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The phase behavior of two mixtures of methane, carbon dioxide, hydrogen sulfide, and water
Fluid Phase Equilibria, 19 (1985) 21-32 Elsevier Science Publishers B.V., Amsterdam
21 -
Printed in The Netherlands
THE PHASE BEHAVIOR OF TWO MIXTURES OF METHANE, CARBON DIOXIDE, HYDROGEN SULFIDE, AND WATER SAM S.-S. HUANG Department (Canada) (Received
*, A.-D. LEU **, H.-J. NG ***
of Chemical November
Engineering,
University
and DONALD
of Alberta,
B. ROBINSQN
Edmonton,
Alberta,
T6G
2G6
7, 1983; accepted in final form July 23, 1984)
ABSTRACT Huang, 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 carried out to determine the composition of the equilibrium phases over a range of experimental conditions within the two- or three-phase envelope for each mixture, and to determine the two- and three-phase boundaries for each mixture within the temperature and pressure range of the study. A variable volume equilibrium cell consisting of a transparent sapphire cylinder was used for the experimental measurements and observations.
INTRODUCTION
Natural gas handling and processing operations are frequently carried out under conditions where an aqueous liquid phase appears. This phase may be in equilibrium with either a hydrocarbon gas phase or a hydrocarbon-rich liquid phase or both. Interest in the composition and behavior of these systems in the presence of relatively large concentrations of carbon dioxide and/or hydrogen sulfide has been prompted in part by the trend toward processing sour mixtures from reservoirs that had hitherto been considered economically 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, 037%3812/85/$03.30
Q 1985 Elsevier Science
Publishers
B.V.
Canada.
22
Kennedy, 1964) and hydrogen sulfide-water (Selleck et al., 1952) systems, reliable information on the behavior of ternary and quaternary mixtures of these compounds is essentially non-existent. Such data are considered to be useful as they provide a severe test of the capability of fluid property predictive 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 to 204 OC at pressures in the 0.4-18.5 MPa range. The primary objectives of the work were: (1) to determine the composition of the equilibrium phases over a range of experimental conditions within the two- or three-phase envelope for each mixture. (2) To determine the two- and three-phase boundaries for each mixture within the temperature and pressure range of the study. The composition of the two mixtures chosen for the study had the following nominal compositions on a mol% basis: Component
Mixture 1
Mixture 2
CHd
1s 30 5 so
S S 40 50
co2
H,S Hz0
The composition liquid-liquid-vapor EXPERIMENTAL
of Mixture 2 assured the existence equilibrium within the study conditions.
of
3
phase
METHOD
Figure 1 schematically shows the experimental measurements and observations were made in a variable volume equilibrium cell consisting of a transparent sapphire cylinder mounted between steel headers. The top header contained the necessary openings and valves for charging the components to the cell, for measuring the cell pressure, and for removing samples for analysis. The bottom header accommodated a movable stainless steel piston 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 the necessary auxiliary lead lines and valves. were mounted inside a controlled temperature air bath. Equilibrium was obtained by mechanically rocking the entire assembly. After charging the cell with the required amount of each component as required to achieve the composition of Mixture 1 or 2, the temperature was set to the required value, and the pressure was controlled by movement of
23
the piston. When equilibrium had been achieved by rocking the cell and bath assembly, the final pressure and temperature were read, and the phases were analyzed by sequentially removing them isobarically and isothermally to the chromatographic system. During the sampling procedure, the total amount of each phase was removed slowly from the cell and expanded into the heated line leading to the chromatographic sampling valve. Thus the lines became thoroughly purged with the gaseous sample and the surfaces had ample time to reach equilibrium with the materials in the sample. This was important particularly for the water. Samples could be taken for analysis at any time during the phase-removal procedure. For phase boundary determinations, the lower dew points along the two-phase boundary for Mixtures 1 and 2 were determined by visual observation. The system was controlled at the desired temperature with the contents of the cell in the single phase. The pressure of the system was then increased or decreased alternately by the movement of the piston in the cell. The formation or disappearance of an opaque cloud or a droplet on the sapphire cell wall due to a slight change in pressure was observed repeatedly. The dew point was considered to be the condition where the appearance and disappearance of the cloud or droplet occurred within c 3.5 kPa.
Fig. 1. Schematic illustration of equilibrium cell and auxiliary equipment.
