Kinetic Effects of Ionic Liquids on Methane Hydrate - Energy & Fuels

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Catalysis and Kinetics

Kinetic effects of ionic liquids on methane hydrate xiaodong shen, Xuebing Zhou, and De-Qing Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03108 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Kinetic effects of ionic liquids on methane hydrate Xiao-dong Shena, Xue-bing Zhoub,c,d,e, De-qing Liang*,b,c,d,e aState

Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of

Technology, Chengdu 610059, China bCAS

Key Laboratory of Gas Hydrate, cGuangzhou Institute of Energy Conversion, dGuangdong

Provincial Key Laboratory of New and Renewable Energy Research and Development, and eGuangzhou

Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR

China ABSTRACT: The kinetic effects of several kinds of ionic liquids (ILs) on the formation of methane hydrate were experimentally investigated on both of macro-scale and micro-scale levels. These ILs were 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm]-BF4), 1-butyl-3-methylimidazolium tetrafluoroborate

([BMIm]-BF4),

1-butyl-3-methylimidazolium

iodide

([BMIm]-I),

N-butyl-N-methylpyrrolidinium tetrafluoroborate ([Py14]-BF4) respectively. Formation temperature, max subcooling, induction time and growth rate of methane gas hydrates were used to evaluate the kinetic effects of ionic liquids in a high pressure cell and flow loop. Evaluated from the indicator of the formation temperature in the high pressure cell, the sequence of inhibition performance of ILs was as following: pure water < 1wt% [EMIm]-BF4 < 1wt% PVP < 1wt% [BMIm]-I < 1wt% [BMIm]-BF4 < 1wt% [Py14]-BF4. ILs couldn’t slow the growth rates of methane hydrate effectively and even promote the gas consumption rates no matter in a high pressure cell or flow loop. RXRD spectra showed that ILs couldn’t change the structure of methane hydrate. Cryo-SEM images showed that there was a porous texture of methane hydrate containing ILs. Raman spectra showed that ILs interact with the cages of methane hydrate. Keywords: methane hydrate, ionic liquids, kinetic effect, formation rate 1. Introduction 1

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Gas hydrates which are referred to as clathrate hydrates are a kind of ice-like solid compounds which consist of hydrogen-bonded water cages encapsulating another molecular species.1 Some small gases (such as methane, ethane and propane) and volatile liquid are easily to form clathrate hydrates with water under suitable thermodynamically conditions (typically high pressure and low temperature).2 When hydrates are formed from water and natural gas, we call them natural gas hydrates. Hammerschmidt was the first to discover gas hydrate formed in the oil pipes and devices and gas hydrates have always been a nasty in the petroleum industry since then.3 As there are always produced water in the oil and gas transporting lines, hydrates readily form which lead to equipment blockage, operational problems, and safety concerns.4-6 People have adopted many methods to prevent the formation of gas hydrates such as heating or thermal insulating, gas dehydration, reducing pressures and addition of hydrate inhibitors.7, 8 Two kinds of inhibitors were commonly used containing thermodynamic hydrate inhibitors (THIs) and low dosage hydrate inhibitors (LDHIs). THIs, such as methanol, ethylene glycol and sodium chloride, could change the activity of water and depress the hydrate three-phase equilibrium line towards lower temperatures and higher pressures.9-16 However, there usually need a great amount of THIs.17 LDHIs are a new type of gas hydrate inhibitors which have gained great attention recently, including kinetic hydrate inhibitors (KHIs) and anti-agglomerates (AAs). As we can see from their names, they are effective at low concentrations (usually less than 1 wt%).7, 18 They allow the formation of gas hydrates while minimizing the risk of plug formation. KHIs could interfere with the crystallization process of gas hydrate resulting in a retarded formation time or slow growth rates.

19-23

In contrast, the AAs

prevent them from agglomerating and accumulating into large masses.7 However, KHIs often do not work at high subcooling which is not applicable in deeper seas.18 What’s more, AAs are usually used in the water/oil emulsion systems and would be invalid beyond a water cut of 50vol%.24 Recently, ionic liquids, as a novel class of gas hydrates inhibitors has been discovered. They have both thermodynamic and kinetic inhibition performance on the formation of gas hydrates25,

26

ILs

usually consist of a large organic cation and organic or inorganic anions. They are generally liquid at room temperatures. Most of them are non-toxic, non-flammable, thermally stable and environmentally friendly.27, 28 Due to their tunable properties, they have gained great attention in various application areas.29-31 As ILs are a type of salts, their thermodynamic inhibition performance is quite promising. A

