulose derivative products.

Dissolving pulp has different chemical and physical properties from kraft pulp. Different type

of hemicelluloses and their quantity present in pulp has most significant chemical variation

and thus has different impact on the processability to the quality of the end products. Different

dissolving pulps possess different chemical and physical properties because dissolving pulp

cannot be categorized by their origin of wood type or process type but also for their purpose

of final use because the demand on quality can vary [6]

.

Most common use of pulp is in paper making but dissolving pulp is the cleanest pulp.

Therefore, dissolving pulp is well suited as a raw material for different types of cellulose

derivative products including viscose textile fiber, rayon, carboxy methyl cellulose, cellulose

ester, cellulose acetate, and staple fibers [16]

. Sulfite pulp production is quite small as

10

compared to kraft pulp, the sulfite pulp production towards pulps for integrated production of

certain papers like wood fee printing and writing papers, tissue papers and grease proof paper

[2]. The dissolving pulp cellulose is distinguished by high reactivity, means cellulose ability to

form filterable solution. Cellulose from sulfite delignification for viscose textile fiber gives

high homogeneity [17]

.

11

1.9 Sulfite Process

Sulfite pulping name was derived from the use of a bisulfate solution as the delignifying

medium. Sulfite pulping process has many advantages like high initial brightness and easy

bleachability of sulfite pulps. Acid sulfite pulping process is the dominant technology for the

production of dissolving pulps and responsible for approximately 70% of the total world

production. The sulfite pulp technology in dissolving pulp production of less purity pulp

grades sufficiency for the regenerated fiber. Manufacturing is based on a favorable economy

because of higher pulp yield, higher reactivity of pulp as compared to a corresponding

prehydrolysis kraft pulp and better bleachability. These are the reasons for the domination of

sulfite pulping process for the production of dissolving pulp [1]

.

Sulfite pulping process is sensitive to the wood species. In acid sulfite pulping, wood species

with low extractive contents are used safely because acid sulfite cook has poor ability to

dissolve extractives [18]

.

During sulfite cooking, phenolic compounds in lignin cause problems in the process because

lignin condensation at low pH or low concentration of SO2 can easily occur. In sulfite process,

acid conditions promote the cleavage of glycosidic bond in the cellulose and hemicelluloses

because of the lower degree of polymerization and amorphous state. Weaker glycosidic bonds

and hemicelluloses are depolimerized easier than the cellulose and dissolved in the cooking

liquor as monosugars. Cellulose chains are also affected by acid hydrolysis during the

cooking process and major depolymerization of cellulose does not happen until the end of the

delignification [6]

.

Normally during the process, cation used is calcium, magnesium, sodium or ammonium.

Different sulfite solutions have different solubilites. These solubilites set the selection of

cation like calcium requires pH 2 to stay in solution, magnesium requires pH 4, sodium and

ammonium sulfite solutions may be strongly alkaline without precipitation. Acid process has

pH 2-3, bisulfate processes operated at pH range 3-5, neutral sulfite processes have pH range

6-9 and alkaline sulfite processes have pH range above 11. [18]

Usually sulphite cooking liquors are analyzed for total SO2, free SO2 and combined SO2.

These are determined by iodometric titration, followed by another titration with NaOH. Total

SO2 sums up the SO2- content in the cooking liquor of SO2, HSO3- and SO32-

. The combined

SO2 value is the measured amount of cation in the system and is defined as the amount of

SO2that is needed to produce XSO3, where X is the cation i.e. Ca2+

, Mg2+

, Na+, or NH

4+.

Calcium can be used for acid sulfite cooking. Calcium in the spent liquor cannot be recovered

for continued used and has therefore been substituted primarily by sodium or magnesium

ions. Magnesium base can be used at higher pH but then in slurry which is gradually

dissolved after chelation by means of dissolved wood material. Due to the formation of this

slurry, it is not possible to use Mg base at large scale. However, Na can be used at all pH

levels. NH4+

can also be used to some extent and at all pH levels but it cannot be recovered

and the pulp is darker than normal. [2]

During sulphite cook, the composition of sulphite liquor varies very much. SO2 is consumed

for the sulphonation of lignin, lignosulphonic acids are dissolved which increase the acidity of

the liquor, etc. The rate at which lignin is sulphonated and hydrolyzed depends on the

composition of the liquors. These rates vary during the cooking process which is very hard to

predict. At 130 ºC maximum sulfonation reaction takes place. Therefore, the cook is heated at

130 C, little longer than the usual heating for the sake of maximum sulfonation [18].

Lignin degrading reactions in the acid sulfite process are characterized in the figure 3 below.

