ABSTRACT
Warsaw University of Technology, Department of Chemical and Process Engineering, Warsaw, Poland
Boris Y.Shekunov
Ferro Corporation, Independence, Ohio, U.S.A.
Supercritical fluids (SCFs) are involved in numerous industrial processes and have a potentially wide field of new applications. Current applications include extraction processes, reaction chemistry and polymerization, food fractionation, waste recycling, soil remediation, cleaning of electronic and optical equipment parts, impregnation, dry powder coating, aerogels, nanotechnology, and crystallization and particle formation of pharmaceuticals and many other powdered products (1). Industrial processes related to energy production as well as liquid rocket, diesel, and the gas turbine engines operate at supercritical conditions for the injected fuel (2). In this chapter we are interested in the mechanisms and phenomena affecting particle formation processes entailing the use of supercritical fluids, with particular reference to the production of fine particles for pharmaceutical applications. A description of different methods for particle formation by means of supercritical fluids can be found elsewhere in this volume, (Chapter 4 by Charbit et al.) and in Refs. 3 to 5. Most of these methods are based on crystallization or precipitation of a solid phase from supersaturated product solutions, whereas supersaturation is created in the system by means of supercritical fluids. Particle size distribution, particle morphology, and many physical properties of the solid state depend on the spatial distribution of the supersaturation and the timing of its evolution. To understand how solute supersaturation is created and how to predict its structure and evolution in time, one needs to consider balances of momentum (flow), species (mixing), energy (heating and cooling), and population (particle size distribution, PSD) and to apply the flux and source terms defined for the supercritical fluids. The physics of fluid behavior in the supercritical regime is different from that observed in gases or liquids, because SCFs exhibit some unique properties that must be taken into account during theoretical modeling, analytical studies, and the design of processing equipment.
