Polymorph selection and process intensification in a continuous crystallization-milling process: A case study on L-glutamic acid crystallized from water

The continuous crystallization of L-glutamic acid from water is studied in a single stage mixed suspension mixed product removal crystallizer that is coupled to a milling loop. We show that the stable polymorph can be obtained in the presence of milling at operating conditions that would lead to the metastable polymorph without milling. The effect is first shown through a series of experiments carried out at different residence times, feed concentrations and in the absence/presence of milling. The experimental observations are rationalized through the use of a population balance equation model. We conclude that the observed effects result from fines generation and the corresponding increase in crystal surface area. Using the model, we obtain a map of polymorphic outcomes and process productivity in dependence of operating conditions. It is shown that the combined milling/crystallization process exhibits higher productivity and an enlarged region of operating conditions where the stable polymorph of L glutamic acid can be reliably obtained in comparison to a conventional crystallization process.