Kinetic Promotion and Inhibition of Methane Hydrate Formation by

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Kinetic Promotion and Inhibition of Methane Hydrate Formation by Morpholinium Ionic Liquids with Chloride and Tetrafluoroborate Anions Wonhee Lee,† Ju-Young Shin,† Ki-Sub Kim,*,‡ and Seong-Pil Kang*,† †

Climate Change Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea ‡ Department of Chemical and Biological Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-si, Chungbuk 27469, Republic of Korea ABSTRACT: Ionic liquids of 1-hydroxyethyl-1-methylmorpholinium chloride (HEMM-Cl) and 1-hydroxyethyl-1-methylmorpholinium tetrafluoroborate (HEMM-BF4) were synthesized to investigate the different effects of anions in the ionic liquids on the methane hydrate formation kinetics. HEMM-Cl and HEMM-BF4 acted as the kinetic hydrate promoter and inhibitor, respectively. The induction time of HEMM-BF4 solutions increased in proportion to the HEMM-BF4 concentration, and both ionic liquids showed thermodynamic hydrate inhibition effect on methane hydrate formation. The X-ray diffraction pattern of hydrates formed in the presence of both ionic liquids showed that there was no influence on, and no incorporation of ionic liquids into, the crystal structure. Using the dynamic light scattering (DLS) technique, HEMM-Cl was revealed not to form micelles, which implies that HEMM-Cl is nonamphiphilic and its hydrate promotion mechanism is different from that of amphiphilic surfactant promoters. HEMM-Cl might distort the rigid hydrate host framework at the surface by the hydrogen bonds of the ions with water molecules, which could promote the methane penetration or inclusion into the growing clathrate hydrate structures, thereby improving hydrate formation. In contrast, the kinetic hydrate inhibition behavior of HEMM-BF4 might be attributed to the hypothesis that BF4− could act as a mobile pseudoguest because the anion cannot be the guest of the hydrate cage owing to the charge imbalance even though it could fit into the structure I hydrate cages.

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INTRODUCTION Modern society consumes a substantial amount of fossil fuels to produce energy by oxidation, which inevitably emits carbon dioxide, the most representative of the greenhouse gases now causing climate change and global warming. Natural gas, mainly composed of methane (CH4), generates the least amount of CO2 among the fossil fuels, because CH4 possesses only one carbon in its molecular structure and the carbon/hydrogen ratio of CH4 is the minimum among all hydrocarbons. Thus, natural gas has been considered the cleanest energy resource among the fossil fuels. Recently, natural gas is produced not only onshore but also in offshore fields to meet the increasing demand of energy supply. In particular, the natural gas hydrates buried in the deep oceans have also attracted considerable attention as a novel energy resource for the future.1 Thus, a variety of research has been performed for efficient, economic production of natural gas from the hydrate reservoirs.2−5 Because offshore wells produce water as well as natural gas, gas hydrate could be formed by the reaction of water with light hydrocarbon gas molecules and block the flow in the pipelines or risers.6 Inhibition of hydrate formation has been achieved using thermodynamic hydrate inhibitors (THIs) by changing the phase equilibrium of hydrate formation.1,6 Methanol (MeOH) or ethylene glycol are conventional THIs that form strong hydrogen bonds (H-bonds) to water molecules, which interferes with water-to-water interaction and inhibits hydrate crystallization.1,6 As another hydrate inhibition strategy, kinetic hydrate inhibitors (KHIs) are applied to delay hydrate crystallization processes, such as nucleation and growth.6 © XXXX American Chemical Society

