Synergetic Effect of Ionic Liquids on the Kinetic Inhibition Performance

Oct 12, 2016 - Energy Fuels , 2016, 30 (11), pp 9162–9169 ... highest hydrate-nucleation-inhibiting performance when it was used alone, further expe...
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Synergetic Effect of Ionic Liquids on the Kinetic Inhibition Performance of Poly(N‑vinylcaprolactam) for Natural Gas Hydrate Formation Wonhee Lee,† Ju-Young Shin,† Ki-Sub Kim,*,‡ and Seong-Pil Kang*,† †

Climate Change Research Division, Korea Institute of Energy Research, 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 S Supporting Information *

ABSTRACT: To identify the synergetic inhibition effects of ionic liquids (ILs) containing tetrafluoroborate anion (BF4−), various ILs, poly(N-vinylcaprolactam) (PVCap), commercially available polymeric hydrate inhibitor, and their mixtures, were tested as kinetic hydrate inhibitors (KHIs) for natural gas hydrate formation. The experimental results revealed that PVCap−IL mixtures exhibited significantly higher KHI performance. In particular, the mixture of PVCap and 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF4) showed the best hydrate inhibition effectiveness, even under higher pressures. As HMP-BF4 also exhibited the highest hydrate-nucleation-inhibiting performance when it was used alone, further experiments were performed using the mixtures of PVCap and HMP-BF4 at various combinational concentrations. As a result of the experiments, the combination of 1.0 wt % PVCap and 0.5 wt % HMP-BF4 was found to provide the longest induction time. The excellent synergetic effect of the ILs on natural gas hydrate inhibition may arise from the prevention of methane-containing 512 cage formation by the ILs, inducing inhibition of metastable structure I hydrate formation. more severe.2,3 Methanol (MeOH) and monoethylene glycol (MEG) are the THIs, most widely used for hydrate flow assurance. However, MeOH is a toxic chemical, and its high vapor pressure makes it difficult to recover.3 Additionally, MeOH acts as a poison, reducing the performance of molecular sieves/catalysts, and generates various problems in downstream processes.3 Unlike MeOH, MEG causes salt precipitation (NaCl)/boiler fouling (NaCl and CaCO3) during the MEG regeneration process,3,4 and its high viscosity obstructs the efficient flow of the hydrocarbon fluids.3 Furthermore, there is usually not enough space to store a large amount of THIs for high-water-cut systems (greater water volume than 70%3) in a floating production storage and offloading (FPSO) unit (used for oil and gas production/processing in offshore fields). Thus, another hydrate inhibition method should be considered to reduce the operating cost arising from the THIs. As an alternative to the THIs, kinetic hydrate inhibitors (KHIs) have been developed and used as a result of its low dosage used for hydrate inhibition and different inhibition mechanism from the THIs. KHIs retard the nucleation and/or growth rate of hydrate crystal formation.3,5 Polymeric chemicals, such as antifreeze proteins [AFPs, also called icestructuring proteins (ISPs)]5−10 and poly-N-vinyllactam polymers,3,5,11−13 have been reported as representative KHIs. Their kinetic hydrate inhibition functions are known to be effective at subcooling below 10 K, which is supported by the

1. INTRODUCTION A variety of fossil fuels, including coal, petroleum, and natural gas, are used not only as energy sources but also as raw materials for useful products in the petrochemical industry. Even though such fuels generate a significant amount of carbon dioxide (CO2), the most representative greenhouse gas, during their oxidation in vehicles or power plants, diverse renewable energies, such as solar, wind, tide, and geothermal energies, cannot fully meet the energy demand of the world at the current level of technology. For this reason, fossil fuels must continue to be supplied; therefore, large amounts of petroleum and natural gas have recently been produced in offshore fields. However, this offshore development produces a large amount of water simultaneously (especially in mature oil fields1), which generates gas hydrate flow assurance issues in the flowlines.2,3 Gas hydrates are easily formed by the mixture of small hydrocarbon molecules and water under flowline conditions (low temperature and high pressure in particular). This blocks the flowlines, prevents efficient fluid flow/oil production, and generates undesirable accidents, leading to casualties and/or equipment damage.2,3 Once natural gas hydrates block the flowlines, tremendous cost and time are required to remove the hydrate plug. Thus, hydrate formation should be well-managed for safe, effective, and economical transport of hydrocarbon fuels. As one of the common chemical hydrate inhibition methods, thermodynamic hydrate inhibitors (THIs), which possess functional groups forming hydrogen bonds in their molecular structures, have been used to avoid or minimize hydrate formation by making the hydrate phase equilibrium condition © XXXX American Chemical Society

