Geochemical Journal, Vol. 35, pp. 59 to 66, 2001
Equilibrium conditions for gas hydrates of methane and ethane
mixtures in pure water and sodium chloride solution
TATSUO M AEKAWA *
Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305-8567, Japan
(Received December 22, 1999; Accepted December 12, 2000)
The equilibrium conditions were experimentally determined for gas hydrate of methane and ethane
mixtures in pure water and 3.0 wt% NaCl solution. The present results indicate that the addition of ethane
stabilizes gas hydrate and shifts the equilibrium conditions to higher temperature and lower pressure. As
ethane concentration increases from 0 vol% to 10 vol%, the equilibrium pressure of gas hydrate decreases
non-proportionally with gas composition, which implies a change in the hydrate structure over this range
of gas composition. The equilibrium temperature of gas hydrates of methane and ethane mixtures in 3.0
wt% NaCl solution is approximately 1.0 K lower than that in pure water at a constant pressure.
ally found. For examples, natural gas hydrates
collected from continental slope sediment in the
Gulf of Mexico contain methane with ethane concentrations up to 12 vol% (Brooks et al., 1986;
Sassen and MacDonald, 1997). The origin of these
gases is suggested to be thermogenic, based on
the large amount of higher hydrocarbons such as
ethane and propane. During such thermogenic
processes, hydrocarbons may form during the thermal dissociation of organic matter at high temperature and great depths (Schoell, 1983).
The equilibrium conditions for gas hydrates
depend on the hydrate-forming gas compositions.
Many experimental data on the conditions for
equilibrium formation of gas hydrates have been
reported (e.g., compiled by Sloan, 1998). Relevant
to this study, Deaton and Frost (1946) investigated
gas hydrates of methane and ethane mixtures and
determined their equilibrium conditions up to temperature of 280.4 K. Naturally occurring gas hydrates commonly contain ethane, but there are few
experimental data of equilibrium conditions for
gas hydrates of methane and ethane mixtures containing up to 10 vol% ethane at higher pressure
and temperature. Maekawa (1998) determined gas
I NTRODUCTION
Gas hydrates are solids composed of a cagelike crystalline lattice of water molecules that enclose gases such as methane, ethane and propane.
The known hydrate structures are Structure I,
Structure II, and Structure H (Sloan, 1998). They
can be formed and are stable at high pressure and
relatively low temperature in the presence of gases
such as methane and other hydrocarbons. In nature, such conditions occur in offshore sediments
in outer continental margins and beneath permafrost. It has been suggested that natural gas hydrates contain a large amount of methane gas in
their lattice and the amount of carbon in natural
gas hydrates may exceed that of organic carbon
in fossil fuel deposits (Kvenvolden, 1988). Naturally occurring gas hydrates are expected to be a
potential resource in the near future.
The hydrocarbons trapped in gas hydrates can
be
formed
either
biogenically
and
thermogenically. Most natural gas hydrates that
have been collected contain methane gas as the
main component. Gas hydrates that include a large
amount of ethane and propane are also occasion*e-mail: [email protected]
59
60
T. Maekawa
hydrate equilibrium conditions of a methane and
ethane mixture containing 8.7 vol% ethane over a
range of temperature and pressure.
This study was conducted to determine experimentally the equilibrium conditions for gas hydrates of methane and ethane mixtures containing
up to 10 vol% ethane at temperatures higher than
280 K and pressures up to 11.0 MPa. These experiments were performed using pure water plus
3.0 wt% NaCl solution, a salinity similar to
seawater.
EXPERIMENTAL APPARATUS
The experimental apparatus and the procedure
employed in this study were essentially the same
as those used by Maekawa and Imai (1996, 2000)
and Maekawa (1998). The experimental apparatus consists of a high-pressure hydrate cell, a magnetic stirrer, a heater and refrigerator, and an optical detection unit for gas hydrate formation and
dissociation (Fig. 1). The hydrate cell where gas
hydrate is synthesized is made of stainless steel.
The hydrate cell has a volume of about 500 ml.
