Methane Separation from Coal Mine Methane Gas by Tetra-n-butyl

Article pubs.acs.org/EF

Methane Separation from Coal Mine Methane Gas by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Formation Dongliang Zhong*,† and Peter Englezos‡ †

Key Laboratory of Low-Grade Energy Utilization Technologies and Systems of Ministry of Education, and College of Power Engineering, Chongqing University, Chongqing 400044, China ‡ Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada ABSTRACT: This work presents new data to further develop the hydrate-based process for the separation of CH4 from the lowconcentration coal mine methane gas (30 mol % CH4/N2) through the formation of tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate. The TBAB semiclathrate hydrate formed from the 30 mol % CH4/N2 gas mixture has more favorable equilibrium conditions than the gas hydrate formed from the same gas mixture. The incipient equilibrium conditions at 0.17, 0.29, and 0.62 mol % TBAB were experimentally determined and reported. The experiments of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture were performed in both semibatch and batch operation under a fixed driving force of 3.5 MPa. The results indicate that CH4 is preferentially encaged into the TBAB hydrate. The use of a 0.29 mol % TBAB solution is preferred over the 0.17 and 0.62 mol % TBAB solutions. The semibatch operation is more effective than the batch operation for the separation of CH4 from the 30 mol % CH4/N2 gas mixture. The CH4 recovery was found to be approximately 25% at 0.29 mol % TBAB concentration in the semibatch operation and the corresponding CH4 concentration released from the TBAB hydrate was nearly 43 mol %. A CH4-rich stream (70 mol % CH4/N2) was obtained after two stages of TBAB semiclathrate hydrate formation.

1. INTRODUCTION Coal mine methane (CMM) refers to methane (CH4) gas that is a constituent of coal mine gas.1 For safety reasons, CMM should be diluted and removed from coal mines through the ventilation system. When the released CMM is mixed with air, the concentration of CH4 in the mixture is approximately in 30−50 mol %, that of O2 is around 10 mol %, and the balance is N2. However, emission of methane to the atmosphere is prohibited because it has a global warming potential (GWP) that is 21 times greater than that of CO2.2,3 The CMM gas is currently utilized as a low-energy-fuel gas by power plants located near coal mines. An alternative way of utilizing the CMM gas is to convert it into a methane-rich gas. Pressure swing adsorption (PSA), cryogenic liquefaction, and membrane separation are among the methods to separate CH4 from the CH4/N2/O2 mixture.4−6 However, the operating cost is significant, and hence, research is carried out to find a lower cost separation method. Gas hydrate crystallization is a potentially low cost gas separation process.7 Gas hydrates are ice-like crystalline compounds formed when small-sized gas molecules (e.g., N2, CH4, and CO2) are encaged in the cavities constructed by hydrogenbonded water molecules.8 The fact that the concentration of a gas component in the hydrate crystal is different than that in the original gas mixture forms the basis for using gas hydrate formation as a gas separation method. CO2 separation from flue gas (CO2/N2/O2), fuel gas (CO2/H2), and sulfur hexafluoride (SF6) from the (SF6/N2) mixture can be accomplished with hydrates.9−12 Since the equilibrium formation pressure of CH4 hydrate is much lower than that of N2 and O2 hydrates at the same temperature, it is likely that CH4 enters the hydrate phase preferentially and therefore can be recovered from the © 2012 American Chemical Society

CH4/N2/O2 gas mixture after decomposing the gas hydrate crystals. The feasibility of this idea for methane separation from the CMM gas has been confirmed by Zhang et al.13 in a lowconcentration CMM system. The CMM gas is defined as a lowconcentration CMM gas when its methane content is below 30 mol %. The incipient hydrate formation pressure at 274.15 K for a low-concentration CMM gas (30 mol % CH4/60 mol % N2/10 mol % O2) is 7.73 MPa.14 This means that a separation method based on hydrates will be expensive because of the compression required to bring the CMM gas to the necessary hydrate formation pressure. The quaternary ammonium salt of tetra-n-butyl ammonium bromide (TBAB) is a chemical that may be used to reduce the hydrae phase equilibrium pressure at a given temperature.15 TBAB forms semiclatharate hydrate with water at atmospheric pressure.16,17 In TBAB hydrate crystals, the hydrophilic anions (Br−) are hydrogen bonded with water molecules and build cavities that are occupied by the hydrophobic tetra-n-butyl ammonium cations (TBA).16,18 The empty dodecahedral cavities (512) can encage small gas molecules (e.g., N2, CH4, and CO2).15,19 Phase equilibrium data for TBAB semiclathrate hydrate formed with low-concentration CMM gas (29.95 mol % CH4/ 60.0 mol % N2/10.05 mol % O2) were reported.20 It was also reported that methane is preferentially encaged in the TBAB semiclathrate hydrate. Thus, it is of interest to exploit the TBAB hydrate as a means to concentrate the CMM gas toward higher methane content. Sun et al.21 formed TBAB semiclathrate hydrate Received: December 24, 2011 Revised: March 9, 2012 Published: March 9, 2012 2098