Received: July 26, 2016 Revised: September 25, 2016

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Energy & Fuels calculation using a standard nucleation theory.5 Commercially available KHIs used in the practical fields are cocktails of unknown composition. Many studies have demonstrated that mixtures composed of KHIs or KHIs with non-KHIs exhibited improvement of kinetic inhibition performance of hydrate formation.14−17 According to some KHI studies, ionic liquids (ILs) were revealed to act as KHIs for structure I (sI) methane (CH4) hydrate formation17 or as synergists for the polymeric KHIs inhibiting structure II (sII) natural gas hydrate formation.16 However, it was reported that ILs exhibited very poor KHI properties for sII hydrate inhibition.16 Thus, the ILs should be used properly with the consideration of their structure-dependent hydrate inhibition characteristics. Several studies proved that many BF4−-containing ILs exhibited better sI CH4 hydrate inhibition performance than general polymeric inhibitors.17−19 However, the kinetic inhibition effectiveness of most ILs has not been tested for sII natural gas hydrate formation. Additionally, the previous study related to the role of ILs as hydrate inhibition synergists used only two imidazolium ILs with BF4−.16 In this regard, we carried out sII natural gas hydrate inhibition tests using a selection of six BF4−-containing ILs. Previously, it was identified that the mixtures of several BF4−-containing ILs with poly(Nvinylcaprolactam) (PVCap), one of the representative polymeric KHIs, exhibited improved kinetic inhibition effectiveness of sI CH4 hydrate formation.17 Hence, in this study, we also conducted natural gas hydrate inhibition tests with PVCap−IL mixtures under two different hydrate formation conditions with the same subcooling of 10 K. From the experimental results, even though the ILs exhibited much lower natural gas hydrate inhibition performance than PVCap, the mixtures of the PVCap and ILs showed similar or much better KHI effectiveness than PVCap alone, in experiments conducted under the two conditions. This will be discussed by consideration of different hydrate inhibition functions of PVCap and ILs. Furthermore, the KHI performance of PVCap and PVCap−IL mixtures was reduced under higher pressure conditions, despite the same subcooling. This phenomenon will also be explained as an aspect of hydrate nucleation probability. Finally, 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF4) and its mixture with PVCap exhibited the best inhibition performance for the natural gas hydrate nucleation among the ILs and PVCap−IL mixtures used in the present work. Naturally, the induction time of the mixtures consisting of HMP-BF4 and PVCap were measured at various combinations of their concentrations to find out the specific ratio of the two chemicals exhibiting the best hydrate inhibition performance.

Table 1. Dry-Based Composition of Synthetic Natural Gas component

composition (%)

CH4 C2H6 C3H8 iso-C4H10 n-C4H10 CO2 N2

90.07 4.18 2.11 0.82 0.65 1.94 0.23

2.2. Synthesis of ILs. Among the ILs used in this study, 1hydroxyethyl-1-methylpyrrolidinium tetrafluoroborate (HEMP-BF4) and 1-butyl-1-methylpyrrolidinium tetrafluoroborate (BMP-BF4) were synthesized by following the same procedures as in previous studies.17,18 The synthesis procedures of HMP-BF4 and 1-hexyl-1methylpyrrolidinium tetrafluoroborate (OMP-BF4) are described in the Supporting Information. 2.3. Procedure. Figure 2 shows the schematic diagram of the apparatus for the natural gas hydrate inhibition tests. This experimental setup is similar to that used in previous studies.17,20 All natural gas hydrate inhibition experiments were performed in a 220 mL stainless-steel reactor designed to be used under high pressure. Pure water or aqueous solutions (90 mL, where PVCap, ILs, or PVCap−IL mixtures were dissolved) were charged into the reactor for the hydrate experiments. The concentrations of the additives in the aqueous solutions were listed in Tables 2 and 3. The reactor was immersed in a bath filled with aqueous ethylene glycol solution, and the temperature of the bath was controlled by an external circulating chiller (RW-3040G, Jeio Tech). A thermocouple and a pressure transducer were installed on the reactor and connected to a computer by which the temperature and pressure in the reactor was recorded during the experiments. Injection and ventilation of the natural gas was repeated 3 times to minimize the gas remaining inside the reactor after the vacuum elimination of vapor. Then, the pressure and temperature of the reactor was set to 50 bar/277.15 K or 77 bar/278.65 K, and natural gas hydrate formation tests were performed under these two conditions. The subcooling temperature of the two hydrate formation conditions was 10 K that was determined by CSMGem. The natural gas from the gas cylinder was pressurized using a gas booster up to the specified pressures. After stabilization of the pressure and temperature conditions, a magnetic drive stirred the pure water or aqueous solutions at 700 rpm. The induction times were measured for natural gas hydrate formation in pure water or aqueous solutions. Every experimental run was carried out 3 times for each liquid sample. The natural gas hydrate inhibition tests were performed in a different reactor. Instead of the reactor mentioned above, a Midiclave reactor (Büchiglasuster) with an internal volume of 1 L was used to obtain the best ratio of PVCap and HMP-BF4 for inhibiting the nucleation of natural gas hydrate. Except for the replacement of the reactor, the other experimental settings were the same as described in Figure 2. Pure water or the aqueous solutions with different ratios of PVCap and HMP-BF4 (300 g) were used to measure the induction time. The concentrations of PVCap and HMP-BF4 are listed in Table 4. The pressure and temperature conditions were set to 77 bar/278.65 K. The experimental procedures followed were the same as those mentioned above.

