Experimental Study On The Effect Of Depressurization Process On The Freezing Of Supercooled Water And Aqueous Solution

Freeze drying has been widely used in pharmaceutical industry, food industry and chemical engineering due to its unique advantages. Controlling the supercooling degree of the prefreezing stage has important influences on the homogeneity of the freeze dried products and the drying rate of the later secondary drying stage. Now nucleation by depressurization appears as a novel method to control the supercooling degree, but its fundamental aspect still lacks. Therefore, this thesis carried out theoretical analysis and experimental research on ice nucleation of supercooled water or aqueous solution by depressurization. The main contents are listed as follows:1) physical model was established for the depressurization process of water in vial with COMSOL software. Numerical simulation was done to study the effects of precooling temperature, depressurization velocity, end pressure and end pressure duration on the temporal change of temperature average and distribution. The effect of depressurization velocity was not significant when it was in the range of 5-20kPa/s. The temperature average and the lowest temperature of the water sample became smaller as the end pressure decreased from 400Pa to 100Pa. In the range studied, i.e. precooling temperature -5～-7℃, end pressure 100-400Pa, as end pressure drops, the lower precooling temperature, the faster the decline in the temperature average and the lowest temperature of the water sample。The temperature average and the lowest temperature of the water sample became smaller as the end pressure duration prolonged from 200s to 400s.2) a test apparatus was set up. The ice nucleation phenomenon at normal pressure was first studied for water sample in vial, then the effects of depressurization velocity and end pressure were further investigated. It was found that too quick depressurization (velocity≥40kPa/s) would cause overshoot; the depressurization velocity had minor impact on ice nucleation in the range of 5-17.5kPa/s; lower end pressure made the sample nucleate at higher average temperature. The observed impacts of depressurization velocity and end pressure on ice nucleation of supercooled water were consistent with theoretical prediction.3) the effects of depressurization velocity and end pressure on ice nucleation of sucrose solution in vial were preliminarily studied. When the solution was precooled to -4℃, the depressurization velocity was controlled at 17.5kPa/s, and the end pressure was set at 340Pa, two kinds of phenomenon were observed. One is that all samples nucleated at the same time in ten minutes, the other is that each sample nucleated at different time and the elapsed time for most of samples to nucleate was longer than10 minutes.