Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed

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Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles Dongliang Zhong, Nagu Daraboina, and Peter Englezos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400676g • Publication Date (Web): 16 Jul 2013 Downloaded from http://pubs.acs.org on July 16, 2013

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Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles Dong-Liang Zhong *, †, ‡, Nagu Daraboina §, Peter Englezos § †

College of Power Engineering, Chongqing University, Chongqing, 400044, China ‡

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, China

§

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada

* Corresponding author. Tel/Fax: +86-23-65102473 E-mail: [email protected] (D.L. Zhong)

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Abstract In the present work the separation of CH4 from low-concentration coal mine methane gas (30 mol% CH4/N2) through hydrate crystallization was investigated in a fixed bed of silica sand particles. The influence of the additive tetrahydrofuran (THF) on hydrate equilibrium conditions and kinetics of CH4 separation was studied as well. The incipient hydrate equilibrium conditions at 1 mol% THF were determined using the isothermal pressure search method. It was found the presence of THF significantly reduced the hydrate equilibrium conditions as compared to those obtained in liquid water with the same gas mixture. CH4 recovery in the water-saturated silica sand bed was considerably low (~12.0%) because N2 molecules might compete with CH4 molecules to enter the hydrate crystals under the high pressure conditions. The addition of THF to the bed of silica sand particles reduced the nucleation time of gas hydrate formed from the 30 mol% CH4/N2 gas and increased the CH4 recovery (~21.4%) significantly. The comparison of CH4 separation between the silica sand bed and the stirred reactor in the presence of THF indicated that CH4 recovery was approximately the same but the conversion of water to hydrate in the THF solution-saturated silica sand bed was largely increased.

Keywords: Gas hydrate; Gas separation; Methane recovery; THF; Fixed bed


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1. Introduction Safety of coal mining operations requires continuous separation and recovery of coal mine methane (CMM) from a mixture of methane with air. CH4 is also a strong greenhouse gas and a source of energy.1-3 Recently, attentions have been paid on the use of gas hydrate formation for gas mixture separation.4-7 Thus, the separation of CH4 from model coal mine methane gas (CH4/N2 or CH4/N2/O2) using gas hydrate crystallization has been considered. Although methane is preferentially incorporated into the hydrate phase compared to other components such as N2 and O2 in the CMM gas, the hydrate formation pressures at given temperatures are very high.8 Some additives such as tetrahydrofuran (THF), tetra-n-butyl ammonium bromide (TBAB), and cyclopentane (CP) are known to promote hydrate formation thermodynamically by shifting hydrate formation conditions to lower pressures and higher temperatures.9-13 Zhang et al.

14

reported incipient equilibrium data of gas hydrate formed with a low-concentration CMM gas in the presence of THF. Sun et al.

15

formed TBAB semiclathrate hydrate from a

simulated CMM gas (46.3 mol% CH4/N2) and concluded that CH4 concentration can be increased from 46.3 to 67.9 mol% through a single-stage hydrate separation in the presence of TBAB. Zhong et al.

16

measured the incipient hydrate formation conditions

for gas hydrate formed with a simulated low-concentration CMM gas (30 mol% CH4/N2) in the presence of TBAB and investigated the kinetics of CH4 separation in a small scale stirred vessel. Zhong et al.

17

also reported the phase equilibrium data of gas hydrate

synthesized from the same gas mixture in the presence of CP and found the CH4 recovery was significantly increased as compared to that obtained in the presence of TBAB. It should be noted that all these investigations were carried out in stirred reactors which are frequently used arrangement for laboratory hydrate studies. However, the agitation of reactor contents would consume a significant proportion of energy that is needed for system operation when the stirred reactors are scaled up for industrial application.18 Recently, studies on gas hydrate formation in a fixed bed of porous media have been


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reported, including phase equilibrium investigations 19-23 and kinetic property studies.24-26 Kang et al.

27

investigated kinetics of natural gas hydrate formed in the fixed bed of

porous silica gel. They concluded that the hydrate formation rate and gas storage capacity were enhanced without mechanical stirring. Li et al.

28

studied the dissociation behavior

of methane hydrate formed in a fixed bed of silica gel and reported the effect of formation pressure, environmental temperature, and pore size on the rate of methane released from the hydrates. Linga et al. 29 formed methane hydrate in a fixed bed of silica sand particles instead of using a stirred vessel and found the conversion of water to hydrate was significantly increased as compared to that in the stirred vessel, indicating more hydrate can be obtained in the porous media system. They also found the rate of hydrate formation was enhanced in a silica sand bed compared to a stirred reactor.30 Zanjani et al. 31

formed gas hydrates using natural gas and a mixture of methane, ethane, and propane

in the presence of a small amount of silica-based porous media and concluded that the addition of the silica-based porous media can considerably increase the gas storage capacity of the hydrate phase. However, to the best of our knowledge, there were no studies conducted in a porous media system for the separation of CH4 from the CMM gas mixture. It is therefore of interest to evaluate the properties of CH4 separation in a bed of silica sand particles against a stirred vessel. The objective of this work is to study the characteristics of CH4 separation from the CMM gas mixture through hydrate formation in a fixed bed of silica sand particles, which includes studying the influence of additive THF on the CH4 separation process and comparing CH4 separation between the fixed bed of silica sand and the stirred reactor. A model CMM gas mixture (30 mol% CH4/N2) will be considered in this study.

2. Experimental Section 2.1 Materials The gas mixture used for this work was UHP grade supplied by Praxair Technology Inc.