2. EXPERIMENTAL SECTION 2.1. Reagent. Synthetic natural gas with an ultrahigh-purity grade was supplied by Anjeon Gas (Korea), and its dry-based gas composition is listed in Table 1. Deionized water was acquired using a water purification apparatus (Zeneer Power II, Human Corporation). Luviskol Plus, which is 60 wt % PVCap (Mn = 5100, and polydispersity = 1.303) solution in ethanol, was purchased from BASF as the PVCap source. After removal of ethanol by a rotary evaporator, the solution was further dried in an oven for more than 24 h to eliminate remaining ethanol. The result was PVCap with a purity of 98.0%. Figure 1 shows the structures of PVCap and six ILs used as KHIs or synergists. Among them, N-ethyl-N-methylimidazolium tetrafluoroborate (EMIM-BF4) and 1-ethyl-1-methylpyrrolidinium tetrafluoroborate (EMP-BF4) were supplied from C-TRI and Sigma-Aldrich, respectively.

3. RESULTS AND DISCUSSION 3.1. Kinetic Hydrate Inhibition Tests Performed at 50 bar and 277.15 K Using PVCap/ILs Containing BF4−. Table 2 lists the average induction times for natural gas hydrate formation in the presence of PVCap, several ILs containing BF4− (at the concentrations of 0.5, 1.0, and 3.0 wt %), or the mixture of PVCap (0.5 wt %) and ILs (1.0 wt %). The initial hydrate formation conditions were at 50 bar and 277.15 K, with subcooling of 10 K. The average induction time data indicate that PVCap and all of the ILs tested in this study exhibited B

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Figure 1. Structures of PVCap and the ILs used for the experiments on kinetic inhibition of natural gas hydrate.

Figure 2. Schematic diagram of the apparatus for the hydrate inhibition tests.

Table 2. Average Induction Time of Natural Gas Hydrate Formation in the Presence/Absence of PVCap, ILs, or the Mixture of PVCap (0.5 wt %) with ILs (1.0 wt %)a induction time (min) additive PVCap EMIM-BF4 HEMP-BF4 EMP-BF4 BMP-BF4 HMP-BF4 OMP-BF4 pure water

0.5 wt % 479 39 24 19 45 95 52

± ± ± ± ± ± ±

51 7 11 10 11 12 25

1 wt % no hydrate 49 51 46 66 132 64

± ± ± ± ± ±

3 wt % 8 11 14 12 13 30

no hydrate 89 ± 8 89 ± 12 65 ± 15 78 ± 15 149 ± 14 70 ± 33 14 (without any additives) ± 6

0.5 wt % PVCap + 1.0 wt % IL no hydrate no hydrate

no hydrate

a The experiments were initially performed at 50 bar and 277.15 K (subcooling = 10 K). “No hydrate” means that hydrate formation was not observed for up to 1500 min (more than 24 h).