24
The dew points and bubble points along the three-phase boundary were determined by graphical analysis. After the system reached the desired pressure 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 to determine the intersection of the two straight lines. The point of intersection was 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 the high hydrogen sulfide content of Mixture 2, it was not possible to prepare mixtures having these overall compositions, store them, and charge them to the equilibrium system without entering the two-phase region. Consequently, two synthetic gas mixtures A and B having the compositions given below Ah (cm) 0 4.A
I t
4.2
I I
2 I
3 I
4 I
5 I
6 I
7 I
6 I
P
20
Ah = h, - ho ho : Reference h,
: Final
height of piston
height of piston
I8
16
6 0 Dew Point observation *
Bubble
Point observation
Ah (cm) Fig. 2. Isothermal phase region.
plot of pressure versus volume for determining
the phase boundary
in the 3
25
were prepared for use in making up the final test Mixtures
1 and 2:
Component
Composition Mixture A
Mol % Mixture B
CH4
29.91 + 0.02 60.03 * 0.02 9.95 f 0.02 0.107
50.07 + 0.02 49.93 + 0.02 0 Trace
CO, H,S N,, C,, C,
To obtain Mixture 1, a known amount of Mixture A was added to the equilibrium cell. After this, the predetermined amount of liquid water required to match the required overall mixture composition was injected into the cell using a small high pressure stainless steel plunger pump calibrated and readable to the nearest 0.005 cm3. The same procedures were followed to prepare Mixture 2 except that the cell was first charged with a known amount of Mixture B, followed by the injection of pure water and pure liquid hydrogen sulfide from separate plunger pumps to make up the required composition for the mixture. EXPERIMENTAL
MEASUREMENTS
The pressure of the equilibrium cell contents was measured using a Statham pressure transducer having a range of O-34.5 MPa. The transducer was calibrated against a Ruska dead weight gauge at room temperature. The pressure of the system was known to within 0.25% of the reading. To protect the transducer diaphragm from corrosion by the wet hydrogen sulfide, a 2.54 cm diameter, 0.005 cm thick gold plated stainless steel plate was placed between the cell contents and the transducer. The temperatures of the cell and the air bath were measured using iron-constantan thermocouples which had been calibrated against a certified platinum resistance thermometer. The temperature of the cell contents was known to within + 0.05 OC. The output from both the transducer and the thermocouple was read on a Hewlett-Packard Model 3490A digital voltmeter. The volume of the equilibrium cell and the dead volume resulting from the pressure connections and the sampling valves and lead lines was determined by expanding nitrogen from a high pressure container at known volume, temperature and pressure into the total evacuated equilibrium cell. From the new pressure, temperature, and known properties of nitrogen, the volume of the cell and the dead volume could be determined. The dead volume represented < 5% of the total cell volume. The volume of the visible portion of the cell above the piston was determined from the known dimensions.
26
The composition of the gas mixtures and the equilibrium phases was determined using a Hewlett-Packard Model 5880A gas chromatograph with a thermal conductivity detector. The samples were eluted through a 3.2 mm OD by 1 m long Porapak QS column with temperature programming from 30 to 60 OC. Methane, carbon dioxide, and hydrogen sulfide were calibrated using a gravimetrically prepared sample of known composition containing 0.2991+ 0.0002 CH,, 0.6003 + 0.002 CO,, and 0.0995 f 0.0002 H,S. The calibrations with water were done using a mixture of carbon dioxide and water sampled at equilibrium with liquid water over a range of temperatures and pressures. The repeatability of the analyses was generally within kO.2 mol%. RESULTS
The experimental test conditions were chosen to cover a range in temperature from 37.8 to 176.7 “C. At each temperature, two to four pressure l-
20 , -
18
I
I
MIXTURE
I
I
COMPOSITION.
=t-b
14.05
f
0.02
l-q
CO,
30.07
f
0.04
Hz0
I
,
MOLE
I
I
I
%
4.97
f
0.01
50.00
f
0.08
16
0
I-
0
A Experimental
Two-phase
Dew Points
Experimental
Two-phase
Conditions
0
TEMPERATURE,
“C
Fig. 3. Experimental phase boundary and experimental pressure- temperature conditions for Mixture 1.
