In the supercritical fluid extraction (SCFE) process, extraction is carried out by a
solvent above its critical pressure and temperature. In this case the solvent used is
carbon dioxide (CO2).
>> A brief description of the SCFE process is given below.
The commercial scale SCFE plant has three extractors, two separators, one CO2
hold-up tank and heat exchangers. The extraction process works in a closed loop
with constant circulation of CO2 in the system. It is a semi-batch operation with
typical batch time of 1.5 to 4 hours. CO2 is liquefied and fed to the hold-up tank
through a heat exchanger (Condenser). Liquid CO2 is then fed to the high
pressure metering pump, through a heat exchanger (sub-cooler) to ensure supply
of CO2 in liquid form. The high pressure liquid from the pump discharge is then
heated in a heat exchanger (pre-heater) to achieve supercritical temperature. The
Co-solvent pump can be used for dosing a small quantity of other solvents such as
ethanol, acetone etc. in to the CO2 stream, if desired. This supercritical fluid then
flows through the extractors. The solid material to be extracted is held in these
extractors with help of perforated baskets for easy handling. Extractors are
equipped with specially designed quick acting closures for easy and fast opening
of extractors. The supercritical CO2 with dissolved extract then flows to separators.
In the separators, lower temperature and pressure conditions are maintained for
recovery of the extract from the CO2 stream. Thus with proper selection of
conditions, fractionation of extract can be achieved by using two separators
operating in series. Separator-1 is maintained at supercritical conditions, where
typically high molecular weight components of the extract precipitate. Whereas in
separator-2 sub-critical conditions are maintained for complete recovery of any
extract from CO2 stream. The CO2 vapour leaving separator-2 is condensed and
fed back to CO2 hold-up tank.
When the solid feed in the extractor needs replacement or when plant is to be shut
down, respective vessels are depressurised to atmospheric pressure. In doing so
maximum possible CO2 is fed back to CO2 hold-up tank through CO2
depressurising line. CO2 from each vessel can be transferred till the pressures
become equal to that of the hold-up tank. Remaining CO2 is then vented off to the
atmosphere.
