Supercritical fluid separations

Keith P. Johnston/ Ph.D./ P.E./ Profe.s.sor of Chemical Engineering, University of Texas (Austin) Member, American In stitute of Chemical Engineers, American Chemical Society, Univ. of Texas Separations Re.search Program. (Supercritical Fluid Separation Proce.s.se.s)... 

TABLE 22-11 Commercial Applications of Supercritical Fluid Separations Technology... 

The two SCFs most often studied—CO2 and water—are the two least expensive of all solvents. CO2 is nontoxic and nonflammable and has a near-ambient critical temperature of 31. UC. CO2 is an environmentally friendly substitute for organic solvents including chlorocarbons and chlorofluorocarbons. Supercritical water (Tc = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. (See also Sec. 15 for additional discussion of supercritical fluid separation processes.)... 

Supercritical fluid separation processes operate at pressures ranging from 1000 to 4000 lb/in.2, pressures that might be considered high, especially in the foods and essential oils industries. However, because of the factors just listed, supercritical fluid extraction has become eco-... 

Since 1929, polychlorinated biphenyls (PCBs) have been produced and used as heat-transfer, hydraulic, and dielectric fluids. Because of their chemical and physical stability, PCBs have been found in many environmental samples. Generally, PCBs have been analyzed by GC with electron-capture detection. There are many reports on subcritical and supercritical fluid extraction of PCBs, but only a few on supercritical fluid separation of PCBs. 

Among the works of supercritical fluid separations of PCBs, UV has been the most popular detector. A Microbore Cig column was used to separate individual PCB congeners in Aroclor mixtures. Density and temperature programming was also utilized for separation of PCBs. Both packed (with phenyl and Cig) and capillary (Sphery-5 cyanopropyl) columns were used in this work. Carbon dioxide, nitrous oxide, and sulfur hexafluoride were tested as mobile phases for the separation of PCBs. 

For most applications, it is not possible to treat supercritical fluid separations, for example extraction, as a well-defined unit operation as is the case for simpler proctssts such as distillation. Instead, research is often needed to characterize the important properties for each specific separation or reaction. However, the recent advances in the molecular understanding of SCF solutions provide some general themes that can be utilized in a semi-quantitative manner to evaluate the potential of both research and applications. 

Ram B. Gupta, Ph.D. Alumni (Chair) Professor of Chemical Engineering, Department of Chemical Engineering, Auburn University Member, American Institute of Chemicd Engineers, American Chemical Society (Supercritical Fluid Separation Processes)... 

---