27
conditions were chosen so that the amount of liquid present at the equilibrium..conditions would be large. enough to make visual observation and chromatographic analysis of each phase possible. A total of nine experimental runs were made on Mixture 1, covering three TABLE
1
Equilibrium phase compositions 1 Pressure MPa 4.82
Temperature OC 37.8
Component
CH, co2 H2S H2O
7.60
37.8
CH, co2 H2S
12.52
37.8
H2O CH, co2 H2S H2O
16.93
37.8
CH, co2 H2S H2O
8.36
107.2
CH, co2 H2S
12.93
107.2
H2O CH, co2 H2S
17.17
107.2
H2O CH, co2 H2S
11.80
176.7
H2O CH., co2 H2S H2O
17.31
176.7
and relative liquid volumes in the 2 phase region for Mixture
CH, co2 H2S H2O
Composition,
Mole fraction
Feed
Liquid
Vapor
0.1494 0.3005 0.0497 0.5004 0.1488 0.2991 0.0494 0.5027 0.1497 0.3010 0.0498 0.4996 0.1495 0.3006 0.0497 0.5002 0.1496 0.3009 0.0498 0.4997 0.1496 0.3009 0.0498 0.4997 0.1496 0.3009 0.0498 0.4997 0.1495 0.3006 0.0497 0.5001 0.1500 0.3015 0.0499 0.4986
2.76~10-~ 9.30 x10-s 5.03 x1o-3 0.9854 4.66~10-~ 0.0121 5.4Ox1o-3 0.9816 7.96 x 1O-4 0.0151 5.95 x10-a 0.9781 9.9Ox1o-4 0.0154 6.08 x10-3 0.9777 3.79x1o-4 6.98~10-~ 3.42x10-’ 0.9894 5.78 x 1O-4 9.59 x lo-’ 4.47 x 10-3 0.9854 7.79x1o-4 0.0113 4.73 x 10-3 0.9834 7.17 x 10-4 7.95x10-3 3.74x 10-3 0.9877 1.10 x 10-3 0.0114 4.78 x lo- 3 0.9827
0.3040 0.5945 0.0998 1.91 x 10-3 0.3031 0.5970 0.0982 1.71 x 1o-3 0.3029 0.5967 0.0985 1.87~10-~ 0.3021 0.5963 0.0996 1.99x 10-s 0.2907 0.5919 0.0967 0.0225 0.2929 0.5920 0.0963 0.0196 0.2935 0.5916 0.0970 0.0179 0.2641 0.5575 0.0848 0.0950 0.2762 0.5520 0.0870 0.0848
Liquid Vol. %I
4.08
7.30
14.5
19.9
5.38
8.66
11.8
5.82
8.65
28
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. All of these experimental conditions were in the vapor-aqueous liquid equilibrium region. For Mixture 2, five experimental runs in the vapor-aqueous liquid equilibrium region were made. These covered two temperatures at 107.2 and 176.7 “C at pressures from 7.6-18.2 MPa. Table 2 presents the results and Fig. 4 shows graphically the conditions. Two experimental runs in the TABLE
2
Equilibrium phase compositions and relative liquid volumes in the 2 phase region for Mixture 2 Pressure MPa
Temperature OC
Aqueous liquid-vapor 7.56 107.2
12.27
107.2
16.92
107.2
11.00
176.7
18.17
176.7
Aqueous liquid-hydrogen 13.00 37.8
Component
Feed CH, CO, Hz8 Hz0
0.0494 0.0493 0.4072 0.4941 CH4 0.0498 CO, 0.0496 H2S 0.4019 H2O 0.4986 CH4 0.0499 co2 0.0498 HzS 0.4009 H2O 0.4994 CH4 0.0502 co2 0.0501 Hz8 0.4012 H20 0.4985 CH4 0.0496 co2 0.0494 H2S 0.4000 Hz0 0.5000 sulfide-rich dense fluid CH4 co2 H2S I.320
16.46
37.8
CH4 co2 H2S H2O
Liquid vol.%
Composition, Mol %
0.0500
0.0498 0.4006 0.4996 0.0500 0.0499 0.4000 0.5001
a Hydrogen sulfide-rich dense fluid phase.
Liquid
Vapor
1.55 x 1o-4 1.25 x 1o-3 0.0304 0.9682 3.32 x 1O-4 2.26 x 1O-3 0.0361 0.9613 6.06X 1o-4 3.34 x lo- 3 0.0392 0.9568 3.50x 1o-4 1.64x1O-3 0.0286 0.9694 7.15 x 1O-4 2.92 x 10 - 3 0.0517 0.9454
0.1182 0.1112 0.7485 0.0253 0.1060 0.1148 0.7528 0.0264 0.1207 0.1176 0.7322 0.0295 0.1092 0.1078 0.6896 0.0938 0.0928 0.0914 0.704 0.113
8.59 x 1O-4 3.62x1O-3 0.0291 0.9666 8.82 x 1O-4 3.81 X lo- 3 0.0281 0.9672
0.0891 a 0.0994 0.8016 9.32~10-~ 0.0891 a 0.1061 0.7958 9.o5x1o-3
6.43
13.0
20.3
6.43
12.3
27.1
28.0
29
aqueous liquid-hydrogen sulfide-rich dense fluid region were made at a temperature 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-hydrogen sulfide-rich liquid-vapor equilibrium region were made at 37.8 OC, 6.3 MPa and 65.6 OC, 8.4 MPa. Table 3 presents the results and Fig. 4 shows graphically the conditions. In addition to directly determining the equilibrium phase compositions and liquid volume fractions in the two- and three-phase regions for these mixtures, phase boundaries were determined by extensive studies. The studies included determining experimental dew points along the two-phase boundary for Mixture 1 and 2 from - 121.4 to 204.4O C at pressures from 0.42 to - 3.7 MPa. The results and presented in Table 4 and shown graphically 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 24
I
I
I
MIXTURE
COMPOSITION,
C”4
22
20
I
5.00
f
I
I
MOLE
%
I
I
1
0.04
CO,
4.98
*
0.05
kt,s
40.09
f
0.03
H,O
49.93
f
0.08 l
I8
. 0
.
