Adsorption–Hydrate Hybrid Process for Methane Separation from a

Sep 16, 2014 - 1. INTRODUCTION. Coal-mine methane (CMM) gas is a mixture of CH4, N2, and. O2 that is released during the process of coal mining. It is...
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Adsorption−Hydrate Hybrid Process for Methane Separation from a CH4/N2/O2 Gas Mixture Using Pulverized Coal Particles Dong-Liang Zhong,†,‡ Dong-Jun Sun,‡ Yi-Yu Lu,*,† Jin Yan,† and Jia-Le Wang‡ †

State Key Laboratory of Coal Mine Disaster Dynamics and Control and ‡College of Power Engineering, Chongqing University, Chongqing 400044, China ABSTRACT: In the present work, an adsorption−hydrate hybrid process was employed to enhance the separation of CH4 from a 30 mol % CH4/60 mol % N2/O2 gas mixture in the presence of pulverized coal particles. The incipient hydrate equilibrium conditions for gas hydrates formed from this CH4/N2/O2 gas mixture were measured in the presence of 1.0 mol % tetrahydrofuran (THF) and reported. Then, the characteristics of CH4 separation efficiency, gas uptake, and rate of gas consumption were studied at different saturations of the coal particles in the presence of 1.0 mol % THF. The experiments were carried out in batch operation with the initial pressure fixed at 3.6 MPa. The temperature was fixed at 277.1 K. The results indicated that hydrate formation is induced immediately by gas adsorption in the coal particles. Unsaturated coal particles perform better than saturated coal particles for CH4 separation from the CH4/N2/O2 gas mixture during the adsorption−hydrate hybrid process. The gas uptake and rate of gas consumption increased greatly as the saturation of the coal particles was reduced from 100% to 40%. In addition, the gas uptake obtained in the presence of pulverized coal particles was significantly increased as compared to those obtained in stirred reactors containing TBAB and CP/SDS. The CH4 recovery obtained at 277.1 K, 3.6 MPa, and 40% saturation of coal particles was about 33.5%, which was higher than those obtained in the presence of TBAB (26.2%) and CP/SDS (33.1%).

1. INTRODUCTION Coal-mine methane (CMM) gas is a mixture of CH4, N2, and O2 that is released during the process of coal mining. It is an unconventional natural gas, and its reserves on the globe are estimated to be 260 × 1012 m3, so it is considered as a potential worldwide energy resource.1 Problems with CMM gas utilization will be encountered when the methane concentration in the gas mixture is below 30 mol %. However, CH4 cannot be vented to the atmosphere because it is a greenhouse gas whose greenhouse effect is 21 times stronger than that of CO2. On the other hand, CMM gas must be removed from coal mines because explosions will occur if the CH4 concentration is in the range of 5−15%. It is therefore of great importance to recover CMM gas from underground coal mines. Because of the low combustion efficiency of CMM gas containing low CH4 concentrations, an alternative method for utilizing the CMM resource is to separate CH4 from the CMM gas mixture. As a result, separation techniques such as pressure swing adsorption (PSA), cryogenic liquefaction, and membrane technology have been developed for CH4 recovery from low-concentration CMM gas, but their major drawbacks are low separation efficiencies and huge energy costs.2,3 Therefore, it is necessary to improve the current separation techniques and develop novel separation methods. As a contribution to these efforts, in this work, we studied the hydrate-based separation process for CH4 separation from CH4/N2/O2 gas mixtures in the presence of porous media. Gas hydrates are icelike crystalline inclusion compounds formed by water and a number of small molecules (CH4, C2H6, CO2, etc.) under suitable temperature and pressure conditions.4 When gas hydrates are formed from a mixture of gases, the concentrations of these gases in the hydrate crystals are different from the concentrations in the original gas mixture. © 2014 American Chemical Society

This is the basis for the utilization of gas hydrate formation/ dissociation as a separation process. This hydrate-based separation process has been extensively studied for CO2 capture from flue gas (CO2/N2)5,6 and fuel gas (CO2/H2)7 because the CO2 capture efficiency in this process is much higher than those in conventional separation processes (chemical absorption, physical absorption, membranes, etc.).8 Recently, Zhang and Wu9 reported the feasibility of using the hydrate-based separation process for CH4 separation from a simulated CMM gas (CH4/N2/O2). They formed gas hydrates from a CH4/N2/O2 gas mixture (26 mol % CH4) in a stirred reactor and found the preferential incorporation of CH4 molecules into the hydrate phase compared to N2 and O2 molecules. However, the limitations that prevent this novel technology from being scaled up from the laboratory to the industrial level are the severe thermodynamic conditions required and the slow hydrate formation kinetics. One possible solution is to use chemical additives such as thermodynamic and kinetic promoters. Tetrahydrofuran (THF) and tetra-nbutyl ammonium bromide (TBAB) are well-known thermodynamic promoters for gas hydrate formation10−12 and have been successfully used to reduce the phase equilibrium conditions for gas hydrates formed from CH4/N2/O2 mixtures.9,13 However, it was reported that the CH4 recovery obtained in the presence of these promoters is considerably low.14 Therefore, the kinetics of gas hydrate formation from CH4/N2/O2 gas Received: Revised: Accepted: Published: 15738

July 6, 2014 September 8, 2014 September 16, 2014 September 16, 2014 dx.doi.org/10.1021/ie502684j | Ind. Eng. Chem. Res. 2014, 53, 15738−15746

