July 21, 2009

Vapor Pressure Vs Temperature

Generally we search too many things for frequently used properties like the vapor pressure or saturation temperature at given condition which is the most common among all.

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Though the famous & simplest correlation of Antoine is available but many a times we do not find correlation constants A, B & C for a given compound. Therefore, I always prefer methods which uses its own thermodynamic property e.g. critical parameters for calculation of any derived property.

So this post is giving a method which uses Tc, PC atc for calculating vapor pressure, though the method is somewhat tedious but can be used once formulated in Excel or in any other program.

The method used in this case is called Gomez - Nieto & Thodos which is based on critical properties.

The equation is

Ln (Pvr^5) = beta [ (1/ Tr^m)-1] + gamma (Tr^n - 1)

Pvr is basically reduced vapor pressure i.e. Pv / Pc
Tr is as usual reduced temperature at T i.e. T/Tc

m = [0.78425 exp (0.089315 * S)] - [8.5217 / exp (0.74826 * S)]
n = 7
beta =-4.267-[221.79/{(S^2.5)(exp(0.03848 S^2.5))}]+[3.8126 / exp(2272.44 /S^3)]
gamma = a * S + b * beta

a = [(1/Trb)-1]/(1-Trb^7)
b = [(1/Trb^m)-1]/(1-Trb^7)
S = Tb * Ln (Pc)/ (Tc - Tb)

Here Pc is in atm & Pvr is in mmHg.

List of other property estimation methods on this Blog.

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