Prediction of Compressibility of Natural Gas Mixtures by Use of the Benedict-Webb-Rubin Equation of State.

Abstract

ford and Phillips, a s was done by Herzog in his original article. Constants were a l s o derived for these equations using experimental parachors but no significant improvement in correlation w a s observed. Hence, it was decided to u s e the calculated parachors throughout since calculated parachors are normally more convenient to use.Equations with the re-evaluated constants are shown in Tables I, 11, and 111, together with their per cent "reliabilities." Reliabilities of t h e estimated T , values were calculated using the equation shown below. T h i s equation gives the maximum deviation to be expected in 95% of the cases.The same approach was used in determining the p c and V, equation reliabilities. d = 2 0 0 {-1 A knowledge of the pressure-volume-temperature properties of natural gas is of great importance in t h e transportation, storage, and u s e of the gas a s a fuel and a s a source of raw materials for chemical syntheses. Because of the variable composition of natural g a s and t h e time and skill required t o determine t h e P-V-T properties in the laboratory, i t h a s long been the goal of research engineers within the industry to find a n accurate method for their calculation. Heretofore, the methods proposed have been of limited application.The tentative standards a s s e t forth by the Natural Gasoline Association of America (25) and the California Natural Gasoline Association (9, 10) are either not of sufficient accuracy or are limited t o relatively low pressures. Following the pseudo-critical concept of Kay (13), and assuming the law of corresponding state valid for any natural gas mixture, Dunkle (12) proposed a method using pure methane a s the basis for evaluating the compressibility of natural g a s mixtures. Dunkle's approach was later used by Zimmerman and Beitler (16, 17). Their 'Present address, University of Bridgeport, Bridgeport, Conn. method i s simple and accurate for most natural gases,and h a s been adopted by the American G a s Association a s a new standard for the natural gas industry (1). However when the ethane and/or nitrogen and carbon dioxide contents are high, this method i s not so accurate a s i s generally desired.The present study was undertaken t o test the application of the Benedict-Webb-Rubin equation of state (4, 5) to the prediction of the compressibility of natural gas mixtures and particularly t o mixtures of high ethane and/or nitrogen and carbon dioxide content. The study was confined to the gaseous phase in the pressure and temperature ranges usually encountered in i t s transportation and storage. The data on natural g a s mixtures compiled' by Zimmetman and Beitler (16, 17j were used for purposes of comparison in the study. CONSTANTS FOR BENEDICT-WEBB-RUBIN EQUATION OF STATEBenedict, Webb, and Rubin ( 5 ) have proposed the following empirical equation for the representation of the P-V-T relations of a pure gas: T a b l e 1. Values of Constants in Benedict-Webb-Rubin Equation o f State for Compounds Constituting Natural Gas Mixtures Units. P = pounds...