Experimental And Numerical Simulation Studies On Heat And Mass Transfer Processes In Freeze-drying

Vacuum freeze-drying technology(freeze-drying for short)is a drying technology in which the material is cooled below the eutectic point temperature to completely solidify the free water in the material into ice,and then heated under the state of near vacuum to directly sublimate the ice crystal into water vapor.Since lyophilized materials have the characteristics of thorough drying,good rehydration performance and porous media,lyophilized materials have been widely used in many fields such as medicine,food,chemical industry,materials,aerospace and so on.However,the development of freeze-drying technology has been restricted by the extremely complex heat and mass transfer process in the freeze-drying process and the large energy consumption of the freeze-drying equipment.Therefore,the research on the main heat and mass transfer process in freeze-drying is of great significance for the development of freeze-drying technology.In this paper,carrots were used as experimental materials to carry out experimental research on the pre-freezing process and drying process in the freeze-drying process.ANSYS FLUENT15.0 software was used to simulate the phase change process and the change of temperature field inside the material in the pre-freezing process,and the cooling process at different pre-freezing temperatures was studied by using the pre-freezing model.COMSOL Multiphysics software was used to simulate the material internal phase change process,temperature field and pressure field in the sublimation drying process,and the sublimation model was used to study the influence of various factors on the sublimation drying efficiency.The main research contents and results are as follows:The freeze-drying experiment of carrot showed that the pre-freezing temperature and blanching pretreatment could affect the porosity,pore size and pore size distribution of the freeze-dried material.The lower the pre-freezing temperature is,the higher the porosity of the freeze-dried products will be.At the same pre-freezing temperature,the porosity of the freeze-dried products will be higher for the blanched materials.Precool stage in the process of freeze drying experiments and numerical simulation study found: in the process of the precool has the greatest effect in the cooling process of thermal effect of material,the material temperature at the centre of the downward trend in general can be divided into three stages: cooling stage,phase changing stage and freezing stage,combined with the experimental data fitting out of the pore diameter of cooling rate and cooling stage negative exponent of power function relationship.The experiment and numerical simulation of sublimation drying in freeze-drying process have found the variation of temperature,pressure and sublimation interface in the material.Using Matlab to fit the numerical simulation data,the results show that the sublimation time and the initial temperature of the freeze-dried bin have an inversely proportional function relationship,the sublimation time and the pressure of the freeze-dried bin have a Fourier series relationship,the sublimation time and material height have a linear relationship,the sublimation time and shelf temperature also have an inversely proportional function relationship.Finally,the cause of uneven frosting in cold trap was found through two-dimensional numerical simulation,and the improvement scheme was put forward to improve the frosting uniformity and temperature uniformity of cold trap.