Typical KHIs belong to the group of polymers with a polyethylene backbone and suspended lactam pendant groups.6 Recently, ionic liquids have received much attention as a novel type of hydrate inhibitor because these chemicals possess dual functions of both thermodynamic and kinetic hydrate inhibition.7−10 The ionic liquids are defined as salts that have low melting points due to their poorly coordinated ions.11,12 They can be designed using unlimited numbers of anions/ cations, and the associated properties are profoundly affected by their specific combinations.11,12 Thermodynamic modeling and phase-equilibria measurements of the gas hydrate formation have been carried out in the presence of various ionic liquids and presented a thermodynamic inhibition effect.13−19 This inhibition behavior is considered to arise from H-bonds between the ions and water molecules, which interrupt hydrate crystallization like general THIs.8,13 Additionally, their synergy effect for the kinetic natural gas hydrate inhibition was revealed by co-utilization of the ionic liquids and a polymeric KHI.20,21 However, the kinetic hydrate inhibition mechanism of ionic liquids has not been clearly demonstrated. While hydrate inhibition has been researched to avoid serious blockages in offshore pipelines or risers, hydrate promotion has been studied for storage of natural gas, involving use of the hydrogen-bonded water cages of the clathrate hydrate as an alternative for gas storage or transport.22−28 In particular, Received: February 2, 2016 Revised: April 7, 2016

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DOI: 10.1021/acs.energyfuels.6b00271 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

room temperature. HEMM-BF4 was obtained by the reaction between HEMM-Cl and sodium tetrafluoroborate, with an equimolar ratio in acetone as a solvent, at 25 °C for 24 h. The sodium chloride was filtrated, and the sodium chloride remaining in the ionic liquid was further reduced by filtration at low temperature with Celite (Aldrich). The final organic liquid phase was tested to confirm the concentration of the residual chloride salt using concentrated silver nitrate solution, and a small amount of silver chloride was precipitated. The 1H NMR (DMSO, δ/ppm, relative to TMS) spectrum included the following peaks: 3.20 (s, 3H), 3.39−3.45 (m, 2H), 3.49−3.58 (m, 4H), 3.87− 3.88 (s, 2H), 3.91−3.93 (s, 4H), 5.32−5.34 (t, 1H). The content of chloride anion was less than 185 ppm. Procedure. Figure 2 shows the experimental equipment used to determine the hydrate phase equilibria and to measure the amount of

natural gas storage as hydrate offers the advantages of low energy consumption and cost reduction, compared with liquefied natural gas (LNG; requires very low temperature) or compressed natural gas (CNG; requires very high pressure).22 To reduce the hydrate formation period and process cost, the promotion of hydrate kinetics has been widely investigated using various surfactants.23−28 In a previous study, the hydrate promotion effect of the ionic liquid, 1hydroxyethyl-1-methylmorpholinium chloride (HEMM-Cl) was investigated and compared with that of sodium dodecyl sulfate (SDS).29 Interestingly, based on previous research, ionic liquids could act as kinetic inhibitors or promoters of hydrate formation.9,20,29 In the present study, the hydrate kinetic inhibition effect of the ionic liquid 1-hydroxyethyl-1-methylmorpholinium tetrafluoroborate (HEMM-BF4) was revealed even though HEMM-Cl possessing the same cation promoted the rate of hydrate formation.29 In other words, directly opposed hydrate formation kinetics was observed by anion replacement of ionic liquids. To understand the hydrate promotion effect of HEMM-Cl, an attempt was made to measure the critical micelle concentration (cmc) of the ionic liquid using the dynamic light scattering (DLS) technique. In addition, the hydrate phase equilibria in the presence of HEMM-BF4 and HEMM-Cl were measured and compared. X-ray diffraction (XRD) patterns of the CH4 hydrates formed in the presence of HEMM-Cl and HEMM-BF4 have also been presented to confirm whether the ionic liquids affected the crystal structure or not. The different effects of the two ionic liquids on the hydrate formation kinetics are discussed.

