Phase Behavior in the Methane-ethane-n-pentane System By G. W. BILLMAN,· B. H. SAGE,· MEMBER AIME AND W. N. LACEY· (New York Meeting, March 1947) ABSTRACT However, these results do not establish THE composition of the coexisting phases in the influence of the nature and amount of the methane-ethane-n-pentane system was the other substances making up a multi­ determined at 100°F. This was accomplished by component system upon the gas-liquid withdrawing samples of the coexisting phases equilibrium constant of anyone com­ under isobaric-isothermal conditions. The rela­ tive amount of each of the components present ponent. A preliminary correlation on the in the liquid and gas phases was determined by basis of vaiues for binary systems was conventional analytical fractionation methods. made2 utilizing such experimental informa­ The phase behavior of this system approxi­ tion as was available. However, there is mates that estimated from generalized corre­ need for much additional information lations which take into account the influence regarding behavior in systems containing of the nature and amount of each of the com­ more than two components. ponents present upon the equilibrium. It was found that for particular pressures and tem­ Study of the phase behavior of ternary Downloaded from http://onepetro.org/trans/article-pdf/174/01/13/2178576/spe-948013-g.pdf by guest on 30 September 2021 peratures the equilibrium constant of each of systems offers a feasible attack upon the the components was influenced significantly by determination of the influence of com­ the composition of the system and this effect position as well as of pressure and tem­ was more pronounced at the lower pressures perature upon the several equilibrium for methane than for the components of greater constants concerned. An investigation of molecular weight. However, at the higher pres­ the phase behavior of the methane­ sures approaching the maximum two-phase propane-n-pentane system at IOOo, 160°, pressure for the system at this temperature the and 220°F was reported.3,. This work composition markedly influences the equilib­ rium constants of all the components at a served to illustrate the marked influence particular pressure and temperature. The re­ of concentration of the other components sults of this study are presented in graphical upon the equilibrium constants. Variations and tabular form. of as much as 50 pct in the equilibrium constants for methane for fixed tempera­ INTRODUCTION ture and pressure were observed in regions The phase behavior of many of the remote from the critical state. Likewise binary systems containing paraffin hydro­ there was a marked variation in the carbon components from methane through equilibrium constants of propane and n-pentane has been investigated during n-pentane as a result of changes in the recent years. In addition the compositions composition. These results indicated the of the coexisting phases in mixtures of crude desirability of investigating additional oil and natural gas have been studied. I ternary systems. The present paper deals Manuscript received at the office of the with a study of the methane-ethane-n- pen­ Institute Feb. 10, 1947. Issued as TP 2232 in PETROLEUM TECHNOLOGY, July 1947. tane system at IOooF, which is similar in • California Institute of Technology, Pasa­ many respects to the work upon the meth­ dena, California. 1 References are at the end of the paper. ane-propane-n-pentane system cited above. 13 14 PHASE BEHAVIOR IN THE METHANE-ETHANE-N-PENTANE SYSTEM METHOD dioxide. The equipment was essentially The compositions of the phases coexisting the same as that used in earlier studies.a.'.8 under isobaric-isothermal conditions were The fractionating columns employed established by withdrawing samples of the in the determination of the composition of gas and liquid phases which had been gas and liquid phase samples had internal brought to thermodynamic equilibrium diameters of 0.12 and 0.18 in. and lengths by prolonged mechanical agitation. The of 4 and 5 ft, respectively. The analyses sample was confined in a steel vessel over were carried out in conventional fashion mercury, the pressure being controlled by except that in the transition region the addition or withdrawal of the latter between one component and another the fluid. After equilibrium had been reached, distillation was repeated in order to check isobaric-isothermal conditions were main­ the completeness of separation of the com­ tained during the withdrawal of the ponent being withdrawn. Comparison of samples by controlled addition of mercury duplicate samples indicates that the over­ to the equilibrium vessel. In general the all analytical uncertainty in the mol fraction of a component was approximately withdrawals were