Chapter 15
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Supercritical Fluids in Environmental Remediation and Pollution Prevention A. Akgerman Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122
During the last decade use of supercritical fluids in environmental applications have increased due to their unique properties. Technologies have already been developed for extraction of organic compounds from aqueous and solid environmental matrices and research on extraction of metals is in progress. In most applications, supercritical carbon dioxide is the solvent of choice because it is environmentally benign, safe, and abundant at a low cost. Recent research focus is on use of supercritical fluids as separation and reaction media. Especially as the reaction media, supercritical fluids offer properties that may significantly affect reaction rates and selectivities. The most important property is the control of solvency power through density. This aspect may be used to eliminate side reactions, exceed thermodynamic yield limitations, or control polymer chain lengths and molecular weight distributions. In the separations area, supercritical fluids can be exploited for adsorptive separations of structurally very similar compounds and for selective extraction of thermally labile compounds from natural products.
A supercritical fluid (SCF) is a fluid at conditions above its critical temperature and pressure. Interest in the extraction of solid and liquid media by SCFs have increased during the last decade due to: (1) environmental problems associated with common solvents (mostiy chlorinated hydrocarbons); (2) the increasing cost of energy intensive separation processes; and (3) the inability of conventional separation processes to provide the necessary separations needed in emerging new industries. The attractive physicochemical properties of SCFs qualify them as a viable alternative to conventional solvents used in extraction processes. At temperatures and pressures above its critical point a pure substance exists in 208
© 1997 American Chemical Society
In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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a state that exhibits gas-like and liquid-like properties. Thefluid'sdensity would be very close to that of a liquid, the surface tension is very close to zero, the diffusivity and viscosity have a value somewhat in between that of a liquid and a gas, and most important the solvent power of a SCF can be varied over a very wide range by small variations intemperatureand/or pressure in the supercritical region. These properties result in several advantages in extraction such as ease of solvent recovery, elimination of residual solvent in the extracted medium, lower pressure drops, and higher mass transfer rates. In extraction of a solute from a matrix (such as water) the choice of the solvent depends on two criteria, its immiscibility with the matrix and the solubility of the solute in the solvent. The solubility in SCFs is a strong function of density. In the vicinity of the critical point, l
