Chemical Engineering Science, Vol.53, No.18, 3189-3197, 1998
Numerical study on sublimation-condensation phenomena during microwave freeze drying
A sublimation-condensation model is developed for freeze drying of unsaturated porous media. The governing equations describing the drying process are solved numerically with variable time step finite-difference method. The sublimation-condensation phenomena during microwave freeze drying are studied numerically for different operating conditions including electric field strength, sample thickness, and vacuum pressure. The results show that the sublimation-condensation effects depend on different drying conditions. For larger electric field strength, vapor transfer in sublimation-condensation region is more intensive and larger part of the vapor is sublimated from this region, so the effect of sublimation-condensation region on drying time is more significant. Too large electric field strength is not necessary for microwave freeze drying. In case of larger sample thickness, the saturation change of the sublimation-condensation region is larger, and the effect of sublimation-condensation region on drying time is more significant. The effect of vacuum pressure on drying time is not so significant in the observed range.