that the induction time increases monotonically with increment in the concentrations for all of the ILs studied (but not linearly proportional). In addition, from the data comparison for the induction time of hydrate formation using EMP-BF4, BMP-BF4, HMP-BF4, or OMP-BF4, HMP-BF4 exhibited the longest

hydrate-nucleation-inhibiting performance, with respect to the increased induction times of the additive-containing solutions in comparison to that of pure water. Figure 3 visualizes the comparison of the average induction times in the presence of only ILs, depending upon the concentration. The figure shows C

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structures possess alkyl chains of different length, from ethyl to octyl groups, in the same methylpyrrolidinium cation. HMPBF4 actually exhibited the greatest hydrate-nucleation-inhibiting effectiveness among the ILs in this study and produced the longest induction time, 95, 132, and 149 min at the concentrations of 0.5, 1.0, and 3.0 wt %, respectively. The nonlinearly increased induction time depending upon the IL concentration, and the maximum induction time affected by the alkyl chain length, implies that natural gas hydrate nucleation is influenced by a diversity of IL characteristics. To understand the KHI mechanisms of ILs in more detail, the interactions occurring between water/guest molecules and anions/cations of ILs should be considered through further experiment and/or simulation studies. Another important point is that all of the average induction times measured in this study for natural gas hydrate formation using only ILs were shorter than those using PVCap at corresponding concentrations. Figure 4 compares the induction

Table 3. Average Induction Time of Natural Gas Hydrate Formation in the Presence of 0.5/1.5 wt% PVCap or the Mixture of PVCap (0.5 wt %) with ILs (1.0 wt %)a additive 0.5 1.5 0.5 0.5 0.5

wt wt wt wt wt

% % % % %

induction time (min)

PVCap PVCap PVCap + 1.0 wt % EMIM-BF4 PVCap + 1.0 wt % HEMP-BF4 PVCap + 1.0 wt % HMP-BF4

28 ± 13 149 ± 32 266 ± 46 no hydrate no hydrate

a

The experiments were initially performed at 77 bar and 278.65 K (subcooling = 10 K). “No hydrate” means that hydrate formation was not observed for up to 1500 min (more than 24 h).

Table 4. Average Induction Time of Natural Gas Hydrate Formation in the Presence of a Mixture of PVCap and HMPBF4 at Various Ratiosa concentration (wt %) PVCap

HMP-BF4

0 0.05 0.5 1 0.1 0.5 1 0 0.1 0.5 1 0 0.1 0.5 1

0 0 0 0 0.1 0.1 0.1 0.5 0.5 0.5 0.5 1 1 1 1

induction time (min) 11 28 76 225 32 78 667 48 84 500 1350 76 93 112 292

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

7 9 13 29 8 13 37 9 13 32 53 13 20 33 41

a

The experiments were initially performed at 77 bar and 278.65 K (subcooling = 10 K).

Figure 4. Average induction time for natural gas hydrate formation in the presence of PVCap, ILs, or the mixture of PVCap (0.5 wt %) with ILs (1.0 wt %). The initial conditions were 50 bar and 277.15 K.

times of hydrate formation in pure water or aqueous solutions containing 0.5 or 1.0 wt % of the three ILs (EMIM-BF4, HEMP-BF4, and HMP-BF4) or PVCap. Even though only 0.5 wt % PVCap retarded hydrate formation as much as 465 min (=479 − 14 min, the difference in the induction times in the presence/absence of 0.5 wt % PVCap), all of the ILs tested inhibited the initial hydrate formation less than 150 min, despite a much higher concentration (3 wt %). The induction time increased to more than 1500 min with 1.0 or 3.0 wt % PVCap. Figure 5 shows that the pressure profiles during natural gas hydrate formation affected by several ILs (1.0 wt%) and PVCap (0.5 wt%). Among the KHIs tested, PVCap definitely exhibited the best hydrate inhibition performance because the existence of PVCap showed the longest induction time period, slowest hydrate growth rate after initial hydrate formation, and the least amount of hydrate formed at the end of the experiment. Despite lower KHI performance of the ILs than PVCap, all of the ILs retarded the initial hydrate formation, which resulted in induction times beyond that for water only. Although the hydrate formation rate and the pressure levels at the end of the experiments show unclear cation-dependent patterns, the pressure traces indicate that all of the BF4−containing ILs are somewhat effective for inhibiting the hydrate growth. As a result of weaker KHI performance of the ILs for

Figure 3. Average induction time for natural gas hydrate formation in the presence of various concentrations of ILs. The initial conditions were 50 bar and 277.15 K.