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EXPERIMENTAL SECTION

Reagents. For synthesis of the ionic liquids, 4-methylmorphorine (Aldrich, 99%), 2-chloroethanol (Aldrich, 99%), and sodium tetrafluoroborate (Sigma-Aldrich, 98%) were used for synthesis of ionic liquids without purification. Acetonitrile (99.9%, Merck) and acetone (99.9%, Merck) were utilized as solvents. Deionized (DI) water was obtained from water purification equipment (Zeneer Power II, Human Corp.). Methane (CH4, 99.99%) supplied from Safety Gas (Seoul, Korea) was used for hydrate formation. Synthesis of Ionic Liquids. Two ionic liquids, HEMM-Cl and HEMM-BF4 were synthesized for this study. Figure 1 shows the synthesis routes of the ionic liquids. The 4-methylmorphorine (15.4 mL) was mixed with an excess amount of 2-chloroethanol (13.4 mL) and reacted in nitrogen atmosphere, at 70 °C for 48 h. Acetonitrile (200 mL) was used as a solvent to yield HEMM-Cl. White crystals (20 g, yield 80%) of HEMM-Cl were generated by recrystallization at

Figure 2. Schematic representation of the apparatus. CH4 consumption for hydrate formation kinetics. A stainless steel reactor (220 mL) was designed to measure pressure and temperature under high pressure. The reactor was immersed in an ethylene glycol bath, of which the temperature was controlled by an external circulator (RW-3040G, Jeio Tech). The pressure and temperature inside the reactor were automatically measured and recorded by a computer during all the experiments. Hydrate−liquid−vapor (HLV) three phase equilibria in the presence of ionic liquids were determined by the measurement of the dissociation temperatures of the hydrates formed at various pressures. An aqueous solution (90 mL) was injected into the reactor in the presence of ionic liquids (28.1 wt % (3.74 mol %) of HEMM-Cl or 10.0, 30.0, 45.5 wt % (0.85, 3.21, 6.06 mol %) of HEMM-BF4). The air inside the reactor was removed by a vacuum pump. Then, the injection and ventilation of CH4 were repeated three times to further remove any residual gases inside the cell. After the reactor was pressurized with CH4 to the desired pressures, the temperature was gradually decreased to induce hydrate formation. The initiation of gas hydrate formation caused declining pressure inside the cell due to the consumption of CH4 arising from hydrate formation. Once the decline in the cell pressure stopped, the pressure was maintained for some period of time. The hydrate formed was dissociated by increasing the cell temperature at a rate of 0.1 K/h. Hydrate dissociation lines intersected with thermal expansion lines of the aqueous solution containing ionic liquids, and the intersections, indicating three phaseequilibrium points, were determined. Aqueous solutions in the presence of 1 wt % of HEMM-Cl and HEMM-BF4 were used to compare the hydrate formation kinetics. Additionally, the induction times of hydrate formation were measured at various concentrations of HEMM-BF4 (0, 0.1, 1, 5, or 10 wt %). The sample solutions were injected into the reactor immersed in the

Figure 1. Synthesis procedure of ionic liquids: (a) HEMM-Cl and (b) HEMM-BF4. B

DOI: 10.1021/acs.energyfuels.6b00271 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels ethylene glycol bath. The reactor temperature was maintained at 274.15 K. The air inside the reactor and other gases dissolved in the solution were removed by a vacuum pump. CH4 was injected and vented three times to further remove gases other than CH4 from the cell. Then, CH4 was pressurized to slightly over 70 bar in the reactor, and this pressure was held for enough time to saturate the solutions. Once the pressure stabilized, the pressure was set to around 70 bar, the initial condition of the experiments. The temperature and pressure of the reactor were recorded by computer, and agitation, provided by a magnetic drive (700 rpm), was initiated for hydrate formation. Each experiment with ionic liquids at various concentrations was repeated three times. A particle size analyzer (Malvern Instruments, Zetasizer Nano ZS90) was used to confirm whether HEMM-Cl aqueous solution could form micelles, using the dynamic light scattering (DLS) technique. It can measure the particle size (diameter) at a 90° scattering angle from 0.3 nm to 5 μm. An attempt was made to measure the critical micelle concentration of the HEMM-Cl solution at various concentrations (20, 50, 100 mM; around 0.36, 0.91, 1.82 wt %, respectively) using the particle size analyzer. To confirm the effect of ionic liquids on the crystal structure of the gas hydrate, the XRD patterns of the two hydrate samples were recorded using a Rigaku D/Max-2500 with low temperature equipment. The samples were obtained from the aqueous solution in the presence of 1 wt % of HEMM-Cl or HEMM-BF4 after CH4 hydrate formation reaction by liquid nitrogen quenching. The hydrates were ground to less than 200 μm. Graphite-monochromatized Cu Kα1 radiation (wavelength, 1.5406 Å) was used as a light source. The XRD measurements were carried out in step mode with a step width of 0.030° and a fixed time of 2 s. The 2θ range was from 5° to 55°. All of the XRD patterns were obtained at 93 K.

