Crystallization process in turbulent supercritical flows

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