An Air-Lift Crystallizer Can Suppress Secondary Nucleation at a Higher Supersaturation Compared to a Stirred Crystallizer

Secondary
nucleation is suppressed in an air-lift crystallizer at levels of
supersaturation where in a stirred crystallizer a clear contribution
of secondary nucleation is visible. A comparison of batch crystallization
of l-ascorbic acid in an air-lift crystallizer and in a stirred
crystallizer is presented. The results demonstrate that at low supersaturation,
secondary nucleation can be suppressed in both the air-lift crystallizer
and the stirred crystallizer. At higher supersaturation, nucleation
starts to dominate in the air-lift crystallizer. At an intermediate
level of supersaturation, a clear contribution of secondary nucleation
in the final product obtained from the stirred crystallizer is visible.
However, experiments with similar conditions in the air-lift crystallizer
show a significantly smaller contribution of secondary nucleation.
The observed enlargement of the operating window in terms of supersaturation
where secondary nucleation is suppressed in an air-lift crystallizer
may have important practical consequences. Air-lift crystallizers
can potentially operate with a higher crystal growth rate and the
operating window for design and automated control can be extended.