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Thermodynamic stability conditions, methane enrichment, and gas uptake of ionic clathrate hydrates of mine ventilation air
Accepted Manuscript Thermodynamic Stability Conditions, Methane Enrichment, and Gas Uptake of Ionic Clathrate Hydrates of Mine Ventilation Air Jianwei Du, Huijuan Li, Liguang Wang PII: DOI: Reference:
S1385-8947(15)00362-9 http://dx.doi.org/10.1016/j.cej.2015.03.046 CEJ 13397
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
26 November 2014 5 March 2015 10 March 2015
Please cite this article as: J. Du, H. Li, L. Wang, Thermodynamic Stability Conditions, Methane Enrichment, and Gas Uptake of Ionic Clathrate Hydrates of Mine Ventilation Air, Chemical Engineering Journal (2015), doi: http:// dx.doi.org/10.1016/j.cej.2015.03.046
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Thermodynamic Stability Conditions, Methane Enrichment, and Gas Uptake of Ionic Clathrate Hydrates of Mine Ventilation Air Jianwei Du1, Huijuan Li, and Liguang Wang* The University of Queensland, School of Chemical Engineering, Brisbane, QLD 4072, Australia.
Abstract: Abatement of methane from mine ventilation air (MVA) is a significant challenge faced by coal mining industry. A promising method for methane capture from gas mixture is clathrate hydrate formation. In search of suitable and costeffective low-dosage promoters for hydrate-based methane capture processes, this paper reports the pressure requirement for the hydrate formation of simulated MVA (0.5 vol% CH4 + 99.5 vol% air) and its potential for methane extraction, in the presence of tri-n-butyl phosphine oxide (TBPO) or tetra-n-butyl ammonium bromide (TBAB) at three different initial loadings (5 wt%, 15 wt%, and 26 wt%). An isochoric equilibrium step-heating pressure search method was used to measure the hydrate phase equilibrium conditions at the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa. It was found that at a given initial loading, TBPO was largely more effective than TBAB in reducing the pressure requirement for hydrate formation of MVA. At a given temperature, the equilibrium pressures of the clathrate hydrates were indifferent to the change in the initial loading of TBPO from 5 wt% to 26 wt%, in contrast to those of TBAB. Gas composition analysis by gas chromatography confirmed that CH4 could be significantly enriched in the ionic clathrate hydrates, and the highest methane enrichment ratio obtained in the present work was 300%, with TBPO at initial loading of 5 wt%. At this relatively low loading, within a given period of 5 hours, TBPO also led to higher gas uptake compared with TBAB. The advantages of TBPO as a promoter of MVA hydrate were discussed. Keywords: Ventilation air methane; Ionic clathrate hydrate; Tri-n-butylphosphine oxide; Tetra-n-butyl ammonium bromide; Phase equilibrium
*Corresponding author: Email: [email protected]; Phone: +61 7 3365 7942. 1 Present address: Center for Hydrate Research, Colorado School of Mines, U.S.A.
1. Introduction Abatement of methane in mine ventilation air (MVA) is a significant challenge faced by coal mining industry. Methane as a greenhouse gas is approximately twentyone times worse than carbon dioxide, so the removal of the methane from mine ventilation air has dual benefits of energy recovery and greenhouse gas mitigation [1]. There are three main sources of methane emissions from coal mining operations: a) mine ventilation air (0.1 – 1 % CH4), b) gas drained from the seam before mining (6095% CH4), and c) gas drained from worked areas of the mine (30 – 95% CH4) [2,3]. Coal mine drainage methane with a high concentration of methane is easy to either flare or to utilize in a similar way to natural gas, whereas utilization of methane from mine ventilation air is hampered by low and fluctuating methane concentration, large gas flow rate, and lack of pipeline infrastructure, and a satisfactory methane enrichment solution has not yet been found. There is a pressing need for concentrating methane from low levels up to requirements of lean-burn methane utilization technologies, typically at least 0.8% - 1.6%. Conventional methane capture methods include solvent adsorption, temperature swing adsorption, pressure swing adsorption, cryogenic separation and membrane separation. But these methods are not effective and have various problems such as constraints to scale-up, high maintenance cost, and intolerance to water vapour and particulate contaminations. Some sorbents such as liquid solvents and nanoporous zeolites have been explored for their effectiveness in dilute and medium concentration methane capture explored [4]. An unconventional alternative is use of structured fluid absorbents such as gas hydrates. Gas hydrates are ice-like crystalline substances, which are made of water molecules acting as the cage forming host and other gas molecules as the captured guest species [5]. Clathrate hydrate-based gas separation has been proved to be a promising method for carbon dioxide or methane capture from gas mixture [5-7]. It involves making carbon dioxide or methane into hydrate (solid phase) and keeping air or other gas molecules in gaseous form, thereby enabling gas separation. That equilibrium formation pressure of CH4 hydrate is much lower than that of N2 and O2
hydrates at the same temperature also implies that CH4 in the CH4-N2-O2 gas mixture might enter the hydrate phase preferentially and thus be captured. Gas hydrate formation usually requires high pressure. Energy requirement for the compression of ventilation air would be high if no thermodynamic promoters are used to reduce the pressure requirement for hydrate crystallization. Some chemicals such as tetrahydrofuran (THF) and cyclopentane (CP) are capable of reducing the hydrate phase equilibrium pressure and have been extensively investigated [8-10], but the high volatility of these chemicals is undesirable for large-scale use. A group of nonvolatile additives, known as semi-clathrate hydrate (SCH) formers or ionic hydrate formers, can lead to lower pressure requirement than THF and CP [11]. The structure of these SCH crystalline solids is featured by the water-anion-framework containing large cavities [12] such as tetrakaidecahedral (51262), pentakaidecahedral (51263), hexakaidecahedral (51264), encaging the alkyl chains of the cations with the interpolation of empty dodecahedral cavities (512) containing small gas molecules such as hydrogen, methane, and carbon dioxide [11, 13, 14]. Shimada et al. [15] and Kamata et al. [16] investigated TBAB and Alekseev et al. [17] reported the crystal structure of tri-n-butyl phosphine oxide (TBPO) that is different from that of TBAB. All these findings showed that unoccupied cages in SCH could trap suitably sized molecules and thus can be used as a vehicle for storing and separating gases. SCHs have drawn increasing interest for their potential applications in gas storage [18-19] and gas separation [20, 21]. Several studies of this kind have addressed coal methane gas with relatively high original methane concentrations (e.g., close to or over 30 mol%) [22-24]. For lean methane-containing gas streams such as MVA, Adamova et al. [25] used a statistical thermodynamic approach to predict the hydrate formation pressure of the water + MVA containing 0.5 vol% CH4 system. And their results suggest that enrichment of methane from MVA can be achieved using the hydratedbased gas separation method. Our recent experimental work has demonstrated the possibility of enriching methane from MVA [26]; however, the required dosage of the tested hydrate promoter, tetra-butyl phosphonium bromide (TBPB), for effectively reducing the hydrate phase equilibrium pressure was high, at 37 wt%.
Addition of thermodynamic promoters such as TBAB [27], tetra-butyl ammonium nitrate (TBANO3) [14], TBPB [28], tetra-amyl ammonium chloride (TAAC) [29] and TBPO [30] can, at a given temperature, allow CH4 semi-clathrate hydrates to form at much lower pressure than N2 semi-clathrate hydrates, suggesting that these promoters are potentially useful for hydrate-based capture of CH4 from the MVA at proper thermodynamic conditions. In search of suitable and cost-effective low-dosage promoters for hydrate-based methane capture processes, this paper reports the pressure requirement for the hydrate formation of simulated MVA and its methane enrichment efficiency, with addition of TBPO or TBAB at three different initial loadings. The results obtained at relatively low dosage have important implications for hydrate-based methane capture technology development.
2. Experimental 2.1. Materials TBAB (99% pure) and TBPO (95% pure) were purchased from Sigma-Aldrich. Simulated ventilation air methane (0.5 vol% CH4 + 99.5 vol% air) was obtained from Coregas. All of these materials were used as received. Deionized water was used to prepare the aqueous solutions of TBPO or TBAB.
