The following article explains the key differences between short path and wiped film distillation equipment and techniques. We discuss the operation of the equipment and also compare and contrast some key distillation operating parameters.
For years, the hemp extraction industry has used two common distillation processes known in the market as short path distillation (SPD) and wiped film distillation (WFD). To create a quality hemp distillate, short path and WFD equipment have become standard practice in the hemp industry and are both essential to the manufacturing process. But, what are the differences between the two and what advantages does each method have to offer producers?
Basic Description of Short Path Distillation
In order to achieve the desired distilled CBD oil product, a typical short path distillation machine uses a round bottom flask, heating mantle, and chilled condenser to evaporate and collect volatile fluids as shown in Figure 1. Winterized and decarboxylated CBD oil is introduced to the round bottom flask which is then heated by the heating mantle to vaporize volatile components from the remaining liquids. Once evaporated, the vaporized components are collected on the chilled condenser.
“In the case of fractional distillation, no film is made by the apparatus and ‘starve feeding’ the short path can greatly improve efficiency with the tradeoff of reduced throughput.” – Dr. Jon Thompson
The residence time that the oil spends exposed to high heat is on the order of 10-60 minutes compared to WFD at 1-3 minutes. This makes short path equipment best for samples that do not degrade easily or for low viscosity mixtures that are capable of boiling at a lower overall temperature.
This equipment can also be used for short path fractional distillation when multiple receivers are used. This means that the terpenes, solvents, and cannabinoids can be separated into different fractions using this equipment. This is something that can not be done on a WFD apparatus. Even though fractionation may sound good in theory, it is difficult to achieve due to overlap of boiling points and eutectics in practice.
Producers may find a short path fractional distillation kit to be simpler, have a smaller footprint, and the equipment certainly comes with a smaller price tag. For these reasons, many producers turn to a short path as a standard route for distilling hemp extraction solvents.
Basic Description of Wiped Film Distillation
Some short path vacuum distillation equipment relies on mechanical means to create a film for efficient evaporation. For example, a rotary evaporator uses a rotating round bottom flask to create a film on the glass walls. A falling film evaporator performs a similar function that uses gravity to create an evaporative film that runs down the heated column.
WFD equipment on the other hand uses mechanical wipers within the machinery to produce the film. This allows for liquids with a higher viscosity and higher boiling points to be distilled and reduce the time spent on a heat source before evaporation.
In the case of WFD, the distillation machine produces a film of oil on a hot surface with wiper blades as seen in Figure 2. Components in the oil evaporate and travel to a nearby condenser and condense on that surface. The wiper is highly engineered to provide a thin film at the evaporator surface that is mixed at the surface as well. This mixing improves mass transfer and efficiency of the separation.
Separation in this format makes for a much more efficient way to achieve a higher evaporation rate of the desired product. The thinner the film gets, the more efficiently cannabinoids and terpenes can be evaporated and collected on a chilled condensation surface.
WFD is often done in two passes per cycle. One pass removes trace components like remaining terpenes, chlorophyll and other plant materials. The remaining mixture would then be re-run through with minor changes in temperature and pressure in the second pass to refine desired cannabinoids like CBD.
Because of this, the WFD process generally creates a higher resolution, increases potency, and creates a consistent distillate for production. Let’s take an even closer look at how a WFD apparatus works.
A Deeper Look at Wiped Film Distillation
As seen in Figure 3., the heated reservoir holds the winterized decarboxylated oil that feeds the still. The feed is pumped through the feed pump and enters the still under high vacuum pressure.
Conditioned vacuum pressure is provided by a mix of roughing vacuum pumps and optional turbo molecular pumps. Cold fingers and vacuum condensers are typically installed in between the vacuum and the still to condense volatile molecules in the vacuum so that a high vacuum gradient exists between the still and the vacuum source.
As the wipers rotate, the oil is spread onto the evaporator wall. The wipers continue to mix the fluid as it moves down the column.
As the liquid is heated, the volatile components begin to evaporate from the evaporator surface (blue bubbles). Once in the gas phase, they randomly migrate through the vacuum to the condenser. Since the condenser is cool, the molecules condense back into a liquid. As the condensate accumulates, it starts to move down the finger by gravity and exits the still via the distillate pump.
The residue on the other hand does not evaporate and continues to be pushed down the evaporator wall by the wipers. It exits the still via the residue gear pump. Thus, the distillate and residue are separated.
The operator may desire either the residue or the condensate (distillate). So, different methods can be run to optimize the recovery of either.
In the case where there is solvent in the oil or low boiling compounds such as terpenes, the vacuum in the still will be limited by the vapor pressure of components that are being evaporated. Some of those compounds will not condense on the condenser and therefore will diffuse through the vacuum and condense on the “cold finger” that is installed just before the vacuum.
When there is an excessive amount of solvent in the oil to be distilled, a pre-processing step may be required. This is where a wiped film evaporator “WFE” or a falling film evaporator may be most optimal.
Comparison of The Two Distillation Techniques
In summary, short path fractional distillation is quite different from WFD based on the above conversation. Table 1. summarizes the discussion and will help guide you decision as to when to deploy each method.
|Distillation Comparison||Short Path (fractional distillation)||WFD|
|Mean free path¹||Long-6-10”||Short – 1-2”|
|Mixing||Stir bar||Wiper blades|
|Residence time for distillation²||5-60 min||Short 1-2 min|
|Changeover Time||20 min||N/A, continuous|
|Film Thickness||No “thin film” is made||1-5 mm typically|
|Typical Vacuum Level||0.1 torr||<0.001 torr|
|Fractionation||Yes||No, unless incorporated with external equipment.|
- The distance a molecule needs to diffuse in the gas phase to reach the condenser.
- Typical residence time for oil to complete a process.
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