Glycol-water coolant is continuously circulated
around the cell, with temperature controlled by a
heater and refrigerator. The temperature of the liquid in the cell is measured with a platinum resistance thermometer, and the gas pressure is mea-
sured with a semi-conductor transducer. A magnetic stirrer agitated the liquid in the cell at a stirring rate of about 300 rpm. At this stirring rate,
we confirmed by using two thermometers that
there is no difference in temperature between the
upper and lower water column in the cell. The
hydrate cell has glass windows on the upper and
lower sides for visual observation and optical detection of gas hydrate formation and dissociation.
The unit used to detect the formation and dissociation of gas hydrate consists of a light source
with a xenon lamp, an optical sensor and an optical power meter. The light beam from the lamp is
introduced into the liquid through the window
from the bottom of the hydrate cell. The optical
sensor on the upper side measures the intensity of
the light penetrating the liquid. When gas hydrate
forms on the surface of the liquid, the light is scattered by the gas hydrate, and the light intensity
detected by the optical sensor sharply decreases.
Similarly, when the gas hydrate dissociates, the
light intensity also sharply increases for the same
reason. Therefore, the formation and dissociation
of gas hydrate can be detected by measuring the
change of light intensity. The estimated accuracy
of temperature and pressure measurements in these
experiments is ±0.1 K and ±0.1 MPa, respectively,
which is caused mainly by manual reading of the
records.
Fig. 1. Schematic diagram of the experimental apparatus, consisting of a high-pressure hydrate cell, a magnetic
stirrer, a heater and refrigerator, and an optical unit to detect gas hydrate formation and dissociation.
Equilibrium conditions for gas hydrates of methane and ethane mixtures
In this experiment, measurable pressure
changes were not detected during hydrate formation and dissociation. This probably indicates that
the hydrate forms only a thin film on the surface
of the liquid, resulting in unmeasurable changes
of gas pressure in the vapor phase. Using this optical system, small amounts of gas hydrate formation are easy to detect. Compared to the conventional isochoric technique which requires the
formation of sufficient hydrate to cause a measurable pressure change, only small amounts of gas
hydrate forms a film on the surface of the solution in our experiments. This gas hydrate formation causes an imperceptible change of gas pressure in the gas phase, but our optical system allows this change to be determined sensitively.
61
sure decrease resulting from this cooling. When
the gas hydrate formed, the light intensity detected
by the optical sensor sharply decreased. The formation of gas hydrate was also confirmed by
visual observation using an optical fiber scope.
The temperature and pressure conditions when gas
hydrate forms are usually different from the equilibrium conditions because the liquid is under a
supercooled state. After gas hydrate formation, the
temperature was raised by 0.25 K per hour. During the heating, the light intensity changed again
and recovered to the original light intensity after
all the gas hydrate dissociated. The equilibrium
condition for gas hydrate was determined by measuring the pressure and temperature of gas hydrate
dissociation, as indicated by the change in transmitted light intensity.
EXPERIMENTAL PROCEDURES
The gases used in this study were all researchgrade purity supplied by Takachiho Chemical Industrial Co. Ltd. Pure methane and gas mixtures
with constant compositions of 98.9 vol% CH 4 +
1.1 vol% C 2H6, 97.9 vol% CH 4 + 2.1 vol% C2H 6,
95.2 vol% CH 4 + 4.8 vol% C 2H6 and 90.2 vol%
CH4 + 9.8 vol% C2 H6 were used. Deionized and
distilled water was used as pure water. To obtain
a 3.0 wt% NaCl solution, appropriate quantities
of sodium chloride were weighed and added to a
known quantity of distilled water. The mixture was
stirred at room temperature to dissolve the sodium
chloride.
In this experiment, gas hydrates form from
pressured gas and pure water or 3.0 wt% NaCl
solution in the hydrate cell. At first, 80 ml of water or NaCl solution was added to the hydrate cell.
The methane and ethane mixture was supplied to
the cell up to the starting pressure. The liquid was
stirred over night to equilibrate the liquid and gas
mixture.
The formation and dissociation of hydrate was
regulated by changing the temperature of the liquid after a constant volume of gas was sealed in
the hydrate cell. The pressure was monitored constantly. The temperature in the hydrate cell was
lowered to form gas hydrate, with a slight pres-
R ESULTS AND DISCUSSION
The equilibrium conditions for gas hydrate of
methane and ethane mixtures in pure water and
3.0 wt% NaCl solution are shown in Figs. 2 and
3, respectively. These data are compared with
those previously published. The equilibrium conditions for methane hydrate in pure water, as compiled by Sloan (1998), are shown in Fig. 2, and
those for 3.0 wt% NaCl solution determined by
Dholabhai et al. (1991) are included in Fig. 3. The
equilibrium conditions for methane hydrate in pure
water and in 3.0 wt% NaCl solutions determined
in this study are in good agreement with the results from other experimental studies. These results indicate that the dissociation pressures of gas
hydrate decrease and the dissociation temperatures
increase as ethane concentrations increase in the
gas mixture.