dx.doi.org/10.1021/ef202007x | Energy Fuels 2012, 26, 2098−2106

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Figure 1. Schematic of the apparatus to assess CMM gas separation. the liquid phase in the crystallizer were measured using the Omega copper-constantan thermocouples (Omega Engineering, Stamford, CT) with an uncertainty of 0.1 K. A valve (Fisher-Baumann) coupled with a PID (proportional−integral−derivative) controller was mounted between the reservoir and the crystallizer and used to regulate the gas flow from the reservoir to the crystallizer in the semibatch operation. The data acquisition system (National Instruments) was used to record experimental data and communicate with the control valve. TBAB Semiclathrate Hydrate Formation from the CH4/N2 Gas Mixture. For the semibatch operation, the crystallizer was filled with 20 cm3 TBAB solution and purged three times with the 30 mol % CH4/N2 gas mixture from the gas cylinder, so that the air remaining in the system could be removed. The crystallizer was pressurized to the desired value once the predetermined temperature of the gas and liquid phase in the crystallizer was achieved. Then, the magnetic stirrer was started at a constant speed (400 rpm) and the data acquisition system was initiated. This was set as time zero for the experiments. During hydrate formation, the crystallizer was connected to the reservoir and kept at a constant temperature and pressure. Thus, a continuous supply of gas was provided. The experiments were kept running for approximately 9 h. Then the gas phase in the crystallizer was sampled at the end of the experiment and the composition was measured with a Varian CP-3800 gas chromatograph. The apparatus is equipped with appropriate instrumentation to allow the calculation of moles of gas mixture consumed in the crystallizer (gas uptake) versus time.11 For the batch operation, the crystallizer was set to the desired temperature and pressure at time zero. Then, the crystallizer was isolated from the reservoir. Thus, the pressure in the crystallizer would drop due to the formation of TBAB semiclathrate hydrate. However, the temperature in the crystallizer was kept constant until the end of the experiment. The nucleation point or induction time was identified on the basis of a sudden temperature rise in the water phase or through the increased gas consumption. The moles of gas mixture encaged in the TBAB semiclathrate hydrate in a semibatch manner are calculated by the following equation:

from a simulated CMM gas (46.25 mol % CH4/N2) and found that CH4 concentration can increase from 46.3 to 67.9 mol % through a single-stage hydrate separation. They also reported the split fraction of CH4 in the hydrate phase and the separation factor in the CH4/N2/TBAB and CH4/N2/TBAB/ SDS systems. The split fraction and separation factor are metrics introduced by Linga et al.12,22 to assess the separation efficiency. Simulated gas is used because the hydrate formation conditions between O2 and N2 hydrate are close,23 and hence it is assumed that the mixture contains only N2. Sun et al. performed the experiments of TBAB semiclathrate hydrate formation for CH4 separation in a batch mode, which is not preferable for the industrial application, and they only used the 0.29 mol % TBAB solution in the experiments. Therefore, the purpose of this work is to assess the separation of CH4 from the 30 mol % CH4/N2 mixture through TBAB semiclathrate hydrate in both semibatch and batch mode. The incipient phase equilibrium conditions for the semiclathrate hydrate will also be determined at three TBAB concentrations. The fundamental information that will be obtained in this work may be used for the development of the hydrated-based CH4 recovery from the low-concentration CMM gas (30 mol % CH4/N2).