2.2. Experimental Methods 2.2.1 Phase equilibrium measurement A home-made non-visual 102 mL stainless steel cylindrical vessel was used to measure the thermodynamic stability conditions of MVA hydrates. Fig. 1 illustrates a schematic diagram of the apparatus, which is essentially the same as in our recent hydrate phase equilibrium studies [26, 30]. Briefly, the temperature inside the reactor can be controlled precisely at 0.1 K. A temperature sensor was inserted into the reactor to measure the temperature of the tested liquid or hydrate phase with an uncertainty of ± 0.03 K. A pressure transducer with accuracy of ± 0.01 MPa was used to measure the gas pressure inside the reactor. A magnetically driven stirrer with rotating speed of 600 rpm was used to agitate the test liquid. The experimental data
were collected using a data acquisition system at 10 seconds intervals. The high pressure reactor was cleaned, dried, and vacuumed prior to introduction of the test solution. The hydrate phase equilibrium measurements were performed at the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa with using the isochoric equilibrium step-heating pressure search method, and more detailed information can be found elsewhere [26, 30]. 2.2.2 Gas composition analysis and gas uptake A 362.5 mL stainless steel vessel (model BR300, Berghof) was used to prepare the samples from the hydrate phase and co-existing gas phase for gas composition analysis. It was also used to study the gas uptake of the hydrate by measuring the changes in gas pressure, with accuracy of ± 0.1 MPa. All the hydrate samples for gas composition analysis were formed from 75 ml test solutions agitated at 600 rpm at 278 K and initial pressure of 4 MPa. The detailed experimental procedure is listed as following: 1) Prior to induction of the test gas, the reactor was degased with a vacuum pump for 10 minutes. More details on this vessel and the relevant experimental procedure can be found elsewhere [31]. 2) After a sufficiently long period for the hydrate formation to be complete (indicated by gas pressure having remained constant for more than 3 hours), samples of the co-existing vapor phase in the headspace were taken. 3) Then the vapor phase was completely evacuated from the reactor by using a vacuum pump (for around 30 seconds), before dissociation of the hydrate at elevated temperature. 4) The gas released from the hydrate phase was sampled using Tedlar gas sampling bags (0.5 liter, SKC). Prior to sampling, each gas sampling bag was vacuumed for 1 – 2 minutes and flushed 3 – 4 times using the gas to be sampled. 5) The gas composition of these samples was analyzed by a gas chromatograph (GC, Shimazu model GC-2014), which is equipped with Alltech Washed Molesieve 5A 80/100 column, with ultra-high purity Argon used as carrier gas. For each gas sample, at least three GC readings were taken and their average was recorded.
3. Results and Discussion 3.1 Thermodynamic stability conditions The reliability and accuracy of our experimental system was given in our recent studies [26, 30], which showed excellent agreement between our phase equilibrium data of TBAB + H2O + N2 or CH4 and those reported by other research groups. In the present work, the phase equilibrium conditions of ventilation air methane hydrate and ventilation air methane + TBAB or TBPO semi-clathrate hydrate at 5 wt%, 15 wt% and 26 wt% are tabulated in Table 1 and plotted in Figures 2 and 3. Also plotted in these figures are the phase equilibrium conditions of air hydrate in the absence of chemical additives. At a given temperature, the MVA hydrate without chemical additives is thermodynamically more stable than air hydrate, which is consistent with the fact that CH4 hydrate is thermodynamically more stable than N2 and O2 hydrates [26]. Pure methane forms structure I hydrate whereas pure nitrogen and oxygen form structure II hydrate [32]. The NMR spectroscopic studies by Lee et al. showed that the hydrate formed from gas mixture of N2 and CH4 at low CH4 concentrations (up to 28.51 mol%) is in Structure I [22]. However, there is no information on the hydrate structure of methane + air and possible structure changes induced by varying methane concentration. Figures 2 and 3 show that both TBAB and TBPO significantly reduced the pressure requirement for forming MVA hydrate. With increasing the initial loading from 5 wt% to 26 wt%, the three phase (hydrate-liquid water-vapor) equilibrium curves of the MVA hydrate with TBAB had a pronounced shift to more thermodynamically stable area, in contrast to those of TBPO. That the pressure requirement for the formation of MVA hydrate with TBPO was not sensitive to the initial loading can be considered compelling for ensuring the operation smoothness of methane recovery from MVA. Figure 4 compares the difference between TBPO and TBAB in lowering the pressure requirement for MVA hydrate formation. At a given temperature and initial loading, the hydrates with TBPO had lower phase equilibrium pressure than those of
TBAB. It is, therefore, suggested that TBPO is more effective than TBAB in reducing the hydrate equilibrium pressure of MVA. This result is consistent with the observed difference between CH4 + TBPO and CH4 + TBAB hydrates [30,33]. Note also that with increasing the initial loading from 5 wt% to 26 wt%, the gap between the TBAB and TBPO curves became smaller as the pressure requirement for the formation of ventilation air methane hydrate was not sensitive to the initial TBPO loading but to the initial TBAB loading.
3.2 Methane Enrichment GC was used to analyze the gas composition (CH4, N2, and O2 content, on a water-free basis) of the hydrate phase formed from 5 wt% and 26 wt% TBAB/TBPO solutions. In the present work, the methane enrichment was simply defined as the ratio of the concentration of CH4 in the hydrate phase to the concentration of CH4 in the feed gas. The gas composition results shown in Tables 2 and 3 (see the original GC graphs in Figures 2S – 4S) suggest that CH4 was enriched in the hydrate phase. Table 2 also shows that at 5 wt%, TBPO gave a better CH4 enrichment (1.50 vol% CH4 in the hydrate phase, a three-fold increase from 0.05 vol% CH4 in the feed gas) compared to TBAB at the same initial loading. Table 3 shows that at 26 wt%, the CH4 enrichment for TBPO and TBAB was close to each other. Increasing the initial loading of TBPO from 5 wt% to 26 wt% would reduce the methane enrichment in the hydrate phase from 300% to 190%. It is, therefore, suggested that separation of methane from MVA can be achieved by clathrate hydrate crystallization aided by TBPO at relatively low dosage.
3.3 Gas Uptake Figure 5 shows the gas uptake by the hydrate phase within five hours after the onset of hydrate formation. The experimental temperature and initial pressure were the same for TBAB and TBPO; presumably, the slightly difference in subcooling would have little impact on gas storage capacity. A larger pressure drop would indicate a higher gas uptake into the hydrate. For TBAB or TBPO, within the first 5
hours, the gas uptake into hydrate increased with increasing the initial reagent loading from 5 wt% to 26 wt%. At 5 wt%, the gas uptake into TBPO SCH was much higher than that of TBAB SCH whereas at 26 wt%, the difference diminished. It appears, therefore, that the impact of initial loading change on the gas uptake into hydrate phase would be smaller for TBPO compared to TBAB. Figure 5 also shows that addition of 26 wt% TBPO allowed the gas uptake to reach a Plateau within 10 minutes. It is likely that further increase in hydrate formation rate can be achieved by adding surfactants, spraying, gas bubbling, or other methods, which is, however, beyond the scope of the present work.