The equilibrium pressures for gas hydrates of
methane and ethane mixtures in pure water at
given temperatures are shown in Fig. 4. This figure also shows the experimental data determined
by Deaton and Frost (1946) and Maekawa (1998).
Our results are in good agreement with those determined by previous studies. The equilibrium
pressures of gas hydrate decrease at each temperature as ethane concentration increases. The addi-
62
T. Maekawa
Fig. 2. Equilibrium conditions for gas hydrates of methane and ethane mixtures in pure water. Solid curves are
thermodynamically predicted by Sloan (1998) for the range of compositions studied.
Fig. 3. Equilibrium conditions for gas hydrates of methane and ethane in 3.0 wt% sodium chloride solution. Solid
curves are those thermodynamically predicted by Sloan (1998).
tion of ethane to methane stabilizes gas hydrate
in these mixtures.
Equilibrium conditions for gas hydrates of
methane and ethane mixtures have been predicted
thermodynamically by several researchers (e.g.,
Sloan, 1998; Bakker, 1998). There are pressure
differences between the experimental results and
thermodynamically predicted results (Fig. 4). The
difference of pressure is no less than 0.5 MPa at
the composition of 95 vol% methane.
Equilibrium conditions for gas hydrates of methane and ethane mixtures
63
Table 1. Equilibrium conditions for gas hydrates of methane and ethane mixture in pure water. Some data of
methane hydrate in pure water was previously reported by Maekawa (1998)
Temperature (K)
100% CH4
274.2
274.3
274.7
275.1
275.9
276.2
276.7
276.9
277.4
277.9
278.4
278.7
279.0
279.4
279.5
279.7
280.4
280.7
280.9
281.9
282.7
283.1
283.9
284.6
285.2
285.9
286.5
287.2
287.7
288.2
Pressure (MPa)
2.9
3.0
3.1
3.2
3.4
3.6
3.8
3.8
4.0
4.2
4.4
4.5
4.7
4.8
4.9
5.1
5.3
5.7
5.8
6.4
6.8
7.1
7.7
8.3
8.9
9.6
10.2
11.2
11.8
12.6
Temperature (K)
Pressure (MPa)
98.9% CH4
275.4
276.8
278.5
280.2
281.3
281.9
283.1
283.8
284.7
285.4
3.0
3.5
4.2
5.1
5.7
6.1
6.9
7.4
8.1
8.9
97.9% CH4
277.0
278.4
279.1
280.3
281.4
282.4
283.0
283.7
284.4
285.2
285.5
285.5
285.9
3.2
3.7
4.0
4.6
5.2
5.8
6.2
6.7
7.3
8.1
8.4
8.9
9.0
The differences between measured and calculated equilibrium pressures may be caused by the
formation of a different structure of gas hydrate
than the Structure I assumed for the calculations
(Sloan, 1998). In addition, the equilibrium pressures at each temperature decrease non-proportionally with gas composition. There appears to
be a systematic transition point of a series of measured equilibrium pressures versus gas composition. The trend of experimental data (Fig. 4) suggests a transition from Structure I to a different
structure over a methane compositional range of
about 99 vol% to 98 vol%.
