Experimental Investigation of Methane Separation from Low

Article pubs.acs.org/IECR

Experimental Investigation of Methane Separation from LowConcentration Coal Mine Gas (CH4/N2/O2) by Tetra‑n‑butyl Ammonium Bromide Semiclathrate Hydrate Crystallization Dong-liang Zhong,* Yang Ye, Chen Yang, Yu Bian, and Kun Ding Key Laboratory of Low-grade Energy Utilization Technologies, Systems of Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400044, China ABSTRACT: In this work, tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate was employed in a batch operation to separate CH4 from low-concentration coal mine methane (CMM) gas with a mole composition of 30% CH4, 60% N2, and 10% O2. TBAB semiclathrate hydrate formed from the low-concentration CMM gas has more favorable incipient equilibrium conditions than gas hydrates formed from the same gas mixture. The experiments were carried out at a fixed pressure of 4.0 MPa, three TBAB mole concentrations of 0.29, 0.62, and 1.38 %, and in the temperature range of 274.75−283.35 K. The effects of TBAB concentrations and subcoolings (ΔTsub) on CH4 separation from the low-concentration CMM gas were investigated. The results indicated that CH4 was preferentially incorporated into the hydrate crystals rather than N2 and O2 in the presence of TBAB. The experimental conditions of the TBAB concentration of 1.38%, 4.0 MPa, and ΔTsub = 7.0 K were most favorable for TBAB semiclathrate hydrate to separate CH4 from the low-concentration CMM gas. The CH4 recovery obtained at these conditions was approximately 27%. Compared with CH4 content in the low-concentration CMM gas, the mole fraction of CH4 in TBAB semiclathrate hydrate was increased to 41%.

1. INTRODUCTION Coal mine methane (CMM) is a term given to the methane gas (CH4) produced or emitted in association with underground coal mining activities. Safety, reduction of greenhouse gas emissions, and energy utilization are three primary incentives for the recovery of CMM from underground coal mines.1−3 Normally, the mole concentration of CH4 in CMM gas mixtures is in the range of 30−50 %, that of O2 is around 10%, and the balance is N2.4 CMM is usually mixed with air and poses an explosion risk if the mole concentration of methane reaches the range of 5−15 %,5 thus a large quantity of ventilation air is provided to reduce the methane content below the lower explosive limit of 5%. However, CMM and the ventilation air evacuated from coal mines should not be emitted to atmosphere, because methane is a greenhouse gas with a high global warming potential 21 times greater than CO2.6 Low-concentration CMM gas is referred to the CMM gas mixture in which the CH4 concentration is less than 30%. Power generation is one approach to use the low-concentration CMM gas. However, variation of methane concentration and supply continuity of the low-concentration CMM gas mixture will affect the continuous and stable operation of the power generation units and increase the complexity of power station operation.7,8 Thus, the operation cost and the efficiency in such power plants are challenges that need to be overcome. An alternative way of utilizing the low-concentration CMM gas is to convert it into a methane-rich gas. Pressure swing adsorption (PSA), cryogenic liquefaction, and membrane separation are examples of technologies that have been developed for the separation of CH4 from CMM gas mixtures.4,9,10 One potential method to separate CH 4 from lowconcentration CMM gas is by gas hydrate crystallization,11 but the equilibrium pressure for gas hydrate formed from such a © 2012 American Chemical Society

