Numerical Simulation Of Equilibrium And Non-equilibrium Phenomena In Freeze Drying Of Porous Media

Freeze-drying is a complex multi-phase transport process of mass-heat coupling.It is still a challenging task to accurately predict the transport behavior of wet content in wet porous media.Based on the local thermal nonequilibrium and/or local mass nonequilibrium hypothesis,three mathematical models of multiphase transport for initial saturated(S0=1.00)/unsaturated(S0=0.28)porous media freeze-drying with Local mass equilibration-local heat balance(LME-LTE),local mass nonequilibration-local heat balance(LMNE-LTE)and local mass nonequilibration-local heat nonbalance(LMNE-LTNE)are established respectively.Using COMSOL Multiphysics software platform based on finite element method,the nonlinear and unsteady PDE governing equations of momentum,heat and mass are numerically solved.The simulation results were compared with the literature data published by the research group to verify the accuracy of equilibrium and non-equilibrium phenomena in the transport process for the prediction results of the model.The results are as follows:The numerical simulation results reproduce the freeze-drying process of mannitol aqueous solution under the experimental conditions,and the local mass nonequilibrium-local heat balance drying curve obtained by the simulation is in good agreement with the experimental results.The characteristics of porous media significantly affect the LMNE effect and LTNE effect.In materials with low initial saturation,mass non-equilibrium effect is more close to the actual phenomenon.In the initial saturated material,the difference between the thermal nonequilibrium hypothesis and the thermal equilibrium hypothesis is more obvious.Therefore,the local mass balance-local heat balance(LME-LTE)model can obtain satisfactory simulation results.The model is optimized by the mass nonequilibrium hypothesis compared with the mass balance hypothesis,and the local mass nonequilibrium--local heat balance(LMNE-LTE)model further improves the accuracy of the simulation results.The local heat nonequilibrium is not significant in freeze drying,and the local mass nonequilibrium--local heat nonequilibrium(LMNE-LTNE)model has low applicability under the existing calculation conditions.The distribution of temperature and saturation is closely related.Freeze-drying is a transfer process of heat and mass coupling.Under typical operating conditions,the relative drying rate of the initial unsaturated material is higher,and the prefabricated pores are beneficial to accelerate the drying rate.At the same time,for the characteristic variables of drying process,the influencing factors are analyzed,which are related to the initial saturation value,pore diameter and saturation change.From the order of mass transfer velocity and Reynolds number,it can be seen that convective heat transfer takes a secondary position.The factor that controls the drying rate of unsaturated material is heat transfer.The factor controlling the drying rate of saturated material is mass transfer.