The most comprehensive guide to supercritical extraction

Cannabis extracts are an important part of the fast-growing cannabis industry, especially in the medical market. Cannabis essential oil, a concentrate of all the active pharmaceutical ingredients in the cannabis plant, is a dynamic substance that can be transformed into many forms for user consumption. As a starting point, extracts can be transformed into products (with a bit of basic chemistry) such as tinctures, transdermal patches, effervescent tablets, beverage powders, suppositories and oral tablets, not to mention standard distillate and drip oils.

There are a variety of suitable solvents available for extracting active ingredients from cannabis - each with its strengths, weaknesses, laboratory infrastructure requirements, and production scale considerations.
This column will explore supercritical carbon dioxide extraction (SCCO2), including its capabilities, laboratory requirements, and features to consider when selecting an extractor.

Medicinal Value
A logical place to begin discussing CO2 extraction (or any type of extraction) is to provide a quick overview of the solutes that will be extracted from the cannabis plant that have medicinal value.

In this growing industry, there are two classes of cannabis substances that have received the most attention: cannabinoids and terpenes.

At least 113 cannabinoids have been isolated, and these molecules weigh between 250 and 350 amu (atomic mass units). Their physical form can be liquid or solid (depending on identity), contain a variety of functional groups, and are not volatile.

Terpenoids are a large and diverse group of compounds produced by plants and some animals. This class of molecules is classified by the number of basal isoprene units. (Isoprene is a common organic compound produced by plants.) In addition, terpenes and their related mixtures are responsible for the pleasant or unpleasant aromas emitted by plants. Terpenoids vary greatly in quality depending on the number of carbon atoms (or isoprene units), can include a variety of functional groups, and are physically liquid or oily.

Flavonoids and carotenoids are also present in cannabis. Although they are not often considered valuable in the cannabis industry, they are well-known biological plant compounds in the nutritional and medical industries. Flavonoids are polyphenolic compounds that give plant extracts their golden yellow and brown color. There are more than 5,000 known flavonoids, which vary in molecular weight and number of functional groups. They are usually pure solids.

Carotenoids are a group of molecules of medicinal importance with more than 600 known components. They tend to have a high molecular weight, contain a variety of functional groups and are orange to red in color.

Finally, many fatty acids and chlorophylls can be extracted from plant material. Although they are not generally considered to have medicinal value in the cannabis industry, there is some evidence of their biological activity in the nutraceutical industry. Fatty acids are typically 16 to 20 carbon atoms long, but can be much larger; they tend to solidify at room temperature and the degree of saturation (i.e., the number of hydrogen-carbon bonds) can vary.

Chlorophyll is the large molecule responsible for the plant's production of sugars from sunlight and water. The amount of chlorophyll varies between 800 and 900 amu, giving the plant extract a green to black color. When chlorophyll is oxidized, it appears black.
The carbon dioxide process
Now that we've covered most of the extractable solutes in cannabis, let's explore the function of carbon dioxide as a solvent.

Before we dive in, a quick review of some of the relevant physical properties of carbon dioxide will be helpful. Carbon dioxide is a gas at standard temperature and pressure. It forms a liquid at pressures above 5 bar (i.e., 73 psi), and its critical point (the vapor-liquid boundary) is 73 bar (1060 psi) at a temperature of 33.1 degrees Celsius.

Here, we will present the solvent properties of carbon dioxide in the supercritical state, because the gaseous state cannot be used as a solvent and the liquid state is not an effective solvent for cannabinoid extraction.

So, what are the characteristics of supercritical carbon dioxide (SCCO2) that make it an effective solvent for cannabis extraction? Supercritical carbon dioxide - and all supercritical fluids - have the density of a liquid, the diffusivity of a gas, and low viscosity (thickness). In simple terms, this means that SCCO2 has: a high solute carrying capacity (i.e., it can hold a lot of material), the ability to penetrate the smallest of spaces (like a gas), and very low resistance to flow. In addition, its polarity and density can be manipulated. Polarity manipulation can be achieved by adding a co-solvent (e.g. ethanol). Density manipulation is the real power of supercritical CO2 as a solvent. While other solvents such as hydrocarbons and ethanol are more effective in stripping cannabinoids and terpenes from plant material, SCCO2 has the unique ability to target specific parts of the parent (plant) material or individual solutes. These processes are possible because SCCO2 density depends on pressure and temperature parameters.