induction time, regardless of the concentration. This implies that the hexyl chain is the optimum length for inducing the best inhibition performance of natural gas hydrate nucleation among the tested ILs. These four ILs were selected because their D

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formation of 512[CH4] is more likely than that of 51262[CH4], from calculations of the interaction and free energies.24 This implies that 512[CH4] is more thermodynamically stable than 51262[CH4].24 From the two previous studies,23,24 it can be stated that more 512[CH4] can be generated than 51262[CH4] as a result of the greater thermodynamic stability of 512[CH4]. This implies that prevention of the formation of 512[CH4] could be the key to effective inhibition of sI hydrate nucleation. Therefore, the ILs containing BF4− may possess an ability to inhibit 512[CH4] formation effectively, which results in their excellent KHI performance for sI hydrate formation, experimentally proven in the previous studies.17,18,21 Because 51262[CH4] is less stable than 512[CH4], the ILs could more effectively inhibit the formation of 51262[CH4]. It should be noted that this study did not consider the stabilities of cages occupied by C2H6 or CO2 because CH4 is the primary component (90.07% of total gas). However, the formation of 51262[C2H6] and 51262[CO2] would influence the KHI performance of the ILs because C2H6 or CO2 hydrates are more stable thermodynamically than CH4 hydrate.2 To demonstrate the effects of the guests on KHI performance or the synergetic effect of the ILs, additional systematically planned experiments were performed, and the results from these will be reported soon. In summary, PVCap and ILs containing BF4− may play different roles in natural gas hydrate inhibition. PVCap prevents sII hydrate nucleation and/or growth by adsorption onto the hydrate surface.3 The ILs inhibit the nucleation of metastable sI hydrate, which would be gradually converted to sII hydrate,21 through effective prevention of the formation of 512[CH4]. These different functions of PVCap and ILs containing BF4− could generate a synergetic effect extending the induction time of natural gas hydrate formation, as shown in Figures 4 and 5. From the experimental results of the previous21 and current studies, the ILs containing BF4− should be used as KHIs for sI hydrate formation or as synergists for improving the sII hydrate inhibition performance of other polymeric KHIs. This result, related to the utilization of the ILs as synergists, corresponds to the research performed by Villano et al.16 They concluded that, even though the ILs used in their study (one of them was EMIM-BF4, also used in this study for comparison) were poor KHIs, they were fair synergists for improving the performance of commercial KHIs for natural gas inhibition.16 3.3. Kinetic Hydrate Inhibition Tests at 77 bar and 278.65 K and Consideration of Pressure-Dependent Induction Time of Natural Gas Hydrate Formation. The KHI test results for PVCap−IL mixtures in section 3.2 exhibited excellent natural gas hydrate inhibition performance at 277.15 K at the subcooling temperature of 10 K. This is considered the maximum subcooling, above which hydrate nucleation cannot be stopped.5 However, the experiments were performed at 50 bar, which is a relatively low pressure. Hence, further KHI tests with PVCap or PVCap−IL mixtures were carried out under higher pressure (77 bar) with the same subcooling (10 K), at 278.65 K. The values of the induction time measured are listed in Table 3 and plotted in Figure 6, which shows significantly reduced induction time of natural gas hydrate formation with 0.5 wt % PVCap from 479 min (50 bar and 277.15 K, in Figure 4) to 28 min (77 bar and 278.65 K). The induction time measured with 1.5 wt % PVCap is 149 min, which is still quite a small value in comparison to the induction time of hydrate formation with 1.0 wt % PVCap measured at 50 bar and 277.15 K (more than 1500 min). With PVCap−EMIM-

Figure 5. Pressure profiles as a function of time arising from natural gas hydrate formation under initial conditions of 50 bar and 277.15 K in the presence of 1 wt % PVCap (0.5 wt%), several ILs (1.0 wt%), and the PVCap (0.5 wt %)−HMP-BF4 (1.0 wt %) mixture.

