by Ed Rosenthal
We all have a basic understanding of CO2. Plants use it. Humans exhale it. At atmospheric pressure, CO2 has no liquid state. It goes from a solid (dry ice) directly to a gas. But at pressures greater than 5.1 standard atmospheres, it does liquefy.
Extract artists exploit the physical properties of liquid and “supercritical” CO2 to extract and concentrate cannabinoids found on marijuana plants. The machines are expensive and require several days of training to learn how to operate, but have a number of advantages. They are far safer to operate than butane extractors, because carbon dioxide is not flammable. And any CO2 left in an extract is harmless, whereas a large amount of residual butane in poorly purged BHO is not healthy.
Large extraction companies are turning to CO2 because the equipment converts large amounts of leaf and trim into some of the cleanest, most valuable cannabis products in the world.
CO2 extracts are four to five times as concentrated as the best buds yet they can be inhaled without the irritation of smoking raw cannabis. CO2 extracts can also be used to make super-potent edibles, topicals, and other marijuana-infused products.
OVERVIEW
CO2 extraction involves using cool, pressurized carbon dioxide in either liquid or supercritical form to strip the psychoactive ingredients from the surface of the cannabis plant. The CO2 is then purged from the solution, leaving the plant’s waxes, fats, lipids, and cannabinoids behind. The process requires tanks of CO2 and an extractor designed to function under the extreme cold and pressure of the process.
Gas is liquefied then pumped through a pressure vessel packed with cannabis. The resulting mix is depressurized in a separator vessel, resulting in the cannabinoids, oils, waxes, and anything else in the solution falling out of the mix as the CO2 exits the vessel and is vented off.
“Supercritical” CO2 extraction refers to extractions that occur beyond the “critical point” of carbon dioxide. Normally we think of CO2 in gas, liquid, or solid form. But past CO2’s critical point, the molecule ceases to exist as a typical solid, liquid, or gas. Supercritical CO2 looks like a very dense fog.
Supercritical fluid has no surface tension. It moves through the vegetative material like a gas and dissolves 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 5000 to 8000 psi.
SFE Diagram.
Illustration: Conor Buckley
DANGER!
CO2 is nonflammable and nontoxic. But the extreme pressure that it is used at can still hurt. People have built their own homemade CO2 extractors for small batches. This undertaking should not be considered lightly. If you are not experienced in the construction of high-pressure/extreme-temperature devices, this is not a good work project for you to undertake.
Wear safety glasses during extraction. You don’t want a leak to hit you in the face. (But just for some perspective, keep in mind that every bar and restaurant has CO2 under pressure to push beer into mugs. Those cylinders in bars are at 800 psi, the beginning of the typical range for cannabis extractions.) At higher pressures, safety glasses will make little difference. When a 10000 psi extractor fails, it creates an enormous bang like a shotgun going off, and can damage anything in the immediate vicinity of the failing part. Technicians have lost their fingers trying to adjust a failing seal in the middle of a high-pressure extraction. Don’t risk it. If you suspect a failure, stop and depressurize the system.
“Subcritical” CO2 extraction uses CO2 in its liquid state, below its critical point. Pressure of between 800 and 1500 psi is applied to CO2 at a temperature of 35°F (2°C)–55°F (13°C). The CO2 liquid is pumped through plant material and it dissolves the oils and terpenes. Then the CO2 is decompressed and returns to its gaseous state. The oils, consisting mostly of cannabinoids, terpenes, and waxes, precipitate out and are collected on a nearby surface.
Supercritical fluid extraction has some advantages over subcritical (liquid) extraction. Supercritical 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. Another advantage is that it can be used to select particular molecules for extraction by manipulating pressure and temperature.
The downside is supercritical fluid CO2 reacts with moisture to form carbonic acid, which turns oils rancid. For this reason all material used for extractions must be completely dry.
Supercritical CO2 extraction is likely the future of legal, marijuana-infused products because manufacturers can pinpoint molecules they want to extract and leave everything else behind. The end product contains virtually no residual CO2, which we inhale with every breath and which is harmless.
While CO2 extractions create no harmful residues, BHO extractions do contain remnants of the extraction solvent, which makes many people reluctant to use these concentrates. Butane extraction has higher yields than CO2, but butane is explosive, while CO2 is not. CO2 has much higher start-up costs than BHO as far as equipment. However, the start-up costs of a fully permitted, legal BHO extraction center can equal or surpass that of CO2 extraction.
This Hi-Flo system from Eden Labs can be used with a 5hp compressor for energy efficiency. The air-drive liquid CO2 pump needs little maintenance.
Photo: Brad Huskinson
A CO2 EXTRACTION METHOD
Equipment
•Plant material (fresh, cured, very dry, and trichome rich, cleaned of all debris)
•Liquid CO2 cylinder with dip tube (a 50-pound cylinder goes for about $20; you’ll need 75 pounds to extract 5 liters)
•CO2 extractor (Eden Labs 5 liter, 2000 psi; Waters SFE 500 (Super Fluid Extractor); Apeks 1500-1L)
TERMS OF EXTRACTION
Carbon dioxide extraction of cannabis fits into a broader practice of plant extraction that dates back hundreds of years. Plant extracts are used extensively in medicine and the perfume industry. The industry breaks plant extracts down in three general categories, each more refined: essential oils, concretes, and absolutes.