2


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great amount of related research work has down recently.15, 32-44 However, as a kind of THIs, limited researches could be found. Chen et al. identified the kinetic enhancement of 1-butyl-3-methylimidazolium tetrafluoroborate on the formation rate of carbon dioxide hydrate which was quite different from the findings by Xiao et al. that some of ILs delay the induction time.45 Luca et al. found that the performance of the imidazolium-based ionic liquids EMIM-BF4 and BMIM-BF4 was poor on sII gas hydrates compared to the commercial KHIs.46 Kim et al. synthesized pyrrolidinium cation-based ionic liquids and found they could tremendously improve induction time while shifting the original equilibrium line.47 Nazari et al. also found the dual functions of ILs and modeled the inhibition kinetics of methane hydrate by low dosage ILs. The related inhibition mechanism was attributed to the ionic nature and electrostatic interactions of ILs with water molecules.48 The team of Kang have conducted systematic investigation on the inhibition performance of ILs and their mixture with polymer KHIs.49-51 They found that imidazolium-based ILs with BF4 anion have the best inhibition performance and have a strong synergetic inhibition effects with polymer KHIs. Tariq et al. reviewed the research progress of IL on the inhibition performance on gas hydrates and found that the longer the alkyl chain of the cation of ILs, the less effective they are as inhibitors.52 In our previous study, we investigated the kinetic effect of N-butyl-N-methylpyrrolidinium bromide on CO2 hydrate and found its complex inhibition performance under different subcooling and in different mass fraction.53 Rasoolzadeh et al. investigated methane hydrate formation induction time in the presence of three ILs (BMIM-BF4, BMIM-DCA, TEACL), and a three parameter semi-empirical model was proposed.54 Recently, Nashed et al. investigated the induction time of nine ILs employing a high pressure micro differential scanning calorimeter and they found some of the ILs had higher inhibition power than PVP.55 Though some research work has been done on the kinetic effects of ILs of hydrate formation, there was hardly a consensus on the kinetic inhibition performance of ILs. The actually inhibition mechanism of ILs on gas hydrate was unclear. Though ILs are a big family in the field of chemistry, but it is new as a kind of hydrate inhibitor. Great amount of research work needs to be done to find the ones with the most efficient inhibition performance. In this work, we systematically investigated the kinetic effects of four ILs employing a high pressure flow loop and a high pressure cell. They belong to the imidazole and pyrrolidone families which are supposed to be effective KHIs.51,52 Some microscopic level equipment, such as P-XRD

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spectrometer, cryo-SEM and Raman spectromether were also used to investigate the inhibition mechanism of methane hydrate. 2. Experimental sections 2.1 Materials Detail information of the chemicals used in the experiment was list in Table 1. Deionized water was made in the laboratory with a resistivity of 18.25 mΩ·cm-1. All solutions were prepared in mass percent fraction. An electronic analytical balance with an uncertainty of ± 0.001 g was used to weigh the mass of the materials. The concentrations of the ionic liquids and other inhibitor solutions were mainly 0.5wt% and 1.0wt%. Table 1. List of the materials used for the experiments. chemicals

abbreviation

1-ethyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium

[EMIm]-BF

Molecular weight 197.97

purity

supplier

≥99%

of

226.02

≥99%

Lanzhou Institute Chemical Physics Lanzhou Institute Chemical Physics

4

[BMIm]-BF

of

tetrafluoroborate

4

1-butyl-3-methylimidazolium

[BMIm]-I

266.12

≥99%

Lanzhou Institute Chemical Physics

of

N-butyl-N-methylpyrrolidinium

[Py14]-BF4

229.07

≥99%

of

tetrafluoroborate PVP K90

Lanzhou Institute Chemical Physics

PVP K90

360000

-

Deionized water Methane gas

H2O CH4

18 16

99.9%

Tokyo Chemical Industry Co., Ltd Laboratory-made Guangzhou Puyuan Gas Co., Guangzhou, China

iodide

2.2. Experimental apparatus The sketch of the high pressure flow loop system was showed in Figure 1.There has been detailed description of the apparatus in our previous publication.56 It mainly contained a U-bend double pipe with a total length of 51.85m and inner diameter of 25.4 mm. The total volume of the flow system was 30.4L. The horizontal section of the pipe was 42.35m long and a section of 12mm-inner diameter pipe with a length of 4.124m was used. It also had visual window section and cooling system. The pressures, temperatures and differential pressures could be recorded in real time. The flow rate and density of hydrate slurry were measured by a mass flow meter. A piston pump was used to recycle the fluid in the

4


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flow loop with a maximum volume flow rate of 2000L/h. All the experimental data was collected using a data-acquisition system at the interval of 10 s.

