7
CHAPTER
Selected Definitions Drying: It can be defmed as an process in which the liquid, generally water, present in a wet solid is removed by application of heat and fmally a liquid free solid product is obtained. Bound Water: It is the moisture content of a substance that exerts a equilibrium vapor pressure lower than that of the pure liquid at same temperature. Unbound water: It is the moisture content of a substance that exerts a equilibrium vapor pressure equal to that of the pure liquid at same temperature. Free Moisture: It is the moisture content of the solid in excess of the equilibrium moisture content Moisture content: It is measured by ratio of weight of dry sample to ratio of weight of water in sample EqulUbrium Moisture Content (EMC): It is the moisture content of the solid when in equilibrium with the given partial pressure of vapor in the gas phase. Critical Moisture Content (CMC): The point at which decrease in drying rate start. Fluidized State: It is the state in which the granules are suspended in the air stream and rise from the bottom. Isothermal Desorption : It is the process of reducing continued at warmer temperature.
the residual moisture content to optimum values when drying is
Desorption: If solid contains more moisture than EMC, then solid will continuously lose water until EMC is reached. Sorption:
If solid contains less moisture than EMC, then solid will continuously absorb water until EMC is reached.
7.1 INTRODUCTION Drying can be defmed as an process in which the liquid, generally water, present in a wet solid is removed by application of heat and finally a liquid free solid product is obtained. Drying is often necessary in various industrial operations to remove moisture from a wet solid. There are different ways to remove liquid. In general, drying is accomplished by thermal techniques and thus involves the application of heat. Nonthermal drying processes eh as squeezing wetted sponge, adsorption by desiccant (desiccation) and extraction are also used. For example: Organic liquids or gases are dried by passing them through bed of silica gel or activated alurnina which act as adsorbents.
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