(394g) 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)