Application of GC-PPC-SAFT EoS to ammonia and its mixtures

Abstract

The GC-PPC-SAFT equation of state (EoS) is a combination of the PC-SAFT EoS proposed by Gross and Sadowski [J. Gross, G. Sadowski, Ind. Eng. Chem. Res. 40 (2001) 1244–1260] and a group contribution method proposed by Tamouza [S. Tamouza et al., Fluid Phase Equilib. 222–223 (2004) 67–76; S. Tamouza et al., Fluid Phase Equilib. 228–229 (2005) 409–419] which was extended to polar molecules by Nguyen-Huynh [D. Nguyen-Huynh et al., Fluid Phase Equilib. 264 (2008) 62–75]. It is here applied to ammonia for which parameters have been regressed on pure component data (vapour pressures and liquid molar volumes), and some mixture vapour–liquid equilibrium data. Ammonia is described with an association scheme containing four associating sites (one donor site on the nitrogen atom and an acceptor site on each hydrogen atom). It also contains a dipole moment. These resulting parameters have then been tested on vapour–liquid equilibrium (VLE) and liquid–liquid equilibrium (LLE) of mixtures with n-alkanes, cycloalkanes, aromatics and small molecules such as methane, hydrogen or nitrogen whose parameters were previously published. The vapour–liquid equilibrium and the mixing enthalpy have also been evaluated for the ammonia+water binary system. For these systems a binary interaction parameter kij and corrective parameters for the cross-association energy wαβ and volume uαβ have been used in some cases.The agreement with experimental data is encouraging as correlations and predictions are both qualitatively and quantitatively satisfactory. In terms of pure component properties, the mean absolute average deviation (AAD) for the vapour pressures is about 2.5% whereas it is about 2.7% for the saturated liquid volumes. Furthermore, predictions on mixtures with n-alkanes, cycloalkanes or aromatics show that LLE lines predicted by GC-PPC-SAFT are perfectly well-located. Finally, results for VLE with n-alkanes, aromatics, small molecules or water are as accurate as it was for previously investigated systems. These results also show that the model is able to correctly represent the sign and the order of magnitude of the mixing enthalpy for the ammonia+water system.For systems containing ammonia with alkylbenzenes, this study have highlighted that a re-adjustment of alkybenzenes groups parameters was necessary. This paper explains the fitting procedure used for the new alkylbenzenes groups parameters and show satisfactory results for VLE and LLE of systems containing ammonia and alkylbenzenes.