Although the phenomenon of supercritical fluids (SCFs) has been known for more than a century, it was not until the early 70's that chemical engineers began to appreciate the potential of SCFs for chemical processes. After an initial burst of enthusiasm where SCFs were hailed as the new panacea, the community settled down to learning their strengths and limitations and began discovering an ever-growing host of useful applications. The primary advantage of the SCF state is that it couples efficient solvation (never as good as in the liquid) with transport properties far superior to liquids (but never as good as those of gases). The earliest work primarily involved extractions with SCF CO2, with food products and environmental applications leading the way, and in each of these the exceptional ability of a solvent to penetrate a matrix made unique processes possible.

In the ensuing years, a plethora of other applications were explored, and many of these are now in commercial use. Perhaps the first application proposed other than extraction was SCF water oxidation of waste streams, which has been thoroughly investigated but has never been widely implemented. However, subsequent work on other reactions in SCFs has generated broader use. Very often the advantage of using SCFs in the reaction system is the vastly improved mass transfer. Other early applications spanned the range from the rehabilitation of old books to miscible flooding for tertiary oil recovery.

A bit later a variety of methods using SCFs for particle formation have been proposed, and many are now in widespread use for applications ranging from drug delivery to nanoparticle formation. In addition there are many applications in coating and cleaning, including such diverse targets as microelectronic and commercial dry cleaning. Other applications abound, such as SCF drying, chromatography, processing of polymers, impregnation and dyeing of polymers, and the processing of renewable resources.

The region in which SCFs display unusual, and very useful properties, is generally where both the density and the compressibility are large (typically TR = 1.01 – 1.1 and PR = 1 – 2). The fluid density must be relatively high for it to be a good solvent, but also the compressibility must be high so that small pressure changes give the large density changes allowing for selective solution or dissolution. The SCF does not dissolve insoluble compounds; rather it enables higher concentrations of dilute solutes by a combination of physical and chemical solvation forces. Typical enhancements for low solubility compounds may be 103-106, and in rare cases, effects as large as 1010or even 1015 have been reported.

The majority of the applications have used SCF CO2, which has a readily accessible critical point at 31ēC and 74 bars, and is inexpensive, nontoxic, and nonflammable. Unfortunately, it is a rather poor solvent, but it can be substantially enhanced by additions of small amounts of cosolvents or modifiers – generally polar or protic compounds. However, a wide range of SCF solvents and cosolvents are available, and will be discussed with the merits and limitations of each. Also we shall discuss the physical and chemical intermolecular forces operating in such systems, and cite examples of chemical engineers taking advantage of these for specific applications.

Finally, the talk will focus on the choice of when to select the use of SCF methods. Generally these are cases where more conventional separations will not serve the purpose, and often where cost alone is not the issue. Rather it could be used where there is the need for a safe solvent, for ease of solvent removal, for the need for enhanced mass transport, for treating a thermally labile product, or even to create a unique form of product.