Ethanol and Its Guidelines
The Food and Drug Administration (FDA) classifies ethanol as a Class 3 solvent with low risk for acute or chronic toxicity in pharmaceutical manufacturing processes where the residual is less than 5,000 ppm or 0.5 percent. The FDA also implies that residual solvents in this category should be limited to 0.5 percent through rigorous quality assurance and quality control programs.
Despite those FDA guidelines, some states have adopted more conservative safety limits suggested by the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH). OSHA and NIOSH set the worker environmental exposure limit for ethanol at 1,000 ppm of Total Weighted Average (TWA) over an eight-hour work shift, which means that some states are allowing only 0.1 percent residual ethanol in extracted products.
The National Fire Protection Association (NFPA), for its part, classifies ethanol as a Class 1 Division 2 Group D flammable liquid. As such, OSHA requires that ethanol vapors be held at 25 percent of the 3.3-percent by volume Lower Explosive Limit (LEL) through adequate ventilation in storage areas. Therefore, areas containing ethanol in production facilities must maintain no more than a 0.83 percent by volume ethanol vapor in the ambient air. (Adequate ventilation—as defined by OSHA and NFPA—is a system capable of cycling a room’s total air volume six times per hour.)
Numerous other considerations apply to the storage and use of ethanol in manufacturing laboratories that could fill a column of their own, so I will simply outline that, when it comes to storage, the maximum flammable cabinet storage is 60 gallons, and the maximum storage permissible outside of a flammable cabinet or storage room is 25 gallons.
Ethanol extraction is a single-stream process that can be conducted under warm or cold conditions. An example of a warm ethanol extraction processes is the Soxhlet technique. This technique essentially boils ethanol in a flask or pot, then condenses the alcohol on a cooled-coil, which then drips through the packed flower material, stripping the cannabinoids and terpenes during the process. The advantage to this approach is that the extraction is time efficient and of relatively low solvent-to-feed ratio. However, the warm-ethanol technique is generally a small-batch approach that extracts chlorophyll/waxes and decarboxylates the cannabinoids due to the heat involved. (Decarboxylation is the conversion of THCA, for example, to THC through heating and agitation that yields carbon dioxide during the process.) Therefore, heated ethanol extractions might require additional dewaxing and clarification steps.
This type of technique is also limited in the number of products it can produce because all the acid-form cannabinoids are decarboxylated during the extraction. While heating ethanol can increase the extraction process’s efficiency, ethanol is a good solvent for extracting terpenes and cannabinoids. Therefore, it can be used as an extraction solvent at room temperature or under supercooled conditions. Using ethanol at room temperature or under cooled conditions are the most common practices because these conditions allow for the retention of cannabinoid acid forms that can be leveraged to manufacture shatters, THCA crystals or THCA-rich oral formulations.
Know the Difference
A few differences exist between the outcomes of room temperature and supercooled ethanol extractions. First, room-temperature extractions generally extract more waxes and pigments than supercooled techniques, which results in additional dewaxing and clarification steps. However, room-temperature extraction techniques are more efficient. In short, ethanol is a very good solvent as it applies to the extraction of cannabinoids and terpenes. In the literature that describes the solubility of cannabinoids in ethanol, there is no definitive carry capacity, but many sources suggest that cannabinoids are soluble in ethanol at a 1:1 ratio (meaning that 1g of THC is soluble in 1mL of ethanol).
Finally, ethanol extraction can be conducted as expensively or inexpensively as the manufacturer desires. It can be conducted in simple vessels where the ethanol/plant material mixture is agitated by hand or in automated extractors that control temperature, inject a specific ethanol volume and undertake inline dewaxing/clarification.
Ultimately, there is always one major problem to address with ethanol extraction beyond the safety requirements: downstream solvent handling.
All types of ethanol extraction require that between 0.6 and 1 gallon of ethanol be used during the extraction process. The reason for this requirement is not due to the solubility of the cannabinoids or terpenes; it is due to the absorbent nature of the plant material. To extract the solutes from the feed material, ethanol must fully saturate the flower or trim. For that reason, a significant volume of ethanol is needed to execute the process with an efficiency rate of more than 90 percent. While some automated machines have built-in processes to minimize the required ethanol volume, the best-case scenario is that the amount of ethanol required ranges from 0.5 gallons to 0.6 gallons per pound.
Machines with automation features have also compensated for the problem of squeezing the ethanol from the plant material by adding spin-of-compression cycles to their processes. This is helpful because recovering all the ethanol from saturated plant material—which holds cannabinoids and terpenes—can be a difficult and messy affair with basic approaches. So, how does one deal with the large amount of ethanol required to extract the cannabinoids from a highly productive cannabis grow? The answer: planning and large solvent-recovery systems.
Scaling for Ethanol Extraction
To help elucidate the laboratory requirements associated with properly scaling ethanol extraction equipment to a grow operation, let’s look at another hypothetical example.
First, let’s assume that the grow is producing 720 pounds of extractable material (i.e., flower/trim) per month. With that information, the extraction system will need to be able to extract a total of 36 pounds of feed material every workday, each month (720 pounds ÷ 20 days). To meet that requirement, the ethanol extraction system will need to process 4.5 pounds per hour of each eight-hour workday. (There are solutions that claim to meet this specification; they often carry a heftier price tag.)
For this example, let’s also assume that we own a supercooled system capable of extracting at the aforementioned rate. That type of machine uses the least amount of ethanol (0.6 gallons/pound) during the process compared to other ethanol extraction technologies. So, the total volume of alcohol to be recovered and processed per day would be 21.6 gallons (36 pounds x 0.6 gallons/pound) or 81.8 liters.
To recover that volume of alcohol, the laboratory would require a large rotary or falling film evaporator (equipment made to gently remove solvents from samples by evaporation). For this example, a rotary evaporator is probably the most cost-effective choice. A large rotary evaporator capable of handling 16 liters/hour would recover the ethanol in about five and a half hours, which means that there would still be room for additional throughput on the extraction side; roughly, 17 pounds of additional feed material throughput per day is feasible under this example.
Ethanol is a solvent capable of extracting cannabinoids and terpenes efficiently. It also has a relatively low boiling point, which makes it easy to remove from final product, and a favorable toxicological profile including FDA limits in the range of 0.5 percent. However, there is a high solvent-to-feed requirement, which can create ethanol storage compliance issues and a need for an expensive, high-throughput rotary evaporator. Finally, ethanol cannot be tailored to separate cannabinoids or terpenes during the extraction phase to work into a predetermined product pipeline. Overall, ethanol extraction is an effective process most suited to high-throughput, bulk-processing laboratories that focus on a few products.