Rapidly Estimating Natural Gas Compressibility Factor

Availableonlineatwww.sciencedirect.com° SciencedirectJournal of Natural Gas Chemistry 16(2007)349-353CIENCE PRESSArticleRapidly Estimating Natural Gas Compressibility FactorAlireza Bahadori*, Saeid Mokhatab, Brian F. Towler31. Department of Chemical Engineering, Curtin University of Technology, GPO Bor 1987, Perth, WA 6845,Australia2. Process Technology Department, Tehran Raymand Consulting Engineers, Tehran, IranS. Chemical and Petroleum Engineering Department, University of wyoming, Laramie, WY, USAManuscript received June 5, 2007; revised July 17, 2007 1Abstract: Natural gases containing sour components exhibit different gas compressibility factor (ehavior than do sweet gases. Therefore, a new accurate method should be developed to account for thesedifferences. Several methods are available today for calculating the Z-factor from an equation of statHowever, these equations are more complex than the foregoing correlations, involving a large number ofparameters,which require more complicated and longer computations. The aim of this study is to developa simplified calculation method for a rapid estimating Z-factor for sour natural gases containing as muchs 90% total acid gas. In this article, two new correlations are first presented for calculating the pseudo-critical pressure and temperature of the gas mixture as a function of the gas specific gravity. Then, a simplecorrelation on the basis of the standard gas compressibility factor chart is introduced for a quick estimationof sweet gases compressibility factor as a function of reduced pressure and temperature. Finally, a newcorrective term related to the mole fractions of carbon dioxide and hydrogen sulfide is developedKey words: natural gas; compressibility factor; sour gas; critical pressure; critical temperature1. Introductionn is the number of kilomoles of the gas, and R is theThe volume of a real gas is usually less than thatThe gas deviation factor, Z, is close to 1 at lowof an ideal gas, and hence a real gas is said to be su- pressure and high temperature, which means that thepercompressible. The ratio of the real volume to the gas behaves as an ideal gas in these conditions.Atideal volume, which is a measure of the amount the standard or atmospheric conditions the gas Z factorgas deviates from perfect behavior, is called the su- is always approximately 1percompressibility factor, sometimes shortened to theThe most common sources of Z-factor valuescompressibility factor. It is also called the gasre experimental measurement, equations-of-stateation factor and is denoted by the symbol Zmethod and empirical correlations. Necessity arisesgas deviation factor is, by definition, the ratio of the when there is no available experimental data for thevolume actually occupied by a gas at a given pressure required composition, pressure, and temperature con-and temperature to the volume it would occupy if it ditions. Several different correlations are available forthis important parameter. The basic correlations useThe real gas equation of state is then written as the中国煤化工PV=ZRTLCNMHstates dictates th…1…3 uvained as a function ofwhere P is the pressure, V is the volume, T is the reduced pressure and reduced temperature The re-absolute temperature, Z is the compressibility factor, duced pressure and reduced temperature are definedCorresponding author. Tel: +61-8-9266-4685: Fax: +61-8-9266-2681: E-mail: alireza bahadori@student. curtin. edu. auAlireza Bahadori et al. Journal of Natural Gas Chemistry Vol. 16 No. 4 2007(Tc) and critical pressure(Pc) of natural gases andPI-P(2) sour gas compressibility correction. The experimentaldata to develop Te and Pc prediction have been usedT(3) from Sutton's work, for gas compressibility factor theStanding-Katz chart 4 data were used to developwhere P and T are reduced pressure and reduced Z"factor correction, and Wichert-Aziz 5] reportedcal pressure and critical temperature of the gas, re- correlation, n usedtemperature, respectively, and Pc and Tc are criti- data have been used to develop sour gas correctionspectively. The values of critical pressure and criti-The values of critical pressure and critical tem-cal temperature can be estimated from the following perature can be estimated from its specific gravity ifequations if the composition of the gas and the critical the composition of the gas and the critical propertiesproperties of the individual components are known: of the individual components are not known.Consid-ering this, in this study, we used a regression analysisPcPca(4) on the experimental data compiled by Sutton [6] toobtain the following second-order fits for the pseudo-K=A1+B178+C172where Pci and Tci are the critical pressure and criticalIn the aforementioned equation, parameter K istemperature of component i, respectively; and yi is defined in Table 1, where Ppc and Tpc are pseudothe mole fraction of component icritical pressure (kPa)and pseudo-critical temperaOnce the critical properties of the mixture are cal- ture(K), respectively, and 1g is the specific gravityculated as stated in Equations (4)and(5), we can use of gas. Equation(6) is valid over the range of specificEquations(2)and 3) to calculate the reduced prop- gas gravities: 0.55