Supersaturation Control and Heterogeneous Nucleation in Membrane Crystallizers: Facts and Perspectives

The interest to combine membrane operations and solution crystallization has grown in the past several years. This approach has been put into practice in several forms of membrane-assisted crystallization (MAC) processes, among which is membrane crystallization (MCr) technology. The main features of MCr are (1) the use of membranes as precision devices to control the composition of the crystallizing solution, by opposing a well-defined and tunable resistance to mass flow occurring in the vapor phase; (2) the action of the porous surface of the membrane as a suitable support to activate heterogeneous nucleation mechanisms; (3) the possibility to induce nucleation and crystal growth in separate sites, thus reducing the risk of membrane fouling even when the same membrane supports heterogeneous nucleation. Thanks to these fundamental options, combined together in a unique apparatus, advantages like (i) improved control of the supersaturation degree and the rate of its generation; (ii) the possibility for the crystallization to be initiated at low supersaturation levels; (iii) the enhancement of the crystallization kinetics; and (iv) improved overall process efficiency can be achieved, even for large and complex molecules like proteins. The most interesting developments and the more exciting perspectives for this novel technology have been reviewed in this paper.