Methane storage in mixed hydrates with tetrahydrofuran

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1 Indian Journal of Chemical Technology Vol. 21, March 2014, pp Methane storage in mixed hydrates with tetrahydrofuran D V S G K Sharma 1, Y Sowjanya 1, V Dhanunjana Chari 1, 2 & P S R Prasad 1, * 1 Gas Hydrates Division, CSIR-National Geophysical Research Institute, Hyderabad , India 2 Department of Physics, Osmania University, Hyderabad , India Received 8 November 2012 ; accepted 31 May 2013 A systematic study on the methane hydrate formation and dissociation with aqueous tetrahydrofuran (~ 6.03 mol %) has been carried out in a stirred reactor. Experiments are conducted with initial methane pressure in the range MPa. Encaging of methane molecules in vacant cages (5 12 ) in mixed hydrate is found to be significantly faster compared to pure methane hydrates. The hydrate formation temperature and methane consumption depend on the initial gas pressure ( MPa) in the reactor and the consumed methane varies from 22.0 (± 1.4) m.mol to 88.0 (± 2.6) m.mol. The nucleation and dissociation temperature of the mixed hydrates is always higher than ice melting temperature. Keywords: Methane hydrates, Methane storage capacity, Phase boundary, Tetrahydrofuran hydrates Gas hydrates are the three dimensional ice-like crystalline compounds, often found in nature. The four essential conditions for their formation are (i) availability of host water molecules, (ii) suitable guest molecules having molecular diameter matching with available space within the host cages, (iii) moderately high pressure and (iv) lower temperature typically in the vicinity of freezing point of ice. The guest molecules are usually smaller hydrocarbons (methane, ethane, propane and iso-butane) CO 2, H 2 S and so on 1,2. Further, smaller molecules like H 2 and He can also form stable clathrates but require an order of magnitude higher pressure than other gas hydrates 3,4. Permafrost regions and the ocean bottom sediments around the globe, where gas hydrates can naturally occur because of the conditions described above, coexist. Methane is the most predominant guest molecule in natural gas hydrates (NGH) but traces of other guest molecules significantly alter the structure and phase stability of methane hydrates 5,6. Although gas hydrates are considered as a nuisance when they occur in oil/gas flow lines, it has been realised that these are a potential energy resource when present in large chunks in natural environment 1,2,7. The global estimate on the amount of energy (methane gas) trapped within natural gas hydrate deposits varies widely, but the most conservative estimates show that *Corresponding author. psrprasad@ngri.res.in the amount of energy in hydrate deposits would be twice to that of all fossil fuel reserves available worldwide 8,9. In summary, the gas hydrates have some useful and devastating properties. Some of the destructive events like blockades in oil/gas pipe lines are often managed by using some inhibitors 7. Another major concern like seafloor collapse or environmental impact of excess methane (due to hydrate dissociation) is still a topic of intensive research 10,11. Typically 163 m 3 of STP equivalent guest molecules can be stored in a unit volume of hydrates and hence this has become an attractive system of fuel gas storage/transportation 2. Additionally, green house gasses like CO 2 can also form stable hydrates and thus the research on gas hydrates gained momentum for separating CO 2 from flue gases and storing it under ocean floors. One of the major technical issues on fuel gas storage in the form of gas hydrates, eventually for storage and transportation applications, is how to increase the mass and heat transfer in the fuel gas-water systems in order to attain a rapid hydrate formation. It has been well documented in the literature that the tetrahydrofuran (THF) can form hydrate having the structure II, with a stiochiometric ratio of THF 17H 2 O 12,13 and also it works as a help molecule for gas hydrate formation Addition of THF largely shifts the hydrate equilibrium conditions and extends the hydrate stability region thus it is known as thermodynamic promoter The research on gas hydrate formation with THF solution has been based on thermodynamics data and there is

