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Fluid Phase Equilibria, 19 (1985) 21-32Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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THE PHASE BEHAVIOR OF TWO MIXTURES OF METHANE,CARBON DIOXIDE, HYDROGEN SULFIDE, AND WATERSAM S.-S. HUANG *, A.-D. LEU **, H.-J. NG *** and DONALD B. ROBINSQNDepartment of Chemical Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6(Canada)(Received November 7, 1983; accepted in final form July 23, 1984)

ABSTRACTHuang, S.S.-S., Leu, A.-D., Ng, H.-J. and Robinson, D.B., 1985. The phase behavior of two

mixtures of methane, carbon dioxide, hydrogen sulfide, and water. Fluid Phase Equilibria,19: 21-32.The behavior of two mixtures of CH,, CO,, H,S and H,O was studied over a temperature

range from - 37.8O to 204OC at pressures in the 0.4-18.5 MPa range. The work was carriedout to determine the composition of the equilibrium phases over a range of experimentalconditions within the two- or three-phase envelope for each mixture, and to determine thetwo- and three-phase boundaries for each mixture within the temperature and pressure rangeof the study.A variable volume equilibrium cell consisting of a transparent sapphire cylinder was usedfor the experimental measurements and observations.

INTRODUCTIONNatural gas handling and processing operations are frequently carried outunder conditions where an aqueous liquid phase appears. This phase may be

in equilibrium with either a hydrocarbon gas phase or a hydrocarbon-richliquid phase or both. Interest in the composition and behavior of thesesystems in the presence of relatively large concentrations of carbon dioxideand/or hydrogen sulf ide has been prompted in part by the trend towardprocessing sour mixtures from reservoirs that had hitherto been consideredeconomically unattractive.Although some reliable data exist on the behavior of the methane-water(Olds et al., 1942; Culberson and McKetta, 1951), carbon dioxide-water(Wiebe and Gaddy, 1941; Todheide and Franck, 1963; Takenouchi and* Present address: Gulf Canada Ltd., Sheridan Park, Ontario, Canada.** Present address: Shell Canada Ltd., Calgary, Alberta, Canada.*** Present address: DB Robinson and Assoc. Ltd., Edmonton, Alberta, Canada.037%3812/85/$03.30 Q 1985 Elsevier Science Publishers B.V.


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Kennedy, 1964) and hydrogen sulfide-water (Selleck et al., 1952) systems,reliable information on the behavior of ternary and quaternary mixtures ofthese compounds is essentially non-existent. Such data are considered to beuseful as they provide a severe test of the capability of fluid propertypredictive correlations.

In the work presented in this paper, the behavior of two mixtures of CH,,CO,, H,S and H,O was studied over a temperature range from -37.8 to204 OC at pressures in the 0.4-18.5 MPa range. The primary objectives of thework were:

(1) to determine the composition of the equilibrium phases over a range ofexperimental conditions within the two- or three-phase envelope for eachmixture.

(2) To determine the two- and three-phase boundaries for each mixturewithin the temperature and pressure range of the study.

The composition of the two mixtures chosen for the study had thefollowing nominal compositions on a mol% basis:Component Mixture Mixture

1 2CHd 1s Sco2 30 SH,S 5 40Hz0 so 50

The composition of Mixture 2 assured the existence of 3 phaseliquid-liquid-vapor equilibrium within the study conditions.EXPERIMENTAL METHOD

Figure 1 schematically shows the experimental measurements and ob-servations were made in a variable volume equilibrium cell consisting of atransparent sapphire cylinder mounted between steel headers. The topheader contained the necessary openings and valves for charging the compo-nents to the cell, for measuring the cell pressure, and for removing samplesfor analysis. The bottom header accommodated a movable stainless steelpiston which was driven manually by an externally mounted hand wheel.The body of the cell was - 2.5 cm inside diameter with a length of 15.2 cm.The working volume of the cell was - 45 cm3. The main cell and all thenecessary auxiliary lead lines and valves. were mounted inside a controlledtemperature air bath. Equilibrium was obtained by mechanically rocking theentire assembly.