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The composition was 30 mol% CH4 and 70 mol% N2, which is a suitable model gas representing the low-concentration CMM gas recovered from coal mines.16, 17 Silica sand with an average diameter of 329 µm (diameter ranges from 150 to 630 µm) and THF with a certified mass purity > 99.0 % were supplied by Sigma-Aldrich. Distilled and deionized water was used in all experimental runs. 2.2 Apparatus and procedure for incipient equilibrium conditions The apparatus used for the measurement of incipient hydrate equilibrium conditions was described elsewhere.32 The isothermal pressure search method

33

was employed to

determine the incipient hydrate formation pressure at a given temperature. The hydrate equilibrium data are necessary for the design of the separation process (selection of operating conditions). Moreover, there are no hydrate equilibrium data available in the literature for the CMM gas mixture (30 mol% CH4/N2) in the presence of THF. We performed hydrate equilibrium experiments to measure the incipient equilibrium data of such system first. 2.3 Silica sand apparatus and procedure for hydrate formation The detailed description of the silica sand apparatus was given elsewhere.29, 34 Briefly, it consists of a cylindrical crystallizer (CR) made of 316 stainless steel (inner diameter, 10.16 cm; height, 15.24 cm; volume, 1236 cm3). In the present work the amount of silica sand placed in the crystallizer was 645 g. The height of the silica sand bed was 4.94 cm. The volume of water required to saturate the silica sand was found to be 0.217 cm3/g,29 filling the interstitial or pore volume of the sand particles. Accordingly, 140 cm3 water (with / without additives) was added to the silica sand bed. To study the effect of the variable volume of silica sand bed on methane separation from the CMM gas by hydrate crystallization, one copper cylinder (CC1) with an inner diameter of 7.62 cm was fit precisely into the cylindrical crystallizer (CR) to reduce the volume of the cylinder. The height of the silica sand was fixed the same as 4.94 cm. Thus, the amount of the silica sand placed into the crystallizer was reduced to 362.8 g and the


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water required to saturate the silica sand was 78.7 cm3. Once the bed of silica sand particles in the crystallizer was set up, four thermocouples with an uncertainty of 0.1 K were positioned (one for the gas phase and three for different positions of the sand bed) and the crystallizer was closed. The crystallizer was pressurized to 0.5 MPa with the CMM gas and depressurized three times to eliminate the presence of any air bubbles in the system. Then the vessel was pressurized with the CMM gas mixture to the desired value and the temperature was allowed to reach the preset value. Once the pressure and the temperature were stabilized, the data acquisition (DAQ) system was initialized and this was time zero for the experiments. All experiments were carried out with a fixed amount of water and gas mixture (batch mode). The temperature was maintained constant by an external refrigerator. As the gas mixture was consumed for hydrate formation, the pressure in the reactor dropped and was recorded by the DAQ system. The data of pressure and temperatures were logged in a computer every 20 s. The experiments were stopped when there was no significant pressure drop in the crystallizer. Subsequently, the gas mixture in the reactor was sampled and the composition was measured using a gas chromatograph with an uncertainty of 0.1% (GC, Varian CP-3800). It is noted that fresh and memory experiments were carried out under each experimental condition. Fresh experiments refer to the experiments using fresh water and sand and memory experiments were performed 4 h after the fresh experiments without replacing the water and sand particles in the crystallizer. 2.4 Stirred reactor and procedure for hydrate formation A small scale stirred vessel (58 cm3) was used to compare the properties of CH4 separation that were obtained in the silica sand apparatus. This stirred apparatus was described by Daraboina, et al.16, 35 Briefly, the experiments were conducted as follows. The reactor was filled with a 20 cm3 THF solution (1 mol% THF). Then it was pressurized to 0.5 MPa with the CMM gas and depressurized three times to remove the air remaining in the system. The temperature was maintained constant by an external


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refrigerator. Once the temperatures of the gas and liquid phase reached the desired values the crystallizer was pressurized to the target pressure. Subsequently, the magnetic stirrer was started at a constant speed (400 rpm) and the data acquisition system was initiated to record the temperatures of the gas and liquid phase and the pressure of the gas phase. This was time zero for the experiments. All experiments were carried out with a fixed amount of solution and gas mixture (batch mode). The experiments were allowed to run until there was no significant pressure drop in the crystallizer. The composition of the gas phase at the end of the experiments was measured using GC as well. It should be noted that fresh THF solutions as well as memory THF solutions were used under each experimental condition. Memory THF solutions refer to the solutions that have prior experience of hydrate formation and were used 4 h after hydrate decomposition. 2.5 Calculation of the amount of gas consumed for hydrate formation As all experiments were carried out in batch operation, the pressure in the crystallizer would drop due to hydrate formation, but the temperature in the crystallizer was maintained constant until the end of the experiment. The hydrate crystallization is an exothermic process, so the nucleation point or induction time was identified on the basis of a sudden temperature rise in the liquid phase and through a rapid increase in gas consumption. The total number of moles of gas mixture in the closed system remains constant and equal to that at time zero (t = 0). Thus, the number of moles of the gas mixture consumed for hydrate formation at any given time (∆nH, gas uptake) is the difference between the number of moles of the gas mixture at t = 0 present in the gas phase of the crystallizer and the number of moles of the gas mixture present in the gas phase of the crystallizer at time t = t and is calculated by eq.1. ∆nH = ng,0 − ng,t = (

PV PV )CR, 0 − ( )CR, t zRT zRT

(1)

where ng is the number of moles of gas mixture (CH4/N2) in the crystallizer (CR) at time t = 0 and time t = t, z is the compressibility factor calculated by Pitzer’s correlation,36 and V is the volume of gas phase in the crystallizer, P and T are the pressure and temperature


