Lec. 2 Dryingdrying

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Separation Processing Chemical engineering Drying of Solids & Classification of Dryers



Drying of solids refer to the final removal of water or another solvent from a wet product

Drying is a reduction in moisture content from an initial

amount to an acceptable amount (EMC equilibrium moisture

content)

Drying is an unit operation in which a liquid usually water is

removed from a wet solid in equipment termed as dryer

Operation often follows Evaporation, Filtration or Crystallization

Objective of drying of solids is the removal of water or any

solvent from wet solids

Drying of Solids

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Drying of solids is a complex process involving several rate

processes occurring co-currently or sequentially

Drying is a Complex Process

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DRYING AS ATHERMAL

PROCESS

MulticomponentMoisture transport

Change of physicalstructure

Transient

Change in

quality

Input

Continuous/

intermittent

Coupled with mass

transfer

Shrinkage

Phase changeChemical/

biochemical

reactions



To reduce cost of transport

To make a material more suitable for handling e.g. fertilizers

& dyestuff

To remove moisture which may lead to corrosion e.g. dryingof gaseous fuels

To improve flow ability and handling of substance

To stabilize moisture sensitive drug such as aspirin

To provide definite properties e.g. free flowing

Importance of Drying

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The drying of solids depend on the following factors :

1. Temperature ↑ → Rate of drying ↑

2. Surface area of dryer. ↑ → Rate of drying ↑

3. Type of dryer (static or moving)

4. Type of material (porous or non porous)

Factors Affecting Drying of Solids

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Evaporation Vs Drying1. The term drying usually refers the removal of relatively smallamounts of water from solid or nearly solid material. The termevaporation is usually limited to the removal of relatively large amountsof water from solutions.2. In most cases drying involves the removal of water at temperatures

below its boiling point, whereas evaporation means the removal of water

by boiling a solution.



Drying

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Selection of the drying method and equipment depend onthe following factors :

1. The heat sensitivity of the product

2. Its physical characteristics prior to drying

3. The nature of the solvent to be removed

4. The need

5. The scale of operation

6. Available sources of heat

Selection Criteria for Drying of Solids

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1. Equilibrium Moisture Content:If any material is exposed to air, at any definite temperature

and humidity, it will either lose or gain water until an

equilibrium point is reached. This is the EMC “Equilibrium

Moisture Content”. It depends on the nature of the material 2. Bound Moisture:a. It is the water present in the material “retained insmall capillaries, adsorbed on surface or as a solution incell wall”

3. Free Moisture:It is the water in excess of equilibrium moisture content(EMC)

Commonly Used Terms in Drying

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Drying

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7 to 8 per cent.



Thousands of different products dried in industry, often new products, new processes, high production rates etc., need new

type of dryer to used

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

Environmental regulations

Need to reduce costs

Need to consider drying system rather than dryer, i.e. Pre-

and post- drying stages are important and often cost more

than dryer

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Why So Many Dryers



There is no simple way of classifying drying equipment

Some are continuous and others operate batchwise; some

agitate the solids and others are essentially unagitated

Some dryers can handle almost any kind of material whilesome are limited to the type of feed

Major divisions may be made among

1. Dryers in which solid is directly exposed to a hot gas which

is usually air

2. Dryers in which heat is transferred to the solid from an

external medium such as condensing steam, usually

through a metal surface with which solid is in contact 12

Classification of Dryers



1. Dryers in which solid is directly exposed to a hot gas whichis usually air

2. Dryers in which heat is transferred to the solid from an

external medium such as condensing steam, usually

through a metal surface with which solid is in contact

3. Dryers that are heated by radiant energy.

Adiabatic or Direct Dryers (heating media is exposed to solid)

Non-adiabatic or Direct Dryers (an external heating media is

used to dry the solid)

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Cont….



Drying mechanism may be made as follows,

1. Mechanical Drying (grain drying, coffee drying etc.)

2. Thermal Drying (biosolids, municipal wastewater solids

etc.)3. Vacuum Drying (paper, long pieces of wood, cereal grains

or corn flakes, washing powder, milk powder etc.)