3.4 Advantages of TBPO Compared to Other Promoters The results presented above concern only two promoters, TBPO and TBAB. It appears that TBPO outperforms TBAB in promoting MVA hydrate formation, in particular at relatively low dosage. It would be worthwhile to further compare TBPO with other promoters of MVA hydrates at 5 wt%, which is of interest to development of cost-effective processes for methane capture. The only promoter for promoting MVA hydrate formation reported in the literature is TBPB [26], including its effect on the thermodynamic stability conditions at various initial loadings and the methane enrichment of MVA hydrates at initial loading of 37.1 wt% TBPB (= 350%), corresponding to the lowest pressure requirement for MVA hydrate formation. Figure 6 compares the three-phase equilibrium condition, methane enrichment, and gas uptake of MVA hydrates in the presence of TBPO, TBAB, and TBPB, respectively, at a relatively low initial loading of 5 wt%. All the experiments for determining methane enrichment and gas uptake were carried out with 75 ml test solutions agitated at 600 rpm at 278 K and initial pressure of 4 MPa. At a given temperature, the effect of promoters on reducing the pressure requirement for MVA hydrate formation follows the order of TBPO > TBPB > TBAB (see Figure 6a). Figure 6b shows that at 5 wt%, TBPO gave higher methane enrichment in the hydrate phase than TBAB and TBPB. As discussed in Section 3.2, increasing TBPO
loading from 5 wt% to 26 wt% would reduce the CH4 enrichment in hydrate phase from 300% to 190%. In contrast, a significant increase in the TBPB loading from 5 wt% to 37.1 wt% would increase the CH4 enrichment in hydrate phase from 206% to 350% [26, 35]. The CH4 enrichment achieved with TBPO at a relatively low dosage (e.g., 5 wt%) would be comparable to that of TBPB at a relatively high dosage (e.g., 37.1 wt%). Figure 6c shows that at 5 wt%, TBPO gave higher gas uptake than TBAB and TBPB did. The gas uptake obtained with 5 wt% TBPO (represented by the pressure drop of 0.238 MPa) was comparable to that of 37.1 wt% TBPB (represented by the pressure drop of 0.275 MPa) [35]. Gholinezhad [20] reported a rather low gas storage capacity for TBPO at 26 wt% and 5.2 MPa, in contrast to the higher gas storage capacity achieved with 37 wt% TBPB at 7.5 - 13.5 MPa. In the present work, however, 26 wt% TBPO gave higher gas uptake than 37.1 wt% TBPB, which was obtained with the initial pressure being 4.0 MPa [35]. This discrepancy might be attributed to difference in hydrate structure between gas pressures above and below 6.5 MPa [30].
3.5 Perspectives of and Implications for Methane Capture from Dilute Sources The ultimate aim of the present work is to explore the possibility of developing a safe and cost-effective methane concentrator based on gas hydrate formation. The present work is an early-stage investigation with focus on chemical additives for concentrating CH4 in mine ventilation air using gas hydrate crystallization. Compared to known hydrate promoter TBAB, the new promoter TBPO not only reduced the pressure requirement for MVA hydrate formation, but also significantly broadens the separation window. Addition of TBPO allowed the pressure requirement for MVA hydrate formation to be comparable to that of hydrate-based CO2 capture process. According to Ref [18], the compression cost is the highest (from 50% to 80%) for CO2 capture via hydrate crystallization. It is also recognized that nearly half of the total power consumption of all the compressors goes to the compression of the inlet gas. The high flow rate of mine ventilation air could be exploited to raise the inlet gas pressure, thus somehow
reducing the compression cost. Further cost cut could be achieved if another additive that allows the hydrates to be formed at lower pressure can be found. It has been reported more recently that THF or TBAB reduces the incipient equilibrium pressure for CO2 hydrate formation and a process has been described that involves three hydrate stages coupled with a membrane-based gas separation process at an operating pressure that is substantially less than the pressure required in the absence of THF, and compression costs were estimated to be reduced from 75 to 53% of the power produced for a typical 500 MW power plant [36]. The importance of this work lies in the use of additives to enhance and expand the range of application of clathrate hydrates, and points to possible new approaches for the design of suitable absorbents under milder conditions. The CH4 concentration of mine ventilation air varies over time. It ranges typically from 0.1 vol% to 1.0 vol%. In the present work, we only tested a typical CH4 concentration sitting in between the two concentration limits. Further research needs to done to understand the effect of CH4 concentration variation on MVA hydrate formation. Compared to MVA at CH4 concentration of 0.5 vol% CH4 or lower, the hydrate formation for MVA at 1.0 vol% CH4 or higher can be more readily formed since the equilibrium formation pressure of CH4 hydrate is much lower than that of N2 and O2 hydrates at the same temperature. Therefore, much more efforts need to be made when CH4 concentrations are below 0.5 vol%. Figure 7 illustrates the concept of the hydrate-based methane concentrator. As shown, one-step separation could result in CH4 enrichment that meets the requirement for lean-burn methane utilization technologies, and the indicative condition for MVA hydrate formation in the presence of chemical additives (e.g. 5 wt% TBPO) is 8 degree Celsius with pressure being higher than 9 atm. A higher CH4 concentration in the product gas can be achieved with multi-stage processes for concentrating methane in mine ventilation air. An advantage of the gas hydrate technology over the membrane and pressure swing adsorption methods includes high throughput and simplicity, which can be the bases for developing a cost-effective process. The hydrate technology is also considered safe as it operates at mild conditions well below
the auto ignition temperature of methane. As the hydrate formation releases heat while the hydrate dissociation absorbs almost the same amount of heat, with using a heat exchanger little external heat is required for the process. Also, there is no need for pretreating the gases by removing the vapor moisture and particulate contaminations such as coal dust and limestone dust because foreign particles could beneficially speed up the hydrate-based process [37]. A hydrate-based methane separation process can have high regeneration performance with all chemicals fully recycled for re-use [38]. And the process can also act as a buffer to cope with variations in methane concentration and ventilation air flow for utilization of ventilation air. Such technological success would provide a pressurized product gas with much higher CH4 concentration to ensure near full utilization of the ventilation air methane, with potentially higher recovery of methane from MVA if blended with drainage gas. According to Ref [39], reducing usage of methane from other sources and increase the proportion of the ventilation air would be beneficial.
4. Conclusions The phase equilibrium conditions of clathrate hydrates formed from simulated ventilation air methane + aqueous solutions of TBAB or TBPO were measured in the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa. It was found that addition of TBAB or TBPO allowed the dissociation conditions of the ventilation air methane hydrate to shift to higher temperatures and lower pressures. The clathrate hydrates of TBPO + simulated MVA are generally more stable thermodynamically than that of TBAB + MVA. At a given temperature, the equilibrium pressures of the hydrates decreased noticeably with increasing TBAB concentration but were indifferent to increase in TBPO concentration. It appears that a relatively low dosage of 5 wt% was efficient for TBPO to reduce the pressure requirement for MVA hydrate formation. Gas composition analysis by gas chromatography also found that addition of 5 wt % TBPO allowed CH4 to be preferentially incorporated into the hydrate phase, with the CH4 enrichment being approximately 3-fold, and TBPO had higher
enrichment of CH4 in the hydrate phase than the same concentration of TBAB. At this relatively low loading, within a given period of 5 hours, TBPO also led to higher gas uptake than TBAB. Increasing the initial loading of either TBPO or TBAB would lead to higher gas uptake. Further comparison between TBPO and another promoter TBPB also found that TBPO performed better in reducing hydrate formation pressure requirement, increasing methane enrichment, and enhancing gas uptake. The results suggest that clathrate hydrate formation with TBPO as a promoter is promising for separating methane from MVA.
Acknowledgements The authors gratefully appreciate the financial support from Australian Research Council under the Discovery Projects scheme (No. 1092846) and thank Prof. Joe da Costa for giving permission for the access to GC in his laboratory. HL gratefully acknowledges The University of Queensland International Scholarship.