2. EXPERIMENTAL SECTION Materials. The CH4/N2 gas mixture used was ultra high purity (UHP) grade and was supplied by Praxair Technology Inc. The composition of the gas mixture was 30 mol % CH4 and 70 mol % N2. The 30 mol % CH4/N2 gas mixture is considered a suitable model gas representing the low-concentration CMM gas recovered from the coal mines. TBAB was supplied by Sigma-Aldrich (catalog number: 86860) with a certified mass purity >99.0%. Deionized water was used in all experimental runs. Apparatus. The apparatus to perform hydrate phase equilibrium measurements was described elsewhere.11 The isothermal pressure search method to determine the incipient phase equilibrium hydrate formation pressure at a given temperature was followed.24 Another apparatus was used to carry out the TBAB hydrate formation experiments to assess the separation of CH4 from the CH4/ N2 gas mixture.25 Figure 1 shows the schematic of the kinetic experimental apparatus. The apparatus consists of a crystallizer (CR) with a volume of 58 cm3, which is made of 316 stainless steel. A 150 cm3 reservoir (R) was used to supply the CH4/N2 gas mixture for TBAB semiclathrate hydrate formation in the crystallizer in a semibatch manner (constant pressure and temperature). Both the crystallizer and the reservoir were immersed in a temperature-controlled water bath. Two Rosemount smart pressure transmitters (Model 3051, Norppac Controls, Vancouver, BC) with an uncertainty of 0.075% of the span 0−15 MPa were employed for the measurement of pressures in the crystallizer and the reservoir. The temperatures of the gas phase and

ΔnH = ng,0 − ng,t =

⎛ PV ⎞ ⎛ PV ⎞ ⎛ PV ⎞ ⎛ PV ⎞ ⎜ ⎟ ⎟ ⎟ ⎟ −⎜ +⎜ −⎜ ⎝ zRT ⎠CR,0 ⎝ zRT ⎠CR,t ⎝ zRT ⎠R,0 ⎝ zRT ⎠R,t (1)

where ng is the number of moles of gas mixture in the crystallizer (CR) and reservoir (R) at time 0 and time t, z is the compressibility factor calculated by Pitzer’s correlation,26 and V is the volume of gas phase in the crystallizer and the volume of reservoir. 2099


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Table 1. Incipient Equilibrium Conditions for Gas Hydrates and TBAB Semiclathrate Hydrate Formed from the 30 mol % CH4/N2 Gas Mixture pure water

TBAB (0.17 mol %)

TBAB (0.29 mol %)

TBAB (0.62 mol %)

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

273.65 274.15

6.9 7.3

276.15 277.15 278.15 279.15 280.15

0.7 1.2 1.9 2.5 3.4

277.15 278.15 279.15 280.15 282.15

0.4 0.6 1.0 1.5 2.7

280.15 281.15 282.15 283.15

0.3 0.6 1.1 1.7

The moles of gas mixture consumed for the formation of TBAB semiclathrate hydrate in the batch manner are calculated by eq 2.

ΔnH = ng,0 − ng,t =

⎛ PV ⎞ ⎛ PV ⎞ ⎜ ⎟ ⎟ −⎜ ⎝ zRT ⎠CR,0 ⎝ zRT ⎠CR,t

(2)

In the semibatch operation, the experiments were carried out at an overpressure (driving force) of 3.5 MPa and three TBAB concentrations (0.17, 0.29, and 0.62 mol %). The corresponding temperatures are 276.15, 277.15, and 280.15 K. The driving force is the deviation of the experiment pressure from the equilibrium value. In the batch operation, the experiments were performed at 277.15 K and 0.29 mol % TBAB with the same driving force 3.5 MPa. It should be noted that fresh TBAB solutions as well as memory TBAB solutions were used. The memory TBAB solution refers to the solution that has experienced TBAB hydrate formation and was used 2 h after the decomposition of the TBAB hydrate. CH4 Recovery and Separation Factor. The CH4 recovery or split fraction (SFr) of methane from the CH4/N2 gas mixture is calculated as follows.12,22

SFr =

H nCH 4 feed nCH 4

(3)

feed H is defined as the moles of CH4 in feed gas and nCH is the where nCH 4 4 moles of CH4 captured in the TBAB semiclathrate hydrate at the end of experiments. In addition, the separation factor (SF) is determined by the following equation.

gas

SF =

H nCH × nN 4 2 gas

H nN × nCH 2

4

(4)

gas nCH is 4

where the moles of CH4 in the gas phase at the end of the kinetic experiment, nN2 gas is the moles of N2 in the gas phase at the end of the kinetic experiment, and nNH2 is the moles of N2 enclathrated in the TBAB semiclathrate hydrate.