After charging the cell with the required amount of each component asrequired to achieve the composition of Mixture 1 or 2, the temperature wasset to the required value, and the pressure was controlled by movement of


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the piston. When equilibrium had been achieved by rocking the cell and bathassembly, the final pressure and temperature were read, and the phases wereanalyzed by sequentially removing them isobarically and isothermally to thechromatographic system.During the sampling procedure, the total amount of each phase wasremoved slowly from the cell and expanded into the heated line leading tothe chromatographic sampling valve. Thus the lines became thoroughlypurged with the gaseous sample and the surfaces had ample time to reachequilibrium with the materials in the sample. This was important particularlyfor the water. Samples could be taken for analysis at any time during thephase-removal procedure.For phase boundary determinations, the lower dew points along thetwo-phase boundary for Mixtures 1 and 2 were determined by visualobservation. The system was controlled at the desired temperature with thecontents of the cell in the single phase. The pressure of the system was thenincreased or decreased alternately by the movement of the piston in the cell.The formation or disappearance of an opaque cloud or a droplet on thesapphire cell wall due to a slight change in pressure was observed repeatedly.The dew point was considered to be the condition where the appearance anddisappearance of the cloud or droplet occurred within c 3.5 kPa.

Fig. 1. Schematic illustration of equilibrium cell and auxiliary equipment.


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The dew points and bubble points along the three-phase boundary weredetermined by graphical analysis. After the system reached the desiredpressure and temperature, a series of equilibrium pressures and volumes(measured in terms of the height of the piston with respect to a fixed point)of the system were measured. Figure 2 shows the values were then plotted todetermine the intersection of the two straight lines. The point of intersectionwas considered to be the dew point or bubble point.MATERIALS USED AND MIXTURE MAKE-UP

In view of the high water content of both experimental mixtures and thehigh hydrogen sulfide content of Mixture 2, it was not possible to preparemixtures having these overall compositions, store them, and charge them tothe equilibrium system without entering the two-phase region. Consequently,two synthetic gas mixtures A and B having the compositions given belowAh (cm)

0 I 2 3 4 5 6 7 6I I I I I I I I I

4. At Ah = h, - hoho : Reference he ight o f p is ton

4. 2 h, : Fina l he ight o f p is ton

0 Dew Poin t observat ion* Bubble Po in t observat ion

Ah (cm)Fig. 2 . Isothermal plot of pressure versus volume for determining the phase boundary in the 3phase region.

P

20

I8

16

6


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were prepared for use in making up the final test Mixtures 1 and 2:Component Composition Mol %Mixture A Mixture BCH4 29.91 + 0.02 50.07 + 0.02CO, 60.03 * 0.02 49.93 + 0.02H,S 9.95 f 0.02 0N,, C,, C, 0.107 Trace

To obtain Mixture 1, a known amount of Mixture A was added to theequilibrium cell. After this, the predetermined amount of liquid waterrequired to match the required overall mixture composition was injected intothe cell using a small high pressure stainless steel plunger pump calibratedand readable to the nearest 0.005 cm3.The same procedures were followed to prepare Mixture 2 except that thecell was first charged with a known amount of Mixture B, followed by theinjection of pure water and pure liquid hydrogen sulfide from separateplunger pumps to make up the required composition for the mixture.EXPERIMENTAL MEASUREMENTS

The pressure of the equilibrium cell contents was measured using aStatham pressure transducer having a range of O-34.5 MPa. The transducerwas calibrated against a Ruska dead weight gauge at room temperature. Thepressure of the system was known to within 0.25% of the reading. To protectthe transducer diaphragm from corrosion by the wet hydrogen sulfide, a 2.54cm diameter, 0.005 cm thick gold plated stainless steel plate was placedbetween the cell contents and the transducer.The temperatures of the cell and the air bath were measured usingiron-constantan thermocouples which had been calibrated against a certifiedplatinum resistance thermometer. The temperature of the cell contents wasknown to within + 0.05 OC.The output from both the transducer and the thermocouple was read on aHewlett-Packard Model 3490A digital voltmeter.The volume of the equilibrium cell and the dead volume resulting fromthe pressure connections and the sampling valves and lead lines was de-termined by expanding nitrogen from a high pressure container at knownvolume, temperature and pressure into the total evacuated equilibrium cell.From the new pressure, temperature, and known properties of nitrogen, thevolume of the cell and the dead volume could be determined. The deadvolume represented < 5% of the total cell volume. The volume of the visibleportion of the cell above the piston was determined from the knowndimensions.


