We have developed a method of predicting the effect of adding a small amount of an entrainer to a supercritical fluid for the extraction of organic compounds from aqueous systems, and we verified the predictions experimentally. The Peng-Robinson equation of state with a modified mixing rule proposed by Panagiotopoulos and Reid was used to qualitatively predict the relative effectiveness of organic cosolvents. The extraction of phenol from water at 298 and 323 K using supercritical carbon dioxide at pressures up to 31 MPa was chosen as a model system. Methanol, chlorobenzene, and benzene were investigated as possible entrainers. Benzene proved to be the most attractive, providing over a 50% increase in the distribution coefficient of phenol.The availability of inexpensive, nontoxic, supercritical solvents such as carbon dioxide and their attractive properties has renewed interest in supercritical extraction as a viable commercial separation technique for removing organic compounds from water. Application has been limited due to the high costs associated with the poor solvent power of certain supercritical fluids and the need for high-pressure equipment. These costs can be substantially lowered with the use of an entrainer (or cosolvent) to increase the solubility of the solute in the supercritical phase, reducing the size of the required extractor. This so-called "entrainer effect" has been reported for a variety of nonaqueous systems in the literature. It is, however, the most poorly understood aspect of supercritical extraction, making selection of an appropriate entrainer difficult. To our knowledge, no work has been done on predicting or experimentally measuring the effect of entrainers on the supercritical extraction of aqueous systems.

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