12

Sulfonation is the main reaction under acidic conditions. Sulfonation reaction is the strong

dependence on pH and is fast reaction at low pH. Hydrolysis reaction is somewhat slow as

compared to the sulfonation reaction and decreases the molecular weight of lignin because

hydrolysis linkages between lignin and carbohydrates. During these reactions, condensation

reaction also takes place but these lignin condensation reactions are undesired processes and

counteracting the delignification. [1]

Temperature is one of the most important parameter because temperature determines the rate

of the delignification in sulfite process. Increase in temperature increases the pH, decreases

SO2 solubility, increases lignin dissolution and helps in decomposition of carbohydrates. In

acid sulfite cooking, hydrolytic carbohydrate reactions take place. In this reaction, cellulose

and hemicelluloses both take part. But attach on cellulose is minor in this reaction due to the

low accessibility of cellulose. However, dissolution of hemicelluloses through hydrolysis is

substantially low. [18]

The dominating sulfite pulping process today is magnesium because the corresponding

aqueous magnesium bisulfate solutions are soluble in a pH range upto 5-6. Magnesium base

sulfite process also has an advantage that the thermal decomposition of MgSO3 occurs at a

rather low temperature, producing only a small amount of sulfide. The obtained magnesium

sulfate from the combustion of magnesium sulfite spent liquor can be decomposed thermally

in the presence of carbon from the dissolved organic substances to render gaseous SO2 and

MgO according to equation 1.

2MgSO4 + C → 2SO2 + 2MgO + CO2 (1)

The chemical reactions during sulfite cooking are complicated system of bonding between

inorganic compounds, lignin, cellulose, hemicellulose, wood extractives and between the

organic compounds exclusively [12]

.

During acid sulfite pulping process, three species are active which are H+, SO2, and HSO

3-.

These species take part in the delignification which is desirable and are not desirable in

carbohydrate reactions. Most important lignin reactions are splitting of α alkyl-aryl ethers and

formation of a carbanion, sulphonation, formation of a benzyl alcohol and condensation of

lignin units.

For the dissolution of lignin, degree of sulphonation is important and the probability for

condensation of lignin increases with a low content of total SO2 or a low amount of combined

SO2 in the cooking liquor. If total SO2 and combined SO2 amounts are very low then the

disturbances appear in the cook and the lignin gets dark and delignification totally stops. This

Lignin

Sulfonation

Dissolution degradation

condensation

SulfitolysisHydrolysis

Figure 3: lignin degradation reactions

13

is called burnt cook or black cook. Whereas SO2 has a strong influence on the delignification

rate, high initial SO2 content in the acid sulphite cooking liquor and maximum pressure gives

a good cooking rate [1]

.

During sulphite cooking, pH of the cook must be well controlled. This could be done by the

addition of correct amount of SO2 for a given case. For instance, in an acid sulphite process

which is used for papermaking, the amount of SO2 is high and pH will be relatively constant

due to the high buffering capacity. However, the combined SO2 is lower in sulfite cook

intended for viscose and the pH of the cook will drop in the end, leading to higher α-cellulose

content. After the main part of the lignin dissolution, lower pH in the final part of the cook

decreases the hemicellulose content. As the kappa number decreases during sulfite pulping

process, yield also decreases to give high amount of cellulose because more amount of

hemicellulose is removed [2]

. When acid sulfite pulping process is used for the dissolving

pulp production, cooking temperature is 140-150 ºC, leading to a high hemicellulose

dissolution. This is beneficial because hemicellulose content should be low in dissolving pulp. [1]

14

2. Material and Method

2.1 Screening of Chips:

Spruce wood chips were collected from StoraEnso Karlstad Sweden, dried and screened in

Metso, Sweden by using SCAN CM 40:88 (46 E). Over large and over thick chips were

separated, whereas the chips that were good in quality and passed through 8mm screening slot

and retained at 7mm round whole tray were collected for experiment. Bark was removed

manually and dryness of chips was checked.