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of the crystallizer. Standard mixing rules were employed in the correlation used for the calculation of critical pressure, critical temperature, and the acentric factor of the gas mixture (CH4/N2). It should be noted that VCR, t at t > 0 is not equal to VCR, 0 due to a difference in volume per mole of water between the aqueous and hydrate phases. However, the bias in the volume of gas phase V was found within 1.5% based on the water conversion to hydrate reported in this work. Therefore, the volume difference between VCR, t (at t > 0) and VCR, 0 was neglected in the present work. The bias in the volume of gas phase V was considered while calculating the standard uncertainty in ∆nH. The rate of hydrate formation is customarily considered to be the rate of gas consumption and is calculated using the forward difference method as follows

 d ∆nH  ∆nH, t+∆t − ∆nH, t , ∆t = 20 s   = ∆t  dt  t

(2)

The average of these rates is calculated every 30 min and reported.

  d ∆nH   d ∆nH   d ∆nH     dt  +  dt  + L +  dt    1  2  m  Average rate of hydrate formation (Rav ) =  , m = 90 m       (3) The conversion of water to hydrate is determined from the gas uptake and the experimental conditions using the following equation. Conversion of water to hydrate (mol%) =

∆nH × Hydrate number × 100% (4) nH 2O

where ∆nH is the number of moles of gas consumed for hydrate formation at the end of the experiment and nH 2O is the total number of moles of water in the system. The hydration number is the number of water molecules per guest molecule. The hydration number of 5.7 was used for CH4/N2 gas mixture forming structure II hydrate in liquid water and 8.5 was used for CH4/N2 gas mixture forming structure II hydrate in the 8 ACS Paragon Plus Environment

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presence of THF.37

2.6 CH4 recovery and separation factor CH4 recovery or split fraction (S.Fr.) of methane from the CH4/N2 gas mixture is calculated as follows.32, 38, 39 CH 4 recovery =

H nCH 4 feed nCH 4

× 100%

(5)

feed H where nCH is defined as the moles of CH4 in feed gas and nCH is the moles of CH4 4

4

incorporated in the hydrate at the end of experiments. In addition, the separation factor (S.F.) is determined by the following equation. S.F. =

H nCH × nNgas2 4 gas nNH2 × nCH 4

(6)

gas where nCH is the moles of CH4 in the gas phase at the end of the kinetic experiment, 4

nNgas2 is the moles of N2 in the gas phase at the end of the kinetic experiment, and nNH2 is

the moles of N2 enclathrated. As defined in eq. 6, the separation factor (S.F.) reflects the extent of CH4 separation from the low-concentration CMM gas. A higher separation factor (S.F.) indicates the capability of gas hydrate to separate CH4 from the CMM gas mixture is stronger.

3. Results and Discussion As reported in our previous work,16, 17 CH4 recovery obtained in the stirred reactor in the presence of CP was significantly increased as compared to that obtained in the presence of TBAB. In the present work we extended our study and performed investigations of CH4 recovery from the CMM gas mixture in a fixed bed of silica sand particles in the presence of CP and TBAB, respectively. The experiments were carried out under the conditions which were reported in the literature.16, 17 140 cm3 aqueous solution (with CP or TBAB) was used to saturate the bed of silica sand particles. However, it was


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found that gas hydrate was not formed from the CMM gas mixture in the bed of silica sand particles in the presence of CP and TBAB. The reason might be that CP is immiscible with water, when it is added to the bed of silica sand particles as a promoter, CP cannot mix with water sufficiently because there is no agitation in the bed of silica sand. Note that cations TBA and anions Br- other than TBAB molecules will participate in the TBAB semiclathrate hydrate formation, so the agitation of TBAB solution might be more important than the increase of gas-liquid interface using the bed of silica sand. However, the agitation was absent in the bed of silica sand. Therefore, THF was used as an alternative thermodynamic promoter instead of CP and TBAB for CH4 recovery from the CMM gas in a fixed bed of silica sand particles. Figure 1 shows the incipient equilibrium conditions for gas hydrate formation for the low-concentration CMM gas (30 mol% CH4/N2) in liquid water and in the presence of 1 mol% THF. The concentration of THF used for this work was 1 mol%. The tested temperature ranged from 276.05 to 283.85 K. As seen in Figure 1, the equilibrium hydrate formation pressures obtained in the presence of THF are significantly reduced as compared to the equilibrium pressures obtained in liquid water at given temperatures. For instance, the equilibrium formation pressure in liquid water is 6.9 MPa at 273.65 K, whereas it is only 0.3 MPa at 276.05 K in the presence of THF (1 mol%). This confirms that the presence of a small amount of THF shifts the equilibrium conditions to low pressures at given temperatures and therefore increases the stability of gas hydrate formed from the CMM gas mixture.14 Table 1 summarizes the experimental conditions and measured induction times for gas hydrate formation using the 30 mol% CH4/N2 mixture. As seen in Table 1, the measured equilibrium points (Teq = 273.65 K, Peq = 6.9 MPa) in liquid water and (Teq = 276.05 K, Peq = 0.3 MPa) at 1 mol% THF were employed for the selection of the pressure/temperature conditions for kinetic experiments. Experiments 1-10 were performed in a fixed bed of silica sand particles and experiments 11-14 were conducted in