4. Freeze Drying (natural mummies, to conserve special

strains, floral preservation)

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Mechanism of Drying



Drying Equipments

15



The dryer may be:

A- Static bed drying:The material is static while hot air is moving above it.

Disadvantages of static bed:

1. Size reduction is needed

2. Conc. will differ from bottom to surface water solublesubstances will concentrate at the surface.

B- Moving bed drying:

The material to be dried is moving inside hot air.

Drying occurs from all surfaces so it has a faster rate.

Advantages of moving bed:

1. ↑ drying rate (10 - 20 times faster).

2. Each granule acts as a single bed. 16

Drying Equipment



Batch tray dryer

It operates by passing hot air over the surface of wet solids

in the trays inside the oven

Thermal efficiency of air is improved by: recirculating theair over the heater

Handle any type of material

Cheaper

Easy to operate

17

Tray Dryers





Continuous dryer

It operates by travelling metal screen through a long

chamber or tunnel

Handle variety of materials

Low cost

Steam consumption is very low

Easy to control the drying conditions

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Screen Conveyor Dryers



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Screen Conveyor Dryer



Continuous dryer

It operates by showering of material to be dried

Turbo dryer consists of a stack of slowly rotating circular

traysMaterial is fed onto the top tray

After one revolution the material is wiped onto the next

lower tray where the operation is repeated

The trays are contained in an enclosure in which heated air

or gas is circulated by internal fans

For Free Flowing, non-sticky, non-dusting products

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Tower Dryers



Handle variety of materials

High cost

Steam consumption is very high

Difficult to control the drying conditions

It occupy small floor space

The solvent can be recovered

Annular shelves rotate at 1 rpm

Capacity up to 10TPH

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Tower Dryers



23

Turbo Dryer



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Continuous dryer

The rotary dryer is a type of industrial dryer employed to

reduce or minimize liquid moisture content

Dryer is made up of a large, rotating cylindrical tube,

usually supported by concrete columns or steel beams

It can handle variety of materials

Efficiency is higher

Power consumption is lower

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Rotary Dryers



The dryer slopes slightly so that the discharge end is lower than the material feed end in order to convey the material

through the dryer under gravity

The material to be dried enters the dryer & as the dryer

rotates the material is lifted up by a series of internal fins,lining the inner wall of the dryer.

When the material gets high enough to roll back off the fins,

it falls back down to the bottom of the dryer, passing through

the hot gas stream as it falls

Radial flights/fins serve to agitate the material and to

provide uniform mixing and drying

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Working Principle of Rotary Dryers



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Fins In Rotary Dryers



This gas stream can either be moving towards the dischargeend, or towards the feed end from the discharge end

The gas stream can be made up of a mixture of air &

combustion gases from a burner, in which the dryer is called a

(direct) heated dryer

The gas stream may consist of air or another gas that is

preheated

Thermal efficiency of the dryer is very high and heat lossthrough the cylinder wall is minimal

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Working Principle of Rotary Dryers



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Rotary Dryer





Steam Tube Rotary Dryer



Metallic oxides

Powdered materials

Insulation materials

Metal powders

Carbon black pigment

Industrial chemicals

Food industry

Mineral drying

32

Application of Rotary Dryers



Wet and sticky products cause clogging of the inlet and

transfer section of the dryer drum

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Limitation of Rotary Dryers



34

Type Residence

time

Main Advantages Major

Limitations• Rotary

(Conventional

direct type)

Axial gas flow

30- 120 min Flexible

Large turndown ratio

•Capital intensive

•Large Foot

prints

•High running

cost

• Rotary

(Indirect or direct

+ indirect)

steam tube dryer

30- 120 min As above

Higher efficiency than

above, Smaller than

above

•Capital intensive

• Not suited for

sticky materials

• Yamato

(Direct gas

injected into

rolling bed)

20-60 • Smaller size due to

higher transfer rates

• Higher thermal

efficiency

Not for fine

particles

Types of Rotary Dryers



Continuous dryer

Nonadiabatic drying

Connected parallel to get greater length

Its rpm is 2-30

Handle too fines and too sticky

Recovery of volatile solvent from solvent wet solids, known

as desolventizers

Thin film dryers

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Screw Conveyor Dryers



A screw conveyor dryer consists of a helical flight fastenedaround a pipe or solid shaft, mounted within a jacketed tubular

or U-shaped trough

Inert gas-swept to carry off the evaporated moisture to the

atmosphere or to condenser

SCD may be operated at atmospheric pressure or under

vacuum

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Screw Conveyor Dryers (SCD)



