Short-path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure. A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. With short path distillation, a decrease of boiling temperature is obtained by reducing the operating pressure. It is a continuous process with very short residence time. The advantage is that the heating temperature can be considerably lower (at reduced pressure) than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.
Short-path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path; 1: Still pot with stirrer bar/anti-bumping granules 2: Cold finger – bent to direct condensate 3: Cooling water out 4: cooling water in 5: Vacuum/gas inlet 6: Distillate flask/distillate.
The process of short path distillation
Short path distillation is actually relatively simple once a thorough understanding of the components and chemical process is reached. Short path distillation will allow the operator to achieve what is known as “distillate”, a pure compound which often takes the form of a clear/golden liquid with little to no taste or color. During the short path processes, and pre-processes discussed here, the original oleoresin will be stripped down to less than a quarter of it’s total active compounds. It will lose all terpenes (smell and flavor) during the process and, the final product will be extremely potent. The end product can be used as a pre-cursor for isolation and crystalization. So what is actually happening inside a short path appartus? The oleoresin will be inside the boiling flask where a combination of temperature and vacuum pressure will force the active compounds to evaporate and turn into a vapor. They will be pulled into the path of the condenser by the vacuum pump where they will re-condense into a solid form and be collected into the individual flasks attached to the receiving cow. This process is quite similar to a rotovap, the difference being the length of the condenser, and what you are actually distilling (ethanol in a rotovap, and oleoresin in a short path).
Components of Short path distillation
–Chiller: The term “chiller” can be miss-leading when talking about short path. A vapor can be recondensed at a temperature that is at least 20 degrees colder than what is was boiled off at. For example, a compound boiled off at 400F could be recondensed at 380F. The 380F temp is “colder” than the 400F that the compound boiled off at but, this is by no means actually cold. When doing short path distillation, the temp range will be about 100C to 250C (212F to 482F) on the heating mantle. This means the short path condenser will be set between 40C and 60C. We prefer 50C (122F) for a starting point (temp differentials regarding specific fractions will be discussed in an advanced post). This means we do not neccessarily need a “chiller”. We just need a device that will keep our desired 50C temperature during the process. This 50C temp will keep our short path from clogging and will be “cold” enough to easily recondense any vapors that travel into the path of the condenser.
–Vacuum Pump: When doing a distillation process, it is imperative that you have a chemically resistant vacuum pump that can acheive extreme depth. The pump should be able to achieve at least 50 micron but the deeper the better. The preferred type of pump for distillation is a rotary vane (oil) pump. Rotary vane pumps offer the best vacuum depth compared to other types of pumps (specialty pumps such as diffusion pumps will be discussed in a later post). Short path distillation can be very chemically abrasive to your vacuum pump. For this reason it is recommended to change your oil before every distillation. This will allow you the best depth and keep your vacuum pump running at peak performance.
-Cold Trap: The cold trap plays the important role of catching any vapors that pass through the condenser. This protects and prolongs the lifespan of your vacuum system.
-Short Path: The Short path itself is made up of 4 major glass components and the heating mantle. These components are: the boiling flask, the short path head, the receiving cow, and the receiving flasks. The heating mantle is similar to a heat bath on the rotovap. This is how you will heat the mixture inside the boiling flask. The head is the key to distillation. Inside the head there are small indentations called “vigreaux”. These create contact points for the vapors to bump as they travel up into the head. They create “theoretical plates” needed for distillation where vapors will become purified before entering the condenser path. The head also encompasses the condenser where the vapors will be pulled to re-condense into that beautiful golden color we all want to see. The three receiving flasks correspond to the three main fractions (single cows, swapable adapters, and head packing will be discussed in an advanced short path post).