It is important to predict vapor pressures correctly for process design. In this study the relationships between vapor pressure and temperature have been considered in terms of accuracy and convenience, resulting in the followings.
Experimental vapor pressure data reported in the literature for monoolefin have been rigorously analyzed and used to determine the constants A, B, C, D and exponent of the following equation in the reduced form:
ln P_r=A+B/T_r+C ln T_r+DT_rⁿ
The error analysis for the above equation has provided us with exponent n, average deviation, four constants for each material and an overall average deviation of 0.41 % for twenty-seven materials.
When exponent n becomes 4 for the best fit, four constants for each material have been determined. The constants A, B and C are correlated with the constant D and the following vapor pressure equation has been obtained, containing only a single constant D:
ln P_r=（0.90008 ln T_r）D³-(4.95122 ln T_r)D²
+[T_r⁴-9.5596 ln T_r-（16/T_r）+15]D+2.26464 ln T_r
where the constant D is found by applying this latter equation at normal boiling point.
When only the normal boiling point and the critical temperature and pressure are required, by employing the latter equation, an overall average deviation of 1.23 % is obtained for 1110 vapor pressure data points comprising 27 materials for monoolefin, between the triple point and critical point. But for 100 mmHg<P<P_c this latter equation was found to reproduce experimental vapor pressures with an overall average deviation of 0.84 %.