Multiphase transport modeling for freeze-drying of aqueous material frozen with prebuilt porosity

A multiphase transport model based on the local mass non-equilibrium assumption was formulated to verify the effect of frozen material with prebuilt porosity on freeze-drying of liquids. An error function form of adsorption-desorption equilibrium relationship was proposed to describe the hygroscopic effect of moist porous media. Simulation results showed excellent agreements with the experimentally measured drying curves for initially saturated (S₀ = 1.00 or zero porosity) and unsaturated (S₀ = 0.28 or 0.69 of porosity) frozen materials. The unified equilibrium relation can be expressed as a polynomial form with a fixed parameter. Almost the same total radiation heat consumptions were attained for the initially saturated and unsaturated frozen samples. The initially prebuilt porosity with larger internal surface area of the unsaturated material can indeed decrease mass transfer resistance and increase energy efficiency of freeze-drying. Volumetric sublimation/desorption was achieved numerically in consistence with experimental findings through the analyses of saturation, temperature and mass source profiles. The mathematical model also provided satisfactory prediction capabilities of the ambient temperature effects on freeze-drying.