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Improving Solvent Recovery in Cannabis and Hemp Extraction

By Alisia Ratliff

Published: Apr 30, 2021   
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“There is always one major problem to address with extraction beyond the safety requirements: downstream solvent handling,” Mark June-Wells, PhD, principal owner of Sativum Consulting Group.

I don’t know about you, but I don’t typically look at my business operation and say, “Hmmm…how much money am I going to throw away today?” The goal of any business is to be profitable and to do so in the cannabis/hemp industry planning the proper design of a processing facility is paramount to your success. These specialized laboratory operations pose specific considerations. There are several ways to extract and refine medicinal cannabinoids from plant biomass using various solvents (carbon dioxide, water, ethanol, butane, etc.). But what does one do with the spent solvent used after the extraction? And why is it important to figure this out?

The importance of solvent recovery

In a cannabis processing facility, various pieces of equipment are used to get to one common goal: producing a cannabinoid-rich consumer product. Using solvent-based methods to extract, process, and refine cannabinoid oils can be extremely costly to an operator that does not consider solvent management. Cannabis processors can mitigate the high cost of solvents and decrease its environmental impacts by procuring equipment that has an effective solvent recycling system. What is solvent recycling? Solvent recycling is when you are recovering and re-using most of the solvent in a process. For example, during supercritical CO2 extraction, 95-to-98 percent of the solvent used during the extraction run goes through a condenser and back to the holding tank free of contaminants to re-use. The other 2-to-5 percent is released into the atmosphere or retained in the spent biomass.          

In order to recycle solvents, you must first recover the solvent properly. Solvent recovery is a form of waste reduction and has a huge impact on keeping operating costs low. If you take 200-proof organic ethanol, for example, the amount you pay in excise tax on a 55-gallon drum can be double the price of the drum itself. With this premium cost, you want to get the most out of every dollar spent. The best way to do this is to recover and recycle the ethanol being used for extraction and refinement as many times as possible. Continuous monitoring of the solvent reuse is also pertinent as an increase in water content over time is the most frequent issue in solvent recycling. Even traces of water can cause unexpected solubility problems, side reactions, or the decomposition of a reactant.

Rotary evaporation

A rotary evaporator (or commonly known as a rotovap) is a piece of equipment used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation. This technique is highly effective for liquid mixtures that separate compounds based upon their boiling points. To increase the throughput of this process a vacuum is applied to create an atmosphere in which the solvent “cold boils.”

There are a few main components to a rotovap: the heating mantle, feed flask, rotor, condenser, receiving flask, and a vacuum pump. A rotovap can operate at atmospheric pressures, however this will prolong the time it takes to evaporate the solvent of choice. For instance, ethanol’s boiling point at atmosphere is 78.4°C, yet under vacuum (100 mmHg) ethanol boils at 31.5°C. Furthermore, exposure to high heat for long periods of time inevitably causing degradation of compounds.

The operation of a rotovap is reasonably simple and only requires proper training in laboratory safety and equipment operation. Typical operation:

  1. The water bath is heated and the liquid mixture is added to the feed flask using vacuum. To ensure ideal evaporation, an extractor should never exceed more than 50 percent of the volume of the flask to alleviate bumping. Flask rotations should cause an even thin film inside the flask and not be aggressive.
  2. Solvent droplets forming on the condenser coils indicate successful evaporation.
  3. The reaction is complete when no more solvent drips from the condenser coils. Using a bright color of ethylene glycol in chillers will make colorless solvent droplets more noticeable.

Many advancements in solvent recovery using rotary evaporation have been made in recent decades. Some systems have a fully automated operation, enclosures for the exposed glassware, multiple condenser options, and larger feed flask sizes. These improvements increase safety, reliability batch over batch, and decrease the level of engagement from the technician. The recovering of solvent using this technique has a capacity constraint that does not allow for uninterrupted operation.

Falling film evaporation

Because of the rotary evaporation limitations, cannabis processors may prefer a continuous style evaporation technique such as falling film evaporation. A falling film evaporator (FFE) is an industrial device used to concentrate solutions, especially with heat sensitive components and evaporate carrier solvents using a special type of heat exchanger.

Operating a FFE needs more technical training on heating components, cooling condensers, and vacuum parameters. There is a delicate balance between all three modules and the temperature/pressure settings. Typical operating considerations:

  1. The liquid mixture is fed from the top of the evaporation column (which can be either glass or stainless steel) and is distributed to flow down the inside of the tube walls as a thin film. At the bottom of this chamber is a collection vessel for the compounds of interest or a gear pump to discharge the material into a collection vessel.
  2. As the film of compounds descends, which is usually more viscous than the carrier solvent, the solvent enters the gaseous phase and travels into the condensing chambers. The vapors then return to the liquid state and travel via a pump to a large solvent collection tank for recycling.
  3. Because of the continuous nature of this equipment, when the evaporation is completed heaters, chillers, vacuum pumps, and gear pumps are shut down.

This technology allows an operator to improve the solvent recovery technique by allowing continuous feed and flow, shorter residence times of highly viscous liquids in the evaporator, continuous collection, and recycling of solvent. Technological advancements such as introducing flow meters as well as automating the pumps are enhancing this recovery technique exponentially.

During the process of refining cannabis oil, you encounter many pieces of equipment that have been used in other industries and for specified applications in laboratories for decades prior to now. Upgrades to solvent recovery equipment and process has not grown as quickly as other tech since most of the systems on the market resemble those 50 years ago. Early in the cannabis industry, the use of bench top rotary evaporators was dominant amongst producers. However, with the increase in demand from consumers, this has rapidly changed. Producers are now interested in equipment with high feed rates, throughput, and ease of use. The advanced technology of quality control sensors, automated pump controls, and enhanced safety features are effectively driving this process from the dark age into the new age.

Alisia Ratliff

Chief Executive Officer of Victus Consulting Ventures LLC.

Alisia is the CEO of Victus Consulting Ventures, a cannabis business consulting firm. With over fifteen years of experience across several industries, Alisia uses a breadth of technical knowledge to help build start-up companies, develop strategic direction, and sustain brands in new emerging markets. She has written for Analytical Cannabis since 2021.


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