Article pubs.acs.org/jced
Phase Equilibria of Clathrate Hydrates Formed with CH4 + N2 + O2 in the Presence of Cyclopentane or Cyclohexane Dong-liang Zhong,*,† Kun Ding,† Chen Yang,† Yu Bian,† and Jun Ji‡ †
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education, and College of Power Engineering, Chongqing University, Chongqing, 400044, China ‡ Merchant Marine College, Shanghai Maritime University, Shanghai, 201306, China ABSTRACT: In this work, the phase equilibrium conditions of gas hydrate formed using the gas mixture of mole fractions 0.3 CH4 + 0.6 N2 + 0.1 O2 in the presence of cyclopentane or cyclohexane were measured and reported in the temperature range of (277.4 to 293.1) K and in the pressure range of (0.89 to 4.87) MPa. The mass fraction of cyclopentane or cyclohexane added into the aqueous solution was w = 0.07. It is found that the equilibrium conditions of gas hydrate formed at the 0.07 mass fraction of cyclopentane or cyclohexane are shifted to high temperatures and low pressures as compared to those obtained using the same gas mixture in pure water. Moreover, the mole fraction of CH4 incorporated into the hydrate phase is increased to approximately 0.5, which is higher than that in the original gas mixture. promoters.8,9 Since CH4, N2, and O2 are main components of the CMM gas mixture, the equilibrium data of gas hydrate formed using the gas mixture of (CH4 + N2 + O2) are necessary and important for kinetic studies of CH4 separation from lowconcentration CMM gas via hydrate formation. Although the phase equilibrium data of pure methane and nitrogen hydrates were measured in the presence of cyclopentane or cyclohexane,10−13 the phase equilibrium data of gas hydrate formed using the gas mixture of (CH4 + N2 + O2) in the presence of cyclopentane or cyclohexane have not yet been reported in the literature. In this work, the phase equilibrium conditions of gas hydrate formed using the gas mixture of mole fractions 0.3 CH4 + 0.6 N2 + 0.1 O2 in the presence of cyclopentane/cyclohexane are measured and reported. Moreover, the phase equilibrium data reported in this work are compared with those obtained in pure water and in TBAB solutions to show the thermodynamic promotion effects of cyclopentane and cyclohexane on hydrate formation using the low-concentration CMM gas.
1. INTRODUCTION Coal mine methane (CMM) gas is a mixture of coal mine methane (CH4) and air, which emits continuously from coal mines.1 The mole fraction of CH4 in this mixture is approximately in the range of 0.3 to 0.5; that of O2 is around 0.1, and the balance is N2. The recovery of methane from such mixtures is a desire to promote safety, reduce emission of greenhouse gases, and provide an additional source of energy.2,3 Gas hydrate crystallization is a potentially low cost method for methane separation from the CMM gas mixture (CH4 + N2 + O2) when a small amount of a suitable additive like tetrahydrofuran (THF) is added to reduce the incipient equilibrium hydrate formation pressure (thermodynamic promoter), because the lower formation pressure would reduce the gas compression cost of the hydrate-based separation method.4 One benefit of using gas hydrate for CH4 separation is that the only material needed to capture CH4 is water and no other hazards or expensive materials are involved. In addition, each volume of hydrate can contain as much as 160 volumes of gas under standard temperature and pressure conditions.5 Tetra-n-butyl ammonium bromide (TBAB) was used as a substitute for THF. The equilibrium conditions of TBAB semiclathrate hydrate formed with the low-concentration CMM gas mixture (0.3 CH4 + 0.6 N2 + 0.1 O2 in mole fraction) have been reported in a recent paper.6 Zhong and Englezos7 presented a conceptual process that employs TBAB with a simulated low-concentration CMM gas mixture of mole fractions 0.3 CH4 + 0.7 N2 and produced a methane-rich stream (0.7 CH4 + 0.3 N2 in mole fraction) using two stages of hydrate formation. However, the CH4 recovery of this process was relatively low, and thus an additive is needed that will not only reduce the hydrate formation pressure but will also capture CH4 from the CMM gas mixture more efficiently. Cyclopentane and cyclohexane are suitable candidates to be investigated because they are effective thermodynamic © 2012 American Chemical Society
2. EXPERIMENTAL SECTION Materials. Table 1 reports the specifications and supplier names of the materials used in this work. The gas mixture with mole fractions of 0.3 CH4 + 0.6 N2 + 0.1 O2 was supplied by Chongqing Rising Gas, China, with a reported composition uncertainty of ± 0.0005. This gas composition was chosen to simulate a typical low-concentration CMM gas mixture recovered from underground coal mines. Cyclopentane and cyclohexane were purchased from Chongqing Oriental Chemical Co., Ltd. with a certified mass purity of 0.95 (gas− liquid chromatography method). The uncertainty in the mass fraction of cyclopentane or cyclohexane is 0.01. The solubility Received: September 18, 2012 Accepted: November 9, 2012 Published: November 15, 2012 3751
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Table 1. Purities and Suppliers of Materialsa material low-concentration CMM gasb cyclopentane cyclohexane
supplier Chongqing Rising Gas Chongqing Oriental Chemical Chongqing Oriental Chemical
purity 0.9995 (mole fraction) 0.95 (mass fraction) 0.95(mass fraction)
were completely decomposed, the gas mixture emitted was sampled and analyzed by gas chromatography (GC).