gas mixture is much higher than pure methane at a given temperature. For example, the equilibrium hydrate formation pressure at 275.15 K for the low-concentration CMM gas with a mole composition of 30% CH4, 60% N2, and 10% O2 is 7.78 MPa, but for pure methane is only 3.11 MPa, predicted using the Chen−Guo model.12 Thus, the operating cost will be high if the CMM gas mixture has to be compressed to the hydrate formation pressure. The use of thermodynamic promoters can shift the equilibrium hydrate formation conditions to lower pressures. Zhang et al.13 used tetrahydrofuran (THF) as a thermodynamic promoter for the separation of CH4 from a low-concentration CMM gas by hydrate formation. They found the incipient equilibrium hydrate formation conditions were greatly reduced with the addition of THF in the water and confirmed the feasibility of recovering CH4 from lowconcentration CMM gas by hydrate formation. Another hydrate promoter is the quaternary ammonium salt of tetra-n-butyl ammonium bromide (TBAB). TBAB can form a semiclathrate hydrate with water molecules at 0.1 MPa and 285.15 K at a mass fraction of 0.4 in aqueous solution. Also, gas molecules such as CH4, N2, and CO2 can be trapped in the small cavities (512) of TBAB semiclathrate hydrate in the pressure range of 1.4−4.1 MPa and in the temperature range of 285.15−289.55 K at the TBAB mole concentration of 0.62%.14−19 The incipient equilibrium conditions for TBAB semiclathrate hydrate formed with the low-concentration CMM gas mixture have been reported by our group in a recent paper.20 Received: Revised: Accepted: Published: 14806

March 12, 2012 October 15, 2012 October 17, 2012 October 17, 2012 dx.doi.org/10.1021/ie300641g | Ind. Eng. Chem. Res. 2012, 51, 14806−14813

Industrial & Engineering Chemistry Research

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2.3. Calculation of the Amount of Gas Mixture Consumed. On the basis of the compositions of the gas mixture at the beginning and at the end of the experiments, the number of moles of the gas mixture incorporated into the TBAB semiclathrate hydrate crystals was calculated by eq 1.

In addition to phase equilibrium conditions, the kinetics of hydrate formation are also important for the development of a hydrate-based process for the separation of CH4 from lowconcentration CMM gas. However, to our knowledge, no kinetic data on the separation of CH4 from low-concentration CMM gas by gas hydrate formation have been published. The purpose of this work is to investigate the effects of subcoolings and TBAB concentrations on the separation of CH4 from the low-concentration CMM gas mixture with a mole composition of 30% CH4, 60% N2, and 10% O2 at a moderate pressure. The kinetic data reported in this work may prove useful in the development of hydrate-based technology for CH4 separation from low-concentration CMM gas.

ΔnH = ng,0 − ng, t =

⎛ PV ⎞ ⎛ PV ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ ZRT ⎠0 ⎝ ZRT ⎠t

(1)

where ng is the number of moles of gas mixture 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 calculated by the Pitzer correlation for the second Virial coefficient (eq 2).21

2. EXPERIMENTAL SECTION 2.1. Materials. The gas mixture used was supplied by Chongqing Rising Gas, China, with a mole composition of 30% CH4, 60% N2, and 10% O2, and with a reported uncertainty in the composition of ±0.05%. The composition of the gas mixture was chosen to simulate the typical low-concentration CMM gas that was recovered from underground coal mines. TBAB was purchased from Chongqing Oriental Chemical Co., Ltd. with a certified mass purity of 99.9%. Deionized water was used in all experimental runs. 2.2. Apparatus and Procedure for Hydrate Formation. A detailed description of the apparatus was given elsewhere in the literature.20 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 gas chromatography (SC-2000, Chongqing Chuanyi Analyzer) with the uncertainty of 0.1% was used to measure the gas composition. The experimental procedure for TBAB semiclathrate hydrate formation was as follows. The volume of the reactor was 600 cm3. Prior to the experiments, the reactor was cleaned with deionized water and dried. It was then filled with 260 cm3 of TBAB solution. The reactor and the tubing were purged with the low-concentration CMM gas mixture at least three times, so that the air remaining in the reactor was flushed out. Once the temperature and the pressure of the contents reached desired values, the reactor was isolated from the gas cylinder by closing the inlet and outlet valves. Then the electromagnetic stirrer was started at a constant speed of 150 rpm. This was considered to be time zero for hydrate formation. Hydrate formation was confirmed by visual observations through the viewing windows. The experimental conditions were maintained for at least 12 h. The composition of the gas mixture remaining in the reactor was measured using the gas chromatograph (GC) at the end of the hydrate formation experiments. Subsequently, the reactor was quickly vented to atmospheric pressure and isolated again by closing the vent valve. Then the reactor was gradually heated to 288.15 K until the hydrate was completely dissociated. Finally, the composition of the gas mixture released from TBAB semiclathrate hydrate was measured with the GC when the temperature and the pressure in the reactor were stabilized. Because the amount of the 1 atm of gas remaining in the cell after venting is about 2% of the total moles of the gas mixture added into the reactor. The maximum uncertainty due to the presence of the 1 atm of gas remaining in the cell was 0.01; therefore, the 1 atm of gas remaining in the cell was ignored during the process of hydrate decomposition.