2 SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN 115 little information available on the hydrate formation kinetics. Recently, THF hydrate based systems have been proved to be useful for separating and storing the methane gas in coal mine methane gas 22,23. Also pellets of pure methane hydrates were prepared with THF hydrates for possible transportation applications 24. In our previous studies, it was shown that the dissociation pressure and temperature conditions critically depend on the concentration of THF in aqueous solution 20,25. In this paper, we report the formation kinetics of CH 4 hydrates in aqueous solution of THF (~6.0 mol%) at different pressures of the range MPa. The phase stability behaviour of the mixed hydrate system in this pressure range and the conversion efficiencies of mixed hydrates are also investigated in a stirred reactor. Experimental Procedure Materials Aqueous solutions were prepared by following the gravimetric method using a Metler Toledo (AB104-S) high accuracy analytical balance. Consequently, the uncertainties on mole fractions are estimated to be below Doubly distilled and deionised water and tetrahydrofuran (98%), supplied by Qualigens (Fine Chemicals, India) were used in this study. Experimental apparatus and procedure Detailed description of experimental layout and procedure has already been described in previous reports 20. Briefly, the main part of the apparatus is a SS-316 cylindrical vessel of 100 ml, which can withstand pressures up to 10 MPa. A stirrer with variable speed was installed in the vessel to agitate the fluids and hydrate crystals inside it. All the experiments were conducted with a fixed speed of 500 rpm. Cold fluid (water + glycol mixture) was circulated around the vessel with the help of Lab Companion (RW-0525G) circulator, to maintain the temperature inside it at a desired level. A platinum resistance thermometer (Pt100) inserted into the vessel was used to measure temperature and check for equality of temperature within measurement uncertainties, which is estimated to be less than 0.2 K. The pressure in the vessel was measured with a WIKA pressure transducer (WIKA, type A-10 for pressure range 0 16 MPa). The vessel containing aqueous solution (approximately 30% by volume of the vessel) was immersed into the temperature controlled bath and the gas was supplied from a cylinder to desired level using Teledyne ISCO Syringe pump (Model 100DX). The vessel was evacuated before introducing any aqueous solution and gas. After obtaining temperature and pressure stability (far above from the hydrate formation region), the valve inline connecting the vessel and the ISCO pump/cylinder was closed. Subsequently, temperature was slowly decreased to form the hydrate and hydrate formation in the vessel was detected by pressure drop. The temperature was then increased in steps of 1.0 K. At each step, temperature was kept constant with sufficient time to achieve equilibrium state in the vessel. In this way, a pressure temperature diagram was obtained for each experimental run, from which we determined the hydrate formation and dissociation pattern. If temperature is increased in the hydrate-forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. If the temperature is increased outside the hydrate region, only a smaller increase in the pressure is observed as a result of the change in phase equilibria of fluids in the vessel. Consequently, the point at which the slope of pressure-temperature data plot changes sharply is considered to be the point at which all hydrate crystals have dissociated. Experiments were conducted at various initial pressure conditions and dissociation points were determined by isochoric pressure search method. In all the experiments the reactor was filled with 30 g aqueous solution of THF and was then pressurised with methane gas. Temperature of circulating bath fluid (glycol + water) was set at 288 ± 1.0 K to bring system into the hydrate formation zone and stirrer was kept at 500 rpm for the entire experimental run. Certain degree of super cooling and overpressure conditions (driving force) are essential to initiate the nucleation of hydrates. It is evident that methane hydrate formation occurs at higher temperatures in the presence of promoter molecules. The molar concentration ( nh,t) of methane gas in the hydrate phase during the experiment at time t, is defined by the following equation: nh,t = ng,0 ng,t = (P 0 V/Z 0 RT 0 ) (P t V/Z t RT t ) (1) where the compressibility factor Z was referred from Perry s Chemical Engineers Handbook. The gas volume (V) was assumed as constant during the

