Numerical Analysis On Freeze Drying Of Initially Porous Material Frozen From Aqueous Solution

Freeze drying plays an irreplaceable role in dehydration industry. Some special foodstuffs, pharmaceuticals, and biological materials, which may not be heated even to moderate temperatures in conventional drying, have to be freeze dried for end use and storage. Major problem of freeze drying, however, is the high energy consumption, which has failed to be effectively solved so far. Drying industry concerns about the product quality and energy consumption of the process. Freeze drying produces the highest quality of final products of all current drying methods. Therefore, reducing the drying time to improve the energy efficiency turns to be the study key point.Freeze drying of initially porous material frozen from aqueous solution was proposed in the present study aiming at improving the process economy. Based on the mechanisms of transmission process and porous media theory, mass and heat flux equations inside the porous material were given, respectively, combining with Whitaker’s volume-average theory. ID and2D governing equations of heat and mass transfer on freeze drying with hygroscopic effect were formulated upon a new adsorption-desorption relationship proposed. The finite volume method with the fully implicit scheme was adopted for discrimination of governing equations. Mannitol, a typical pharmaceutical excipient, was selected as the solute in aqueous solution to be dried. Spherical and cylindrical samples were used in the simulation.Results show that the idea proposed is feasible and the drying time decreases with the increase in the initial porosity, εo(l-So). There exists the shortest drying time corresponding to about0.30~0.35of an initial saturation for a spherical sample. Heat and mass transfer mechanisms were analyzed and the drying rate-controlling factor was discussed according to the profiles of temperature and saturation. Examination on variations of effective mass diffusivity, Ks and effective heat conductivity, λ+KtAH shows that the drying rate-controlling factor changes from mass transfer to heat transfer at the later stage of drying with increase in the instantaneous porosity, so(1-S).