3. RESULTS AND DISCUSSION Incipient Equilibrium Conditions for TBAB Semiclathrate Hydrate Formation from the CH4/N2 Mixture. Table 1 presents the incipient equilibrium conditions for gas hydrates and TBAB semiclathrate hydrate formed from the low-concentration CMM gas (30 mol % CH4/N2) in the temperature range 276.15−283.15 K. The formation of TBAB semiclathrate hydrate was carried out at three TBAB concentrations: 0.17, 0.29, and 0.62 mol %. Figure 2 shows the incipient equilibrium hydrate formation conditions for the gas mixture (30 mol % CH4/N2) obtained in pure water and in TBAB solutions. As seen in Figure 2a, the incipient equilibrium hydrate formation conditions obtained in pure water are in excellent agreement with the prediction by CSMHYD,27 suggesting the apparatus and the procedure adopted for the

Figure 2. Incipient equilibrium hydrate formation conditions for the 30 mol % CH4/N2 gas mixture obtained in pure water and in TBAB solutions (a) and only in the TBAB solutions (b).

measurement of the incipient equilibrium hydrate formation conditions can produce reliable phase equilibrium data. In addition, the equilibrium hydrate formation pressures for the 30 mol % CH4/N2 gas mixture measured in the TBAB solutions are significantly reduced, as compared to the pressures in pure water. Thus, TBAB is a potential promoter molecule to lower the required pressure to form hydrate from the low-concentration CMM gas (30 mol % CH4/N2). It should be noted that the 30 mol % CH4/N2 gas forms structure II gas hydrate in pure water,27 but it forms semiclathrate hydrate in TBAB solutions. 2100


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Table 2. Experimental Conditions along with Measured Induction Times for TBAB Semiclathrate Hydrate Formed from the 30 mol % CH4/N2 Gas Mixture in Semibatch Mode

a

expt. no.

sample state

TBAB (mol %)

Texp (K)

Pexp (MPa)

Peq (MPa)

ΔPa (MPa)

tind (min)

CH4 frac. in gas phase (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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

0.17

276.15

4.2

0.7

3.5

0.29

277.15

3.9

0.4

3.5

0.62

280.15

3.8

0.3

3.5

56.3 27.3 97.3 67.3 41.7 27.7 87.0 10.3 57.3 6.7 48.3 29.3 34.0 17.3 78.0 19.3 54.7 7.3

27.3 26.7 26.9 26.7 26.8 27.0 25.1 25.1 25.2 25.3 25.1 25.2 25.8 26.6 26.4 26.9 26.4 25.8

Driving force ΔP = Pexp − Peq.

TBAB Semiclathrate Hydrate Formation in a Semibatch Manner. Table 2 summarizes the experimental conditions, measured induction times, and CH4 fraction in the gas phase at the end of the experiments. As seen in Table 2 and as expected, the induction times obtained in memory solutions are shorter than those obtained in fresh solutions. As shown in Table 2, the induction time was shortened from 56.3 min (experiment 1) to 27.3 min (experiment 2) at 0.17 mol % TBAB concentration. In the reproduced experimental sets, the induction times obtained in memory solutions (experiments 4 and 6) were also shortened, as compared with those obtained in fresh solutions (experiments 3 and 5). This result can also be observed at 0.29 (experiments 7−12) and 0.62 mol % TBAB concentrations (experiments 13−18). It is also noted that the induction time is stochastic in character. For example, the induction times obtained in the three fresh 0.29 mol % TBAB solutions are 87.0 (experiment 7), 57.3 (experiment 9), and 48.3 min (experiment 11), respectively. Figure 3 shows the gas uptake curve (moles of the 30 mol % CH4/N2 gas mixture supplied from the reservoir) during the hydrate formation experiment carried out at 3.9 MPa, 277.15 K, and 0.29 mol % TBAB concentration along with the temperature profile of the gas and aqueous phases in the crystallizer. As seen in the figure (experiment 7), a strong temperature spike in the water phase marks the nucleation point of the semiclathrate hydrate. Then, the heat released is gradually removed by the fluid in the water bath; thus, the temperature in the solution gradually reaches a steady state. As seen in Figure 3b (experiment 8), a sudden temperature rise in the water phase can also be observed at the nucleation point, but the intensity of the temperature increase is much weaker than that observed in Figure 3a. The comparison of temperature rise suggests a stronger hydrate nucleation occurs in the fresh solution, which is correlated with the greater gas consumption. As seen in Table 3, the moles of the gas mixture consumed at the nucleation point is 0.0006 mol in the fresh solution (experiment 7), whereas only 0.0002 mol is consumed in the memory solution (experiment 8).

