The Volumetric Behavior of Natural Gases Containing Hydrogen Sulfide and Carbon Dioxide

Abstract

Published in Petroleum Transactions, AIME, Volume 219, 1960, pages 54–60. Abstract Experimental data have been obtained on the volumetric behavior of ternary mixtures of methane, hydrogen sulfide and carbon dioxide at temperatures of 40°, 100° and 160°F up to pressures of 3,000 psia. The results indicate that the compressibility factors for this system do not agree with compressibility factors for sweet natural gases at the same pseudo-reduced conditions. The deviation increases as the temperature and methane content decrease. Discrepancies of up to 35 per cent were observed. A careful analysis has been made of the existing published data on compressibility factors for binary systems containing light hydrocarbons and hydrogen sulfide or carbon dioxide. It has been found that the deviation of actual from predicted compressibility factors for methane-acid gas mixtures is a function of the methane content and the pseudo-critical properties of the mixture. The ratio between actual compressibility factors for methane-acid gas mixtures and compressibility factors for sweet natural gases at the same pseudo-reduced conditions has been correlated over a range of pPr from 0 to at least 7 and a range of pTr from about 1.15 to at least 2.0 with an error not exceeding 3 per cent and over most of the range within 1 per cent. The validity of the correlation for mixtures containing appreciable heavier hydrocarbons has not been fully established, but it is shown to be preferable than the use of a correlation based only on hydrocarbons. Introduction Although a relatively accurate method for predicting compressibility factors of pure materials is provided by charts based on reduced properties and the assumption that the compressibility factor is a unique function of Tr, Pr and zc, the determination of the correct values of compressibility factors for gas mixtures is somewhat difficult. Two general methods of dealing with gaseous mixtures have been proposed. The first assumes a direct or modified additivity of certain properties of the mixture in terms of the properties of the individual components. Examples of this method are based on the familiar laws of Dalton and Amagat. The second method averages the constants of an equation of state applicable to the pure components. Both of these methods are of limited value in engineering calculations because the first usually provides reliable answers only over narrow ranges of pressure and temperature and the second is cumbersome to handle.