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a stirred vessel. Experiments 1-2 were carried out in the water-saturated silica sand bed at Texp = 273.65 K and Pexp = 9.0 MPa (∆P =Pexp-Peq= 2.1 MPa). Interestingly, although experiments 1-2 were maintained under the experimental conditions for 96 h, no pressure drop and temperature increase were observed in the crystallizer. Moreover, the composition of the gas mixture at the end of the experiments was similar to the original gas mixture (seen in Table 2). This is an evidence indicating that gas hydrate was not formed at Texp = 273.65 K and Pexp = 9.0 MPa. A likely reason for this is that the driving force (∆P = Pexp - Peq) is low. Therefore, the driving force at 273.65 K was increased to ∆P = 3.1 MPa for the following experiments (experiments 3-6). In addition, as seen in Table 1, the induction times obtained in memory solutions were shorter than those obtained in fresh solutions, indicating the onset of hydrate nucleation was reduced due to the ‘memory effect’ in memory solutions. Figure 2 shows the temperature and gas uptake curves for gas hydrate formation from the 30 mol% CH4/N2 gas mixture in the fixed bed of water-saturated silica sand particles at 273.65 K and 10.0 MPa (∆P = 3.1 MPa). The gas uptake is the amount of the gas mixture consumed for hydrate formation, which was calculated by eq.1 and then divided by the moles of water added in the system. Thus, the normalized gas uptake was obtained for comparison. As seen in the figure, a sudden temperature increase was observed at 721 min and a rapid increase in the gas uptake was seen at the same time. This indicates the onset of hydrate nucleation and the time is defined as induction time. It should be noted that T1, T2, and T3 in the figure correspond to the bottom, middle, and top position of the silica sand bed, respectively. The temperature increase in T1, T2, and T3 were observed at the nucleation point, indicating hydrate nucleation occurred simultaneously in different locations of the silica sand bed. Later on, temperature spikes can also be seen in T1 but the intensity was weaker than that at the induction point. This can be explained by the multiple nucleation of gas hydrate occurred in the bed of silica sand particles. This result is in good agreement with observations in the literature.29 Figure 3 shows the temperature


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and gas uptake curves for gas hydrate formation from the 30 mol% CH4/N2 gas mixture in a small size of water-saturated silica sand bed (CR+CC1) at 273.65 K and 10.0 MPa. Similarly, an abrupt temperature increase indicates the occurrence of hydrate nucleation, and the gas consumption increases suddenly at the nucleation point. Table 2 summarizes the experimental results including final gas uptake (mol of gas / mol of water), CH4 content in the gas phase (mol%), CH4 recovery, separation factor (S.F.), and water conversion to hydrate. The final gas uptake was the gas consumption at the end of the experiments for hydrate formation and was also normalized by taking into account the amount of water for the experiments. The combined standard uncertainties in the final gas uptake were calculated from the uncertainties in P, T, V, and gas composition (x) and were presented in Table 2 as well. The water conversion to hydrate, CH4 recovery, and separation factor (S.F.) were calculated by eq.4, eq.5, and eq.6, respectively. As seen in Table 2, two experiments (experiments 5-6) were performed in a smaller size of silica sand bed (362.8 g) to examine the effect of volume variation of silica sand bed on methane recovery. The bed of silica sand was saturated by liquid water and the height of the sand bed was maintained the same as experiments 3-4 (4.94 cm). It was found the average final gas uptake decreased from 0.0301 to 0.0263 mol of gas/mol of water as the volume of the silica sand bed was reduced. This indicates that the amount of gas hydrate formed from the CMM gas was reduced in the small volume of silica sand bed. This result can be explained by the fact that the water volume and the gas-liquid interface in the reactor decreased when the size of the silica sand bed is decreased. The gas-liquid interface is an important factor for hydrate formation. The hydrate formation may not be enough in the small silica sand bed and therefore the gas uptake in the small size of silica san bed is reduced. Figure 4 shows the comparison of rate of hydrate formation obtained in two volumes of silica sand bed (CR and CR+CC1). The rate of hydrate formation was calculated by eqs. 2-3. As seen in Figure 4, the rate of hydrate formation in the small volume of silica


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sand bed drops quickly and reduces below 0.001 mol of gas/mol of water/h at about 3 h. Although the rate of hydrate formation in the large volume of silica sand bed shows a decreasing tendency as well, it surpasses the rate of hydrate formation in the small volume of silica sand bed at 3 h and maintains at a higher value as compared to the small volume of silica sand bed. Then both rates of hydrate formation reach a plateau at about 30 h and the values are approximately close to zero. This can be explained by the fact that the area of gas-water interface increases as the volume of silica sand bed is increased. As a result, more hydrate nucleation sites are available in the large volume of silica sand particles. Therefore, the process of hydrate formation is prolonged in the large volume of silica sand bed. Interestingly, it was also found in Table 2 that CH4 recovery was decreased from 12.7% to 11.3% when the volume of silica sand bed was reduced, indicating a large volume of silica sand bed can promote the separation of CH4 from the CMM gas. Note that CH4 molecules are more soluble than N2 molecules in liquid water. When a large volume of silica sand bed is used, the water volume and the gas-liquid interface are increased. As a result, more CH4 molecules might dissolve into the liquid water and more hydrate will form. Thus, the CH4 recovery is increased in a large volume of silica sand bed. Therefore, the large volume of silica sand bed was preferred and used for the experiments 7-10 based on the reported rate of hydrate formation and CH4 recovery. The results also indicate that for mathematical modeling purposes the size of the sample matters as was suggested in the literature.29 Figure 5 shows the temperature and gas uptake curves for gas hydrate formation in the large volume of silica sand bed which was saturated by 140 cm3 THF solutions (1 mol% THF). As seen in the figure, temperature spikes at 8.7 min indicate the onset of hydrate nucleation. Similar to the experiments carried out in water-saturated silica sand particles (Figure 2), temperature spikes were observed in T1, T2 and T3 at the nucleation point, indicating hydrate nucleation occurred simultaneously at different depths of the silica sand bed. This is also an evidence of multiple nucleation event of hydrate formation in