Figure 3. Temperature and gas uptake profile of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture at 277.15 K, 3.9 MPa, and 0.29 mol % TBAB concentration in a semibatch mode: (a) fresh solution (experiment 7); (b) memory solution (experiment 8). 2101


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Table 3. Experimental Results for TBAB Semiclathrate Hydrate Formed from the CH4/N2 Gas Mixture in a Semibatch Modea expt. no.

sample state

TBAB (mol %)

moles consumed at nucleation point (mol)

final gas consumed (mol gas/mol water)

CH4 recovery (%)

SF

water conversion to hydrateb (mol %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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

0.17

0.0007 0.0005 0.0009 0.0007 0.0007 0.0004 0.0006 0.0002 0.0007 0.0001 0.0008 0.0006 0.0007 0.0003 0.0006 0.0004 0.0006 0.0002

0.0056 0.0056 0.0056 0.0056 0.0056 0.0055 0.0060 0.0063 0.0064 0.0061 0.0067 0.0064 0.0063 0.0061 0.0068 0.0062 0.0064 0.0060

16.9 18.8 18.3 18.8 18.4 17.9 25.0 25.2 25.0 24.4 25.7 25.0 23.5 20.7 22.3 19.7 21.7 23.0

3.9 5.6 5.0 5.6 5.2 4.9 9.9 8.6 8.1 8.3 7.8 7.9 5.9 4.3 4.1 3.6 4.4 6.3

10.8 10.6 10.7 10.6 10.7 10.5 11.5 12.0 12.1 11.7 12.7 12.2 12.0 11.6 13.0 11.7 12.1 11.4

0.29

0.62

a CH4/N2 composition of the gas mixture was 30 mol % CH4 and 70 mol % N2. bHydration number 38 is used for the calculation of water conversion.28,29

The gas uptake curve in Figure 3a can be divided into three stages according to the slope variation. The first stage is the gas dissolution stage that starts from time zero (0 min) to the nucleation point (87 min). The second stage is the growth of TBAB semiclathrate hydrate, which begins from the nucleation point (87 min) to approximately 300 min. The slope of the gas uptake curve at the second stage is steeper than the first and third stages and the gas consumption is sharply increased from 0.0006 to 0.0027 mol, suggesting a fast growth of TBAB semiclathrate hydrate occurs at this stage. The third stage begins at approximately 300 min and stops at the end of the experiment (526.7 min). The gas uptake curve is nearly kept horizontal at this stage, indicating the termination of the TBAB semiclathrate hydrate formation due to the fact that there is a substantial hydrate mass in the vessel that reduces the gas/ liquid mass transfer. In Figure 3b, the gas dissolution stage can not be clearly seen in the gas uptake curve because of a short induction time (10.3 min). Similarly, the slope of the gas uptake curve is steep at the stage of TBAB semiclathrate hydrate formation (10.3 to 300 min) and the gas consumption is significantly increased from 0.0002 to 0.0028 mol. Figure 4 shows a reproduced experimental set of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture at 3.9 MPa, 277.15 K, and 0.29 mol % TBAB (experiments 11 and 12). Likewise, a stronger temperature spike can be observed in the water phase of the fresh solution (Figure 4a) as compared to that obtained in the memory solution (Figure 4b). Interestingly, because the induction time in the fresh solution is short (48.3 min), the gas dissolution stage (from time zero to the nucleation point) is not distinctively exhibited in the gas uptake curve obtained in the fresh solution, which is different from the one having a long induction time (Figure 3a). Therefore, there are two patterns for a gas uptake curve corresponding to short and long induction times. Figures 5 and 6 show the gas uptake curves of the TBAB semiclathrate hydrate formation experiments performed at 4.2 MPa, 276.15 K, and 0.17 mol % TBAB concentration along

