ABSTRACT
Microwave drying has several advantages over conventional hot air drying, such as higher drying rate, minimal heating at locations with less water thus reducing overheating of locations where, heating is not required. However, for microwave drying to be more useful at the industrial level, it needs information on moisture diffusion models that could describe the process accurately. The diffusion coefficient of a food is material property and its value depends upon the conditions within the material. Effective moisture diffusivity describes all possible mechanisms of moisture movement within the foods, such as liquid diffusion, vapor diffusion, surface diffusion, capillary flow, and hydrodynamic flow (Kim and Bhowmik, 1995). Availability of such correlations and models, verified by experimental data, will enable engineers and operators to provide optimum solutions to aspects of drying operations such as energy use, operating conditions, process control, and without undertaking experimental trials on the system [9]. In particular, thin-layer Equations contribute to the understanding of the heat and mass transfer phenomena and computer simulations for designing new processes and improving existing commercial operations [12]. Thin-layer drying models can be categorized as theoretical, semi-theoretical, and empirical [5]. Models within the latter two categories consider only external resistance to moisture transfer [15] and neglect the effect of variation in sample temperature on the drying process [6]. The models are
generally derived by simplifying general series solutions of Fick’s second law and are only valid within the drying conditions for which they have been developed. However, they require short time, as compared with theoretical thin-layer equations, and do not require assumptions regarding sample geometry, mass diffusivity, and conductivity. Such models include the Page [1], Henderson and Pabis [2], two-term Sharaf-Eldeen, Blaisdell, and Hamdy [4], approximation of diffusion (Yaldiz and Ertekin [13]), and Midilli, Kucuk, and Yapar Equations [14].