Z = 1 + B0

Pr P + ωB1 r Tr Tr

(2)

where the equations of Abbott were used for B0 and B1. 2.4. CH4 Recovery and Efficiency. Split fractions and separation factors were two metrics used by Linga et al.22,23 to assess the separation efficiency of a gas mixture by gas hydrate crystallization. In this work, the CH4 recovery or split fraction (R) of CH4 was calculated as follows: R=

H nCH 4 feed nCH 4

× 100% (3)

where nfeed CH4 is the moles of CH4 supplied into the crystallizer and nHCH4 is the moles of CH4 captured in hydrate at the end of the experiments. There are three components (CH4, N2, and O2) included in the low-concentration CMM gas mixture and the TBAB semiclathrate hydrate may not distinguish between N2 and O2, so the separation factor (S) in this work was determined by the following equation: S=

H nCH (n Ngas2 + nOgas2 ) 4 gas nCH (n NH2 + nOH2) 4

ngas CH4,

ngas N2 ,

(4)

ngas O2

where and are the moles of CH4, N2, and O2 in the gas phase at the end of the experiments, respectively. The symbols nHN2 and nHO2 are the moles of N2 and O2 incorporated into the hydrate crystals at the end of the experiments, respectively. As defined in eqs 3 and 4, the split fraction (R) reflects the efficiency of CH4 recovered from the low-concentration CMM gas by hydrate formation, and the separation factor (S) represents the extent of separating CH4 from the lowconcentration CMM gas. A higher separation factor (S) indicates the ability of the hydrate to separate CH4 from the CMM gas mixture is stronger.

3. RESULTS AND DISCUSSION Incipient equilibrium conditions for TBAB semiclathrate hydrate formed with the low-concentration CMM gas mixture at three TBAB mole concentrations of 0.29, 0.62, and 1.38 % were reported in our previous work.20 It was found that the incipient equilibrium hydrate formation conditions obtained in the presence of TBAB were significantly reduced as compared to that obtained in pure water. In this work, two more incipient equilibrium points for TBAB semiclathrate hydrate formed with the same gas mixture were measured using the same procedure. The results are shown in Figure 1. The incipient equilibrium 14807

dx.doi.org/10.1021/ie300641g | Ind. Eng. Chem. Res. 2012, 51, 14806−14813


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data were correlated, and the equilibrium temperatures at a pressure of 4.0 MPa were determined. The incipient equilibrium temperatures (Teq) at 4.0 MPa were 283.75, 286.05, and 288.35 K for 0.29, 0.62, and 1.38 % TBAB solutions, respectively. Table 1 summarizes the experimental conditions for CH4 separation from the low-concentration CMM gas mixture by TBAB semiclathrate hydrate crystallization. The experiments were carried out at a fixed pressure of 4.0 MPa, three TBAB mole concentrations of 0.29, 0.62, and 1.38 %, and at temperatures between 274.75 and 283.35 K. Three subcoolings (ΔTsub) of 5.0, 7.0, and 9.0 K were used at each TBAB concentration. Subcooling (ΔTsub) was defined as the difference between the phase equilibrium temperature (Teq) and the experiment temperature (Texp) at a given pressure (ΔT sub = Teq − Texp). It should be noted that fresh TBAB solutions as well as “memory”24−26 TBAB solutions were used. The memory TBAB solution refers to the solution that has experienced TBAB semiclathrate hydrate formation and was used 4 h after the dissociation of TBAB semiclathrate hydrate. The measured induction times (tind) for fresh and memory TBAB solutions are also shown in Table 1. It is generally

Figure 1. Incipient phase equilibrium conditions for TBAB semiclathrate hydrate formed from the low-concentration CMM gas with different TBAB mole concentrations.

