selection of dryers

8/4/2019 Selection of Dryers

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Classification and Selection of

Industrial Drying Systems

Dr. B. N. Thorat

Reader in Chemical Engineering

UICT

Refresher Course in Chemical Engineering for Plant Personnel

9th and 10th December, 2005

DEFINITION OF DRYING

Converts liquids/ solid/ paste into a solid product by evaporation of

liquid into vapor phase via application of heat. (Sometimes converts

solid moisture into vapor by sublimation eg. Freeze drying with

application of heat.)

Note: Mechanical dewatering (filtration, sedimentation, Centrifugation

etc.) is much cheaper (upto 100 times cheaper than drying)

DRYING AS A

THERMAL

PROCESS

Multicomponent

Moisture transportChange of physical

structure

Transient

Change in

quality

Input

Continuous/

intermittent

Coupled with

mass

transfer

Shrinkage

Phase changeChemical/

biochemical

reactions

Drying of solids is a complex process involving several rate processes

occurring co-currently and or sequentially.

MATERIAL BEING

DRIED

Moisture

Heat input

Liquid diffusion

Vapor diffusion

Capillary flow (Permeability)

Knudsen diffusion (Mean free path < pore dia.)

Surface diffusion

Poiseuille flow

Combination of above

Conduction

Convection

Radiation

Dielectric

Combined mode

Various modes of moisture and heat transport

Over 200 types of dryers in industrial use.

Diverse products- physical, chemical properties vary widely for feed

and products.

Involves transient energy/ mass and momentum transport through

porous media, with phase change, with/ without chemical/

biochemical reactions.

No universal drying theory exists.

Little opportunity for generalization.

Minor changes of moisture content result in large changes in physical

properties (eg. Fluidization of dry vs. wet particles).

Why is drying of solids so complex?

Thousands of different products dried in industry, often new products,

new processes, high production rates etc. need new dryers.

Various fuels (gas, oil, electricity, flue gases, waste heat etc.).

Environmental regulations.

Need to reduce costs.

Need to consider drying system rather than dryer, ie. Pre- and post-

drying stages are important and often cost more than dryer.

Why so many dryers?


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MAIN DRYER TYPES

I. Direct (Convective)

DirectDryer

Hot gas

Wet product Dry product

Humid gas

Drying medium directly contacts material to be dried and carries

evaporated moisture.

II. Indirect (Contact, Conduction)

Gas flow (low)

Wet product Dry product

Vacuum or low gas flow

Heat supplied

by heat exchanger

(through metal wall)

III. Radiant

Wet feed Dry product

Vacuum or low gas flow to carry evaporated moisture away.

IV. Microwave or RF

Electromagnetic energy absorbed selectively by water (volumetric heating).

Heater (radiant)

Typically less than 50% of total heat supplied in most direct dryers

is used for evaporation. Water is the most common solvent removed

in dryers.

Top 10 Reasons Why

Drying R&D has been Ignored

1. Very old hence well-understood operation.

2. Conceptually simple (so design must be simple!).

3. Misleading coverage in standard texts and handbooks.

4. Too easy (? ).

5. Too difficult (for academics conversant in transport phenomena!).

6. Vendors should do R&D.

7. Excessively long lifetime of dryers.

8. Low capital costs.

9. Low energy costs as it was thought but not now!!!

10. Lack of legislatives support.

Top 10 Reasons for

Innovation in Drying

1. Better product quality.

2. Improved productivity.

3. Environmentally-friendly process.

4. Improved economics.

5. Reduced energy consumption.

6. Better control .

7. Flexibility (high turn-down ratios).

8. Multi-processing capacity.

9. Safer operation.

10. New products / processes.

Classification of Dryers

Solid Exposure to Heat Conditions

Typical residence time

within dryer

Dryers

0- 10

sec

10- 30

sec

5- 10

min

10- 60

min

1- 6

hr

Convection

Belt conveyor dryer X

Flash dryer X

Fluid bed dryer X

Rotary dryer XSpray dryer X

Tray dryer (batch) X

Tray dryer (continuous) X

Conduction

Drum dryer X

Steam jacket rotary dryer X

Steam tube rotary dryer X

Tray dryer (batch) X

Tray dryer (continuous) X

Granular material - 0.05 to 5 mm

Pastelike materials - 0.1 to 50m S olutions or suspensions - 1 0- 5 0 m- F ines

- 0.1-10 m- Ultrafines

-


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Classification of Granular Material

Group Pore size (nm) Drying time insuspended state

Types of dryersrecommended

I > 100 0.5 - 3.0 sec. Cyclone dryers

Flash dryersTwo- stage flash dryers

II 100 - 6 3 - 30 sec. Two- stage flash dryersFast spouted bed

III 6 - 4

4 - 2

0.5 - 2 min.