Figure 4. Temperature and gas uptake profile of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture at 277.15 K, 3.9 MPa, and 0.29 mol % TBAB concentration in a semibatch mode: (a) fresh solution (experiment 11); (b) memory solution (experiment 12). 2102


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Figure 5. Temperature and gas uptake profile of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture at 276.15 K, 4.2 MPa, and 0.17 mol % TBAB concentration in a semibatch mode: (a) fresh solution (experiment 1); (b) memory solution (experiment 2).


with the temperature profiles of the gas and liquid phases in the crystallizer. A similar behavior of TBAB semiclathrate hydrate formation is observed at 0.17 mol % TBAB concentration as that observed at the 0.29 mol % TBAB concentration. In the case of long induction time, three stages including gas dissolution stage, TBAB hydrate growth stage, and final stage are distinctively exhibited in the gas uptake curve (Figure 6). However, in the case of short induction time, the gas dissolution stage can not be clearly seen in the gas uptake curve (Figure 5). It should be noted that the growth stage of TBAB semiclathrate hydrate is shortened as compared to that conducted at 0.29 mol % TBAB. For instance, as shown in Figures 5 and 6, the gas uptake curve begins to reach a plateau at approximately 250 min, whereas Figures 3 and 4 show that the gas uptake curve becomes horizontal at approximately 300 min. Meanwhile, the consumption of the gas mixture at 0.17 mol % TBAB is reduced when compared with that obtained at 0.29 mol % TBAB. As shown in Table 3, the average final gas mixture consumption in the 0.17 mol % TBAB solution is 0.0056 mol gas/mol water, but for the 0.29 mol % TBAB solution the final gas consumption is 0.0063 mol gas/mol water. This result indicates that more TBAB semiclathrate hydrate is formed as the TBAB concentration is increased from

0.17 to 0.29 mol %. Thus, the process for TBAB semiclathrate hydrate formation is prolonged. In addition, for the 0.17 mol % TBAB solution, a higher temperature spike can be seen in the liquid phase of the fresh solution as compared to that exhibited in the memory solution, suggesting a stronger nucleation of TBAB semiclathrate hydrate occurs in the fresh solution. This is consistent with what was observed in the 0.29 mol % TBAB solutions. Figures 7 and 8 show the gas uptake curves of the TBAB semiclathrate hydrate formation experiments carried out at 3.8 MPa, 280.15 K, and 0.62 mol % TBAB concentration. Two different patterns of the gas uptake curve can also be seen in the case of short induction time (Figure 7) and long induction time (Figure 8a). It should be noted that the growth stage of TBAB semiclathrate hydrate is nearly complete at the same point as that obtained in the 0.29 mol % TBAB solutions. For instance, as shown in Figures 7 and 8, the gas uptake curve begins to reach a plateau at approximately 300 min that equals to the time point observed in the 0.29 mol % TBAB solution. As seen in Table 3, the average final gas mixture consumed in the 0.62 mol % TBAB solution is the same as that obtained in the 0.29 mol % TBAB solution (0.0063 mol gas/mol water). Thus, in this vessel configuration, a limit is reached beyond which the 2103


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to 0.17, 0.29, and 0.62 mol % TBAB. This result indicates that the 0.29 mol % TBAB performs better than the 0.17 and 0.62 mol % TBAB mixtures for CH4 recovery from the 30 mol % CH4/N2 gas mixture in the semibatch manner. It can also be seen in Table 3 that the average final gas mixture consumed for the TBAB semiclathrate hydrate formation is 0.0056, 0.0063, and 0.0063 mol gas/mol water corresponding to the 0.17, 0.29, and 0.62 mol % TBAB. The water conversion to semiclathrate hydrate is 10.7%, 12.0%, and 12.0% for the 0.17, 0.29, and 0.62 mol % TBAB solution, respectively. It is indicated that more TBAB semiclathrate hydrate is formed as the TBAB concentration is increased from 0.17 to 0.29 mol %. In addition, combined with the reduction of the CH4 fraction in the gas phase (seen in Table 2) and the increase of the CH4 recovery (seen in Table 3), it is found that more CH4 is encaged in the TBAB semiclathrate hydrate as the TBAB concentration is increased from 0.17 to 0.29 mol %. Interestingly, the amount of TBAB semiclathrate hydrate is kept constant as the TBAB concentration is increased from 0.29 to 0.62 mol %, but the CH4 recovery is reduced from 25.1% to 21.8%. This result suggests that since no more hydrate forms the recovery is not improved and that N2 competes with CH4 to enter the dodecahedral cavities in the TBAB