6
A Dew
Point (L2v)
0 Dew
Point (L, L2V)
0 Bubble
Point (L, L2V,
0 Experimental
Conditions
P
I
0
50
100
TEMPERATURE,
Fig. 4. Experimental Mixture 2.
150
2oc
“C
phase boundaries and experimental pressure-temperature
conditions for
Temperature “C
37.8
65.6
Pressure MPa
6.26
8.43
H2O
CO, H*S
CH4
CH, CO, H,S H*O
Component
0.0504 0.0503 0.3986 0.5008 0.0501 0.0499 0.4016 0.4984
Feed 4.90x 1o-4 3.50x 1o-3 0.0284 0.9677 3.85x 1O-4 2.72~10-~ 0.0321 0.9684
L*
L, 0.0653 0.1049 0.8197 0.0101 0.0580 0.0904 0.8287 0.0212
H ,O liquid
H,S liquid
Composition, Mole fraction
Equilibrium phase compositions and relative volumes in the 3 phase region for Mixture 2
TABLE 3
0.3213 0.1739 0.5028 2.14~10~~ 0.1872 0.1484 0.6557 8.66x1o-3
Vapor V
19.6
31.3
L,
15.0
17.7
L*
65.4
51.0
V
31 TABLE
4
Phase boundary data along the lower dew point boundary for Mixture 1 Feed composition Mol 8
Temperature OC
pressure MPa
CH, CO,
0.1497 0.3010 0.0498 0.4995
198.8 190.6 176.7 169.8
3.25 2.70 1.94 1.67
H2S
0.1495 0.3005 0.0497
163.1 148.8 135.0
1.39 0.95 0.64
H2O
0.5003
121.1
0.42
H2S
Hz0 CL-L, CO*
TABLE
5
Phase boundary data along the lower dew point boundary for mixture 2 Feed composition Mol W
Temperature OC
Pressure MPa
CL% CO, Hz8 Hz0
0.0502 0.0501 0.3996 0.5001
120.8 148.6
0.42 0.97
CL-L, CO* H2S
0.0500 0.0499 0.3999
176.7 190.7 204.7
1.95 2.70 3.70
H2O
0.5002
TABLE
6
Three-phase dew point and bubble point data for Mixture 2 Mixture Composition Mole fraction
Dew -points Temperature “C
Pressure MPA
CH, co2 H,S H20
0.0504 0.0502 0.3980 0.5014
38.7 49.0 59.9
3.59 4.50 5.72
CH, co2 H*S Hz0
0.0498 0.0496 0.4040 0.4966
71.0 82.1
7.23 9.41
CH,
0.0501 0.0499 0.3998 0.5002
co2
H*S HP
Bubble points 37.7 48.8 59.9 71.1 76.0
7.81 8.65 9.47 10.21 10.52
32
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 at pressures from 3.6 to - 9.4 MPa. Bubble points were determined at temperatures from 38.9 to 76.7 “C and at pressures from 7.8 to - 10.5 MPa. The results are presented in Table 6 and shown graphically in Fig. 4. It will be noted from the tables that there are minor changes in overall composition for each experimental point. These result from the fact that a new charge of material was required after each analysis. The slight differences in overall composition represent the ability of the system and procedures to reproduce a desired composition. ACKNOWLEDGMENTS
The financial support received for this work from the Gas Processors Association and the National Research Council of Canada is gratefully acknowledged. REFERENCES Culberson, O.L. and McKetta, Jr., J.J., 1951. Phase equilibria in hydrocarbon-water systems. Trans. AIME, 192: 223-226, 297-300. systems. Olds, R-H., Sage, B.H. and Lacey, W.N., 1942. Phase equilibria in hydrocarbon Composition of the dew-point gas of the methane-water system. Ind. Eng. Chem., 34: 1223-1227. Selleck, F.T., Carmichael, L.T. and Sage, B-H., 1952. Phase behavior in the hydrogen sulfide-water system. Ind. Eng. Chem., 44: 2219-2226. Takenouchi, S. and Kennedy, G.C., 1964. The binary system H,O-CO, at high temperatures and pressures. Am. J. Sci., 262: 1055-1074. Tedheide, K. and Franck, E.U., 1963. Das Zweiphasengebiet und die Kritische Kurve im System Kohlendioxid-Wasser bis zu Drucken von 3500 bar. Z. Phys. Chem. (Frankfurt am Main) 37: 387-401. Wiebe, R. and Gaddy, V-L., 1941. Vapor phase composition of carbon dioxide-water mixtures at various temperatures and pressures to 700 atmospheres. J. Am. Chem. Sot., 63: 475-477.
