Abstract:

Cooling crystallization is a classical solution crystallization process, which is often used to separate substances whose solubility varies greatly with temperature and prepare high quality crystal products. Direct cooling leads to uncontrollable nucleation rate and poor crystal quality. Industrial crystallization processes are often controlled by adding crystal seeds within the metastable zone to induce nucleation. The process of seeds preparation is complex. And a successful seeding process depends on a lot of factors, such as the size distribution, amount, time point and experience of operators, which decreases the batch repeatability of product quality. In this work, a polytetrafluoroethylene (PTFE) hollow fiber membrane module was used to provide heat exchange interface for the crystallization solution and the cooling liquid, and the temperature of the crystallization solution decreased to form a relatively uniform supercooling gradient near the membrane surfaces. Heterogeneous nucleation occurred at a low supersaturation, realizing a new type of membrane-assisted seeding for ammonium persulfate cooling crystallization. The seeds generated in the membrane module were circulated into the batch crystallizer and continue to grow, decoupling the nucleation and growth process. The images captured by the on-line crystallization monitoring system confirmed that seed crystals with good morphology and narrow size distribution can be obtained by controlling two operating parameters, namely the membrane-involved temperature and the duration. Compared to the direct cooling crystallization, at a similar cooling rate, the crystal products prepared by membrane-assisted seeding processes had larger mean crystal sizes, narrower distributions and smoother surfaces. Thus, membrane-assisted cooling crystallization exhibits good nucleation control capability, and is expected to realize automatic preparation and addition of seeds, which opens up a new direction for the industrial cooling crystallization process design of high value-added crystal products.

Key words: cooling crystallization, membrane crystallization, seeds, process control, on-line monitoring

摘要：

冷却结晶是经典的溶液结晶过程，常用于分离溶解度随温度变化较大的物质，制备高品质晶体产品。直接进行降温会导致成核速率不可控，得到的晶体产品质量差。在工业中通常选择在溶液结晶介稳区内投放适量晶种来诱导成核，但晶种制备过程复杂，而且成功的添加晶种过程取决于晶种的粒度分布、数量、投放时机和操作人员的经验等因素，降低了产品质量的批次重复性。本文利用聚四氟乙烯（PTFE）中空纤维膜组件为结晶溶液和冷却液提供换热界面，结晶溶液温度降低，在膜界面处形成较均匀的过冷度梯度，进而在低过饱和度下发生异相成核，实现膜辅助添加晶种的过硫酸铵冷却结晶过程调控。膜组件中产生的晶种进入结晶釜中继续生长，将成核和生长过程进行解耦。在线结晶检测系统捕捉到的照片证实了通过控制膜组件使用温度和时长两个操作参数便可得到具有较好的形貌、较窄的粒度分布的晶种。相比直接冷却结晶，在相近的降温速率下，膜辅助添加晶种过程制备的晶体产品具有更大的平均粒径，且粒度分布更集中，表面更加光滑。因此，膜辅助冷却结晶呈现了良好的成核控制能力，有望实现晶种自动制备和添加功能，为高附加值晶体产品的冷却结晶过程开发提供了新方向。

关键词: 冷却结晶, 膜结晶, 晶种, 过程控制, 在线监测