gas consumption cannot be increased pointing to the need for a better gas/water contact mode. Table 3 summarizes the moles of the gas mixture consumed at the nucleation point, final gas consumed, CH4 recovery, separation factor (SF), and water conversion to semiclathrate hydrates for the experiments of separating CH4 from the 30 mol % CH4/N2 gas mixture through TBAB semiclathrate hydrate formation in the semibatch manner. The moles of the gas mixture consumed at the nucleation point and the final gas consumption are calculated using eq 1. The CH4 recovery and the separation factor (SF) are calculated using eqs 3 and 4, respectively. As seen in Table 3, at each TBAB concentration, the number of moles of the gas mixture consumed at the nucleation point in the fresh solution is larger than that consumed in the memory solution, indicating a strong nucleation of TBAB semiclathrate hydrate in the fresh solution. Therefore, a higher temperature spike is observed at the nucleation point in the fresh solution as compared to that exhibited in the memory solution. As seen in Table 3, the CH4 recovery corresponding to 0.17, 0.29, and 0.62 mol % TBAB is 18.2%, 25.1%, and 21.8%, respectively. The separation factor (SF) of CH4 from the 30 mol % CH4/N2 gas mixture is 5.0, 8.4, and 4.8 corresponding 2104


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Table 4. Experimental Conditions and Results for TBAB Semiclathrate Hydrate Formed from the CH4/N2 Gas Mixture at 277.15 K and 0.29 mol % TBAB in a Batch Modea expt. no.

sample state

tind (min)

CH4 fraction in gas phase (mol %)

final gas consumed (mol gas/mol water)

CH4 recovery (%)

SF

water conversion to hydrateb (mol %)

19 20 21 22 23 24

fresh memory fresh memory fresh memory

49.0 36.3 45.7 7.3 13.7 7.7

25.4 25.8 26.4 26.5 25.8 26.3

0.0056 0.0057 0.0056 0.0055 0.0055 0.0054

23.8 22.6 20.7 20.1 22.1 20.6

8.8 6.9 5.4 5.2 7.3 6.0

10.7 10.8 10.6 10.4 10.4 10.2

a CH4/N2 composition of the gas mixture was 30 mol % CH4 and 70 mol % N2). The initial pressure was 3.9 MPa. The driving force based on the initial pressure was 3.5 MPa. bHydration number 38 is used for the calculation of water conversion.

semiclathrate hydrate as the TBAB concentration is increased from 0.29 to 0.62 mol %. TBAB Semiclathrate Hydrate Formation in a Batch Manner. Experiments in a batch mode at 0.29 mol % TBAB concentration were carried out for comparison. The conditions are summarized in Table 4. TBAB semiclathrate hydrate formation was conducted at 277.15 K and 0.29 mol % TBAB concentration. The initial pressure was 3.9 MPa. The driving force based on the initial pressure was 3.5 MPa, which is the same as that used in the semibatch operation. Figure 9 shows the gas uptake curve, and a similar behavior to that in the semibatch manner is observed. For example, a higher temperature spike can be seen in the water phase of the fresh solution when compared to that observed in the memory solution (Figure 9b). This temperature spike corresponds to the nucleation point of the TBAB semiclathrate hydrate, and the induction time is determined by this sudden temperature rise. In addition, Figure 9a shows that the gas uptake curve is also composed of three stages. The first stage is the gas dissolution stage that starts from time zero to the nucleation point (45.7 min). The second stage is the growth of TBAB semiclathrate hydrate with a steep slope which begins at the nucleation point (45.7 min) and ends at approximately 300 min. In this stage, the consumption of the gas mixture is sharply increased from 0.0005 to 0.0058 mol. Then, the gas uptake curve reaches a plateau in the third stage, suggesting the termination of the TBAB semiclathrate hydrate. It should be noted that the gas dissolution stage can not be clearly seen in the gas uptake curve in case of short induction time (Figure 9b). Table 4 also summarizes the CH4 fraction in the gas phase at the end of the experiment, the final gas mixture consumed, CH4 recovery, separation factor (SF), and water conversion to semiclathrate hydrates for the experiments of separating CH4 from the 30 mol % CH4/N2 gas mixture in the batch manner. As seen in Table 4, the average final gas mixture consumed for TBAB semiclathrate hydrate formation is 0.0056 mol gas/mol water, which is smaller than the value obtained at 0.29 mol % TBAB in the semibatch manner (0.0063 mol gas/mol water, seen in Table 3). This result indicates that less TBAB semiclathrate hydrate is formed in the batch manner. At the same time, the CH4 concentration in the gas phase at the end of the experiment (∼26 mol %) is a little higher than that obtained in the semibatch manner (∼25 mol %), suggesting that less CH4 enters the dodecahedral cavities in the TBAB semiclathrate hydrate in the batch operation. In addition, with the operation mode varying from semibatch to batch, the CH4 recovery is decreased from 25.1% to 21.7%, the separation factor (SF) is reduced from 8.8 to 6.6, and the water conversion to semiclathrate hydrate is also reduced from 12.0% to 10.5%. Thus, the results indicate that under the same experimental