Solute-CO2 interactions are solute-specific. Each solute in the mixture (i.e., the parent plant material) has a unique solubility profile that is related to the density of SCCO2; there exists a density at which a specific solute becomes highly soluble in SCCO2. This is known as the crossover phenomenon. It is characterized by an exponential increase in the solubility of the solute in SCCO2. Because the crossover point is solute-specific - if the critical density of the target solute is known - then temperature and pressure gradients can be used to remove them individually.

We can also look at this crossover phenomenon from another perspective. Imagine using temperature and pressure settings that result in the extraction of all solutes from your feed, and then lowering the density downstream of the extraction location. This process is called retrograde dissolution and can be used to separate the components of the SCCO2/solute mixture.

Essentially, the process begins with a very high density of SCCO2, followed by successive pressure reductions, resulting in a continuous decrease in SCCO2 density throughout the process. As this process occurs, certain solutes are no longer dissolved and are collected at specific locations (i.e., separation vessels).

This ability to target or separate solutes in the mixture is the most valuable feature of SCCO2 extraction. Other favorable features of CO2 extraction include that it is generally considered safe (i.e., high exposure limits), it is relatively inexpensive, and it can be obtained with high purity from many sources.

Considerations for Carbon Dioxide Systems
So, what are the important features of supercritical CO2 Extraction Machine? As mentioned earlier, density, which is determined by pressure and heat, is a physical characteristic that determines the efficiency and separation of SCCO2 extraction. Therefore, three variables are most important.

Maximum pressure rating
The ability to measure the temperature of the CO2 (rather than the surface of the vessel), and
High wattage heater.
These characteristics are important because high pressures must be obtained to deliver heat in an efficient manner and to know the temperature of the CO2 in real time to adjust the density appropriately.

The extractor should also have a pump/flow monitoring system to assess the quality of the CO2 delivered to the extraction vessel. In addition, the pump should have the ability to deliver high flow rates to the parent material in the extraction vessel. This is because an important calculation variable for optimizing a supercritical CO2 extractor is the ratio of the mass of CO2 used in the extraction process to the mass of the parent material - typically a ratio of 50 or more is required to achieve 90 to 95 percent extraction integrity.

Finally, separation vessels with higher maximum pressure ratings are extremely important because they allow technicians to use a variety of pressures when developing separation (i.e., product development) protocols.
A disadvantage of SCCO2 extraction is that many waxes and fatty acids are also soluble in supercritical CO2. This is an important issue from a manufacturing perspective, as these materials need to be removed during the refining process prior to product development. This is achieved through a process called "winterization" which takes advantage of the differential solubility of waxes and cannabinoids in solvents at low temperatures (i.e. -30 degrees Celsius or lower).

The winterization process is often the slowest part of the refining process if the infrastructure does not match the productivity of the extractor. Standard protocols use funnels and filter paper in conjunction with vacuum. Depending on the throughput, this process can take from 4 to 8 hours. In addition, it must be repeated several times to account for the dissolution of the wax back into the ethanol during the long filtration process. However, the redundancy and lengthiness of this process can be circumvented by using a low-pressure cartridge filter technique that can quickly process large volumes of winterized material while controlling the temperature.

The next consideration is solvent recovery after the winterization process. It is important that the solvent recovery unit be sized to match your winterization/filtration productivity. Most commonly, a rotary evaporation system is used to recover the winterization solvent. It is also important that the system be sized to match your pre-solvent recovery step production and extraction rates.

To put these considerations into perspective, here is an example of a production system followed by the identification of a bottleneck.

First, it is necessary to lay out the assumptions.

Extractor input of 2000 g
Return rate of 0.18
Two extractions per day
Running time of five days per week.
Based on these assumptions, the yield per run is 360 grams per day and 3,600 grams per week. Therefore, with a 10:1 ratio of winterized solvent to extract, the total amount of material to be filtered is 36 liters.

The material can be cleaned of wax in 34 minutes with a positive pressure filtration system capable of 125 liters of water flow per hour and an equal volume of solvent. The final volume of solvent recovered is 72 liters, which can be recovered in four and a half hours using a rotary evaporator capable of handling 16 liters per hour. Evaluating these numbers indicates that your post-processing equipment is capable of extracting a week's worth of extract in approximately five hours. Therefore, your extraction parameters or extraction machine is the bottleneck of the production system described.

While this analysis oversimplifies the process in some ways, it does exemplify the importance of planning your total production system to meet the output of each stage, as capital may be better utilized to obtain a system with higher overall output. An unbalanced system can result in production equipment sitting idle for a period of time, which is not the best use of capital, labor or equipment.