bonds increases, solubility decreases. Substances belonging to the same chemical class are dissolved in the SC phase in order of their increasing boiling points. The use of cosolvents or entrainers such as water or methanol enhances the solubility of substances normally not miscible with SC C02.

maining solutes in the SC phase are separated from C02 in the last separator and collected as product 2. The C02 is now at a low pressure, and carries no dissolved material. It is recycled by going through the pump, and reentering the extraction vessel. Therefore, multiple stages can be used to fractionate different compounds from the original feed. However, fractionation of extracts is possible only if the constituents exhibit large differences in vapor pressure.

As an alternative to fractionation by pressure reduction, the process can also be operated at constant pressure with temperature changes in the SCF to cause separation. Such a system is shown in Figure 4.

Operation Characteristics

The conditions of pressure and temperature for extraction and fractionation applications using C02 vary greatly with the materials to be extracted. Typical ranges of pressure and temperature for food applications are given in Figure 5. To evaluate the technical and economic feasibility for SCF extraction of a specific material, a pilot-plant study is carried out to determine factors such as solubility and selectivity properties, yields, quality, and operation condi-

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