Table 1. Experimental Conditions for TBAB Semiclathrate Hydrate Formation from the Gas Mixture with a Mole Composition of 30% CH4, 60% N2, and 10% O2a

a

exp no.

TBAB (%)

Pexp (MPa)

Teq (K)

Texp (K)

ΔTsubb (K)

solution state

tind (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

0.29

4.0

283.75

278.75

5.0

276.75

7.0

274.75

9.0

281.05

5.0

279.05

7.0

277.05

9.0

283.35

5.0

281.35

7.0

279.35

9.0

fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory fresh fresh fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory fresh memory

74.0 15.3 253.7 95.2 55.3 8.7 202.0 17.3 48.3 13.7 180.7 64.5 no hydratec no hydratec 43.5 23.7 37.7 21.6 61.1 5.5 22.8 8.67 163.7 32.3 109.0 63.0 136.0 8.0 135.5 39.5 44.6 11.8 185.5 41.7

0.62

1.38

4.0

4.0

286.05

288.35

Standard uncertainties u are u(T) = 0.1 K and u(P) = 10 kPa. bΔTsub = Teq − Texp. cDid not nucleate for 24 h. 14808



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Table 2. Experimental Results of TBAB Semiclathrate Hydrate Formation from the Gas Mixture with a Mole Composition of 30% CH4, 60% N2, and 10% O2a

a

exp no.

TBAB (%)

ΔTsub (K)

xgas CH4(%)

xHCH4(%)

R (%)

u(R) (%)

ΔnH (mol)

u(ΔnH) (mol)

S

u(S)

uncertainty (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

0.29

5.0

27.3 27.1 27.1 27.1 26.8 26.5 26.2 26.5 27.1 27.1 27.5 28.0

41.1 39.1 43.2 42.2 40.2 42.3 44.5 43.1 39.1 41.5 41.0 40.3

19.2 19.9 21.9 21.4 23.4 23.5 26.6 25.0 19.9 20.0 18.3 17.1

1.3 1.3 1.5 1.4 1.6 1.6 1.8 1.7 1.3 1.3 1.2 1.1

0.0736 0.0744 0.0803 0.0762 0.0954 0.0952 0.0955 0.0941 0.0750 0.0738 0.0676 0.0704

0.0049 0.0049 0.0053 0.0051 0.0064 0.0064 0.0064 0.0063 0.0050 0.0049 0.0045 0.0046

4.94 5.14 4.70 4.69 4.54 4.55 5.01 4.90 5.22 5.24 4.73 4.26

0.33 0.34 0.31 0.31 0.30 0.31 0.34 0.33 0.35 0.35 0.31 0.28

3.1 4.3 4.6 4.1 3.5 2.7 3.7 4.1 4.4 3.6 3.9 4.0

26.9 26.8 26.7 26.4 26.8 27.2 27.5 27.3 27.1 26.9 27.1 27.4 26.4 26.5 26.6 26.5 26.8 27.2 27.4 27.3

43.7 40.5 41.2 41.4 39.3 41.0 39.3 40.5 38.8 39.2 42.6 41.9 40.7 40.1 39.9 40.6 38.2 38.7 38.3 39.1

22.7 22.9 24.8 23.1 20.6 19.6 20.3 21.1 20.8 23.8 21.8 20.7 27.3 26.7 25.4 25.4 19.8 20.6 20.9 20.5

