Simulation of particle formation during the rapid expansion of supercritical solutions

Experimental and theoretical results for the rapid expansion of supercritical solutions (RESS-process) are presented. Experiments were carried out with the solvents carbon dioxide (CO(2)) and trifluoromethane (CHF,) and the organic solutes cholesterol, benzoic acid and griseofulvin. It is shown that for each of these solutes it is possible to produce particles in the size range of 200 nm which leads to improved dissolution. Furthermore, to demonstrate the higher bioavailability of the micronized pharmaceutical griseofulvin as of the original material, results of a dissolution experiment are shown. Besides the experiments the flow and particle formation and growth in the expansion unit (inlet-capillary nozzle-supersonic free jet) of the RESS-process is modelled numerically. The model to calculate the flow field consists of mass, momentum and energy balances as well as the extended generalized Bender equation of state (egB-EoS). The flow is assumed to be steady, one-dimensional, viscid and non-adiabatic. The general dynamic equation (GDE) is included to model the particulate phase. The results of the modelling show that particle formation occurs in most of the cases mainly in the supersonic free jet and that the main mechanism for particle growth is coagulation. Furthermore, it can be inferred from the high particle number concentration at the Mach disk that particle growth by coagulation continues in the subsonic part of the free jet (expansion chamber). Experimental results with variation of additional air supply to the expansion chamber are presented to confirm the theoretical results. (C) 2001 Elsevier Science Ltd. All rights reserved.