Crystallization is one of the most important unit operations in process engineering and is applied in many fields such as chemical, pharmaceutical, and electronic industries. However, despite the importance of the crystallization operation, the currently-used techniques still suffer from certain limitations affecting the quality of the final product, the repeatability of the production, and the polymorphic phase formed. Membrane processes have been recently proposed as some of the most promising techniques for improving the performance of crystallization. Pervaporation, based on the use of a dense selective membrane, is one of the innovative membrane technologies of growing interest for application in crystallization processes. The mixture to be separated is upstream of the membrane, while the permeate side is put under vacuum, allowing for the selective transfer of species by vaporization. When employed in crystallization, the membrane allows, for example, the removal of the solvent from the solvent/antisolvent mixture and, thus, provides the supersaturation necessary to induce the crystallization. The objectives of this study are to determine the operating parameters controlling the final properties of the product to be crystallized (crystal size and polymorphic form) and to identify the limiting parameters of the process. To achieve the objectives of this study, a semi-continuous experimental set-up has been developed. L-glutamic acid is selected as the model compound as it crystallizes under two well-known monotropic polymorphs: the stable β-polymorph and the metastable α-polymorph. Firstly, a comprehensive study of the solubility of α and β forms of L-glutamic acid is performed by gravimetric method, in different water/ethanol mixtures and at temperatures ranging from 283 to 343 K. The solubility results yield the reference values for the study of the crystallization by the pervaporation process in each of the studied water/ethanol mixtures. Secondly, the performances of the selected commercial membranes for the separation of the water/ethanol mixtures are evaluated: an organic flat membrane (PDMS) and a hybrid tubular membrane (HybSi®). The experimental results obtained suggest that the HybSi® membranes are more suitable for the desired application because the separation factors are favorable to the selective removal of water. A final experimental study is achieved to examine the influence of different operating conditions (permeate pressure, solution temperature, and initial water/ethanol composition…) on the performance of the pervaporation process applied to the crystallization of L-glutamic acid. The results obtained demonstrated the feasibility of the pervaporation process applied to crystallization despite the occurrence of concentration polarization and fouling phenomena that limit the performance of the process. However, this technology allows the preferential and reproducible crystallization of a polymorphic form thanks to an appropriate choice of operating conditions.
Keywords: crystallization, polymorphism, pervaporation, dense and hybrid membranes, semi-continuous process, process intensification.