Numerical Verification And Experimental Comparision On Freeze Drying Of Initially Unsaturated Porous Material

Compared with many other drying methods, freeze drying technology displays various advantages. It has been widely applied in the field of medical health care, food industry and new materials in recent years. Especially for some special materials, freeze drying is their only treatment method. Due to its excellent performance, it has been an important technology in the21century. However, the cost of good quality is high energy consumption. Despite its numerous advantages, the disadvantage of high energy consumption is a global challenge, and it has not been solved effectively, which limits its further application. Therefore, reducing drying time, lowering energy cost has been the key to promote widely industrial application of freeze drying technology.The idea of freeze drying of initially unsaturated media was adopted. Heat and mass transfer equations within porous material was derived based on classical transfer theory and combined with Whitaker’s volume-average theory. Finite volume method with fully implicit scheme was adopted for the numerical solution of sphere and cylinder material’s freeze drying. Mannitol was selected as primary solute in solution. Simulation of material’s freeze drying with different initial saturation was conducted under the same quality and moisture content. Process mechanism was analysed and simulation and experiment results were compared.Results show that, freeze drying of initially unsaturated material could effectively shorten drying time and lower energy cost. Temperature and saturation distribution can be obtained during drying simulation of different material under the same operation. The simulation result of sphere material shows that drying time shorer with the decrease of initial saturation in large range, which indicates drying process is transferred from mass transfer control to heat transfer control. The result of cylinder material shows that drying time shorer with the decrease of initial saturation, and naturally occurred sublimation interface exist in radial and circumferential direction. Experiment results show that, the idea is feasible in sugar alcohol’s freeze drying, and drying time decreases with the increase of temperature. Numerical simulation of the experiment process was carried, drying time between simulation and experiment result was almost unanimous, and for such material geometry, radial direction’s sublimation is dominant.