analysis method
3. RESULTS AND DISCUSSION The experimental data are reported in Tables 2 and 3 and are plotted in Figures 1 and 2. The mass fraction of cycloypentane/
GC GC GC
Table 2. Phase Equilibrium Data for the Hydrates Formed Using the Gas Mixture of Mole Fractions 0.3 CH4 + 0.6 N2 + 0.1 O2 at the Cyclopentane Mass Fraction w = 0.07a
a Deionized water was used in all experiments. bMole fractions of this gas mixture were 0.3 CH4, 0.6 N2, and 0.1 O2.
of cyclopentane and cyclohexane in water is 0.01 % at 293.15 K and 0.006 % at 298.15 K, respectively. Deionized water was used in all experimental runs. Apparatus and Procedure. A detailed description of the apparatus was given elsewhere in the literature.6 The volume of the reactor was 600 cm3. The reactor was immersed into a temperature-controlled cryostat to maintain the temperature of the contents at a desired value. Two platinum resistance thermometers with the uncertainty of ± 0.1 K were used to measure the gas and liquid temperatures, respectively. A pressure transducer (Banna Electronics Inc., USA) with the uncertainty of ± 0.01 MPa was used to measure the pressure in the vessel. A data acquisition unit (Agilent 34970A) was used to collect temperature and pressure data during the experiment process. A gas chromatograph (SC-2000, Chongqing Chuanyi Analyzer) with the uncertainty of ± 0.001 was used to measure the gas composition. The incipient phase equilibrium hydrate formation conditions were determined using the isochoric step-heating method,6,14 which is similar to the T-cycle method.15−17 Prior to the experiments, the reactor was cleaned with deionized water and dried. It was then filled with 140 cm3 pure water and 14 cm3 cyclopentane or cyclohexane. The reactor and the tubing were purged to atmospheric pressure with the lowconcentration CMM gas mixture at least three times; thus the air remaining in the reactor was flushed out. Once the temperature and the pressure reached desired values, the reactor was separated from the gas cylinder by closing the inlet and outlet valves. Subsequently, the electromagnetic stirrer was started at a constant speed of 150 rpm. Hydrate formation was visually observed through the viewing windows. Then the vessel was heated slowly with a step of 0.1 K to decompose the hydrates until the solution became transparent and only an infinitesimal amount of hydrate crystals were seen in the solution. At least 4 h were maintained during each heating step to obtain an equilibrium state at which the gas and liquid temperatures and the pressure were stabilized. When the infinitesimal amount of hydrate crystals did not change the size, the heating process was continued with a step of 0.1 K. When the very small amount of hydrate crystals disappeared after the stepwise heating, this point (the temperature of the solution and the pressure in the vessel) was considered as the incipient equilibrium dissociation point. To confirm the incorporation of CH4 into the hydrate phase, we analyzed the gas composition of the hydrates at the end of the experiments. When hydrate formation was observed in the reactor, the experimental conditions were maintained for about 10 h until the temperature and the pressure did not change any more. The gas mixture remaining in the reactor was quickly evacuated at the end of hydrate formation, and then the reactor was heated to decompose the hydrates. When the hydrates
a
T/K
P/MPa
288.0 288.5 289.2 290.3 291.3 291.9 293.1
0.89 0.99 1.19 1.58 1.91 2.19 2.60
Standard uncertainties u are u(T) = 0.1 K and u(P) = 10 kPa.
