Lab Vacuum Short Path Distillation is a distillation technique that involves the distillate traveling a short distance, often only a few centimeters, and lab glass short path distillation is usually done at reduced pressure. A classic example would be a distillation involving the distillate traveling 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. The vacuum short path distillation 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.
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).
Operating Procedure of Short Path Distillation
This operating procedure is meant to be a basic starting point for anyone new to short path distillation. Once mastered, you can adjust the parameters to suit your specific needs. Temperatures and procedures will vary slightly with material and experience of operator.
-Before attemtping distillation, a full winterization and decarbing procedure should be done to remove any residual solvents and inactive compounds from the extracted oil (this will be disccused in another post).
-Place PTFE stir bar in flask and using the supplied glass funnel, begin loading material into boiling flask.
-Once material is loaded, begin assembling all glassware and adjust stands to secure glassware. Make sure to place a small amount of grease on each male joint. To spread grease, turn the glass in a circular motion. You should see the grease spread completely around the joint. Place keck clips on respective joints. The receiving cow should be oriented so that the first fraction will go into the first receiving flask, not the middle receiving flask.
-Unit setup will begin with attaching the heater/chiller to inlet and outlet ports on the short path. Once connected, set the temperature to 50C.
-Connect your vacuum pump to one of the 4 ports on the supplied vacuum manifold. The manifold will allow you to isolate each individaul component of the short path while leaving you one extra valve to let atmosphere in and out.
-Connect the cold trap to the receiving cow and then connect the other cold trap port to the manifold. Fill the cold trap with Dry Ice (placing the cold trap receiving flask in a dish with dry ice as well will help keep the trap contents from vacuum boiling).
-Connect the supplied gauge to the manifold. Make sure all valves on the manifold are in the open position before beginning.
-Insert thermometer probe into the thermometer adapter on the head. To secure the thermometer, tighten down the plastic cap so that the gasket becomes compressed inside. You must put some force into this to create a tight vacuum seal. Secure the probe so that it is slightly below the condenser opening.
-Insert the thermal probe into the adapter on the boiling flask and repeat the same process as above to create a vacuum seal. You want to secure the probe so that is as deep as possible into the flask without disturbing the PTFE stir bar.
-Check to make sure all connections are secure and all components are in working order. At this point you are ready to begin operation. (If you have insulation rope, wrap the boiling flask and the bottom quarter of the head. Leave vigreaux visible to observe reaction.)
-Close the valve on the manifold that is open to atmosphere. As your vacuum pump begins to pump down, examine the gauge to make sure there are no leaks.
-At this point you may begin heat and stirring. Turn on heat and set to 60C. Turn on stirring and set to approximately 200 RPM. (If using an analog heating mantle, set the heating knob to the middle position. You can leave it at this temp if using an analog mantle for the duration of the distillation. If you see little to no reaction, bump the knob up slightly past the halfway position until the reaction begins.)
-Once the temperature reaches 50C, increase the mantle temp to 140C. You may see some reaction in the head at this point, and your cold trap will most likely be dripping. As the mantle heats up, the thermal transfer of energy from the mantle to the boiling flask will be most prominent and you may see the temps on the head thermometer changing rapidly. This is okay. You should allow the mantle to heat up to about 10 degrees less than your set point. This gradual increase in temeprature will keep the mantle from overshooting.
-Once the mantle reaches 130C, set the temp to 220C (some material might need slightly higher temps to complete distillation. Temps above 250C should be avoided.)
-You should start to see a reaction in the head around 180-190C or slightly before. This is your first “heads” fraction. The first fraction will be all unwanteds. Great care should be taken to isolate this fraction from others. You will notice this fraction as being darker with reds, oranges, and possibly blues.
-You will know this fraction is over when you see a slight color change towards golden in the head. The reaction will increase in the head. This is your “main body” fraction. Allow this fraction to self clean the head for a minute or two. Then, rotate the cow to the middle receiving flask. Reaction in the head may vary but should never stop. Increase stirring speed to 400-800 RPM as fluid level decreases in the flask. (Increase temps no more than 10 degrees at a time if no reaction is present. If minimal reaction occurs, then you are still on the initial “heads” fraction. Be careful to not turn the cow to early as you will ruin the clean receiving flask for the main body.)
-Now is a somewhat tricky part but do not worry, each time you distill you will become better at it. The final “tails” fraction is a mixture of the first and second fractions and should be saved for re-distillation at a later time. Ideally, you want to turn the receiving cow to the final flask before the “tails” hit. This will keep your “main body” as pure as possible. You will notice a very obvious color change when the “tails” fraction hits. If you see a color change, and you have not yet turned the receiving cow to the final flask, then do so now. Again, it is optimal to try and turn the receiving cow right before this fraction hits.
-As fluid level decreases even more, set stirring to 900-1000+ RPM. You may need to compensate with a bit more heat as well. Increase the temp 10 more degrees if reaction has subsided. Once reaction stops, and distillate has stopped flowing, the process is complete. Stop the heat, stop the stir, and stop vacuum pump. Open the valve to let atmosphere into the system.