This study considers the use of ethanol as a co-solvent in a carbon dioxide extraction of cannabinoids and its effects on extraction time and overall raw yield. A control test was initially conducted to calculate the volumetric flow of liquid and supercritical CO2 and to record the amount of material collected. Additional runs were then performed with the injection of a 5% by volume stream of ethanol, first with hops and then confirmed on cannabis. Upon separation of the oil from the ethanol it was determined that the oil was extracted up to 300% faster and with roughly 2% greater overall yield.
There is much discussion around increasing solubility power of a solution with the assistance of a co-solvent, but few, if any, studies consider the increase in solubility of CO2 with the presence of ethanol as a co-solvent. Often, various solvents are mixed together to obtain a larger amount of solute before it’s saturated. However, unlike many experiments involving co-solvents, CO2 as a solvent cannot exist at room temperature and pressure. That means, to determine if the solubility of the solution has increased, a dynamic research model is needed instead of the typical static models that can be used with other solvents. An increase in the solubility of the solution is just one concern. The ability of CO2 to select different cannabinoid fractions at specific times must still be retained, proving the introduction of ethanol can improve extraction times without detracting from the flexibility offered by CO2 as a solvent. Otherwise, a straight ethanol extraction might be chosen over a carbon dioxide and ethanol co-solvent extraction. Throughout this study, two key factors were considered: increase solubility as measured by an increase in yield, and extraction selectivity as measured by color and viscosity. The oils that are lighter in color and less viscous offer a decrease in post-processing time, making them more desirable to many end users. These more volatile components will separate out sooner than the waxes, fats, and chlorophylls that contribute to a darker, thicker end product.
This experiment was completed with a beta co-solvent module that consisted primarily of an ethanol tank and pump with some instrumentation and a check valve to ensure unidirectional flow of the ethanol into the system (see Figure 1). The ethanol tank supplied the ethanol pump with room pressure and room temperature ethanol. The pump would then increase the pressure of the ethanol from atmospheric to the operating pressure of the carbon dioxide system. A variable frequency drive would control the speed of the positive-displacement pump, and therefore the flow rate of the ethanol, to allow for a specific injection quantity. Volumetric percentages were chosen based on the standard for measuring co-solvent efficiencies. Finally, the necessary pressure valves and reliefs were added to connect this module to a CO2 extraction system. The piping and instrumentation diagram of the co-solvent test system is shown in Figure 1. A CO2 flow meter, placed in line between the separators and the pump on the closed loop CO2 extraction system, was used to monitor the flow of CO2. This returned a standard cubic feet per minute (SCFM) for the CO2, used to calculate a L/min flow rate based on liquid or supercritical CO2 at either 1200 PSI and 75 °F or 1800 PSI and 105 °F. The closed loop system allowed for the conversion of SCFM (mass flow) to L/min (volumetric). (See upper right for Figure 1, click to enlarge.)
The connection point between the co-solvent module and the CO2 extraction system was located just before a temperature control heat exchanger (red arrow in), which is located immediately before the inlet tubing for the extractor, where the raw material is located. (See upper right for Figure 2, click to enlarge.) This structure would allow the ethanol to mix with the CO2 stream, thus avoiding a temperature drop and ensuring the mixture was homogenized before going into the extractor. The CO2 and co-solvent mixture would then travel through the raw material and collect the oil before traveling to the separator. There the saturated solution would be depressurized, allowing the mixture of oil and ethanol to fall out of the solution and allowing the gaseous CO2 to return to be recompressed by the diaphragm compressor.
This experiment was performed on both hops and cannabis, at both subcritical and supercritical parameters. For the subcritical test, parameters of 1200 PSI and 75 °F were used. For the supercritical test, parameters of 1800 PSI and 105 °F were selected.
This was done to match the standard operating parameters and capabilities of the CO2 system. Any subcritical and supercritical parameters could have been used. Due to the increased pressure requirements and the associated losses in volumetric efficiency in the CO2 compressor, the main difference between the subcritical and supercritical runs was the CO2 flow rate. In turn, the ethanol flow rates differed. But since both subcritical and supercritical were compared to their control tests, and not to each other, this did not factor into the findings. The original experiment at our facility in Ohio was done using hops because of regulations in Ohio prohibiting testing with cannabis. The hops used were Columbus hops. In each test, hop pellets were ground, their input weights recorded, and then loaded into the extractor to be used as the raw material. A control test was done on both sets of data points before the co-solvent test was run. During this test, data samples were taken every 10 min and weights were recorded to get a yield curve in grams of oil versus time for a CO2 only extraction. The sample time started only after the system was up to pressure and flow had begun between the extractor and the separator. A representative portion of every sample was kept to evaluate the control extraction relative to the co-solvent extraction in terms of both color and consistency. After the control tests were completed, the same tests were replicated, with the only difference being the addition of a 5% by volumetric flow ethanol stream. This ethanol stream was started only after the system was up to pressure and flow had begun. Samples were again taken every 10 min, with each sample containing both botanical oil and ethanol. The samples were weighed and kept apart from one another until separation of the oil from the ethanol. Each individual ethanol and oil sample was then put through a rotary evaporator to separate the ethanol from the extracted oil. Both the weight of the oil and the volume of the ethanol were recorded. The ethanol volume was recorded to verify the flow rate of the ethanol pump since there was no ethanol flow meter present during the test runs. In the absence of an ethanol flow meter for this experiment, measurements of ethanol used were taken throughout the process to verify that the amount used was consistent with the pump motor speed. Once the control and experimental hops tests were completed, subsequent control and experimental tests were run in Pueblo, Colorado to verify the results on cannabis. The cannabis strain used was a high tetrahydrocannabinol (THC) strain that had been decarboxylated prior to the extraction test. Test cannabis had also been ground to approximately 100 µm for the test runs. The CO2 extraction system used in conjunction with the cannabis test had a higher CO2 flow rate than the one used with the hops tests, but the ethanol flow rate was increased to keep the volumetric percentages the same.