Passive and Active Hydrocarbon Recovery in Cannabis Extraction
One of the most important steps of successful hydrocarbon extraction (see our complete guide) is the recovery of the hydrocarbon solvent that is used during the cannabis extraction process.
Solvent recovery is generally associated with ethanol but this article specifically pertains to the recovery of hydrocarbon solvents in a closed-loop cannabis extraction system.
To properly recover the hydrocarbon solvent, the collection vessel of the extraction system must be heated which forces the solvent into a vapor state and builds positive pressure inside the collection vessel. The vapor solvent is then moved back to the solvent tank by use of a mechanical pump, or by the temperature differential between the solvent tank and the collection vessel. This is where the terms “active” and “passive” recovery are used.
Active recovery refers to the use of a mechanical pump that pulls the vapor solvent from the collection vessel and then pushes it back into the solvent tank. Passive recovery refers to the use of the pressure and temperature differential between the collection vessel and the solvent tank to create a similar push and pull effect without the use of any mechanical systems between the two.
Below, we will look more in-depth at both of these types of hydrocarbon solvent recovery.
Active hydrocarbon solvent recovery systems require a mechanical pumping mechanism between the collection vessel and solvent tank.
These mechanical pumps require back pressure to operate at an optimal recovery rate. This backpressure is created by the evaporation of the solvent inside the collection vessel. As the collection vessel builds pressure, the vapor solvent is then pulled by the pumping mechanism into its inlet where it is then forced outward at a higher rate of pressure. The vapor then moves into a condensing coil where it returns to a liquid state and finally back into the solvent tank or storage vessel.
There are many types of recovery pumps on the market today for cannabis extraction, all of which are effective means of recovering hydrocarbon solvents. They often have different internal pumping mechanisms such as pistons or diaphragms and utilize belt drive or direct drive motors.
Larger pumps are capable of handling higher vapor loads and recovering solvent faster but they come with higher price tags. They may also require higher back pressure to operate effectively as well as larger chillers to properly condense large amounts of solvent vapor that are exiting the pump. Smaller recovery pumps are readily available and are used in many other industries such as HVAC.
The one thing all these mechanical recovery pumps have in common is that they are explosion-proof and rated for C1D1 hazardous locations. This makes them safe for use with highly flammable solvents such as propane and butane when they are properly installed.
Passive hydrocarbon solvent recovery systems do not use any kind of mechanical pumping mechanism.
Instead, they use the differential in temperature and pressure between the collection vessel, condensing coil, and solvent tank. As the collection tank is heated and positive pressure is created, the condensing coil and/or solvent tank are chilled using dry ice or a mechanical chiller which creates a vacuum and puts them under negative pressure.
This difference between positive pressure and negative pressure as well as temperature forces the vapor to be pulled towards the solvent tank where it is condensed back into a liquid and can be used again.
Effective passive recovery requires the correct cooling capacity to maintain the temperature of the condensing coil and/or the solvent tank. As warm vapor that is pressurized enters the condensing coil and/or solvent tank, it puts a heat load on the cooling mechanism. Whether it means adding more dry ice or using a sufficiently large chiller, the system must be properly set up to allow efficient condensation of the vapor solvent.
Photo Credit: Engineering ToolBox, (2008). Propane Butane Mixture – Evaporation Pressure. [online] Available at: https://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html [Accessed 07/01/2021]
Solvent Recovery System Conclusions:
Active and passive solvent recovery systems both require the extraction system to be set up properly. Most systems are set up to perform one type of recovery or the other. This may include larger chillers for better condensation of vapor solvent, sieves to remove moisture, or powerful heat exchangers to create higher evaporation rates and higher back pressures.
Both types of solvent recovery systems are very efficient when performed properly. The size of the extraction system, throughput, and dry ice availability are all important factors to understand when considering both types of solvent recovery.
Looking to incorporate solvent recovery into your lab’s operations?
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