Drying
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 liquid to an acceptably low value. Drying is usually the final step in a series of operations, and the produce from a dryer is ofen ready for final packing.
Water or other liquids may be removed from solids mechanically by filtration or centrifugation or thermally by vaporization. It is generally cheaper to remove water mechanically than thermally. The utmost use should be made of these mechanical processes. In a simple instance a centrifugal will remove 2 tons of water from a ton of granular solid with a power cost of about 10 kwh;the steam equivalent of this will be at most 300 lb. If the water is evaporated in a steam-heated dryer, the steam consumption must be at least 4,480 lb. Thus it is advisable to reduce the moisture content as much as possible before feeding the material to a heated dryer.
The moisture content of a dride substance varies from product to product. Occasionally the product contains no water. More commonly, the product does contain some water. Dried table salt,for example, contains about 0.5 per cent water,dried coal about 4 per cent,and dried casein about 8 per cent. Drying is a relative term and means only that there is a reduction in moisture content from an initial value to a final one.