2.2 Preparation and analysis of cooking liquor:

The cooking liquor was prepared by mixing 5 g of MgO and distilled water was added to a

total weight of 800 g. Mixer was started with the speed adjusted to 250 rpm. Then 50 g of SO2

was added to the solution and the small valve was closed in order to stop the SO2-gas flow.

The total and free amount of SO2 in the solution was then determined by titration analysis. For

this purpose, approximately 50 ml of distilled water was added to the E-flask and 1 ml of the

cooking liquor was added with the help of pipette. 5 ml of starch (10 g/l) , 5 ml of Kl (1 M)

and 7-10 drops of methyl red (0.1%) as an indicator were added in the flask. Solution was

titrated with KlO3 (1/60 M). The amount of KlO3 was registered when the solution turned

blue. Then one or two drops of Na2S2O3 (0.2 M) were added and titrated with NaOH (0.1 M).

The amount of NaOH was registered when the solution turned yellow. The total SO2, free

SO2, bound SO2 and total free SO2 were calculated by using equations A, B, C, D (Appendix)

respectively.

2.3 Acid Sulfite cooking:

200 g dry chips of 93% dryness were added to the autoclave. The bottom valve was opened at

half rotation. The difference of autoclave was weighed before and after 30 minutes steaming.

The charge of 24 % of total SO2 and the total amount of liquid (cooking acid+ water) of 800 g

was poured to each autoclave as the liquor to wood ratio was 4:1.The amount of cooking acid

and water was calculated by using equations (E) and (F) shown in Appendix.

Then the autoclave was closed with a lid ensuring that there was no leakage from the upper

and lower valves of the autoclave. Autoclaves were placed in PEG bath with 90 C as its initial

temperature. The temperature of PEG- bath was ramped up at the rate of 2.5 ºC /min upto 115

ºC for 30 minutes in order to get better sulfonation reaction. The temperature of PEG-bath

was ramped up at the rate of 0.41, 0.58 and 0.75 C/min up to 140 ºC, 150 ºC and 160 ºC

respectively. Pressure of autoclaves at 140 C, 150 C and 160 C was 6 bar, 7 bar and 8 bar

respectively. The cooking times for 140 C were 2, 3, 5, 8, 17 hours. Cooking times for 150 ºC

were 1 ½, 2, 3, 4, 5 and 6 hours, whereas cooking times for 160 C were 2, 3 and 4 hours.

The autoclaves were taken out from the PEG-bath after cooking and cooled down. The spent

liquor was collected from each autoclave for pH and total residual SO2.

For analysis of total residual SO2, 50 ml distilled water was poured in E flask and pipette 10ml

of residual liquor into flask. 5 ml starch and 5 ml KI were added and titrated with KIO3 until

the solution turned dark blue. The amount of KIO3 was then registered and total residual SO2

was calculated by using equation (G) shown in Appendix.

The washed pulp samples were soaked overnight in distilled water and screened after

disintegrating according to ISO 5263-1:2004. The screened pulp samples were then

centrifuged for 10 minutes. The reject was kept overnight in the oven at 105 ºC. The yield was

15

calculated by the equation (I) shown in Appendix 1. Kappa number, limiting viscosity and R18

of screened pulp were measured according to ISO 302:2004(E), ISO/FDIS 5351:2009(E) and

ISO 699-1982(E) respectively.

3. The Process flow diagram of experiment

Figure 4: process flow diagram of experiment

Cooking

Acid24 % of total

SO2

Cent

rifug

ation

Swedish

Spruce

Chips

Cellulose 40 %

hemicellulose 28.5 %

Lignin 27.7 %

extractives 3.5 %

Screening(SCAN CM

40:94) over 7 mm Round

holes

Cooking

at [140 ,150, 160]

oC

Pressure [6, 7, 8] bar

for Sulfonation (115

oC , 30 mints)

Steami

ng

(2.5

bar, 30

mints)

Washing

Dissolvi

ng Pulp

Defibrillation

(ISO 5263-1:2004) Screening Cent

rifug

ation

Dissolving Pulp

Analysis

pH of

spent

liquor

Total

Residue SO2 Reject Total

Yield

Kappa no.