accomplished with devia­ Downloaded from http://onepetro.org/trans/article-pdf/174/01/13/2178576/spe-948013-g.pdf by guest on 30 September 2021 tions from isobaric conditions of less than 0.002, while a precision of 0.001 was usually 3 psi. Mechanical agitation was dis­ easily obtainable. continued during the withdrawal process At low pressures it is possible that very and it is believed that the divergences from small amounts of liquid phase on the walls the original equilibrium state as a result of the equilibrium chamber near the sample of slight changes of pressure did not cause outlet were carried out when a sample of significant error In the experimental the gas phase was withdrawn. At higher results. pressures this difficulty was less probable. The temperature of the equilibrium cell After considering these somewhat intangi­ was kept at desired values by immersing ble quantities in addition to the uncer­ it in an agitated oil bath, thermostatically tainties in pressure, temperature, and controlled by use of suitable electronic composition it appears that, when a com­ circuits to give variations of less than ponent was present only in small quantity, 0.3°F with respect both to time and the uncertainty in mol fraction is probably location within the bath. Temperatures less than 0.005. were related to the International Platinum Scale by a mercury-in-glass thermometer MATERIALS which had been compared with the tem­ The methane used in this investigation perature indications of a standardized, was obtained from the Buttonwillow field strain-free platinum resistance thermom­ in California. As received, the sample eter. It is believed that the temperature was saturate, with water and contained of the equilibrium mixture was known approximately 0.003 mol fraction carbon within 0.2°F. dioxide and 0.0004 mol fraction heavi:r The pressure during the approach to hydrocarbon. Combustion analyses have equilibrium and during the withdrawal of indicated that nitrogen and other non­ the samples was measured by means of a combustible materials are present in this commercial type of piston-and-cylinder gas in negligible amounts. Before use, the pressure balance with an estimated uncer­ methane was dried and purified by passing tainty of 2 psi or less. This instrument it at a pressure of 300 psi or more through was calibrated against a precision pressure layers of calcium chloride, potassium balanceS which in turn had been calibrated hydroxide, activated charcoal, and ascarite. against the known vapor pressure of carbon The partially purified methane was then G. W. BILLMAN, B. H. SAGE AND W. N. LACEY 15 passed through a copper coil immersed of the coexisting phases for the experi~ in a mixture of acetone· and solid carbon mentally studied mixtures have been dioxide. connected by dotted lines. For the pur~ The ethane was procured from the pose of systematic portrayal of the be~ Carbide and Carbon Chemicals Corpora­ havior of the system, solid combining tion and when received it contained sub­ lines are shown for even values of a stantial amounts both of more and less parameter C which is defined by Eq 1.3 volatile components. This crude ethane C _ X 2 was purified by repeated low temperature [Il - (X2 + X 5) fractionation in which the first and last tenths of the overhead product were In this equation, X 2 and Xs represent the discarded. Analytical measurements upon mol fractions of ethane and n-pentane, the purified material indicated that it respectively, in the mixture. did not contain more than 0.002 mol TABLE I-Experimentally Determined Com­ fraction of material other than ethane. positions of Coexisting Phases The n-pentane was obtained from the Downloaded from http://onepetro.org/trans/article-pdf/174/01/13/2178576/spe-948013-g.pdf by guest on 30 September 2021 Phillips Petroleum Company whose special Pres· I Gas Phase. I Liquid Phase. Meth-IEthan n-Pen- Meth-IEth I n-Pen- analysis upon a similar sample indicated s~~i' ane e I tane ane ane tane the material to contain approximately 0.005 100°F mol fraction isopentane and less than 0.001 mol fraction of other hydrocarbon 500 0.904" 0.0377 0.0583 0.154 0. 0284 0.818 0.652 0.297 0.0519 0.115 0.223 0.662 impurities. Care was exercised to a void 0.517 0·431 0.0511 0.0947 0·327 0·578 0.275 0.681 0.0440 0.0550 0.514 0·431 contamination of the sample with air and a 0.000 0.965 0.0349 0.0000 0.736 0.264 further precaution was taken by sub­ 1,000 0.762 0.188 0.0499 0.263 0.211 0.5.6 mitting the n-pentane to prolonged boiling 0.674 0.282 0.0454 0.246 0.314 0.440 0.596 0.360 0.0443 0.237 0.396 0.368 after addition to the equilibrium apparatus. 0.548 0·405 0.0468 0.224 0·444 0.331 0·483 0·473 0.0438 0.214 0.515 0.271 Addition of the material to the equilibrium 0·394 0·562 0.0441 0.198 0.60.