Use of ab initio interaction energies for the prediction of phase equilibria in the system nitrogen–ethane

Abstract

Ab initio molecular orbital methods have been used for calculations of interaction parameters of the UNIQUAC and NRTL activity coefficient model to predict phase equilibria in the system nitrogen–ethane. The UNIQUAC and NRTL model with ab initio parameters, calculated on the MP4 and QCISD(T) theory level, have been used in different gE-mixing rules to predict high-pressure vapor–liquid- (VLE) and vapor–liquid–liquid-equilibria (VLLE) by the Peng–Robinson and the Soave–Redlich–Kwong equation of state. The results have been compared to predictions based on UNIFAC with the PSRK-mixing rule. The results using ab initio-UNIQUAC are poor. However, ab initio-NRTL gives good VLE-predictions with all gE-mixing rules and with both equations of state. Only in the temperature range below 133 K, where the VLLE occur, is ab initio-NRTL inferior to the predictions by UNIFAC.