Table 3. Phase Equilibrium Data for the Hydrates Formed Using the Gas Mixture of Mole Fractions 0.3 CH4 + 0.6 N2 + 0.1 O2 at the Cyclohexane Mass Fraction w = 0.07a
a
T/K
P/MPa
277.4 278.6 280.0 280.4 281.5 282.4 283.3
1.71 2.19 2.72 2.89 3.51 4.10 4.87
Standard uncertainties u are u(T) = 0.1 K and u(P) = 10 kPa.
cyclohexane added into the aqueous solution was w = 0.07. The equilibrium data of nitrogen hydrate and methane hydrate obtained in the presence of cyclopentane/cyclohexane were employed to compare with the experimental data reported in
Figure 1. Phase equilibrium conditions for gas hydrate formed with different hydrate-forming gases in the presence of cyclopentane. △, nitrogen, Tohidi et al.;10 ■, nitrogen, Mohammadi and Richon;11 ▲, low-concentration CMM gas, this work; ⧫, methane, Tohidi et al.;10 ○, methane, Sun et al.12 3752
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Figure 2. Phase equilibrium conditions for gas hydrate formed with different hydrate-forming gases in the presence of cyclohexane. ●, nitrogen, Tohidi et al.;13 △, nitrogen, Mohammadi et al.;11 ■, lowconcentration CMM gas, this work; ▲, methane, Tohidi et al.;13 ○, methane, Sun et al.12
Figure 3. Phase equilibrium conditions for gas hydrate formed with the low-concentration CMM gas mixture (0.3 CH4 + 0.6 N2 + 0.1 O2 in mole fractions) in the presence of different thermodynamic promoters. △, pure water, predicted using Chen-Guo model;19 ●, cyclohexane, w = 0.07, this work; ▲, TBAB, w = 0.07; ■, cyclopentane, w = 0.07, this work.
fi = x if i0 (1 −
this work. Because the low-concentration CMM gas hydrate was formed at a low concentration of cyclopentane/cyclohexane (w = 0.07), most of the cyclopentane/cyclohexane may be in the vapor phase instead of being present in the liquid hydrocarbon phase during the process of hydrate formation. Zhang and Lee18 found that structure II hydrates were formed in the presence of cyclopentane and confirmed that small molecules such CO2 only occupy the small cavities while cyclopentane molecules are trapped in the large cavities. Similarly, for the CH4/N2/O2 gas mixture used in this work, small-sized CH4 molecules are expected to occupy the small cavities of the sII hydrate, and cyclopentane/cyclopentane molecules are incorporated into the large cavities. As seen in Figure 1, the equilibrium pressure at which the hydrates formed with the low-concentration CMM gas is increased with the increase of the temperature. In addition, the equilibrium conditions of the low-concentration CMM gas hydrate are found between pure nitrogen hydrate and methane hydrate in the presence of cycloypentane. This indicates that gas hydrate formed with the low-concentration CMM gas is more stable than nitrogen hydrate, and less stable than methane hydrate in cycloypentane solutions. Table 3 presents the equilibrium conditions of gas hydrate formed with the low-concentration CMM gas at the cyclohexane mass fraction w = 0.07. The experimental data are also plotted in Figure 2. Similarly, as seen in Figure 2, the equilibrium pressure at which the hydrates formed with the low-concentration CMM gas is increased with the increase in temperature. The phase equilibrium conditions of the lowconcentration CMM gas hydrate are observed between those of pure nitrogen hydrate and methane hydrate in the presence of cyclohexane. Figure 3 compares the equilibrium conditions of the lowconcentration CMM gas hydrate formed in the presence of different thermodynamic promoters. The Chen−Guo model19 and Patel−Teja equation of state were used to determine the equilibrium conditions for low-concentration CMM gas hydrate formed in pure water. The equations used in the Chen−Guo model are given as follows:
∑ θj)α (1)
j
∑ θj = j
∑j fj C j 1 + ∑j fj C j
(2)
∑ xj = 1.0 (3)
j
⎛ −∑ A ijθj ⎞ ⎡ ⎛ ⎞⎤ j ⎟ ·⎢A′exp⎜ B′ ⎟⎥ f i0 (T ) = exp⎜⎜ ⎟ ⎝ T T − C ′ ⎠⎦ ⎝ ⎠⎣
(4)
f01(T)
