Principles of supercritical fluid systems (5) What is supercritical fluid extraction?

November 8, 2019

Supercritical fluid extraction (SFE) is a method in which supercritical fluid as an extraction medium is added to substances containing target components, and extraction is performed based on differences in solubility. In particular, the use of supercritical carbon dioxide as an extraction medium offers many advantages in a variety of fields, including short extraction times, simplified operation, and improved extraction efficiency compared with the organic solvent extraction method. Also, solvent removal following extraction is much easier, allowing more accurate component concentrations to be determined.

Because the critical temperature of carbon dioxide is only 31 °C, extraction is possible close to room temperature. Furthermore, extraction can be carried out in a carbon dioxide atmosphere without oxygen. This means that this method can be used for substances that are temperature-sensitive or easily oxidized. Supercritical carbon dioxide is effective at dissolving lipid-soluble or low-polar compounds, but it can be adapted to the extraction of highly polar components by adding alcohols such as methanol or organic solvents such as acetonitrile and tetrahydrofuran as auxiliary solvents (referred to as entrainers).

In recent years, it has attracted attention as an environmentally friendly extraction method that does not use harmful organic solvents, in line with the concept of Green Chemistry.

Table 3 Comparison of conventional organic solvent and supercritical carbon dioxide extraction and pretreatment methods

Organic solvent Disadvantages
Soxhlet extraction & purification Uses harmful organic solvents
Solvent extraction & purification Process is complicated and time-consuming
Long pre-processing time
Higher sample concentration required
Trace residual solvent
Supercritical carbon dioxide Advantages
Carbon dioxide Uses safe carbon dioxide
Carbon dioxide & ethanol, etc. Simple process that can be automated
Fast pre-processing and extraction
Selective extraction possible
Online SFE-SFC
Prevention of sample oxidation, operation at low temperature

 

Table 4 Applications of supercritical carbon dioxide extraction

Field Material Purpose
Food Coffee beans Decaffeinated
Tea Flavor ingredient
Hop Hop extract
Red pepper Spicy extract, pigment
Fish meal Fish oil
Fish oil EPA, DHA
Plant Essential oils, pigments, medicinal ingredients
Vegetable oil Vitamin E
Pharmaceutical Natural products Medicinal ingredients, bioactive substances
Tablet Residual solvent
Chemical Coal, oil, energy
Plastic, rubber, polymer
Metal
Inorganic compounds and ceramics
Aerogel
Electrical / electronic parts
Low boiling point component
Polymerization solvent
Monomer
Oligomer
Impurities
Nasty smell
Developing agent
Degreasing
Dehydration
Solvent removal
Washing
Others Cigarettes Nicotine
Soil Organic matter

 

Supercritical carbon dioxide extraction system

The basic configuration and flow diagram of the SFE system are shown in Fig. 6. The system is configured with a liquefied carbon dioxide feed pump (2), an entrainer feed pump (5), a high-pressure vessel (8), a thermostatic bath (10), an optional detector for monitoring the extraction process, and a back-pressure regulator valve for controlling the extraction pressure (11).

Fig. 6 Configuration of supercritical fluid extraction system

For the high-pressure-resistance vessel, a column-type or cup-type design can be selected according to the state of the extracted sample, and various capacities are available. The extraction conditions can be set by changing the pressure, temperature, and modifier solvent type and amount as parameters. It is also possible to attach a viewing cell that allows real-time observation of sample extraction and reaction processes under supercritical conditions from the observation window.

Fig. 7  Pressure vessels used in SFE system (Upper left: column type, Lower left: glass type, Center: tower type, Right: observation type)