Multiple unit processing using sub- and supercritical fluids

Abstract

The past twenty years has seen an expansion of the “critical fluid” technology platform with respect to using or combining multiple types of unit operations and compressed fluids in both their sub- and supercritical states. Although supercritical fluid extraction (SFE) utilizing supercritical carbon dioxide (SC-CO 2) has seen considerable industrial use, the high capitalization cost of building and running such production plants promotes expanding their use and functionality with respect to multi-unit operations and multiple fluids. An evaluation of the current status of implementing such an approach is provided, and a perspective of future needs suggested based on the senior author’s 40 years of experience in critical fluid processing of agriculturally-derived materials and natural products. The use of hot compressed water is no longer limited to just supercritical water processing, but can be advantageously exploited over a wide range of temperatures and appropriate pressures to include extraction and reaction of targeted substrates, including bioactive natural products, biomass conversion for renewable fuels, and synthesis of chemicals. As noted previously, combining SFE with fractionation methods using critical fluids, and/or reaction chemistry in critical fluid media can produce a variety of extracts or products [J.W. King, Sub- and supercritical fluid processing of agrimaterials: extraction, fractionation and reaction modes, in: E. Kiran, P.G. Debenedetti, C.J. Peters (Eds.), Supercritical Fluids: Fundamentals and Applications, Kluwer Publishers, Dordrecht, The Netherlands, 2000, pp. 451–488.] however the modification of processing equipment such as expellers, extruders, and particle production devices extend the use of the critical fluid technology platform. Sequential unit processing using multiple fluids also has been reported although demonstrated to a limited degree—however combining both compressed CO 2 and water in a unit process can also impart some unique processing possibilities. The concept of multiple unit and fluid processing supports an environmentally benign or “green” production platform and is consistent with processing sustainable materials. Integration of critical fluids into existing processing concepts such as biomass conversion, biorefineries, and synthesis based on methyl ester intermediates are discussed.