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mixtures must be studied so as to improve our understanding of how to increase the CH4 separation efficiency. The use of porous media has been found to be efficient in enhancing hydrate formation as compared to the use of stirred reactors.15,16 Linga et al.17 compared the performance of natural gas hydrate formation in a fixed bed of silica sand and in a stirred vessel. They found that the rate of hydrate formation in the fixed bed of silica sand was significantly increased. Park et al.18 investigated the feasibility of using the hydrate-based process for CO2 capture from flue gas in the presence of porous silica gels and THF. They concluded that the use of silica gels results in increased gas consumption and a higher conversion of water to hydrates. Zhong et al.19 reported a comparison of CH4 separation from a 30 mol % CH4/N2 gas mixture by hydrate formation in a stirred reactor and in a fixed bed of silica sand in the presence of 1.0 mol % THF. They concluded that the gas uptake in the fixed bed of silica sand was greatly increased, but the CH4 recovery (∼21.4%) obtained in the fixed bed of silica sand was the same as that obtained in the stirred reactor. Porous media such as silica sand and silica gels are commonly used for the promotion of hydrate formation, but the results obtained in these systems in terms of CH4 recovery and gas uptake are not satisfactory for CH4/N2/O2 gas mixtures. Zhang et al.20 employed an adsorption-based hybrid method for the separation of CO2/CH4 gas mixtures using activated carbon as the adsorbent. They found that the separation performance was greatly influenced by the water content in the activated carbon and the initial gas−solid ratio. It is well-known that the adsorption ability of CH4 is stronger than those of N2 and O2 in activated carbons and coals.21,22 In this work, we employed pulverized coal particles as novel porous media for gas hydrate formation and examined the hydrate-based process for CH4 separation from a model CMM gas mixture (30 mol % CH4, 60 mol % N2, and 10 mol % O2). The purpose of this work was to investigate the kinetics of CH4 separation from a CMM gas mixture by hydrate formation in saturated and unsaturated coal particles in the presence of 1.0 mol % THF. The characteristics of CH4 separation efficiency, gas uptake, and rate of gas consumption were studied at different saturations during the adsorption−hydrate hybrid process. The adsorption behaviors of saturated and unsaturated coal particles were also studied. New data reported in this work are of great importance in helping elucidate how to improve hydrate-based separation processes for CH4 recovery from low-concentration CMM gas mixtures.

Table 1. Properties of Pulverized Coal Particles

a

property

value

pore diameter (nm) particle size distribution (mm) pore volume (cm3/g) surface area (m2/g) density (g/cm3) water saturationa (cm3/g)

6.4 0.5−3.0 0.0063 3.933 0.941 0.281

Amount of water needed to saturate the pulverized coal particles.

which is the amount of water needed to completely fill the interstitial or pore volume of the coal particles. This result is also included in Table 1. Deionized water was used in all experimental runs. 2.2. Experimental Apparatus. The experimental apparatus is illustrated in Figure 1. The stainless-steel reactor was a 600 cm3 vessel immersed in a temperature-controlled water bath. An adjustable-speed electromagnetic stirrer was inserted into the reactor for solution agitation and was removed for hydrate formation in a fixed bed of porous media. Two platinum resistance thermometers with an uncertainty of 0.1 K were used to measure the gas and liquid temperatures. A pressure transducer (Yokogawa Electric Corporation, Tokyo, Japan) with an uncertainty of 0.06% in the range of 0−10 MPa was used to measure the pressure of the gas phase. A gas chromatograph (GC-2014, Shimadzu Corporation, Kyoto, Japan) with an uncertainty of 0.1 mol % was used to determine the composition of the gas mixture at the end of each experiment. 2.3. Experimental Procedure for Hydrate Formation. The incipient phase equilibrium conditions for gas hydrates formed form CMM gas in the presence of THF were measured using the isochoric step-heating method. Kinetic investigations of gas hydrate formation in the presence of THF indicated that 1.0 mol % THF is an optimum concentration for the separation of gas mixtures (CO2/N2, and CO2/H2) as compared to other THF concentrations.24,25 As a result, 1.0 mol % THF was selected for the thermodynamic and kinetic experiments in this study. A detailed description of the experimental procedure is available in the literature.13,26 Briefly, gas hydrates were initially formed at a given pressure and temperature in 1.0 mol % THF solution. Then, the formed gas hydrates were decomposed at a heating rate of 0.1 K every 4 h. When the very small amount of hydrate crystals disappeared after stepwise heating in the reactor, the temperature and pressure in the reactor at this moment were determined as the incipient hydrate equilibrium dissociation point. Kinetic experiments in this study were carried out in batch operation on the basis of the measured incipient phase equilibrium conditions. The experimental procedure was as follows: Prior to the experiments, the reactor was cleaned with deionized water and dried. Then, 320 g of pulverized coal particles was placed in the crystallizer, and these particles were saturated completely or partially with the prepared 1.0 mol % THF solution. The saturated and unsaturated coal particles were used separately in this work for CH4 separation from the CH4/N2/O2 gas mixture. The reactor and its tubing were purged three times with the CH4/N2/O2 gas mixture to flush out the air remaining in the system. Once the temperature and pressure in the reactor had reached the desired values, the crystallizer was isolated from the gas cylinder by closing the inlet and outlet valves. This was considered to be time zero for