Viscosity

R18

16

4. Results & Discussion

In this study, spruce wood was processed by acid sulfite process. Different dissolving pulps

were produced by varying the cooking temperature and cooking time and different properties

of spent liquor and produced pulps were investigated like pH, residual total SO2, reject, yield,

kappa number, limiting viscosity and R18. The dissolving pulp was produced at 140 ºC, 150

ºC and 160 ºC at different cooking time intervals with the same cooking liquor charge until

black cook was produced at each temperature. Physical appearance of the cooked pulp at

different time and different temperatures are shown in figures 5, 6 and 7.

When the pulp was cooked at 140 ºC for two hours, the pulp gave a bit stiff feeling and the

pulp was getting softer when it was prolong cooked. After sometime pulp started detoriating

and black cook was obtained at 17 hours.

When the pulp was cooked at 150 ºC for one and half hour, the pulp was soft in feeling. As

the cooking time increased, pulp was getting harder and black cook was obtained at five

hours. When the pulp was cooked at 160 ºC for one hour, the pulp was soft in feeling and

when cooking time was increased and at three hour, black cook was obtained.

2 hours 3 hours 5 hours 8 hours 17 hours

Figure 5: different cooked pulp samples at 140 ⁰C and different time intervals.

1, 5 hours 2 hours 3 hours 4 hours 5 hours 6 hours

Figure 6: different cooked pulp samples at 150 ⁰C and different time intervals.

1 hour 2 hours 3 hours

Figure 7: different cooked pulp samples at 160 ⁰C and different time intervals.

Different reject and R18 samples were obtained at black cook are shown in figures 8 and 9.

Due to lignin deposition on fibers it became difficult to pass through screen slots and it also

increased the weight of reject. Lignin deposition on fibers also made difficulty during R18

test.

17

0 2 4 6 8 10 12 14 16 18

0,5

0,7

0,9

1,1

1,3

1,5

1,7

Time (hrs)

pH

at

25

oC

140 °C

150 °C

160 °C

17 hrs, 140 ⁰C 5 hrs, 150 ⁰C 6 hrs, 150 ⁰C 3 hrs, 160 ⁰C

Figure 8: different reject samples at different critical times and temperatures

R18, 17 hrs, 140 ⁰C R18, 6 hrs, 150 ⁰C R18, 3 hrs, 160 ⁰C

Figure 9: different R18 samples at different critical times and temperatures

4.1 Effect of cooking time and temperature on spent liquor pH

During acid sulfite cooking, lignin and hemicelluloses were dissolved simultaneously. As the

cooking time increased at a specific temperature, spent liquor pH had decreased. Longer the

cooking time, lower will be the spent liquor pH because during cooking, acetic acid is formed

from acetylated polysaccharide of the hemicelluloses. Acetic acid formation enables the

hydrolysis for the dissolution of the hemicelluloses and cleavage of lignin carbohydrate

bonds. [19, 20, 21, 22] as shown in figure 10.

Figure 10: effect of cooking time and temperature on pH of spent liquor

18

0 2 4 6 8 10 12 14 16 18

0,5

5,5

10,5

15,5

20,5

25,5

Time (hrs)

Tota

l Res

idu

al S

O2 (

g SO

2/l

)

140 °C

150 °C

160 °C

0 2 4 6 8 10 12 14 16 18

0,5

10,5

20,5

30,5

40,5

50,5

60,5

70,5

Time (hrs)

Tota

l Yie

ld (

%)

140 °C

150 °C

160 °C

4.2 Effect of cooking time and temperature on total residual SO2

Figure 11 shows the effect of cooking time on total residual SO2. As the cooking time had

increased, the total residual SO2 decreased. As the cooking begins, SO2 consumption starts

which is too quick to dissolve the lignin and hemicelluloses. But after certain time when

lignin and hemicelluloses amount decreases in the wood, SO2 is not too effective to get rid of

these hemicelluloses and lignin because of the acetic acid formation in the spent liquor and

some amount of SO2 remains unreacted. The amount of total residual SO2 was too high at 140

ºC and cooking time two hours gives high amount of total residual SO2 probably because of

the excessive temperatures and time is required to react the pulp with the chemicals. At 150

ºC total amount of SO2 was not varied too much because this high temperature allowed better

chemical reaction of the pulp. When black cook condition was reached at 150 ºC and five

hour cooking time, no further change was observed in total residual SO2 because of lignin

condensation. At high cooking temperature 160 ºC and high pressure 8 bar facilitates, the

pulping reaction occured for a shorter time. However when the cooking time was increased

from one hour to three hour lignin condensation started and resulted black cook.

