CO2 extraction uses carbon dioxide, compressed until it behaves like a liquid solvent, to dissolve cannabinoids and terpenes out of dried cannabis or hemp; when the pressure is released the CO2 turns back into gas and leaves the extract behind. It runs in two regimes. Supercritical extraction works above CO2’s critical point of about 31 °C and 74 bar, where the fluid has liquid-like dissolving power and gas-like flow. Subcritical extraction works below that point, with CO2 as a cold compressed liquid. Operators pick pressure, temperature and time to steer which compounds come out, and the finished crude is usually refined further before it becomes a product.
The critical point, with numbers
The critical point is where the separate liquid and gas phases of a substance stop existing. For CO2 the reference values are well established. The NIST Chemistry WebBook lists a critical temperature of 304.18 K and a critical pressure of 73.80 bar from one of its evaluated datasets; a narrative review of cannabis processing gives 31.06 °C and 73.83 bar. In practical terms, about 31 °C and 74 bar.
That low critical temperature is the reason CO2 is used at all. Water can also become supercritical, but only around 647 K and 22.1 MPa, far beyond what is useful for heat-sensitive plant compounds. The same review notes that CO2 is non-flammable, non-toxic, easy to remove and relatively cheap, which is why it became the default supercritical solvent for botanicals.
Supercritical vs subcritical
| Subcritical | Supercritical | |
|---|---|---|
| Conditions | Below about 31 °C and/or 74 bar | Above both |
| State of CO2 | Compressed liquid | Supercritical fluid |
| Dissolving power | Lower, gentler | Higher, adjustable with pressure |
| Typical use | Volatile fractions, low-temperature runs | Full-range extraction, targeted fractions |
| Trade-off | Weaker solvent, less extracted per run | More waxes carried over at high pressure |
The subcritical side is often marketed as better for terpenes. The underlying physics supports “gentler”, since the solvent is colder and weaker, but how much difference it makes to a specific extract depends on the material and the run settings, so treat blanket claims with caution.
Why CO2 is selective
In the supercritical range, density climbs towards that of a liquid while viscosity stays close to that of a gas. Density largely controls how much a fluid can dissolve, so raising pressure raises solvent strength, and lowering it narrows what comes out.
A 2022 study on industrial hemp flowers in the journal Pharmaceuticals shows how this plays out. Working at 60 °C, the authors found that a relatively low 131.2 bar was best for monoterpenes (the class that includes myrcene, limonene and alpha-pinene), extracts richer in sesquiterpenes (the class that includes beta-caryophyllene) came at 319.7 bar, and CBD extraction peaked at 284.78 bar. Yields across their runs ranged from 0.75% to 8.83% depending on pressure and temperature. That is the practical meaning of “tunable”: one machine, different settings, different chemical profiles.
Some processes also add a co-solvent, usually ethanol, to the CO2 stream. It raises total yield, but a hemp purification study notes that it does so partly by co-extracting chlorophylls and other compounds, which then have to be removed.
Winterization
Higher-pressure CO2 runs carry over plant waxes and lipids along with cannabinoids. Winterization removes them: the crude is dissolved in ethanol, chilled so the waxes precipitate, filtered cold, and the ethanol is evaporated off. One published hemp protocol held the extract, dissolved in ethanol to 10% by weight, at −20 °C for 24 to 48 hours before filtering. This matters because “CO2 extract” on a label often means CO2 crude plus an ethanol refining step, not CO2 alone.
Advantages and drawbacks
Advantages:
- No flammable solvent in the extraction vessel, and CO2 leaves the product as a gas when pressure drops.
- Selectivity can be adjusted with pressure and temperature, as the hemp study shows.
- Low working temperatures compared with many thermal processes.
Drawbacks:
- High-pressure equipment is expensive to buy, certify and maintain.
- Yields vary widely with settings, so results depend heavily on operator skill.
- Crude often still needs ethanol winterization, which brings back a flammable solvent downstream.
“CO2 extraction machine for home”
This is one of the most common searches around the topic, so a direct answer: a supercritical CO2 extractor is not a household appliance, and small size does not change that. The system has to hold CO2 above about 74 bar just to reach the supercritical range, and working pressures in published cannabis studies run far higher, around 131 to 320 bar in the hemp work above.
The UK Health and Safety Executive’s guide to pressure systems lists what happens when such equipment fails: blast from the release of compressed gas, impact from failing parts and flying debris, and contact with the released contents. It also explains that most pressure systems need a written scheme of examination drawn up by a competent person, and that only very small systems are exempt. Commercial CO2 extractors are built, inspected and operated inside that kind of framework, which is a large part of why they cost what they do.
For a home user the arithmetic rarely works either. The equipment cost, plus certification, maintenance, CO2 supply and the post-processing steps described above, is aimed at producers running large volumes of material. And in Thailand, making an extract above 0.2% THC by weight falls under the Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569, which reserves production licences for Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542 (1999), government agencies and the Thai Red Cross Society. This post does not recommend or compare any machines or manufacturers.
Common mistakes
- Assuming “supercritical” means “stronger product”. It describes the solvent state, not the potency of the finished oil.
- Assuming CO2 extracts never meet another solvent. Many go through ethanol winterization.
- Treating low temperature as low risk. The hazard in CO2 extraction is stored pressure, not heat.
Check the current Thai rules and your local pressure-equipment requirements before acting on anything here.