Probe into the adaptability of local equilibrium/non-equilibrium assumptions in simulating freeze-drying of porous media
A multiphase transport model was formulated to explore the adaptability of local thermal and mass equilibrium/non-equilibrium (LTE/LTNE and LME/LMNE) assumptions in simulating freeze-drying of porous media. Saturated and unsaturated samples with the same initial mass and moisture content but different initial saturations were used in freeze-drying experiments and numerical simulations. Four pairwise combinations between LTE/LTNE and LME/LMNE were tested using the commercial software of COMSOL Multiphysics based on the finite element method. Results showed that the LTE-LMNE assumption achieved excellent agreements between numerical and experimental drying curves with RMSEs being 1.81% for the saturated sample and all less than 1.00% for the unsaturated samples. Based on distributions of saturation and temperature, and variations of effective mass diffusivity and thermal conductivity, the rate-controlling factor was determined to be mass transfer for the saturated material and heat transfer for the unsaturated one. Convective heat transfer played an unimportant role in freeze-drying. The LTE-LMNE based model provided satisfactory predictive capabilities under different conditions. Appropriately increasing the operating temperature could boost the drying rate. Changing the chamber pressure had insignificant effects with drying time differences only below 3%. The smaller the initial saturation, the shorter the drying time. The initially unsaturated frozen material with preformed pores significantly enhanced the freeze-drying process and improved the process economy.
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