Continuous Crystallization of L-Ascorbic Acid: Integration of Airlift Crystallizer with Membrane Distillation-Experiments and Results

Continuous Crystallization of L-ascorbic acid:

Integration of Airlift Crystallizer with Membrane Distillation-Experiments and Results

F. Anisi,¹ H.J.M Kramer^{1 1} Department of Process and Energy, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Delft, the Netherlands

Crystallization is one of the oldest important separation and purification processes in production of pharmaceutical compounds. The common problems in crystallisation processes are often include large variations in the product quality and inflexibility of the process. It is due to the entanglement of different phenomena such as primary nucleation, growth, secondary nucleation and so on. For a better control on the secondary nucleation and the growth of the crystals an airlift crystallizer has been developed, tested and analysed. Batch experiments with airlift crystallizer showed that rate of secondary nucleation is two orders of magnitude lower in such a crystallizer compared to conventional stirred crystallizer. Figure 1 shows how bimodal the CSD from stirred crystallizer is which confirms the higher rate of secondary nucleation in the stirred crystallizer. Therefore, it is a promising alternative for conventional ones.[1]

Membrane distillation technology can be used for supersaturation generation as an alternative to evaporation. Since it is not necessary to work with high temperatures, membranes are suitable options for thermo-labile compounds. During crystallization experiments supersaturation has been generated and controlled with continuous solvent removal and fed to the crystallizer. Seed crystals have grown by consuming the supersaturation.

Therefore, a continuous membrane-assisted crystallization using an airlift crystallizer is potentially believed to be more controllable, energy efficient and to produce a more uni-modal crystal size distribution.

In this contribution the airlift crystallizer and membrane-assisted unit are integrated in a skid. The set-up is capable of performing batch and continuous crystallization with airlift or stirred crystallizer. Continuous crystallization of L-ascorbic acid has been done in the airlift and stirred crystallizer while membrane controls the supersaturation during the process. The integrated system has been also modelled and various process conditions are tested.

Based on modelling results experiments are designed and show that generally use of membrane unit decreases the level of secondary nucleation but it is mainly suppressed in the airlift crystallizer rather than stirred one. The higher the residence time the larger the crystals grow. The results from the experiments are used to validate the model and furthermore to estimate the kinetic parameters and optimize the process via design of new experiments.

 

Figure 1:Crystal size distribution for different batch seeded cooling crystallization of L-ascorbic acid a) stirred crystallizer b) airlift crystallizer

  USERPROPERTY  \* MERGEFORMAT  [1]      R. Lakerveld, J. Krochten, H. Kramer, An airlift crystallizer can suppress secondary nucleation at a higher supersaturation compared to a stirred crystallizer, Cryst. Growth Des.  14, 7 (2014)