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the fixed bed of silica sand particles. Then significant temperature increases in T2 were also observed at 27.4, 27.8, and 31.2 h, indicating an intermittent hydrate formation at the same depth. Gas hydrate was quickly formed at the beginning of the experiment when THF was present in the liquid solution. As a result, the liquid water at the same depth cannot be fully converted to gas hydrate at one time due to the transport of gas molecules was prevented by the hydrate film. Therefore, the hydrate formation process was prolonged at the same site. It should be noted that the gas uptake was observed to increase rapidly at the beginning of the experiment. This is probably because the induction time was short and hence the stage for gas molecules dissolving into the liquid phase was not observed. Interestingly, the presence of THF in the bed of silica sand particles accelerated the process of hydrate nucleation. As seen in Table 1, the induction times obtained in the presence of THF (experiments 7-10) were greatly reduced as compared to those obtained in pure water (experiments 3-6). The average value of induction times in the presence of THF was 8.5 min while that for pure water was 887.8 min. The presence of THF also promoted the CH4 recovery from the CMM gas in the bed of silica sand particles. The CH4 recovery obtained in the bed of THF solution-saturated silica sand particles (1 mol% THF) was increased to 21.4%, whereas the CH4 recovery obtained in the bed of water-saturated silica sand particles was only 12% (seen in Table 2). The separation of CH4 from the CMM gas by hydrate formation was also compared between the bed of silica sand and the stirred reactor in the presence of THF. As seen in Table 1, experiments 11-14 were carried out in the small scale stirred vessel and the experimental conditions were maintained the same as experiments 7-10. Figure 6 shows the comparison of gas uptake for hydrate formation from the 30 mol% CH4/N2 gas between the bed of silica sand and the stirred reactor. As seen in the figure, the final gas uptake obtained in the THF solution-saturated silica sand (experiment 7) was higher than that obtained in the stirred reactor using the same THF solution (1 mol% THF,


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experiment 11). The final gas uptake for experiments 7 and 11 was 0.0128 and 0.0056 mol of gas / mol of water, respectively. This indicates that the amount of gas hydrate formed from the 30 mol% CH4/N2 gas mixture was increased in silica sand bed as compared to the stirred vessel. Moreover, CH4 recovery for experiment 7 was 21.4% which was similar to that obtained in experiment 11 (21.1%). Therefore, the fixed bed of silica sand particles can be used as an effective arrangement for CH4 separation from the CMM gas mixture as compared to the stirred reactor. However, further research is needed to reveal the mechanism of CH4 separation from the CMM gas mixture in silica sand particles, so as to further increase the CH4 recovery. Interestingly, as seen in Figure 6, the final gas uptake obtained in the water-saturated silica sand bed was 0.0288 mol of gas / mol of water, which was two times larger than that obtained in the THF solution-saturated silica sand bed (0.0128 mol of gas / mol of water). This is because the experiments using the water-saturated silica sand particles were performed at a high pressure of 10 MPa, so more CH4 and N2 molecules occupied the small and large cages of the structure II hydrate. As a result, the gas uptake was increased as compared to the experiments that were carried out at a low pressure of 3.4 MPa using the THF solution-saturated silica sand particles. However, as seen in Table 2, it was found the CH4 recovery obtained in the water-saturated silica sand bed (12.6% for experiments 3) was much lower than that obtained in the THF solution-saturated silica sand bed (21.4% for experiments 7). The reason is because experiment 3 was carried out with liquid water at a high pressure of 10.0 MPa and this high pressure condition would be favorable for N2 molecules to compete with CH4 molecules to occupy the small and large cages of the structure II hydrate. Therefore, the amount of CH4 molecules occupied the small and large cavities of the hydrate were reduced and the methane recovery decreased accordingly. Figure 7 shows the comparison of rate of hydrate formation between the bed of silica sand and the stirred reactor. As seen in the figure, for the experiments carried out in the


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Page 16 of 30

presence of THF, the rate of hydrate formation in the stirred vessel (experiment 11) was larger than that in the bed of silica sand (experiment 7) at the nucleation point, but it decreased faster and dropped below the rate of hydrate formation in the bed of silica sand at 4.5 h. Then it continued decreasing and nearly approached zero at 10 h. This is probably due to that in the stirred vessel hydrate crystals first nucleate in the bulk liquid and quickly grow at the gas/liquid interface. With the accumulation of gas hydrate at the gas/liquid interface, it is difficult for gas molecules to transport through the hydrate phase to the hydrate/liquid interface, thus the progress of hydrate growth might be interrupted. On the contrary, as seen in the figure, although the rate of hydrate formation in the bed of silica sand exhibited a decreasing tendency as well, it approximately reached a constant value at about 5 h and decreased slowly after 25 h. This indicates that the process of hydrate formation in the bed of silica sand was extended compared to the stirred vessel and as a result a higher conversion of water to hydrate was obtained. The conversion of water to hydrate for experiments 7 and 11 was 25.6% and 11.2%, respectively (seen in Table 2). In addition, small jumps in the rates of hydrate formation (experiments 3 and 7) were also observed during the decreasing period. This is in accordance with the temperature spikes observed after the nucleation point in the bed of silica sand particles, suggesting the occurrence of multiple hydrate nucleation in the bed of silica sand particles. Although our laboratory-scale experiments showed that the silica-sand fixed bed was more effective than the stirred reactor for CH4 separation from the CMM gas mixture, the superiority of the silica-sand fixed bed in industrial-scale operations needs to be investigated in future. Due to the absence of any advection in the fixed bed during batch operations, the cooling of the bed for discharging the heat generated by hydrate formation is

inevitably

dependent

only

on

the

heat

conduction

through

the

sand/aqueous-liquid/hydrate composite. Thus, the cooling of the fixed bed would become increasingly ineffective, thereby causing a sharp decrease in hydrate formation rate per


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unit volume of the bed when the reactor increases to an industrial scale. Placing metal grids in the fixed bed of silica sand might be an efficient method for the enhancement of heat conduction and should be studied in an industrial-scale reactor in future.

