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ScienceDirect Energy Procedia 105 (2017) 5048 – 5054
The 8th International Conference on Applied Energy – ICAE2016
CO2 Hydrates – Effect of Additives and Operating Conditions on the Morphology and Hydrate Growth Hari Prakash Veluswamya, Kulesha Priyalal Premasinghea, Praveen Lingaa* a
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore
Abstract In this paper, we present the kinetics of CO2 hydrate formation in presence of additives at different operating conditions that result in the formation of pure sI hydrate, pure sII hydrates and a mixture of sI and sII hydrates. Visual observations of different hydrates formed are presented with the associated CO2 uptake achieved under different experimental conditions. We observe a striking contrast in hydrate formation behavior observed for CH 4 hydrate and CO2 hydrate in presence of tetrahydrofuran (THF) under similar diving force and operating conditions. Based on our experiments, it can be inferred that hydrate formation kinetics in presence of the THF is highly influenced by the type of guest gas. © 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 peer-review of under responsibility of ICAE Peer-reviewand/or under responsibility the scientific committee of the 8th International Conference on Applied Energy. Keywords: gas hydrates; formation kinetics; carbon dioxide capture; hydrate morphology; promoter
1. Introduction Clathrate hydrates are inclusion compounds that encompass guest molecules in host water cages. Guest molecules can be predominantly gases like methane, ethane, carbon dioxide etc., or even organic compounds like acetone, tetrahydrofuran, cyclopentane etc. Hydrates are ice-like, crystalline and nonstoichiometric compounds wherein the guest molecules are held intact only by weak Van der Waals force [1]. Due to these peculiar characteristics and other significant advantages offered including high volumetric gas storage capacity, environmental friendly nature and moderate operating conditions for formation, hydrates find applications in many areas including energy storage [2, 3], desalination [4], cold storage [5], carbon capture and sequestration (CCS) [6]. CO2 in presence of water at certain conditions of temperature (1-2 deg C) and pressure (2-3 MPa) forms a standard sI type hydrate structure. Investigation of kinetics of CO2 hydrate formation in presence of different additives has been an active research area in the last decade due CO2 capture applications.
* Corresponding author. Tel.:+65-66011487; fax:+65-67791936. E-mail address:
[email protected].
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.2017.03.1019
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Additives can be predominantly classified into kinetic and thermodynamic promoters. Thermodynamic promoters are additives that participate in hydrate formation thereby will shift the equilibrium curve resulting in more moderate conditions during hydrate formation (lower pressure and higher temperature than that of pure CO2 hydrates). Kinetic promoters (commonly ‘surfactants’ or ‘amino acids’) on the other hand, alter the interfacial properties during gas/liquid contact due to which increased hydrate formation rates are achieved. These kinetic promoters have no effect on the phase equilibrium curve. Tetrahydrofuran (THF) is a prominent sII hydrate structure forming guest studied employed as a thermodynamic promoter for different guest gases like methane, hydrogen and CO2 hydrates [7-10]. Torrre et al. [11] investigated CO2 hydrates formation kinetics in presence of kinetic promoter (SDS) and thermodynamic promoter (THF) and reported enhanced kinetics with the combination of two types of additives in comparison to the usage of individual additives and pure CO2 hydrates. The concentration of THF and SDS used were 4 wt% (equivalent to 1 mol%) and 0.3 wt% respectively. Both batch type and semi-continuous mode of reactor operation were employed. Lirio et al. [12] also studied the kinetics of CO2 hydrate formation in presence of these both additives but at a higher concentration of THF- 5 mol% and lower concentration of SDS 0.05 wt%. They have studied kinetics at two different pressures and two temperatures. Both these studies included experiments performed under temperatures that resulted in the formation of mixed sI and sII hydrate domain. For clear understanding, Figure 