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Drying of Process

Materials

F. Taufany, PhD

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DefinitionRemoval of small amounts of water or other organic liquids (eg., solvents) from

the wet materials (solids); converting a solid, semi-solid or liquid feedstockinto a solid product by evaporation of the liquid into a vapor phase viaapplication of heat (ie., convection, conduction, radiation).

This definition thus excludes: Conversion of a liquid phase into a concentrated liquid phase (evaporation).

Mechanical dewatering operations such as filtration, centrifugation,

sedimentation, etc.An essential operation in the chemical, agricultural, biotechnology, food,

polymer, ceramics, pharmaceutical, pulp and paper, mineral processing,and wood processing industries.

Preservation of food: (a) enzymes and (b) organisms couldnt survive in moisturecontent

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Involved components1. Water: phase diagram

2. Air: psychrometry

3. Solid: moisture content

Phase diagram of water

Triple point at 0.0098 C and 0.64 kPa

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Involved components: Water-phase diagram

Phase diagram of water

Air, vacuum, and freeze dryer

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Involved components1. Water: phase diagram

Example: Vapor-liquid equilibrium of water

Rating dryer: heat energy requirement,

efficiency???

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Examples of Heat Energy Calculations1. Air drying

2. Vacuum drying

3. Free drying

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Involved Components: Air-Psychrometry

1. Psychrometry:

Humidity

Humidity chart

Wet bulb temperature

Adiabatic saturation temperature

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1. The capacity of air for moisture removal depends on its humidity and its temperature.

The study of relationships between air and its associated water is called psychrometry.

2. Humidity (H) is the measure of the water content of the air; precisely Hof air-watervapor mixture is defined as the kg of water vaporcontained in 1 kg of dry air.

absolute humidity.

3. Air is said to be saturated with water vapor at a given Tand Pif its His a maximum

under these conditions. If further water is added to saturated air, it must appear as liquid

water in the form of a mist or droplets. Under conditions of saturation, the partial

pressure of the water vapor in the air is equal to the saturation vapor pressure of water

at that temperature. Hence, saturation humidity HS is

4. Percentage humidity

Involved Components: Air-Psychrometry

AS

ASS

pP

pH

28.97

18.02

A

A

A

A

pP

p

pP

pH

28.97

18.02

airmolair/kgkg28.97

1

OHmolkg

OHkg18.02

airmolkg

OHmolkg

airdrykg

OHkg

2

222

SP

H

HH 100

Water vapor is in equilibrium with liquid water

Gibbs Dalton law; total pressure (P) = partial pressure =pA + pB

pB

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5. Percentage relative humidity is defined as 100 times of the ratio of the partial pressure

of the water vapour in the air (pA

) to the partial pressure of saturated water vapour at

the same temperature (pAS). HP HR

Psychrometry: Humidity. Contd

A

As

AS

A

A

A

A

A

SP

pP

pP

p

p

pP

ppP

p

H

HH

28.97

18.0228.97

18.02

100100AS

AR

P

pH 100 HP HR

Example:

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Dew point at HP100%H

T(dry bulb)

TW(wet bulb)

HP

Psychrometry

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Adiabatic saturation temperature: steady state temperature (i.e., recirculated water)

attained when large amounts of water is contacted with entering gas under adiabatic

condition.


SSSSSSSS HTTcHTTc )()(

Adiabatic process

Enthalpy balance: total enthalpy of the entering gas mixture = enthalpy of the entering gas mixture

005.1

S

S

S

S c

TT

HH

sprayoptional

TS < Tand HS > H

Sensisble heat latent heat

Slope in humidity chart

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HS

H

T(dry bulb)TS

HP

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Wet bulb temperature: steady state nonequilibrium temperature reached when small

amounts of water is contacted under adiabatic condition by a continuous stream of

gas.


005.196.0/

W

yB

W

W kMh

TT

HH

Energy balance: latent heat of evaporation on the wick = convective heat flowing from the gas

stream at Tto the wick (at lowerTW)

Large amount

Assumed no changes in its properties

Small amount

Note: For air/water system only, TS TW

Psychrometric ratio

Where h=heat transfer coefficient, MB=MW of air, ky=mass-tansfer coefficient, =latent heat of vaporization

005.1

S

S

S

S c

TT

HH

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Dew point at HP100%H

T(dry bulb)

TW(wet bulb)

HP

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Equilibrium Moisture Content of Materials

Moisture content of given materials

after being contacted with large

excess air (H & T)

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Equipment for drying: Batch1. Tray dryer

2. Vacuum-shelf dryers

The lumpy solid or pasty solid is spread out, generally

quite thinly depth (10 100 mm), is uniformly spread

out on trays in which the drying takes place. Heating

may be by an air current sweeping across the trays, by

conduction from heated trays or heated shelves on

which the trays lie, or by radiation from heated surfaces.

Most tray dryers are heated by air, which also removes

the moist vapours.

10 - 20 % fresh air

Almost similar to tray only that is operated under vacuum.Useful for handling materials which are (a) expensive dry, (b)

temperature sensitive/easily oxidable, (c) even for the materials with toxic

or valuable solvents

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Equipment for drying: Batch3. Freeze dryer The material is held on shelves or belts in a chamber that isunder high vacuum. In most cases, the food is frozen

before being loaded into the dryer. Heat is transferred to the

food by conduction or radiation and the vapour is removed

by vacuum pump and then condensed. In one process,

given the name accelerated freeze drying, heat transfer is

by conduction; sheets of expanded metal are inserted

between the foodstuffs and heated plates to improve heattransfer to the uneven surfaces, and moisture removal. The

pieces of food are shaped so as to present the largest

possible flat surface to the expanded metal and the plates

to obtain good heat transfer. A refrigerated condenser may

be used to condense the water vapour.

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Equipment for drying: Continous1. Continuous Tunnel dryer

The solids are placed on

trays/trucks (operated in series)

which move continuously through a

tunnel with hot gases passing over

the surface of each tray. The hot air

flow can be countercurrent,

cocurrent or combination.

FOOD DRYING

The wet granular solids are

conveyed as a layer 25-150 mm

deep.

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Equipment for drying: Continous2. Roller (rotary drum) dryer

The food (ex: potato slurry) is spread over the

surface of a heated drum. The drum rotates, with the

food being applied to the drum at one part of the

cycle. The food remains on the drum surface for the

greater part of the rotation, during which time the

drying takes place, and is then scraped off. Drum

drying may be regarded as conduction drying.

Example: POTATO FLAKES

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Equipment for drying: Continuous3. Rotary dryer The foodstuff is contained in a horizontal inclined

cylinder through which it travels, being heated either by

air flow through the cylinder, or by conduction of heat

from the cylinder walls. In some cases, the cylinder

rotates and in others the cylinder is stationary and a

paddle or screw rotates within the cylinder conveying

the material through.
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