4. Conclusions The recovery of CH4 from low-concentration CMM gas (30 mol% CH4/N2) by hydrate crystallization was investigated in a fixed bed of silica sand particles. The CH4 recovery obtained in the water-saturated silica sand bed was considerably low and N2 molecules might compete with CH4 molecules to enter the hydrate crystals at high pressure conditions. However, the use of 1 mol% THF solution to saturate the bed of silica sand particles accelerated the progress of hydrate formation and increased the CH4 recovery greatly. The comparison of CH4 separation between the silica sand and the stirred reactor in the presence of THF indicated that CH4 recovery was nearly the same but the conversion of water to hydrate in the THF solution-saturated silica sand bed was significantly increased. Therefore, the interruption of hydrate growth due to the hydrate cap at the reaction interface can be avoided in the bed of silica sand particles. Future work is needed to understand the mechanism of CH4 separation form the CMM gas mixture in the bed of silica sand in the presence of THF or to explore new additives to increase the CH4 recovery in the bed of silica sand particles.

Acknowledgement The financial support from the National Natural Science Foundation of China (No. 51006129), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Visiting Scholar Foundation of the State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University (No. 2011DA105287-FW201211), and Chongqing Science and Technology Commission (cstc2013jcyjA90012) is greatly appreciated.


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References (1) Olajossy, A.; Gawdzik, A.; Budner, Z.; Dula, J., Methane separation from coal mine methane gas by vacuum pressure swing adsorption. Chem. Eng. Res. Des. 2003, 81, 474-482. (2) Krzysztof, W., Harnessing methane emissions from coal mining. Process Saf. Environ. 2008, 86, 315-320. (3) Karacan, C. O.; Ruiz, F. A.; Cote, M.; Phipps, S., Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int. J. Coal Geol. 2011, 86, 121-156. (4) Zhang, L. W.; Chen, G. J.; Sun, C. Y.; Fan, S. S.; Ding, Y. M.; Wang, X. L.; Yang, L. Y., The partition coefficients of ethylene between hydrate and vapor for methane + ethylene + water and methane + ethylene + SDS + water systems. Chem. Eng. Sci. 2005, 60, 5356-5362. (5) Sun, C. Y.; Ma, C. F.; Chen, G. J.; Zhang, S. X., Experimental and simulation of single equilibrium stage separation of (methane plus hydrogen) mixtures via forming hydrate. Fluid Phase Equilib. 2007, 261, 85-91. (6) Zhang, L. W.; Chen, G. J.; Sun, C. Y.; Ding, Y. M.; Yang, L. Y., The partition coefficients of ethylene between vapor and hydrate phase for methane + ethylene + THF + water systems. Fluid Phase Equilib. 2006, 245, 134-139. (7) Fan, S. S.; Li, S. F.; Wang, J. Q.; Lang, X. M.; Wang, Y. H., Efficient Capture of CO2 from Simulated Flue Gas by Formation of TBAB or TBAF Semiclathrate Hydrates. Energy Fuels 2009, 23, 4202-4208. (8) Zhong, D. L.; Ye, Y.; Yang, C., Equilibrium conditions for semiclathrate hydrates formed in the CH4 + N2 + O2 + tetra-n-butyl ammonium bromide systems. J. Chem. Eng. Data 2011, 56, 2899-2903. (9) Tohidi, B.; Danesh, A.; Burgass, R. W.; Todd, A. C., Equilibrium data and thermodynamic modelling of cyclohexane gas hydrates. Chem. Eng. Sci. 1996, 51, 159-163. (10) Arjmandi, M.; Chapoy, A.; Tohidi, B., Equilibrium data of hydrogen, methane, nitrogen, carbon dioxide, and natural gas in semi-clathrate hydrates of tetrabutyl ammonium bromide. J. Chem. Eng. Data 2007, 52, 2153-2158.


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(11) Sun, C. Y.; Chen, G. J.; Zhang, L. W., Hydrate phase equilibrium and structure for (methane plus ethane plus tetrahydrofuran plus water) system. J. Chem. Thermodyn. 2010, 42, 1173-1179. (12) Mohammadi, A. H.; Richon, D., Phase equilibria of binary clathrate hydrates of nitrogen plus cyclopentane/cyclohexane/methyl cyclohexane and ethane plus cyclopentane/cyclohexane/methyl cyclohexane. Chem. Eng. Sci. 2011, 66, 4936-4940. (13) Meysel, P.; Oellrich, L.; Raj Bishnoi, P.; Clarke, M. A., Experimental investigation of incipient equilibrium conditions for the formation of semi-clathrate hydrates from quaternary mixtures of (CO2+N2+TBAB+H2O). J. Chem. Thermodyn. 2011, 43, 1475-1479. (14) Zhang, B.; Wu, Q., Thermodynamic promotion of tetrahydrofuran on methane separation from low-concentration coal mine methane based on hydrate. Energy Fuels 2010, 24, 2530-2535. (15) Sun, Q.; Guo, X.; Liu, A.; Liu, B.; Huo, Y.; Chen, G., Experimental study on the separation of CH4 and N2 via hydrate formation in TBAB Solution. Ind. Eng. Chem. Res. 2011, 50, 2284-2288. (16) Zhong, D. L.; Englezos, P., Methane separation from coal mine methane gas by tetra-n-butyl ammonium bromide semiclathrate hydrate formation. Energy Fuels 2012, 26, 2098-2106. (17) Zhong, D. L.; Daraboina, N.; Englezos, P., Recovery of CH4 from coal mine model gas mixture (CH4/N2) by hydrate crystallization in the presence of cyclopentane. Fuel 2013, 106, 425-430. (18) Linga, P.; Kumar, R.; Lee, J. D.; Ripmeester, J.; Englezos, P., A new apparatus to enhance the rate of gas hydrate formation: Application to capture of carbon dioxide. Int. J. Greenh. Gas Con. 2010, 4, 630-637. (19) Handa, Y. P.; Stupin, D. Y., Thermodynamic properties and dissociation characteristics of methane and propane hydrates in 70-.ANG.-radius silica gel pores. J. Phys. Chem. 1992, 96, 8599-8603. (20) Smith, D. H.; Wilder, J. W.; Seshadri, K., Methane hydrate equilibria in silica gels with broad pore-size distributions. AlChE J. 2002, 48, 393-400. (21) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W., Experimental measurement of methane and carbon dioxide clathrate hydrate equilibria in mesoporous silica. J. Phys. Chem. B 2003, 107, 3507-3514.