3 116 INDIAN J. CHEM. TECHNOL., MARCH 2014 experiments, i.e. the volume changes due to phase transitions were neglected. ng,0 and ng,t represent the number of moles of feed (methane) gas at 303 K taken as zero time and in the gas phase at time t respectively. Pressure (P) and temperature (T) were logged at a fixed time interval (60 s) as the hydrate formation/dissociation progresses. The hydrate formation could be divided into three regions for convenience. In region 1, the change in vapour concentration of gas was almost constant, mainly due to real gas nature of the methane molecules and the typical time we allowed for this stage being around min. In the second stage (region 2), the temperature remained constant under steady state condition for about 9 10 h after hydrate nucleation for its maximum conversion/ growth. There was a decrease in the gas concentration due to maximum pressure drop in the reactor vessel for hydrate formation. Thereafter, in region 3, once the hydrate formation was completed it is similar to region 1. The plot between n(ch 4 ) vs. time (inset of Fig.1a) in these three regions provide necessary information about the rate of hydrate formation and the total gas adsorbed during hydrate formation. Results and Discussion Figure 1 shows the methane content in vapor phase (m.mol) and temperature (T) variations with time during initial 900 min (i.e. hydrate formation) with pure water alone and in aqueous THF solution. Corresponding n temperature (n T) trajectories (in both hydrate formation and dissociation stages) are shown as the insets. Since our goal is to get an insight into the formation kinetics for hydrate forming systems with co-guest molecules (THF), we conducted all the experiments with about 6 mol% THF solutions. It has been reported in the literature that the resultant structure for the mixed hydrates as Fig. 1 Observed variations in methane in vapor phase ( ) and temperature ( ) with time inside the reactor. (a) water + methane system; and aqueous THF (6.03 mol%) solution + methane system at different initial pressures (b) 8.21 MPa, (c) 6.21 MPa and (d) 1.88 MPa. Corresponding n(ch 4 )-temperature trajectory during hydrate formation and dissociation is shown as inset

4 SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN 117 sii, with THF molecules occupying cages and CH 4 molecules in 5 12 cages 13,26. Formation kinetics in mixed hydrates and the overall methane storage in sii is compared with pure methane hydrate system under identical experimental conditions. Further, the stirring (at 500 rpm) was continuously on throughout the experimental duration (during hydrate growth and dissociation) to promote a faster nucleation and growth of methane hydrates 27,28. It is known that the outside of hydrate formation zone, pressure in the reactor vessel monotonically decreases with temperature, however within the hydrate formation zone an abrupt pressure drop is observed even at constant temperature. Because of heat of formation (exothermic) during the hydrate nucleation, we observed peaks in the temperature vs. time profiles. The height and duration of the peaks depend on the amount of formation of heat and the mass and heat transfer rates of the hydrate nucleation at different pressures. As can be seen from Fig. 1, the hydrate nucleation is more sluggish for the water + methane system. Comparatively for water + methane + THF system a very sharp nucleation occurs within the first one hour of cooling and slowly the system attains the equilibrium temperature i.e. ~ 288 K (Figs 1b d). It is clear that the consumption of 90% of methane to hydrates occurs significantly faster in methane + THF + water system. It takes around 425 min for pure hydrates (Fig. 1a) while it is much faster, i.e. 150, 160 and 175 min respectively for the mixed hydrate system at three different initial pressure conditions (8.21, 6.21 and 1.88 MPa at 303 K). It is observed that the methane intake into THF hydrates at other pressures also follows a similar trend and which is significantly faster compared to pure hydrates. In Fig. 2, we plotted the phase stability conditions measured in this study and compared these with the literature data 17. The phase stability point is determined by following the procedure described by Sloan and Koh 1. The data points plotted in figure are the hydrate dissociation points following the isochoric conditions. The results are correlated well with the literature data and the mixed hydrates could be formed at much lower pressures in the temperature range K. Mixed hydrates are