Estimation of the Water Content of Sour Natural Gases
Authors: Robinson J.N. , Wichert E. , Moore R.G. , Heideman R.A. 
Published: December 2007

Equilibrium water contents of sour natural gases are predicted using the Soave modification of the Redlich-Kwong equation of state. Calculated results are compared with published experimental data and with 180 field observations that were collected from the natural gas industry of western Canada and France.

Computer-generated curves are provided to enable field calculation of the equilibrium water content of sour natural gases over a greater range of conditions than is possible with other methods.

Introduction

Natural gases containing significant quantities of acid gas are encountered frequently in western Canada. Estimates of the water content of these sour gases are required for the design of plant and pipeline facilities. This paper describes an pipeline facilities. This paper describes an equation-of-state method for predicting water content and presents a summary of field data gathered for testing the model.

Three methods are currently available for estimating the water content of sour natural gases. In the procedure outlined by the Gas Processors Suppliers Assn. (GPSA), the estimated water content of a sour gas is a molar average of the solubility of water in the hydrocarbons, hydrogen sulfide, and carbon dioxide. The water-content curves for H2S and CO2 are based on experimental data for the binary mixtures H2O-H2S and H2O-CO2, respectively. Both these binaries display liquid-liquid equilibria at temperatures and pressures common in processing applications, and the water content read for the acid gas components often corresponds to the solubility of water in a nonaqueous liquid phase rather than in a vapor phase. In general, the predicted water content of sour natural gases is predicted water content of sour natural gases is high when based on these experimental curves.

Maddox presents a method similar to the GPSA procedure. The difference is that the effect of procedure. The difference is that the effect of liquid-liquid separation has been removed from the H2O-CO2 and H2O-H2S binary data by smoothing. Campbell is credited with generating the curves, which terminate at 204 atm (3,000 Psi). Use of the modified curves in the molar averaging method can be expected to result in low estimates of the water content at elevated pressures.

A semi-empirical correlation based on calculated mixture properties was developed by Sharma and Campbell. This method yields satisfactory estimates of water content for sour gases having total acid gas concentrations less than 15 percent, at temperatures between 300 (80 deg. F) and 344 K (160 deg. F), and at pressures less than 136 atm (2,000 psi).

The major limitation of these three methods is that there is no basis for extrapolation to high acid gas concentrations or to more extreme conditions of temperature and pressure. When an equation of state is used to estimate the water content, as we have done in this paper there is, in principle, no limitation on the range of conditions that can be considered.

It has been demonstrated previously that an equation of state can be used to describe waterhydrocarbon systems and the two binary pairs H2O-CO2 and H2O-H2S. In this paper, the Soave modification of the Redlich-Kwong equation has been used to calculate the water content of sour natural gases. Comparison is made with 180 field measurements taken in western Canada and in France.

THE EQUATION OF STATE

Soave presented a modification of the Redlich-Kwong equation in which the coefficient a was correlated against reduced temperature and acentricity so as to match vapor-pressure data of pure components. We have used this equation to pure components. We have used this equation to describe both the liquid and vapor phases.

Evelein et al. found that the vapor pressure of water was not predicted with satisfactory accuracy by the Soave correlation, and presented alternative values of a for water.

SPEJ

P. 281