Calculation of critical points for natural gas mixtures with the GERG-2008 equation of state

•We modify an algorithm for the calculation the critical points of mixtures from equation of state.•The new algorithm is robust in the calculation of critical points of mixtures.•We use the algorithm to calculate critical points using different equations of state.•We analyze the performance of the GERG-08 equation state in the calculation of critical points of natural gas mixtures.

Critical points of natural gas mixtures containing linear alkane molecules (C1-n-C8), branch alkanes (i-C4-i-C5), nitrogen (N2), and carbon dioxide (CO2) are calculated using a criterion based upon the Helmholtz free energy. We use the GERG equation of state with its mixing rules together with needed derivatives evaluated numerically. A multivariable iterative method is used to solve the system of non-linear equations that appear in this procedure. Results are checked for stability using mechanical, diffusional and global stability tests. Our results are compared with the literature values. Maximum error for the temperature and pressure are 1.301 and 11.087% respectively. The predictive capability of the GERG-2008 equation of state is compared with PR, SRK, PSRK, SPHCT, and PC-SAFT equations of state. For binary mixtures, all equations of state are capable of representing global phase behavior types I to IV. Results show that the GERG-2008 equation of state with binary interaction parameters is adequate for the prediction of critical points.

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