2 - 20 min.

Vortex dryersBatch dryersFluid bedVibrated fluid bedBatch dryers

IV Ultra-micropores, particlesize 1 - 2 mm

Particle size > 2 mm

10 - 60 min

40 - 90 min.

> 90 min.

Vibrated fluid bedMultistage fluid bedBatch dryersBatch dryers

Suspended state dryersnot recommended

Always think DRYING SYSTEM

not just DRYING

DRYING SYSTEM

Pre-drying

StagesDrying

Post-drying

Stages

Feeders

Pre-forming

(extrusion,

pelletizing)

Backmixing

Metering

Blending

Mechanical

dewatering

Solar collector /

storage

May involve

chemical reactions. Cooling

Agglomeration

Solvent recovery

(if applicable)

Grinding

Gas cleaning

(cyclone, filters,

scrubbers, etc.)

Product

collection

packaging

Continuous---Good15Spray

ContinuousFairGoodGood-185Spin Flash

ContinuousFairGoodPoor-33Rotary

(indirect)

ContinuousFairGoodFair-750Flash

Continuous--FairGood22Film Drum

ContinuousGood-Fair-30Band

ContinuousGoodGood--130FBD

BatchPoorFairPoor-10Double Cone

BatchGood---7.5Forced

Convection

(through flow)

OperationGranules,

pellets

PowdersPastesFluid, liquid

suspension

Evap. Rate

(kg/m2/hr)

Dryers

Product Classification and Dryers Types

as an aid to SelectionFalling rate Constant rate

Critical moisture, XcEquilibrium moisture, Xe

Zone- BZone- C Zone- A

Idealized Drying Rate Curve

Rate of

drying

0X = moisture content, kg moisture/ kg BDS (bone dry solids)

DRYER CHOICE BASED ON SOLIDS DRYING CHARACTERISTICS

Zone A(Constant Rate)

Zone B(Falling Rate)

Zone C(Internal diffusion)

Turbo- tray Turbo- tray Turbo- trayRotary Rotary R otary

Belt Belt Belt--------- Fluid Bed (Spray) Fluid BedSpray Spray - -------

Flash Flash - -------

dX

dt

STEP 1- BASIC CHOICES

Batch Or Continuous

Batch dryers favored by:

Low throughput (under 50 Kg/H)

Long residence time (I.E. Mainly falling-rate drying)

Batch equipment upstream and downstream

Requirement for batch integrity

Continuous dryers favoured by opposite conditions

EXAMPLE - Fluidized Bed Dryers

Sub types:

Batch: Well-Mixed (WM)

Continuous: Well- Mixed, Plug- Flow (PF), Multi- Stage (MS)

Options: Internal Heating Coils, Split Distributor, Vibration, Feed End Rake,

Expanded Freeboard, Special Distributor, etc.

Flowsheet options:

Gas Recycle, Backmixing, Direct Firing, Self- Inerting

Cost criteria:

Simple types cheapest, most options add to cost.

Vibro fluidized beds

Have high capital cost/ low energy costs

Payback periods (depend on if dryer is new or replacing an existing

operating unit)


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STEP 3- SUBTYPES AND REFINEMENTS

Choice Between Subtypes

Feed difficult to fluidized-Use WM, MS (WM before PF),

If PF used need ; rotating Rake, Backmixing, Vibrated Feed Region

Narrow X0 specification - Prefer in order PF= MS= WM

X0 much less than X1- prefer PF and MS to WM.