Essential Oils: An essential oil is a concentrated hydrophobic liquid containing the volatile aroma compounds and other “essential” molecules of a plant—including its lipids and, in the case of pot, its cannabinoids. Essential oils are obtained by pressure or steam, water, or dry distillation.
Concretes: Essential oil extracts made from hydrophobic solvent are called concretes and are a mixture of oil, waxes, resins, and other oil-soluble plant material, like THC. They can be hard, malleable, or viscous depending on wax content.
Absolutes: Concretes treated with another solvent to remove their waxes and leave just the fragrant, essential oil are called absolutes. They’re usually highly concentrated viscous liquids but can be solid or semisolid.
•Industrial CO2 pumps, rated anywhere from 5–10000 psi, .01–50 ml/min flow rate, +/-2% pressure accuracy; stainless steel fluid path
•Heater
•Chillers
•Condensing coil
•Cold gloves
•Pyrex dishes
Pre 97 Bubba CO2 Oil by CO2 Made requires little additional processing beyond the extraction process. CO2 concentrates are made without heavy metals, hydrocarbon materials, or residual contaminants.
Introduction to Gear
Here are the basic components of a CO2 supercritical extractor (SFE machine) from beginning to end:
•CO2 supply—your tanks of CO2
•Cooling bath/heat exchanger—to ensure CO2 is in liquid state when it goes to pump
•CO2 pump—the heart of the system; receives liquid CO2 at 750 psi and takes it to supercritical levels
•Heater/heat exchanger—used to raise pressurized CO2 to supercritical temperatures
•Extraction vessel—where CO2 and plant matte
r meet and extraction occurs
•Back-pressure regulator—reduces pressure, turning supercritical CO2 back to a gas, causing precipitation of solutes
•Fraction collector/collection vessel—where precipitated solutes collect
•Vent—where CO2 comes out of the system
•CO2 condenser/chiller—used to reliquefy CO2 for recycling
System Setup and Checks
These units range in size from desktop to garage-sized and need to be placed in a structure that has enough power and ventilation, ideally something like a mechanic’s bay. Smaller systems run on 110V, but larger systems usually use 220V circuits. The pumps, heat exchangers, and other equipment generate heat and noise, so the space must be able to accommodate it.
Each equipment manufacturer’s machines have their own procedures and techniques. For that reason, the equipment comes with comprehensive guides. Some manufacturers provide personal training, since the process can be quite complex.
Key Tips
•Make sure the seals are tight, but don’t overtighten or cross-thread them.
•Use a procedures guide, a safety checklist, and the same extraction process every time.
•Do not guess about procedures because it can be very dangerous.
Blasting
After your system is all set up and you are trained on it, follow the basic extraction steps below.
1.Warm everything up: Start up the chiller and the heat exchangers. They need a bit of time to warm water or cool coolant to their set points. Bringing hundreds of pounds of stainless steel extraction equipment to its specific operating temperatures can take 3–4 hours.
2.Load the extraction vessel with your prepared material and bolt the lid.
3.Pressure test the system for 15–20 minutes to make sure there are no leaks. Keep a watchful eye on the gauges looking for sudden pressure drops or temperature changes.
4.Extract: Send liquid CO2 through the pump, which raises its pressure to 5000–8000 psi. The 80°F (27°C) heater makes the liquid go supercritical. The CO2 mixes with the trim in the extraction vessel, dissolves the plant’s oils, and flows out to the back-pressure regulator.
5.Precipitate: The back-pressure regulator takes the supercritical CO2 solution from 5000+ psi to 45 psi, flashing it back to a gas and causing the essential oil to fall out.
6.Vent/Recirculate: CO2 is then vented outside the building or sent to the compressor/chiller that reliquefies the CO2 and sends it back to the beginning of the system. It can take 20 minutes to an hour to fully extract from trim, depending on vessel size.
7.Depressurize the extraction vessel and open the drain valve to receive the oil.
8.Clean: Run the system dry to clean it. Clean in between different strains and at the end of the day.
Tips
You’ll get more consistent results at the low end of the supercritical range. The higher you go in the supercritical range, the hotter and more soluble the CO2 becomes, which can burn cannabinoids, and extract plant matter.
As gas drains from the CO2 tank, it depressurizes and cools. This can create bubbles or voids in the line, which can cause flow problems. Prevent these bubbles by not using nearly empty CO2 tanks. Use a scale to keep an eye on how much CO2 is left in a tank. When it gets to a certain weight indicating it is low, swap it out.
Keep the temperature in the extraction room at a consistent 65°F (18°C) and keep humidity as low as possible. Don’t extract in hot environments; the ambient heat will work its way into your system and sap your yield.
Purchase an American-made extractor. You’re going to need the manufacturer’s staff to train you, and you’re going to be calling them when you run into problems. An SFE machine is a very hands-on product.