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The composition of the gas mixtures and the equilibrium phases wasdetermined using a Hewlett-Packard Model 5880A gas chromatograph witha thermal conductivity detector. The samples were eluted through a 3.2 mmOD by 1 m long Porapak QS column with temperature programming from30 to 60 OC. Methane, carbon dioxide, and hydrogen sulfide were calibratedusing a gravimetrically prepared sample of known composition containing0.2991+ 0.0002 CH,, 0.6003 + 0.002 CO,, and 0.0995 f 0.0002 H,S. Thecalibrations with water were done using a mixture of carbon dioxide andwater sampled at equilibrium with liquid water over a range of temperaturesand pressures. The repeatability of the analyses was generally within kO.2mol%.R E SU L T S

The experimental test conditions were chosen to cover a range in tempera-ture from 37.8 to 176.7 C. At each temperature, two to four pressure

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I I I I I , I I IMIXTURE COMPOSITION. MOLE %=t-b 14.05 f 0.02 l-q 4.97 f 0.01CO, 30.07 f 0.04 Hz0 50.00 f 0.08

0

0

A Exper imenta l Tw o-phase Dew Poin ts0 Exper imenta l Tw o-phase Condi t ions

TEMPERATURE, CF i g . 3. E x p e r i m e n t a l p h a s e b o u n d a r y a n d e x p e r i m e n t a l p r e s s u r e - t e m p e r a t u r e c on d i t i on s fo rM i x t u r e 1 .


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conditions were chosen so that the amount of liquid present at the equi-librium..conditions would be large. enough to make visual observation andchromatographic analysis of each phase possible.

A total of nine experimental runs were made on Mixture 1, covering threeTABLE 1Equilibrium pha se compositions and relat ive liquid volumes in th e 2 phas e region for Mixtu re1Press ure Tempera tur e Component Composition, Mole fractionMPa OC Feed Liquid Vapor

Liqu id Vol. %I

4.82 37.8

7.60 37.8

12.52 37.8

16.93 37.8

8.36 107.2

12.93 107.2

17.17 107.2

11.80 176.7

17.31 176.7

CH, 0.1494 2.76~10-~ 0.3040 4.08co2 0.3005 9.30 x10-s 0.5945H2S 0.0497 5.03 x1o-3 0.0998H2O 0.5004 0.9854 1.91 x 10-3CH, 0.1488 4.66~10-~ 0.3031 7.30co2 0.2991 0.0121 0.5970H2S 0.0494 5.4Ox1o-3 0.0982H2O 0.5027 0.9816 1.71 x 1o-3CH, 0.1497 7.96 x 1O-4 0.3029 14.5co2 0.3010 0.0151 0.5967H2S 0.0498 5.95 x10-a 0.0985H2O 0.4996 0.9781 1.87~10-~CH, 0.1495 9.9Ox1o-4 0.3021 19.9co2 0.3006 0.0154 0.5963H2S 0.0497 6.08 x10-3 0.0996H2O 0.5002 0.9777 1.99x 10-sCH, 0.1496 3.79x1o-4 0.2907 5.38co2 0.3009 6.98~10-~ 0.5919H2S 0.0498 3.42x10- 0.0967H2O 0.4997 0.9894 0.0225CH, 0.1496 5.78 x 1O-4 0.2929 8.66co2 0.3009 9.59 x lo- 0.5920H2S 0.0498 4.47 x 10-3 0.0963H2O 0.4997 0.9854 0.0196CH, 0.1496 7.79x1o-4 0.2935 11.8co2 0.3009 0.0113 0.5916H2S 0.0498 4.73 x 10-3 0.0970H2O 0.4997 0.9834 0.0179CH., 0.1495 7.17 x 10-4 0.2641 5.82co2 0.3006 7.95x10-3 0.5575H2S 0.0497 3.74x 10-3 0.0848H2O 0.5001 0.9877 0.0950CH, 0.1500 1.10 x 10-3 0.2762 8.65co2 0.3015 0.0114 0.5520H2S 0.0499 4.78 x lo- 3 0.0870H2O 0.4986 0.9827 0.0848