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[27] A.H. Mohammadi, A. Eslamimanesh, V. Belandria, D. Richon, Phase equilibria of semiclathrate hydrates of CO2, N2, CH4, or H2 + Tetra-n-butylammonium bromide aqueous solution, J. Chem. Eng. Data, 56 (2011) 3855-3865. [28] T. Suginaka, H. Sakamoto, K. Iino, Y. Sakakibara, R. Ohmura, Phase equilibrium for ionic semiclathrate hydrate formed with CO2, CH4, or N2 plus tetrabutylphosphonium bromide, Fluid Phase Equilibr., 344 (2013) 108-111. [29] L.-L. Shi, D.-Q. Liang, Phase Equilibrium Conditions for the Double Semiclathrate Hydrate Formed with Tetraamylammonium Chloride Plus CH4, CO2, or N2, J. Chem. Eng. Data, 59 (2014) 3705–3709. [30] J. Du, L. Wang, Equilibrium Conditions for Semiclathrate Hydrates Formed with CO2, N2, or CH4 in the Presence of Tri-n-butylphosphine Oxide, Ind. Eng. Chem. Res., 53 (2014) 1234-1241. [31] J. Du, H. Li, L. Wang, Effects of ionic surfactants on methane hydrate formation kinetics in a static system, Adv. Powder. Technol., 25 (2014) 1227-1233. [32] D.W. Davidson, Y.P. Handa, C.I. Ratcliffe, J.S. Tse, B.M. Powell, The ability of small molecules to form clathrate hydrates of structure II, Nature, 311 (1984) 142-143. [33] J.-H. Cha, E.S. Kim, K.S. Lee, J.W. Kang, J.W. Kang, K.-S. Kim, J. Chem. Eng. Data 58 (2013) 3494−3498. [34] A.H. Mohammadi, B. Tohidi, R.W. Burgass, Equilibrium Data and Thermodynamic Modeling of Nitrogen, Oxygen, and Air Clathrate Hydrates, J. Chem. Eng. Data, 48 (2003) 612-616. [35] Jianwei Du, Huijuan Li, Liguang Wang, unpublished data. [36] P. Linga, A. Adeyemo, P. Englezos, Environ. Sci. Technol. 42 (2008) 315-320.[37] H. Li and L. Wang, Hydrophobized particles can accelerate nucleation of clathrate hydrates, Fuel 140 (2015) 440445. [38] H. Li, Clathrate hydrate formation in the presence of promoters. PhD thesis, The University of Queensland, 2015. [39] S. Su, A. Beath, H. Guo, C. Mallett, An assessment of mine methane mitigation and utilisation technologies, Prog. Energ. Combust., 31 (2005) 123-170.
Figure captions Figure. 1. Schematic of the experimental apparatus used for phase equilibrium measurements (not drawn to scale). Figure 2. Experimental phase equilibrium data for three concentration with TBAB, this work: ■ (black), 5 wt% TBAB + Ventilation Air Methane; ● (red), 15 wt% TBAB + Ventilation Air Methane; ▲ (blue), 26 wt% TBAB + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. Ref [26] □ (black), water + Air, ref [34] Figure 3 Experimental phase equilibrium data for three concentration with TBPO: ■ (black), 5 wt% TBPO + Ventilation Air Methane; ● (red), 15 wt% TBPO + Ventilation Air Methane; ▲ (blue), 26 wt% TBPO + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. ref[26] □ (black), water + Air, ref [34]. Figure 4. Comparison between TBAB and TBPO at different concentrations: a) 5 wt%; b) 15 wt%; c) 26 wt%. The symbols represent experimental data: ■ (black), TBAB + Ventilation Air Methane + water; ● (red), TBPO + Ventilation Air Methane + water. The lines are drawn to guide the eye. Figure 5. Pressure drop versus time in the presence of TBAB and TBPO at different initial loadings. The error bars represent the standard deviation obtained from two independent experiments. Figure 6. Comparison of a) thermodynamic stability conditions, b) methane enrichment in the hydrate phase, and c) gas uptake, for TBPO, TBAB, and TBPB at initial loading 5 wt%. The TBPB data in a) were adapted from Ref [26] and those of b) and c) were adapted from Ref [35]. The error bars represent the standard deviation obtained from two independent experiments. Figure 7. Conceptual diagram of CH4 enrichment via hydrate crystallization. The product gas released from hydrate decomposition can be fed to lean-burn turbines.
Table 1. Phase Equilibrium Data of simulated Ventilation Air Methane in the presence of TBAB and TBPO
5 wt% T/K
277.61 281.23 283.44 285.51 286.98
0.39 3.38 6.88 12.46 17.61 5 wt%
T/K 281.46 285.41 289.26 291.84 294.42
Simulated Ventilation Air Methane + TBAB + H2O 15 wt% P/MPa T/K P/MPa T/K
283.35 284.23 285.93 288.15 289.77 291.47
0.81 2.62 5.46 9.6 13.32 18.35
T/K 286.42 290.46 292.49 294.72
P/MPa 2.58 7.67 11.64 18.25
P/MPa
284.46 285.32 287.51 290.69 294.03
Simulated Ventilation Air Methane + TBPO + H2O 15 wt%
P/MPa 0.89 3.07 7.74 12.47 19.11
26 wt%
T/K 279.62 283.72 285.43 288.67 290.79 291.8 293.55 295.54
0.98 2.29 5.93 11.87 18.28 26 wt%
P/MPa 0.23 1.08 2.26 5.67 8.26 10.77 13.83 18.43
Table 2. Composition of Gases (on a Water-Free Basis, mean ± 1 standard deviation) in different phases in presence of additives with 5 wt% TBAB TBPO
Hydrate phase Co-existing gas phase Hydrate phase Co-existing gas phase
CH4 1.00 + 0.02 0.49 + 0.00 1.50 + 0.01 0.50 + 0.00
N2 74.5 + 0.3 78.9 + 0.3 69.3 + 0.1 79.4 + 0.2
Note: Feed gas composition = 0.50 vol % CH4 + 78.6 vol % N2 + 20.9 vol % O2.
O2 24.5 + 1.1 20.6 + 0.1 30.1 + 0.1 20.4 + 0.0
Table 3. Composition of Gases (on a Water-Free Basis, mean ± 1 standard deviation) in different phases in presence of additives with 26 wt%
TBAB TBPO
Hydrate phase Co-existing gas phase Hydrate phase Co-existing gas phase
CH4 0.94 + 0.00 0.48 + 0.02 0.95 + 0.03 0.45 + 0.02
N2 72.1 + 0.6 79.1 + 0.0 73.6 + 0.3 80.4 + 0.9
Note: Feed gas composition = 0.50 vol % CH4 + 78.6 vol % N2 + 20.9 vol % O2.
O2 27.2 + 0.8 20.3 + 0.2 25.7 + 0.2 19.8 + 0.3
Ventilation Data logger
Relief Booster
P
T
Gas cylinder Compressed air Vacuum Pump PC
Temperature control Magnetic stirrer circulator
Figure. 1. Schematic of the experimental apparatus used for phase equilibrium measurements (not drawn to scale).
50
40
P/MPa
30
20
10
0 270
275
280
285
290
295
T/K
Figure 2. Experimental phase equilibrium data for three concentration with TBAB, this work: ■ (black), 5 wt% TBAB + Ventilation Air Methane; ● (red), 15 wt% TBAB + Ventilation Air Methane; ▲ (blue), 26 wt% TBAB + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. Ref [26] □ (black), water + Air, ref [34]
50
40
P/MPa
30
20
10
0 270
275
280
285
290
295
300
T/K
Figure 3 Experimental phase equilibrium data for three concentration with TBPO: ■ (black), 5 wt% TBPO + Ventilation Air Methane; ● (red), 15 wt% TBPO + Ventilation Air Methane; ▲ (blue), 26 wt% TBPO + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. ref [26] □ (black), water + Air, ref [34].
20
a)
b)
c)
18 16 14
P/MPa
12 10 8 6 4 2 0
280
285
290
295
280
285
290
295
280
285
290
295
T/K
Figure 4. Comparison between TBAB and TBPO at different concentrations: a) 5 wt%; b) 15 wt%; c) 26 wt%. The symbols represent experimental data: ■ (black), TBAB + Ventilation Air Methane + water; ● (red), TBPO + Ventilation Air Methane + water. The lines are drawn to guide the eye.