21 -
Printed in The Netherlands
THE PHASE BEHAVIOR OF TWO MIXTURES OF METHANE, CARBON DIOXIDE, HYDROGEN SULFIDE, AND WATER SAM S.-S. HUANG Department (Canada) (Received
*, A.-D. LEU **, H.-J. NG ***
of Chemical November
Engineering,
University
and DONALD
of Alberta,
B. ROBINSQN
Edmonton,
Alberta,
T6G
2G6
7, 1983; accepted in final form July 23, 1984)
ABSTRACT Huang, 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 carried out to determine the composition of the equilibrium phases over a range of experimental conditions within the two- or three-phase envelope for each mixture, and to determine the two- and three-phase boundaries for each mixture within the temperature and pressure range of the study. A variable volume equilibrium cell consisting of a transparent sapphire cylinder was used for the experimental measurements and observations.
INTRODUCTION
Natural gas handling and processing operations are frequently carried out under conditions where an aqueous liquid phase appears. This phase may be in equilibrium with either a hydrocarbon gas phase or a hydrocarbon-rich liquid phase or both. Interest in the composition and behavior of these systems in the presence of relatively large concentrations of carbon dioxide and/or hydrogen sulfide has been prompted in part by the trend toward processing sour mixtures from reservoirs that had hitherto been considered economically 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, 037%3812/85/$03.30
Q 1985 Elsevier Science
Publishers
B.V.
Canada.
22
Kennedy, 1964) and hydrogen sulfide-water (Selleck et al., 1952) systems, reliable information on the behavior of ternary and quaternary mixtures of these compounds is essentially non-existent. Such data are considered to be useful as they provide a severe test of the capability of fluid property predictive 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 to 204 OC at pressures in the 0.4-18.5 MPa range. The primary objectives of the work were: (1) to determine the composition of the equilibrium phases over a range of experimental conditions within the two- or three-phase envelope for each mixture. (2) To determine the two- and three-phase boundaries for each mixture within the temperature and pressure range of the study. The composition of the two mixtures chosen for the study had the following nominal compositions on a mol% basis: Component
Mixture 1
Mixture 2
CHd
1s 30 5 so
S S 40 50
co2
H,S Hz0
The composition liquid-liquid-vapor EXPERIMENTAL
of Mixture 2 assured the existence equilibrium within the study conditions.
of
3
phase
METHOD
Figure 1 schematically shows the experimental measurements and observations were made in a variable volume equilibrium cell consisting of a transparent sapphire cylinder mounted between steel headers. The top header contained the necessary openings and valves for charging the components to the cell, for measuring the cell pressure, and for removing samples for analysis. The bottom header accommodated a movable stainless steel piston 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 the necessary auxiliary lead lines and valves. were mounted inside a controlled temperature air bath. Equilibrium was obtained by mechanically rocking the entire assembly. After charging the cell with the required amount of each component as required to achieve the composition of Mixture 1 or 2, the temperature was set to the required value, and the pressure was controlled by movement of
23
the piston. When equilibrium had been achieved by rocking the cell and bath assembly, the final pressure and temperature were read, and the phases were analyzed by sequentially removing them isobarically and isothermally to the chromatographic system. During the sampling procedure, the total amount of each phase was removed slowly from the cell and expanded into the heated line leading to the chromatographic sampling valve. Thus the lines became thoroughly purged with the gaseous sample and the surfaces had ample time to reach equilibrium with the materials in the sample. This was important particularly for the water. Samples could be taken for analysis at any time during the phase-removal procedure. For phase boundary determinations, the lower dew points along the two-phase boundary for Mixtures 1 and 2 were determined by visual observation. The system was controlled at the desired temperature with the contents of the cell in the single phase. The pressure of the system was then increased or decreased alternately by the movement of the piston in the cell. The formation or disappearance of an opaque cloud or a droplet on the sapphire cell wall due to a slight change in pressure was observed repeatedly. The dew point was considered to be the condition where the appearance and disappearance of the cloud or droplet occurred within c 3.5 kPa.
Fig. 1. Schematic illustration of equilibrium cell and auxiliary equipment.
