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Drying.ppt - DRYING It is a mass transfer process resulting...

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DRYINGDRYING
It is a mass transfer process resulting in the removal of moisture (water) or another solvent, by evaporation from a solid, semi-solid or liquid to end in a solid state. To achieve this, there must be a source of heat.Drying is most commonly used in manufacturing as a unit process in the preparation of granules, which can be dispensed in bulk or converted into tablets or capsules.
Purposes of dryingIn pharmaceutical technology, drying is carried out for one or more of the following reasons:To avoid or eliminate moisture which may lead to corrosion and decrease the product or drug stability.To improve or keep good properties of a material, e.g. flowability, compressibility.To reduce the cost of transportation of large volume materials ( liquids).To make the material easy or more suitable for handling.The final step in: Evaporation- Filtration- Crystallization.
Moisture content of wet solidsThe moisture content of a wet solid is expressed as kilograms of moisture associated with 1 kg of the moisture. Thus a moisture content of 0.4 means that 0.4 kg of removable water is present per kg of the solid. It is sometimes calculated as percentage moisture content.Total moisture content:This is the total amount of liquid associated with a wet solid. The easily removable water is known as the free moisture content, and the moisture which is more difficult to remove is the equilibrium moisture content.The easily removable water is known as unbound water.Unbound water:This water exists as a liquid and exerts its full vapour pressure, it can be removed readily by evaporation. During a drying process this water is easily lost but the resulting solid is not completely free from water molecules.
Equilibrium moisture content: The moisture content present in a solid under steady-state ambient conditions is termed the as equilibrium moisture content. Its value changes with temperature, humidity and the nature of the solid.Bound water :Part of the moisture present in a wet solid may be adsorbed on surfaces of the solid or be adsorbed within its structure to such an extent to prevent it from developing its full vapour pressure and from being easily removed by evaporation. Such moisture is described as “bound” and is more difficult to remove than unbound water.
DRYING CURVE
Constant-rate period:It is found that the evaporation rate from the drying bed is similar to that of the solvent alone from a free liquid surface under the same conditions, indicating that the evaporation takes place from the wet surface of the solid, and that the surface remains wet in this period as a result of the liquid being replaced from below as fast as it is vaporized Controlling factors in this period are the rate at which heat can be transferred and the rate of removal of the vapour.
First falling-rate period: As moisture is removed from the surface, a point will be reached when the rate of vaporization is insufficient to saturate the air in contact with the surface. Under these conditions, the rate of

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Term
Fall
Professor
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Tags
Chemical Engineering, FBD, Fluidized bed

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