Wide size distribution- Use Vibration, Special Distributor, Expanded

Freeboard, Fines Collections

Small particles, low gas velocity- consider internal coils

WM: WELL- MIXED; PF= PLUG FLOW

Small Scale Lab Tests

Small- scale tests give valuable information:

Drying kinetics- drying rates (parametric effects)

Equilibrium moisture content- effect of t,h (humidity)

Microscopic examination- surface, agglomeration

Lab-scale rotary evaporator- overheating, balling, adhesion

Rotating drum tester- attrition, dustiness

Cohesion and adhesion- handling, sticky point

Vital to have a representative sample of final material

Not necessary to carry out all of above tests in all cases

Selection: Proper selection is crucial. Best designed wrong dryer is still a poor choice.

For granular solids, for example, numerous choices exist.

Type Advantages/ limitations

Rotary- Convective (direct only) Flexible/ high operating cost; large volume; high

carryover of fines

Rotary- Convection/ Conduction (steamtube)

Flexible/ better efficiency/ low fines carryover/ expensive/large volume

Fluid bed (Convective) Compact/ more efficient/ less flexible

Fluid bed (Combined mode) Compact/ efficient

Vibrated Fluid Bed More efficient, flexible, for friable materials, polydisperse

solids

Vibrated Bed with Immersed

Exchangers

More efficient, less carryover, low blower power

Spouted Bed Dryer Compact, limited range of operation, high efficiecy, high

blower power, limited capacity

Centrifugal Fluid Bed Very high transfer rates, expensive

Rotary Tray (Turbo Dryer) Gentle handling, flexible

Spiral Dryer (Flash with Indirect

heating)

High drying rates, more expensive

Flash/ Pneumatic Dryer Flash + Fluid

Bed

For surface moisture removal only

Jet- Zone Dryer (layers of particlesfluidized by impinging jets)

High drying rates, flexible

Through Dryers (hot air through beds ofparticles)

Expensive for fine particles, non-uniform drying

Vacuum Dryer For heat- sensitive solids, expensive

Combination Dryers

Definitions

R.H. 50%

EMC

Bound moisture

Free moisture

Content

X Moisture Content(dry basis)X*

Unbound moisture

100%

T= Constant

Ls= Dry mass

A= Area

C.R.P.

F.R.P.

X

XcritX*

R =

-Lsd X

A dt

Drying rate curve

0

CLASS I: Glass beads sand, clay, mixtures of sand and clay, calcium carbonate, silica gel, paper pulp, leather, pig

manure. CLASS II: glass beads, ceramic tiles, clay, silica gel. CLASS III: organic liquid in glass beads, for

example,(a) benzene and n-propanol, (b) n-pentanol. CLASS IV: glass beads, polystyrene beads. CLASS V: sand,

plastic-clay mix, silica-brick mix, whiting slab, ceramic plate, leather, lactose granulation. CLASS VI: special case

of CLASS I: Schlunder reports that for molecular sieve. CLASS VII: CLASS I with different curvature during bthe

period of decreasing drying rate, aluminium silicate particles as a function of air temperature, and sand and paper

pulp as a function of thickness of the sample. CLASS VIII: (a) fir wood; (b) cypress wood. CLASS IX: (a) paper,

wool, aluminiumstearate dough; (b) potatoes, tapioca tuber, and rice flour.CLASS X: (a) rye bread, yeast; (b) butter

and margarine. CLASS XI: (a) wheat corns; (b) and (c) the same for lower X normalized tothe initial drying rate for

(a). CLASS XII: limestone granules saturated with (a) water, (b) 0.05-M NaCl. Similar behavior, due to crust

formation, has been observed for p[lastertiles and for clay.

XII

XIXIX

VIIIVIIVIV

IVIIIIII

a

b

a

b

a

b

a

b

a b

c

c

a

b

Examples Of Normalized Drying- Rate Curves For Different Types Of Media

Turbo Tray Dryers

Suitable for granular feeds, operate with rotating shelves and force

convection of air above the shelves.

The Dryer can have 30+ trays and provide large residence time.

Hermetic sealing is possible for solvent recovery.


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Rotary Dryer

Combined cascade motion

with heat & mass transfer.

Large capital & operating cost.

Used in fertilizers,

pharmaceutical, lead & zinc

concentrate for smelting,

cement.

Size 0.3 to 5 m diameter &

2 to 90 m length.

Steam Tube Rotary Dryer

Pneumatic Conveying (Flash) Dryers Tunnel Dryers

FBD Dryers - VariationsRotocone Dryers (Batch)

Agitated Dryers

Drying of fine & moderately

wet materials such as

gypsum, pigments, and

dyestuffs.

