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 ofdrying (particles of milk).

The key problem in spray drying which has not been solved yet is the determination of drying kinetics and degradation kinetics for heat sensitive products. The lack of appropriate experimental investigations is due to technical problems in carrying them out. The residence time of particles in the spray dryer does not usually exceed 30 seconds, and is often even shorter. Thus the whole process of dehydration, formation of solid structure, degradation, etc. takes a very short time. Therefore, a few attempts made so far to determine the kinetics of product drying and degradation have been restricted to the analysis of relevant parameters only at the dryer inlet and outlet, e.g. Alizondo and Labuza (1974), Johnson and Etzel ( 1994).

Step I of this project is a preliminary stage to enable extensive studies on kinetics and degradation of dewatering of selected products in a disperse system when particle residence time does not exceed a few seconds.

As follows from the research we have carried out so far, the range of measurements performed in the experimental rig must be extended by increasing the residence time of sprayed material in the measuring section and reducing the risk of material deposition on the walls. To achieve this the diameter of the measuring section should be changed from 30 to 50 cm which would enable drying in a broad range of initial process parameters (mainly in a wide range of feeding rates and atomization angles).