0.4
Pressure Drop (MPa)
26 wt% TBPO 0.3
26 wt% TBAB 5 wt% TBPO 0.2
0.1
5 wt% TBAB 0.0
0
1
2
3
4
5
Time (Hours)
Figure 5. Pressure drop versus time in the presence of TBAB and TBPO at different initial loadings. The error bars represent the standard deviation obtained from two independent experiments.
a)
20
TBPB
TBAB
P / MPa
15
TBPO 10
5
0
276
278
280
282
284
286
288
290
292
294
296
T/K
Methane enrichment (%)
300
b)
250 200 150 100 50 0
TBAB
TBPO
TBPB
0.40
Pressure drop (MPa)
0.35
c)
0.30
TBPO
0.25 0.20 0.15
TBAB
0.10
TBPB
0.05 0.00
0
1
2
3
4
5
Time (hours)
Figure 6. Comparison of a) thermodynamic stability conditions, b) methane enrichment in the hydrate phase, and c) gas uptake, for TBPO, TBAB, and TBPB at initial loading 5 wt%. The TBPB data in a) were adapted from Ref [26] and those of b) and c) were adapted from Ref [35]. The error bars represent the standard deviation obtained from two independent experiments.
Concentrator Water & Chemical
Ventilation air
Heat exchange
Hydrate Formation (8 °C, > 9 atm)
Hydrate slurry
Hydrate Dissociation (Ambient condition)
Product gas (CH4 enriched)
Tail gas (CH4 depleted)
Figure 7. Conceptual diagram of CH4 enrichment via hydrate crystallization. The product gas released from hydrate decomposition can be fed to lean-burn turbines.
Highlights:
Hydrate-based CH4 removal from mine ventilation air was studied
An isochoric pressure-search method was used to measure hydrate stability condition
3-fold methane enrichment was obtained at a relatively low chemical dosage
S1385-8947(15)00362-9 http://dx.doi.org/10.1016/j.cej.2015.03.046 CEJ 13397
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
26 November 2014 5 March 2015 10 March 2015
Please cite this article as: J. Du, H. Li, L. Wang, Thermodynamic Stability Conditions, Methane Enrichment, and Gas Uptake of Ionic Clathrate Hydrates of Mine Ventilation Air, Chemical Engineering Journal (2015), doi: http:// dx.doi.org/10.1016/j.cej.2015.03.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermodynamic Stability Conditions, Methane Enrichment, and Gas Uptake of Ionic Clathrate Hydrates of Mine Ventilation Air Jianwei Du1, Huijuan Li, and Liguang Wang* The University of Queensland, School of Chemical Engineering, Brisbane, QLD 4072, Australia.
Abstract: Abatement of methane from mine ventilation air (MVA) is a significant challenge faced by coal mining industry. A promising method for methane capture from gas mixture is clathrate hydrate formation. In search of suitable and costeffective low-dosage promoters for hydrate-based methane capture processes, this paper reports the pressure requirement for the hydrate formation of simulated MVA (0.5 vol% CH4 + 99.5 vol% air) and its potential for methane extraction, in the presence of tri-n-butyl phosphine oxide (TBPO) or tetra-n-butyl ammonium bromide (TBAB) at three different initial loadings (5 wt%, 15 wt%, and 26 wt%). An isochoric equilibrium step-heating pressure search method was used to measure the hydrate phase equilibrium conditions at the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa. It was found that at a given initial loading, TBPO was largely more effective than TBAB in reducing the pressure requirement for hydrate formation of MVA. At a given temperature, the equilibrium pressures of the clathrate hydrates were indifferent to the change in the initial loading of TBPO from 5 wt% to 26 wt%, in contrast to those of TBAB. Gas composition analysis by gas chromatography confirmed that CH4 could be significantly enriched in the ionic clathrate hydrates, and the highest methane enrichment ratio obtained in the present work was 300%, with TBPO at initial loading of 5 wt%. At this relatively low loading, within a given period of 5 hours, TBPO also led to higher gas uptake compared with TBAB. The advantages of TBPO as a promoter of MVA hydrate were discussed. Keywords: Ventilation air methane; Ionic clathrate hydrate; Tri-n-butylphosphine oxide; Tetra-n-butyl ammonium bromide; Phase equilibrium
*Corresponding author: Email: [email protected]; Phone: +61 7 3365 7942. 1 Present address: Center for Hydrate Research, Colorado School of Mines, U.S.A.
1. Introduction Abatement of methane in mine ventilation air (MVA) is a significant challenge faced by coal mining industry. Methane as a greenhouse gas is approximately twentyone times worse than carbon dioxide, so the removal of the methane from mine ventilation air has dual benefits of energy recovery and greenhouse gas mitigation [1]. There are three main sources of methane emissions from coal mining operations: a) mine ventilation air (0.1 – 1 % CH4), b) gas drained from the seam before mining (6095% CH4), and c) gas drained from worked areas of the mine (30 – 95% CH4) [2,3]. Coal mine drainage methane with a high concentration of methane is easy to either flare or to utilize in a similar way to natural gas, whereas utilization of methane from mine ventilation air is hampered by low and fluctuating methane concentration, large gas flow rate, and lack of pipeline infrastructure, and a satisfactory methane enrichment solution has not yet been found. There is a pressing need for concentrating methane from low levels up to requirements of lean-burn methane utilization technologies, typically at least 0.8% - 1.6%. Conventional methane capture methods include solvent adsorption, temperature swing adsorption, pressure swing adsorption, cryogenic separation and membrane separation. But these methods are not effective and have various problems such as constraints to scale-up, high maintenance cost, and intolerance to water vapour and particulate contaminations. Some sorbents such as liquid solvents and nanoporous zeolites have been explored for their effectiveness in dilute and medium concentration methane capture explored [4]. An unconventional alternative is use of structured fluid absorbents such as gas hydrates. Gas hydrates are ice-like crystalline substances, which are made of water molecules acting as the cage forming host and other gas molecules as the captured guest species [5]. Clathrate hydrate-based gas separation has been proved to be a promising method for carbon dioxide or methane capture from gas mixture [5-7]. It involves making carbon dioxide or methane into hydrate (solid phase) and keeping air or other gas molecules in gaseous form, thereby enabling gas separation. That equilibrium formation pressure of CH4 hydrate is much lower than that of N2 and O2
hydrates at the same temperature also implies that CH4 in the CH4-N2-O2 gas mixture might enter the hydrate phase preferentially and thus be captured. Gas hydrate formation usually requires high pressure. Energy requirement for the compression of ventilation air would be high if no thermodynamic promoters are used to reduce the pressure requirement for hydrate crystallization. Some chemicals such as tetrahydrofuran (THF) and cyclopentane (CP) are capable of reducing the hydrate phase equilibrium pressure and have been extensively investigated [8-10], but the high volatility of these chemicals is undesirable for large-scale use. A group of nonvolatile additives, known as semi-clathrate hydrate (SCH) formers or ionic hydrate formers, can lead to lower pressure requirement than THF and CP [11]. The structure of these SCH crystalline solids is featured by the water-anion-framework containing large cavities [12] such as tetrakaidecahedral (51262), pentakaidecahedral (51263), hexakaidecahedral (51264), encaging the alkyl chains of the cations with the interpolation of empty dodecahedral cavities (512) containing small gas molecules such as hydrogen, methane, and carbon dioxide [11, 13, 14]. Shimada et al. [15] and Kamata et al. [16] investigated TBAB and Alekseev et al. [17] reported the crystal structure of tri-n-butyl phosphine oxide (TBPO) that is different from that of TBAB. All these findings showed that unoccupied cages in SCH could trap suitably sized molecules and thus can be used as a vehicle for storing and separating gases. SCHs have drawn increasing interest for their potential applications in gas storage [18-19] and gas separation [20, 21]. Several studies of this kind have addressed coal methane gas with relatively high original methane concentrations (e.g., close to or over 30 mol%) [22-24]. For lean methane-containing gas streams such as MVA, Adamova et al. [25] used a statistical thermodynamic approach to predict the hydrate formation pressure of the water + MVA containing 0.5 vol% CH4 system. And their results suggest that enrichment of methane from MVA can be achieved using the hydratedbased gas separation method. Our recent experimental work has demonstrated the possibility of enriching methane from MVA [26]; however, the required dosage of the tested hydrate promoter, tetra-butyl phosphonium bromide (TBPB), for effectively reducing the hydrate phase equilibrium pressure was high, at 37 wt%.