4.3 Effect of cooking time and temperature on total yield

Figure 11: effect of cooking time and temperature on total residual SO2 in Spent liquor

Figure 12: effect of cooking time and temperature on total yield

19

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30

35

40

45

Time (hrs)

Kap

pa

no

. 140 °C

150 °C

160 °C

Figure 12 shows the relation between cooking time and total yield at different cooking

temperatures. When cooking temperature and cooking time increased total yield had

decreased because high temperature and more cooking time helps the cooking liquor to

dissolve more lignin and hemicellulose. Therefore low total yield was obtained because better

penetration and higher rate of delignification was obtained.

Pulp yield also depends on the degradation of carbohydrates and these carbohydrates are

degraded by peeling, chain cleavage and the dissolution of short chain carbohydrates. During

cooking hemicelluloses which mainly consist of glucomannan and xylan are degraded. Both

are degraded at specific conditions and reduce the pulp yield.

As shown in the figure 12 lower temperature gives high yield as compared to high

temperature. This is because rate of delignification is much higher at higher temperature as

compared to lower temperature and at higher temperature we get lower yield because higher

temperature also affects and damages the fiber which contributes in decreasing the yield.

Higher temperature also affects the lignin condensation because at higher temperature,

chemical penetration takes place rapidly which also reduces the yield. When the cooking

temperature increases by 10 ºC, the rate of delignification doubles which also reduced the

total yield. At higher temperature, degradation of both hemicelluloses and celluloses occur

which results in reducing the total yield.

4.4 Effect of cooking time and temperature on kappa number

Figure 13 shows the relationship between kappa number and time. As the cooking

temperature had increased, kappa number decreased because lignin decreased in the pulp,

resulting lower kappa number. When pulp is cooked for longer time, condensation of lignin

on fiber takes place which results in black cook. When the condensed lignin is precipitated

onto the fiber it resists the delignification from wood. That is why black cook is obtained after

a certain time. As cooking time increases dissolution of lignin takes place which results in

reducing the combined SO2 or total SO2. This leads towards the lignin condensation to give a

black cook. Cooking temperature at 140 ºC and pressure 6 bar prolongs the lignin

condensation i.e. seventeen hours because the consumption of cooking liquor is slow as

compared to the high temperature i.e.160 ºC and 8 bar. At higher temperature, lignin

condensation started after a short time period as in the case of 160 ºC three hours shown in

figure 13.

Figure 13: effect of cooking time and temperature on kappa number

20

0 2 4 6 8 10 12 14 16 18

50

250

450

650

850

1050

1250

Time (hrs)

Vis

cosi

ty (

ml/

g)

140 °C

150 °C

160 °C

0 2 4 6 8 10 12 14 16 18

60

65

70

75

80

85

90

95

Time (hrs)

R1

8 %

140 °C

150 °C

160 °C

4.5 Effect of cooking time and temperature on limiting viscosity

Limiting viscosity gives the idea about the degradation of the cellulose. During acid sulfite

cooking hemicelluloses and short chain carbohydrates are degraded easily and dissolved in

the cooking liquor. During cooking some of the cellulose is also degraded. Limiting viscosity

gives the average length of the cellulose chains. The greater the cellulose chain length is, the

higher the limiting viscosity and the degree of polymerization will be. As we increased the

cooking time and temperature, long chain of carbohydrates had also shortened to reduce the

pulp viscosity. It is clear from the figure 14 that lower temperature has higher viscosity as

compared to higher temperature because higher temperature degrades the cellulose. At higher

cooking temperature i.e. 160 ºC for a longer cooking time i.e. three hours, limiting viscosity

was found to be very low due to heavy chain cleavage.

4.6 Effect of cooking time and temperature on R18

Figure 14: effect of cooking time and temperature on limiting viscosity

Figure 15: effect of cooking time and temperature on R18

21

0 2 4 6 8 10 12 14 16 18

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

Time (hrs)

Rej

ect

(%) 140 °C

150 °C

160 °C

at 140 ⁰C ,2 hr = 9,07

To measure the pulp purity, R18 method is now widely used. For R18 analysis, pulp samples

were treated with 18% sodium hydroxide solution to dissolve the alkali soluble impurities.

