Equations of State with Group Contribution Binary Interaction Parameters for Calculation of Two-Phase Envelopes for Synthetic and Real Natural Gas Mixtures with Heavy Fractions

Khashayar Nasrifar,Nejat Rahmanian

doi:10.2516/ogst/2017044

Abstract

Three equations of state with a group contribution model for binary interaction parameters were employed to calculate the vapor-liquid equilibria of synthetic and real natural gas mixtures with heavy fractions. In order to estimate the binary interaction parameters, critical temperatures, critical pressures and acentric factors of binary constituents of the mixture are required. The binary interaction parameter model also accounts for temperature. To perform phase equilibrium calculations, the heavy fractions were first discretized into 12 Single Carbon Numbers (SCN) using generalized molecular weights. Then, using the generalized molecular weights and specific gravities, the SCN were characterized. Afterwards, phase equilibrium calculations were performed employing a set of (nc + 1) equations where nc stands for the number of known components plus 12 SCN. The equations were solved iteratively using Newton's method. Predictions indicate that the use of binary interaction parameters for highly sour natural gas mixtures is quite important and must not be avoided. For sweet natural gas mixtures, the use of binary interaction parameters is less remarkable, however.

Highlights

Information provided by vapor-liquid equilibrium is important in gas processing [1, 2] and the related environmental protection [3]
Prior to phase equilibrium calculations, the unknown fraction needs to be discretized into a certain number of Single Carbon Numbers (SCN) and the SCN need to be characterized by available methods [8,9,10]
We extend the application of PPR78 in calculating phase envelopes to real natural gas mixtures with unknown fractions

Summary

Introduction

Information provided by vapor-liquid equilibrium is important in gas processing [1, 2] and the related environmental protection [3]. While phase envelope calculations for synthetic and lean natural gas mixtures with a few known components do not often pose problem [4], for real gas mixtures with unknown fractions calculations are increasingly more complicated [5]. A real gas mixture collected from well-head is often reported by a few known components plus an unknown fraction [7]. Prior to phase equilibrium calculations, the unknown fraction needs to be discretized into a certain number of Single Carbon Numbers (SCN) and the SCN need to be characterized by available methods [8,9,10]. As the equilibrium measurements for real gas mixtures are difficult to measure, expensive and limited, numerical methods using Equations of State (EoS) have become imminent [13, 14]

Methods

Results

Conclusion