2. EXPERIMENTAL SECTION 2.1. Materials. A gas mixture with a composition of 30 mol % CH4, 60 mol % N2, and 10 mol % O2 was supplied by Chongqing Rising Gas with a reported uncertainty in the composition of ±0.05 mol %. This gas composition was used to simulate the typical low-concentration CMM gas recovered from underground coal mines. THF was purchased from Chongqing Oriental Chemical Co., Ltd., with a certified mass purity higher than 99%. Pulverized coal particles with a density of 0.941 g/cm3 were purchased from Chongqing Zhongliangshan Mining Company. Properties such as pore diameter and surface area were measured using a surface area and porosity analyzer (Micromeritics ASAP 2010, Norcross, GA) and are reported in Table 1. The surface area and pore diameter were determined based on Brunauer−Emmett−Teller (BET) analysis. The water saturation of the coal particles was determined using the method reported in the literature,23 15739

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Figure 1. Schematic of the experimental apparatus for CH4 separation from a CH4/N2/O2 gas mixture by hydrate formation in a fixed bed of pulverized coal particles.

the experiments of hydrate formation. The temperature and pressure were recorded on a data acquisition unit (Agilent 34970A) and logged into a computer every 10 s. The experiments were stopped when no pressure drop in the crystallizer was observed. The compositions of the gas mixtures remaining in the vessel and decomposed from the gas hydrates at the end of the experiments were analyzed by gas chromatography (GC). To obtain reproducible results, three parallel experiments were carried out under each set of experimental conditions. 2.4. Calculation of the Amount of Gas Mixture Consumed. Based on the compositions of the gas mixture at the beginning and end of the experiments, the number of moles of the gas mixture consumed during the adsorption−hydrate hybrid process was calculated as ΔnH = ng,0 − ng, t

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

conversion (mol %) (ΔnH − Δn THF) × hydration number = × 100 n H 2O

where ΔnTHF is number of moles of THF consumed for hydrate formation and the hydration number of 5.67 was used for structure II hydrates formed from the CH4/N2/O2 gas mixture.4 The consumption of the gas mixture in this study was caused by the adsorption of the gas mixture and the incorporation of the gas mixture into the hydrate phase. The rate of gas consumption was calculated using the forward difference method as ΔnH, t +Δt − ΔnH, t ⎛ dΔnH ⎞ ⎜ ⎟ = , ⎝ dt ⎠ t Δt

Pr P + ωB1 r Tr Tr

⎡ ⎢ R av = ⎢ ⎢ ⎣

ΔnH n H 2O

(5)

dΔnH dt 1

dΔnH dt 2

( ) +( )

m

+ ··· +

dΔnH dt m

( )

⎤ ⎥ ⎥, ⎥ ⎦

m = 180 (6)

It should be noted that the rate of hydrate formation is customarily considered to be the rate of gas consumption after the occurrence of hydrate nucleation and can be calculated using eqs 5 and 6 as well. 2.5. CH4 Recovery and Efficiency. The split fraction and separation factor are two parameters proposed by Linga et al.28 to assess the separation efficiency of a gas mixture by gas hydrate formation. In this work, the CH4 recovery or split fraction (R) of CH4 was calculated as follows

(2)

where the equations of Abbott were used for B0 and B1. The gas uptake was normalized by considering the amount of the water used in the experiment and was calculated as follows ΔnH,normalized =

Δt = 10 s

The averages of these rates (Rav) were calculated every 30 min and reported as

(1)

where ng,0 and ng,t represent the numbers of moles of the gas mixture in the crystallizer at time 0 and time t, P is the pressure in the crystallizer, T is the temperature of the gas phase, V is the volume of the gas phase, and Z is the compressibility factor calculated by the Pitzer correlation for the second virial coefficient27 Z = 1 + B0

(4)

R=

(3)

where ΔnH is the number of moles of the gas mixture consumed at the end of the experiment and nH2O is the number of moles of water used in the experiment. In the presence of THF, the conversion of water to hydrate was calculated as follows

H nCH 4 feed nCH 4

× 100% (7)

nfeed CH4

where is the number of moles of CH4 supplied to the crystallizer and nHCH4 is the number of moles of CH4 trapped in the hydrate crystals at the end of the experiments. Three gas components (CH4, N2, and O2) are included in lowconcentration CMM gas, and the hydrate formed from CMM 15740

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Table 3. Experimental Conditionsa and Measured Induction Times for CH4 Separation from a CH4/N2/O2 Gas Mixtureb by Hydrate Formation in the Presence of Pulverized Coal Particles and THF

gas is not able to distinguish between N2 and O2 because the phase equilibrium pressure for O2 hydrate is very close to that of N2 hydrate. Therefore, the separation factor (S) in this work is defined by the equation S=

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

ngas CH4,

ngas N2 ,

(8)

ngas O2

where and are the numbers of moles of CH4, N2, and O2, respectively, in the gas phase at the end of the experiments. The symbols nHN2 and nHO2 are the numbers of moles of N2 and O2, respectively, incorporated in the hydrate crystals at the end of the experiments.