Drying of pharmaceuticals -

tableting formulation

Maximum capacity 10 m3.

Evap. rate 2-7 kg/hr.m2


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Microwave Dryers Used in ceramics industries,

foods & pharmaceuticals to

drive of last traces of moisture.

Paddle Dryers

Provides drying time

upto several hours.

Suitable for pastelike

& granular material.

Evap. rate upto

10 kg/hr.m2

Vacuum Dryers Heat Sensitive MaterialsScrew Conveyer Dryer

Freeze Dryer


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BN

A

V/

P

ReO

O+=

221

Two phase theory for fluidization Thorat and Joshi, 2001

I & ECFixedbed

Incipient

fluidization

Aggregative

fluidization

V< VONo particle

mixing

V = VOModerate particle

mixing

V> VOV


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INTRODUCTION

Drying of Foods To increase the shelf life, to reducepackaging cost

Acceptable final moisture content 5 to 12%

Quality Changes occurs during drying of FOODS

Factors affecting structural Properties

1. Drying Method

2. Drying Conditions

3. Moisture Content

Mostly Used Drying Techniques in

Food Industry

Conventional Air Dryer

Osmotic Dehydration

Freeze Dryer

Microwave Dryer

Spray Dryer

Fluid Bed Dryer

Heat Pump Dryer

Infrared Dryer

Case study : Shrimp drying

Shrimp (Prawn) is one of the most popularseafood products

Sun Drying a traditional way, results inpoor product quality

High local and export market

Superior quality and energy minimizationcould result in value addition to product

Different combinations of drying can be

used

Combinatorial Drying for Shrimps

Osmoticdehydration

15% w/v NaCl30 min450C

4 to 2 kg/kg db

Heat pumpdrying

17% RH40-440C

X=2 to 0.5 kg/kg db

Infra-reddrying

Max. 20 min600C

X=0.2 db

What is Heat Pump

Why heat pump + Drying

Heat savings

Product Quality

Recovery of solvent

How it works ?

Heat pump drying

Evaporator

Condenser

Dryer Compressor

Simple Scheme


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Refrigerant: R134A (Dichloro Difluoro Ethane)

Enhancement of performance using bypass coil

Dehumidified air

17 % RH

40-440C

Shrimps taken out after Heat Pump drying

693.7542.7830.40.2890.608Rice powder5.

75055.7830.750.2650.798Suji4.

7905230.80.2260.655Rice rawa

(Semolin)

3.

688.745530.50.230.65Chickpea

(=0.945

mm)

2.

712.4149.9630.30.240.67

Chickpea (mix)

1.

Temp(C)DriedRawMaterials

Bulk Density

(kg/m3)

Angle of ReposeWater ActivityNo.

Water Activity, Angle of repose& Bulk densities after drying of Materials @ 60 C

3 MotorGearBox

N2

Wetmaterial

N2

Hot water

Hot

water

Screw

Dry

Product

Screw

Dryer developed at UICT:

Screw Conveyor Dryer

Motor

Gear box

Jacketed dryer

Material outlet

Material inlet

Bearing support

Gas inlet

Actual SCD set-up


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Dryer Throughput

Where,

C = Screw conveyor throughput, m3/h

= degree of fullness

Dsc = screw diameter, m

Dsh = shaft diameter, m

r = radial clearance, m

P = screw pitch, m

t = flight thickness, m

N = screw speed, rpm

Note: The allowable loading or degree of fullness and screw

speed are limited by the material characteristics

( )( ) 60NtPD2r)(D4

C2

sh

2

sc +=

Degree of fullness or dryer loading

Furnace slag, dry

sand,

alumina etc.

Abrasive and poor

flowability

15 %Class IV

Dry ash, cement,

salt,

charcoal,

crushed

chalk etc.

Similar in size and

flowability to Class

II solids, but more

abrasive

30 %Class III

Baking powder,

pulverized

coal, corn

grits etc.

Non-abrasive, less free

flowing than Class I,

small lumps mixed

with fines

30 %Class II

Wheat, flour,

graphite etc.

Light, free-flowing,

non-abrasive

45 %Class I

ExamplesMaterial

characteristics

Degree of

fullness

Material

type

Note: Class I materials fill the trough deeply, permitting a higherrotating speed than heavier and more abrasive materials

Thank You

selection of dryers

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