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(22) Kang, S. P.; Lee, J. W.; Ryu, H. J., Phase behavior of methane and carbon dioxide hydrates in meso- and macro-sized porous media. Fluid Phase Equilib. 2008, 274, 68-72. (23) Lee, S.; Seo, Y., Experimental measurement and thermodynamic modeling of the mixed CH4 + C3H8 clathrate hydrate equilibria in silica gel pores: Effects of pore size and salinity. Langmuir 2010, 26, 9742-9748. (24) Seo, Y. T.; Moudrakovski, I. L.; Ripmeester, J. A.; Lee, J. W.; Lee, H., Efficient recovery of CO2 from flue gas by clathrate hydrate formation in porous silica gels. Environ. Sci. Technol. 2005, 39, 2315-2319. (25) Sun, X.; Mohanty, K. K., Kinetic simulation of methane hydrate formation and dissociation in porous media. Chem. Eng. Sci. 2006, 61, 3476-3495. (26) Kneafsey, T. J.; Tomutsa, L.; Moridis, G. J.; Seol, Y.; Freifeld, B. M.; Taylor, C. E.; Gupta, A., Methane hydrate formation and dissociation in a partially saturated core-scale sand sample. J. Pet. Sci. Technol. 2007, 56, 108-126. (27) Kang, S. P.; Lee, J. W., Formation characteristics of synthesized natural gas hydrates in mesoand macroporous Silica Gels. J. Phys. Chem. B 2010, 114, 6973-6978. (28) Li, X. S.; Zhang, Y., Study on dissociation behaviors of methane hydrate in porous media based on experiments and fractional dimension shrinking-core model. Ind. Eng. Chem. Res. 2011, 50, 8263-8271. (29) Linga, P.; Haligva, C.; Nam, S. C.; Ripmeester, J. A.; Englezos, P., Gas hydrate formation in a variable volume bed of silica sand particles. Energy Fuels 2009, 23, 5496-5507. (30) Linga, P.; Daraboina, N.; Ripmeester, J. A.; Englezos, P., Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel. Chem. Eng. Sci. 2012, 68, 617-623. (31) Zanjani, N. G.; Moghaddam, A. Z.; Nazari, K.; Mohammad-Taheri, M., Increasing the storage capacity and selectivity in the formation of natural gas hydrates using porous media. Chem. Eng. Technol. 2012, 35, 1973-1980. (32) Linga, P.; Kumar, R. N.; Englezos, P., Gas hydrate formation from hydrogen/carbon dioxide and


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nitrogen/carbon dioxide gas mixtures. Chem. Eng. Sci. 2007, 62, 4268-4276. (33) Englezos, P.; Hall, S., Phase equilibrium data on carbon dioxide hydrate in the presence of electrolytes, water soluble polymers and montmorillonite. Can. J. Chem. Eng. 1994, 72, 887-893. (34) Daraboina, N.; Linga, P., Experimental investigation of the effect of poly-N-vinyl pyrrolidone (PVP) on methane/propane clathrates using a new contact mode. Chem. Eng. Sci. 2013, 93, 387-394. (35) Daraboina, N.; Linga, P.; Ripmeester, J.; Walker, V. K.; Englezos, P., Natural gas hydrate formation and decomposition in the presence of kinetic inhibitors. 2. stirred reactor experiments. Energy Fuels 2011, 25, 4384-4391. (36) Smith, J. M.; Van ness, H. C.; Abbott, M. W., Introduction to Chemical Engineering Thermodynamics. Mcgraw-Hill, Inc.: New York, 2001. (37) Sloan, E. D.; Koh, C. A., Clathrate hydrates of natural gases. 3rd ed.; CRC Press: 2008. (38) Linga, P.; Adeyemo, A.; Englezos, P., Medium-pressure clathrate hydrate/membrane hybrid process for postcombustion capture of carbon dioxide. Environ. Sci. Technol. 2007, 42, 315-320. (39) Daraboina, N.; Ripmeester, J.; Englezos, P., The impact of SO2 on post combustion carbon dioxide capture in bed of silica sand through hydrate formation. Int. J. Greenh. Gas Con. 2013, 15, 97-103.