24
The dew points and bubble points along the three-phase boundary were determined by graphical analysis. After the system reached the desired pressure 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 to determine the intersection of the two straight lines. The point of intersection was 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 the high hydrogen sulfide content of Mixture 2, it was not possible to prepare mixtures having these overall compositions, store them, and charge them to the equilibrium system without entering the two-phase region. Consequently, two synthetic gas mixtures A and B having the compositions given below Ah (cm) 0 4.A
I t
4.2
I I
2 I
3 I
4 I
5 I
6 I
7 I
6 I
P
20
Ah = h, - ho ho : Reference h,
: Final
height of piston
height of piston
I8
16
6 0 Dew Point observation *
Bubble
Point observation
Ah (cm) Fig. 2. Isothermal phase region.
plot of pressure versus volume for determining
the phase boundary
in the 3
25
were prepared for use in making up the final test Mixtures
1 and 2:
Component
Composition Mixture A
Mol % Mixture B
CH4
29.91 + 0.02 60.03 * 0.02 9.95 f 0.02 0.107
50.07 + 0.02 49.93 + 0.02 0 Trace
CO, H,S N,, C,, C,
To obtain Mixture 1, a known amount of Mixture A was added to the equilibrium cell. After this, the predetermined amount of liquid water required to match the required overall mixture composition was injected into the cell using a small high pressure stainless steel plunger pump calibrated and readable to the nearest 0.005 cm3. The same procedures were followed to prepare Mixture 2 except that the cell was first charged with a known amount of Mixture B, followed by the injection of pure water and pure liquid hydrogen sulfide from separate plunger pumps to make up the required composition for the mixture. EXPERIMENTAL
MEASUREMENTS
The pressure of the equilibrium cell contents was measured using a Statham pressure transducer having a range of O-34.5 MPa. The transducer was calibrated against a Ruska dead weight gauge at room temperature. The pressure of the system was known to within 0.25% of the reading. To protect the transducer diaphragm from corrosion by the wet hydrogen sulfide, a 2.54 cm diameter, 0.005 cm thick gold plated stainless steel plate was placed between the cell contents and the transducer. The temperatures of the cell and the air bath were measured using iron-constantan thermocouples which had been calibrated against a certified platinum resistance thermometer. The temperature of the cell contents was known to within + 0.05 OC. The output from both the transducer and the thermocouple was read on a Hewlett-Packard Model 3490A digital voltmeter. The volume of the equilibrium cell and the dead volume resulting from the pressure connections and the sampling valves and lead lines was determined by expanding nitrogen from a high pressure container at known volume, temperature and pressure into the total evacuated equilibrium cell. From the new pressure, temperature, and known properties of nitrogen, the volume of the cell and the dead volume could be determined. The dead volume represented < 5% of the total cell volume. The volume of the visible portion of the cell above the piston was determined from the known dimensions.
26
The composition of the gas mixtures and the equilibrium phases was determined using a Hewlett-Packard Model 5880A gas chromatograph with a thermal conductivity detector. The samples were eluted through a 3.2 mm OD by 1 m long Porapak QS column with temperature programming from 30 to 60 OC. Methane, carbon dioxide, and hydrogen sulfide were calibrated using a gravimetrically prepared sample of known composition containing 0.2991+ 0.0002 CH,, 0.6003 + 0.002 CO,, and 0.0995 f 0.0002 H,S. The calibrations with water were done using a mixture of carbon dioxide and water sampled at equilibrium with liquid water over a range of temperatures and pressures. The repeatability of the analyses was generally within kO.2 mol%. RESULTS
The experimental test conditions were chosen to cover a range in temperature from 37.8 to 176.7 “C. At each temperature, two to four pressure l-
20 , -
18
I
I
MIXTURE
I
I
COMPOSITION.
=t-b
14.05
f
0.02
l-q
CO,
30.07
f
0.04
Hz0
I
,
MOLE
I
I
I
%
4.97
f
0.01
50.00
f
0.08
16
0
I-
0
A Experimental
Two-phase
Dew Points
Experimental
Two-phase
Conditions
0
TEMPERATURE,
“C
Fig. 3. Experimental phase boundary and experimental pressure- temperature conditions for Mixture 1.
