Thermodynamic Analysis Of Porous Materials Vacuum Freeze-Drying Process

Vacuum freeze-drying(lyophilization)technology has been widely used in food,medicine,biochemical engineering and other fields because of its outstanding advantages.There is a heat and moisture cross process and a water phase transition process in the material vacuum freeze-drying system.However,for the “reverse transfer phenomenon” of moisture content from low moisture content to high moisture content and heat from low temperature to high temperature transfer during the heat and moisture cross process,and the energy conversion between heat and mass transfer processes during the water phase transition process,the current material drying research did not given a reasonable explanation.This paper use the generalized field synergy principle and the thermodynamic coupling mechanism to analyze the vacuum freeze-drying process,and reveal the implied thermodynamic mechanism of the drying process.Firstly,the vacuum freeze-drying process of porous materials was analyzed,include three stages: pre-treatment,vacuum freeze-drying and post-treatment,the influence factors mainly include material pre-treatment and material drying environment.The thermodynamic analysis of the core drying stage(pre-freezing,sublimation-drying and desorption-drying stage)of porous materials shows that the essence of material drying is the phase transition of water,and the phase transition of water include heat and mass transfer processes,and there is a heat and moisture cross effect between heat and mass transfer processes.Then,start from the study of heat and mass transfer processes in porosity materials under low temperature and pressure,the heat and moisture cross effect in the vacuum freeze-drying process can promote the heat and mass transfer processes,and the heat and mass transfer processes promote and enhance each other,which conforms to the generalized field synergy principle.The heat and mass transfer model of vacuum freeze-drying in different stages were established when consider the porosity and the heat and moisture cross effect,based on the model,the temperature and moisture changes in the material during the drying process were discussed.After the temperature of the whole material decreases rapidly,it slowly rises to the drying environment temperature.After the moisture content decreases rapidly,it continues to go through a long period of slow decline.Secondly,the thermodynamic coupling analysis of the water phase transition process shows that the spontaneous heat transfer process in the water phase transition exothermic process drives the non-spontaneous mass transfer process of chemical potential increase,and the spontaneous mass transfer process of chemical potential decrease in the water phasetransition endothermic process drives the non-spontaneous heat transfer process.Based on the generalized Carnot theorem,the energy change in the process of water phase transition was analyzed,which include the heat transfer and the generalized do work process besides the heat transfer process.The liquefaction of water vapor into water,the condensation of water into ice(frost)and the condensation of water vapor into ice(frost)contain the characteristics of the spontaneous heat transfer process drives the non-spontaneous generalized do work process,which conforms to the generalized Carnot heat engine model;while ice(frost)melts into water,water vaporizes into water vapor,and ice(frost)sublimes into water vapor contain the characteristics of the spontaneous generalized do work process drives the non-spontaneous heat transfer process,which conforms to the generalized Carnot heat pump model.Finally,based on the generalized Carnot heat engine model,the cold surface frost formation process of the vapor condenser was analyzed.The heat transfer process of positive entropy production rate in the phase transition process drives the generalized do work process of negative entropy production rate,and the main factors affect frost formation are temperature difference and heat transfer intensity.The sublimation process of ice crystals was analyzed based on the generalized Carnot heat pump model.The generalized do work process of positive entropy production rate in the phase transition process drives the heat transfer process of negative entropy production rate,and the key to the ice crystal sublimation is the generalized do work process.