The integration of membrane emulsification and cooling crystallization of glycine

Cooling crystallization is commonly applied as a purification and product-formation technology in many branches of chemical industry. Control over crystal quality attributes such as the crystal size distribution (CSD) is important for the performance of the downstream process and possibly for the efficacy of the final product. The CSD is affected by various crystallization phenomena, which are difficult to manipulate in a bulk volume due to the lack of control over the crystallization conditions at the relevant scale. One strategy to improve control over the local crystallization conditions is to confine crystallization inside small droplets, which is a form of process miniaturization.[1] The idea is to control the size of crystals inside droplets by limiting the available supersaturation for growth and possibly by physically constraining crystal growth due to the restricted droplet size. When successful, such a strategy can provide the same growth conditions to all crystals in the process allowing for a uniform CSD of the product. Indeed, cooling crystallization inside the droplets of an emulsion can give a certain control over the CSD, which is difficult to achieve in conventional bulk crystallization.[2,4,6] Existing studies[2-6] used conventional methods for emulsification such as an overhead stirrer. However, scale-up of such approach can be difficult. Furthermore, since in emulsion crystallization the problem of controlling the CSD has essentially been replaced with the problem of controlling the droplet size distribution of an emulsion, flexible methods for creating the emulsion with supersaturated droplets of uniform size need to be developed. In this work, we present a novel process concept, which integrates membrane emulsification[7] and cooling crystallization to address these challenges. The objective of this work is to develop and characterize an integrated membrane emulsification and cooling crystallization process for glycine. Droplets of uniform and controllable size that will act as small crystallizers are generated by flowing a hot feed solution through the pores of a membrane. Subsequently, the droplets are cooled to crystallize the solute. In absence of droplet coalescence or breakage, a crystal should grow until either the supersaturation inside the droplet is depleted, or until the largest crystal dimension matches the droplet diameter, which allows for control of the crystal size via the droplet size. The key novelty of this work is the use of membrane emulsification to produce monodisperse droplets of controllable size.