Abstract:

The crystallization process is an energy-efficient solid-liquid separation and purification technology in chemical unit operation, which is widely used in food, medicine, dye and other production processes. The crystallization of sodium acetate was studied by ultrasound, image and optical turbidimetry simultaneously, in which the crystal size distribution (CSD) was obtained by ultrasonic forward model combined with the optimum regularization technique (ORT) algorithm. By implementing a series of experiments at various stirring and cooling rates, the crystallization temperature and particle size were measured, and the crystal shape was also determined via image analysis. The results showed that the faster cooling rate the higher crystal growth rate, and the faster stirring rate promoted crystal growth at the initial stage but the average size decreased due to particle wear and breakage in the later stage. The crystallization temperature detected by ultrasonic method was slightly lower than that of optical turbidimetry under the same condition but the deviation was within 15%. In addition, the crystal growth and the morphology had been directly observed and characterized by the image method, which yielded a close trend with ultrasonic method, and the deviation of median diameter measured by these two methods was less than 15% once the crystal precipitated steadily. As a whole, the optical methods are more suitable for local characteristic measurement in the early stage compared with the ultrasonic method, and the latter is more appropriate for probing the continuous process of particle size change.

Key words: sodium acetate, ultrasonic spectra, image method, crystallization, particle size distribution, measurement

摘要：

结晶过程是化工单元操作中一种高效节能的固液分离与提纯技术，广泛应用于食品、医药、染料等生产过程。结合超声法、图像法和光学浊度法研究了无水醋酸钠的结晶过程，通过超声模型结合最优正则化反演算法求解晶体颗粒粒度分布，在线测量了不同降温速率和搅拌速率下的析晶温度、晶体尺寸和形貌变化。结果表明：较快的降温速率可促进晶体生长，搅拌速率较快初期晶体尺寸较大，但后期由于颗粒磨损致平均尺寸减小；相同条件下光学浊度法较超声法较早测得析出晶体，实验中析晶温度偏差低于15%；图像法直观观察到了晶体生长过程并较好表征了晶粒形貌，与超声法具有接近的晶粒增长趋势，当晶体稳定析出后两者体积中位径偏差在15%以内，均较好地反映了实验条件对结晶晶体尺寸的影响。光学方法可更精细体现结晶初期特征，而在结晶后期超声法仍可对粒径进行有效测量。

关键词: 无水醋酸钠, 超声谱, 图像法, 结晶, 粒度分布, 测量