1.5 1.5 1.7 1.6 1.4 1.3 1.3 1.4 1.4 1.6 1.4 1.4 1.8 1.8 1.7 1.7 1.3 1.4 1.4 1.4

0.0834 0.0820 0.0921 0.0852 0.0694 0.0691 0.0814 0.0805 0.0711 0.0713 0.0784 0.0797 0.1116 0.1085 0.0941 0.0929 0.0725 0.1020 0.0783 0.0744

0.0056 0.0055 0.0062 0.0057 0.0046 0.0046 0.0054 0.0053 0.0047 0.0047 0.0052 0.0053 0.0075 0.0073 0.0063 0.0062 0.0048 0.0068 0.0052 0.0049

5.12 5.10 4.67 4.91 5.51 5.12 4.35 4.48 5.17 5.47 4.71 4.45 4.65 4.70 4.65 4.73 5.59 4.29 4.39 4.56

0.34 0.34 0.31 0.33 0.37 0.34 0.29 0.30 0.34 0.36 0.31 0.29 0.31 0.32 0.31 0.32 0.37 0.28 0.29 0.30

2.3 3.7 4.1 3.6 4.8 3.5 3.9 3.6 4.4 4.8 4.5 4.2 3.4 3.3 3.7 4.1 7.0 4.0 4.7 3.3

7.0

9.0

0.62

5.0 7.0

9.0

1.38

5.0

7.0

9.0

Standard uncertainties u are u(T) = 0.1 K, u(P) = 10 kPa, and u(x) = 0.1%. bCalculated by (|ng,0 − (nH + ng,t)/ng,0|) × 100%.

a weak correlation between the induction time and the subcooling. Table 2 shows the experimental results of TBAB semiclathrate hydrate formation with the low-concentration CMM gas mixture. The CH4 concentration in the gas phase (xgas CH4) was measured at the end of hydrate formation, and the CH4 concentration in the hydrate phase (xHCH4) was measured at the end of hydrate dissociation. The final gas consumption (ΔnH), CH4 recovery (R), and separation factor (S) were calculated using eqs 1−4. The combined standard uncertainties in ΔnH, R, and S, i.e., u(ΔnH), u(R), and u(S) were calculated from the uncertainties in P, T, V, and Z and were presented in Table 2, respectively. Table 2 shows that for the experiments carried out at a given TBAB concentration and subcooling, the final gas consumption in the fresh solution experiment was almost the same as that in the memory solution experiment (with a discrepancy of about 2%). For example, the average final gas consumptions obtained at the TBAB concentration of 0.29% and ΔT sub = 5.0 K were 0.0770 and 0.0753 mol for the fresh and memory solution, respectively. This indicates the final gas consumption was not affected by the solution state when

accepted that the induction times for gas hydrate formed without promoters are shorter in memory water.27,28 This was also found in the present work. For example, the average induction time for the experiments carried out in the fresh solutions with a TBAB concentration of 0.29% was 135.7 min, while the average induction time for the memory solutions was 35.8 min. This effect was also observed in the 0.62% TBAB experiments (15−22) and 1.38% TBAB experiments (23−34). It should be noted that semiclathrate hydrate did not nucleate in the 0.62% TBAB solutions at the subcooling ΔTsub = 5.0 K (experiments 13−14 in Table 1), and therefore, no induction times were obtained. As seen in Table 1, the induction time obtained at a given TBAB concentration did not exhibit a clear trend with the increased subcooling. For the 0.29% TBAB solutions (fresh), the induction times decreased from 163.9 to 114.5 min as the subcooling increased from 5.0 to 9.0 K. Similarly, the induction times obtained with the 1.38% TBAB solutions (fresh) decreased from 136.4 to 115.1 min as the subcooling increased from 5.0 to 9.0 K. However, the induction times for the 0.62% TBAB solutions (fresh) slightly increased from 40.6 to 42.0 min when the subcooling increased from 7.0 to 9.0 K. This indicates 14809



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Figure 2. CH4 recovery from the low-concentration CMM gas by TBAB semiclathrate hydrate formed with different TBAB mole concentrations.