natural gas hydrate than that of PVCap, the utilization of ILs without additional KHIs is not recommended for natural gas hydrate inhibition or management. 3.2. Kinetic Hydrate Inhibition Tests Performed at 50 bar and 277.15 K Using PVCap−IL Mixtures and the Different Roles of PVCap and ILs for Natural Gas Hydrate Inhibition. Figure 4 also includes the induction times measured from the hydrate inhibition tests using various PVCap−IL mixtures under the initial hydrate formation conditions of 50 bar and 277.15 K. All of the induction times (using the PVCap−IL mixtures) presented in Figure 4 exceed 1500 min (the maximum time that we attempted to measure). The pressure trace of the hydrate inhibition test using the mixture of PVCap and HMP-BF4 was indicated by the red line in Figure 5, and no pressure variation was observed during the experiment. This result is related to the enhanced KHI performance by the addition of an IL to PVCap and is quite similar to that reported in a previous study.21 While ILs containing BF4− showed relatively weak effectiveness for sII gas hydrate inhibition, the PVCap−IL mixtures exhibited excellent KHI performance. Ohno et al. reported that abundant CH4 in sII-forming gas mixture can form metastable sI CH4 hydrate as a result of easy contact of CH4 with water, and then, the metastable sI hydrate gradually interconverted to sII of natural gas.22 Our previous study found that the ILs containing BF4− showed further inhibition or a synergetic effect when used with PVCap by preventing the metastable sI hydrates.21 Because PVCap alone is not effective for preventing sI hydrate17,18,21 that would be interconverted to sII hydrate,22 the prevention of the metastable phase by ILs can significantly extend the induction time and would lead to the results shown in Figures 4 and 5. For a better understanding of the synergetic effect of the ILs containing BF4− by inhibiting metastable sI hydrate, the progress of sI CH4 hydrate formation and the thermodynamic stability of a large 51262 cage containing CH4 (51262[CH4]) and a small 512 cage containing CH4 (512[CH4]) should be considered. Subramanian et al. demonstrated that more 512[CH4] was formed than 51262[CH4] at the initial stage of CH4 hydrate formation, using time-resolved Raman spectroscopy.23 According to the density functional theory (DFT), the E

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time, indicating very poor KHI performance.16 However, the induction times for natural gas hydrate formation using various BF4−-containing ILs at the same IL concentration as in this study exhibited positive values for all cases, as listed in Table 2. Villano et al. performed the experiments at 85−90 bar with subcooling of 10.5−10.6 K,16 which is more severe hydrate formation conditions than in this study (50 bar and 10 K of subcooling). Hence, a higher pressure, indicating more gaseous reactant, and subcooling would better facilitate the natural gas hydrate nucleation and formation in their KHI tests compared to the tests in this study. In the case of the mixtures of PVCap−HEMP-BF4 and PVCap−HMP-BF4, the hydrate-nucleation-inhibiting performance was quite remarkable. The induction time of natural gas hydrate formation using the two PVCap−IL mixtures was still longer than 1500 min, as shown in Figure 6, despite the higher pressure condition (77 bar). In addition, Figure 7 shows the stagnant pressure profile during the hydrate inhibition experiment with PVCap−HMP-BF4 (the pressure profile was the same in the presence of PVCap−HEMP-BF4). HEMP-BF4 is an excellent KHI and synergist for hydrate inhibition because the hydroxyl group in its cation could effectively interfere with the hydrogen bonds between water molecules.18 Thus, it is considered a benchmark material.17,18,21 However, HMP-BF4 possesses a much more hydrophobic cation, HMP+, because it does not have any hydrophilic functional groups. Additionally, HMP-BF4 also exhibited the best hydrate-nucleation-inhibiting performance among the four pyrrolidinium ILs having different lengths of alkyl chains (from ethyl to octyl). Thus, the hexyl chain is the optimum length for the most effective hydrate nucleation inhibition. Even though the physical properties of the two cations are quite different, KHIs containing them showed excellent inhibition performance. This indicates that HMP-BF4 could also be another type of benchmark material that follows a different mechanism or interacts with water or hydrocarbon molecules in a different way from HEMP-BF4. Therefore, further research on the interactions or free energy calculations during natural gas hydrate formation and inhibition should be carried out to shed light on the hydrate inhibition mechanism of the ILs. 3.4. Kinetic Hydrate Inhibition Tests with Various Ratios of PVCap and HMP-BF4 at 77 bar and 278.65 K. Among the ILs or PVCap−IL mixtures tested in this study, HMP-BF4 and PVCap−HMP-BF4 exhibited the best inhibition performance of natural gas hydrate nucleation. Thus, further hydrate inhibition tests were conducted using KHIs with various ratios of PVCap and HMP-BF4. For more practical results, these experiments were carried out in the Midiclave reactor with a larger internal volume of 1 L, following the same procedures as for the previous tests in this study. Table 4 lists the compositions of KHIs used for additional hydrate inhibition tests and the resultant induction times. As a result of the tests, the KHI mixture consisting of 1.0 wt % PVCap and 0.5 wt % HMP-BF4 exhibited 1350 min of induction time, which is the best natural-gas-hydrate-nucleation-inhibiting performance. Panels a and b of Figure 8 show three-dimensional (3D) scatter and mesh plots, respectively. These display the variation of the induction time affected by the hydrate-nucleationinhibiting effectiveness of the KHIs, which is dependent upon the concentration ratios of PVCap and HMP-BF4. Within the composition range performed in this study, a 2:1 weight ratio of PVCap/HMP-BF4 is likely to be recommended as a treatment to inhibit the formation of natural gas hydrate.