3. RESULTS AND DISCUSSION The incipient equilibrium data of gas hydrates formed from a 30 mol % CH4/60 mol % N2/O2 gas mixture in the presence of THF are not available in the literature. Table 2 reports the

expt no.

liquid water (cm3)

saturation (%)

xgas CH4 (mol %)

xHCH4 (mol %)

tind (min)

1 2 3 4 5 6 7 8 9 10 11 12

90 90 90 72 72 72 54 54 54 36 36 36

100 100 100 80 80 80 60 60 60 40 40 40

23.2 23.7 23.6 24.6 25.1 24.9 24.2 24.1 23.9 24.0 23.7 23.8

44.0 43.5 44.3 45.3 45.2 44.9 46.5 46.6 45.7 45.3 45.0 45.6

2.5 2.5 2.7 2.0 2.2 2.0 1.0 1.2 1.0 0.7 0.7 0.8

a

Constant conditions: Texp = 277.1 K, Pexp = 3.6 MPa, THF content = 1.0 mol %, loading of coal particles = 320 g. Standard uncertainties u: u(T) = 0.1 K, u(P) = 6 kPa. bGas composition: 30 mol % CH4, 60 mol % N2, and 10 mol % O2.

Table 2. Incipient Equilibrium Conditions for Gas Hydrates Formed from a CH4/N2/O2 Gas Mixturea in the Presence of THF liquid waterb

a b

THF (1.0 mol %)

Teq (K)

Peq (MPa)

T (K)

P (MPa)

277.1 284.6 286.1 287.9 288.4 289.1

9.55 23.14 27.71 34.28 36.40 39.50

277.1 284.6 286.1 287.9 288.4 289.1

0.35 1.58 2.0 2.58 3.08 3.68

%). This result indicates the preferential incorporation of CH4 into the hydrate phase compared to N2 and O2 in the presence of 1.0 mol % THF. Thus, the hydrate-based separation process employing pulverized coal particles can be used as a potential technology for CH4 recovery from CMM gas mixtures. As reported in Table 3, the average values of the induction times corresponding to 100%, 80%, 60%, and 40% water saturation of the pulverized coal particles were 2.6, 2.1, 1.1, and 0.7 min, respectively. This indicates that hydrate nucleation occurred faster when the saturation of the coal particles was reduced. Therefore, the duration of hydrate formation in the presence of pulverized coal particles might be shortened at the low water saturations. Table 4 shows the results obtained in the kinetic experiments, including the rate of gas consumption, final gas uptake, standard uncertainties in the final gas uptake, water conversion to hydrate, CH4 recovery (R), and separation factor (S). As can be seen, r30 min indicates the rate of gas consumption at 30 min after the initiation of the experiments (mol/h). The final gas uptake was normalized by considering the amount of water used in the experiments (mol of gas/mol of water) and calculated by eq 3. The combined standard uncertainties in the final gas uptake were calculated according to the uncertainties in P, T, V, and Z. Water conversion to hydrate was calculated by eq 4. CH4 recovery (R) and separation factor (S) were calculated using eqs 5 and 6, respectively. Figure 2 shows typical temperature and gas uptake profiles for the experiment performed in the presence of saturated pulverized coal particles (experiment 1 in Table 3). As shown in Figure 2a, the first temperature spike indicates the onset of hydrate nucleation due to the exothermic nature of hydrate formation. The time corresponding to this temperature spike was identified as the induction time. Figure 2b is an enlarged plot of Figure 2a during the first 40 min after the initiation of the experiment. It can be seen clearly in Figure 2b that the temperature began to increase at 2.5 min, and this time was determined as the induction time. Meanwhile, it can be observed in Figure 2a that the gas uptake started to increase rapidly at the nucleation point, indicating the beginning of gas hydrate growth. Interestingly, small temperature spikes were

Gas composition: 30 mol % CH4, 60 mol % N2, and 10 mol % O2. Predicted by the Chen−Guo model.13

measured incipient equilibrium conditions for gas hydrates formed from this gas mixture in the presence of 1.0 mol % THF. As can be seen, the incipient hydrate equilibrium pressure at a given temperature was significantly reduced as compared to that obtained in liquid water using the same gas mixture. This result confirms that THF can be used as an effective thermodynamic promoter to reduce the hydrate equilibrium conditions for the CH4/N2/O2 gas mixture. The incipient equilibrium pressure (Peq) obtained in the presence of 1.0 mol % THF was 0.35 MPa at 277.1 K, and subsequent kinetic experiments were performed based on this equilibrium point. Table 3 presents the experimental conditions for CH4 separation from the CH4/N2/O2 gas mixture by hydrate formation in a fixed bed of pulverized coal particles. The experiments were carried out at 277.1 K and 3.6 MPa in batch operation. The driving force (overpressure in this work, ΔP = Pexp − Peq) obtained based on the initial experimental pressure was 3.25 MPa at 277.1 K. Saturated (100% saturation) and unsaturated (80%, 60%, and 40% saturation) pulverized coal particles were used to examine the characteristics of CH4 separation from the CH4/N2/O2 gas mixture by hydrate formation in the presence of 1.0 mol % THF. Table 3 also reports the CH4 concentrations remaining in the reactor (xgas CH4) H and decomposed from the hydrates (xCH4) at the end of the experiment, as well as the induction time of hydrate nucleation H (tind). It can be seen that xgas CH4 was lower and xCH4 was higher than the CH4 concentration in the original gas mixture (30 mol 15741

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Table 4. Experimental Results for CH4 Separation from a CH4/N2/O2 Gas Mixturea by Hydrate Formation in the Presence of Pulverized Coal Particles and THF

a

expt no.

