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Principle In general drying a solid means the removal of relatively small amounts of water or other liquid from the solid material, to reduce the content of residual Inquid to an acceptably low value. Water or other liquids may be removed from solids mechanically by process of centrifuges or thermally by vaporization. The liquid content of the dried substance varies from product to product. Occasionally the product contains on hquid and is called done-dry The solids to be dried may be in many different forms-flakes granules,crystal, powders, slabs or continuous sheets. The liquid to be vaporized may be on the surface of the solid or it may be entirely inside the solid, as in solvent removal from polymer, or it may be partly outside and partly inside. The feed to some dryers is a liquid in which the solid is suspended as particles or is in solution. The dried product may be able to stand rough handling and a very hot environment. Definitions (1) Moisture content: (wet basis ) The moisture content of a solid is defined as the ratio of wt. of moisture the wt of the wet solid. Moisture content (dry basis): It is defined as the ratio of weight of moisture content to the weight of dry sold (2) Equilibrium moisture This is the moisture content of the solid when at equilibrium with a given partial pressure of vapor.
If a wet solid is exposed to a continuous supply of fresh gas containing a fixed partial pressure of the vapor, the solid will either lose moisture by evaporations or gain moisture from the gas until the vapor pressure of the moisture of the solid equals the partial pressure of vapor in the gas. The solid and the gas are in equilibrium, and the moisture content of the solid is termed its equilibrium moisture content at a prevailing conditions. (3) Bound moisture:Solids This refers to the moisture contained by a substance, which exerts an equilibrium vapour pressure less than that of the pure liquid at the same temperature. (4) Unbound moisture: This refers to the moisture contained by a substance which exerts an equilibrium vapour pressure equal to that of the pure liquid at the same temperature (5) Free moisture: Free moisture is that moisture contained by a substance in exerts of the equilibrium moisture i.e. X-X. Only free moisture cam be evaporated or removed. Mechanism of Drying of Solids When surface evaporation occurs, there must be a movement of moisture from the depths of the solid to the surface. The natures of the movement influence the drying rate during the falling rate periods. There are two theories are available to explain the moisture movement within the solid. Diffusion theory: โ The moisture distribution in a solid is as shown in the above figure. โ In which the local moisture content is plotted against distance from the surface. โ It differs somewhat from the theoretical curve. โ Diffusion of moisture may result because of concentration gradients between the depth of the solid, where the concentration is high, and the surface, where it is low. โ These gradients are setup during drying from the surface. โ This method of moisture movement is limited to cases lie soap, glue,gelatin and etc. โ Also this method limited to certain cases where bound moisture is being dried like, clays, flour, textiles, paper and wood
โ A typical moisture distribution curve for a porous solid is shown above figure โ A point of inflection divides the curve into two parts, one concave upwards and one concave downward โ This is entirely different from diffusion mechanism โ Moisture flows through porous solids by capillarity โ A porous material contains a complicated network of interconnecting pores and channels โ The cross section of these pores varies โ At the surface the mouths of pores of various sizes. โ As moisture is removed by evaporation, a meniscus across each pore is formed, which sets up capillary forces by the surface tension. โ This force provides the driving force for the movement of water through the pores towards the surface. โ The strength of the capillary forces at given point depends upon the curvature of meniscus โ Small pores develop greater capillary force than the larger ones.
โ Therefore, small pores can pull water out of the large pores. โ As water at surface is depleted, the large pores tend to empty first. โ Air must displace the water so removed. This air enters either through the mouth of larger pores or from the sides of the solid. โ The pores are progressively depleted of water and at the critical point the surface layer of water begins to recede into the solid. โ The dry points on the surface of the solid begin to emerge from the liquid and the area available for mass transfer from the solid into the air decreases. โ Therefore the rate based on total surface, is less than that in the constant rate period. โ The rate decreases continuously as the fraction of dry surface increases. Rate of Drying
โ Consider a bed of solids (wet) over which air is calculated. โ Assume that the temperature, humidity velocity and direction of flow of the air are constant that is drying under constant drying condition. โ As time passes the moisture content typically falls as shown by curve (A) in the above figure. โ The curve becomes linear after a short period during which the feed is heated to vaporization temperature. โ Finally the curve (A) levels off. โ The drying rate shown by graph (B), it is horizontal for much of its length, indicating that the drying rate is constant. โ โ Later it curves down wards and eventually when the material come to its equilibrium moisture content, reaches zero.
