A population balance model of the solution-mediated phase transition of citric acid

The Solvent-Mediated Anhydrous to Monohydrate Phase Transition (SMPT) of citric acid in water was monitored using in situ Raman spectroscopy and image analysis. The solid phase composition, solute concentration, and crystal size distribution were measured during experiments designed to investigate the different steps of the SMPT process: dissolution of the metastable form, nucleation, and growth of the stable form. A Population Balance Model (PBE) was developed to depict the time evolutions of the two populations of particles involved. Nucleation and growth phenomena were analyzed and described through simplified kinetic laws. The related kinetic parameters were estimated through the nonlinear least squares minimization of model-experiments prediction errors. The overall PBE model was finally found to satisfactorily describe the SMTP process; the time variations of the solute concentration and solid phase composition in particular. It is clearly shown that the secondary nucleation of stable monohydrate particles depends on both the supersaturation anti the overall solid content. Surprisingly, as far as the nucleation of stable particles in the presence of metastable solid is concerned, the rate of particle generation was found to exhibit identical dependency on the metastable and stable content, a feature which remains to be further investigated and explained. (c) 2007 American Institute of Chemical Engineers.