In botanical oil extractions, supercritical CO2 extraction (sometimes called supercritical fluid extraction or SFE) extracts oils from the plant material using different phases (liquid and gas) of carbon dioxide as the extracting solvent. CO2 is unique because its solvency power can change by simply adjusting the temperature and pressure during the extraction. When the temperature and pressure of the CO2 are above 1083psi AND88F, the CO2 is considered supercritical (high pressure/high temperature). If the temperature is drops below 88F, the CO2 changes to a liquid and is referred to as subcritical (low pressure/low temperature).
Supercritical is a phase that has a unique combination of gas and liquid-like properties – it can effuse through solids like a gas (meaning it has low surface tension and can get deep into the nooks and crannies of the plant material to get access to the oils within), and dissolves materials like a liquid (important for extraction!). Supercritical CO2 is sometimes described as a “cloud” because it is cloudy and you can’t see through it.
During an extraction, the CO2 is pumped into the extraction vessel containing the plant material to extract the oil. Then the CO2 and oil are pumped and decompressed through an orifice into a separator vessel converting it back to gas. The gaseous CO2 naturally separates from the denser, heavier oil which falls to the bottom of the separator vessel into a collection cup. The CO2 continues its journey through the closed-loop CO2 extractor, circulating until the run is complete, then flows back into the gas cylinder. About 90% of the CO2 is recovered during an extraction.
Supercritical vs Subcritical
Supercritical being a much stronger solvent than subcritical CO2, allows for faster and more complete extractions. This means the lighter oils as well as the not-so-desirable higher molecular weight materials (like waxes, paraffins, lipids and resins) are extracted. These fatty waxes, etc. will then need to be removed from the extract by a process called winterization (described below).
The advantage of subcritical over supercritical is that the extracted oil is pure, free of fats, waxes, and lipids so less post processing work, if any, is required. The purer oil doesn’t need to be further refined and can be used just as it is in vape pens, edibles, and other products. Subcritical oil is full of terpenes (smell) and flavonoids (taste) which are proving very popular among cannabis consumers. Subcritical extractions take longer than supercritical, but with the new Apeks Duplex system that is no longer the case. This system — optimized for fast subcritical extractions — extracts 1 pound in 20 minutes, which is comparable to a supercritical extraction.
Because all Apeks Mid-Range and High Production Series systems extract at both subcritical and supercritical parameters, processors can perform a “fractional extraction” by first extracting at lower temperatures and pressures to remove the lighter, temperature-sensitive volatile oils, and then subsequently extract the same material at a higher pressure and temperature to extract the remaining denser, heavier oils (waxes, etc.).
Supercritical vs Subcritical
The winterization process is commonly used to separate unwanted compounds from supercritical extractions. To winterize, the extract (which has the consistency of runny peanut butter) is mixed well with 200-proof alcohol, and then put in a deep freezer overnight. The next day, the mixture is poured into a funnel through a filter paper, trapping the waxes and lipids. This process can be done several times to completely filter all the unwanted elements. Once the oil is filtered, the alcohol is removed by heating the oil extract using a rotary evaporator. Alcohol boils at a different temperature than oil, so it evaporates, leaving the pure oil behind. Although effective, winterization is a labor-intensive process, requiring additional equipment.
Watch a video about winterization:
Short Path Distillation/Fractional Distillation
Customers can further refine oil extracts using a process called short path distillation whereby individual compounds are separated by applying heat. For example, in cannabis oil extracts, CBD can be separated from THC. Each compound boils at a different temperature. When the oil is reheated, each compound can be siphoned off at the relevant temperature.
Unless the plant material (cannabis) was heated before extraction (a chemical process called decarboxylation), the THCa in the plant remains inert, meaning it has no psychotropic elements. Preserving the THCa is essential if the extract is used in certain medicines. THCa converts to THC when heated. This is called decarboxylation. Non-decarboxylated oil is used in vape pens – when the oil is heated, the THC activates. Processors often decarboxylate their source material in an oven before extraction. This increases the yield of the extracted oil now containing THC used for non-smoking products, such as edibles, tinctures, and topical creams. Any post processing to further refine the extracts, must be done very carefully to avoid overheating the oil (thermal degradation).
To avoid thermal degradation during the extraction process, Apeks systems are engineered for cold separation processing. The vessels are water jacketed, which keeps the temperature below 40 F. Keeping the plant material cool is essential for high-quality oils. Our customers attribute Apeks systems and cold separation for allowing them to create consistent, pure, award-winning oils.
Apeks Supercritical systems can run at high pressures (up to 5000 psi), which sounds dangerous, but it’s not, because each machine is built with multiple levels of safety protection. Manufactured in an ASME-certified facility, every vessel features a non-isolable spring-loaded safety relief valve to prevent accidental over-pressurization and is safety tested before it’s installed. And the systems don’t require additional overheads for safety equipment. They can be placed in a room with no special venting. Only a CO2 monitor (pictured left) is required in case of a leak.
CO2 Extraction: Safer, Purer, Cleaner
There are a number of advantages to extracting with CO2, whether subcritically or supercritically. One is that no traces of the solvent are left behind versus other methods of extraction using hydrocarbons. Purer, cleaner oils are especially important for medicinal products. Additionally, CO2 is not a volatile solvent so there’s no need for special safety equipment. Finally, CO2 is relatively cheap, and Apeks systems recover and recycle 90% of the CO2 used in each extraction.