(194d) Crystal Shape Engineering through Cycles of Dissolution and Growth

The impacts of particle shape on end-use efficacy and downstream processing efficiency has driven industrial and academic efforts to control and/or manipulate the shapes obtained by crystallization processes. Strategies for controlling crystal shape have focused primarily on chemical routes; with shapes optimized through either the use of additives or engineered solvent selection. However, the chemical design space for crystallization may be limited and/or the additional separation/purification steps required to remove the additives or engineered solvents may be uneconomical. In addition, regulatory compliance may be an obstacle to such approaches. In 2007, Snyder et al. proposed a nonchemical strategy for modifying crystal shape; using cycles of growth and dissolution to attain shapes which were otherwise unachievable through a single growth step.

A series of experiments are presented that were performed to provide proof of concept for this approach. These experiments used a small quiescent crystallizer (volume ~1mL), with 0.1 °C accuracy temperature control. The thermal control was achieved using a Peltier cell connected to a PI controller. The control strategy employed resulted in rapid convergence to the desired temperature. Crystal shapes were recorded throughout using an inverted microscope. The systems investigated were adipic acid and paracetamol, both grown from aqueous solutions.

In the adipic acid system, the shape became elongated in the [010] directions after cycling; with a ~150% change to the aspect ratio obtained after 43 cycles of growth and dissolution. For this case cycle times were varied between 25 and 60 minutes for each growth and dissolution step, with the experiment lasting 64 hours. The super and under saturations for each cycle were 14.3 and -7.5%, respectively. Having proved that the concept works; the challenge is now to optimize operating strategies in order to achieve desired shape changes in a minimum time.

References

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Snyder, R. C., S. Studener and M. F. Doherty (2007). Manipulation of crystal shape by cycles of growth and dissolution. AIChE J, 53, 1510-1517.