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

Example 4-3: Comparison of phase envelope predictions for the ethane + n-pentane mixture

Various models can be used to predict phase envelope. For the ethane + n-pentane mixture, the Reamer et al. [1] data of 1960 is available. Bubble and dew curves are represented on figure 1 with solid dots and empty dots respectively.

Given that ethane is supercritical at this temperature, and a critical point is observed on the diagram, only an equation of state (homogeneous approach) can be used.

The cubic Peng-Robinson EoS with binary interaction parameters (BIP) equal to zero gives a very good approximation of the complete envelope. There is a small under-prediction of the critical point but the general shape is correct. A small improvement can be made using the Nishiumi et al. [2] predicted BIP. An even better improvement is possible using fitted BIP obtained directly from experimental data.

Finally, a modern equation of state such as SAFT, in this case using the group contribution version (GC-SAFT), allows accurate representation of the whole envelope based only on predictive behaviour of molecules as described by Nguyen-Huynh et al. (2009)[3].

image Figure 1: Phase envelope of ethane + pentane equilibrium (experimental and models) at 344.26 K.

References

[1] REAMER H.H., SAGE B.H., LACEY W.N., Phase Equilibria in Hydrocarbon Systems. Volumetric and Phase Behavior of the Ethane-n-Pentane System, Journal of Chemical & Engineering Data, 1960, 5, n°1, p. 44-50. http://dx.doi.org/10.1021/je60005a012

[2] NISHIUMI H., ARAI T., TAKEUCHI K., Generalization of the binary parameter of the Peng and Robinson equation of state by component family, Fluid Phase Equilibria, 1988, 42,p. 43-62. http://dx.doi.org/10.1016/0378-3812(88)80049-9

[3] NGUYEN-HUYNH D., PASSARELLO J.P., TOBALY P., In Situ Determination of Phase Equilibria of Methyl Benzoate plus Alkane Mixtures Using an Infrared Absorption Method. Comparison with Polar GC-SAFT Predictions, Journal of Chemical and Engineering Data, 2009, 54, n°6, p. 1685-1691. http://dx.doi.org/10.1021/je800757j