Study On The Mass And Heat Transfer In Liquid Vacuum Evaporation Freezing Process

Freezing by vacuum evaporation is one of the most important ways to freeze the solution. The fundamental principle of freezing by vacuum evaporation is based upon that liquid is able to evaporate rapidly and the latent heat of phase change with the liquid's evaporation which lows the temperature of the liquid remained under the situation of vacuum until it is frozen by the vacuum evaporation. That is applied mostly in the process of manufacturing nm-powdery by chemical solution lyophilization. Compared with other ways of freezing, there are many advantages of freezing by vacuum evaporation including low costs, high rate of freezing, rapid controllable process, and keeping the chemical homogeneity of the solute in the solution away from being destroyed. This thesis makes an intensive study of the characteristics and mechanism of the heat and mass transfer during the process of freezing by vacuum evaporation.This thesis is, on the basis of a summary of the development of vacuum cooling home and abroad, practice of experiments with theory, gives a further study on the process of freezing by vacuum evaporation.During the research with experiments on the process of freezing by vacuum evaporation of solution, the experimental equipments were improved. The affiliation of the electronic balance for mass measurement, vacuum gauge for pressure measurement, and the thermocouple for temperature measurement increase the accuracy of measurement of experimental, and realize the rapid survey of quality, pressure and temperature. The ties of matter parameter of the experimental materials were measured and the weight, surface temperature of solution and pressure in vacuum tank were recorded. The variation of weight, temperature and pressure of the aqueous solution of copper sulphate, sodium chloride and pure water with different consistency in the whole process of FVE, boiling and freezing point during the experiment was analyzed with fixed amount. The curved line of variation of the weight, temperature and pressure in vacuum tank with time was described. The heat transmittal mode inside the solution is defined during the process of FVE. The influence of the ability of permanent gas and condensing chamber to seize vapor in the air in the whole process of FVE; the influence of the trapping ability of the cold trap to FVE; and the comparison between the time and energy consumption during FVE and arid stage after directly frozen.For the research on the theory of FVE, the lumped parameter model and macroscopical energy identical model were set up. The temperature-weight curved line was simulated to describe the variation of the weight with temperature. The numerical value by simulating two models in the boiling and freezing point was analyzed and compared. Simultaneously, the model of dynamics of gas flow was established, the curved line of variation with time of weight, temperature and pressure was simulated, the rate and amount of gas flow during FVE was calculated, and finally the method and theoretical basis to describe FVE of solution were summarized and concluded.Comparing with each others, the experimental curved line fits well with the simulation one. It analyzed the mechanism in the process of FVE, and provided convenience for other solutions' extensive industrial production in the field of FVE.