Figure 6. Average induction time for natural gas hydrate formation under initial conditions of 77 bar and 278.65 K, in the presence of PVCap or mixtures of PVCap (0.5 wt %) with ILs (1.0 wt %).

BF4, the induction time was also reduced from more than 1500 to 266 min. Figure 7 shows the decrease in pressure due to

Figure 7. Pressure profiles as a function of time arising from natural gas hydrate formation in the presence of PVCap or several PVCap−IL mixtures. The experiments were initially performed at 77 bar and 278.65 K.

hydrate formation in the presence of 0.5 or 1.5 wt % PVCap or PVCap−EMIM-BF4 mixture. With PVCap−EMIM-BF4, the amount of the hydrate formed was much less than those in the presence of only PVCap. This is caused by the enhanced inhibition performance of the PVCap−IL mixture. Here, it is necessary to explain why the performance of KHIs was reduced under a higher pressure condition with the same subcooling. Because the temperature difference of the two hydrate formation conditions is only 1.5 K, the pressure increase is not profoundly affected by the temperature increment. However, the pressure increase from 50 to 77 bar indicates the higher density gaseous reactant for hydrate formation. Thus, a higher pressure will generate more hydrate nucleation sites by increasing the number of gas guests, during the hydrate crystal nucleation period; namely, an increased hydrate nucleation probability would lead to reduction of the induction time. According to the research performed by Villano et al., 0.5 and 1.0 wt % BF4−-containing ILs (one of them is EMIM-BF4, the same ionic liquid used in this study) exhibited zero induction F

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Figure 8. (a) Three-dimensional scatter plot representing induction times of natural gas hydrate inhibition tests in the presence of PVCap and HMPBF4 at various concentrations and (b) three-dimensional mesh plot of the induction times of natural gas hydrate formation depending upon the concentrations of PVCap and HMP-BF4. The experiments were initially performed at 77 bar and 278.65 K.

Notes

4. CONCLUSION On the basis of our previous work, ILs containing BF4− anion showed a good ability to inhibit the nucleation of CH4 hydrates, and accordingly, we extended the hydrate inhibition tests for natural gas using various BF4−-containing ILs or PVCap−IL mixtures in this study. The experimental results revealed that the natural gas hydrate inhibition performance of the ILs was much lower than that of PVCap when the ILs were used alone. However, the PVCap−IL mixtures showed a drastic increase in their hydrate inhibition effectiveness, greater than that of PVCap alone. The synergetic inhibition effect might originate from the function of the ILs, preventing the nucleation of metastable sI hydrate by inhibiting 512[CH4] formation. Additional hydrate inhibition experiments at higher pressure showed a reduced induction time of hydrate formation, which arose from a pressure-driven increase in the amount of gaseous reactant, inducing more hydrate nucleation sites. Among the tested ILs and PVCap−IL mixtures, HMP-BF4 and PVCap− HMP-BF4 exhibited the greatest hydrate-nucleation-inhibiting performance. Thus, further hydrate inhibition tests using mixtures of various compositions were carried out. As a result, the mixture of 1.0 wt % PVCap and 0.5 wt % HMP-BF4 was found to exhibit the best KHI performance within the composition range tested. We are going to perform more experimental tests that reflect the real fluid production circumstances in the near future.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (10045068) of Korea Evaluation Institute of Industrial Technology funded by the Ministry of Trade, Industry and Energy.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01830.

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REFERENCES

Synthesis procedures of HMP-BF4 and OMP-BF4 (PDF)

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DOI: 10.1021/acs.energyfuels.6b01830 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01830 Energy Fuels XXXX, XXX, XXX−XXX