The pump is the heart of the SFE system, and must be designed to withstand operating pressures and temperatures for prolonged periods of time. Waters, Apeks, and Eden Labs sell appropriate pumps. The pump is the part of the system that is most likely to require servicing, and they can run tens of thousands of dollars. Broken pumps are the chief cause of downtime. Prevent this by having a backup or replacement pump so the system can continue functioning while the pump is being serviced.
CO2 RESOURCES
Apeks
ApeksSupercritical.com
Eden Labs
EdenLabs.com
Waters Corporation
Waters.com
You can reuse your CO2 a couple times. A compressor and chiller reliquefies CO2, and charcoal microfilters scrub out the water and other impurities. Still it will eventually have to be vented off. It is still suitable to use for enriching gardens with CO2.
Purging
With CO2 you don’t really have to purge unless you are using a high amount of cosolvent. (Cosolvents, often ethyl alcohol, are used in some machines.)
Raw CO2 oil comes out of the drain valve bright red, yellow, orange, or amber. If it’s green, chlorophyll has been extracted. Most likely one of five things is happening: the pressure is too high; the temperature is too high; the trim is old, stale, or wet. Moisture changes the pH of CO2, making it more acidic and polar. Acidic, polar solvents extract chlorophyll and other undesired elements.
If used in an open system, the CO2 evaporates to a gas from its supercritical or liquid state and returns to the atmosphere. Vent it outside or it can kill you. The process yields a thick oil. Strain and processing can play a role in determining how much wax is in the raw oil.
When the paste is left sitting in the open air it slowly settles into a blob, sort of like a bead of water on a hydrophobic surface, or into an oil-like consistency.
You can further alter this product for use in vape pens or edibles. Commercial manufacturers sometimes add propylene glycol to the oil to make it runnier for flowing into pens and cartridges. However, we don’t recommend that you ingest one of the main ingredients of windshield washer fluid. By changing the techniques you can make different consistencies, such as a honeycomb-like crumble consistency, or slow-flowing or hardened oils by dewaxing the material (see below).
The Coldfinger from Eden Labs enables distillation at low temperatures and creates faster extraction.
Photo: Eden Labs
Winterization/Dewaxing
Winterization in chemistry refers to the removal of waxes from a solution, usually by means of cold temperature. There are several ways to dewax CO2 oil in the depressurization stage of extraction. One way is to treat CO2 oil with another solvent, usually alcohol, to dewax it. Winterization boosts THC levels from 50%–70% to 80%–90%, but the increase comes at the expense of terpenes, which are lost in the process.
Decarboxylation
In order to use CO2 concentrates in edibles, they must first be decarboxylated. Decarboxylation is a process that uses heat to change raw THCA, CBDA, and other cannabinoid acids to their psychoactive, nonacid form. Decarboxylating a CO2 extract requires taking it up to 220°F (104°C)–240°F (115°C) for a certain period of time depending on the amount of oil you have. Even though CBD is nonpsychoactive, you still need to use heat to remove the acid from the CBD or your body cannot use it as it would if smoked.
Variations
Strain, growing environment, harvest time, curing method and length, and batch size all play roles in varied outcomes of CO2 extraction.
One strain may extract twice as fast as another strain. Strains vary in their yield, color of the oil, and consistency. Cannabinoid ratios play a role in solubility, color, taste, and smell.
Using all buds, small nugs, trim, or a combination thereof also results in different quality oils.
Keep variables to a minimum to get a repeatable outcome. Fully automated computerized systems like an Apeks or Waters CO2 SFE machine drastically reduce variability and produce more consistent oil compared to manual systems.
Storage/Pressing
Store CO2 extracts in a cool, dark place in a silicone container, which concentrates don’t stick to. Parchment paper can also be used because concentra
tes don’t adhere to it. They do stick to Pyrex and stainless steel.
Long-term exposure to air or light causes unwanted reactions with the active ingredients.
CO2 oil or honeycomb—the two most common consistencies—do not need pressing. The trichome heads were already broken in the violent extraction process and the active compounds dissolved into the solution before being chilled out of it.
People are using CO2 oil in vape pens, oil rigs, heath stones, or over a bowl of flowers. The flowers act as a sponge and prevent the heated CO2 extract—which liquefies—from running through a pipe.
Eden Labs
Company Spotlight
Eden Labs founder Fritz Chess created the company in 1996 to reinvent standard scientific extraction equipment. The first was the Coldfinger Home Unit, an updated version of the standard Sohxlet extractor used in labs since 1921. The Coldfinger method takes advantage of the evaporation and condensation principle that creates rainfall. A perforated basket of herb is suspended between a cooled condenser and a heated pool of solvent. The heat causes solvent evaporation, which then liquefies on the cooler condenser and saturates the herb with solvent, resulting in a steady drip of extract into the bottom of the flask.
In 1997, Eden Labs created a CO2 extractor based on the Coldfinger design, but it remained slow and required maintenance. In 2012, they started research and development for the Hi-Flo, a unique supercritical CO2 system. After a year’s worth of R&D, the Eden Labs team redesigned the pumping system, the solvent recovery system, and the vessel closures to create a more efficient CO2 design.