Motor Gear

Box

N2

Wet

material

N2

Hot water

Hot

water

Screw

Dry

Product

Screw

Schematics of Screw Conveyor Dryer



Free-flowing to non free-flowing materials

Fine powder to lumpy materials

Sticky and Fibrous materials

Materials which become friable at some stage during thedrying process

Dusty and toxic materials

Materials which are difficult to handle

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Application of SCD



Drum dryer

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Spray dryer

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A conventional spray dryer

consists of the following maincomponents:

•Drying chamber (1)

•Hot air system and air

distribution (2)

•Feed system (3)

• Atomizing device (4)•Powder separation system

(5)

•Pneumatic conveying and

cooling system (6)

•Fluid bed after-drying/cooling

(7)

•Instrumentation and

automation (8)



10/23/2013 | 41

Spray Drying

Classification of spray dryer

◦ Height-to-diameter ratio (2:1 or 5:1)

◦ Air flow type

◦ Atomizer type

Rotary or centrifugal disk

High pressure nozzle

Pneumatic atomizer

Spray drying procedure



10/23/2013 | 42

Spray drying procedure

Red arrows shows

heated air

Blue arrows represents

liquid droplets

forming particles



10/23/2013 | 43

Spray drying stages

Three fundamental stages are involved

to accomplish spray drying

1. The liquid feed is atomized into fine

droplets

2. Liquid evaporation from the droplet,

resulting in final (dried) product

3. The dried product is separated from thegas stream and collected in a vessel



10/23/2013 | 44

Advantages of spray drying

Food encapsulation

Production of hollow particles

Production of porous material

Control of physical properties Production of micrometer-sized low

melting point materials

Flexible process Temperature sensitive materials can be

dried

Parameters affecting product



10/23/2013 | 45

Parameters affecting product

properties

Four main parameters affect spray

drying

1. Direction of flow of inlet gas

2. Temperature of the gas

3. Degree and uniformity of atomization

4. Degree of aeration of feed



10/23/2013 | 46

Industrial uses of spray drying

Pharmaceutical industry

Food industry e.g.◦ Egg drying

◦ Animal feeds

◦ Juices

◦ Yeast & yogurt

◦ Milk

◦ Cake mixes

◦ Creamers

Material processing◦ Powder particles of metals & semiconductors



Most industrial dryers handle particulate solids

In adiabatic dryers, the solids are exposed to the gas by the

following ways:

1. Cross-circulation Drying

2. Through-circulation Drying

3. Solids are showered downward and gas upward

4. Fluidized bed Drying

5. Solids are completely entrained and then mechanical

separated

6. Spray Drying 47

Solids Handling in Dryers



In Nonadiabatic dryers, the solids are exposed to the hotsurface or source of heat by the following ways:

1. Stationary or slowly moving horizontal hot surface

2. Solids are moved over a heated surface

3. Solids are moved by gravity over an inclined heated

surface

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Solids Handling in Dryers



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There is no single theory of drying that covers all materials

and dryer types

No unified approach

General principles

Dryer are not designed by user

Dryer are designed by manufacturer

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Principles of Drying



Pre-drying

StagesDrying

Post-drying

Stages

• Feeders

• Pre-forming

(extrusion)

• Backmixing

• Metering

• Blending

• Mechanicaldewatering

storage

• May involve

chemical reactions.

• Cooling

• Agglomeration

• Solvent recovery

(if applicable)

• Grinding

• Gas cleaning

(cyclone, filters,scrubbers, etc.)

• Product

collection

packaging

Stages in Drying System



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Temperature Patterns in Dryers

The way in which temperatures vary in a dryer depends

upon the

Nature and liquid content of the feedstock

Temperature of heating medium

Drying time

Allowable final temperature of dry solids

Drying Periods



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Drying Periods

• Necessary to remove free moisture from

the surface and moisture from the interior of

the material

•If moisture as a function of time, a smoothcurve is obtained

•Drying rate curve varies with the structure

and type of material

•There are two well-defined zones:

•AB, where the rate of drying is constant

•BC, where there is a steady fall in the rate

of drying as the moisture content is reduced.