Figure 3. Comparison of pressure profiles as a function of time during CH4 hydrate formation in the presence of 1 wt % of HEMM-Cl or HEMM-BF4 at 274.15 K.

this experiment using HEMM-BF4 was 58 min, and the reactor pressure dropped only 3.2 bar for 170 min. Figure 4 shows the

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RESULTS AND DISCUSSION Previous research reported that some ionic liquids containing Cl−, such as EMIM-Cl (1-ethyl-3-methylimidazolium chloride) and BMIM-Cl (1-butyl-3-methylimidazolium chloride), exhibited kinetic hydrate inhibition.8 Moreover, several ionic liquids containing BF4−, such as EMIM-BF4 (1-ethyl-3methylimidazolium tetrafluoroborate), BMP-BF4 (1-ethyl-1methylpyrrolidinium tetrafluoroborate), and HEMP-BF4 (1hydroxyethyl-1-methylpyrrolidinium tetrafluoroborate), showed much higher hydrate inhibition performance than those containing other types of anions including dicyanamide, trifluoromethanesulfonate, ethylsulfate, and halides.7−9 However, promotion of hydrate formation in the presence of HEMM-Cl was revealed in a previous study even though HEMM-Cl possesses a halide anion.29 Thus, the hydrate formation kinetics of aqueous solutions with HEMM-Cl and HEMM-BF4 were compared in this work, because of the particular kinetic inhibition effect of BF4− on the hydrate formation. Figure 3 shows the pressure traces during hydrate formation with aqueous solution containing 1 wt % ionic liquids, (a) HEMM-Cl or (b) HEMM-BF4, and (c) without any additives. The initial subcooling of these experiments was to ∼8.9 K, which is the driving force for hydrate formation. Without ionic liquids, formation of CH4 hydrate started immediately after agitation, which continuously decreased the reactor pressure from 69.4 to 62.9 bar for 200 min. In the presence of 1 wt % HEMM-Cl, it took only 2 min for the reactor pressure to decrease from 70.1 to 62 bar, which implies that CH4 consumption was much more rapid due to facilitated hydrate formation. This rapid hydrate formation was observed in multiple experiments. However, the use of 1 wt % HEMM-BF4 delayed the onset of hydrate formation and retarded the growth rate, unlike in two previous experiments. The induction time of

Figure 4. Average induction time of CH4 hydrate formation in the presence of different concentrations of HEMM-BF4 at 70 bar and 274.15 K.

induction time of CH4 hydrate formation in the presence of different concentrations of HEMM-BF4 (up to 10 wt %). The graph indicates that the induction time monotonically increased with increase in the concentration of HEMM-BF4. Despite the unique hydrate promotion effect of HEMM-Cl, the previous study only provided experimental results.29 Because most kinetic hydrate promoters are surfactants,23−31 it is necessary to consider whether the hydrate promotion mechanism of HEMM-Cl could be similar to those of general surfactants or not. In several studies, mechanisms were proposed for the enhanced hydrate formation rate induced by the surfactants. According to the research of Zhong and Rogers, gas hydrate formation in the sodium dodecyl sulfate (SDS) aqueous solution was dramatically promoted above the cmc of the surfactant.23 They explained that the increase in the hydrate formation rate originated from the gaseous hydrocarbon solubilized in the micelles, which induced subsurface hydrate formation.23 However, the cmc of SDS in their study was C