18% sodium hydroxide dissolves pulp contents other than the crystalline cellulose and the

value of R18 gave the amount of alphacellulose. R18 test at room temperature gives the

information about the pulp quality like the amount of hemicellulose present in the pulp and

the extent of cellulose degradation. As the cooking time increases more and more

hemicelluloses and lignin are removed from the pulp which gives the high amount of

alphacellulose. But when cooking time is increased to more than some extent, cellulose is

degraded which also dissolves in the 18% sodium hydroxide solution. At 160C and three hour

time, pulp has high amount of lignin condensation on it which has very low R18 value because

fibers were damaged and large amount of lignin were deposited on the black cook. While at

140C and eight hour cooking time, the value of R18 was maximum but when pulp was more

cooked at 140 ºC and seventeen hours, the value of R18 decreased shown in figure 15 because

lignin condensation and fibers damaged on the pulp.

4.7 Effect of cooking time and temperature on reject

Figure 16 shows the relationship between reject and cooking time at different cooking

temperatures. As the cooking temperature and time increased, the reject percentage had

increased due to the dissolution of lignin and hemicelluloses. In case of 140 ºC and two hour,

the reject amount was so high because rate of delignification was too slow at that condition,

therefore less amount of lignin and hemicelluloses dissolved. However, when the cooking

time was prolonged for 140 ºC, reject percentage decreased because of favorable conditions of

cooking. But after that particular point reject amount increased because longer cooking time

favors the deposition of lignin on the fiber surface which was difficult to pass through the

screening slots same as with temperature 150 ºC. In case of 160 ºC reject percentage was very

low for the first hour because lignin and hemicelluloses dissolved into the cooking liquor. But

at three hours, amount of reject was negligible because all the cooked pulp was like slurry

which easily passed through the screening slots.

Figure 16: effect of cooking time and temperature on reject

22

8

3

2

0 2 4 6 8 10 12 14 16 18 20

135

140

145

150

155

160

165

Time (hrs)

Tem

pe

ratu

re (

oC

)

140 °C

150 °C

160 °C

Area of Acceptable cooks

4.8 Black cooks at different cooking temperatures and times

Figure 17 shows the end points of the pulp cooking at specific liquor charge. At 140 ºC the

black cook was obtained at seventeen hour pulp cooking. At 150C the black cook was

obtained at five hour pulp cooking. At 160 ⁰C the black cook was obtained at three hour pulp

cooking. Lower temperature gives longer time for lignin condensation during pulp cooking.

140 ⁰C is the safest temperature to obtain different pulp properties while temperature at 160

⁰C is highly risky because after one hour cooking lignin condensation chances are more to

produce the black cook.

In sulfite cooking SO2 has strong influence on delignification rate. Therefore in the beginning

of the cooking, rate of delignification is much higher as compared to the later cooking. This is

because initially higher amount of combined SO2 is available for the chemical reaction and as

the combined SO2 consumes in the reaction rate of delignification decreases. Dissolution of

hemicellulose is high at 140-160 ºC.

Figure 17: Black cooks at different temperatures & times

23

5. Conclusion

Higher cooking temperature and cooking time gave faster both lower pH and total

residual SO2.

Higher cooking temperature and cooking time gave faster lower pulp yield.

Kappa number decreased with increasing cooking time and temperature but after a

certain point kappa number increased due to lignin condensation.

Limiting viscosity of the pulp decreased with increasing cooking temperature and

time.

Reject percentage decreased with increasing cooking temperature and time and reject

percentage had increased in black cook area.

R18 value gave higher alphacellulose content and had increased with increasing

cooking temperature and time until the cellulose was not affected by cooking

conditions.

6. Recommendation for future work

Sulfite cooking could be done with different combined SO2 charges to minimize the lignin

condensation at high cooking temperature.

7. Acknowledgement

I take this opportunity to express my profound gratitude and deep regards to my supervisor

Ulf Germgärd and assistant supervisor Niklas Kvarnlöf for their constant supervision and

sincere guidance. I also thank Stora-Enso Skoghall for providing spruce wood chips and

special thanks to Metso Paper Karlstad for providing chip screening facility.