1 presents equilibrium curves for pure CO2 and CO2+THF systems along with the indication of experimental conditions used for the current study. Recently, Veluswamy et al. [13] had reported enhanced methane hydrate formation kinetics in presence of THF promoter at lower pressures of 3.0 MPa and 283.2 K with high methane storage capacity in unstirred tank reactor (UTR). The objective of the current study was to evaluate if such similar enhancement of hydrate formation kinetics could be observed for CO2 hydrates in presence of THF in UTR. This is of practical relevance for several applications pertaining to gas hydrate technology for gas separations involving CO2 streams like CO2 capture from flue, fuel, land fill and bio gas streams. Also, the difference in gas uptake and kinetics under different hydrate structure forming experimental conditions were also documented with associated morphology observations. In the current study, we chose experimental conditions for hydrate formation such that it is possible to form CO 2 hydrates of sI structure, sII structure and a combination of both structures. 2. Experimental Section CO2 gas cylinder of 99.8% purity purchased from SOXAL Pte Ltd, Tetrahydrofuran (THF) of 99.7% purity obtained from Fisher Chemicals, and 99% pure sodium dodecyl sulfate (SDS) from AMRESCO along with deionised water obtained from Elga micromeg deionizaton apparatus were used in all experiments. Experimental setup used was the same as detailed in Veluswamy et al. [14]. Briefly, the reactor has an internal volume of 142 ml and fitted with two marine type viewing windows (at the front and back) to allow visual observation of the reactor contents during hydrate formation/dissociation. All experiments performed were of batch type, with or without the application of stirring (envisaged by magnetic stirrer along with stirrer bar) during experimental trials. Data Acquisition (DAQ) system supplied by National Instruments was used to record the temperature and pressure data for every 20 s throughout the experiment.
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3. Results and Discussion The experimental conditions chosen for this work are represented in Figure 1 along with the thermodynamic phase equilibrium data of pure CO2 and THF (5 mol%)/CO2. CO2 hydrate formation in presence of stoichiometric 5.6 mol% THF was studied at 3.0 MPa and 283.2 K and the visual observations during the hydrate formation experiment during the first 60 min from nucleation are presented in Figure 2a-d.
Figure 1. Phase equilibrium plots for CO2+water system [15] and CO2+THF+water [16] system along with representation of experimental conditions used in the present study
Figure 2. Visual observations of mixed CO2/THF and CH4/THF hydrates under similar experimental conditions
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As seen from the figure, only limited hydrate formation is observed and the total CO2 uptake recorded for this experiment was only 2.9 mmol of gas/mol of water after 2 h from nucleation. It should be noted that the operating conditions and driving force of hydrate formation were similar to that used in the recent study by Veluswamy et al. [13] that had resulted in rapid methane/THF hydrate formation. A likely comparison of hydrate formation images at similar time interval starting from methane/THF/water system is also presented in figure 2w-z. (From the data presented in [13], methane gas uptake of about 65 mmol of gas/mol of water after 2 h from nucleation with same initial volume of solution taken). The visual observations for the same time period in figure 2 show the extensive hydrate formation between THF/CH4 and THF/CO2 system. Also quantitatively, the gas uptake for THF/CO2 system was about 20 times lower. Both methane and CO2 in presence of water alone form sI hydrate structure and in presence of THF under studied experimental conditions they can form only sII hydrate structure. Thus, despite the similar driving force and experimental conditions, the behaviour of two guest gases – CO2 and CH4 in presence of THF is entirely different. This behaviour is also in sharp contrast to the solubility of guest gases studied- CO2 has higher solubility in THF solution in comparison to the limited solubility of methane in THF solution (CO2 solubility in water is approximately 80 times higher than that of methane). THF promoter is shown to demonstrate a