The processing of raw cannabis plants to extract cannabinoids and other compounds can be carried out using a variety of methods. Some of the most popular methods are extraction using CO2, a hydrocarbon (usually butane), or ethanol. Which one is used depends on a variety of factors, including the amount of raw material to be processed, and the final application of the product, for example what form it will take and whether it’s for recreational or medicinal use.
In this article, we’ll focus on the method of ethanol extraction. Ethanol extraction can be carried out by a number of methods including cold, warm, or room-temperature processing. Some ethanol extraction processes involve the use of a rotary evaporator setup to remove the ethanol from the extract solution.
Here, we take a closer look at ethanol extraction for cannabis applications, with a particular focus on ethanol removal using a rotary evaporator. We’ll include information about the steps involved in ethanol extraction, optimal conditions for removing ethanol, and misconceptions about the ethanol removal process.
Ethanol Extraction Methods for Cannabis Applications
Here’s a bit more information about the main methods of ethanol extraction:
- Room temperature process: This involves raw cannabis being soaked in ethanol to pull soluble compounds from the plant matter into the solvent. The cannabis is then removed and the solvent is evaporated.
- Cold process: The cannabis plant is soaked in ethanol that has been chilled. This prevents undesirable compounds such as waxes and pigments from solubilizing, but it also means extraction of desired compounds is slower.
- Hot process: This involves using hot ethanol to solubilize compounds. The Soxhlet technique is a popular example of hot ethanol extraction. This uses a specialized piece of equipment called a Soxhlet extractor. However, this is difficult to scale up and results in undesirable compounds, such as bitter-tasting chlorophyll ending up in the solvent.
Ethanol extraction is popular for various reasons. It can be used to extract a full spectrum of compounds from cannabis plants, or the process can be tweaked to prevent certain compounds from solubilizing. Equipment costs for ethanol extraction are generally lower than for other extraction methods. Another plus is that the ethanol can be recovered and recycled, allowing for cost savings.
In addition, ethanol extraction can be scaled, so it’s suitable for processing large volumes of cannabis. In some ethanol extraction procedures, you’re simply soaking the plant matter in a vat, so the major limitation is the size of the vessel. Depending on regional or industry regulations, you’re also limited by the acceptable volume of ethanol that can be safely used and stored at a given time. One more limitation is the rate of solvent removal you’re able to achieve (more on that below).
Basic Steps for Ethanol Extraction
For basic room temperature or cold process ethanol extraction of cannabis (without specialized equipment), here are the steps involved:
- The plant is soaked in ethanol, either in a bag (like a giant teabag) or loose. Certain compounds within the plant will solubilize in the ethanol. The specific compounds that will be extracted will depend on the makeup of the plant and the temperature of the ethanol. For example, cannabinoids such as THC (tetrahydrocannabinol) and CBD (cannabidiol) solubilize easily even in cold ethanol. Other compounds such as terpenes (fragrant oils) and plant waxes won’t solubilize in cold ethanol but will solubilize in room temperature ethanol.
- After sufficient time, the plant is removed from the mixture, either by removing the “teabag” or some other means of filtering.
- In some processes, the next step is “winterization.” This involves adding more ethanol to the extract, then cooling it rapidly so that lipids precipitate out. Note that this is not typically needed in cold process ethanol extraction as the lipids won’t have entered the solution.
- Next, the solvent must be removed from the solution. This is where evaporation comes in. One popular method to remove the ethanol is to use a rotary evaporator, although other distillation methods may be used.
- What is left is a high-purity cannabis oil. However, there will still be some residual solvent, even after evaporation. This is often removed by drying the oil in a vacuum oven. There are also documented cases of using a hotplate stirrer to purge residual ethanol from cannabis oil.
Using a Rotary Evaporator to Remove Ethanol
Rotary evaporators (rotovaps) are efficient at removing volatile solvents from non-volatile samples. They lower the boiling point of the solvent and reduce exposure to heat (especially important for temperature-sensitive compounds). They are also environmentally-friendly and allow for the recovery and reuse of the solvent. With the right equipment and controls, rotovap processes have excellent repeatability.
Optimal Rotovap Operating Conditions for Ethanol Removal
The recommended vapor temperature for ethanol extraction is 25–30°C. Boiling ethanol at 30°C requires a pressure of 123 mbar, and boiling at 25°C requires a pressure of 95 mbar.
