Drying refers to an operation in which the moisture of a substance is removed by thermal means the removal of a relatively small amount of water or other liquids from the solid material. The removal of moisture by thermal means is more costly than mechanical means e.g. filtration and pressing. The moisture content of a solid on a dry basis is X then the moisture content on a wet basis is X / (1 + X). The moisture present in a substance that has an equilibrium vapor pressure the same as that of pure liquid at a unique temperature is called unbound moisture.
In drying operation moisture that is evaporated is free moisture, a solid can be dried completely by passing over it completely dry air. In the constant-rate-period moisture that is evaporated is unbound moisture. When the average moisture content of a solid is equal to the critical moisture content, the drying rate is equal to the rate at constant-rate-period. If drying occurs by convective heat and mass transfer (conduction and radiation effects are absent), the temperature at the solid surface is equal to the wet-bulb temperature of the gas.
Let a certain drying operation dry bulb temperature of the air is 60oC and the surface temperature of the solid is 30oC. Assume that conduction and radiation effects are negligible otherwise uniform conditions if the dry bulb temperature of the air is increased to 90oC, the drying rate during the constant rate period will increase by a factor of 2.
The drying rate during the constant rate period for the use of cross-circulation drying of a solid depends on the velocity of the drying gas, the gas temperature, and the humidity of the drying gas. In a certain process, a solid will be dried from all surfaces. Under this condition, the rate of drying during the constant rate period will be independent of thickness. The drying time between fixed moisture contents in the constant rate period will be directly proportional to the thickness.
If the drying rate, N, is plotted in the ordinate and the free moisture, X, is plotted in the abscissa, then the N vs. X curve will definitely pass through (0, 0) [NC: drying rate at constant – rate – period; Xe: equilibrium moisture content] Assume that the rate-of-drying curve in the entire falling-rate-period is linear. Then the drying time between fixed moisture contents in the falling rate period is directly proportional to the solid thickness. When drying occurs due to the movement of moisture through a solid, the drying time between fixed moisture contents is directly proportional to the square of the thickness
The drier that is most suitable for drying milk is a spray drier.
Rotary dryers are employed for drying solids that are free-flowing and granular. The critical moisture content is a function of material properties, the rate of drying during the constant rate period, and particle size. A continuous rotary dryer is filled with the material to 10 to 15% of the shell volume. In countercurrent rotary driers, the exit gas temperature is usually lower than the product temperature.
For drying a heat-sensitive material in a rotary dryer, the gas flow should be co-current. Rotary dryers are usually operated with a negative internal pressure to prevent dust and vapor escape through the rotating seals. Drying of detergent solution to produce detergent powder is accomplished in a spray drier. Rotary dryers are usually rotated at peripheral speeds of 0.2 to 0.5 m/s. The length-to-diameter ratio of most rotary driers is around 4 to 10. Prilling towers are used for drying urea.
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