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temperatures of 37.8, 107.2, and 176.7 OC at pressures from 4.8 to 17.3 MPa.Table 1 presents the results and Fig. 3 shows graphically the conditions. Allof these experimental conditions were in the vapor-aqueous liquid equi-librium region.

For Mixture 2, five experimental runs in the vapor-aqueous liquid equi-librium region were made. These covered two temperatures at 107.2 and176.7 C at pressures from 7.6-18.2 MPa. Table 2 presents the results andFig. 4 shows graphically the conditions. Two experimental runs in theTABLE 2E qu i li b r i um ph ase com pos i t i ons an d r e l a t i ve l iqu i d vo l um es i n t h e 2 ph ase r eg i on fo r M i xt u r e2P r e s s u r e T e m p e r a t u r e C o m p o n e n t Composi t ion , Mol % Liquid vol .%MP a OC Feed L i qu i d VaporA queous l i qu i d - vapo r7.56 107.2 CH, 0.0494 1.55 x 1o-4 0.1182 6.43

CO, 0.0493 1.25 x 1o-3 0.1112Hz8 0.4072 0.0304 0.7485Hz0 0.4941 0.9682 0.0253

12.27 107.2 CH4 0.0498 3.32 x 1O-4 0.1060 13.0CO, 0.0496 2.26 x 1O-3 0.1148H2S 0.4019 0.0361 0.7528H2O 0.4986 0.9613 0.0264

16.92 107.2 CH4 0.0499 6.06X 1o-4 0.1207 20.3co2 0.0498 3.34 x lo- 3 0.1176HzS 0.4009 0.0392 0.7322H2O 0.4994 0.9568 0.0295

11.00 176.7 CH4 0.0502 3.50x 1o-4 0.1092 6.43co2 0.0501 1.64x1O-3 0.1078Hz8 0.4012 0.0286 0.6896H20 0.4985 0.9694 0.0938

18.17 176.7 CH4 0.0496 7.15 x 1O-4 0.0928 12.3co2 0.0494 -.92 x 10 3 0.0914H2S 0.4000 0.0517 0.704Hz0 0.5000 0.9454 0.113

Aqu eous l iqu i d - hyd r ogen su l fi de -r i ch den se f lu i d13.00 37.8 CH4 0.0500 8.59 x 1O-4 0.0891 a 27.1

co2 0.0498 3.62x1O-3 0.0994H2S 0.4006 0.0291 0.8016I.320 0.4996 0.9666 9.32~10-~

16.46 37.8 CH4 0.0500 8.82 x 1O-4 0.0891 a 28.0co2 0.0499 3.81 X lo- 3 0.1061H2S 0.4000 0.0281 0.7958H2O 0.5001 0.9672 9.o5x1o-3

a H ydr ogen su l fi de -r i ch den se fl u i d ph ase .


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29aqueous liquid-hydrogen sulfide-rich dense fluid region were made at atemperature of 37.8 OC and pressures of 13.0 and 16.5 MPa (Table 2 and Fig.4). Two experimental runs in the three-phase aqueous liquid-hydrogensulfide-rich liquid-vapor equilibrium region were made at 37.8 OC, 6.3 MPaand 65.6 OC, 8.4 MPa. Table 3 presents the results and Fig. 4 showsgraphically the conditions.In addition to directly determining the equilibrium phase compositionsand liquid volume fractions in the two- and three-phase regions for thesemixtures, phase boundaries were determined by extensive studies. The stud-ies included determining experimental dew points along the two-phaseboundary for Mixture 1 and 2 from - 121.4 to 204.4O C at pressures from0.42 to - 3.7 MPa. The results and presented in Table 4 and showngraphically in Fig. 3 for Mixture 1, and in Table 5 and Fig. 4 for Mixture 2.The study also included determining dew points and bubble points and an

2 4

2 2

2 0

I 8

6

I I I I I I I I 1MIXTURE COMPOSITION, MOLE %C4 5.00 f 0.04

CO, 4.98 * 0.05kt,s 40.09 f 0.03H,O 49.93 f 0.08

0

.

