Efficient CO2 Capture from a Simulated Shale Gas using Tetra-n-butylphosphonium Bromide Semiclathrate Hydrate

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ScienceDirect Energy Procedia 105 (2017) 4904 – 4908

The 8th International Conference on Applied Energy – ICAE2016

Efficient CO2 Capture from a Simulated Shale Gas Using Tetra-n-butylphosphonium Bromide Semiclathrate Hydrate Zheng Li a,b, Dong-Liang Zhong a,b,*, Yi-Yu Lu a, Jin Yan a a

State Key Labrotory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China b College of Power Engineering, Chongqing University, Chongqing 400044, China

Abstract In this work, tetra-n-butylphosphonium bromide (TBPB) semiclathrate hydrate was employed to capture CO2 from a simulated shale gas (40 mol% CO2/CH4) for the first time. New liquid-hydrate-vapor phase equilibrium data at 5, 10, and 20 wt% TBPB were experimentally determined and reported. Hydrate formation kinetics and CO2 separation efficiency at different TBPB concentrations were studied at a fixed initial pressure of 2.8 MPa with the temperature varying from 278.1 K to 283.4 K. The results indicated that 20 wt% TBPB solution was more favorable for CO2 separation compared to 5 and 10 wt% TBPB solutions as the largest separation factor (29.3) was obtained. The preferential enclathration of CO2 into the TBPB semiclathrate hydrate indicates that the hydrate-based CO2 capture from the CO2/CH4 gas mixture is more efficient in the presence of TBPB as compared to that in the presence of tetrahydrofuran (THF). © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewofunder responsibility of of ICAE Peer-review under responsibility the scientific committee the 8th International Conference on Applied Energy.

Keywords: Gas hydrates; Gas separation; Tetra-n-butylphosphonium bromide; Carbon dioxde capture; Shale gas

1. Introduction Shale gas is an important unconventional natural gas resource which is widely distributed around the world [1]. Developing shale gas industry is considered as one of the transitional energy strategies filling the gap between conventional fossil fuels and future renewable energies. Hydraulic fracturing, the state of the art shale gas production technique, may hinder the development of shale gas industry due to many environmental concerns [2]. Recently, a potential technology that explores shale gas using supercritical

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

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.977

Zheng Li et al. / Energy Procedia 105 (2017) 4904 – 4908

carbon dioxide (SC-CO2) injection may be a good solution to this issue [3]. However, shale gas (mainly methane) might be contaminated by the injected CO2 during the production process. In this case the shale gas recovered is thus a gas mixture containing CO2 and CH4 gas [4], so the major challenge of this technology is how to efficiently remove CO2 from the CO2/CH4 gas mixture. The hydrate-based gas separation (HBGS) process is a potential approach for CO2 removal from CO2containing gas mixtures compared to other CO2 capture techniques [5]. Gas hydrates are nonstoichiometric crystalline compounds in which hydrogen-bonded water molecules form cavities to capture small gas molecules at low temperature and high pressure conditions [6]. Since the major component in hydrate structures is water, the hydrate-based CO2 capture process has been considered as an environmental friendly method. The concept of CO2 separation from gas mixtures by forming hydrates was first proposed at the end of twentieth century [7]. Many studies have been done to improve this novel technology because the high pressure conditions for hydrate formation will cause considerable amounts of energy consumption in gas compression [7-10]. In order to obtain high-temperature and low-pressure conditions for hydrate formation, thermodynamic promoters such as tetrahydrofuran (THF), cyclopentane (CP), and quaternary ammonium salts were employed [4-9, 11]. Linga et al. conducted studies on post and pre-combustion CO2 capture (CO2 removal from the CO2/N2 and CO2/H2 gas mixture) using hydrate formation with the help of these chemical additives. It was found that the operating pressure for hydrate formation was greatly reduced in the presence of these promoters [12-14]. For more important and detailed information, one can reference their recent review article by Babu et al. [15]. However, it was found that CO2 and CH4 enclathration in the small 512 cavities of sII hydrate was quite fair in the presence of THF or CP [16, 17], and the CO2 separation efficiency was compromised when 1.0 mol% THF was added in aqueous water [18]. Therefore, there is an ongoing effort to screen high performance promoters for CO2 separation. Recently, it was reported that tetra-n-butylphosphonium bromide (TBPB) is a new additive. The three phase (liquid-vapor-hydrate) equilibria of TBPB semiclathrate hydrate formed with CO2 or CH4 were extensively studied [19-21]. However, to our best of knowledge, the hydrate-based CO2 capture from the CO2/CH4 gas mixture in the presence of TBPB has not been performed. The objective of this work is to evaluate the performance of CO2 removal from the simulated shale gas (40 mol% CO2/CH4) via a HBGS process at three different TBPB concentrations (5, 10, and 20 wt%). The fundamental data of hydrate phase equilibrium conditions, gas uptake, and CO2 recovery and separation factor in the presence of TBPB were produced and reported. 2. Experimental section The simulated shale gas containing 40 mol% CO2 and 60 mol% CH4 was supplied by Chongqing Rising Gas with an uncertainty of 0.05 mol%. TBPB was purchased from Shanghai Aladdin Co. Ltd. (CAS: 3115-68-2) with a certified mass purity of 98.0%. Distilled and deionized water was used in the experiments. Detailed description of the experimental apparatus, procedure, and the calculation of normalized gas uptake, CO2 recovery and separation factor was given in the previous work [4, 18]. 3. Results and discussion The isochoric pressure search method was used to determine the phase equilibrium conditions for TBPB semiclathrate hydrate formed with the CO2/CH4 gas mixture. The equilibrium temperature at 0.3 MPa was 278.1, 280.9 and 283.4 K corresponding to 5, 10, and 20 wt% TBPB, respectively. In this work, the experiments were carried out at a fixed initial pressure of 2.8 MPa and in the temperature range of (278.1-283.4 K) with the TBPB concentration changing from 5 to 20 wt%. As a result, all experiments can be compared at a fixed initial driving force (overpressure) of 2.5 MPa. Fig. 1(a) shows the phase

