The problem with distilling heavy compounds
Many valuable botanical compounds boil at temperatures high enough to destroy them. Heat them to their atmospheric boiling point and what comes over the condenser is degradation product rather than the compound you wanted. Distillation of these materials is therefore a race between volatility and decomposition.
Two levers, used together
Short path distillation pulls both levers at once. Deep vacuum drops the boiling point far below the atmospheric value, so the compound evaporates at a temperature it can survive. And the physical distance between the evaporating surface and the condenser is reduced to a few centimetres, so vapour reaches the condenser almost immediately instead of travelling up a column. The material is therefore held at temperature for a very short time.
Residence time is the point. A compound can tolerate a temperature for seconds that would destroy it over minutes.
Taking fractions
A distillation run is normally taken in three parts. The heads come over first and carry the most volatile material, including residual solvent and light terpenes. The main fraction follows and is the product. The tails are the heaviest material left behind. Where the cut points fall is a process decision, and it is what determines the profile of the finished distillate.
Feed preparation decides the result
Short path distillation does not fix a bad feed. Crude that still carries wax will foul the evaporator and drag wax into the fractions, which is why winterization comes first. Residual solvent should be removed before the run rather than during it. A clean, dewaxed, degassed feed is the single largest factor in how well a distillation run goes.
Lab to production
Short path distillers are built in the same laboratory equipment factory as the rotary evaporators and jacketed glass reactors, so a refining recipe proven at laboratory scale can be carried onto larger equipment with the same parameters.