synergistic behaviour enhancing hydrate formation in presence of methane in unstirred configuration compared to highly soluble CO2 guest gas under studied experimental conditions. Another plausible reason could be that the smaller methane molecule (diameter 4.36 A) can readily occupy the small cages of sII structure unlike CO2 (having slightly larger diameter 5.12 A) to occupy the small cages of sII hence resulting in decreased gas uptake despite the higher solubility. A recent molecular dynamic simulation study of methane hydrate in presence of THF has been documented by Wu et al. [17] highlighting the synergism between THF and CH4. Addition of 0.01 wt% SDS surfactant had shown to retard the methane gas uptake to 23 mmol gas/mol of water after 2 h from nucleation despite initial rapid kinetics of hydrate formation observed [13]. The same concentration of SDS surfactant when employed with CO2/THF system resulted in slightly increased gas uptake of 12.7 mmol gas/mol of water at the end of 2 h from nucleation. On further increasing concentration to 0.5 wt% SDS surfactant, it was possible to achieve an improvement in gas uptake of 27.5 mmol gas/mol of water (only 42% of what could be achieved with methane despite higher solubility of CO2 gas). Figure 3 presents morphology observations during hydrate formation in presence of surfactant with CO2 and CH4 (data/video for methane data based on study by Veluswamy et al. [13])
Figure 3. Visual observations of mixed CO2/THF and CH4/THF hydrates in presence of SDS surfactant
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We also performed hydrate formation experiments at 274.2 K and 3.0 MPa in presence of stoichiometric 5.6 mol% THF and CO2. As shown in Figure 1, these operating conditions envisage the formation of hydrates having both sI and sII structures. Under these conditions, it could be seen that gas uptake recorded was about 5.1 mmol of gas/mol of water, only slightly better than the gas uptake recorded at 283.2 K. Morphology of hydrate formation observed at 274.2 K were similar to that at 283.2 K and hence not presented. Despite the higher driving force (ΔT) available at 274.2 K and the possibility to form both sI and sII hydrate structures, rapid CO2 hydrate formation and increased gas uptake did not occur unlike the scenario observed with methane guest gas in presence of THF at both 283.2 K and 274.2 K. We also performed pure CO2 hydrate formation at 3.0 MPa and 274.2 K (in absence of THF promoter). Though there is reduced driving force (1.6 MPa in comparison to 2.4 MPa available at 283.2 K in presence of THF promoter), the gas uptake achieved was 11.8 mmol of gas/mol of water at the end of 2 h from nucleation, higher than that observed at 283.2 K and 274.2 K (in presence of THF). This reaffirms our observation that CO2 occupancy in 512 cages of sII structure in presence of THF is weak thus resulting in a significantly low gas uptake for THF/CO2 system. Visual observations (Figure 4a-e) show slightly more hydrate growth above the interface and the water below the interface remains clear unlike in presence of THF wherein it gets converted to hydrate as observed in figure 2d. In presence of 0.05 wt% SDS (optimal SDS concentration reported for improved gas uptake by et al.) the growth behaviour entirely changes as presented in Figure 4 (v-z). We observe improved kinetic promotion with hydrate growth above the gas/liquid interface and gas uptake of about 16.9 mmol gas/mol of water at the end of 2 h.
Figure 4. Visual observations of pure CO2 hydrates with and without SDS surfactant
Gas uptake profiles at the end of 60 min for all experiment trials conducted are presented as bar plots in figure 5 [13]. It can be observed from figure 5 that CO2 uptake without THF is better in comparison to with THF at both 274.2 K and 283.2 K (at least 2-4 times higher). In presence of low concentration of SDS (0.01-0.05 wt%) though initial kinetics is better for system with THF, the final gas uptake was observed to be higher for system without THF at comparable pressure driving force. Thus presence of THF did not characteristically result in improved mixed CO2 hydrate formation kinetics in sharp contrast to that observed for mixed CH4 hydrate formation kinetics.
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Figure 5. CO2 uptake profiles for experimental trials with different additives at 3.0 MPa starting pressure.
