| Freeze concentration has been identified as an energy efficient and green process,which is widely applied in food and chemical industry. Although the ice crystals obtained by freeze crystallization route are relatively, there are so many problems such as small size,nonuniform size distribution and so on in industry. So that the ice crystals are unable to meet the market needs. In this study, we conducted the experimental and theoretical systematic research on the thermodynamics and kinetics for the ice crystals in potassium nitrate aqueous solutions. In addition, investigated the effect of various of factors on the size of ice crystals during the freeze crystallization.Firstly, the thermodynamics of ice crystals in potassium nitrate aqueous solutions was investigated. The determination method of freezing point was developed and verified, the freezing point data in potassium nitrate aqueous solutions were determined by the experimental methods. The results showed that the freezing point data was fitted by Extend UNIQUAC model, the temperature of the freezing point is inversely proportional to the radius of ions with the presence of impurities. Meanwhile, the metastable zone of ice crystals was monitored by visual method and the effect of cooling rate and the species of impurities were investigated. The width of the metastable zone increase with increase in valence of cations and decrease in valence of anions. The simulation model of metastable zone width is derived by means of the classical nucleation theory. The calculated results agree well with the experimental data, and the nucleation order of ice crystal in potassium nitrate aqueous solutions was estimated to be 1.46.Secondly, the crystallization kinetics of ice crystals in potassium nitrate aqueous solutions was investigated by a batch crystallizer. Crystallization kinetic equations(MJ2 size-dependent model) were fitted with the experimental data by 1stOpt software by Marquardt and Universal Global Optimization regression method. Growth and nucleation rate equations were obtained. The main factors of crystal nucleation and growth are the supersaturation and suspension density.Finally, in the view of the above, the crystal size control of ice crystal was determinedly investigated in batch freezing crystallization process. The effects of operational parameters, such as cooling rate, stirring rate and seeding policy, on the crystal size distribution were studied. The results showed that these factors have a great influence on the crystal size distribution. |
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