Non-ideality of the system NH3-H-2-N-2. Comparison of equation of state and simulation predictions with experimental data

In this study, experimental PVT data of pure NH3, H-2, N-2 and He are used to extract parameters for a three-parameter semi-empirical equation of stat e (EOS) for a pure substance and interaction parameters for the exponential -6 (exp-6) potential. For ammonia, the experimental pressure and liquid den sity at 323 K in the liquid-vapour coexistence region and the experimental density at the same temperature and 9500 bar are taken as inputs in the fit ting procedure. For the other species, supercritical data at pressures up t o 10000 bar are selected. It is found that the EOS is not able to simultane ously fit both the liquid-vapour coexistence data and the high pressure reg ion of ammonia. In contrast, the use of Monte Carlo simulations with an opt imised set of exp-6 parameters leads to good agreement both at low and high pressure. The quality of the fits to H-2 and N-2 data using the EOS is sig nificantly worse than that using the optimised exp-6 potential because the EOS requires physically unreasonable parameters for a good fit. Despite the higher deviations of the EOS results, their corresponding predicted equili brium constants for the synthesis of ammonia from H-2 and N-2 in the indust rial range agree just as well with the experimental data. Furthermore, the predicted critical point is slightly closer to the experimental value (a de viation of 10% in the critical temperature). Simulations with the exp-6 pot ential are performed for the system H-2-He-NH3-N-2 at pressures and tempera tures occurring in the deep atmosphere of Jupiter. Comparison between previ ous ideal calculations and the simulation predictions indicates that the ex pected concentration of N-2 at 2300 K is overestimated by about a factor of three when ideality is assumed.