Low Temperature Phase and Volumetric Behavior of Natural Gases

Published in Petroleum Transactions, AIME, Volume 195, 1952, pages 279–284. An experimental method and apparatus for the study of the low-temperature phase and volumetric behavior of volatile mixtures are described. The phase diagrams, including the critical points, and the volumetric behavior of two natural gases at temperatures from -40°F to -200°F and pressures to 1200 lb/sq in are given. Introduction Knowledge of the phase and volumetric behavior of light hydrocarbon mixtures at low temperatures is becoming of increasing importance as interest in low-temperature separation processes increases. Ethane and ethylene are already being removed from natural and refinery gases by low-temperature distillation, and removal of nitrogen from natural gases by this process has been proposed as a means of increasing the transmission capacity of existing pipe lines. While existing correlations of equilibrium vaporization constants may be used to predict the vapor-liquid equilibrium behavior of such mixtures at lower pressures, high-pressure phase computations often require knowledge of the critical behavior of the system involved. Although correlations have been developed for the prediction of the critical temperature and pressure of certain types of hydrocarbon systems, they are not applicable to very volatile systems such as nitrogen-bearing natural gases. This paper describes the initial stages of an investigation which had as its main purpose the study of the low-temperature phase and volumetric behavior of such mixtures. The experimental phase diagrams, critical points, and volumetric behavior of two natural gases are presented here. The published low-temperature vapor-liquid equilibrium data for hydrocarbons and mixtures of hydrocarbons with nitrogen and helium are summarized in Table 1. Most of these data are for binary systems, and the data of Stutzman and Brown, at 100 lb/sq in. are the only data on a natural gas. The data of Bloomer and Parent on the methane-nitrogen system, which have been published since this study was completed, are a valuable addition to the knowledge in this field. Prior to the publication of these data the only critical data on systems having critical temperatures below the ice point were those of Ruhemann, on the methane-ethane system, and those of Eilerts, et al, on a gas-condensate system. These data alone were not sufficient to allow accurate estimation of the critical behavior of the very volatile systems of engineering interest.