Membrane crystallizers in multicomponent systems

Crystallization is an excellent technique for purification of chemical species by solidification from a liquid mixture: many materials are marketed in crystalline form and large amounts of product may be obtained from impure solutions in one processing step. In a competitive market a product has to meet increasingly stringent demands concerning the quality in terms of crystal size distribution, degree of agglomeration and morphological properties of the particles. On this basis, a study on redesigning traditional crystallizers by using microporous hydrophobic polymeric membranes in a Membrane Crystallizer unit and, in particular, on the possibility of recovering fumaric acid in aqueous solutions of malic acid is presented. The application at industrial level of fumaric acid, obtained using by-products resulting from the production of phtalic anydride, is limited by its low solubility. In order to make it useful, it is enzimatically converted to L-malic acid, which is more soluble, and is used as an acidulant in fruit and vegetable juices, jams, infant foods, etc. Results obtained from an enzymatic membrane reactor at laboratory scale show the ability to achieve a steady-state conversion degree up to 80%. In this case, the experimental work is focussed on the recovery of unreacted fumaric acid by introducing a membrane crystallization stage downstream the reactor. Simultaneous developments of the two main conceptual aspects of this operation are emphasised: one concerning the transport of matter and energy trough membrane, other the physical and chemical phenomena that occur during crystallization. Considering the energetic exchange between cold and hot streams at the membrane module, theoretical efforts are devoted to the determination of both axial and radial temperature profiles, the latter affected by polarisation phenomena. Population balance equation is applied in order to estimate, given experimentally determined particle size distributions, the parameters of a set of equations describing the underlying kinetic processes and to predict, given a set of kinetic relations, the resulting particle size distributions as function of operative conditions, chemical and physical properties of the solution flowing along the membrane fibers, structural membrane parameters.