Hydrocarbons such as n-propane and n-butane are pressurized into a liquid state and used as solvents for the extraction of non-polar cannabinoids. The process is typically performed by passing the pressurized gas through the cannabis material contained in an extraction vessel. To avoid dragging the plant material, the biomass is retained by a mesh or filter at one end of the container (Oroskar et al., 2019).
Butane extraction is one of the most popular extraction methods due to its great versatility and potency, reaching cannabinoid concentrations of up to 90%. Butane extraction is the most used technique for the production of currently available cannabinoid concentrates, such as the popular “shatter”, a viscous substance with a high THC concentration (Oroskar et al., 2019; Radoiu et al., 2020).
Non-polar solvents like n-butane do not dissolve polar compounds such as chlorophyll, which is an advantage over ethanol extraction. Therefore, butane is an ideal solvent for selectively extracting neutral cannabinoids and terpenes. However, butane has a higher tendency to co-extract waxes that must be removed in further purification steps. In addition, the volatile and non-polar butane and propane have higher toxicity compared to polar solvents when inhaled since they are rapidly absorbed through the respiratory system and the blood-brain barrier due to their high lipophilicity; thus, they concentrate in lipid-rich organs such as the liver and the brain, causing effects on the nervous system (Sironi et al., 2016). Moreover, butane and propane are highly flammable; thus, safety facilities are required due to the risk of explosion. The difficulty in scaling up is another drawback to the use of hydrocarbon solvents for the extraction of cannabinoids. Hydrocarbon extraction is performed in batches and the scalability of the process is only feasible by the addition of multiple units (Radoiu et al., 2020). Moreover, the solvents used must be pure to prevent the presence of toxic hydrocarbons or heavy metals in the final product (Ayres, 2016; Keller, 2018).
Some examples of hydrocarbon extraction can be found in the patent literature. For example, US20200102283A1 (Dibble and Cole, 2020) describes a method of obtaining crystallized THCA (purity >95%) by first pouring butane (600 ml) at 10°C or colder for 5–10 min into an extraction column packed with optionally dried plant material (50 g–200 g; 3–12 L/kg) with 10% THCA + THC. This process can be repeated several times, obtaining an extract with 75% THC + THCA, an extraction yield of 12%, and 90% (THC + THCA) recovery from plant material. The obtained solvent extract is subjected to a cooling step to remove pigments and lignocellulosic impurities. THCA crystals with a purity >95% are then obtained by crystallization.
Another patent using a hydrocarbon-based solvent, such as butane, reports the extraction of cannabinoids from cannabis plant material but provided no data on its efficiency. The patent, US9789147B2 (Jones, 2017), describes in detail a closed-cycle extraction system that allows solved recovery. The butane contained in a solvent reservoir is chilled (−17°C to −78°C), condensed into the liquid phase, and then flows into an extraction chamber containing the plant material. Butane washes through the material, dissolving the extractable compounds, forming a solution that is collected in a collection reservoir, which is at a higher temperature to volatilize the solvent and remove it from the extracted compounds, leaving no residues. The resulting extract can be further purified, while the gaseous butane is returned to the solvent chamber and condensed. The process comprises a cold refinement chamber to solidify co-extracted waxes, which can be filtered from the extract along with solids and heavier oil components.
Invention US10888596B1 reports the use of propane as the preferred extraction solvent to produce extracts with high THC concentrations (high quality and purity) in the form of shatter, wax, or budder for dabbing or vaping (Hindi, 2021). According to the method, the starting material is frozen and dehydrated plant material, which is placed in a chamber with a chiller column (only to obtain a shatter product) so that the extraction occurs at −5 to −40°C to immobilize the lipids and water-soluble molecules within the plant material to make shatter. The gaseous solvent is pumped into the chamber and passed through a filter and the plant material at low pressure to maintain the solvent in a liquid state. The gaseous solvent is collected in a vessel and removed by applying heat so that the solvent is evaporated and pumped back into the gas holding tank. To produce a shatter product, the plant material “cookie” (a dry, crystalline, powdery round disc) is scraped from the chamber, powdered, and heated under vacuum at 21–43°C for 15 min to 48 h. Then, the powder is heated at 71–98°C under atmospheric pressure with a heat gun until a clear and transparent shatter product is obtained with 70–98% THCA/THC. To produce a wax product, the “cookie” is heated to 51–63°C to evaporate residual moisture and solvent gas. During the heating, the “cookie” is stirred until a waxy substance is obtained. Similarly, a budder product is produced by heating the “cookie” to 54–57°C to remove residual solvent or moisture. The “cookie” is stirred during heating to produce a budder product with a creamier texture. The authors did not provide any performance data such as extraction yield or efficiency. According to them, the final product, whether a shatter, wax, or budder, is free of chlorophyll and residual solvent. In addition to propane, other suitable solvents are butane and isobutane.
The lack of patents using gaseous hydrocarbons such as butane or propane as the extraction solvent is striking. To our knowledge, only patent US20200102283A1 (Dibble and Cole, 2020) has provided numerical data on the efficiency of butane extraction; thus, it is the only reference that can be used to compare the performance of butane to other solvents in the patent literature. While butane provides cannabinoid recovery and extraction yields in the same range as ethanol or hexane, the use of gaseous hydrocarbon solvents has several advantages; i.e., high cannabinoid concentrations, no solvent residues to remove the requirement for an evaporation step, and no co-extraction of chlorophylls compared with ethanol) Nevertheless, the co-extraction of waxes involves a mandatory winterization step when the target product is a cannabinoid isolate, whereas the intrinsic characteristics of the solvent such as toxicity and flammability, which implies the use of safety facilities, diminishes the desirability of gaseous hydrocarbons as extraction solvents.
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