Addition of thermodynamic promoters such as TBAB [27], tetra-butyl ammonium nitrate (TBANO3) [14], TBPB [28], tetra-amyl ammonium chloride (TAAC) [29] and TBPO [30] can, at a given temperature, allow CH4 semi-clathrate hydrates to form at much lower pressure than N2 semi-clathrate hydrates, suggesting that these promoters are potentially useful for hydrate-based capture of CH4 from the MVA at proper thermodynamic conditions. In search of suitable and cost-effective low-dosage promoters for hydrate-based methane capture processes, this paper reports the pressure requirement for the hydrate formation of simulated MVA and its methane enrichment efficiency, with addition of TBPO or TBAB at three different initial loadings. The results obtained at relatively low dosage have important implications for hydrate-based methane capture technology development.
2. Experimental 2.1. Materials TBAB (99% pure) and TBPO (95% pure) were purchased from Sigma-Aldrich. Simulated ventilation air methane (0.5 vol% CH4 + 99.5 vol% air) was obtained from Coregas. All of these materials were used as received. Deionized water was used to prepare the aqueous solutions of TBPO or TBAB.
2.2. Experimental Methods 2.2.1 Phase equilibrium measurement A home-made non-visual 102 mL stainless steel cylindrical vessel was used to measure the thermodynamic stability conditions of MVA hydrates. Fig. 1 illustrates a schematic diagram of the apparatus, which is essentially the same as in our recent hydrate phase equilibrium studies [26, 30]. Briefly, the temperature inside the reactor can be controlled precisely at 0.1 K. A temperature sensor was inserted into the reactor to measure the temperature of the tested liquid or hydrate phase with an uncertainty of ± 0.03 K. A pressure transducer with accuracy of ± 0.01 MPa was used to measure the gas pressure inside the reactor. A magnetically driven stirrer with rotating speed of 600 rpm was used to agitate the test liquid. The experimental data
were collected using a data acquisition system at 10 seconds intervals. The high pressure reactor was cleaned, dried, and vacuumed prior to introduction of the test solution. The hydrate phase equilibrium measurements were performed at the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa with using the isochoric equilibrium step-heating pressure search method, and more detailed information can be found elsewhere [26, 30]. 2.2.2 Gas composition analysis and gas uptake A 362.5 mL stainless steel vessel (model BR300, Berghof) was used to prepare the samples from the hydrate phase and co-existing gas phase for gas composition analysis. It was also used to study the gas uptake of the hydrate by measuring the changes in gas pressure, with accuracy of ± 0.1 MPa. All the hydrate samples for gas composition analysis were formed from 75 ml test solutions agitated at 600 rpm at 278 K and initial pressure of 4 MPa. The detailed experimental procedure is listed as following: 1) Prior to induction of the test gas, the reactor was degased with a vacuum pump for 10 minutes. More details on this vessel and the relevant experimental procedure can be found elsewhere [31]. 2) After a sufficiently long period for the hydrate formation to be complete (indicated by gas pressure having remained constant for more than 3 hours), samples of the co-existing vapor phase in the headspace were taken. 3) Then the vapor phase was completely evacuated from the reactor by using a vacuum pump (for around 30 seconds), before dissociation of the hydrate at elevated temperature. 4) The gas released from the hydrate phase was sampled using Tedlar gas sampling bags (0.5 liter, SKC). Prior to sampling, each gas sampling bag was vacuumed for 1 – 2 minutes and flushed 3 – 4 times using the gas to be sampled. 5) The gas composition of these samples was analyzed by a gas chromatograph (GC, Shimazu model GC-2014), which is equipped with Alltech Washed Molesieve 5A 80/100 column, with ultra-high purity Argon used as carrier gas. For each gas sample, at least three GC readings were taken and their average was recorded.
3. Results and Discussion 3.1 Thermodynamic stability conditions The reliability and accuracy of our experimental system was given in our recent studies [26, 30], which showed excellent agreement between our phase equilibrium data of TBAB + H2O + N2 or CH4 and those reported by other research groups. In the present work, the phase equilibrium conditions of ventilation air methane hydrate and ventilation air methane + TBAB or TBPO semi-clathrate hydrate at 5 wt%, 15 wt% and 26 wt% are tabulated in Table 1 and plotted in Figures 2 and 3. Also plotted in these figures are the phase equilibrium conditions of air hydrate in the absence of chemical additives. At a given temperature, the MVA hydrate without chemical additives is thermodynamically more stable than air hydrate, which is consistent with the fact that CH4 hydrate is thermodynamically more stable than N2 and O2 hydrates [26]. Pure methane forms structure I hydrate whereas pure nitrogen and oxygen form structure II hydrate [32]. The NMR spectroscopic studies by Lee et al. showed that the hydrate formed from gas mixture of N2 and CH4 at low CH4 concentrations (up to 28.51 mol%) is in Structure I [22]. However, there is no information on the hydrate structure of methane + air and possible structure changes induced by varying methane concentration. Figures 2 and 3 show that both TBAB and TBPO significantly reduced the pressure requirement for forming MVA hydrate. With increasing the initial loading from 5 wt% to 26 wt%, the three phase (hydrate-liquid water-vapor) equilibrium curves of the MVA hydrate with TBAB had a pronounced shift to more thermodynamically stable area, in contrast to those of TBPO. That the pressure requirement for the formation of MVA hydrate with TBPO was not sensitive to the initial loading can be considered compelling for ensuring the operation smoothness of methane recovery from MVA. Figure 4 compares the difference between TBPO and TBAB in lowering the pressure requirement for MVA hydrate formation. At a given temperature and initial loading, the hydrates with TBPO had lower phase equilibrium pressure than those of
TBAB. It is, therefore, suggested that TBPO is more effective than TBAB in reducing the hydrate equilibrium pressure of MVA. This result is consistent with the observed difference between CH4 + TBPO and CH4 + TBAB hydrates [30,33]. Note also that with increasing the initial loading from 5 wt% to 26 wt%, the gap between the TBAB and TBPO curves became smaller as the pressure requirement for the formation of ventilation air methane hydrate was not sensitive to the initial TBPO loading but to the initial TBAB loading.
3.2 Methane Enrichment GC was used to analyze the gas composition (CH4, N2, and O2 content, on a water-free basis) of the hydrate phase formed from 5 wt% and 26 wt% TBAB/TBPO solutions. In the present work, the methane enrichment was simply defined as the ratio of the concentration of CH4 in the hydrate phase to the concentration of CH4 in the feed gas. The gas composition results shown in Tables 2 and 3 (see the original GC graphs in Figures 2S – 4S) suggest that CH4 was enriched in the hydrate phase. Table 2 also shows that at 5 wt%, TBPO gave a better CH4 enrichment (1.50 vol% CH4 in the hydrate phase, a three-fold increase from 0.05 vol% CH4 in the feed gas) compared to TBAB at the same initial loading. Table 3 shows that at 26 wt%, the CH4 enrichment for TBPO and TBAB was close to each other. Increasing the initial loading of TBPO from 5 wt% to 26 wt% would reduce the methane enrichment in the hydrate phase from 300% to 190%. It is, therefore, suggested that separation of methane from MVA can be achieved by clathrate hydrate crystallization aided by TBPO at relatively low dosage.
3.3 Gas Uptake Figure 5 shows the gas uptake by the hydrate phase within five hours after the onset of hydrate formation. The experimental temperature and initial pressure were the same for TBAB and TBPO; presumably, the slightly difference in subcooling would have little impact on gas storage capacity. A larger pressure drop would indicate a higher gas uptake into the hydrate. For TBAB or TBPO, within the first 5
hours, the gas uptake into hydrate increased with increasing the initial reagent loading from 5 wt% to 26 wt%. At 5 wt%, the gas uptake into TBPO SCH was much higher than that of TBAB SCH whereas at 26 wt%, the difference diminished. It appears, therefore, that the impact of initial loading change on the gas uptake into hydrate phase would be smaller for TBPO compared to TBAB. Figure 5 also shows that addition of 26 wt% TBPO allowed the gas uptake to reach a Plateau within 10 minutes. It is likely that further increase in hydrate formation rate can be achieved by adding surfactants, spraying, gas bubbling, or other methods, which is, however, beyond the scope of the present work.
