Vapor-liquid equilibrium of light hydrocarbon multicomponent systems by solving the Rachford-Rice equation

Abstract

Mixed-refrigerants are required for cryogenic cooling systems, as well as for refrigeration systems, in order to provide enhanced cooling performances and to comply with new global environmental protection regulations. Thus, the vapor–liquid equilibrium (VLE) of light hydrocarbon (LHC) multicomponent systems is studied numerically by solving the Rachford-Rice flash equations using the Peng-Robinson equation of state and the van der Waals mixing rules. Among many other methods for calculating VLE of mixtures, the presented method does not involve numerical iterations, there aren’t any convergence problems, and it is a fast responding method, which allows extensive investigations of many compositions at various conditions. VLE results are presented for binary systems of nitrogen, methane, ethane, propane, n-butane, and carbon dioxide at temperatures between 240 K and 330 K; and ternary systems of nitrogen, methane, ethane, propane, n-butane, and carbon dioxide at temperatures between 220 K and 330 K and pressures between 5 bar and 40 bar. All the computational results are validated against experimental results, which are taken from literature, and the maximum obtained deviation between calculated and experimental results is 4.03 %, while most of the deviations are less than 2.0 %. As the number of components in the system increases, the deviations between calculated and experimental results increase, although the liquid and the vapor line trends are maintained. In the current research, seven different alpha functions are examined, and the results show that they all provide similar accuracies. The presented numerical method may be used by any equation of state, it is numerically stable, and successfully validated against reported experimental results; consequently, it is recommended for the development of future mixed-refrigerants.