Humidification And Drying

Humidification: Humidification operation is a example for an inter phase transfer of mass and energy, when a gas and a pure liquid are brought into contact. In humidification process - liquid is transfer to gas phase - it is vaporizes In dehumidification process - liquid is transfer from gas phase to liquid phase - It is condenses

Basic Terms in Humidification Moles of water Moles of air p A  Partial pressure of water pt  Total pressure M A  Molecular weight of water M B  Molecular weight of air

Ys  Absolute Humidity at saturation condition p A  Equillibrium vapour pressure

Basic Terms in Humidification Unsaturated vapor-gas mixture Dry Bulb Temperature - Temperature of air at unsaturated condition Wet bulb Temperature - Temperature of air at saturated condition Re lative saturation or relative humidity p RH  A pA p A partial pressure of A p A partial pressure of A at saturation condition

Percentage of saturation or Percentage absolute humidity Y Y  100 or  100 Ys Ys

Basic Terms in Volume Humidification

of unit mass of dry gas and its accompanying vapor at the prevailing temperature and pressure

Heat required to raise the temperature of unit mass of gas and its accompanying vapor by one degree at constant pressure

Adiabatic saturation process

Air

Air Water

Psychrometric chart

TDP

Tad

or

T2

Applications of Humidification •

Cooling of liquid (Cooling Towers)

•

Cooling of gases

•

Recovery of vapors from gas mixtures

•

Dehumidification in pharmaceutical industries

•

Application in drying technology

Drying The separation operation of drying converts a solid, semi-solid or liquid feedstock into a solid product by evaporation of the liquid into a vapor phase via application of heat is applied Drying •Chemical Industries, •Agricultural, •Biotechnology, •Food, •Ceramics, •Pharmaceutical, •Pulp and paper, •Mineral processing, and •Wood processing industries. Drying is more energy-intensive unit operations •due to the high latent heat of vaporization and •the inherent inefficiency of using hot air as the (most common) drying medium.

Basic Terms in Drying Moisture Content: (Wet basis) Weight of moisture per unit weight of wet substance Moisture Content: (dry basis) Weight of moisture per unit weight of bone dry substance Constant rate drying period: Rate of drying per unit dry area is constant. It occurs during surface water evaporation Falling rate drying period: Rate of drying per unit dry area is varies with respect to time. It occurs when evaporation of water present in the pores starts Critical moisture content It is the moisture content when the constant rate drying period ends and falling rate drying period starts.

Equilibrium moisture •At a given temperature and pressure, the moisture content of the solid is equilibrium with the gas-vapor mixture. It is the limiting moisture content to which a given material can be dried under specific temperature of air and humidity. Bound moisture: Liquid is physically or chemically bounded in the solid pores and hence it exert a vapor pressure lower than that of pure liquid at the same temperature. Unbound moisture: Moisture in solid which exerts vapor pressure equal to that of pure liquid at the same temperature. Free moisture content: Moisture content in excess of the equilibrium moisture content at given air humidity and temperature

Determination of drying time Drying time

Ms





Ms (Xi  XC ) A NC



cons tan t rate period Mass of solids

 Xc  Xe Ms ( X c  X e ) ln  X X A NC f e  falling rate period

A  Drying surface X i  Intial moisture content XC

 Critical moisture content

Xe



Equilibrium moisture content

Xf



Final moisture content

NC



Drying rate

NC



k y  Ys  Y 



k y is mass transfer coefficient



Ys is Absolute Humidity at saturation condition



Y is Absolute Humidity









Crystallization

Crystallization   is the process of formation of solid crystals precipitating from a solution. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.mechanical and rheological Many physical, chemical, properties of solid materials depend on the size and shape of the particle. examples : •pigments for paintings (TiO2), •catalysts, •pharmaceuticals, •food products,

Solubility (kg of solute/kg of solvant)

Cs(T) generally increases with temperature

Temperature

(°C)

C Principle of cooling crystallization is purely thermal transition from an undersaturation state (T1) to a supersaturation state (T2)

T T

T

2

1