Supercritical Fluid Extraction – More Pros than Cons
Supercritical Fluid Extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent. It is considered by many to be the most efficient mechanism to extract botanical components. SFE can be used as a sample preparation step prior to further analysis by other techniques, such as High Performance Liquid Chromatography, or on a larger scale to either strip unwanted material from a product or collect a desired product. With cannabis, for example, the goal is to collect CBD, THC, terpenes and other compounds while stripping away unwanted residues, like pesticides, chlorophyll and waxes, and leaving behind the biomass.
In an earlier blog post, I discussed the characteristics of a supercritical fluid and how they can effuse through solids like a gas and dissolve materials like a liquid. I also described how supercritical fluids are capable of breaking down structures where they can be separated or fractionated. And, I also noted that CO2 is the ideal solvent to be used in a supercritical fluid extraction. Now let’s take a closer look at the equipment required to perform a supercritical fluid extraction.
A typical SFE system includes a pump for the CO2, a pressure cell to contain the sample, a means of maintaining pressure in the system, and a collection vessel(s). The first step in the SFE process is to take gaseous CO2 and run it through a chamber where it will be subjected to extremely low (-70°F) temperatures and enough pressure to cause the gas to change into a fluid. Within this state, CO2 exhibits special properties that when reheated and pressurized become supercritical. In this state, the supercritical fluid passes through a chamber containing the raw cannabis material. Because of its unique properties, this supercritical fluid can pass through the raw cannabis very easily while gently dissolving the membrane of the trichomes to capture their many active compounds.
Next, the extraction process occurs when the compound-enriched solvent passes into another pressurized separation vessel, only this time the pressures and temperatures will fluctuate in order to fractionate the compounds such as cannabinoids and terpenes away. The dissolved material is swept from the extraction cell into a separator (or a series of in-line separators) at lower pressure, and the extracted material settles out. The CO2 extraction provides the ability to separate individual compounds as the extraction is taking place. Different molecular weight compounds drop out at different temperatures and pressures which enables for the fractionation to take place in different separators. The CO2 can then be cooled, recompressed and recycled, or discharged to atmosphere.
A research review from the International Journal of Chemical Science written by G.N. Sapkale et.al.1, provides an excellent overview of the Supercritical Extraction Process. The review outlines in detail the many advantages of SFE:
Environmental improvement and reduced product contamination: SFE is an alternative to liquid extraction using solvents such as hexane or dichloromethane. In conventional liquid-liquid extractions, there will always be some residual solvent left in the extract and matrix, and there can be some level of environmental contamination from their use. In contrast, carbon dioxide is easy to remove simply by reducing the pressure, leaving almost no trace, and it is also environmentally benign.
Selectivity: The solvent strength of a supercritical fluid can be varied by changes in the pressure and temperature which enables to the process to be fine-tuned to collect compounds of interest. We’ll talk more about this in future posts.
Speed: Extractions can be completed in just 10 to 60 minutes.
Purity: A supercritical fluid can separate for analytes by simple releasing pressure. Supercritical fluids are cheap, inert and nontoxic. Thus, they are readily disposed of after an extraction is completed by allowing them to evaporate into the atmosphere.
SFE Equipment Considerations
There are many considerations regarding the configuration of SFE equipment itself. What are the most desirable mechanical properties of the extractor ̶ vessel configuration and size, tubing size, configuration of piping, valves, heat exchangers and other components of the extractor. For example, the extraction system can incorporate one, two, three or more extraction vessels (what is the optimal size of the vessel?), multiple collection vessels, and other mechanical options that most efficiently accomplish the extraction based on the composition of the materials.
The advantages of SFE are clear and well documented over the years. However, the equipment is expensive and there is some level of expertise required to operate perform an extraction to achieve the highest efficiency. An ROI analysis will help determine whether it is more cost effective to contract an outside lab to perform the extractions or for you to make the investment in equipment, disposables and operator training.