Simulation Of The Phase Change Heat Transfer During Freeze Drying

Freeze drying technology has been widely studied and applied in the field of biomedicine,complex phase change heat transfer process are involved in all stages of freeze drying,which has a serious effect on the quality of freeze-dried products,so it is necessary to study the phase change heat transfer characteristics during the freeze drying process of biological materials.Based on the thermophysical mechanism of lyophilization,a simulation model of freeze drying preservation(including freezing and sublimation drying and desorption drying)of biomaterial was established,the numerical simulation method of heat and mass transfer process,crystallization,sublimation during the drying stage was studied.According to the simulation results,the effects of freeze drying conditions on drying time and residual water content of biomaterials were analyzed in detail.The concrete work includes:(1)The principle of phase change heat transfer and simulation method in freezing process during the freeze-dried process was studied.According to the heat transfer control equation and nonisothermal crystallization kinetics equation,the freezing process of the material was modeled by using the finite element method,the temperature distribution and variation in the material during the cooling process are simulated.The distribution and growth of ice crystals in the cell suspension were predicted during the freezing process.Based on this method,the crystallization inside the solution at with different heights is predicted and the suitable loading heights is determined.(2)The principle of heat and mass transfer and the simulation method in the sublimation drying stage during freeze-dried process was studied.Based on the multidimensional dynamic mathematical model,the heat and mass transfer and phase transition process during the sublimation drying stage was modeled by using a combination of finite element method and dynamic grid technology,the dynamic change process of the sublimation interface,temperature,pressure,and the bound water content in the material of the sublimation drying stage was simulated.The influence of freeze-drying conditions on the heat and mass transfer during the sublimation drying stage was analyzed,the effect of drying conditions on sublimation drying time and the bound water content were obtained by single factor control variable method.(3)The principle of heat and mass transfer and simulation method in the desorption drying stage was studied.Based on the multi-dimensional dynamic mathematical model,the finite element method is used to model the desorption drying stage.The changes of temperature,pressure and water content in the material in the desorption drying stage were studied.Analysis of residual water residue in different drying conditions,And predict the time needed to finish the drying process.The influence of drying conditions on the heat and mass transfer process of biomaterials in the whole drying process was analyzed,A variety of red blood cell freeze-drying procedures were set up,and the experimental results under the same conditions were compared with the simulation results,which had good consistency.Based on this research method,the dynamic distribution of the temperature,heat flux,water vapor pressure and the bound water are visualized during the whole process of freeze-drying.The influence of several operation parameters(sample volume,drying chamber pressure and heating plate temperature)on freeze-drying process can be quantitatively analyzed,the proposed method provides an effective tool for predicting and optimizing the process of freeze-drying biological materials.