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equilibria of the CO2/CH4 mixed hydrate formed in TBPB solutions. It can be seen that the phase equilibrium data reported in this work agree well with that measured using differential scanning calorimeter (DSC) by Sales Silva et al. [21]. Fig. 1(b) shows the average gas uptake at induction time and the total gas consumption obtained at different TBPB concentrations. It can be seen from the figure that gas consumption at induction time seems to have no links with the TBPB concentration. It is interesting to note that the total gas uptake obtained at 10 wt% TBPB was higher than that obtained at 5 wt% and 20 wt% TBPB. Fig. 2(a) shows the comparison of CO2 recovery (R) and separation factor (S) obtained at different TBPB concentrations. As seen, the highest CO2 recovery was 50.2% in 10 wt% TBPB solution but the separation factor (8.0) obtained was the lowest. On the contrary, the experiment conducted at 20 wt% TBPB concentration shows the highest separation factor (29.3) although CO2 recovery was compromised (42.1%). This indicates the preferential enclathration of CO2 molecules into the dodecahedral cavities of TBPB semiclathrate hydrate at 20 wt% TBPB. Thus, it is concluded that the optimum TBPB concentration was 20 wt% under the experimental conditions employed in this work. 15

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Fig. 1. (a) Hydrate phase equilibrium conditions for the CO2/CH4 mixture at 5, 10, and 20 wt% TBPB; (b) Effect of TBPB concentration on gas uptake at induction time and the total gas uptake.

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Fig. 2. (a) Effect of TBPB concentration on CO2 separation efficiency; (b) Comparison of CO2 separation efficiency in THF solution and TBPB solution at the same operating pressure.

Fig. 2(b) shows a comparison of CO2 separation efficiency obtained in THF solutions and TBPB solutions at the same overpressure (ΔP = 2.5 MPa). The amount of solution used in the two systems was also the same (140 cm3). As seen from the figure, CO2 recovery obtained at the optimum TBPB

