OpenSpecEngineering

Finds the vapor pressure of a pure liquid at a given temperature from the Antoine equation. It shows the result in mmHg, kPa and atm. Process engineers use it for tank breathing and emission estimates, flash and boiling-point checks, vacuum design and the vapor term in a pump’s NPSH available.

Equation

log₁₀P = A − B/(C + T)

P is the vapor pressure in mmHg and T the temperature in °C. A, B and C are empirical constants fitted to measured data for one compound [1]. They only work with the units they were fitted in. Choose a Compound and the page fills in A, B and C from Poling, Prausnitz and O’Connell [3]. Their constants are for P in bar and T in kelvin, so the page adds log₁₀(750.062) to A to give mmHg and °C. Choose Custom to type your own constants. The defaults are a widely quoted set for water from 1 to 100 °C.

Limits

• Each compound’s constants hold only over the temperature range they were fitted to. The page shows that range and warns when the temperature falls outside it. Outside the range the result is an extrapolation and can be badly wrong, especially near the critical point.
• The equation is for a pure compound. For a mixture, use Raoult’s law with each component’s vapor pressure and mole fraction.
• The page rejects a temperature where C + T is zero, because the equation has no value there.

Common mistakes

• Mixing constant sets. Published tables use log₁₀ or ln, bar, kPa or mmHg, and °C or K. Typing constants from one set into an equation with another gives nonsense. Convert them to log₁₀, mmHg and °C before you use Custom.
• Treating vapor pressure as a gauge pressure. It is absolute.
• Using one set of constants for a whole range when the source splits it in two. Check the source’s range.

Worked example

A toluene storage tank can warm to 60 °C in summer sun. What vapor pressure must its vent and emission estimate allow for?

1. Choose Compound Toluene (C₇H₈), 13 to 136 °C. A, B and C fill in.
2. Enter Temperature 60 °C.
3. Press Solve. The Solution shows: Vapor Pressure: 139.144 mmHg
   Vapor Pressure: 18.551 kPa
   Vapor Pressure: 0.183084 atm
   Range of the fitted constants: 13.3 to 136.5 °C

Check by hand with the constants in bar and kelvin [3]: log₁₀P = 4.05043 − 1327.62/(60 + 217.625) = −0.7316, so P = 0.1855 bar = 18.55 kPa. As a second check, the same constants give 759.8 mmHg at 110.6 °C, toluene’s normal boiling point. The NIST WebBook [2] lists data you can compare against.