24

8. References:

1. Sixta, H. (2006): Hand book of pulp, vol 1, Weinheim: Wiley-VCH verlag GmbH &

Co. Germany.

2. Ek, M., Gellerstedet, G., Henriksson, G. (2012): Ljungberg text book pulp and paper

chemistry & technology, Stockholm: Fiber and Polymer Technology, KTH, Sweden

3. Sjödahl, R.G. (2006): Some aspects on the effects of dissolved wood components in

kraft pulping. Doctor Dissertations, KTH, Sweden

4. Kassberg, M. (1999): The Swedish forest industry, In pulp manufacture-a review,

published by: Skogsindustrins Utbildning i Markaryd, Sweden pp 7-11

5. Schild, G., Sixta, H., Estova, L. (2010): Multifunctional alkaline pulping,

delignification and hemicellulose extraction. Cellulose chemistry and technology Vol

4 (1) pp 35-45

6. Strunk P. (2012): Characterization of cellulose pulps and the influence of their

properties on the process and production of viscose and cellulose ethers, Doctor

Dissertation, Umeå University, Sweden

7. Christoffersson, K. E. (2005): Dissolving pulp-Multivariate characterization and

analysis of reactivity and spectroscopic properties. 28-01-2005. Doctor Dissertations,

Umeå University, Sweden

8. Ciolacu, D., Ciolacu, F., Popa, V.I. (2011): Amorphous cellulose structure and

characterization. Cellulose chemistry and technology 45 (1-2) pp 13-21

9. Ciolacu, D. (2007): On the supramolecular structure of cellulose allomorphs after

enzymatic degradation. Journal of optoelectronics and advanced materials Vol. 9 (4)

pp 1033-1037

10. Schädel, C., Blöchl, A., Richter, A., Hoch, G. (2012): Quantification and

monosaccharide composition of hemicelluloses from different plant functional types.

Plant physiology and biochemistry. Vol. 48 (1) pp 1-8

11. Lisa X., Lia, (2010): Bio products from sulfite pulping. Master´s Thesis, University of

Washington, USA

12. Rydholm, S. A. (1985): Pulping processes, New York, USA: Robert E. Krieger

Publishing Company

13. Almlöf H. (2010): Extended mercerization prior to carboxymethyl cellulose

preparation, Licentiate thesis, Karlstad University, Sweden

14. Gehmayr, V., Sixta, H. (2011): Dissolving pulp from enzyme treated kraft pulps for

viscose application. Lenzinger berichte Vol. 89 (1), pp 152-160

25

15. Sarwar, J., M., Sabina, R., Nasima, C., D., A., Al-Maruf, A., (2008): Alternative

pulping process for producing dissolving pulp from jute. Bio Resources Vol. 3 (4), pp

1359-1370, Austria.

16. Sixta, H., Schild, G., (2009): A new generation kraft process. Lenzinger Berichte Vol.

87(1) pp 26-37

17. Mazur, N., A., Zubakhina, N., L., Khizhnyak L., G. (2006) Evaluation of the quality of

cellulose for chemical processing. Fiber chemistry Vol. 38 (1) pp 23-25

18. Sten H., Bengt O. L., Ulla S. (1953): The rate dominating reaction of the

delignification of wood powder with sulfite solutions. Svensk Papperstidning Vol. 56

(17) pp 645-690

19. Paredes, H., J., J. (2009): The influence of hot water extraction on physical and

mechanical properties of OSB, 01-01-2009. Doctor Dissertation, University of Maine,

Orono

20. Tunc, M., Heiningen, V., A., R., P. (2008): Hemicellulose extraction of mixed

southern hardwood with water at 150 °C. Effect of time. Industrial engineering and

chemistry research, 47 (18), pp 7031-7037

21. Liu, Z., Ni. Y., Fatehi, P., Saeed, A. (2011): Isolation and cationization of

hemicelluloses from pre hydrolysis liquor of kraft based dissolving pulp production

process. Biomass bioenergy 35(5), pp 1789-1796

22. Hage, R., E., Chrusciel, L., Desharnais, L., Brosse, N. (2010): Effect of autohydrolysis

of Miscanthus x giganteus on lignin structure and organosolve delignification.

