橙汁冷冻浓缩动力学模型的研究

Kinetic model of freeze concentration of orange juice

为了降低果汁在冷冻浓缩过程中由于冰晶夹带造成的的损失并提高浓缩速度,需要获得冷冻浓缩过程中冰晶的增长规律。以橙汁为原料,采用二级冷冻浓缩方法,利用冰与水溶液之间的固液相平衡的原理,将水分从液态转变为固态的冰,使橙汁的浓度从13°Bx浓缩到34°Bx。该试验中,测定了橙汁的浓度及其对应的冻结点温度的关系,结果表明:在试验范围内,冻结点与浓度呈线性关系并随浓度的升高而降低。该文还研究了在冰晶中所夹带的可溶性固形物与橙汁浓度的关系,所获得分配系数K表明,冰晶中所夹带的可溶性固形物随浓度的提高而增加。根据Fick扩散方程式建立了冰晶增长动力学模型,模型显示冰晶增长速率与冰晶质量成正比,同时也受到最大冰晶量的抑制,采用Powell法,对试验数据回归模型参数、采用龙格—库塔法进行数值积分求得模型解,计算结果与试验结果吻合,该模型有助于为建立冰晶增长的质量、热量传递之间的平衡提供一定的理论依据和试验参考,加深对橙汁冷冻浓缩过程的认识。

In order to decrease the juice loss during freeze concentration progress of ice crystals and improve the concentration speed,it＇s essential to obtain the growth law of ice crystals.Orange juice was concentrated from originally 13 to 34 Bx by two-stage freeze concentration which was based on the solid-liquid phase equilibrium.The relationship between orange juice concentration and its freezing temperature was investigated.Experimental results showed there was linear correlation between freezing temperature and concentration.And the freezing point decreased with concentration increasing.It was also researched the connection between soluble solids and concentration of orange juice.The obtained distribution coefficient K indicated that soluble solids in crystals increased with juice concentration raise.According to Fick diffusion equation,The kinetic model of ice crystals growth was developed,which showed that the growth rate of ice was proportional to the mass of ice and was inhibited by the maximal mass of ice.Powell method was adopted to search the parameters of the model and it＇s solution was obtained with numerical integrition using Runge-Kutta method.The results obtained from the kinetic model were coincident with that of experiments.The established model can provide theoretical basis and reference on equilibrium of heat transfer and ice crystals growth and be helpful to understand the mechanism of freeze concentration of orange juice.