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Page 22 of 30

Table 1. Experimental Conditions along with Measured Induction Times for Gas Hydrate Formed from the 30 mol% CH4/N2 Gas Mixture a.

a

Exp. No

Solution (cm3)

THF (mol%)

Solution state

Texp (K)

Pexp (MPa)

∆P b (MPa)

Induction time (min)

Running time (h)

1

140

0

fresh

273.65

9.0

2.1

no hydrate

96.0

2

140

no hydrate

96.0

3

140

721.0

88.7

4

140

memory

158.7

84.8

5

78.7

fresh

2640.0

87.4

6

78.7

memory

31.3

64.3

7

140

14.0

36.3

8

140

memory

8.7

43.5

9

140

fresh

6.0

38.9

10

140

memory

5.1

26.2

11

20

9.5

12.0

12

20

memory

2.0

8.0

13

20

fresh

7.1

8.1

14

20

memory

1.5

12.2

fresh 0

1.0

1.0

fresh

fresh

fresh

273.65

276.05

276.05

10.0

3.4

3.4

3.1

3.1

3.1

Standard uncertainties u are u(T) = 0.1 K, u(P) = 11 kPa. Experiments 1-10 were carried

out in a fixed bed of silica sand particles and experiments 11-14 were carried out in a small scale stirred vessel. b

Driving force ∆P = Pexp - Peq, Pexp was the pressure of the gas phase at the start of the

experiments.


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Energy & Fuels

Table 2. Experimental Results for Gas Hydrate Formed from the 30 mol% CH4/N2 Gas Mixture in Silica Sand Particles and in a Stirred Vessel a.

a

Exp. No

THF (mol%)

∆P (MPa)

1

0

2.1

Final gas uptake Uncertainty CH4 content in CH4 recovery (mol of gas / mol (mol of gas / mol gas phase, xCH4 (%) of water) of water) (mol%)

S.F.

Water conversion to hydrate (mol%)

-

-

29.2

-

-

-

-

-

29.1

-

-

-

0.0288

0.0020

27.6

12.6

21.1

17.6

4

0.0314

0.0021

27.7

12.9

7.9

19.2

5

0.0250

0.0017

27.8

11.1

25.4

15.3

6

0.0276

0.0019

27.8

11.5

12.8

16.8

0.0128

0.0011

24.8

21.4

18.0

25.6

8

0.0115

0.0010

24.6

21.4

60.5

23.0

9

0.0140

0.0012

24.5

22.0

41.3

28.0

10

0.0122

0.0010

25.2

20.9

23.3

24.4

0.0056

0.0005

25.9

21.2

7.3

11.2

12

0.0053

0.0004

25.3

21.6

10.0

10.6

13

0.0045

0.0004

26.1

20.3

7.2

9.0

14

0.0060

0.0005

26.0

22.2

4.9

12.0

2 3

0

7

1.0

11

1.0

a

3.1

3.1

3.1

Standard uncertainties u are u(T) = 0.1 K, u(P) = 11 kPa, and u(xCH4) = 0.1%.


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14 12 10

P (MPa)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

30 mol% CH4 / N2

8

[37]

Predicted by CSMHYD Liquid water (0 mol% THF) 1.0 mol% THF

6 4 2 0 273

275

277

279

281

283

285

T (K)

Figure 1. Incipient equilibrium conditions for gas hydrate formed from the 30 mol% CH4/N2 gas mixture in liquid water and in the presence of THF.


Page 25 of 30

0.035

274.5

Gas uptake T1

Experiment 3

T3 Nucleation point

274.0

0.025 0.020 0.015

273.5 0.010 0.005

tind = 721 min 273.0 0

10

20

30

Gas uptake (mol of gas / mol of water)

0.030

T2 Temperature (K)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

50

60

70

0.000 80

Time (h)

Figure 2. Temperature and gas uptake profiles for hydrate formation from the 30 mol% CH4/N2 gas mixture in a bed of water-saturated silica sand particles at 273.65 K and 10.0 MPa.


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274.5

0.035

Experiment 5 (CR+CC1)

0.030

T1 T2

Nucleation point

0.025

T3

274.0

0.020 0.015 273.5 0.010 0.005

tind = 2640 min 273.0 0

10

20

30

40

50

60


Gas uptake

Temperature (K)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

70

0.000 80

Time (h)

Figure 3. Temperature and gas uptake profiles for hydrate formation from the 30 mol% CH4/N2 gas mixture in the small bed of water-saturated silica sand particles (CR+CC1) at 273.65 K and 10.0 MPa.


Page 27 of 30

0.005

0.004

Rate of gas consumpiton (mol / mol of water / h)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.003

CR CR + CC1

(Experiment 3) (Experiment 5)

0.002

0.001

0.000 0

10

20

30

40

50

60

70

Time (h)

Figure 4. Comparison of rate of hydrate formation from the 30 mol% CH4/N2 gas in a fixed bed of silica sand. Time zero in the plot corresponds to the nucleation point (induction time) for the experiments.


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0.015

278.0

Experiment 8

T2 T3

277.0

0.010

Nucleation point tind = 8.7 min

276.5

0.005

276.0


Gas uptake T1

277.5

Temperature (K)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

275.5

275.0

0.000 0

10

20

30

40

Time (h)

Figure 5. Temperature and gas uptake profiles for gas hydrate formation form the 30 mol% CH4/N2 gas mixture in silica sand particles at 276.05 K and 3.4 MPa and in the presence of THF (1 mol% THF).


Page 29 of 30

0.03


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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Silica sand (liquid water, experiment 3) Silica sand (1 mol% THF, experiment 7) Stirred vessel (1 mol% THF, experiment 11) 0.02

0.01

0.00 0

5

10

15

20

25

30

35

Time (h)

Figure 6. Comparison of gas uptake for hydrate formation from the 30 mol% CH4/N2 gas in the bed of silica sand and in the stirred reactor.


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0.0035 0.0030

Rate of gas consumpiton (mol / mol of water / h)

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 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

0.0025

Silica sand (experiment 3) Silica sand (experiment 7) Stirred vessel (experiment 11)

0.0020 0.0015 0.0010 0.0005 0.0000 0

5

10

15

20

25

30

Time (h)

Figure 7. Comparison of rate of hydrate formation between the bed of silica sand and the stirred reactor. Time zero in the plot corresponds to the nucleation point (induction time) for the experiments.


Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed

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