Study on the response law between cooling rate and temperature with process parameters during vacuum freezing of liquid

Vacuum freezing has high freezing efficiency, fast cooling rate, and no special cold source is required, it can be easily carried out on general vacuum freeze-drying equipment. Nevertheless, it is difficult to accurately control the freezing temperature and cooling rate during vacuum freezing on traditional freeze-drying equipment. In order to control the freezing temperature and cooling rate, the mathematical model of vacuum freezing process is established by using the basic theory of phase change heat and mass transfer and the gas state equilibrium equation. The iterative method is used to calculate and simulate the relationship between the three controllable process parameters on the general freeze-drying equipment: evaporation area, pumping speed and liquid thickness with the freezing temperature and cooling rate. The experimental results obtained using the experimental equipment are largely consistent with the theoretical results. It provides a theoretical foundation for controlling the freezing temperature and cooling rate during the vacuum freezing process. •Vacuum freezing has high freezing efficiency, fast cooling rate, and no special cold source is required.•The mathematical model of heat and mass transfer during vacuum freezing process was developed.•The relationships between cooling rate and temperature with pumping speed, liquid thickness and evaporation area were obtained.•Using vacuum freezing method, the maximum cooling rate is about 20℃/min and the final freezing temperature can reach -42℃.