PEF-Drying 制药工程与设备基础　Pharmaceutical Equipment Fundamentals　教学课件　.pptVIP

Pharmaceutical Equipment Fundamentals Drying Drying Drying is a mass transfer process consisting of the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid. Often the final operation in a manufacturing process, carried out immediately prior to packaging or dispatch. During drying, To avoid crystals damage or products contamination. To prevent shrinkage (paper), cracking (wood), loss of flavour (fruit) What’s for? to reduce the cost of transport to make a material more suitable for handling, as soap powders, dyestuffs and fertilisers to provide definite properties, as maintaining the free-flowing nature of salt to remove moisture which may otherwise lead to corrosion, eg. drying of gaseous fuels or benzene prior to chlorination. The final product must be a solid, either as a continuous sheet (e.g., paper), as long pieces (e.g., wood), particles (cereal grains, corn flakes) or in powder form (sand, salt, washing powder, milk powder). To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In bioproducts (food, grains, vaccines), and pharmaceuticals, the solvent to be removed is almost invariably water. Drying Process In some products having a relatively high initial moisture content, an initial linear reduction of the average product moisture content as a function of time may be observed for a limited time, often known as constant drying rate period. Usually, in this period, it is surface moisture outside individual particles which is being removed. If drying is continued, the slope of the curve, the drying rate, becomes less steep (falling rate period) and eventually tends to the horizontal at very long times. The product moisture content is then constant at the equilibrium moisture content, where it is in dynamic equilibrium with the dehydrating medium. In the falling rate period, water migration from the product interior to the surface is mostly by molecular diffusion