Closed loop systems: Butane vs. Supercritical CO2


Most of what consumers know as marijuana concentrate, “dabs,” “wax,” or BHO is made with a butane extraction system. Butane is an inexpensive solvent, so it’s the most common one used by amateur extractors. Though BHO can be safely made in “closed-loop” systems—where the butane gas is trapped, recycled and reused in a controlled, sealed environment—it’s sometimes processed in a much cheaper “open blast” system that’s highly explosive, and extremely dangerous for non-professionals.

In the past, extracting of any kind was a great way to monetize a moldy crop or leftover trim that might otherwise be useless; but we now realize that most extraction methods are actually garbage in, garbage out, and the quality of nugs or sweet leaf used in the extraction ends up reflected in the quality of the concentrate—leading to a huge variety in the quality of BHO on the market. BHO varies greatly in color and texture, from yellow shatter to dark-green wax.

This happens because butane doesn’t discriminate about what it will pull out of the plant matter. Along with any THC, butane will also pull mold, mildew, and even naturally-occurring plant waxes into the concentrate that can be toxic when vaporized. BHO is also known for containing some residual amount of butane, even after purging, because it doesn’t completely evaporate. And no butane gas is sold as food- or medical-grade, so all canisters of butane contain at least some trace amount of lighter fluid lubricant—resulting in BHO that also contains these lubricating chemicals.

Supercritical Carbon Dioxide

But there are other solvents available besides butane for making concentrates. Supercritical carbon dioxide, or CO2, is a super-solvent: a low-toxicity, low-temperature, relatively stable compound that can extract from plant material without denaturing the product. With the right equipment and professional oversight, producers create more consistent products with CO2 that don’t contain residual solvent.

Currently, there aren’t many CO2 concentrates available on the market, partly because supercritical extractions—operating under extremely high pressure—require a higher initial investment in equipment and workspace. But this higher barrier to entry also makes CO2 extraction much safer than butane, as extractors are required to work with closed systems in specially-designed, vented rooms, when most open-blast butane extractions are performed in an apartment living room.

CO2 also offers extractors more control over what is extracted from the plant. One of the primary criticisms consumers have of BHO concentrates is that it just “gets you high.” Butane primarily pulls THC out of the plant material, leaving behind the cannabinoids and terpenes (chemicals in marijuana that contribute to aroma, and some claim provide additional medicinal effects) that have made marijuana flower popular with the medical community.

With CO2, producers—usually working alongside a chemist—have the option of extracting more selectively from the plant by applying varying pressures, temperatures, and co-solvents. This gives CO2 concentrates the potential for a more well-rounded flavor that better reflects the original marijuana product, and contains fewer chemical byproducts. CO2 as a super-solvent also destroys molds, mildew, pests, and bacteria as it passes through the plant material, making it a great option for processing and selling less-than-Grade-A cannabis flower. (CO2 cannot, however, erase chemicals or pesticides present in the original plant.)

Consumer reviews of CO2 extractions are mixed, in part due to mislabeling. Amateur BHO extraction is illegal, but with only a handful of licensed extractors actually operating, most of the BHO on the market is actually amateur-made. To keep up appearances with law enforcement, amateur BHO products are often labeled and sold by dispensaries as CO2 extractions, instead—so not many consumers are aware of the distinction.

It takes time to get a CO2 operation up and running, but as the market for concentrates grows and consumers develop a more refined palette, the potential for CO2 will flourish.

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