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CO2 Essential Oil Extraction: Supercritical vs. Subcritical

CO2 extraction is an essential oil extraction process that has only been available in the past 100 years, especially in recent years due to the extraction of cannabis, it has become more widely known. So that if an essential oil store does not have CO2 oil, it will make people doubt the product grade and product quality of the store.
But when we mention CO2 extraction, many people think of supercritical CO2 extraction, and few people know that it also has a cousin, subcritical CO2 extraction. Both variations retain the plant’s beneficial compounds with no residual solvent contamination, creating a more palatable, high-quality product. 
In today’s article, we mainly clarify what is supercritical CO2 extraction, what is subcritical CO2 extraction, its principle, and its characteristics.

Two types of CO2 essential oil extraction equipment

There are two types of equipment for CO2 essential oil extractors: supercritical and subcritical. This means that there are two methods of CO2 essential oil extraction, supercritical CO2 fluid essential oil extraction, and subcritical CO2 liquid essential oil extraction.
It should be pointed out that, the extraction of essential oils in subcritical CO2 essential oil extraction equipment, can only have one extraction process: subcritical.
But both of the above are compatible with supercritical CO2 essential oil extraction equipment: an initial run of terpene removal may be performed first by an experienced operator in a subcritical extraction environment. In a subcritical environment, operators use low pressure and low temperatures to retain the most vulnerable compounds in plant biomass.

What is Subcritical CO2 Extraction?

Unlike supercritical CO2, subcritical carbon dioxide is in a liquid state.
The pressure of between 800 and 1500 psi is applied to CO2 at a temperature of 35˚F (2˚C)–55˚F (13˚C), putting the CO2 liquid below its critical point.

Subcritical CO2 extraction is the same kind of process; only it requires less pressure and lower temperature (non-supercritical fluid) than supercritical CO2 extraction. It occurs below the critical point (less than 1,083 psi and lower than 88˚F) where CO2 is in the form of a liquid. While subcritical CO2 has decreased solvent power, this can be advantageous, as it allows for more selectivity in the extraction process.

Subcritical CO2 is ideal for extracting terpenes and other more volatile compounds from plant biomass, and many of the more undesirable components (fats, waxes, and chlorophyll) are not readily soluble in subcritical CO2, subcritical CO2 extraction enables fractional distillation to produce plant essential oils rich in various special substances beneficial to the human body.

Subcritical CO2 Extraction of Cannabis and Hemps

Subcritical CO2 extraction produces a substance with a consistency closer to molasses, which, as mentioned below, preserves additional cannabinoids besides CBD.

Subcritical CO2 Extraction is useful because subcritical extraction is capable of fractionation, producing oils rich in CBD, THC, and other cannabinoids. The cooler temperatures used in subcritical also mean there is minimal decarboxylation that occurs in the process, preserving the acid forms of CBD and THC (CBDA and THCA) that are naturally present in the plant. The major drawback is time—subcritical CO2 extraction generally takes two to four times longer than supercritical to get the same yield.

What is Supercritical CO2 Extraction?

Agarwood essential oil supercritical CO2 extraction equipment
Agarwood essential oil supercritical CO2 extraction equipment

The CO2 gas we buy from the store is actually a liquid because its pressure is 60 bar, At this pressure and room temperature, CO2 is in a liquid state. However, when heated above 31.1˚C (critical temperature) and 1,071 psi (critical pressure), which has physical characteristics of both gas and liquid, allowing it to effuse through material like a gas while retaining the solvent properties of a liquid and becomes a so-called supercritical CO2 fluid.

Beyond the critical point of CO2, molecules no longer exist in their typical solid, liquid, or gaseous form. Supercritical carbon dioxide looks like a dense fog. Supercritical CO2 fluids have no surface tension. It can pass through the plant material in a gaseous state and dissolves the trichomes.

Carbon dioxide’s supercritical state results from a combination of temperature and pressure. CO2’s supercritical range begins at 87˚F (30˚C) and 1070 psi. (By contrast, water goes supercritical at 700˚F [371˚C] and 4000 psi.) Supercritical fluid extraction (SFE) is usually performed at 800 to 5,000 psi.

Supercritical CO2 Extraction Principle

The supercritical CO2 extraction is a type of essential oil extraction that uses carbon dioxide as the solvent. Unlike traditional solvent extraction, however, no residue is left behind which generally makes the resulting oils purer and safer for use.

This supercritical CO2 fluid s injected into the container loaded with biomass, used as the solvent to extract the oils from the plant material and can slide into porous materials in addition to dissolving them, all desirable active components of plant biomass are extracted.

Next, the compound-enriched solvent passes into another pressurized separation vessel, as pressures and temperatures fluctuate, collected the essential oils with the carbon dioxide returning to its natural gaseous state, leaving behind only the extracted essence of the plant.

Finally, the remaining CO2 is transferred to a condenser vessel where the temperature and pressure allow the fluid to stabilize back into a gas.

Supercritical CO2 Extraction of Cannabis and Hemps

In the application of cannabis and hemp extraction, the supercritical fluid passes through a chamber containing raw cannabis material, gently dissolving the membrane of the trichomes to capture their active compounds. The compound-rich solvent is then delivered into a vessel which lowers the pressure and temperature, separating the cannabinoids and terpenes. Finally, the remaining CO2 is transferred to a condenser vessel where the temperature and pressure allow the fluid to stabilize back into a gas.

Supercritical CO2 Extraction vs. Supercritical CO2 Extraction

  • In the application of cannabis and hemp extraction, supercritical co2 extraction produce a substance with the consistency of peanut butter. Higher temperatures and pressures used in supercritical co2 extraction can extract much larger molecules such as omega 3 and 6 lipids, chlorophyll and waxes which can then be removed through a process called winterization—leaving behind just cannabinoid oil. Subcritical CO2 extraction produces a substance with a consistency closer to molasses, which, as mentioned above, preserves additional cannabinoids besides CBD.
  • While supercritical extractions produce a greater yield and take less time than subcritical extractions, they are too harsh for some terpenes—resulting in a loss or a transformation of these compounds.
  • Subcritical CO2 extraction is the same kind of process; only it requires less pressure and lower temperature (non-supercritical liquid) than supercritical CO2 extraction. This process is longer, less efficient and produces smaller yields, but it retains and protects fragile constituents like essential oils, terpenes and other sensitive chemicals within the plant, which is highly desirable when producing full-spectrum cannabis products.
  • Supercritical CO2 fluids are faster extractors because of their low viscosities and high diffusiveness. CO2 is pumped through the extracting material 3 to 10 times using supercritical, rather than 10 to 40 times using subcritical.
  • Supercritical CO2 extraction can be used to select particular molecules for extraction by manipulating pressure and temperature.
  • Supercritical fluid CO2 reacts with moisture to form carbonic acid at pressures over 5000 PSI, which can turns oils rancid. For this reason, all material used for extractions must be completely dry.

Which is better?

Supercritical CO2 extraction was the most commonly acknowledged method of CO2 extraction, but as more emphasis is placed on full-spectrum plant products and the role of terpenes in cannabis products, subcritical CO2 extractions have challenged common thought and practice, scientists refer to as the “entourage effect,” a hotly debated topic of the cannabis industry.

Works Cited

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