4. Conclusions We present the visual observations during hydrate formation observed for CO 2 hydrates with and without thermodynamic (THF) and kinetic (SDS) promoters. Experimental conditions were chosen such that it is possible to form pure sI, pure sII and a mixture of sI/sII hydrates. THF in stoichiometric concentration did not result in the improvement of kinetics/gas uptake unlike the synergistic effect associated with rapid hydrate formation kinetics achieved for CH4 study. Thus, guest gas plays a key role in influencing the activity of thermodynamic promoter despite similar driving force and operating conditions of hydrate formation. 5. Acknowledgements The work was funded in part under the Energy Innovation Research Programme (EIRP, Award No. NRF2015EWTEIRP002-002), administrated by the Energy Market Authority (EMA) and funded by the National Research Foundation (NRF) of Singapore. 6. References [1] Sloan ED, Koh CA. Clathrate Hydrates of Natural Gases. 3rd ed. New york: CRC press, Taylor & Francis Group; 2008. [2] Gudmundsson JS, Parlaktuna M, Khokhar A. Storing natural gas as frozen hydrate. SPE Production and Facilities. 1994;9:69-73. [3] Veluswamy HP, Kumar R, Linga P. Hydrogen storage in clathrate hydrates: Current state of the art and future directions. Applied Energy. 2014;122:112-32. [4] Park K-n, Hong SY, Lee JW, Kang KC, Lee YC, Ha M-G, et al. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Desalination. 2011;274:91-6.
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[5] Wang X, Dennis M, Hou L. Clathrate hydrate technology for cold storage in air conditioning systems. Renewable and Sustainable Energy Reviews. 2014;36:34-51. [6] 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. [7] Veluswamy HP, Linga P. Macroscopic kinetics of hydrate formation of mixed hydrates of hydrogen/tetrahydrofuran for hydrogen storage. International Journal of Hydrogen Energy. 2013;38:4587-96. [8] Linga P, Adeyemo A, Englezos P. Medium-pressure clathrate hydrate/membrane hybrid process for postcombustion capture of carbon dioxide. Environmental Science and Technology. 2008;42:315-20. [9] Kang S-P, Lee H. Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements. Environmental Science & Technology. 2000;34:4397-400. [10] Linga P, Kumar R, Lee JD, Ripmeester J, Englezos P. A new apparatus to enhance the rate of gas hydrate formation: Application to capture of carbon dioxide. International Journal of Greenhouse Gas Control. 2010;4:630-7. [11] Torré J-P, Ricaurte M, Dicharry C, Broseta D. CO2 enclathration in the presence of water-soluble hydrate promoters: Hydrate phase equilibria and kinetic studies in quiescent conditions. Chemical Engineering Science. 2012;82:1-13. [12] Lirio CFdS, Pessoa FLP, Uller AMC. Storage capacity of carbon dioxide hydrates in the presence of sodium dodecyl sulfate (SDS) and tetrahydrofuran (THF). Chemical Engineering Science. 2013;96:11823. [13] Veluswamy HP, Kumar S, Kumar R, Rangsunvigit P, Linga P. Enhanced clathrate hydrate formation kinetics at near ambient temperatures and moderate pressures: Application to natural gas storage. Fuel. 2016;182:907-19. [14] Veluswamy HP, Wong AJH, Babu P, Kumar R, Kulprathipanja S, Rangsunvigit P, et al. Rapid methane hydrate formation to develop a cost effective large scale energy storage system. Chemical Engineering Journal. 2016;290:161-73. [15] Robinson D, Metha B. Hydrates in the propanecarbon dioxide-water system. Journal of Canadian Petroleum Technology. 1971;10. [16] Seo Y, Kang S-P, Lee S, Lee H. Experimental Measurements of Hydrate Phase Equilibria for Carbon Dioxide in the Presence of THF, Propylene Oxide, and 1,4-Dioxane. Journal of Chemical & Engineering Data. 2008;53:2833-7. [17] Wu J-Y, Chen L-J, Chen Y-P, Lin S-T. Molecular dynamics study on the nucleation of methane + tetrahydrofuran mixed guest hydrate. Physical Chemistry Chemical Physics. 2016;18:9935-47. Biography: Praveen Linga is an Associate Professor in the Department of Chemical and Biomolecular Engineering at the National University of Singapore. His research interests are in the areas of clathrate (gas) hydrates, pertaining to energy recovery, carbon dioxide capture, storage & utilization (CCS & U), desalination and energy storage.
