Short-Path, Molecular Distillation
Distilling the extracted oil allows us to separate the terpenes and cannabinoids from the chlorophyll and other non-volatile materials that are not desirable to have in the final product. The apparatus spreads the material into a thin film on the wall of the still where the lighter molecules are vaporized and separated from the heavier molecules. The short-path refers to the fact the the compounds only stay in the vapor phase for a very short distance as they travel the inch from the still wall to the internal condenser. "Short-path" distillation has become the de-facto term to refer to any distillation process applied to a crude cannabis extract. There are really two different types of distillation that can be done and it's important to understand the purpose of each.
Cannabinoids are sensitive to heat and will degrade as a function of temperature and time. In order to minimize the degradation we can reduce the ambient pressure so the compounds will boil at a lower temperature, and we can minimize the amount of time they are in the vapor phase, at this higher energy level. A true short-path still heats the material in a very thin film so the most efficient heat transfer can occur. The material is forced across the heating surface in a thin film so that the residence time, or time that the material spends in contact with the heat source, is kept very short. Once the compounds evaporate, they travel a very short distance, in most cases about an inch, and then condense and return to their lower energy state. The entire design of the still is to protect temperature sensitive materials from degrading during the distillation. Using a true short-path still to purify CBD hemp extracts gives the best yields and a highly purified oil. There is much confusion in the industry around short-path distillation and in fact, the term is almost always incorrectly used to refer to fractional distillation.
In the cannabis and hemp industries, fractional distillation is almost always incorrectly referred to as short-path distillation, despite it's purpose being much different. A short-path still evaporates and condenses the compound one time which means it isn't very good at separating compounds with similar boiling points. Conversely, a fractional still has a refluxing column where the compounds evaporate and condense many times allowing better separation of similar volatility compounds. In fact, it's even possible to isolate specific cannabinoids to a high purity using fractional distillation. Of course, the many cycles of evaporating and condensing, heating and cooling, mean that the cannabinoids are in the vapor phase, and at a high energy where they quickly degrade, for a much longer time period than in short-path distillation. The result is a loss in yield as a percentage of the cannabinoids are destroyed by the high heat, and there is also some degradation of terpenes which can lead to some unpleasant smells and tastes in the final product. If you are willing to accept the poor yield, fractional distillation can be an effective way to create high purity isolates but it is not a great way to make full-spectrum, terpene-rich medicinal products.