Figure 1. Schematic of the hydrate flow loop system. 1: gas cylinder; 2: piston pump; 3: gas-liquid separator; 4: gas compressor; 5: mass flow meter; 6: camera; 7: visual window; 8: tank for liquid; V1– V8: valve; P: pressure sensor; T: thermocouple; DP: differential pressure transducer The sketch of the high pressure cell system was showed in Figure 2. Similar apparatus was use in our previous work.15, 53 Its main part was a 316 stainless steel cell with an effective volume of 100mL. There were cooling system, gas buffer and agitation system. The temperatures and pressures were monitored by a platinum resistance thermometer with a maximum uncertainty of ± 0.1 K and a pressure transducer with an uncertainty of ± 0.025 MPa, respectively. The experimental data was collected every 10 seconds by an Agilent data collector.

PC DA

PT TS R

CB

SS

MS

GB

VP GC

5


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Figure 2. Schematic diagram of experimental apparatus: PC, personal computer; DA, data acquisition system; PT, pressure transducer; TS, temperature sensor; R, reactor; CB, cool bath ; SS, stirring seed; MS, magnetic stirrer; GB, gas buffer, VP, vacuum pump; GC, gas cylinder. Raman spectrograms of methane gas hydrates were measured by a Raman spectromether (Horiba, LabRAM HR) at -70℃ in atmospheric pressure, and a 523 nm Ar+ laser was used. X-ray diffraction (XRD) spectrograms of gas hydrate were determined by a PXRD spectrometer (PANalytical, X’Pert Pro MPD) at -70℃ in atmospheric pressure, and the 2θrange was from 5°to 60°. The topographical microstructures of gas hydrate were obtained by cryo-SEM (Hitachi, S-4800) at -140 ℃ . Detailed description of these apparatuses could be found in some of the literatures.21, 57-61 2.3. Experimental method and procedure The formation temperature and max subcooling of methane hydrates in the high pressure cell were measured using a temperature ramp method. The experimental procedure was as following: first, wash the cell with deionized water and the experimental aqueous solution three times. Then add 30mL of the experimental aqueous solution prepared or pure water into the cell. Next, turned on the cool both and adjust the temperature of the system to the 298.15K. Meantime, turned on the magnetic stirrer to promote heat transfer. Then evacuate the system with a vacuum pump for about five minutes and flush it with the experimental methane gas for three times. Then introduce enough methane gas into the reaction cell from the gas buffer until 9 MPa. Subsequently, close the inlet valve and decrease the temperature of the system by a rate of 8.5K/h. This was regarded as the start of every experiment run. There should be an abrupt temperature increase and a rapid change of the pressure slope which indicated the formation of methane hydrate. The induction time and growth rate of the methane hydrate in the high pressure cell were conducted using an isochoric method. There was detail description about the procedure in our previous publication. 53 The cleaning and washing process was the same as that in the temperature ramp method. 30mL of the experimental aqueous solution prepared or pure water was used. The experimental temperature was cooled to be 277.15K. Then methane gas was added into the cell until the desired experimental pressure. Subsequently, the inlet valve was closed and the solution was agitated by the magnetic stirrer again at a fixed stirring rate. This was regarded as the start of every experiment run. Hydrate formation could be inferred by an abrupt temperature increase or a rapid pressure drop. As the pressure didn’t drop which meant that no more methane hydrate continued to form, the hydrate samples 6


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would be collected from the cell under the protection of liquid nitrogen and nitrogen gas properly. The samples were stored in liquid nitrogen for microscopic test. The induction time and growth rate of the methane hydrate in the high pressure flow loop were investigated using an isothermal and isobaric method. There was detail description about the procedure in our previous publication.56 The main procedure was cleaning of the loop and addition of 25L of the experimental solution. Then the temperature of the system was cooled to the set point and the pump would be stopped. Next was the addition of methane gas and the pump would be started at fixed frequency and the data acquisition was commenced. A special regulating valve could help maintain a constant pressure in the flow loop. Gas hydrate formation could be inferred from the change of temperature and pressure or just be observed through the transparent sapphire section. The experimental run would be stopped when the pressure drops increased rapidly or increased beyond the measure range. 3. Results and discussion The temperature ramp method was used to measure the formation temperature and max subcooling of methane hydrates in the high pressure cell. A series of experiments were conducted for the aqueous solutions with or without the presence of ILs. The initial pressure and temperature was 9MPa and 298.15K and was decreased to 268.15K by a rate of 8.5K/h. The mass fraction of the aqueous solution with the presence of ILs was 1wt%. Subcooling was defined as the difference between the system temperature and the hydrate equilibrium temperature.62 The equilibrium phase diagram of methane hydrates was calculated by the CSMGem prgram.2 Though ILs could depress the equilibrium conditions of methane hydrate to higher pressures or lower temperatures as they were a kind of salts, this effect was very weak when the mass fraction was relatively low and could be ignored.15 A typical run of the temperature ramp method was shown in Figure 3. When the temperature decreased to 275.35K, there was a sharp change of pressure slope which indicated the formation of methane hydrate. The increase of temperature wasn’t so obvious as a result of the great thermal conductivity and small amount of hydrate formation. As the temperature continued to decrease, great amount of ice was formed which was indicated by the sharp increase of temperature and pressure. This was more obvious in the diagram of P-T in Figure 4.