formed at much lower methane pressure i.e. ~ 0.88 MPa and the formation temperature is around 288 K. It is clear from Fig.2 that all the mixed hydrates formed at methane pressures ranging from 0.88 MPa to 8.21 MPa are stable upto temperatures K and thus it could be advantageous to reform the methane hydrates in aqueous THF of sii stoichiometry rather than using lower mole fractions of THF 21. However, lowering the THF contents significantly alters the hydrate dissociation temperature Kida et al. 24 had recently reported the dissociation behaviour of granular samples of pelletized powdered mixtures of methane or ethane and THF hydrate through a temperature ramping method and concluded that presence of THF clathrate hydrates enhances the restraining effect on the dissociation of pure methane or ethane hydrates up to temperatures above the melting point of ice. In our previous studies, we also noticed a significant variation in dissociation of pure methane hydrates under the influence of THF hydrates 20. At lower concentrations of THF the hydrate formation occurs in two steps, namely hydrates in first stage (~T=295 K) were mostly with sii structure and in the second stage (~T=275 K) were with si structure. There are no si type hydrates at THF concentrations more than mole fractions and the dissociation temperature is also found to be higher by about 18 K. As already discussed the formation kinetics are significantly faster for mixed hydrates. Further concern is about the overall methane gas that could be stored in mixed hydrate systems. In Fig. 3, we show the hydrate yield in mixed hydrates and compared them with pure methane hydrates. As already reported by Kida et al. 24 there is no noticeable differential increment in the constant volume pressure vessel due to the dissociation of pure THF hydrates and the pressure increment/ decrement is because of methane gas release/ consumption in the hydrate system. The hydrate yield is computed from Fig. 2 The phase boundary curve for the mixed hydrates with THF and methane [( ) present study and (+) data of ref.16]

5 118 INDIAN J. CHEM. TECHNOL., MARCH 2014 the observed methane gas consumption in the experiments to the expected values with stiochiometric compositions as described in the literature 20. We assume methane occupancy in small (5 12 ) and large ( ) cages for pure hydrates (si). Further in mixed hydrates (sii) the small (5 12 ) cages are occupied by methane molecules while the large ( ) cages are populated with THF molecules and therefore unit cell compositions for the two respectively are 8CH 4 46H 2 O and for mixed hydrates it is 16CH 4 8THF 136H 2 O. As shown in Fig.3, hydrate yield is more at higher driving force (methane pressure measured at 303 K) and the behaviour could empirically be correlated to second order polynomial with a constraint that no hydrate yield at zero driving force. On the other hand, the methane consumption (hydrate yield) in mixed hydrates is always higher than in pure methane hydrates. The expected methane consumption (mass) in si stiochiometry is 0.135, while the same for mixed hydrates with THF in sii stiochiometry is Therefore, it may be advantageous to use THF hydrates as a medium for storing the methane gas, in particular; the present study shows that the formation kinetics is much faster and gives superior methane consumption compared to pure methane hydrates. Additional advantage is that the formation and dissociation of mixed hydrates occur at much convenient temperatures (~ K) even at much lower methane pressure (~0.9 MPa). Another concern is about the dissociation p-t conditions at which significant amount of methane gas is released from hydrates. Figure 4 shows the methane gas released at different temperatures. First the mixed hydrates are formed (at different initial methane pressures) by the procedure described. The amount of methane consumed in mixed hydrates is measured as 88 (±2.6), 73 (±1.4), 67 (±2.2), 53 (±2.8), 44 (±1.4) and 22 (±1.4) m.mol at initial methane pressure of 8.21, 6.21, 4.25, 3.16, 1.88 and 0.88 MPa. It is well known that the hydrate yield is more at higher gas (guest) pressures 1,2. As already explained, insignificant pressure variation for longer duration indicates the saturation in hydrate conversion and then reactor is cooled to about 265 K. Residual methane gas is completely removed by maintaining the reactor vessel at constant temperature ~265 K. Now the temperature is slowly increased at a rate of 0.05 /min and the pressure