3.4 Advantages of TBPO Compared to Other Promoters The results presented above concern only two promoters, TBPO and TBAB. It appears that TBPO outperforms TBAB in promoting MVA hydrate formation, in particular at relatively low dosage. It would be worthwhile to further compare TBPO with other promoters of MVA hydrates at 5 wt%, which is of interest to development of cost-effective processes for methane capture. The only promoter for promoting MVA hydrate formation reported in the literature is TBPB [26], including its effect on the thermodynamic stability conditions at various initial loadings and the methane enrichment of MVA hydrates at initial loading of 37.1 wt% TBPB (= 350%), corresponding to the lowest pressure requirement for MVA hydrate formation. Figure 6 compares the three-phase equilibrium condition, methane enrichment, and gas uptake of MVA hydrates in the presence of TBPO, TBAB, and TBPB, respectively, at a relatively low initial loading of 5 wt%. All the experiments for determining methane enrichment and gas uptake were carried out with 75 ml test solutions agitated at 600 rpm at 278 K and initial pressure of 4 MPa. At a given temperature, the effect of promoters on reducing the pressure requirement for MVA hydrate formation follows the order of TBPO > TBPB > TBAB (see Figure 6a). Figure 6b shows that at 5 wt%, TBPO gave higher methane enrichment in the hydrate phase than TBAB and TBPB. As discussed in Section 3.2, increasing TBPO
loading from 5 wt% to 26 wt% would reduce the CH4 enrichment in hydrate phase from 300% to 190%. In contrast, a significant increase in the TBPB loading from 5 wt% to 37.1 wt% would increase the CH4 enrichment in hydrate phase from 206% to 350% [26, 35]. The CH4 enrichment achieved with TBPO at a relatively low dosage (e.g., 5 wt%) would be comparable to that of TBPB at a relatively high dosage (e.g., 37.1 wt%). Figure 6c shows that at 5 wt%, TBPO gave higher gas uptake than TBAB and TBPB did. The gas uptake obtained with 5 wt% TBPO (represented by the pressure drop of 0.238 MPa) was comparable to that of 37.1 wt% TBPB (represented by the pressure drop of 0.275 MPa) [35]. Gholinezhad [20] reported a rather low gas storage capacity for TBPO at 26 wt% and 5.2 MPa, in contrast to the higher gas storage capacity achieved with 37 wt% TBPB at 7.5 - 13.5 MPa. In the present work, however, 26 wt% TBPO gave higher gas uptake than 37.1 wt% TBPB, which was obtained with the initial pressure being 4.0 MPa [35]. This discrepancy might be attributed to difference in hydrate structure between gas pressures above and below 6.5 MPa [30].
3.5 Perspectives of and Implications for Methane Capture from Dilute Sources The ultimate aim of the present work is to explore the possibility of developing a safe and cost-effective methane concentrator based on gas hydrate formation. The present work is an early-stage investigation with focus on chemical additives for concentrating CH4 in mine ventilation air using gas hydrate crystallization. Compared to known hydrate promoter TBAB, the new promoter TBPO not only reduced the pressure requirement for MVA hydrate formation, but also significantly broadens the separation window. Addition of TBPO allowed the pressure requirement for MVA hydrate formation to be comparable to that of hydrate-based CO2 capture process. According to Ref [18], the compression cost is the highest (from 50% to 80%) for CO2 capture via hydrate crystallization. It is also recognized that nearly half of the total power consumption of all the compressors goes to the compression of the inlet gas. The high flow rate of mine ventilation air could be exploited to raise the inlet gas pressure, thus somehow
reducing the compression cost. Further cost cut could be achieved if another additive that allows the hydrates to be formed at lower pressure can be found. It has been reported more recently that THF or TBAB reduces the incipient equilibrium pressure for CO2 hydrate formation and a process has been described that involves three hydrate stages coupled with a membrane-based gas separation process at an operating pressure that is substantially less than the pressure required in the absence of THF, and compression costs were estimated to be reduced from 75 to 53% of the power produced for a typical 500 MW power plant [36]. The importance of this work lies in the use of additives to enhance and expand the range of application of clathrate hydrates, and points to possible new approaches for the design of suitable absorbents under milder conditions. The CH4 concentration of mine ventilation air varies over time. It ranges typically from 0.1 vol% to 1.0 vol%. In the present work, we only tested a typical CH4 concentration sitting in between the two concentration limits. Further research needs to done to understand the effect of CH4 concentration variation on MVA hydrate formation. Compared to MVA at CH4 concentration of 0.5 vol% CH4 or lower, the hydrate formation for MVA at 1.0 vol% CH4 or higher can be more readily formed since the equilibrium formation pressure of CH4 hydrate is much lower than that of N2 and O2 hydrates at the same temperature. Therefore, much more efforts need to be made when CH4 concentrations are below 0.5 vol%. Figure 7 illustrates the concept of the hydrate-based methane concentrator. As shown, one-step separation could result in CH4 enrichment that meets the requirement for lean-burn methane utilization technologies, and the indicative condition for MVA hydrate formation in the presence of chemical additives (e.g. 5 wt% TBPO) is 8 degree Celsius with pressure being higher than 9 atm. A higher CH4 concentration in the product gas can be achieved with multi-stage processes for concentrating methane in mine ventilation air. An advantage of the gas hydrate technology over the membrane and pressure swing adsorption methods includes high throughput and simplicity, which can be the bases for developing a cost-effective process. The hydrate technology is also considered safe as it operates at mild conditions well below
the auto ignition temperature of methane. As the hydrate formation releases heat while the hydrate dissociation absorbs almost the same amount of heat, with using a heat exchanger little external heat is required for the process. Also, there is no need for pretreating the gases by removing the vapor moisture and particulate contaminations such as coal dust and limestone dust because foreign particles could beneficially speed up the hydrate-based process [37]. A hydrate-based methane separation process can have high regeneration performance with all chemicals fully recycled for re-use [38]. And the process can also act as a buffer to cope with variations in methane concentration and ventilation air flow for utilization of ventilation air. Such technological success would provide a pressurized product gas with much higher CH4 concentration to ensure near full utilization of the ventilation air methane, with potentially higher recovery of methane from MVA if blended with drainage gas. According to Ref [39], reducing usage of methane from other sources and increase the proportion of the ventilation air would be beneficial.
4. Conclusions The phase equilibrium conditions of clathrate hydrates formed from simulated ventilation air methane + aqueous solutions of TBAB or TBPO were measured in the temperature range of (277.61 to 295.54) K and pressure range of (0.23 to 19.11) MPa. It was found that addition of TBAB or TBPO allowed the dissociation conditions of the ventilation air methane hydrate to shift to higher temperatures and lower pressures. The clathrate hydrates of TBPO + simulated MVA are generally more stable thermodynamically than that of TBAB + MVA. At a given temperature, the equilibrium pressures of the hydrates decreased noticeably with increasing TBAB concentration but were indifferent to increase in TBPO concentration. It appears that a relatively low dosage of 5 wt% was efficient for TBPO to reduce the pressure requirement for MVA hydrate formation. Gas composition analysis by gas chromatography also found that addition of 5 wt % TBPO allowed CH4 to be preferentially incorporated into the hydrate phase, with the CH4 enrichment being approximately 3-fold, and TBPO had higher
enrichment of CH4 in the hydrate phase than the same concentration of TBAB. At this relatively low loading, within a given period of 5 hours, TBPO also led to higher gas uptake than TBAB. Increasing the initial loading of either TBPO or TBAB would lead to higher gas uptake. Further comparison between TBPO and another promoter TBPB also found that TBPO performed better in reducing hydrate formation pressure requirement, increasing methane enrichment, and enhancing gas uptake. The results suggest that clathrate hydrate formation with TBPO as a promoter is promising for separating methane from MVA.
