(536a) Crystallization of L-Ascorbic Acid in a 18-L Airlift Crystallizer

Crystallization is applied extensively in the chemical and pharmaceutical industry to produce and purify solid particles. The performance of the process and final product are closely related to specific properties of the crystals such as purity, crystal size distribution (CSD), and shape. However, the design and control of industrial crystallization processes to manufacture a crystalline product with tailored properties remains a major challenge. Many interacting physical phenomena occur in solution crystallization processes, involving for example primary nucleation, growth and attrition of crystals. In current industrial equipment, those physical phenomena take place in the same environment and are, therefore, strongly entangled. Novel designs are needed to improve control over individual physical phenomena.

Attrition refers to the birth of new crystals caused by collisions of crystals, which is often undesirable as it reduces the crystal size and broadens the CSD. Recently, a new crystallization concept involving an airlift, which eliminates the need for a stirrer, has been tested for crystallization of ammonium sulphate from water¹. The results demonstrate that attrition can be significantly suppressed compared to crystallization in a conventional stirred crystallizer.

Although these results are very promising for further development, the full potential of the concept remains largely unclear. An airlift crystallizer offers an exciting opportunity to bring the control of crystal product quality to a new level particularly for systems that are very sensitive to the mechanical impact of a stirrer. In particular, pharmaceutical compounds that are mostly produced batch-wise and in stirred-tank crystallizers often suffer from such sensitivity, which limits flexibility to produce a desired CSD due to uncontrolled nucleation. In order to apply an airlift system for crystallization successfully, the process conditions need to be optimized. Especially the control of the flow regime and the design of the sparger are essential for the performance of this new type of crystallizer.

The objective of the present study is to investigate experimentally the performance of an 18-L airlift crystallizer for crystallization of L-ascorbic acid from water in terms of hydrodynamic behavior and final product quality for seeded batch crystallization. First, the hydrodynamic behavior of the system has been investigated as function of the operating conditions and the design of the sparger via several in-line density measurements and in-line imaging. The results demonstrate that favorable hydrodynamic conditions in which the injected nitrogen is only present in the riser of the system can be achieved via a proper design of the sparger for a broad range of operating conditions. Secondly, the attainable product quality in terms of shape and CSD has been investigated via in-line imaging and off-line measurement of the supersaturation profile and CSD. A systematic comparison between the crystallization performance in an 18-L airlift crystallizer and crystallization in a conventional 5-L stirred crystallizer has been conducted. The results demonstrate a significant improvement of crystal shape and reduced nucleation in the airlift crystallizer. Furthermore, a clear distinction between grown seeds and new nuclei in both crystallizers allow us for the first time to quantitatively distinguish various forms of nucleation of the studied model system. Thirdly, the experimental data has been used to develop kinetic expressions for growth and nucleation, which allows for model-based optimization of the system to reveal the full potential of both a conventional crystallizer and airlift crystallizer. Finally, prospects for extending the system to continuous flow-mode will be discussed.

 References

¹ A. Soare, R. Lakerveld, J.van Royen, G. Zocchi, A. I. Stankiewicz, H. J. M. Kramer. Minimization of Attrition and Breakage in an Airlift Crystallizer. Ind.Eng.Chem.Res. 2012, 51, 10895-10909.

Acknowledgement

This work was supported by the European Commissions Framework 7 program through the OPTICO consortium