Figure 3. Temperature profile and gas uptake curve for the experiment carried out at 276.15 K, 4.0 MPa, and with the 0.29% TBAB solution (mole fraction).

might block the mass transfer of gas molecules to the liquid. Table 2 also shows that the final gas consumptions obtained at ΔTsub = 7.0 K and at three TBAB concentrations of 0.29, 0.62, and 1.38 % were 0.0951, 0.0857, and 0.1018 mol, respectively. This indicates that the 1.38% TBAB solutions are better than the 0.29 and 0.62 % TBAB solutions for the capture of lowconcentration CMM gas mixture. As defined in eq 4, the separation factor (S) reflects the extent of separating CH4 from the low-concentration CMM gas. A higher separation factor indicates the ability of the hydrate to separate CH4 from the CMM gas mixture is stronger. The average values of CH4 separation factor (S) were 4.83, 4.91, and 4.78 for three TBAB concentrations of 0.29, 0.62, and 1.38 %, respectively (seen in Table 2). Compared to the value reported by Sun et al.29 who formed semiclathrate hydrate with 0.29% TBAB solutions to separate CH4 from a gas mixture with a composition of 46.25% CH4 and 53.75% N2, it

experiments were carried out at the same temperature and pressure conditions. However, the final gas consumption at a given TBAB concentration is dependent on the subcooling. For example, the average values of the final gas consumption obtained with the 0.29% TBAB solutions were 0.0761, 0.0951, and 0.0717 mol corresponding to ΔT sub = 5.0, 7.0, and 9.0 K, respectively. Likewise, the average values of the final gas consumption in the 0.62% TBAB solutions were 0.0857 and 0.0751 mol corresponding to ΔTsub = 7.0 and 9.0 K, respectively. The same trend can also be seen in the 1.38% TBAB solutions. This result indicates that ΔT sub = 7.0 K was an optimum subcooling for TBAB semiclathrate hydrate to capture the low-concentration CMM gas as compared to ΔT sub = 5.0 and 9.0 K. This is probably due to the fact that when the sucooling was increased from 7.0 to 9.0 K more TBAB semiclathrate hydrate formed and accumulated at the gas/liquid interface, which 14810



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observed in the 0.62 and 1.38 % TBAB solutions. This indicates that TBAB semiclathrate hydrate is better at separating CH4 from low-concentration CMM gas at a subcooling of 7.0 K than subcoolings of 5.0 and 9.0 K. Figure 2 also shows the effect of TBAB concentrations on CH4 recovery from the lowconcentration CMM gas. It can be seen that the TBAB concentration of 1.38% has the highest CH4 recovery (R) among the three TBAB concentrations of 0.29, 0.62, and 1.38 %, and the CH4 recovery (R) obtained at the TBAB concentration of 0.29% is higher than that obtained with the 0.62% TBAB solutions. Figure 3 shows the temperature profile and gas uptake curve obtained at 276.15 K, 4.0 MPa, and at the TBAB mole concentration of 0.29% (experiment 5 in Table 1). The induction time was identified by a sudden temperature rise in the water at 55.3 min. The gas uptake is the number of moles of the gas mixture captured by the semiclathrate hydrate formed in the presence of TBAB, which was obtained using eq 1. As seen in the figure, the gas uptake rate rapidly increases to about

was found the CH4 separation factors are very close although O2 was added to the CH4/N2 gas mixture. This indicates that TBAB semiclahtrate hydrate can separate CH4 effectively from the low-concentration CMM gas but may not distinguish N2 and O2 during the process of gas capture. This is probably due to the fact that the equilibrium conditions of gas hydrate formed with N2 or O2 are close.30 In addition, a mole balance was conducted to compare the moles of CH4, N2, and O2 added to the reactor (ng,0) with the gas remaining in the cell (ng,t) and in TBAB semiclathrate hydrate (nH). As seen in Table 2, the uncertainties are less than 5%, thus the uncertainty in the moles of CH4, N2, and O2 is not a major contributor to the variation of separation factors (S). Figure 2 illustrates how the CH4 recovery (R) varies with subcooling. For the experiments performed with 0.29% TBAB solutions, the CH4 recoveries obtained at subcoolings of 5.0, 7.0, and 9.0 K were 20.6, 25.0, and 19.1 % with fresh solutions, while the CH4 recoveries obtained with memory solutions were 20.7, 24.3, and 18.6 %, respectively. The same trend was also 14811



