Drying and Degradation Kinetics of Heat Sensitive Products in Spray Drying

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Czeslaw Strumillo, Ireneusz Zbicinski
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Particle Formation
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Spray drying is one of the best theoretically developed drying methods, particularly in the area of calculation of hydrodynamics in the two-phase flow: solid particles-gas.

The process of spray drying is, however, such a complex phenomenon that so far no model describing it correctly has been proposed. The gravest errors in the calculation of spray drying are caused by an incorrect determination of drying kinetics and improper model of flow turbulence (Bahu, 1992).

Due to broad variety of materials being dried in spray dryers, it seems difficult to develop general principles concerning the kinetics of water removal from these materials (Masters, 1991). It is necessary to determine individual drying kinetics for each material separately. In a typical schematic of spray drying, a particle stops shrinking and the formation of a rigid structure starts when a critical moisture content is reached. Since that moment on the particle may not change its size, it may break down, disintegrate and agglomerate. All these phenomena affect the coefficients of aerodynamic drag and heat and mass transfer which have a significant influence on the drying process. Some materials may behave in a quite different way, e.g. the colloidal-capillary-porous bodies reveal high resistance to vapour diffusion on the surface; this may cause swelling of the particle in the initial period of drying (particles of milk).