variations are continuously logged at 10 s intervals. As shown in Fig. 4, the total amount of methane gas released from mixed hydrate system is 88, 73, 68, 52, 43 and 21 m.mols. As expected, higher amount of methane gas is stored in hydrates formed at higher pressures. The methane gas is released from mixed hydrates at a rate ranging from 0.30 m.moles/min to 0.64 m.moles/min and significant gas release starts always above 275 K (higher than ice melting temperature), indicating that the mixed hydrate systems could hold the methane molecules at much higher temperatures even at 0.1 MPa pressure. It is well known that the stability temperature for methane hydrates at this pressure is normally around 193 K. In other words, the stability of mixed hydrates is significantly higher than pure methane hydrates 20, 24. Fig. 3 Hydrate yield in the mixed hydrate ( ) and pure methane hydrate systems ( ) at different initial gas pressures. Best fit for observed data is shown as solid line Fig. 4 Methane gas recovered upon dissociation of mixed hydrates

6 SHARMA et al.: METHANE STORAGE IN MIXED HYDRATES WITH TETRAHYDROFURAN 119 Conclusion Mixed hydrate formation and dissociation behavior has been studied in methane and aqueous THF system. The formation is found to be much quicker in methane and aqueous THF as compared to that in pure methane hydrates. Typically about 90% of methane gas is consumed in mixed hydrates within 200 min, while similar amount is consumed in about 500 min in pure methane hydrates. Experimental observations show that the overall hydrate yield in mixed hydrates (sii structure with THF molecules in cages & CH 4 molecules in 5 12 cages) is always more than that in pure methane hydrates (si structure with CH 4 molecules in 5 12 & cages). It is also observed that significant amount of methane gas release occurs from mixed hydrates at temperatures higher than the ice melting temperature. Thus, the mixed hydrate system with aqueous THF of sii stiochiometry could be used as medium to store and transport the methane gas, because of its ability to accommodate methane molecules in vacant small cages even at lower pressures with fast formation kinetics and higher dissociation temperature (>275 K). Acknowledgement The authors gratefully acknowledge the funding support from the Department of Science & Technology, Ministry of Earth Sciences (gas hydrates programme), Directorate of Hydrocarbons (NGHP) and DRDO ASL, Government of India, through sponsored projects. References 1 Sloan E D & Koh C A, Clathrate Hydrates of Natural Gases, 3 rd edn(crc Press, Taylor & Francis Group, Boca Raton) Koh C A, Sloan E D, Sum A K & Wu D T, Annu Rev Chem Biomol Eng, 2 (2011) Dyadin Y A, Larionov E G, Aladko E Y, Manakov A Y, Zhurko F V, Mikina T V, Komarov V Y & Grachev E V, J Struct Chem, 40 (1990) Mao W L, Mao H K, Goncharov A F, Struzhkin V V, Guo Q Z, Hu J Z, Shu J F, Hemley R J, Somayazulu M & Zhao Y S, Science, 297 (2002) Sloan E D, Nature, 426 (2003) Lu H, Seo Y, Lee J W, Moudrakovski I, Ripmeester J A, Chapman N R, Coffin R B, Gardner G & Pohlman J, Nature, 445 (2007) Jamaluddin A K M, Kalogerakis N & Bishnoi P R, J Pet Sci Eng, 5 (1991) Makogon Y F, Holditch S A & Makogon T Y, J Pet Sci Eng, 56 (2007) Milkov A V, Earth Sci Rev, 66 (2004) Kelland M A, Energy Fuels, 20 (2006) Kvenvolden K A, Proc Natl Acad Sci, 96 (1999) Mak T C & McMullan R K, J Chem Phys, 42 (1965) Kim D Y, Park J, Lee J W, Ripmeester J A & Lee H, J Am Chem Soc, 128 (2006) Seo Y T, Lee H, Moudrakovski I & Ripmeester J A, Chem Phys Chem, 4 (2003) Zhang Q, Chen G J, Huang Q, Sun C Y, Guo X Q & Ma Q L, J Chem Eng Data, 50 (2005) Papadimitriou N I, Tsimpanogiannis I N & Stubos A K, J Chem Phys, 131 (2009) De Deugd R M, Jagar M D & de Swaan Arons J, AIChE Journal, l47 (2001) Susilo R, Alavi S, Ripmeester J A & Enzlezos P, Fluid Phase Equilibria, 263 (2008) Mohammadi A H & Richon D, Ind Eng Chem Res, 48 (2009) Chari V D, Sharma D V S G K & Prasad P S R, Fluid Phase Equilibria, 315 (2012) Strobel T A, Koh C A & Sloan E D, Fluid Phase Equilibria, 280 (2009) Zhang B & Wu Q, Energy Fuels, 24 (2010) Ma Q L, Chen G J & Zhang L W, J Chem Eng Data, 54 (2009) Kida M, Jin Y, Narita H & Nagao J, Phys Chem Chem Phys, 13 (2011) Prasad P S R, Chari V D, Sharma D V S G K & Murthy S R, Fluid Phase Equilibria, 318 (2012) Prasad P S R, Sowjanya Y & Prasad K S, J Vibra Spec, 50 (2009) Vysniauskas A & Bishnoi P R, Chem Eng Sci, 38 (1983) Kim H C, Bishnoi P R, Heidenmann R A & Rizvi S S H, Chem Eng Sci, 42 (1987) 1645.