Comparison of modified alpha functions of the PR-EoS for volume and enthalpy prediction of natural gases.

André F Gonçalves,Andréa O.S Da Costa,Esly F Da Costa Junior

doi:10.1590/0001-3765202020190603

Abstract

Proper simulation of processes of the natural gas industry such as dehydration, liquefaction and regasification require accurate prediction of thermodynamic properties of the working fluids. For such processes, cubic equations of state are the calculation methods most frequently employed. Among them, the Peng-Robinson equation is usually the one recommended for gas, refinery and petrochemical applications in many simulators. Numerous works have been proposed in order to improve the temperature dependence relation of the attraction parameter of the equation - the so called alpha function. In this work, five currently available alpha functions are evaluated for the prediction of molar volumes and enthalpies of natural gas samples. Additionally, parameters of one of the models are readjusted to volumetric data of methane, in order to represent its supercritical behavior more accurately. Experimental data of 44 mixtures are compared with calculated results. Van der Waals mixing rules are used, with binary interaction parameters set as zero. In the case of the original alpha function, it is also tested how the inclusion of non-zero binary parameters affects the predictions. The extended Saffari-Zahedi model presents the smallest average deviation for the molar volumes (1.35%). For the enthalpy calculation, the inclusion of the binary parameters results in deviation values of 2.62% for gas-gas transitions and 4.44% for gas-liquid transitions.

Highlights

Increasing global energy demand and growing concern with environmental issues accelerate the development of clean and economical energy sources
Apart from that model, for the first four experimental points, the inclusion of the binary interaction parameters (BIPs) resulted in a small improvement in the prediction of specific volumes
It was investigated if a better performance of the Peng-Robinson equation could be achieved by improving the temperature dependence relation – or alpha function – of its attraction parameter

Summary

Introduction

Increasing global energy demand and growing concern with environmental issues accelerate the development of clean and economical energy sources. Low greenhouse gas and air pollutant emissions make the natural gas an attractive possibility, in comparison with other fossil fuels (Qyyum et al 2018). The U.S Energy Information Administration expects the natural gas global consumption to grow 1.9% annually. Projections of the same agency indicate that by 2030 natural gas will surpass coal as the second most consumed fuel (U.S EIA 2016). For long-distance trading, natural gas is liquefied and stored in cryogenic tanks of ships (Wang et al 2011). The design and optimization of liquefaction processes are usually conducted with software packages like Aspen Hysys, Aspen Plus and Honeywell UniSim Design, which employ accurate thermodynamic models for the estimation of physical properties of natural gas and refrigerant fluids (Yuan et al 2015). Saffari & Zahedi (2013) consider that the favorable balance between precision, simplicity and computational time of cubic equations of state (EoSs) – especially Peng-Robinson (PR), SoaveRedlich-Kwong (SRK) and Lee-Kesler-Plocker

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Conclusion