DOI: 10.1021/acs.energyfuels.6b00271 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels determined by induction time during hydrate formation23 rather than general methods to measure the cmc of surfactants, such as surface tension/conductivity measurement or DLS technique. This implies that the cmc of SDS determined in the previous study was quite suspicious. According to the research conducted by Watanabe et al.32 and Zhang et al.,33 SDS did not form micelles in their hydrate formation experiments because of too high Krafft point of SDS. Thus, the micelles are unlikely to be involved in the promotion of the hydrate formation rate.32,33 Okutani et al. insisted on a “the capillarity-driven hydrateformation mechanism” in which water is supplied to porous hydrate layers by capillary force, leading to continuous contact between gas and water, rather than a micelle-driven mechanism.31 Despite the extensive research and efforts to reveal the surfactant effect on the hydrate formation kinetics, its mechanism is not clearly understood. However, the proposed mechanisms are based on the basic nature of the surfactants; that is, the surfactants are amphiphilic.34 If HEMM-Cl possessed similar structures or exhibited similar properties to surfactants, it might follow the similar hydrate promotion mechanism of surfactants. Even though micelles are unlikely to play any roles in hydrate promotion,32,33 micellization is a unique phenomenon of general surfactants due to their amphiphilic structure.34 Thus, if HEMM-Cl does not form micelles, its structure has to be different from those of amphiphilic surfactants, which leads to a different hydrate promotion mechanism of HEMM-Cl from those of surfactants. It was known that most surfactants exhibit a cmc less than 100 mM.34 Hence, an attempt was made to measure the cmc of HEMM-Cl using 20, 50, and 100 mM (around 0.36, 0.91, and 1.82 wt %, respectively) HEMM-Cl solutions by the DLS technique to confirm whether HEMM-Cl follows a mechanism similar to that of surfactants. It was not possible to detect the existence of any types of micelles, which might be spherical, lamellar, vesicular, or bicontinuous structures, using the nanoparticle size analyzer. This result indicates that HEMMCl is a nonamphiphilic chemical, and thus, the promotion effect of HEMM-Cl on the hydrate formation would not follow the same mechanism of amphiphilic surfactants such as SDS. (It should be noted that a cmc of HEMM-Cl was attempted to be measured by DLS technique to check whether HEMM-Cl is amphiphilic or nonamphiphilic.) Due to the unique structures of surfactants consisting of hydrophilic heads and hydrophobic tails, they generally form micelles to minimize the free energy of solutions.34 Unlike the surfactants, HEMM+ does not possess distinguishable hydrophilic and hydrophobic parts, because cyclic ether and hydroxyl groups make the entire structure of the cation more hydrophilic than other cations such as EMIM+ or BMIM+. The oxygen sites of HEMM+ and Cl− itself can be hydrogen-bonded with water molecules in the solutions. Thus, it is necessary to consider the hydrate formation kinetics in the presence of polar organic or ionic materials that can form Hbonds with water. McLaurin et al. reported that low concentrations of various thermodynamic inhibitors, such as MeOH, ammonia (NH3), glycerol, sodium hydroxide (NaOH), hydrogen chloride (HCl), and sodium chloride (NaCl), can facilitate CH4 hydrate formation occurring in powdered frozen solutions.35 Additionally, CH4 hydrate has been produced at high yields with additives,35 as in the previous studies using surfactants,23−27 and in this study with HEMM-Cl. According to molecular dynamics simulation, MeOH molecules replaced water

molecules at the ice surface to form a MeOH solution/ice interface, which facilitated the penetration of CH4 molecules into the MeOH solution layer.35 They suggested a possible mechanism by which MeOH penetration into the ice lattice generates Bjerrum defects due to the different numbers of Hbonds between MeOH and water molecules, which also assisted the CH4 entry into the ice lattice.35 Then, the water molecules in the ice structure may be rearranged to form structure I (sI) CH4 hydrate.35 Another simulation result showed that the MeOH inclusion in the large cage of the sI hydrate distorted the clathrate structure.36 In summary, molecular dynamics simulations in previous studies showed the distortion of rigid ice or hydrate structure due to the strong H-bonds between MeOH and water molecules.35,36 Additionally, MeOH also exhibited the kinetic hydrate promotion behavior and produced high hydrate yield in liquid solution at low concentrations (