•B is the critical moisture content.

In Curve 2,

•DE represents constant rate

•EF and FC are falling rate , first and final

respectively.

•The drying of soap gives rise to a curve of

type 1 and sand to a curve of type 2.

R t f d i



Rate of drying

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R t f d i



Rate of drying

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Ti f d i



Time for drying

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Time of drying



Time of drying

57



R.H. 50%

Bound moisture

Free moisture

Content

X Moisture Content (dry basis)X*

Unbound moisture

100%

T= Constant

Drying Rate Curve



Dryers Typical residence time

within dryer 0 - 10

sec

10 - 30

sec

5 - 10

min

10 - 60

min

1 - 6

hr

Convection

1. Belt conveyor dryer X

2. Flash dryer X

3. Fluid bed dryer X

4. Rotary dryer X

5. Spray dryer X

6. Tray dryer (batch) X

Tray dryer (continuous) X

Conduction

1. Drum dryer X

2. Steam jacket rotary dryer X3. Steam tube rotary dryer X

4. Tray dryer (batch) X

5. Tray dryer (continuous) X

Classification of Dryers – Solid Exposure to Heat Conditions



Dryers Evap. Rate

(kg/m2/hr)

Fluid, liquid

suspension

Pastes Powders Granules,

pellets

Operation

Forced

Convection

(through flow)

7.5 - - - Good Batch

Double Cone 10 - Poor Fair Poor Batch

FBD 130 - - Good Good Continuous

Film Drum 22 Good Fair - - Continuous

Flash 750 - Fair Good Fair Continuous

Rotary

(indirect)

33 - Poor Good Fair Continuous

Spin Flash 185 - Good Good Fair Continuous

Spray 15 Good - - - Continuous

Product Classification and Dryers Types as an aid to Selection

Cl ifi ti f S lid



Group I: Nonporous or Capillary- porous solids with large

pore sizes. Only free moisture is removed e.g. NaCl,

Group II: Uniformly and non-uniformly porous materials

with pores sizes of 6nm. Contains free moisture, eg.

Phenobarbital, Sodium perborate

Group III: Microporous or colloidal- capillary- porous

material of size 4-6 mm, eg. Glucose, sulfadimethoxine and of

size 2-4 mm, eg. Calcium gluconate

Group IV: Ultra-microporous materials. Pore sizecomparable to molecule size. Intense drying is required to

remove moisture content as low as 0.2- 0.1% or less

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Classification of Solids

Classification of Granular Material



Group Pore size (nm) Drying time in

suspended state

Types of dryers

recommendedI > 100 0.5 - 3.0 sec. Cyclone dryers

Flash dryers

Two- stage flash dryers

II 100 - 6 3 - 30 sec. Two- stage flash dryers

Fast spouted bed

III 6 - 4

4 - 2

0.5 - 2 min.

2 - 20 min.

Vortex dryersBatch dryers

Fluid bedVibrated fluid bed

Batch dryers

IV Ultra-micropores, particle

size 1 - 2 mm

Particle size > 2 mm

10 - 60 min

40 - 90 min.

> 90 min.

Vibrated fluid bed

Multistage fluid bedBatch dryers

Batch dryers

Suspended state dryers

not recommended

Classification of Granular Material



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Heat Transfer in Dryers

Calculation of Heat Duty

Heat must be applied to a dryer to accomplish the following

1. Heat the feed to the vaporization temperature

2. Vaporize the liquid

3. Heat the solids to their final temperature

4. Heat the vapor to its final temperature

5. Heat the air or gas to its final temperature

Heat transfer coefficients



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Equilibrium Moisture Curves at 25 ◦C



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Equilibrium Moisture Curves at 25 C



When both heat and mass transfer involved, the mechanism

of drying depends upon

Nature of solids & Method of contacting the solids and

the heating medium

Drying conditions are carefully controlled in the

production of pharmaceuticals and fine chemicals

Drying under constant drying conditions

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Cross-circulation Drying



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