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Figure 1 Phase diagram of a pure compound. See the description in the text.

• the economical and practical interest of simple solvent recycling is the same as in industrial supercritical fluid extraction (SFE).

The extensive study of the properties and characteristics of supercritical fluids is not the subject of this article. The main properties relevant to their use in Prep-SFC are good solvent power (comparable to liquids), low viscosity and high diffusion coefficients (intermediate between gases and liquids) and straightforward modulation of solvent properties by pressure adjustment (it is possible to transform a supercritical fluid to a gas by isothermal depressur-ization).

The principle and characteristics of Prep-SFC can be described with a phase diagram. Figure 1 shows the eluent cycle in a Prep-SFC process.

At point 1, the eluent is supercritical and the chromatographic separation takes place in the column. At the column outlet, the eluent is depressurized (and heated) to the gas phase at point 2. While in a gas phase the eluent is cleaned and pure fractions collected. The cleaned gaseous eluent is then recycled, through condensation (point 3), recompression (point 4) and heating to the operating temperature (point 1).

This is a description of the simplest Prep-SFC process that uses a pure compound as a supercritical eluent. However, it is often necessary to use a mixture as the supercritical eluent. The secondary component of the eluent is most often a liquid solvent ('the modifier'). A Prep-SFC process that includes a modifier differs from the simple scheme without a modifier and both processes will be described here.

Prep-SFC processes can be classified according to scale. Micro-Prep-SFC is basically the adaptation of an analytical chromatograph to collect small frac tions (microgram or milligram size) and is not described here. Laboratory-Prep-SFC is concerned with the purification of larger amounts (hundreds of milligrams or grams). It requires a specific technology, but cannot be considered to be large-scale SFC. It is described briefly later. Pilot-Prep-SFC is intended for the purifications of kilograms per day on columns having an internal diameter of between 5 and 15 cm. It can be used for scale-up studies or even for some small industrial productions. Production-Prep-SFC is for the purification of hundreds of kilograms to tens of metric tons per year. This scale of production takes full advantage of the economics of Prep-SFC.

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