Bioresources technology 101(23), pp 9321-9329

23. Borrega, M., Nisminen, K,. Sixta, H. (2011): Effects of hot water extraction in a batch

reactor on the delignification of birch wood. Bio Resources Vol. 6 (2) pp 1890-1903

26

9. Appendix:

9.1 Equations:

Equations for calculating the total, bound and free concentration of SO2 in the solution:

Total SO2: g SO2/l (A)

Free SO2: g SO2/l (B)

Combined SO2: g SO2/l (C)

Total free SO2: g SO2/l (D)

Where a = amount of KlO3 used during titration

b = amount of NaOH used during titration

The amount of cooking liquor

(E)

Where

e = Charge 24 % of total SO2

f = amount of dry wood chips

g = concentration of the total SO2

The amount of water to add (F)

Where

h = amount of water present in chips (93% dryness)

i = amount of water due to steaming

Total Residual SO2: g SO2/l (G)

Where

j = amount of KlO3 used during titration

Rejects (%)

(H)

Yield %:

(I)

R18

(J)

Where

m1 = oven dry mass of alkali insoluble fraction in grams

m2 = mass of the test portion calculated on an oven dry basis in grams

27

9.2 Summary of Results

Table 2: Results of different parameter of acid sulfite pulping at 140 ⁰C and different time intervals

with standard deviation. Standard deviation is based on four samples.

Time

(hrs)

pH of

spent

liquor at

25 ⁰C

Total

Residual

SO2 (g

SO2/l)

Reject

(%)

Total

Yield

(%)

Kappa

no.

Viscosity

(ml/g)

R18 (%)

2 1,55

±0,84

20,03

±0,27

18,15

±0,84

58

±0,28

33

±0,84

1079

±10

84,28

±0,12

3 1,45

±0,35

12,10

±0,30

4,44

±0,34

50

±0,38

23

±0,34

993

±2

85,94

±0,45

5 1,33

±0,13

11,14

±0,14

0,38

±0,62

43

±0,32

9

±0,23

766

±6

87,37

±0,08

8 1,17

±0,32

10,88

±0,32

0,12

±0,31

34

±0,14

5

±0,48

430

±4

90,71

±0,18

17 0,70

±0,24

7,68

±0,01

0,70

±0,02

18

±0,21

11

±0,23

264

±7

80,48

±0,34

Table 2: Results of different parameter of Acid sulfite pulp at 150 ⁰C and different time intervals

Time

(hrs)

pH of

spent

liquor at

25 ⁰C

Total

Residual

SO2 (g

SO2/l)

Reject

(%)

Total

Yield

(%)

Kappa

no.

Viscosity

(ml/g)

R18 (%)

1,5 1,50

±0,15

9,96

±0,29

2,58

±0,15

46

±0,10

16

±0,15

776

± 5

82,45

±0,10

2 1,40

±0,52

9,60

±0,13

1,85

±0,52

43

±0,28

14

±0,52

651

±12

86,24

±0,21

3 1,34

±0,35

9,16

±0,50

0,95

±0,35

36

±0,19

11

±0,35

415

± 7

90,13

±0,47

4 1,29

±0,42

8,96

±0,32

0,88

±0,12

34

±0,32

6

±0,43

292

±8

90,58

±0,09

5 1,23

±0,21

7,96

±0,48

0,93

±0,34

27

±0,12

8

±0,30

185

±10

87,56

±0,51

6 1,20

±0,43

7,96

±0,51

1,32

±0,24

24

±0,50

9

±0,12

170

±4

79,81

±0,42

Table 3: Results of different parameter of acid sulfite pulping at 160 ⁰C and different time intervals

Time

(hrs)

pH of

spent

liquor at

25 ⁰C

Total

Residual

SO2 (g

SO2/l)

Reject

(%)

Total

Yield

(%)

Kappa

no.

Viscosity

(ml/g)

R18 (%)

1 1,35

±0,39

13,54

±0,02

0,54

±0,33

48

±0,33

13

±0,39

754

±4

87,35

±0,31

2 1,12

±0,31

8,87

±0,36

0,60

±0,31

30

±0,49

7

±0,31

166

±6

73,82

±0,26

3 0,90

±0,97

4,48

±0,22

0,42

±0,69

26

±0,25

15

±0,97

115

±13

72,10

±0,35