27
conditions were chosen so that the amount of liquid present at the equilibrium..conditions would be large. enough to make visual observation and chromatographic analysis of each phase possible. A total of nine experimental runs were made on Mixture 1, covering three TABLE
1
Equilibrium phase compositions 1 Pressure MPa 4.82
Temperature OC 37.8
Component
CH, co2 H2S H2O
7.60
37.8
CH, co2 H2S
12.52
37.8
H2O CH, co2 H2S H2O
16.93
37.8
CH, co2 H2S H2O
8.36
107.2
CH, co2 H2S
12.93
107.2
H2O CH, co2 H2S
17.17
107.2
H2O CH, co2 H2S
11.80
176.7
H2O CH., co2 H2S H2O
17.31
176.7
and relative liquid volumes in the 2 phase region for Mixture
CH, co2 H2S H2O
Composition,
Mole fraction
Feed
Liquid
Vapor
0.1494 0.3005 0.0497 0.5004 0.1488 0.2991 0.0494 0.5027 0.1497 0.3010 0.0498 0.4996 0.1495 0.3006 0.0497 0.5002 0.1496 0.3009 0.0498 0.4997 0.1496 0.3009 0.0498 0.4997 0.1496 0.3009 0.0498 0.4997 0.1495 0.3006 0.0497 0.5001 0.1500 0.3015 0.0499 0.4986
2.76~10-~ 9.30 x10-s 5.03 x1o-3 0.9854 4.66~10-~ 0.0121 5.4Ox1o-3 0.9816 7.96 x 1O-4 0.0151 5.95 x10-a 0.9781 9.9Ox1o-4 0.0154 6.08 x10-3 0.9777 3.79x1o-4 6.98~10-~ 3.42x10-’ 0.9894 5.78 x 1O-4 9.59 x lo-’ 4.47 x 10-3 0.9854 7.79x1o-4 0.0113 4.73 x 10-3 0.9834 7.17 x 10-4 7.95x10-3 3.74x 10-3 0.9877 1.10 x 10-3 0.0114 4.78 x lo- 3 0.9827
0.3040 0.5945 0.0998 1.91 x 10-3 0.3031 0.5970 0.0982 1.71 x 1o-3 0.3029 0.5967 0.0985 1.87~10-~ 0.3021 0.5963 0.0996 1.99x 10-s 0.2907 0.5919 0.0967 0.0225 0.2929 0.5920 0.0963 0.0196 0.2935 0.5916 0.0970 0.0179 0.2641 0.5575 0.0848 0.0950 0.2762 0.5520 0.0870 0.0848
Liquid Vol. %I
4.08
7.30
14.5
19.9
5.38
8.66
11.8
5.82
8.65
28
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. All of these experimental conditions were in the vapor-aqueous liquid equilibrium region. For Mixture 2, five experimental runs in the vapor-aqueous liquid equilibrium region were made. These covered two temperatures at 107.2 and 176.7 “C at pressures from 7.6-18.2 MPa. Table 2 presents the results and Fig. 4 shows graphically the conditions. Two experimental runs in the TABLE
2
Equilibrium phase compositions and relative liquid volumes in the 2 phase region for Mixture 2 Pressure MPa
Temperature OC
Aqueous liquid-vapor 7.56 107.2
12.27
107.2
16.92
107.2
11.00
176.7
18.17
176.7
Aqueous liquid-hydrogen 13.00 37.8
Component
Feed CH, CO, Hz8 Hz0
0.0494 0.0493 0.4072 0.4941 CH4 0.0498 CO, 0.0496 H2S 0.4019 H2O 0.4986 CH4 0.0499 co2 0.0498 HzS 0.4009 H2O 0.4994 CH4 0.0502 co2 0.0501 Hz8 0.4012 H20 0.4985 CH4 0.0496 co2 0.0494 H2S 0.4000 Hz0 0.5000 sulfide-rich dense fluid CH4 co2 H2S I.320
16.46
37.8
CH4 co2 H2S H2O
Liquid vol.%
Composition, Mol %
0.0500
0.0498 0.4006 0.4996 0.0500 0.0499 0.4000 0.5001
a Hydrogen sulfide-rich dense fluid phase.
Liquid
Vapor
1.55 x 1o-4 1.25 x 1o-3 0.0304 0.9682 3.32 x 1O-4 2.26 x 1O-3 0.0361 0.9613 6.06X 1o-4 3.34 x lo- 3 0.0392 0.9568 3.50x 1o-4 1.64x1O-3 0.0286 0.9694 7.15 x 1O-4 2.92 x 10 - 3 0.0517 0.9454
0.1182 0.1112 0.7485 0.0253 0.1060 0.1148 0.7528 0.0264 0.1207 0.1176 0.7322 0.0295 0.1092 0.1078 0.6896 0.0938 0.0928 0.0914 0.704 0.113
8.59 x 1O-4 3.62x1O-3 0.0291 0.9666 8.82 x 1O-4 3.81 X lo- 3 0.0281 0.9672
0.0891 a 0.0994 0.8016 9.32~10-~ 0.0891 a 0.1061 0.7958 9.o5x1o-3
6.43
13.0
20.3
6.43
12.3
27.1
28.0
29
aqueous liquid-hydrogen sulfide-rich dense fluid region were made at a temperature 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-hydrogen sulfide-rich liquid-vapor equilibrium region were made at 37.8 OC, 6.3 MPa and 65.6 OC, 8.4 MPa. Table 3 presents the results and Fig. 4 shows graphically the conditions. In addition to directly determining the equilibrium phase compositions and liquid volume fractions in the two- and three-phase regions for these mixtures, phase boundaries were determined by extensive studies. The studies included determining experimental dew points along the two-phase boundary for Mixture 1 and 2 from - 121.4 to 204.4O C at pressures from 0.42 to - 3.7 MPa. The results and presented in Table 4 and shown graphically 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 24
I
I
I
MIXTURE
COMPOSITION,
C”4
22
20
I
5.00
f
I
I
MOLE
%
I
I
1
0.04
CO,
4.98
*
0.05
kt,s
40.09
f
0.03
H,O
49.93
f
0.08 l
I8
. 0
.
