KINETICS OF MICROWAVE DRYING OF A FREE-FLOWING ORGANIC MATERIAL V. A. Kalender’yan, I. L. Boshkova, and N. V. Volgusheva UDC 664.723.047 The kinetics of drying of a dense buckwheat layer in a microwave electromagnetic field of frequency 2.45 GHz has been investigated for different amounts of the material charged into a working chamber. Analysis of the kinetics curves has shown that the drying of the material studied is divided into the periods of heating, drying with a constant rate, and drying with a decreasing rate. The influence of the power supplied as well as the mass and dimensions of a sample on the rate of its drying has been investigated and a formula for calculating this rate has been obtained. It has been established that, in the process of drying of a disperse material, the amount of microwave energy converted into heat energy depends not only on the mass of a sam- ple, but also on the thickness and area of its surface layer. Generalized equations for calculating the moisture content in a layer of a free-flowing material and its temperature have been obtained. The microwave drying of free-flowing materials possessing the properties of polar dielectrics, among which are agricultural products, holds considerable promise because it provides the following advantages as compared to other forms of drying of these materials: a heat energy is generated directly in the material being dried and therefore is used rationally; the drying is safe for the environment, is realized at a high rate [1], and provides a high quality of the product obtained [2]. It is known that it is not economically advantageous to completely replace convective drying by microwave drying [3]; however, microwave drying can be used to advantage at the finite stage of the process, where the conditions of heat and mass transfer become inappropriate for convective drying. It has been established that it is well to use microwave drying for materials with a moisture content changing from 0.18 to 0.13 kg/kg in the process of drying [3], which corresponds to the moisture-content range of cereals. At the same time, it is difficult to realize a drying process in a microwave field because the main mechanisms of heat and mass transfer are not clearly under- stood. As the investigation object, we used buckwheat because this cereal is produced in large volumes, is in popular demand in the world market, and should match the high quality requirements [4]. The aim of the present work is to investigate the kinetics of drying of a dense buckwheat layer in a microwave field in different regimes and under dif- ferent geometric conditions. Investigations were carried out with the use of a microwave apparatus including a working chamber of vol- ume 20 dm 3 supplied with power from a magnetron with a generation frequency of 2.45 GHz and an output power of 80–400 W. The time dependences of the integral moisture content and temperature of the material studied were deter- mined with the use of experimental cells representing cylindrical reservoirs, the side walls and bottom of which were made in the form of a net transparent to radio waves. The cells were installed at a height of 0.07 m from the base of the chamber, which allowed the vapor formed to emerge from them in all directions. We investigated buckwheat sam- ples with an initial moisture content of 0.2–0.25 kg/kg, an initial temperature of 17–26 o C, and a mass of 0.05–1.2 kg. The thickness of the buckwheat layer in a cell was 0.008–0.048 m, the diameter of this layer was 0.11–0.276 m, and the area of its open surface (from which vapor was removed) was 8⋅10 -3 –94⋅10 -3 m 2 . A cell with the material studied was placed into the microwave chamber. After a certain time, varying from 30 sec to 5 min depending on the power supplied, the amount of evaporated water was determined by the gravimetric method and the moisture content of the buckwheat was calculated. The temperature of the buckwheat was measured with the use of copper-constantan thermo- couples that were placed in the layer immediately after the shutdown of the magnetron. Journal of Engineering Physics and Thermophysics, Vol. 79, No. 3, 2006 Odessa State Academy of Cold, 1/3 Dvoryanskaya Str., Odessa, 65082, Ukraine; email: ira_boshkova@mail.ru. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 3, pp. 123–127, May–June, 2006. Original article submit- ted November 30, 2004; revision submitted June 10, 2005. 1062-0125/06/7903-0547 2006 Springer Science+Business Media, Inc. 547