Supercritical CO2 Extraction Technology
When a gas is compressed to a sufficiently high pressure, it becomes liquid. If, on the other hand, the gas is heated beyond a specific temperature, no amount of compression of the hot gas will cause it to become a liquid. This temperature is called the critical temperature and the corresponding vapor pressure is called the critical pressure. These values of temperature and pressure define a critical point, which is unique to a given substance.
This “fluid” now takes on many of the properties of both gas and liquid. It is the region where the maximum solvent capacity and the largest variations in solvent properties can be achieved with small changes in temperature and pressure.
The state of the substance is called supercritical fluid (SCF) when both the temperature and pressure exceed the critical point values as schematically described in a pressure-temperature phase diagram.
Supercritical fluid CO2 extraction (SCFE) process flow is shown in the diagram. The feed, generally ground solid, is charged into the extractor. CO2 is fed to the extractor through a high-pressure pump (100 to 350 bar). The extract-laden CO2 is sent to a separator (120 to 50 bar) via a pressure reduction valve. At reduced temperature and pressure conditions, the extract precipitates in the separator, while CO2, free of any extract, is recycled to the extractor.
SCFE for solid feed is a semibatch process in which carbon dioxide flows in a continuous mode, whereas the solid feed is charged in the extractor basket in batches. Separation is often carried out in stages by maintaining different conditions in two or three separators for fractionation of the extract, depending on the solubility of the components and the desired specifications of the products. Similarly, by varying the pressure, it is possible to alter the solvent power of the extractant, the effect of which is equivalent to changing the polarity of an extraction solvent.
Thus a production plant can have flexible operating conditions for multiple natural products and it is also possible to obtain different product profiles from a single botanical material by merely using a single solvent, namely, supercritical CO2.