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concentration (41.1%) was lower than that obtained in the THF solution (49.9%). However, the separation factor obtained in TBPB solution (29.3) was 8.4 times larger than that in the THF solution (3.5). Considering the operating temperature was 277.2 K in THF solution and 283.4 K in the 20 wt% TBPB solution, so the HBGS process for CO2 capture from CO2/CH4 with high CO2 separation efficiency can be achieved at mild operating conditions using the 20 wt% TBPB solution. 4. Conclusion In this work, TBPB semiclathrate hydrate was formed to enhance the hydrate-based CO2 separation from the CO2/CH4 gas mixture. The phase equilibrium data of TBPB semiclathrate hydrate formed with the CO2/CH4 gas mixture were measured at 5, 10, and 20 wt% TBPB, and the effect of TBPB concentration on hydrate growth and CO2 separation efficiency was investigated. It was found that 20 wt% TBPB solution performs better than 5 and 10 wt% TBPB solutions and the separation factor was 8.4 times larger than that in THF solutions. Acknowledgements The financial support from the Natural Science Foundation of China (No. 51676021), the Ministry of Education Innovation Research Team (IRT13043), National Key Basic Research Program of China (No. 2014CB239206), and Chongqing University Postgraduates’ Innovation Project (CYS15009) is appreciated. References [1] Yuan J, Luo D, Feng L. A review of the technical and economic evaluation techniques for shale gas development. Applied Energy. 2015;148:49-65. [2] Wang Q, Chen X, Jha AN, Rogers H. Natural gas from shale formation - The evolution, evidences and challenges of shale gas revolution in United States. Renewable & Sustainable Energy Reviews. 2014;30:1-28. [3] Wang H, Li G, Shen Z. A Feasibility Analysis on Shale Gas Exploitation with Supercritical Carbon Dioxide. Energy Sources Part A-Recovery Util Environ Eff. 2012;34:1426-35. [4] Zhong DL, Li Z, Lu YY, Sun DJ. Phase Equilibrium Data of Gas Hydrates Formed from a CO 2 + CH4 Gas Mixture in the Presence of Tetrahydrofuran. Journal of Chemical & Engineering Data. 2014;59:4110-7. [5] Ma ZW, Zhang P, Bao HS, Deng S. Review of fundamental properties of CO2 hydrates and CO2 capture and separation using hydration method. Renewable and Sustainable Energy Reviews. 2016;53:1273-302. [6] Sloan ED, Koh CA. Clathrate hydrates of natural gases. 3rd ed. Florida, Boca Raton: CRC press; 2008. [7] Yang MJ, Jing W, Zhao JF, Ling Z, Song YC. Promotion of hydrate-based CO2 capture from flue gas by additive mixtures (THF (tetrahydrofuran) + TBAB (tetra-n-butyl ammonium bromide)). Energy. 2016;106:546-53. [8] Li XS, Zhan H, Xu CG, Zeng ZY, Lv QN, Yan KF. Effects of Tetrabutyl-(ammonium/phosphonium) Salts on Clathrate Hydrate Capture of CO2 from Simulated Flue Gas. Energy & Fuels. 2012;26:2518-27. [9] Fan SS, Li SF, Wang JQ, Lang XM, Wang YH. Efficient Capture of CO2 from Simulated Flue Gas by Formation of TBAB or TBAF Semiclathrate Hydrates. Energy & Fuels. 2009;23:4202-8. [10] Babu P, Kumar R, Linga P. Medium pressure hydrate based gas separation (HBGS) process for pre-combustion capture of carbon dioxide employing a novel fixed bed reactor. International Journal of Greenhouse Gas Control. 2013;17:206-14. [11] Xu CG, Li XS. Research progress of hydrate-based CO2 separation and capture from gas mixtures. RSC Adv. 2014;4:1830116. [12] Linga P, Kumar R, Englezos P. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chemical Engineering Science. 2007;62:4268-76. [13] Ho LC, Babu P, Kumar R, Linga P. HBGS (hydrate based gas separation) process for carbon dioxide capture employing an unstirred reactor with cyclopentane. Energy. 2013;63:252-9. [14] Babu P, Chin WI, Kumar R, Linga P. Systematic Evaluation of Tetra-n-butyl Ammonium Bromide (TBAB) for Carbon Dioxide Capture Employing the Clathrate Process. Ind Eng Chem Res. 2014;53:4878-87. [15] Babu P, Linga P, Kumar R, Englezos P. A review of the hydrate based gas separation (HBGS) process for carbon dioxide pre-combustion capture. Energy. 2015;85:261-79.

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Biography Zheng Li is currently a Ph.D. student at the College of Power Engineering, Chongqing University (China). He is mainly working on the hydrate-based gas separation process for CO2 capture from the CO2/CH4 gas mixture.