6
A Dew
Point (L2v)
0 Dew
Point (L, L2V)
0 Bubble
Point (L, L2V,
0 Experimental
Conditions
P
I
0
50
100
TEMPERATURE,
Fig. 4. Experimental Mixture 2.
150
2oc
“C
phase boundaries and experimental pressure-temperature
conditions for
Temperature “C
37.8
65.6
Pressure MPa
6.26
8.43
H2O
CO, H*S
CH4
CH, CO, H,S H*O
Component
0.0504 0.0503 0.3986 0.5008 0.0501 0.0499 0.4016 0.4984
Feed 4.90x 1o-4 3.50x 1o-3 0.0284 0.9677 3.85x 1O-4 2.72~10-~ 0.0321 0.9684
L*
L, 0.0653 0.1049 0.8197 0.0101 0.0580 0.0904 0.8287 0.0212
H ,O liquid
H,S liquid
Composition, Mole fraction
Equilibrium phase compositions and relative volumes in the 3 phase region for Mixture 2
TABLE 3
0.3213 0.1739 0.5028 2.14~10~~ 0.1872 0.1484 0.6557 8.66x1o-3
Vapor V
19.6
31.3
L,
15.0
17.7
L*
65.4
51.0
V
31 TABLE
4
Phase boundary data along the lower dew point boundary for Mixture 1 Feed composition Mol 8
Temperature OC
pressure MPa
CH, CO,
0.1497 0.3010 0.0498 0.4995
198.8 190.6 176.7 169.8
3.25 2.70 1.94 1.67
H2S
0.1495 0.3005 0.0497
163.1 148.8 135.0
1.39 0.95 0.64
H2O
0.5003
121.1
0.42
H2S
Hz0 CL-L, CO*
TABLE
5
Phase boundary data along the lower dew point boundary for mixture 2 Feed composition Mol W
Temperature OC
Pressure MPa
CL% CO, Hz8 Hz0
0.0502 0.0501 0.3996 0.5001
120.8 148.6
0.42 0.97
CL-L, CO* H2S
0.0500 0.0499 0.3999
176.7 190.7 204.7
1.95 2.70 3.70
H2O
0.5002
TABLE
6
Three-phase dew point and bubble point data for Mixture 2 Mixture Composition Mole fraction
Dew -points Temperature “C
Pressure MPA
CH, co2 H,S H20
0.0504 0.0502 0.3980 0.5014
38.7 49.0 59.9
3.59 4.50 5.72
CH, co2 H*S Hz0
0.0498 0.0496 0.4040 0.4966
71.0 82.1
7.23 9.41
CH,
0.0501 0.0499 0.3998 0.5002
co2
H*S HP
Bubble points 37.7 48.8 59.9 71.1 76.0
7.81 8.65 9.47 10.21 10.52
32
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 at pressures from 3.6 to - 9.4 MPa. Bubble points were determined at temperatures from 38.9 to 76.7 “C and at pressures from 7.8 to - 10.5 MPa. The results are presented in Table 6 and shown graphically in Fig. 4. It will be noted from the tables that there are minor changes in overall composition for each experimental point. These result from the fact that a new charge of material was required after each analysis. The slight differences in overall composition represent the ability of the system and procedures to reproduce a desired composition. ACKNOWLEDGMENTS
The financial support received for this work from the Gas Processors Association and the National Research Council of Canada is gratefully acknowledged. REFERENCES Culberson, O.L. and McKetta, Jr., J.J., 1951. Phase equilibria in hydrocarbon-water systems. Trans. AIME, 192: 223-226, 297-300. systems. Olds, R-H., Sage, B.H. and Lacey, W.N., 1942. Phase equilibria in hydrocarbon Composition of the dew-point gas of the methane-water system. Ind. Eng. Chem., 34: 1223-1227. Selleck, F.T., Carmichael, L.T. and Sage, B-H., 1952. Phase behavior in the hydrogen sulfide-water system. Ind. Eng. Chem., 44: 2219-2226. Takenouchi, S. and Kennedy, G.C., 1964. The binary system H,O-CO, at high temperatures and pressures. Am. J. Sci., 262: 1055-1074. Tedheide, K. and Franck, E.U., 1963. Das Zweiphasengebiet und die Kritische Kurve im System Kohlendioxid-Wasser bis zu Drucken von 3500 bar. Z. Phys. Chem. (Frankfurt am Main) 37: 387-401. Wiebe, R. and Gaddy, V-L., 1941. Vapor phase composition of carbon dioxide-water mixtures at various temperatures and pressures to 700 atmospheres. J. Am. Chem. Sot., 63: 475-477.