Why deep vacuum lowers the boiling point

A liquid boils when its vapor pressure equals the pressure around it. The Clausius Clapeyron relation, ln(P / Pref) = (minus dH / R) x (1 / T minus 1 / Tref), says vapor pressure climbs steeply with temperature, so cutting the surrounding pressure a thousandfold drops the boiling point by hundreds of degrees.

This model uses illustrative values for a cannabinoid-like molecule: a boiling point of 175 °C at 0.05 mbar and a heat of vaporization of 95 kJ/mol. With those numbers the same compound would need well over 400 °C at atmospheric pressure, hot enough to break it down, which is why processors distill under deep vacuum. Real values depend on the compound and the still.

Residence time and two passes

The residence time readout divides an assumed 5 mL of oil held on the wall by the feed rate (a simplified holdup model). Wiped film units are prized because oil spends only seconds on the hot surface, limiting heat damage. Feed too fast and the film thickens, which blurs the separation.

Research groups often run two passes: a first pass at milder vacuum strips terpenes and residual ethanol, and a second pass at deep vacuum (tens of microns of mercury) separates cannabinoids from waxes and pigments. Stills run inside licensed facilities with pressure rated glass or steel, guarded hot surfaces and cold trap procedures.