A Dew Point (L2v)0 Dew Point (L, L2V)0 Bubble Point (L, L2V,0 Experimental Conditions P

l

.

I0 50 1 0 0 150 2ocTEMPERATURE, C

Fig. 4. Experimental phase boundaries and experimental pressure-temperature conditions forMixture 2.


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TABLE 4Phase boundary data along the lower dew point boundary for Mixture 1Feed composition Temperature pressureMol 8 OC MPaCH, 0.1497 198.8 3.25CO, 0.3010 190.6 2.70H2S 0.0498 176.7 1.94Hz0 0.4995 169.8 1.67CL-L, 0.1495 163.1 1.39CO* 0.3005 148.8 0.95H2S 0.0497 135.0 0.64H2O 0.5003 121.1 0.42TABLE 5Phase boundary data along the lower dew point b oundary for mixture 2Feed compositionMol WCL% 0.0502CO, 0.0501Hz8 0.3996Hz0 0.5001CL-L, 0.0500CO* 0.0499H2S 0.3999H2O 0.5002

TemperatureOC120.8148.6

176.7190.7204.7

PressureMPa0.420.97

1.952.703.70

TABLE 6Three-phase dew point and bubble point data for Mixture 2Mixture Composition Dew -pointsMole fraction Temperature

CPressureMPA

CH,co2H,SH20CH,co2H*SHz0CH, 0.0501co2 0.0499H*S 0.3998HP 0.5002

0.05040.05020.39800.50140.04980.04960.40400.4966

38.7 3.5949.0 4.5059.9 5.72

71.0 7.2382.1 9.41

Bubble points37.748.859.971.176.0

7.818.659.47

10.2110.52


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examination of the critical region for the three-phase region for Mixture 2.Dew points were determined at temperatures from - 38.9 to 82.2O C and atpressures from 3.6 to - 9.4 MPa. Bubble points were determined at tempera-tures from 38.9 to 76.7 C and at pressures from 7.8 to - 10.5 MPa. Theresults are presented in Table 6 and shown graphically in Fig. 4.

It will be noted from the tables that there are minor changes in overallcomposition for each experimental point. These result from the fact that anew charge of material was required after each analysis. The slight dif-ferences in overall composition represent the abili ty of the system andprocedures to reproduce a desired composition.ACKNOWLEDGMENTS

The financial support received for this work from the Gas ProcessorsAssociation and the National Research Council of Canada is gratefullyacknowledged.REFERENCESCulberson, O.L. and McKetta , J r., J .J., 1951. Pha se equilibria in hydrocar bon-water systems.

Tra ns . AIME, 192: 223-226, 297-300.Olds, R-H., Sage, B.H. and Lacey, W.N., 1942. Pha se equilibria in hydrocar bon systems.

Composition of the dew-point gas of the meth an e-water system. Ind. Eng. Chem., 34:1223-1227.

Selleck, F.T., Car michael, L.T. an d Sage, B-H., 1952. Ph ase beha vior in th e hydrogensu lfide-wat er system . Ind. En g. Chem ., 44: 2219-2226.

Takenouchi, S. and Kennedy, G.C., 1964. The binary system H,O-CO, at high temper at uresan d pres sur es. Am. J . Sci., 262: 1055-1074.Tedheide, K. an d Fra nck, E .U., 1963. Das Zweiphasengebiet und die Kritische Kurve im

System Kohlendioxid-Wasser bis zu Drucken von 3500 bar. Z. Phys. Chem. (Fran kfurt amMain) 37: 387-401.

Wiebe, R. an d Gadd y, V-L., 1941. Vapor ph as e composition of carbon dioxide-watermixtures at various temper at ures an d pressu res to 700 at mospheres. J . Am. Chem. Sot.,63: 475-477.

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