Drying

DRYING Part 1

References Transport processes and Separation Process Principles; 4th

edition, by Christie J. Geankoplis McCabe W. M., Smith J. C. and Harriott P., Unit Operations of

Chemical Engineering, 7th Ed., McGraw Hill, 2005.

Drying Objectives:

To understand the basic principles of drying

To understand the theories associated with drying processes

To be able to deal with heat and material balances and related mathematical models of drying processes

To be familiar with drying equipment and the principle of their operation

Drying: Definition and Principles Drying generally means removal of relatively small amounts

of water from a wet solid. The water is usually removed as a vapor by air at temperature below its boiling point. Differences between Drying & Evaporation

Evaporation refers to removal of relatively large amounts of water from a solution. The water is removed as vapor at its boiling point. In evaporation the water is removed by heating up to its boiling point, while in drying water is removed by circulating air or some other gas over the material.

Dried Substance BONE-DRY : product contain no liquid

DRIED TABLE SALT 0.5% H2O

Dried Coal 4% H2O

Dried Casein (Milk protein) 8% H2O

Dried Substance Drying as a preservation technique (biological materials, esp.

food) at < 10 % wt water content

Microorganisms that cause food spoilage and decay cannot grow and multiply in the absence of water

Many enzymes that cause chemical changes in food and other biological materials cannot function without water.

Usually < 5% wt water content to preserve flavour and nutrition

Solid Particles to be Dried many different forms: SOLID

flakes granules crystals powders slabs continuous sheets

Liquid with suspended solid

Application in Industry Drying of polymers:

1. Wet powders, where the particles have been formed: PVC 2. Emulsions and solutions, where the particles are formed during the drying process: Acrylic Resins, Paraformaldehyde.

Reasons for Drying To reduce the cost of transport and

storage To make the material suitable for

handling To avoid the presence of moisture

which may lead to corrosion To prevent(or inhibit) microorganisms

and hence preserve the food

Mechanism of Drying Wet Material Drying involve both

a. Heat b. Mass transfer Heat must be transferred to the material to be dried in order to

supply the heat required for vaporization of the moisture.

Mass transfer is involved in the diffusion of water through the material to the evaporation surface and subsequent evaporation of water from the surface and diffusion of the resultant vapor into the passing air stream.

Mechanism of Drying Material The mechanism of moisture transport in different solids may be broadly classified into: Transport by capillary forces Liquid diffusion Pressure induced transport Vapour diffusion The mechanism that dominates depends on the nature of he solids, its pore structure and the rate of drying

Rate of Drying can be accelerated by:

Introducing a high temperature radiating heat source into drying chamber. Reducing the thickness of material being dried. Increasing the air velocity will speed up the rate of drying . Dehumidifying the inlet air (thus increasing the humidity

differential)

For effective drying, air should be hot, dry and moving.

Vapour Pressures of Water and Humidity Vapour pressure of water (PA)

Boiling occurs when: The vapor pressure of a liquid is the equilibrium pressure of a vapor

above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container

Vapour pressure of water( PA) = Total pressure (P) above the water surface water boils

Water at 100 oC the PA=101.3 kPa (1atm) it will boil at 100oC if P=101.3 kPa

At 65.6oC the PA=25.7 kPa it will boil at 65.6 oC if P =25.7 kPa If water is held at 65.6oC in a room at 101 .3 kPa, the vapour pressure of

water will again be 25.7 kPa. This illustrates an important property of the vapour pressure of water.

Humidity and Humidity chart Humidity is the quantity of water vapour present in air. It can be

expressed as an absolute, saturated or a relative value

Humidity Ratio by Mass (Absolute humidity ) The humidity of an air-water vapour mixture is defined as the Kg of

water vapour contained in 1 kg of dry air at any given temperature . Humidity ratio expressed by mass:

Humidity and Humidity in chart Humidity depends only on the vapour partial pressure (PA) of water

in the air and the total pressure (P). Using the Mwt of water (A) as 18.02 and of air as 28.97, the humidity H in kg H2O/Kg dry air, as follows;

Humidity and Humidity Chart

Humidity and Humidity chart Absolute (Actual) Humidity (H)

Saturation humidity (Hs):

Percentage Humidity: Defined as 100 times the actual humidity (H) of the air divided by

saturated (Hs) at the same temperature and pressure

Humidity and Humidity chart Absolute (Actual) humidity Saturation humidity (Hs) Percentage Humidity (Hp)

Relative Humidity of Air( RH) It is defined as the ratio of the mass of water vapour present in the unit volume of the air at certain temperature to the maximum mass of the water vapour that can be accommodated in the unit volume of air when it is saturated .

Relative Humidity RH = Amount of water vapour in unit volume of unsaturated

air/amount of water vapour in unit volume of saturated air RH = Partial Pressure of water vapour in unsaturated air/ Partial pressure of

water vapour in saturated air

Humidity and Humidity chart Example 1:

The air in the room is 26.7 C and pressure of 101.3 kPa and contains water vapour with partial pressure PA = 2.76 kPa.