saturation (%)

r30 min (mol/h)

final gas uptake (mol of gas/mol of water)

uncertainty (mol of gas/mol of water)

water conversion to hydrate (mol %)

R (%)

S

1 2 3 4 5 6 7 8 9 10 11 12

100 100 100 80 80 80 60 60 60 40 40 40

0.0177 0.0169 0.0170 0.0192 0.0186 0.0188 0.0272 0.0273 0.0273 0.0303 0.0505 0.0394

0.0167 0.0161 0.0163 0.0193 0.0178 0.0182 0.0208 0.0228 0.0244 0.0314 0.0313 0.0313

0.0012 0.0011 0.0011 0.0013 0.0012 0.0013 0.0015 0.0016 0.0017 0.0022 0.0022 0.0022

40.2 39.9 39.9 41.7 40.9 41.2 42.6 43.7 44.6 48.5 48.6 48.5

38.9 37.5 37.9 32.4 31.5 32.2 32.3 33.5 34.9 32.7 33.9 33.9

4.4 4.4 4.5 4.1 3.5 4.0 5.1 4.7 4.9 5.8 6.6 6.5

Gas composition: 30 mol % CH4, 60 mol % N2, and 10 mol % O2. Standard uncertainties u: u(T) = 0.1 K, u(P) = 10 kPa.

confirmed by the rate of gas consumption obtained at 30 min. As can be seen in Table 3, the average value of r30 min at 80% saturation was 0.019 mol/h, which was higher than that obtained at 100% saturation (0.017 mol/h). Figure 4 shows typical temperature and gas uptake profiles for the experiment carried out in the presence of unsaturated pulverized coal particles with 40% saturation (experiment 10 in Table 3). As can be seen in the figure, only one temperature spike was observed during the hydrate formation process, and this process was shorter than those occurring at 80% and 100% saturations. For example, the gas uptake began to turn flat at 5 h (0.028 mol of gas/mol of water, 90% of the total gas uptake), whereas it reached a plateau at 12.5 h for 80% saturation (90% of the total gas uptake in Figure 3) and at 21 h for 100% saturation (90% of the total gas uptake in Figure 2). One reason for this difference is that the rate of hydrate growth was higher at 40% saturation because more gas was absorbed before hydrate nucleation. Another reason is probably that less water was used at 40% saturation as compared to other saturations. Thus, the hydrate formation process was shortened with the reduction of the water saturation. It was also observed that the temperature spike at the nucleation point increased significantly from 277.1 to 281.5 K and the intensity of this temperature spike was much higher than those observed in the pulverized coal particles with 100% and 80% saturations. This is probably because more gas was absorbed at 40% saturation before hydrate nucleation, hence resulting in the formation of more gas hydrates at the nucleation point. The adsorption behaviors between saturated and unsaturated coal particles were compared under the experimental conditions of 277.1 K and 3.6 MPa. For these experiments, 320 g of pulverized coal particles was fully or partially saturated by liquid water in the absence of THF, ensuring that no gas hydrates were formed with the 30 mol % CH4/60 mol % N2/ O2 gas mixture under the experimental conditions. The results are presented in Figure 5. As can be seen in Figure 5a, the gas uptake increased quickly at the beginning of the experiment and began to increase slowly 6 h later (90% of the total gas uptake). Also, the amount of the gas mixture adsorbed in the unsaturated coal particles (1.08 × 10−4 mol of gas/g of coal at 40% saturation) was 1.3 times larger than that adsorbed in the saturated coal particles (0.85 × 10−4 mol of gas/g of coal at 100% saturation). This is probably because more void spaces such as the micropores and interstitial spaces between coal

also observed between 7.5 and 20 h (seen in Figure 2a), and the gas uptake continued to increase rapidly during this period. At the same time, as indicated by the arrows in Figure 2c, sudden increases in the rates of hydrate formation were observed to accompany these temperature spikes. This is evidence of multiple hydrate nucleation occurring in the fixed bed of pulverized coal particles. It should be noted that the phenomenon of multiple hydrate nucleation has also been observed in other porous media systems such as silica sand and silica gels, and the gas consumption is normally increased because of the multiple hydrate nucleation.29 As can be seen in Figure 2a, the gas uptake began to increase slowly at 21 h (0.0152 mol of gas/mol of water, 90% of the total gas uptake) and reached a plateau at the end of the experiment. The temperature curve was observed to turn flat at the same time. This indicates the completion of hydrate growth in the saturated pulverized coal particles. Note that the temperature and gas uptake profiles obtained in experiments 2 and 3 exhibited the same trend as those obtained in experiment 1. We also carried out experiments in the presence of unsaturated coal particles (80%, 60%, and 40% saturation) and investigated the influence of the unsaturation of the coal particles on hydrate formation from the CH4/N2/O2 gas mixture. Figure 3a shows the temperature and gas uptake profiles for the experiment carried out at 80% saturation of the coal particles (experiment 5 in Table 3). Figure 3b is an enlarged plot of Figure 3a during the first 120 min after the initiation of the experiment. As for the saturated pulverized coal particles (Figure 2), the first sharp temperature spike indicates the onset of hydrate nucleation, and the gas uptake was observed to increase rapidly at this nucleation point (Figure 3b). The induction time was 2.2 min, as reported in Table 3. Multiple temperature peaks can also be clearly seen between 8 and 12 h, and the gas uptake continued to increase rapidly during this period (Figure 3a). This indicates that multiple nucleation of gas hydrates also occurred in the unsaturated coal particles and agrees with what was observed in the presence of saturated coal particles. Interestingly, the intensity of the temperature spike at the nucleation point (2.2 min) was observed to be higher than that observed for the saturated pulverized coal particles (Figure 2). This is probably because more methane was adsorbed in the unsaturated coal particles, resulting in a higher rate of hydrate formation, and therefore, a greater amount of gas hydrates formed. This explanation is 15742

