The estimation of vapor pressure uses a theory developed by Gomez-Thodos [22], [23], [24]. This estimation method is reported to be valid for reduced temperatures down to 0.5. The following equations are used in estimation:
(3–42) |
The normal boiling point provides the correlation
 | (3–43) |
where
 | (3–44) |
 | (3–45) |
and the enthalpy compressibility factor is:
 | (3–46) |
For non-polar compounds:
 | (3–47) |
where D* is zero except for the special cases He (D*=0.41815) H2 (D*=0.19904) and Ne (D*=0.02319).
For polar compounds:
 | (3–48) |
 | (3–49) |
For water and alcohol:
 | (3–50) |
 | (3–51) |
For the last two chemical categories the correlation to get beta is:
 | (3–52) |
M is the molecular weight, Tb is the boiling temperature, Tc is the critical temperature, Pc is the critical pressure. Any quantity with the subscript of ‘r’ means that it is ‘reduced’ which means it is divided by the corresponding critical quantity, for example, Tr=T/Tc.