What About a Wiped Film?
By Emmett McGregor, SciPhy CEO
When scaling distillation from the Lab to a Factory Plant it is necessary to leave behind batch Pot Still distillation and embrace a continuous mode of operation. The tool of choice? The Wiped Film Evaporator, but to be more specific the task of cannabinoid oil distillation is most often put in the pipes of a Short Path Wiped Film Evaporator. Why is that?
To Understand SPD, Start with WFE
To understand essential advantages of the Short Path Wiped Film Evaporator (SPD) we first need to look at its often overlooked, less specialized cousin often simply referred to as a Wiped Film Evaporator (WFE) or Thin Film Evaporator (TFE). The WFE is a very capable tool for continuous separations by distillation. A liquid mixture is pumped into the top of a heated column and wiped or rolled onto the surface where it forms a thin film. This film has a very high surface area to volume ratio on both its surface in contact with the evaporator, and the film surface which is exposed to the chamber of the WFE. When we distill hemp oil, we utilize vacuum to lower the boiling point of the target compounds and encourage them to evaporate at a relatively lower temperature. A WFE then passes this vapor out of a port and into a condenser mounted external to the body of the evaporator.
Because the condenser is separate from the evaporator body it can have as much surface area, and as large an internal volume as the engineer might choose. For this, and other reasons we will explore later, a WFE can handle a substantially higher vapor load than an SPD. Many low molecular weight compounds have very high gas to liquid volume ratios, meaning that for every liter of liquid you evaporate you will need to handle several times that volume of gas, sometimes as much as 100 times or more the starting volume of the liquid. That is a lot of vapor! For this reason we at SciPhy Systems recommend the use of a WFE for degassing and devolitization, sometimes just referred to as “deterping” in the trade.
In most cases both the WFE and SPD rely solely on the condenser to handle this vapor volume and maintain the target vacuum depth while under operation. The vacuum pump is used to achieve this vacuum depth when the process starts, and to remove any non-condensable gas, but really it is the condenser that keeps the vacuum down!
Pressure Drop: The Advantage of SPD
Why then use an SPD, a configuration in which the condenser is located inside the evaporator body, and so is limited in size and surface area? The answer lies in the pressure drop. While a WFE’s condenser can be sized and configured any number of ways external to the evaporator body, it still is located outside the evaporator, and as such the vapor must travel a relatively far distance from where it is generated, to where it is condensed. Typically the vapor passes through a vapor port that is smaller than the diameter of the evaporator, and often makes a turn of at least 90 degrees, more often 180. This design constraint means that no matter how large a surface area the condenser supports, there will always be a pressure drop between the evaporator body and the condenser itself, meaning that the evaporator body will never reach a vacuum depth below that pressure differential.
What is the solution? Put the condenser inside the evaporator body where the vapor can take a line-of-site path from point of vaporization to point of condensation experiencing near zero pressure drop. The SPD is the king of deep vacuum evaporation, routinely achieving and maintaining vacuum depths in the micron range, and allowing for even greater reductions in the vapor pressure of our target compounds. If you want to evaporate cannabinoids and keep the temperature down below the temperatures where degradation becomes a concern, an SPD is your best bet. The down side? That internal condenser takes up space, and must convey the condensate down and out of the body effectively using gravity, the combined effect of which is that it can’t handle the vapor volume a WFE would hog down easily.
Now you can easily see why we at SciPhy Systems presently recommend our clients consider at minimum a two stage distillation skid when manufacturing cannabinoid distillate: a first pass WFE to degas and devoletalize the high volume high boilers, and a second stage SPD to get down to deep vacuum and evaporate those heavy cannabinoids away from the heavier fats, waxes, and other contaminants. Then again, perhaps you want to get creative, and we can manufacture you a Hybrid unit with both kinds of condenser on a single evaporator body? Is there a WFE design with very low pressure drop AND very high vapor capacity? These exciting topics will have to wait for a future post.
Want to see our most popular SPD? Check out the Binary WFE/SPD here.
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