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Figure 3. Temperature and gas uptake profiles for hydrate formation in a fixed bed of unsaturated pulverized coal particles (80% saturation) in the presence of 1.0 mol % THF. The experiment was performed at 277.1 K and 3.6 MPa. (a) Temperature and gas uptake curves obtained between 0 and 18 h. (b) Enlarged temperature and gas uptake curves between 0 and 120 min after the initiation of the experiment.

Figure 2. Profiles of temperature, gas uptake, and rate of gas consumption obtained in a fixed bed of saturated coal particles in the presence of 1.0 mol % THF. The experiment was performed at 277.1 K and 3.6 MPa. (a) Temperature and gas uptake curves obtained between 0 and 40 h. (b) Enlarged temperature and gas uptake curves during the first 40 min after the initiation of the experiment. (c) Rate of gas consumption obtained between 0 and 25 h.

Figure 4. Temperature and gas uptake profiles for hydrate formation in a fixed bed of unsaturated pulverized coal particles (40% saturation) in the presence of 1.0 mol % THF. The experiment was performed at 277.1 K and 3.6 MPa.

particles were available for gas adsorption when less water filled the coal particles. As a result, more of the gas mixture was adsorbed on the coal surfaces or filled in the micropores. This also means that the concentration of the gas mixture at the gas−solid−liquid interfaces or in the vicinity of the interfaces was higher at 40% saturation than at 100% saturation. In Figure 5b, strong temperature spikes can be observed for both the saturated and unsaturated coal particles at the beginning of the adsorption process. This indicates the exothermic property of

gas adsorption in coal particles. However, the intensity of the temperature spike at 40% saturation was higher than that at 100% saturation, indicating that more of the gas mixture was adsorbed at 40% saturation and, thus, released more heat. Then, the temperature was restored to the set value of 277.1 K as the heat was removed by the coolant in the water bath. It should be noted that the difference in the gas uptakes at 40% 15743

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particles was 34.3% (experiments 1−12), which was greater than the amounts obtained in the systems of TBAB/H2O (26.2%) and cyclopentane (CP)/sodium dodecyl sulfate (SDS)/H2O (33.1%).14,30 This result indicates that the adsorption−hydrate hybrid process employing pulverized coal particles greatly enhanced the separation of CH4 from the CH4/N2/O2 gas mixture. Figure 6 shows a comparison of gas uptake obtained during the adsorption−hydrate hybrid process using saturated and

Figure 6. Comparison of the gas uptake obtained during the adsorption−hydrate hybrid process using saturated and unsaturated coal particles in the presence of 1.0 mol % THF. The experiments were performed at 277.1 K and 3.6 MPa. The CH4/N2/O2 gas mixture used was 30 mol % CH4, 60 mol % N2, and 10 mol % O2. Figure 5. (a) Gas uptake and (b) temperature profiles for the adsorption of a CH4/N2/O2 gas mixture in saturated and unsaturated pulverized coal particles. The experiments were performed at 277.1 K and 3.6 MPa.

unsaturated coal particles in the presence of 1.0 mol % THF. It can be seen that the gas uptake increased with decreasing water saturation in the coal particles. The average values for the final gas uptake obtained at 100%, 80%, 60%, and 40% saturation were 0.0164, 0.0184, 0.0227, and 0.0313 mol of gas/mol of water, respectively (Table 4). It should be noted that the gas uptake is composed of two parts, namely, the adsorption of the gas mixture in the coal particles and the incorporation of the gas mixture into the hydrate phase. Therefore, the consumption of the gas mixture during the adsorption−hydrate hybrid process can be divided into two stages. The first stage is from the beginning of the experiment to the hydrate nucleation point, which is dominated by gas adsorption. As can be seen in Figure 6, the gas uptake at the hydrate nucleation point was 0.004, 0.005, 0.008, and 0.016 mol of gas/mol of water for 100%, 80%, 60%, and 40% saturations, respectively. This is because the gas−solid contact area for gas adsorption was largest at 40% saturation among all of the water saturations used, so the gas uptake caused by adsorption was highest at 40% saturation. The second stage is from the hydrate nucleation point to the end of the experiment. Gas uptake in this stage is dominated by gas hydrate formation. As shown in Figure 6, the gas uptake in this stage was 0.0124, 0.0134, 0.0147, and 0.0153 mol of gas/ mol of water for 100%, 80%, 60%, and 40% saturations, respectively. This indicates that more hydrates were formed as the water saturation decreased. The hydrate formation mechanism at low water saturation might be that more gas molecules are adsorbed and accumulate at the gas−solid−liquid interface (Figure 5), and the water films created on the

