A study on redesigning traditional crystallizers by using microporous hydrophobic membranes and, in particular, the realization of a separation unit aiming to recover the unreacted fumaric acid leaving a biocatalytic membrane reactor for the production Of L-malic acid is presented. The development of two main conceptual aspects of this operation are analysed: one concerning the transport of matter and energy through membrane, other the physical and chemical phenomena that occur during crystallization. From experimentally determined particle size distributions and induction time measurements, the parameters of a set of equations describing both heterogeneous and secondary nucleation processes, as well as growth rate, have been evaluated. Finally, population balance equation has been applied in order to predict the evolution of CDS in time. (C) 2003 Elsevier Science B.V. All rights reserved.