The 20/40/60 rule offers a general guideline for rotovap operating conditions. Under this rule, the bath should be set to around 20 degrees higher than your desired vapor temperature. In turn, the condenser temperature should be set at around 20 degrees lower than the desired vapor temperature. For ethanol extraction, the chiller is usually set to around 0°C, and the bath temperature is typically about 50°C.
Knowledge of these conditions along with rates of ethanol evaporation will help you decide what equipment you need to use.
Choosing Your Rotary Evaporator Equipment
Here are some tips for deciding which rotary evaporator, chiller, and vacuum pump to go with.
One of the main things you’ll be looking at when shopping for a rotovap is the size, of which they come in a broad range. For example, the LabTech EV311H Rotary Evaporator has a maximum capacity of 2 L, while the Calpha RE-550B Rotary Evaporator has a maximum flask size of 50 L.
Bear in mind that the rotation flask should not be more than half full. For example, with a 20L rotation flask, you can expect to process 10L of extract solution at a time.
Other factors to look at when deciding on a rotovap include the types of displays and controls and whether or not they have integrated vacuum control.
Choosing the right chiller for your rotovap setup is important. The chiller needs to have sufficient cooling capacity to cool the solvent. Note that the cooling capacity decreases as the setpoint temperature is decreased, so you need to be aware of the cooling capacity of the unit at your desired setpoint temperature. Most manufacturers list multiple cooling capacities in their specifications. We discuss cooling capacity of recirculating chillers in-depth in a separate post.
Selecting a suitable pump for your rotovap setup is important to avoid issues such as causing a safety risk, damaging the pump, or losing solvent. The pump needs to provide sufficient vacuum pressure and flow rate to create an efficient evaporation process.
However, an oversized pump can cause problems. Safety risks are an especially critical consideration when using a solvent like ethanol. For example, using an oversized pump could cause the pump to become flooded with ethanol. The vapor will clear too quickly out of the condenser without condensing, and enter the pump. You could end up with flammable solvent spurting out of the back of the pump.
Vacuum control is an important factor too, especially if you want to create the most efficient process possible and maximize the recovery of ethanol. Find out more about choosing a vacuum pump in our dedicated post.
Rotovap Sizes and Evaporation Rates
A variety of factors can affect evaporation rates, including:
- Vessel size
- Rotation speed
- Water bath temperature
- Vacuum level
While the last three of those can be tweaked with most rotovap setups, rotovap models are limited in terms of the size of the vessel that can be used.
Manufacturers may list evaporation rates for some solvents in their specifications or you can contact them directly to find out rates for ethanol and other solvents. For example, here are ethanol evaporation rates for some of the Across International models we carry:
- Ai SolventVap SE13 Rotary Evaporator (Capacity of 5 L): 1 gallon/hour
- Ai SolventVap SE26 Rotary Evaporator (Capacity of 10 L): 1.5 gallon/hour
- Ai SolventVap SE53 Rotary Evaporator (Capacity of 20 L): 2 gallon/hour
- Ai SolventVap SE130 Rotary Evaporator (Capacity of 50 L): 4 gallon/hour
Rates will vary by manufacturer and model. For example, a 20 L Heidolph rotovap has an ethanol evaporation rate about 50 percent faster than the 20 L Ai model, due to higher rotation speed and other design factors.
Misconceptions in Ethanol Removal for Cannabis Applications
When dealing with a rotovap setup, there are some common misconceptions:
- A recirculating chiller should be run as cold as it can go. This is not true. When you lower the set temperature, the cooling capacity of the system decreases. Instead of setting the chiller to the lowest temperature, you should use the 20/40/60 rule, as discussed above.
- You want as deep a vacuum as possible. This is also not true. Too deep a vacuum can result in the solvent having too low of a boiling point. You could end up with uncontrollable boiling and possibly bumping or foaming. The product will escape the evaporation flask and you'll have to clean your system and start over.
- The pump should be run at full speed. If the pump speed is too high, the solvent won’t have time to condense into the condensing flask. Instead, it travels to the pump, which can become flooded with solvent, damaging the pump and creating a safety risk. This also impacts your solvent recovery rate as you can’t recover solvent from inside a pump.
- You need as high a water bath temperature as possible. Similar to having too deep a vacuum, this could increase the evaporation rate of the solvent too much and overload the condenser. Too high a bath temperature can also cause thermal decomposition of temperature-sensitive compounds in your product.