Study On The Growth Kinetics Of Ice Crystal In Freeze Concentration Of Watermelon Juice And The Optimization And Enlargement Of Equipment

As watermelon juice was the material, the influencing factors of crystallization and the purity of ice crystal while freeze concentration were systematically studied, combined with the ice crystal growth characteristics of multilevel freeze concentration was studied to determine the end point of freeze concentration. The kinetic equation of multilevel freeze concentration was established for further understanding the quality of ice crystal growth and energy transmission. On the basis, pilot equipment was set, and the overall heat transfer coefficient and cold energy efficiency were analyzed for the improvement of the freeze concentration equipment. The brief introduction of whole research was as follow:1. As watermelon juice was the material, the effects of coolant temperature, juice concentration, ice-removed time and the speed of scraper rotational on crystallization and the purity of ice crystal while freeze concentration were studied. The results showed that ice crystal could be formatted while the differences in temperature between coolant temperature and freezing points of juice were achieved. The effect of coolant temperature was significant, the best temperature was at -15℃～-19℃. The higher concentration, the lower crystallization and purity of ice crystal were got. The ice crystal removed timely was helpful for efficiency improving, and the time was at 25 min～30 min. While the speed of scraper rotational was at 60 r/min to 105 r/min, that could be obtained more ice crystal and lower loss of juice.2. The freezing point curve and frozen rate curve were studied to determine the end point of freeze concentration. The multilevel freeze concentration was carried on, and the kinetic equation was be verified followed the principle of diffusion. The results showed that when the progression of freeze concentration was higher, theβand k got lower. That meant juice concentration got higher, the resistance of quality and energy of juice were higher, the growth rate of ice crystal was lower.The kinetic equation of first grade freeze concentration: The kinetic equation of second grade freeze concentration:The kinetic equation of third grade freeze concentration:3. The refrigeration system and the juice freezing system of freeze concentration were design for the pilot equipment, and the overall heat transfer coefficient and cold energy efficiency were analyzed. The results showed that the range of refrigerant flow was at 0.317 m~3/h~7.405 m~3/h, the calculated value and measured value of overall heat transfer coefficient K were 194.07~509.36 W/m~2·K and 196.16~516.26 W/m~2·K. The effects of heat transfer could be improved by increasing the range of refrigerant flow or shortening the distance of the inner tube and outer tube of the heat exchangers. When the refrigerant flow was 7.405 m~3/h, the mass of ice crystal could be reached at 31.095㎏/h and the cold energy efficiency could be reached at 78.1%.