Crystallization process in turbulent supercritical flows

A model compound paracetamol was crystallized from EtOH solution using supercritical CO2 as a dispersing and extracting solvent in a turbulent jet flow at the Reynolds numbers similar to 10(5), pressures up to 300 bar and temperature interval between 35 and 85 degrees C. The particle size and morphology were analysed using SEM and AeroSizing techniques. Two different crystallization mechanisms are observed as a function of the CO2 density and miscibility in the CO2/ethanol system. This phase behavior results in a surface transition of particles at density about 250 kg/m(3). The solubility function of paracetamol strongly affects the crystallization behavior, leading to an increase of particle size by a factor of 10(2) and a decrease of particle nucleation density by a factor of 10(4) under conditions of high pressure and temperature. Estimated time constants of the mass-transfer process suggest that at high flow velocities investigated particle growth is the time-limiting step, although solvent extraction at low CO2 densities and large-scale inhomogeneity in the flow are likely to affect the crystallization behavior. (C) 1999 Elsevier Science B.V. All rights reserved.