Calculate the following

Absolute Humidity (H) Saturation Humidity (Hs) Percentage Humidity (Hp) Percentage relative Humidity (RH)

Vapour Pressure of water and Humidity Saturated Air At certain T Saturated Air: Is the maximum

amount of moisture that the air can hold at particular temperature, it depends on the air temperature

Vapour Pressure of water and Humidity

The higher the temperature of air, the more quantity of the moisture it can absorb

The higher the water vapour (moisture) content within the air, the higher the partial pressure of the water vapour (PA) within the air

Saturated Air There are 2 ways for air to become saturated:

Evaporating water until air is saturated when water evaporates, the vapour pressure increases . If water evaporates until the vapour pressure is equal to the saturation vapour pressure, then the air is saturated .

Saturated Air Evaporating Water until air is saturated

Saturated Air Decreasing the temperature of air until it becomes saturated

The saturation vapour pressure (PAS) is a function of temperature. If the temperature decreases until it is equal to the PA, then the air is saturated and RH = 100%

Saturated Air Decreasing the temperature of air

until it becomes saturated

Dew Point Dew Point temperature: is the temperature at which a given mixture

of air and water vapour would be saturated

Humid heat of an air water vapour mixture The humid heat (Cs) : is the amount of heat in J(or kJ) required

to raise the temperature of 1 kg dry air plus the water vapour present by 1 K or 1 C.

The heat capacity of air and water vapour can be assumed constant over the temperature ranges

Humid Volume of an air-water vapour mixture Humid volume (VH ): is defined as the volume of unit mass of dry

air with accompanying water vapour at a given temperature and pressure.

If H is the absolute humidity of a sample air at atmospheric pressure and temperature T oC, its humid volume can be calculated from the following equation assuming ideal gas behaviour (volume of 1 kmol of gas at 1 atm and 0 oC =22.4 m3)

= 128.97+ 18.02 22.41273 , in m3 per kg dry air where T is in K = 2.83 10 3 + 4.56 10 3

Total Enthalpy of an air-water Vapour Mixture The total enthalpy of 1 kg dry air plus its water vapour is (Hy)

J/kg dry air. If To is the datum temperature chosen for both components, the total enthalpy is the sensible heat of the air- water vapour mixture plus the latent heat in J/kg or kJ/kg water vapour of the water vapour at To

Dry Bulb and Wet Bulb Temperature Dry Bulb temperature:

This is the temperature of air measured by a thermometer whose bulb is dry, i.e. not in touch with water or any other liquid. This is true temperature of air

Wet Bulb temperature:

Its the temperature attained by a small amount of evaporating water in a manner such that the sensible heat transferred from the air to the liquid is equal to the latent heat required for the evaporation.

Its measured by passing air over the bulb of thermometer which is covered with a cloth-wick saturated with water

Humidity chart of an air-water vapour mixture

Example Air entering a dryer has a temperature (dry bulb temp) of 60oC

and a dew point of 26 .7oC.Using the humidity chart, determine the actual humidity H , percentage humidity Hp, humid heat, and humid volume.

Example Air entering a dryer has a temperature (dry bulb temp) of 60oC and a

dewpoint of 26 .7oC . Using the humidity chart, determine the actual humidity H, percentage humidity Hp, humid heat, and humid volume .

Adiabatic Saturation of Air If a given gas mixture at T and H is contacted with water for a

sufficient long time in an adiabatic saturator, it will leave saturated at HS and TS.

These values are determined by following the adiabatic saturation line going through point H and T, until it reaches the 100 % saturation line .

If contact is not sufficient, the leaving mixture will be less than 100 % saturation line .

Example

Example

Example

Wet Bulb Temperature and humidity example A water vapour air mixture having a dry bulb temperature of

60oC is passed over a wet bulb, and the wet bulb temperature obtained is Tw=29. 5oC. What is the humidity of the mixture? Assuming that wet bulb temperature ( Tw) = adiabatic saturation temperature ( TS) .

Wet Bulb Temperature and humidity example

Wet Bulb Temperature and humidity example A water vapour air mixture having a dry bulb temperature of

60oC is passed over a wet bulb, and the wet bulb temperature obtained is Tw=29. 5oC. What is the humidity of the mixture? Assuming that wet bulb temperature ( Tw) = adiabatic saturation temperature ( TS) .

Solution: 0.0135kg H2O/kg dry air

DRYINGReferencesDryingDrying: Definition and PrinciplesDried SubstanceDried SubstanceSolid Particles to be DriedApplication in IndustryReasons for DryingMechanism of Drying Wet MaterialMechanism of Drying MaterialRate of Drying can be accelerated by:Vapour Pressures of Water and HumidityHumidity and Humidity chartHumidity and Humidity in chart Humidity and Humidity ChartHumidity and Humidity chartHumidity and Humidity chartHumidity and Humidity chartSlide Number 20Slide Number 21Vapour Pressure of water and HumidityVapour Pressure of water and HumiditySaturated AirSaturated AirSaturated AirSaturated AirDew PointHumid heat of an air water vapour mixtureHumid Volume of an air-water vapour mixtureTotal Enthalpy of an air-water Vapour MixtureDry Bulb and Wet Bulb TemperatureHumidity chart of an air-water vapour mixtureExampleExample Adiabatic Saturation of AirExampleExampleExampleWet Bulb Temperature and humidity exampleWet Bulb Temperature and humidity exampleWet Bulb Temperature and humidity example