86A-2

H. FENG¹, J. TANG¹, and O. A. Plumb². (1) Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120, (2) College of Engineering, University of Wyoming, Laramie, WY 82071

Microwave drying involves a simultaneous transport of heat, mass, and momentum, accompanied by a volumetric heat generation. The internal heat generation results in a buildup of gas/vapor pressure gradient that distinguishes microwave drying from other drying methods. It also produces a positive temperature gradient, in contrast to the temperature gradient when dried from the product surface by conventional hot-air methods. A heat and mass transfer analysis for microwave drying is therefore complicated by the need to consider the effect of the internal vapor generation. The problem becomes even more complicated when considering the interaction between the microwave and the material. A comprehensive heat and mass transfer analysis that takes into account the contribution of the positive pressure gradient to moisture migration is highly desirable to understand the underlying physics involved in microwave drying and to predict the temperature and moisture changes during the drying.

The objective of this study was to develop a comprehensive heat and mass transfer model for microwave drying of hygroscopic porous food products.

The moisture transport mechanisms were analyzed by examining the microstructure and possible transport passages in the material. The transport mechanisms for free water, bound water, and vapor were studied separately. The governing drying equations were developed at a macroscopic level using averaged parameters and variables. A total gas pressure equation was introduced to account for the internal vapor generation in microwave drying.

A three-equation drying model was developed that can account for the transport of free water, bound water, and vapor in a hygroscopic porous material. The scaling analysis was used to simplify the drying equations. Bound water diffusivity and the capillary pressure were found to be negligible.

The model developed provided a comprehensive solution for the heat and mass transfer in microwave drying. It can be used for hygroscopic and nonhygroscopic media. It can also be used for microwave heating.