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hydrate formation stages and used as natural gas (∼90% CH4) if the hydrate-based process is greatly improved. Therefore, further studies are needed to increase the CH4 recovery and improve the hydrate-based process for CH4 separation in the presence of TBAB.

0.076 mol at 2 h and then decreases, plateauing at 0.0932 mol at about 5 h. Figure 4 shows the temperature profile and gas uptake curve obtained at 279.05 K, 4.0 MPa, and at the TBAB mole concentration of 0.62% (experiment 15 in Table 1). The induction time is 43.5 min, and the gas uptake shows the same trend as that observed in Figure 3. Figure 5 shows the temperature profile and gas uptake curve obtained at 281.35 K, 4.0 MPa, and at the TBAB mole concentration of 1.38% (experiment 28 in Table 1). The induction time is 8.0 min. Because this experiment was carried out with a memory solution, the induction time was much shorter than that obtained with the fresh solution (experiment 27). Similarly to the other experiments, the gas uptake increases quickly to 0.087 mol at about 2 h but then decreases plateauing at about 5 h. Although the gas uptake curves in Figures 3−5 were obtained at three different TBAB concentrations of 0.29, 0.62, and 1.38 %, the shape of the gas uptake curves was almost the same. This indicates that the process of low-concentration CMM gas capture by TBAB semiclathrate hydrate experienced three stages of the hydrate rapid growth stage (∼2 h), plateauing stage (∼5 h), and the final stage (∼12 h). It should be noted that the rate of hydrate growth at a fixed subcooling is dependent on TBAB concentration. For example, as seen in Figures 3−5, under the subcooling ΔTsub = 7.0 K, the rates of hydrate growth (assessed over the 30 min after the nucleation point) obtained with the TBAB solutions of 0.29, 0.62, and 1.38 % were 0.0295, 0.0299, and 0.0656 mol/h, respectively. Figure 6 shows the CH4 concentration of the gas mixture released from the TBAB semiclathrate hydrate. As seen in the

4. CONCLUSIONS In this work, we have used TBAB semiclathrate hydrate to separate CH4 from low-concentration CMM gas in a batch operation. The feasibility of recovering CH4 from the lowconcentration CMM gas mixture by hydrate formation in the presence of TBAB was confirmed. The effects of TBAB concentrations and system subcoolings on CH4 separation were studied. However, further studies are needed in order to enhance the efficiency of CH4 recovery by TBAB semiclathrate hydrate and improve the hydrate-based technology for CH4 separation from low-concentration CMM gas.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-23-65102473. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (No. 51006129) is greatly appreciated.

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REFERENCES

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Figure 6. CH4 concentration of the gas mixture released from TBAB semiclathrate hydrate.

figure, compared with the CH 4 content in the lowconcentration CMM gas mixture, CH4 concentration of the gas mixture decomposed from TBAB semiclathrate hydrate is increased to about 41%, indicating CH4 was preferentially incorporated into the TBAB semiclathrate hydrate crystals rather than N2 and O2. Therefore, it is a practical approach to form TBAB semiclathrate hydrate for CH4 separation from the low-concentration CMM gas. Tentative experiments showed that CH4 concentration of the gas mixture released from TBAB semiclathrate hydrate could be increased to about 93% after three stages of hydrate formation at 1.38% TBAB concentration, 4.0 MPa, and 281.35 K. It is believed that the lowconcentration CMM gas can be concentrated with three 14812



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Experimental Investigation of Methane Separation from Low

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