Supercritical Fluid Extraction With Carbon Dioxide (SC-CO2)

Supercritical fluid extraction or pressurized fluid extraction as it is sometimes referred to is a technology that compares extremely favourable with more traditional and conventional methods. These are hydrodistillation, steam distillation and solvent extraction The best and really only fluid used which behaves in a ‘supercritical’ way is carbon dioxide and much of this article is devoted to that material.

A supercritical fluid is any substance at a pressure and temperature above its critical point. Under these conditions, a supercritical fluid exhibits intermediate properties of gas and liquid. Due to its gas-like character, a supercritical fluid has better penetration and solvent power than liquids, and can diffuse easily through solid materials with
a rapid mass transfer rate (Khaw et al., 2017).

Whilst working on seed oil sources for a client, it’s become increasingly obvious that supercritical extraction using carbon dioxide is a highly useful technology for extracting seed oils in particular (Wang and Weller, 2006). The food industry uses the technology extensively for extracting lipids, oils, flavours, colours and bioactive materials.  It’s environmentally friendly compared to the various forms of solvent extractions that have been traditionally used such as hexane, and it’s cost effectiveness has improved. It also has benefits over steam distillation and mechanical pressing.  One of the benefits is the excellent recovery possible over a short period of operating time. It is also a relatively gentle method  allowing chemically sensitive extracts to be produced which would often be subject to thermal damage or solvent reactions if other methods were used. The intriguing physical properties of supercritical fluids where they have both gas- and liquid properties has produced an exciting technology which is still to be expanded (Reverchon, 1997).

The significant properties of supercritical carbon dioxide are its absence of any toxicity, non-flammability, low corrosivity, general chemical inertness and its low cost of production combined with high purity. As well as being odourless, its a property which means it leaves no residues or contamination of the extract which has been seen with hexane or toluene solvent extraction. The solvent is simply removed by reducing pressure. Compared to other supercritical fluids, it has a low critical temperature  (31.1°C) and pressure (7.38 MPa) (Wang and Weller, 2006). Compared to the liquid solvents, it appears to offer higher extraction rates because it improves mass transfer properties, by having zero surface tension combined with extremely low viscosity and higher diffusion coefficients. When the surface tension is very low, the supercritical fluid can penetrate the pores of the material readily which improves contact.

Oil extraction is very well covered in the literature and success depends on a balance between operating temperature and pressure. To manage selectivity and power, the best efficiency for extraction is achieved using sounding studies and a statistical design of experiments. Response surface methodologies seem to be the most appropriate when selecting process parameters.   A few good examples illustrate the point such as the extraction of vitamin E from wheat (Ge et al., 2002) and seed oils from pumpkin (Mitra et al., 2009). In time, legislation may well dictate this method supersede others to minimise solvent contamination and improve the footprint for production.

The Supercritical Fluid Process

1. Sample Preparation: Similar to pressurized solvent extraction, the plant material is cleaned, dried, and ground into smaller particles.
2. Extraction Cell: The ground plant material is loaded into an extraction cell or vessel.
3. Supercritical Fluid: CO2 is pressurized to its supercritical state and used as the extraction fluid.
4. Extraction: The supercritical CO2 is passed through the extraction cell, where it extracts essential oil compounds from the plant material.
5. Separation: The essential oil is separated from the supercritical CO2 by reducing the pressure, causing the CO2 to return to its gas state, leaving behind the essential oil.
6. CO2 Recycling: The CO2 can be recycled and reused in the extraction process, reducing waste and making it a greener extraction method.

Advantages of Supercritical Fluid Extraction (PFE)

1. High Selectivity: Supercritical CO2 has excellent selectivity, allowing for the extraction of specific compounds while leaving other components intact.
2. No Residual Solvents: Supercritical CO2 is used in its gaseous state, leaving no residual solvents in the final essential oil product.
3. Environmentally Friendly: PFE using supercritical CO2 is considered environmentally friendly as CO2 is a naturally occurring and non-toxic substance.
4. Mild Operating Conditions: The low temperatures used in PFE prevent the degradation of heat-sensitive compounds, preserving the overall quality of the essential oil.

Manufacturers of Supercritical Fluid Extraction Equipment

1. Waters Corporation: Waters is a prominent company in the analytical and extraction industry. They offer the Waters® SFE and Waters® Prep SFC systems, which are used for supercritical fluid extraction and chromatography applications.
2. Thar Process: Thar Process specializes in the design and manufacturing of supercritical fluid systems, including supercritical CO2 extractors. They provide extraction solutions for various industries, including pharmaceuticals, nutraceuticals, and food processing.
3. Applied Separations: Applied Separations offers supercritical fluid extraction and processing equipment, including their Supercritical Fluid Extractor (SFE) systems, tailored to the needs of research and industry applications.
4. IES (Isolate Extraction Systems Inc.): IES is a company dedicated to developing and manufacturing advanced CO2 extraction equipment, including supercritical CO2 extractors for the cannabis and hemp industries.
5. Eden Labs: Eden Labs is a well-established manufacturer of extraction equipment, including supercritical CO2 extractors. They offer systems designed for various applications, from herbal extracts to essential oils.
6. Apeks Supercritical: Apeks Supercritical specializes in CO2 extraction technology and provides systems for cannabis and hemp processing. They offer a range of systems, from smaller-scale units to larger industrial extractors.
7. ExtraktLAB: ExtraktLAB is a leading provider of CO2 extraction equipment, particularly focused on the cannabis and hemp industries. They offer scalable, high-efficiency systems for different extraction needs.
8. Supercritical Fluid Technologies, Inc. (SFT): SFT is a company dedicated to supercritical fluid processing, offering a range of supercritical CO2 extraction systems for various applications.

References

Ge, Y., Ni, Y., Yan, H., Chen, Y., Cai, T., (2002) Optimization of the supercritical fluid extraction of natural vitamin E from wheat germ using response surface methodology. J. Food Sci. 67 (1), pp. 239–243

Khaw, K.-Y., Parat, M.-O., Shaw, P. N., & Falconer, J. R. (2017). Solvent supercritical fluid technologies to extract bioactive compounds from natural sources: A review. Molecules, 22(7), pp. 1186 (Article)

Lang, Q., Wai, C.M., (2001) Supercritical fluid extraction in herbal and natural product studies – a practical review. Talanta  53, pp.  771–782.

Mitra, P., Ramaswamy, H.S., Chang, K.S. (2009) Pumpkin (Cucurbita maxima) seed oil extraction using supercritical carbon dioxide and physicochemical properties of the oil. J. Food Engineering, 95 pp. 208–213

Reverchon, E., (1997) Supercritical fluid extraction and fractionation of essential oils and related products. J. Supercritical Fluids 10 pp.  l–37

Wang, L., Weller, C.L., (2006) Recent advances in extraction of nutraceuticals from plants. Trends in Food Sci. Technol. 17, pp. 300–312.