Freeze Drying (Lyophilization) Under Vacuum
Freeze drying removes water from a frozen product by turning ice directly into vapour. Because it never melts, the product keeps its structure, which is why the method is used for vaccines, biologics, coffee, food and archive material.
Sublimation below the triple point
Below 6.1 mbar (the triple point of water) liquid water cannot exist; ice turns straight into vapour. A freeze dryer keeps the chamber well below the vapour pressure of the ice so the vapour flows away and is captured on a condenser that is colder still.
Primary and secondary drying
In primary drying, shelf heat supplies the energy for sublimation while the chamber pressure is held typically around 0.05–0.3 mbar. The ice front recedes through the product. In secondary drying, the temperature is raised and the pressure lowered further to remove water bound to the dry material.
The condenser and the pump
The condenser, not the pump, removes almost all the water vapour: it must be colder than the product so its ice has a lower vapour pressure. The vacuum pump mainly removes air and non-condensable gases, which is why two-stage rotary vane or scroll pumps are typical, with a Roots booster on large production units.
Typical pressures
| Process stage | Pressure |
|---|---|
| Triple point of water | 6.1 mbar (611.7 Pa) |
| Primary drying, chamber | ≈ 0.05 – 0.3 mbar (typical) |
| Secondary drying | ≈ 0.01 – 0.05 mbar |
| Household / hobby freeze dryers (typical) | ≈ 0.4 – 0.7 mbar |
Vapour pressure of ice (sublimation pressure)
| Ice temperature | mbar | Microns | Pa |
|---|---|---|---|
| -10 °C | 2.6 | 1,950 | 260 |
| -20 °C | 1.03 | 774 | 103 |
| -25 °C | 0.633 | 475 | 63.3 |
| -30 °C | 0.38 | 285 | 38 |
| -40 °C | 0.128 | 96.3 | 12.8 |
| -50 °C | 0.0394 | 29.5 | 3.94 |
| -60 °C | 0.0108 | 8.11 | 1.08 |
Suitable vacuum pumps
References & further reading
- D. M. Murphy & T. Koop. Review of the vapour pressures of ice and supercooled water for atmospheric applications. Q. J. R. Meteorol. Soc. 131, 1539 (2005). Sublimation pressure over ice below 0.01 °C.
- 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.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
Frequently asked questions
What vacuum does a freeze dryer need?
The chamber must stay below the vapour pressure of the ice. With product at −30 °C that is 0.38 mbar; in practice primary drying runs at roughly 0.05–0.3 mbar.
Why must the condenser be colder than the product?
Vapour flows from higher to lower vapour pressure. A condenser at −60 °C has a far lower ice vapour pressure than product at −30 °C, so water migrates from the product to the condenser.
Can freeze drying work above 6.1 mbar?
No. Above the triple point pressure the ice would melt instead of subliming, and the product would lose its structure.
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