Introduction
Crystallization is an extremely important separation unit operation, which is used in the production of highly specialized fine solid products1-3. Cooling crystallization, one of classic crystallization process, is mostly controlled by adding seed crystals within the metastable zone to induce nucleation 4-7. Successful artificial seeding operation depends on a lot of aspects, seed size and amount, time point and place of addition, and experience of the operator, etc. However, conventional cooling crystallization that requires external seed adding procedure are suffered the risk of the secondary nucleation, which will limit the crystal purity, morphology and particle size distribution11,12, etc.
Recently, researchers are working to select and add seed by using molecular sieves or additional physical fields (electric, magnetic, etc.)8-11. However, none of these methods can achieve the automate seeding with accurate temperature control. In addition, nucleation induced via seeding crystals and crystal growth kinetics in the crystallizer determine the essential solid product quality and fundamentally influence further downstream processing (solid-liquid separation, drying, etc.). Decoupling the competition between nucleation and crystal growth from space and time aspect simultaneously is the core concern for all the researcher.
As a highly designable and environmentally friendly material, membrane obtains a great development in many fields related to crystallization process12-15. One of the most impressive applications is the membrane served as a heterogeneous nucleation interface to trigger the nucleation process16,17. When the membrane unit concentrating the solution via selectively mass transfer of the solvent, the supersaturated solution on the membrane surface become nucleation and the formed particles can auto-detach from the surface under proper dynamic force field18,19. This finding is of importance for the accurate control of the mass transfer related crystallization process (evaporative crystallization, antisolvent crystallization, e.g.).
Inspiring transfer features of hollow fiber membrane also shed light on the heat exchange and the relevant process control20. In addition, with the high packing density, hollow fiber membrane module ensures the high manufacture capacity for potential industrial applications21-23. The total heat transfer coefficient of the hollow fiber membrane module can be as high as 2000 W/(m2·K); the ratio between the heat transfer coefficient of hollow fiber membrane heat exchangers verse the volume was 2 to 15 times higher than that of commercial metal heat exchangers, showing impressive application advantages24-26. Moreover, the potential advantage of hollow fiber membrane module on the accurate heat exchange and temperature distribution is an interesting topic. All the above properties of the hollow fiber membrane can potentially benefit the cooling crystallization control via heterogenous nucleation and high heat transfer efficiency, which had not fully unfolded and in-depth investigated.
Thiourea, a fundamental chemicals in many industrial fields27-31, was commonly manufactured via cooling crystallization. Nowadays, high-purity thiourea plays more and more irreplaceable role as the electrocatalytic materials and battery materials32,33, which raised the urgent requirement on accurate nucleation and growth control of its cooling crystallization process34,35.
In this work, we proposed a new cooling crystallization control mechanism to prepare high-purity thiourea crystals via introducing the polymeric hollow fiber membrane module. The membrane module is functioning as the key devices for inducing the nucleation and self-seeding. Poly-tetrafluoroethylene (PTFE) and poly-ethersulfone (PES) hollow fiber membrane were investigated on their thermal conductivities and inducing nucleation properties. In this membrane-assisted cooling crystallization (MACC), the surface induced nucleation and accurate self-seeding will be validated from theoretical and experimental aspects. The feasible accurate and automatic control MACC process was then compared with the conventional cooling crystallization (with seeding and none seeding) in terms of thiourea crystal purity, morphology and crystal size distribution (CSD) to fully reveal its advantages in nucleation and crystal growth control.