Figure 9. Temperature and gas uptake profile of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture at 277.15 K and 0.29 mol % TBAB concentration in a batch mode (initial pressure 3.9 MPa): (a) fresh solution (experiment 21); (b) memory solution (experiment 22).

condition (277.15 K, 3.9 MPa, and 0.29 mol % TBAB concentration), the semibatch operation is more effective than batch operation for separating CH4 from the 30 mol % CH4/N2 gas mixture through TBAB semiclathrate hydrate formation. This result can probably be explained by the fact that during the batch operation the pressure in the crystallizer continuously drops and the CH4 fraction decreases in the gas phase; therefore, less TBAB semiclathrate hydrate is formed and less CH4 is enclathrated into the TBAB semiclathrate hydrate. This work focuses on the first stage of forming TBAB semiclathrate hydrate for the separation of CH4 from the 2105


Energy & Fuels

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ACKNOWLEDGMENTS The financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Natural Science Foundation of China (No. 51006129) is greatly appreciated. The authors also thank PhD candidate Nagu Daraboina for his help with the experiments.

low-concentration CMM gas (30 mol % CH4/N2). It is found that the CH4 concentration is increased from 30 mol % to nearly 43 mol % through measuring the composition of the gas mixture decomposed from the TBAB semiclathrate hydrate that was formed at 277.15 K, 3.9 MPa, and 0.29 mol % TBAB concentration in a semibatch mode. Then, we assessed a second separation stage by forming TBAB semiclathrate hydrate from the 43 mol % CH4/N2 gas mixture under the same experimental conditions (277.15 K, 3.9 MPa, and 0.29 mol % TBAB concentration, semibatch mode) and found that the CH4 concentration is increased from 43 to 70 mol % in the second stage. Thus, a medium-pressure conceptual process with the use of TBAB is proposed and illustrated in Figure 10. A membrane

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REFERENCES

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Figure 10. A hybrid process for methane recovery from the lowconcentration CMM gas in the presence of TBAB as a promoter. The first and second stages are performed at 277.15 K and 3.9 MPa.

or vacuum pressure swing adsorption (PSA) process could be employed to deal with the lean CH4 stream. The CH4-rich stream obtained in the second stage of gas hydrate process can be used as natural gas resource, and can also be concentrated further by one more hydrate-based process before it is converted to other gas products.

4. CONCLUSIONS The use of TBAB semiclathrate hydrate to separate methane from the low-concentration CMM gas (30 mol % CH4/N2) is presented in this work. In addition to more favorable equilibrium conditions, the TBAB semiclathrate hydrate formation from the 30 mol % CH4/N2 gas mixture showed that CH4 is preferentially encaged into the TBAB semiclathrate hydrate. The use of a 0.29 mol % TBAB solution is preferred over the use of the 0.17 and 0.62 mol % TBAB solutions. Semibatch operation is more effective than batch operation for the CH4 separation from the 30 mol % CH4/N2 gas mixture. The CH4 recovery was found to be approximately 25% at 0.29 mol % TBAB concentration in the semibatch operation and the corresponding CH4 concentration released from the TBAB semiclathrate hydrate was nearly 43 mol %. A CH4-rich stream (70 mol % CH4/N2) was obtained after two stages of TBAB semiclathrate hydrate formation, which can be used as natural gas resource or converted to other gas products.

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