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

Example 3-7: Evaluate critical temperatures using the group contribution methods of Joback and Gani

This example shows how to calculate a number of important properties using group contributions. One excel file is available for the Joback method, and one for the Marrero-Gani method (more complex). We shall discuss the example of the calculation of the critical temperature for n-octane, n-propyl-benzene and n-propyl-cyclohexane.

Analysis:

Solution:

Use the excel for the Joback method [1]:

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

And for the Marrero-Gani method [2]:

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

n-octane:

It is composed of 2 (CH3) groups and 6 (CH2) groups. It has no second order groups.

n-propyl benzene:

The group decomposition is different depending on the method.

n-propyl cyclohexane:

Again, the group decomposition depends on the method:

image Figure 1: Examples of second order group decomposition according to the Marrero-Gani group contribution method

The final group decomposition of n-propyl cyclohexane is : 1st order: 1 (CH3) group, 2 (CH2) groups, 5 cyclic (CH2) groups and 1 cyclic (CH) group. On top of that, the second order group called ‘CHcyc-CH2’ must be added. The resulting calculation is available on the excel sheet, with a final value of Tc = 630.89 K. If only first order had been used, the calculation would have given Tc = 625.06 K. The database value is 639.15K, showing that the use of second order groups helps, but is not enough to make it a tool of great predictive quality.

References

[1] JOBACK K.G., REID R.C., Estimation of Pure Component Properties from Group Contributions, Chemical Engineering Communications, 1987, 57, p. 233-243. http://dx.doi.org/10.1080/00986448708960487

[2] MARRERO J., GANI R., Group-contribution based estimation of pure component properties, Fluid Phase Equilibria, 2001, 183, p. 183-208. http://dx.doi.org/10.1016/S0378-3812(01)00431-9