24.3 What is Supercritical Fluid Extraction?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
24.3 What is Supercritical Fluid Extraction?
Refer to Section 19.3 for some critical data on common supercritical fluids.
Supercritical fluids have been investigated since last century, with the
strongest commercial interest initially focusing on the use of supercritical
toluene in petroleum and shale oil refining during the 1970s. Supercritical
water is also being investigated as a means of destroying toxic wastes, and
as an unusual synthesis medium [1]. The biggest interest for the last decade
has been the applications of supercritical carbon dioxide, because it has
a near-ambient critical temperature (31C), thus biological materials can
be processed at temperatures around 35C. The density of the supercritical
CO2 at around 200bar pressure is close to that of hexane, and the solvation
characteristics are also similar to hexane, thus it acts as a non-polar
solvent. Around the supercritical region CO2 can dissolve triglycerides at
concentrations up to 1% mass. The major advantage is that a small reduction
in temperature, or a slightly larger reduction in pressure, will result in
almost all of the solute precipitating out as the supercritical conditions
are changed or made subcritical. Supercritical fluids can produce a
product with no solvent residues. Examples of pilot and production scale
products include decaffeinated coffee, cholesterol-free butter, low-fat meat,
evening primrose oil, squalene from shark liver oil. The solvation
characteristics of supercritical CO2 can be modified by the addition of an
entrainer, such as ethanol, however some entrainer remains as a solvent
residue in the product, negating some of the advantages of the "residue-free"
extraction.
There are other near-ambient temperature supercritical fluids, including
nitrous oxide and propane, however there are safety issues with some of them.
There are several introductory texts on supercritical fluid extraction,
including some the ACS Symposium series [2-4]. There are also a large
number of articles on applications of the technique, including processing [5],
extraction of natural products [6], and chemical synthesis [7]. The major
concentration of information occurs in the various proceedings of the
International Symposium on Supercritical Fluids [8]. There is also a Journal
of Supercritical Fluids.
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