Drying: Principles and Practice
The solid/liquid separation operation of drying converts a solid, semi-solid, or liquid feedstock into a solid product by evaporation of the liquid via application of heat. In the special case of freeze drying, which takes place below the triple point of the liquid being removed, drying occurs by sublimation of the solid phase directly into the vapor phase. This definition thus excludes conversion of a liquid phase into a concentrated liquid phase (evaporation); mechanical dewatering operations such as filtration, centrifugation, sedimentation, etc.; supercritical extraction of water from gels to produce extremely high porosity aerogels (extraction); or so-called drying of liquids and gases by use of molecular sieves (adsorption), etc. Phase change and production of a solid phase as an end product are essential features of the drying process. Drying is an essential operation in the chemical, agricultural, biotechnology, food, polymer, ceramics, pharmaceutical, pulp and paper, mineral processing, and wood processing industries.