Drying is an important unit operation. Freeze drying is the most expensive process of all drying operations. To improve the process economics by reducing drying time is one of the key research topics. Microwave heating has proved to enhance the freeze drying rate. However, when the dielectric loss factor (a property directly affects the adsorption of microwave energy) of the solid material is low, the effect of microwave heating is insignificant. In addition, microwave heating may cause collapse of the solid matrix if there is an uncontrolled non-uniform microwave adsorption because of the 4000 times higher loss factor of water than ice....[ Read more ]

Drying is an important unit operation. Freeze drying is the most expensive process of all drying operations. To improve the process economics by reducing drying time is one of the key research topics. Microwave heating has proved to enhance the freeze drying rate. However, when the dielectric loss factor (a property directly affects the adsorption of microwave energy) of the solid material is low, the effect of microwave heating is insignificant. In addition, microwave heating may cause collapse of the solid matrix if there is an uncontrolled non-uniform microwave adsorption because of the 4000 times higher loss factor of water than ice.

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. The hygroscopic effect was quantified by taking the vapor pressure of water as the product of the moisture saturation and vapor pressure of pure water. The models were solved numerically by using the finite-deference technique with internal movable boundaries. Simulation results show that the dielectric material can significantly enhance microwave freeze drying of aqueous solutions. The drying time can be more than 30% shorter than those of conventional freeze drying and/or ordinary microwave freeze drying. Based on profiles of the temperature, ice saturation, vapor concentration and pressure, the mechanisms of heat and mass transfer inside the material were analyzed, and the drying rate-controlling factor was discussed.

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)=S^1/2. Theoretical predictions of the improved model showed good agreements with experimental results.