硝酸钾溶液冷冻浓缩过程冰晶生长机理的研究

发布时间：2018-05-06 16:30

本文选题：硝酸钾水溶液 + 冰晶　；参考：《天津科技大学》2017年硕士论文

【摘要】：冷冻浓缩是一种绿色、节能的结晶方式,被广泛地应用于食品和化工等领域。虽然由冷冻浓缩得到冰晶的生产路线比较成熟,但是目前国内由冷冻浓缩技术得到的冰晶存在晶体粒度小且粒度分布不均匀等问题,不能满足市场需要。本文系统的对硝酸钾水溶液冰晶的结晶热力学和动力学进行了实验和理论研究,同时考察了各种因素对硝酸钾水溶液冰晶粒度的影响。首先,对硝酸钾水溶液的冰晶进行结晶热力学的研究。实验建立并验证了测定冰点温度的方法,并通过实验建立的测定冰点温度的方法测定了硝酸钾水溶液的冰点数据,实验结果表明,硝酸钾水溶液的冰点数据较好的符合Extend-UNIQUAC模型,向硝酸钾水溶液中掺加杂质,杂质离子半径越小,其冰点温度越低。同时用直接观察法测定了硝酸钾水溶液的介稳区,并考察了杂质离子、降温速率对介稳区宽度的影响。结果显示,阳离子杂质的价态越高,介稳区宽度越宽,阴离子反之。并利用基于经典成核理论所推导的介稳区宽度模型方程对试验数据进行模拟计算,模拟结果与实验值符合较好,计算得到硝酸钾水溶液的冰晶成核级数为1.46。其次,对硝酸钾水溶液的冰晶进行动力学研究。采用间歇动态法测定了硝酸钾水溶液冰晶的结晶动力学,实验结果表明,硝酸钾水溶液冰晶的生长与粒度相关,采用MJ2模型,通过矩量变换法,使用1stOpt软件中的麦夸特法(Levenberg-Marquardt)和通用全局优化法对实验数据进行了拟合,得到了硝酸钾水溶液冰晶结晶的成核与生长速率的动力学方程,从拟合的结果可以看出,过饱和度AC与悬浮密度MT是影响晶体成核与生长的主要因素。最后,结合硝酸钾水溶液冰晶的结晶热力学和动力学分析结果,对硝酸钾水溶液冰晶的粒度控制进行了研究。实验考察了降温速率、搅拌速率及晶种添加量对硝酸钾水溶液的冰晶的粒度及粒度分布的影响。实验结果表明:降温速率、搅拌速率、晶种等因素对产品的粒度分布有着重要的作用。
[Abstract]:Freezing concentration is a green and energy-saving crystallization method, which is widely used in food and chemical industry. Although the production route of freezing concentrated ice crystals is relatively mature, the current domestic ice crystals obtained by freezing concentration technology have some problems such as small crystal size and uneven distribution of grain size, which can not meet the needs of the market. In this paper, the crystallization thermodynamics and kinetics of potassium nitrate aqueous solution ice crystals have been studied systematically, and the effects of various factors on the grain size of potassium nitrate aqueous solution ice crystals have also been investigated. Firstly, the crystallization thermodynamics of ice crystals in potassium nitrate aqueous solution was studied. The method of measuring freezing point temperature was established and verified, and the freezing point data of potassium nitrate aqueous solution were determined by the method established by experiment. The experimental results showed that the freezing point data of potassium nitrate aqueous solution were in good agreement with Extend-UNIQUAC model. The smaller the impurity radius is, the lower the freezing temperature is. At the same time, the metastable zone of potassium nitrate aqueous solution was determined by direct observation, and the influence of impurity ion and cooling rate on the width of metastable zone was investigated. The results show that the higher the valence state of cationic impurity is, the wider the width of metastable region is, and the other is anion. The experimental data are simulated and calculated by using the metastable zone width model equation derived from the classical nucleation theory. The simulation results are in good agreement with the experimental data, and the nucleation order of ice crystals in potassium nitrate aqueous solution is calculated to be 1.46. Secondly, the kinetics of ice crystals in potassium nitrate aqueous solution was studied. The crystallization kinetics of ice crystals in potassium nitrate aqueous solution was determined by batch dynamic method. The experimental results show that the growth of ice crystals in potassium nitrate aqueous solution is related to the grain size. The MJ2 model and the moment transformation method are used to determine the crystallization kinetics of ice crystals in potassium nitrate aqueous solution. Using Levenberg-Marquardt method in 1stOpt software and general global optimization method, the experimental data were fitted and the kinetic equations of nucleation and growth rate of ice crystal crystallization of potassium nitrate aqueous solution were obtained. Supersaturation AC and suspended density MT are the main factors affecting the nucleation and growth of crystals. Finally, the particle size control of potassium nitrate aqueous solution ice crystal was studied by combining the results of thermodynamics and kinetics analysis. The effects of cooling rate, stirring rate and seed content on the grain size and distribution of ice crystals in potassium nitrate aqueous solution were investigated. The results show that the cooling rate, stirring rate, seed and other factors play an important role in the particle size distribution of the product.
【学位授予单位】：天津科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ131.13;O78

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