In conventional thermodynamic predictions,
methane and ethane mixtures are assumed to form
Structure I hydrate because pure methane or pure
Temperature (K)
Pressure (MPa)
95.2% CH4
277.4
279.3
280.1
281.9
282.7
283.6
284.8
286.0
286.7
287.4
287.9
288.5
289.3
289.4
2.7
3.2
3.5
4.3
4.7
5.3
6.2
7.2
7.8
8.4
9.1
9.8
10.9
11.1
90.2% CH4
275.6
275.6
277.9
277.9
280.0
281.8
282.3
284.0
285.5
286.7
287.5
287.6
288.4
289.3
289.6
1.7
1.7
2.3
2.3
2.8
3.5
3.6
4.5
5.3
6.1
6.8
6.9
7.8
8.7
9.1
ethane each form Structure I hydrate. However, a
recent spectroscopic study on gas hydrates found
that methane and ethane mixtures may form Structure II hydrate (Subramanian et al., 2000). Based
on Raman and nuclear magnetic resonance (NMR)
spectroscopic investigation, they identified a
change of methane and ethane hydrates from
Structure I to Structure II between 72.2 mol% and
75.0 mol% methane at 274.2 K, and found Structure II hydrate can form from a mixture of more
than 75 mol% methane. Their findings agrees with
the implication from the present experimental results that a gas hydrate different from Structure I
may form when the methane composition is less
than 99 vol%.
64
T. Maekawa
Table 2. Equilibrium conditions for gas hydrates of methane and ethane
mixture in 3.0 wt% NaCl solution
Temperature (K)
Pressure (MPa)
100% CH4
276.4
277.6
279.1
279.8
280.4
281.7
283.1
4.0
4.5
5.3
5.7
6.1
6.9
7.9
97.9% CH4
277.4
278.8
280.1
280.9
281.7
282.6
283.1
283.7
3.8
4.3
5.0
5.5
6.0
6.7
7.0
7.6
Temperature (K)
Pressure (MPa)
95.2% CH4
280.2
281.1
282.5
283.7
284.7
285.9
286.7
287.3
4.0
4.5
5.3
6.1
6.9
8.0
8.9
9.5
90.2% CH4
281.4
283.1
284.4
285.9
287.1
288.1
3.7
4.5
5.2
6.3
7.4
8.4
Fig. 4. Equilibrium pressures for gas hydrates of methane and ethane mixtures in pure water at temperatures of
277.6 K, 280.4 K, 283.2 K and 286.0 K. Dashed curves are spline fits of experimental data. Solid curves are
thermodynamically predicted by Sloan (1998) over the range of temperature.
Figure 5 shows the equilibrium temperatures
for gas hydrates of methane and ethane mixtures
in pure water and 3.0 wt% NaCl solution at pressures of 4 MPa and 7 MPa. The equilibrium tem-
peratures of gas hydrate increase at each pressure
as ethane concentration increases. It is known that
the stability of gas hydrate in saline solution decreases as the concentration of salt in solution in-
Equilibrium conditions for gas hydrates of methane and ethane mixtures
65
Fig. 5. Equilibrium temperatures for gas hydrates of methane and ethane mixtures in pure water and 3.0 wt%
NaCl solution at pressures of 4 MPa and 7 MPa. Solid and dotted curves are thermodynamically predicted by
Sloan (1998) for pure water and for 3.0 wt% NaCl solution, respectively.
creases (Kobayashi et al., 1951; Dholabhai et al.,
1991; Maekawa et al., 1995). The equilibrium temperature differences of gas hydrates in pure water
and 3.0 wt% NaCl solution are approximately 1.0
K over the compositional range investigated in this
study. The results also indicate the possibility of
a hydrate structural transition over the investigated
range of gas composition in saline solution close
to seawater salinity.
CONCLUSIONS
Equilibrium conditions for gas hydrate dissociation for methane and ethane mixtures dissolved
in pure water and 3.0 wt% NaCl solution were
experimentally obtained in the temperature range
of 275–290 K and the pressure range of 1.7–11.0
MPa. The addition of ethane stabilizes gas hydrates and shifts the equilibrium conditions to
higher temperature and lower pressure. The equilibrium temperature differences of gas hydrates
in pure water and 3.0 wt% NaCl solution are approximately 1.0 K at constant pressure over the
compositional range studied. Differences between
theoretical and experimental results suggest that
methane and ethane mixtures may form a hydrate
distinctly different from Structure I hydrate, as
commonly assumed. Equilibrium curves of pressure versus gas composition indicate that the
change of hydrate structure from Structure I may
occur over the methane compositional range of 99
vol% to 98 vol%.
Acknowledgments—I thank Drs. K. A. Kvenvolden
and S. Circone, U.S.G.S., and Dr. J. W. Hedenquist,
for their comments which improved the paper, and Dr.
N. Imai, Geological Survey of Japan, for helpful comments on an earlier version.
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