and 100% saturations became larger at longer time durations, but no obvious temperature difference was observed during this period. One reason is that the pulverized coal particles at 40% water saturation can adsorb more gas than those at 100% saturation because of their larger gas−solid contact surfaces. Another reason is that the adsorption process proceeds slowly with the elapse of time, so that the adsorption heat is very small and cannot result in a sudden increase in the temperature. Interestingly, it was found that the temperature spikes caused by gas adsorption simultaneously occurred with the temperature spikes observed during the adsorption−hydrate formation process (seen in Figures 2 and 4). This result indicates that the gas adsorption by coal particles induced the nucleation of gas hydrates. It was calculated that the number of moles of CH4 adsorbed in the unsaturated coal particles (40% saturation) accounted for 54.1% of the total CH4 consumption during the adsorption−hydrate hybrid process, whereas the number of moles of CH4 adsorbed at 100% saturation accounted for 38.3% of the total number of moles of CH4 captured. Obviously, a larger amount of gas hydrates will be formed at the hydrate nucleation point when a greater number of moles of the gas mixture is adsorbed. This is why temperature spikes with higher intensities were observed at the hydrate nucleation point in the presence of unsaturated coal particles than in the presence of saturated coal particles (Figures 2−4). As can be seen in Table 4, the average CH4 recovery obtained in the presence of coal 15744

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transfer of gas molecules to the reaction interface from the gas phase. It was also observed that the rate of gas consumption at the four saturations decreased below 0.01 mol/h between 4 and 12 h and reached approximately zero at the end of the experiments. Because gas consumption after the occurrence of hydrate nucleation is dominated by hydrate formation, the observations between 4 and 12 h indicate the completion of gas hydrate growth. It should be noted that the growth of gas hydrates was interrupted by mass transfer at the end of the experiments and, hence, the completion of gas hydrate growth in this study does not mean the full conversion of liquid water to gas hydrates. Figure 8 shows a comparison of the CH4 recovery from the CH4/N2/O2 gas mixture and the final gas uptake obtained by

interstitial surfaces of coal particles are thinner at low water saturation compared to those at high water saturation. As a result, gas hydrates nucleate quickly at the gas−solid−liquid interfaces, and a higher conversion of the thinner water films to gas hydrates can be obtained at low water saturation. Therefore, a greater number of moles of the gas mixture are incorporated into the hydrate phase at low water saturation. In the case of high water saturation, the number of gas molecules adsorbed at the gas−solid−liquid interfaces is smaller than that at low water saturation (Figure 5). As a result, the rate of hydrate nucleation is lower than that at low water saturation. When the gas molecules adsorbed at the gas−solid−liquid interfaces are consumed for hydrate nucleation, the transfer of gas molecules from the gas phase to the reaction interface is needed to sustain the growth of gas hydrates. However, it is difficult for gas molecules to diffuse through hydrate films to the hydrate− liquid interfaces. Therefore, further conversion of thick water films into gas hydrates might be hindered. This is why the average water conversion to hydrate was reduced from 48.5 to 40 mol % as the water saturation increased from 40% to 100% (seen in Table 4). Figure 7 shows a comparison of the rate of gas consumption obtained during the adsorption−hydrate hybrid process in the

Figure 8. Comparison of the methane recovery from a CH4/N2/O2 gas mixture and the final gas uptake obtained by hydrate formation in the presence of TBAB, CP/SDS, and THF/coal particles.

hydrate formation in the systems of TBAB, CP/SDS, and THF/coal particles. In the present work, the unsaturated coal particles (40% saturation) were preferred to enhance CH4 separation from a CH4/N2/O2 gas mixture by gas hydrate formation at 277.1 K and 3.6 MPa. The average final gas uptake obtained under these experimental conditions was 0.0313 mol of gas/mol of water, which is 4.5 times larger than that obtained in the presence of TBAB14 (0.007 mol of gas/mol of water) and 3.5 times larger than that obtained in the presence of CP/ SDS (0.0088 mol of gas/mol of water).30 In addition, the CH4 recovery obtained under these conditions was 33.5%, which is also higher than those obtained in the presence of TBAB (26.2%) and CP/SDS (33.1%). The results presented in Figure 8 indicate that the separation of CH4 from a CH4/N2/O2 gas mixture is greatly enhanced by the adsorption−hydrate hybrid process employed in this study. Therefore, gas hydrate formation coupled with other chemical processes such as adsorption might become a promising method for increasing the CH4 separation efficiency from CH4/N2/O2 gas mixtures in the near future.

Figure 7. Comparison of the rates of gas consumption obtained in the presence of saturated and unsaturated coal particles. The experiments were performed at 277.1 K and 3.6 MPa. The CH4/N2/O2 gas mixture used was 30 mol % CH4, 60 mol % N2, and 10 mol % O2.

presence of 1.0 mol % THF. It can be seen that the rate of gas consumption decreased with increasing water saturation during the first 1.5 h. For instance, the average rate of gas consumption at 30 min was 0.04, 0.027, 0.019, and 0.017 mol/h for 40%, 60%, 80%, and 100% water saturations of the coal particles. One proper explanation for this result is that the concentration of the gas mixture adsorbed in the vicinity of the gas−solid− liquid interfaces was higher at low water saturation and, thus, a higher rate of hydrate nucleation was induced at low water saturation. During the period between 1.5 and 4 h, the rate of gas consumption at 40% saturation dropped dramatically as compared to the rates at other saturations. This can be explained by the fact that gas hydrates grew faster at 40% saturation during the first 1.5 h and, as a result, more gas hydrates formed and accumulated on the gas−solid−water interfaces. Then, the further growth of gas hydrates was hindered because the formed hydrate layers blocked the