Acknowledgements The authors gratefully appreciate the financial support from Australian Research Council under the Discovery Projects scheme (No. 1092846) and thank Prof. Joe da Costa for giving permission for the access to GC in his laboratory. HL gratefully acknowledges The University of Queensland International Scholarship.
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Figure captions Figure. 1. Schematic of the experimental apparatus used for phase equilibrium measurements (not drawn to scale). Figure 2. Experimental phase equilibrium data for three concentration with TBAB, this work: ■ (black), 5 wt% TBAB + Ventilation Air Methane; ● (red), 15 wt% TBAB + Ventilation Air Methane; ▲ (blue), 26 wt% TBAB + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. Ref [26] □ (black), water + Air, ref [34] Figure 3 Experimental phase equilibrium data for three concentration with TBPO: ■ (black), 5 wt% TBPO + Ventilation Air Methane; ● (red), 15 wt% TBPO + Ventilation Air Methane; ▲ (blue), 26 wt% TBPO + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. ref[26] □ (black), water + Air, ref [34]. Figure 4. Comparison between TBAB and TBPO at different concentrations: a) 5 wt%; b) 15 wt%; c) 26 wt%. The symbols represent experimental data: ■ (black), TBAB + Ventilation Air Methane + water; ● (red), TBPO + Ventilation Air Methane + water. The lines are drawn to guide the eye. Figure 5. Pressure drop versus time in the presence of TBAB and TBPO at different initial loadings. The error bars represent the standard deviation obtained from two independent experiments. Figure 6. Comparison of a) thermodynamic stability conditions, b) methane enrichment in the hydrate phase, and c) gas uptake, for TBPO, TBAB, and TBPB at initial loading 5 wt%. The TBPB data in a) were adapted from Ref [26] and those of b) and c) were adapted from Ref [35]. The error bars represent the standard deviation obtained from two independent experiments. Figure 7. Conceptual diagram of CH4 enrichment via hydrate crystallization. The product gas released from hydrate decomposition can be fed to lean-burn turbines.
Table 1. Phase Equilibrium Data of simulated Ventilation Air Methane in the presence of TBAB and TBPO
5 wt% T/K
277.61 281.23 283.44 285.51 286.98
0.39 3.38 6.88 12.46 17.61 5 wt%
T/K 281.46 285.41 289.26 291.84 294.42
Simulated Ventilation Air Methane + TBAB + H2O 15 wt% P/MPa T/K P/MPa T/K
283.35 284.23 285.93 288.15 289.77 291.47
0.81 2.62 5.46 9.6 13.32 18.35
T/K 286.42 290.46 292.49 294.72
P/MPa 2.58 7.67 11.64 18.25
P/MPa
284.46 285.32 287.51 290.69 294.03
Simulated Ventilation Air Methane + TBPO + H2O 15 wt%
P/MPa 0.89 3.07 7.74 12.47 19.11
26 wt%
T/K 279.62 283.72 285.43 288.67 290.79 291.8 293.55 295.54
0.98 2.29 5.93 11.87 18.28 26 wt%
P/MPa 0.23 1.08 2.26 5.67 8.26 10.77 13.83 18.43
Table 2. Composition of Gases (on a Water-Free Basis, mean ± 1 standard deviation) in different phases in presence of additives with 5 wt% TBAB TBPO
Hydrate phase Co-existing gas phase Hydrate phase Co-existing gas phase
CH4 1.00 + 0.02 0.49 + 0.00 1.50 + 0.01 0.50 + 0.00
N2 74.5 + 0.3 78.9 + 0.3 69.3 + 0.1 79.4 + 0.2
Note: Feed gas composition = 0.50 vol % CH4 + 78.6 vol % N2 + 20.9 vol % O2.
O2 24.5 + 1.1 20.6 + 0.1 30.1 + 0.1 20.4 + 0.0
Table 3. Composition of Gases (on a Water-Free Basis, mean ± 1 standard deviation) in different phases in presence of additives with 26 wt%
TBAB TBPO
Hydrate phase Co-existing gas phase Hydrate phase Co-existing gas phase
CH4 0.94 + 0.00 0.48 + 0.02 0.95 + 0.03 0.45 + 0.02
N2 72.1 + 0.6 79.1 + 0.0 73.6 + 0.3 80.4 + 0.9
Note: Feed gas composition = 0.50 vol % CH4 + 78.6 vol % N2 + 20.9 vol % O2.
O2 27.2 + 0.8 20.3 + 0.2 25.7 + 0.2 19.8 + 0.3
Ventilation Data logger
Relief Booster
P
T
Gas cylinder Compressed air Vacuum Pump PC
Temperature control Magnetic stirrer circulator
Figure. 1. Schematic of the experimental apparatus used for phase equilibrium measurements (not drawn to scale).
50
40
P/MPa
30
20
10
0 270
275
280
285
290
295
T/K
Figure 2. Experimental phase equilibrium data for three concentration with TBAB, this work: ■ (black), 5 wt% TBAB + Ventilation Air Methane; ● (red), 15 wt% TBAB + Ventilation Air Methane; ▲ (blue), 26 wt% TBAB + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. Ref [26] □ (black), water + Air, ref [34]
50
40
P/MPa
30
20
10
0 270
275
280
285
290
295
300
T/K
Figure 3 Experimental phase equilibrium data for three concentration with TBPO: ■ (black), 5 wt% TBPO + Ventilation Air Methane; ● (red), 15 wt% TBPO + Ventilation Air Methane; ▲ (blue), 26 wt% TBPO + Ventilation Air Methane; ○ (red), deionized water + Ventilation Air Methane. ref [26] □ (black), water + Air, ref [34].
20
a)
b)
c)
18 16 14
P/MPa
12 10 8 6 4 2 0
280
285
290
295
280
285
290
295
280
285
290
295
T/K
Figure 4. Comparison between TBAB and TBPO at different concentrations: a) 5 wt%; b) 15 wt%; c) 26 wt%. The symbols represent experimental data: ■ (black), TBAB + Ventilation Air Methane + water; ● (red), TBPO + Ventilation Air Methane + water. The lines are drawn to guide the eye.
0.4
Pressure Drop (MPa)
26 wt% TBPO 0.3
26 wt% TBAB 5 wt% TBPO 0.2
0.1
5 wt% TBAB 0.0
0
1
2
3
4
5
Time (Hours)
Figure 5. Pressure drop versus time in the presence of TBAB and TBPO at different initial loadings. The error bars represent the standard deviation obtained from two independent experiments.
a)
20
TBPB
TBAB
P / MPa
15
TBPO 10
5
0
276
278
280
282
284
286
288
290
292
294
296
T/K
Methane enrichment (%)
300
b)
250 200 150 100 50 0
TBAB
TBPO
TBPB
0.40
Pressure drop (MPa)
0.35
c)
0.30
TBPO
0.25 0.20 0.15
TBAB
0.10
TBPB
0.05 0.00
0
1
2
3
4
5
Time (hours)
Figure 6. Comparison of a) thermodynamic stability conditions, b) methane enrichment in the hydrate phase, and c) gas uptake, for TBPO, TBAB, and TBPB at initial loading 5 wt%. The TBPB data in a) were adapted from Ref [26] and those of b) and c) were adapted from Ref [35]. The error bars represent the standard deviation obtained from two independent experiments.
Concentrator Water & Chemical
Ventilation air
Heat exchange
Hydrate Formation (8 °C, > 9 atm)
Hydrate slurry
Hydrate Dissociation (Ambient condition)
Product gas (CH4 enriched)
Tail gas (CH4 depleted)
Figure 7. Conceptual diagram of CH4 enrichment via hydrate crystallization. The product gas released from hydrate decomposition can be fed to lean-burn turbines.
Highlights:
Hydrate-based CH4 removal from mine ventilation air was studied
An isochoric pressure-search method was used to measure hydrate stability condition
3-fold methane enrichment was obtained at a relatively low chemical dosage