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CHE 591
PROCESS ENGINEERING 1
DRYING OF PROCESS MATERIAL
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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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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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VAPOR PRESSURE OF WATER
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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VAPOR PRESSURE OF WATER
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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)
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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