4. CONCLUSIONS In this work, a novel adsorption−hydrate hybrid process was used to enhance CH4 separation from a 30 mol % CH4/60 mol % N2/O2 gas mixture in the presence of pulverized coal particles and 1.0 mol % THF. The characteristics of the CH4 separation efficiency, gas uptake, and rate of gas consumption were studied at different water saturations of the coal particles. It was found that hydrate formation was induced by gas 15745

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(13) 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. (14) Zhong, D. L.; Ye, Y.; Yang, C.; Bian, Y.; Ding, K. Experimental Investigation of Methane Separation from Low-Concentration Coal Mine Gas (CH4/N2/O2) by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Crystallization. Ind. Eng. Chem. Res. 2012, 51, 14806. (15) Babu, P.; Yee, D.; Linga, P.; Palmer, A.; Khoo, B. C.; Tan, T. S.; Rangsunvigit, P. Morphology of Methane Hydrate Formation in Porous Media. Energy Fuels 2013, 27, 3364. (16) Dicharry, C.; Duchateau, C.; Asbai, H.; Broseta, D.; Torre, J. P. Carbon Dioxide Gas Hydrate Crystallization in Porous Silica Gel Particles Partially Saturated with a Surfactant Solution. Chem. Eng. Sci. 2013, 98, 88. (17) 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. (18) Park, S.; Lee, S.; Lee, Y.; Lee, Y.; Seo, Y. Hydrate-Based PreCombustion Capture of Carbon Dioxide in the Presence of a Thermodynamic Promoter and Porous Silica Gels. Int. J. Greenhouse Gas Control 2013, 14, 193. (19) Zhong, D. L.; Daraboina, N.; Englezos, P. Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles. Energy Fuels 2013, 27, 4581. (20) Zhang, X. X.; Liu, H.; Sun, C. Y.; Xiao, P.; Liu, B.; Yang, L. Y.; Zhan, C. H.; Wang, X. Q.; Li, N.; Chen, G. J. Effect of Water Content on Separation of CO2/CH4 with Active Carbon by Adsorption− Hydration Hybrid Method. Sep. Purif. Technol. 2014, 130, 132. (21) Wu, J.; Zhou, L.; Sun, Y.; Su, W.; Zhou, Y. Measurement and Prediction of Adsorption Equilibrium for a H2/N2/CH4/CO2 Mixture. AlChE J. 2007, 53, 1178. (22) Ribeiro, R. P.; Sauer, T. P.; Lopes, F. V.; Moreira, R. F.; Grande, C. A.; Rodrigues, A. E. Adsorption of CO2, CH4, and N2 in Activated Carbon Honeycomb Monolith. J. Chem. Eng. Data 2008, 53, 2311. (23) 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. (24) 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. (25) Lee, H. J.; Lee, J. D.; Linga, P.; Englezos, P.; Kim, Y. S.; Lee, M. S.; Kim, Y. D. Gas Hydrate Formation Process for Pre-Combustion Capture of Carbon Dioxide. Energy 2010, 35, 2729. (26) Li, X. S.; Xia, Z. M.; Chen, Z. Y.; Yan, K. F.; Li, G.; Wu, H. J. Equilibrium Hydrate Formation Conditions for the Mixtures of CO2 + H2 + Tetrabutyl Ammonium Bromide. J. Chem. Eng. Data 2010, 55, 2180. (27) Smith, J. M.; Van ness, H. C.; Abbott, M. W. Introduction to Chemical Engineering Thermodynamics; McGraw-Hill, Inc.: New York, 2001. (28) Linga, P.; Kumar, R.; Englezos, P. The Clathrate Hydrate Process for Post and Pre-Combustion Capture of Carbon Dioxide. J. Hazard. Mater. 2007, 149, 625. (29) Babu, P.; Kumar, R.; Linga, P. A New Porous Material to Enhance the Kinetics of Clathrate Process: Application to Precombustion Carbon Dioxide Capture. Environ. Sci. Technol. 2013, 47, 13191. (30) Zhong, D. L.; Ding, K.; Yan, J.; Yang, C.; Sun, D. J. Influence of Cyclopentane and SDS on Methane Separation from Coal Mine Gas by Hydrate Crystallization. Energy Fuels 2013, 27, 7252.

adsorption in the coal particles and occurred immediately as the experiments were started. Unsaturated coal particles performed better than saturated coal particles for CH4 separation from the CH4/N2/O2 gas mixture. The gas uptake and rate of gas consumption were greatly increased as the saturation of coal particles was reduced from 100% to 40%. The gas uptake obtained in the presence of pulverized coal particles was significantly increased as compared to those obtained in stirred reactors using TBAB and CP/SDS. The CH4 recovery obtained at 277.1 K, 3.6 MPa, and 40% saturation of coal particles was around 33.5%, which is higher than those obtained in the presence of TBAB (26.2%) and CP/SDS (33.1%).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-23-65112372. Fax: +86-23-65106440. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education Innovation Research Team (IRT13043), the National Key Basic Research Program of China (No. 2014CB239206), and the National Natural Science Foundation of China (No. 51006129) is greatly appreciated.

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REFERENCES

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