Vacuum Distillation
Distilling under vacuum lowers the boiling point of every component, so heat-sensitive or high-boiling substances can be separated without decomposing. The same principle runs a laboratory rotary evaporator and a refinery vacuum column.
Choosing the pressure
A liquid boils when its vapour pressure equals the pressure above it. Pick the vapour temperature you want, then look up the pressure at which your solvent boils at that temperature. The boiling point calculator does this for water and common solvents.
The delta-20 rule (rotary evaporators)
A widely used rule of thumb: heating bath about 20 °C above the desired vapour temperature, condenser coolant about 20 °C below it. For a vapour temperature of 40 °C that means a 60 °C bath and 20 °C coolant. Set the vacuum so the solvent boils at the vapour temperature.
Industrial vacuum distillation
Refineries distil the heavy residue of the atmospheric column under vacuum, typically at a few tens of mbar at the column top, to recover gas oils without cracking them. Edible oils, fatty acids, essential oils and pharmaceutical intermediates are purified the same way. Large plants usually combine steam ejectors or liquid ring pumps with condensers.
Typical pressures
| Process stage | Pressure |
|---|---|
| Rotary evaporator, common solvents | ≈ 10 – 500 mbar (solvent-dependent) |
| Refinery vacuum column (top) | A few tens of mbar |
| Short-path / molecular distillation | ≈ 10⁻³ – 1 mbar |
Solvent boiling points at reduced pressure (°C)
| Pressure (mbar) | Water | Ethanol | Methanol | Isopropanol (IPA) | Acetone | n-Hexane | Toluene |
|---|---|---|---|---|---|---|---|
| 200 | 60.1 | 42.2 | 28.2 | 46.5 | 15.5 | 24.8 | 61.9 |
| 100 | 45.8 | 29.3 | 15.2 | 33.6 | 1.5 | 9.8 | 45.3 |
| 50 | 32.9 | 17.5 | 3.4 | 22.0 | -11.0 | -3.5 | 30.5 |
| 20 | 17.5 | 3.5 | -10.6 | 8.1 | -25.5 | -18.9 | 13.2 |
| 10 | 7.0 | -6.1 | -20.1 | -1.3 | -35.2 | -29.2 | 1.7 |
Suitable vacuum pumps
References & further reading
- NIST. NIST Chemistry WebBook, SRD 69. National Institute of Standards and Technology. Antoine coefficients and phase-change data for solvents.
- IAPWS. SR1-86(1992): Revised Supplementary Release on Saturation Properties of Ordinary Water Substance. International Association for the Properties of Water and Steam. Saturation vapour pressure equation used for liquid water.
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
Frequently asked questions
Why use vacuum distillation instead of normal distillation?
Lower pressure means lower boiling temperatures, so substances that would decompose, polymerise or oxidise at their normal boiling point can be distilled gently and with less energy.
What vacuum pump is best for a rotary evaporator?
A chemically resistant (PTFE) diaphragm pump with a vacuum controller covers most solvents. Oil-sealed pumps need a cold trap to protect the oil from solvent vapour.
How do I find the right pressure for my solvent?
Enter the solvent and the desired boiling temperature in the boiling point calculator in "required vacuum" mode; it returns the pressure in mbar and Torr.
Other applications
HVAC Evacuation: Pulling a Deep Vacuum
How to evacuate an HVAC/R system: removing air and moisture, why 500 microns, decay tests, hose and pump tips, and the physics of boiling water out.
Freeze Drying (Lyophilization) Under Vacuum
How freeze drying works: sublimation below the triple point, chamber and condenser pressures, ice vapour pressure, drying phases and the right vacuum pumps.
Vacuum Furnaces and Heat Treatment
Heat treatment in vacuum furnaces: bright surfaces, brazing, sintering, typical pressures, partial pressure, gas quenching, pumps and leak-up rate tests.
Vacuum Pressure Impregnation (VPI)
Vacuum pressure impregnation for windings, porous castings and wood: evacuation, resin flooding, pressurisation, curing, typical pressures and pumps.
Vacuum Sewer Systems
How vacuum sewers move wastewater with air pressure instead of gravity: interface valves, the vacuum station, typical pressures, pumps and where they fit.