Select thermodynamic models for process simulation
A Practical Guide to a Three Steps Methodology

Example 3-1: Vapour pressure predictions of methane using different formulas

Compare the results of the various correlations with the DIPPR correlation.

In the following tables, the characteristic properties necessary for the evaluation of each expression is given. Remember that the normal boiling point is measured at 101.325 kPa. The various parameters of the equations are:

Table 1: Properties of methane used as parameters in vapour pressure equations
Parameters Tc Pc ω Tb T3 P3
Value 190.564 4599.0 0.0115478 111.66 90.694 11.696
Table 2: Parameters for various vapour pressure equations for Methane
Parameter A B C D E
Antoine-dimensionless, equation (3.12b) 5.3157 -5.3008 -0.0063
Antoine-API, equation (3.12) 8.6775 -911.234 -6.34
DIPPR, equation (3.13) 38.664 -1314.7 -3.3373 3.0155E-05 2
Wagner, equation (3.14) -6.00435 1.1885 -0.834082 -1.22833

(Temperature in K - Pressure in Pa)

We recall some equations of chapter 3:

Antoine's equation:


Dimensionless Antoine's equation:


DIPPR equation:


Wagner's equation:






See complete results in file (xls):

Some help on nomenclature and tips to use this file can be found here.

Tables for each equation are constructed and compared with the DIPPR set of calculated values. Note that the DIPPR equation does not predict exactly the critical point, the normal bubble point and the triple point; there are relative deviations around 0.2 to 0.6 %. Additionally, the acentric factor (‘omega point’ shown in figure 1) predicted by this equation is not exactly the same as that given in the database.

Relative differences are plotted in figure 1 over the full temperature range, from triple point to critical point for six different equations:

image Figure 1: Precision of various equations for the vapour pressure of methane against the DIPPR equation.

An additional curve (not shown in the book) can be constructed based on a basic formulation (Antoine's equation in this application but any other expression can be used). In this case, the parameter C is a parameter to be optimized while A and B have to satisfy the equations on 2 selected points (for example triple and critical points). Thus, the following equations have to be solved:


We find that:


Both parameters can be automatically determined once the value of C is fixed. Then an error minimization procedure can be executed on parameter C to obtain the optimum value while satisfying the restriction criteria.


R. L. Rowley, W. V. Wilding, J. L. Oscarson, Y. Yang, N. A. Zundel, T. E. Daubert, R. P. Danner, DIPPR® Data Compilation of Pure Compound Properties, Design Institute for Physical Properties, AIChE, New York, NY (2003).

Riazi, M. R. Characterization and Properties of Petroleum Fluids; American Society for Testing and Materials: Philadelphia, 2005.