Vapor–liquid–liquid equilibria of nitrogen + ethane by molecular simulation

Abstract

Vapor–liquid, liquid–liquid, and vapor–liquid–liquid equilibria of the binary system nitrogen + ethane are investigated systematically for temperatures between 105 and 260 K and pressures between 15 and 150 bar. Force field models are used that have been parametrized beforehand solely to vapor–liquid equilibrium data and are augmented by a single binary parameter ξ = 0.974 that has also been adjusted in a preceding work to vapor–liquid equilibrium data at 260 K. The molecular mixture model is tested with a focus on its predictive power with respect to liquid–liquid equilibria. For that purpose, more than 4000 state points are sampled around the three-phase curve with molecular simulations in the isobaric–isothermal (NpT) and canonic (NVT) ensemble. Despite the large distance from its original adjustment point, the mixture model yields sound results for vapor–liquid equilibria at low temperatures and is capable of predicting the large miscibility gap of nitrogen + ethane with an average deviation of ±0.025 mol mol–1. Furthermore, the thermodynamic factor is sampled with Kirkwood–Buff integration and is also used for phase equilibrium calculations. The simulation results are compared with the high-accuracy multifluid GERG-2008 equation of state (EOS) and the Peng–Robinson EOS.