Page 71 - Handbook of Natural Gas Transmission and Processing Principles and Practices

2.1. Introduction
Natural gases have so many industrial applications that a major challenge is to develop a
thermodynamic model able to predict the phase equilibrium of these mixtures. Such predictions are
indeed essential for the processing of natural gases and the design of transportation facilities. As
stated by Louli et al. (2012), the most commonly used models for this purpose are cubic equations of
state (EoS) and especially the ones issued from the Redlich–Kwong and Peng–Robinson classes. The
statistical associating fluid theory (SAFT)-type models are a promising class of EoS emerging since
the late 1980s. Although no industrialized version of SAFT EoS really exists nowadays (by
industrialized model, it is meant to be a model applicable to a large variety of chemicals and related
industrial applications), proof has been given that the Perturbed-Chain SAFT (PC-SAFT) EoS can be
successfully used to model natural gas phase envelopes.
As an alternative class of models for natural gases, Kunz and Wagner (2012) developed the
Groupe Européen de Recherches Gazières (GERG)-2008, an equation of state expressed in terms of
Helmholtz energy applicable to species classically involved in natural gases (methane, nitrogen,
carbon dioxide, ethane, propane, n-butane, i-butane, n-pentane, i-pentane, n-hexane, n-heptane, n-
octane, n-nonane, n-decane, hydrogen, oxygen, carbon monoxide, water, hydrogen sulfide, helium,
and argon) and their related mixtures. Because this model relies on a very large set of adjustable
parameters, a good accuracy can be reached explaining why it was adopted as an ISO standard.
The goal of this chapter is to describe and analyze the various EoS models usable for the
correlation and prediction of phase equilibrium and energetic properties (enthalpy, heat capacities,
and so forth) of natural gases. In the last part of this chapter, some of these models are compared on
the basis of the reproduction of experimental phase-envelope data of real or synthetic natural gases
(SNGs).

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