Having an agitation method is very important during distillation – especially when we have a multitude of compounds in the boiling flask to separate. This is due to relative volatility – which is the comparison of varying vapor pressures of the compounds in the crude. Modulation of the stir bar speed helps separation of more volatile compounds from the rest of the pot. It also keeps the temperature of the boil more evenly distributed throughout the flask.
Increasing the agitation causes more vapor to flow into the system which sometimes may drop vacuum performance, so ramp up slowly to save your pump. Experiencing a rapid decline in vacuum depth when increasing RPMs is a clear indicator of ongoing distillation of volatiles – ideally this needs to conclude prior to pulling your main body of cannabinoid distillate or there may be unwanted odor to the finished product.
Egg-shaped bars work exceptionally well in round bottom flasks because they conform to the shape of the vessel. Rare-earth, which is a highly magnetized compound, works best to stay engaged to the magnetic drive of the mantle – Rare earth stir-bars are worth the extra cost. You also want to get the largest (longest) stir bar possible to maximize the continuous homogeneity of the boil;
- 5’’ length for 22 or 20L SPD systems
- 5’’ – 3’’ length for 12, 10 and 5L SPD systems
If you aren’t sure about a new stir-bar you’ve purchased for your system, throw it into an empty boiling flask (or one filled 1/3rd with water) and set into the mantle – this will allow you to observe the RPM threshold as you ramp up the speed. If doing this process with water, there should be a very clear vortex in the center of the flask with the liquid visibly shifting rapidly. Note the speed at which the motion of the stir-bar becomes erratic and it eventually loses its position – this will be your maximum RPM, try to stick to ¾ of your max during distillation.
When distilling crude, you should aim to increase stir bar speeds as quickly as possible while progressing through the fractions (without knocking your bar out of position). This is important to prevent bumping, and to thin out the more viscous compounds allowing the low-boiling fractions to escape the flask.