Optimal Control Policy of the Microwave Primary Freeze Drying of Random Solids

Abstract

The optimal strategy of microwave energy input in freeze drying is proposed to keep the temperature of the material external surface, ice core, and sublimation interface as high as possible. This should ensure maximum process efficiency without violating the set constraints. The safety factors are introduced on electric field strength to avoid corona discharge. The microwave power is considered either constant or time variable during the process. A mathematical model of the microwave freeze-drying process is formulated with time-varying transport and dielectric properties of the frozen and dried layers. This model takes into account unknown a priori sublimation front temperature T S and mass concentration of water vapor C S at moving ice front. Comparative analysis of the various process operation conditions is performed to determine the optimal control policy for microwave freeze drying. It is demonstrated that applying an optimal control strategy for limiting process conditions significantly reduces the total microwave freeze-drying time in the primary stage.