| In this dissertation, a novel approach - dielectric-material-assisted microwave freeze drying was examined theoretically and experimentally. A dielectric sphere or bar is first frozen with the solution, and then the frozen material is freeze dried with microwave heating. The dielectric material absorbs the microwave energy first and conducts heat to its surrounding in a controlled manner. It has advantages of high product quality, increased drying rate and easy operation. This idea is the first of its kind.; Two mathematical models of coupled heat and mass transfer were developed based on Luikov' system of equations and Whitaker's theory. One considers the sublimation-reverse sublimation effect in the unsaturated region, using skim milk and lactose solution as materials, without accounting bound water removal as a separated drying stage. The other one considers the hygroscopic effect in the secondary drying stage using again skim milk as a representative material.; A laboratory-scale freeze drying apparatus with microwave heating ability was designed, constructed and assembled. Experimental studies were performed for conventional freeze drying and dielectric-material-assisted (SiC) microwave freeze drying of mannitol. More than 20% of drying time was saved for dielectric-material-assisted microwave freeze drying compared to conventional freeze drying under the operating conditions tested. Comparisons between experimental results and model predictions indicate that the hygroscopic relation used in the original model overestimated the experimental data. In fact, the coefficient to account for the hygroscopic effect in freeze drying of mannitol solution should be the square root of moisture saturation, i.e., ratio of equilibrium vapor pressure in adsorption to that in thermodynamics: f(S)= S1/2. Theoretical predictions of the improved model showed good agreements with experimental results. |
