drying of process material

8/13/2019 Drying of Process Material

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CHE 591

PROCESS ENGINEERING 1

DRYING OF PROCESS MATERIAL

1 Fuzieah Subari, Jan 2011


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COURSE OUTCOME (CO1)

Students should be able to :

Apply mass transfer and separation process principles in

solving unit operations such as adsorption,

chromatography, drying, evaporation, ionexchange, crystallization and membrane separation

processes



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DRYING OF PROCESSES MATERIALS

1. Introduction and methods of drying

2. Vapor pressure of water and humidity

3. Equilibrium moisture content of materials

4. Rate of drying curves

5.

Calculation methods for constant-rate & falling-ratedrying periods

6. Combined convection, radiation & conduction heattransfer in constant rate period

7. Drying in falling-rate period by diffusion and capillary

flow8. Equation for various types of dryers

9. Freeze-drying of biological materials



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INTRODUCTION TO DRYING

Purpose ;1. removal of water from process materials and other

substances

2. Removal of other organic liquids e.g benzene from solids

Different from evaporator

Remove small amount of water either thermally or

mechanically (presses, centrifuging and etc.) as vapor by air.



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INTRODUCTION TO DRYING

In dehydration of biological materials (foods) usedpreservation technique

With the absence of water (reduce to below 10wt%)

1. Microorganisms cause food spoilage and decay cant grow

2. Enzymes cause chemical changes in food cant function

Stored in extended period of time



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2

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METHODS OF DRYING

Classification of drying methodsBatch Continuous

Classification according to physical conditions

Heat is added by direct contact with

Heated air at Patm.

Vapor formed remove by air

Vacuum dryingEvaporation at low pressure

Heat is added indirectly by

contact with metal

wall/radiation

Freeze drying

Water is sublimed from

frozen material



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EQUIPMENTS FOR DRYING

1. Tray dryer2. Vacuum-shelf indirect dryers

3. Continuous tunnel dryers

4. Rotary dryers

5. Drum dryers

6. Spray dryers

7. Drying of crops and grains



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VAPOR PRESSURE OF WATER

Important to define properties of mixtures of watervapor and air

Should have the knowledge on:

1. Concentration of water vapor in air under various T& P

2. Thermal properties of these mixtures

3. The changes occurring when these mixtures are in

contact with water/ wet solids in drying



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Humidification The transfer of water from liquid phase into gaseous

mixture of air and water vapor

Dehumidification

Reverse transfer.

Water vapor is transferred from vapor state to liquid

state



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HUMIDITY

Definition :the humidity, H of an air-water vapormixture : kg of water vapor contained in 1 kg of dry air

H depends on partial pressure, pA of water vapor in the air

and total pressure P



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SATURATION HUMIDITY

For saturated air (air in which the water vapor is inequilibrium with liquid water at given P and T)

Partial pressure = vapor pressure,pAS



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PERCENTAGE & PERCENTAGE RELATIVE HUMIDITY

Defined as 100 times the actual humidity of the airdivided by the humidity Hs if the air were saturated at

the same T and P

The amount of saturation of air-water vapor mixture at

partial pressures



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Brainstorming

Is HR = HP ?



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Solve problem 9.3-1 & 9.3-2



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DEW POINT

Dew point???

Given a case

A mixture of water vapor at 26.7oC and saturation vaporpressure, pAS = 3.5 kPa

Dew point @3.5kPa = 26.7o

C

A mixture of air-water vapor @ 3.5kPa and 37.8oC would NOTsaturated

Reason: air would be saturated at cooling T = 26.7oC, furthercooling some water vapor would CONDENSE since the

partial pressure must be less than saturation vaporpressure



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HUMID HEAT, CS

Definition: the amount of heat in J required to raise thetemperature of 1 kg air plus the water vapor present

by 1K or 1oC.

The heat capacities of air and water vapor is assumed

CONSTANT. 1.005 kJ/kg dry air.K1.88 kJ/kg water vapor.K

Hence, CS



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HUMID VOLUME,

Definition: total volume in m

3

of 1 kg of dry air plus thevapor it contains at 1 atm abs pressure and the given

gas temperature.

For SATURATED air-water vapor mixture, H=HS and is saturated

volume



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TOTAL ENTHALPY, Hy

Definition : the total enthalpy of 1 kg of air plus itswater vapor is Hy j/kg dry air.

If referred at base temperature, To = 0oC



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HUMIDITY CHART

Is a convenient chart of the properties of air-watervapor at 1 atm abs pressure

H plotted vs actual temperature of the air-water vapormixture

The curve marked 100% running upward to the rightgives the saturation humidity, Hs as a function oftemperature

Any point below saturation line represent unsaturatedair-water vapor mixtures



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HUMIDITY CHART

The curved lines below 100% saturation line runningupward to the right represent unsaturated mixtures of

definite percentage humidity, Hp

Going downward vertically from saturation line at agiven T, the line between 100% saturation and zero

humidity H is divided evenly into 10 increments of

10% each

All Hp and Hs line can be calculated from the data for

vapor pressure of water.



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Solve problem 9.3-3



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ADIABATIC SATURATION TEMPERATURE, Ts

Is the T of the water being recirculated reaches asteady state T.

In humidity chart, this line plotted through point HSand Ts on the 100% saturation curve and other pointof H and T.

If a given gas mixture at T1 and H1 is contacted longtime in adiabatic saturator it will leave Hs1 and Ts1 its

determined by following adiabatic saturation linethrough point T1 and H1 until intersects the 100%saturation line.



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Solve problem 9.3-5



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WET BULB TEMPERATURE

Is the steady-state nonequilibrium T reached whensmall amount of water is contacted under adiabatic

conditions by a continuous stream of gas.

The adiabatic saturation lines can be used as wet bulblines for water vapor not for other vapor e.g benzene.

The wet bulb determination used to measure the

humidity of an air-water vapor mixture.



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Solve problem 9.3-8



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EQUILIBRIUM MOISTURE CONTENT (EMC) OF

MATERIALS

Wet solid containing moisture is brought into contactwith a stream of air at constant H and T.

Excess of air is used and the condition remain constant

Solid was exposed at sufficiently long time to reachequilibrium.

The solid attain a definite moisture content

Equilibrium moisture content



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Data of EMC for inorganic and biological materials

1. Typical data for various materials

1. For material contains MORE moisture than its equilibrium

value in contact with a gas at given humidity & T, it will DRY

until it reach equilibrium value

2. For material contains LESS moisture than its equilibrium value

it will ADSORB water until it reach equilibrium value.2. Typical food materials

at low relative humidity the EMC is greatest for food materials

high in protein, starch and high molecular weight polymer BUT

lower for food materials high in soluble solids

3. Effect of temperature

The EMC of solid decrease with an increase of temperature



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Refferences

1. http://www.marspedia.org/images/d/d8/Phase_dia

gram_water.png

2. Geankoplis. Transport processes and separation

process principles (includes unit operations)

http://www.marspedia.org/images/d/d8/Phase_diagram_water.pnghttp://www.marspedia.org/images/d/d8/Phase_diagram_water.pnghttp://www.marspedia.org/images/d/d8/Phase_diagram_water.pnghttp://www.marspedia.org/images/d/d8/Phase_diagram_water.png

drying of process material

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