Formation of small organic particles by RESS: experimental and theoreticalinvestigations

The RESS process (rapid expansion of supercritical solutions) is an innovat ive and promising technology to produce small particles with narrow particl e size distribution and it offers interesting applications for heat-sensiti ve organic compounds such as certain pharmaceuticals. RESS experiments were carried out with an apparatus suitable for temperatures up to 600 K and pr essures up to 60 MPa. Until now, carbon dioxide has been used as a supercri tical solvent and naphthalene, cholesterol and benzoic acid as solutes. The se experiments led to particle sizes in the range of 1.5 to 3 mu m for naph thalene, between 0.8 and 1.2 mu m for benzoic acid, and always less than 0. 35 mu m in the case of cholesterol. The diameter and number concentration o f the particles are measured in our apparatus in situ and online with a 3-w avelength-extinction measurement technique, in contrast to the usual offlin e examination techniques reported in the literature. Besides the experiment al investigations, the RESS process is modelled numerically, considering th e three parts as capillary inlet - capillary - freejet. The one-dimensional time-independent flow model for the pure solvent includes heat-exchange in the capillary and the freejet, friction in the capillary and isentropic fl ow in the capillary inlet area. The resulting pressure and temperature chan ges along the expansion path are used to calculate the solute solubility in the solvent and the supersaturation of the real mixture with the Peng-Robi nson equation of state. The results of these calculations are expansion rat es, P, of 10(7) is, cooling rates, T, of 10(9) K/s and theoretical supersat urations of about 10(5) to 10(8) at residence times of less than 10(-6) sec onds in the supersonic freejet. Moreover, in the present paper, some import ant aspects of the RESS process - phase behaviour of the non-ideal, dilute supercritical mixture, application of the classical nucleation theory and t he mechanism of particle growth - will be analysed. (C) 1999 Elsevier Scien ce B.V. All rights reserved.