The Antoine constants A′, B′, and C′ for calculating in eq 4 are presented in Table 4. As seen in Figure 3, the equilibrium pressure obtained in pure water was in the range of (5.55 to 9.7) MPa corresponding to the temperature range of (277.4 to 283.3) K, while the equilibrium pressure obtained at the 0.07 mass fraction of cyclohexane decreased to the range of (1.71 to 4.87) MPa under the same temperature conditions. The equilibrium P−T data obtained at the 0.07 mass fraction of cyclopentane were in the range of (0.89 to 2.6) MPa and (288 to 293.1) K. Therefore, the presence of 0.07 mass fraction of cyclopentane or cyclohexane in aqueous solutions has significantly reduced the equilibrium pressure at a given temperature. So cyclopentane or cyclohexane can be used as an effective thermodynamic promoter for methane separation from the low-concentration CMM gas mixture via hydrate formation. As also seen in Figure 3, the equilibrium pressure at fixed temperature in the presence of the cyclopentane mass fraction w = 0.07 is lower than those obtained at the cyclohexane mass fraction w = 0.07 and at the TBAB mass fraction w = 0.07. For example, the equilibrium pressure at 289.2 K is 1.19 MPa and 8.79 MPa for the cyclopentane and TBAB solution, respectively. Thus, the thermodynamic effect of cyclopentane is better than cyclohexane and TBAB. From the perspective of industrial application, the operating conditions of low pressures and high temperatures are preferred, and therefore less energy 3753
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Table 4. Antoine Constants for Calculating f i0(T) in eq 4 structure I A′/MPa
gas CH4 N2 O2
structure II
B′/K
C′/K
−6591.43 −5286.59 −5353.95
10
1584.4·10 97.939·1010 62.498·1010
experimental conditions for hydrate formation promoters
w
T/K
P/MPa
x
cyclopentane
0.07
286.7
1.99 3.48 5.08 3.50 5.01
0.505 0.533 0.543 0.508 0.516
278.6
5.2602·10 6.81654·1023 4.3195·1023
B′/K
C′/K
−12955 −12770 −12505
4.08 −1.10 −0.35
data were determined with the isochoric step-heating method and were in the range of (0.89 to 4.87) MPa and (277.4 to 293.1) K. The equilibrium conditions of gas hydrate formed in the presence of cyclopentane or cyclohexane are shifted to high temperatures and low pressures as compared to those obtained using the same gas mixture in pure water. The mole fraction of CH4 incorporated into the hydrate phase is increased to approximately 0.5. Therefore, cyclopentane and clyclohexane can be used as effective thermodynamic promoters for methane separation from the low-concentration CMM gas mixture by hydrate formation.
Table 5. Mole Fraction of Methane (x) in the Hydrates Formed Using the Gas Mixture of Mole Fractions 0.3 CH4 + 0.6 N2 + 0.1 O2 at the Cyclopentane/Cyclohexane Mass Fraction w = 0.07a
0.07
23
27.04 31.65 25.93
will be consumed in methane recovery from the lowconcentration CMM gas mixture. Table 5 shows the mole fraction of CH4 (x) incorporated into the hydrates at the cyclopentane or cyclohexane mass
cyclohexane
A′/MPa
■
AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +86-23-65102473. E-mail:
[email protected] (D.L. Zhong). Funding
The financial support from the National Natural Science Foundation of China (No. 51006129) is greatly appreciated.
a
Standard uncertainties u are u(T) = 0.1 K, u(P) = 10 kPa, and u(x) = 0.001.
Notes
The authors declare no competing financial interest.
fraction w = 0.07. The mole fraction of CH4 was determined by measuring the gas composition of the hydrates. Combined with the CH4 composition (xt) of the gas mixture remaining in the vessel at the end of the experiment, the quantity of CH4 (ΔnH) trapped in the hydrates can be calculated as follows: ⎛ P x ·V ⎞ ⎛ Px ·V ⎞ ΔnH = ng,0 − ng,t = ⎜ 0 0 0 ⎟ − ⎜ t t t ⎟ ⎝ Z0RT0 ⎠0 ⎝ ZRTt ⎠t
■
(5)
where ng is the number of moles of CH4 in the crystallizer at time 0 and time t, P is the pressure in the crystallizer, T is the temperature of gas phase, V is the volume of gas phase, and Z is the compressibility factor determined by the Pitzer correlation for the second virial coefficient (eq 2).20 Z = 1 + B0
Pr P + ωB1 r Tr Tr
(6) 0
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1
where the equations of Abbott were used for B and B . As seen in Table 5, the mole fraction of CH4 trapped in the hydrates exceeds 0.5 in the presence of both cyclopentane and cyclohexane, which is higher than that in the original lowconcentration CMM gas mixture (x = 0.3). This indicates that methane was effectively recovered from the low-concentration CMM gas mixture using hydrate formation when a small amount of cyclopentane or cyclohexane was added to water. Therefore, cyclopentane or cyclohexane can be considered as a preferable promoter for methane separation from lowconcentration CMM gas mixture using the hydrate-based method.
4. CONCLUSIONS The equilibrium conditions of the 0.3 CH4 + 0.6 N2 + 0.1 O2 gas hydrate were measured in the presence of the cyclopentane/cyclohexane mass fraction w = 0.07. The equilibrium 3754
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