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Figure 3. Change of temperature and pressure with time in the temperature ramp method

Figure 4. Diagram of P-T in the temperature ramp method

Figure 5. Change of pressure of methane with elapse of time in pure water system at 6 MPa and 277.15K The isochoric method was used to measure the induction time and growth rate of the methane hydrate in the high pressure cell. It is known that the pressure variation usually undergoes three stages for an enclosed hydrate formation system. And a typical change of pressure with the elapsed time in 8


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pure water system was shown in Figure 5. The first rapid drop stage indicated the dissolving of gas into the aqueous solution. Then the stable stage represented the induction progress of hydrate nucleation and the second rapid drop stage referred to the growth of hydrates. The time from the point of the hydrate equilibrium conditions to the start of the second rapid drop stage was usually taken as the induction time.55, 63 The isothermal and isobaric method was used to measure the induction time and growth rate of the methane hydrate in the high pressure flow loop. There was detailed description about the method of the calculation of methane consumed during hydrate formation in the high pressure cell or flow loop in our previous works.53, 56 3.1. Effects of ILs on the formation temperature and max subcooling of methane hydrate in the high pressure cell. Table 2. Formation temperatures and max subcoolings of methane hydrate with or without ILs Mass fraction (wt%)

Formation pressure (MPa)

Formation temperature (K)

Pure water

-

8.15

280.46

8.14

280.87

PVP K90

1

7.96

276.25

7.89

274.59

7.89

274.67

7.92

275.55

8.00

277.27

7.92

275.50

7.79

272.04

7.69

270.08

8.02

277.10

7.76

271.97

7.82

273.46

7.70

270.70

7.73

271.02

7.66

269.53

7.68

270.05

[EMIm]-BF

1

4

[BMIm]-B

1

F4 [BMIm]-I

[Py14]-BF4

1

1

Mean Formation pressure (MPa) 8.15

Mean formation temperature (K) 280.66

Mean max subcooling (K) 4.12

Relative standard deviation (%) 0.10

7.91

275.17

9.32

0.34

7.95

276.11

8.43

0.36

7.78

271.94

12.38

0.66

7.76

272.04

12.25

0.51

7.69

270.20

14.00

0.28

The formation temperature and max subcooling of methane hydrate with or without the presence of ILs was listed in Table 2. The initial pressure and temperature were all 9MPa and 298.15K and the

9


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temperature was decreased to 268.15K by a rate of 8.5K/h. Subcooling was defined as the difference between the system temperature and the hydrate equilibrium temperature.62 From Table 2 we could see that the repeatability of all the experiments was very good and the max relative standard deviation didn’t exceed 1%. This demonstrated the reliability of the experimental data. Evaluated from the indicators of the formation temperature and max subcooling, the sequence of inhibition performance of ionic liquid was as following: pure water < 1wt% [EMIm]-BF4 < 1wt% PVP < 1wt% [BMIm]-I < 1wt% [BMIm]-BF4 < 1wt% [Py14]-BF4. All the ILs and PVP had inhibition performance compared with pure water system. However [EMIm]-BF4 was a weaker kinetic inhibitor than PVP. The inhibition performance [BMIm]-I and [BMIm]-BF4 was much better than that of PVP. It should be noted that [Py14]-BF4 had the highest inhibition performance as a single hydrate inhibitor. When the initial pressure and temperature of every experiment were the same, the formation temperature and max subcooling of methane hydrates could give the identical evaluation of the inhibition performance. Their relation was shown in Figure 6.

Figure 6. Relationship between the mean max subcooling and the mean formation temperature 3.2. Effects of ILs on the induction time of methane hydrate in the high pressure cell. Table 3. Induction time of methane hydrate with or without ILs.

Pure water

Mass fraction (wt%) -

Initial pressure (MPa) 6

Experimental temperature (K)

Induction time (min)

277.15

65.7

Mean induction time (min) 683.5

Relative standard deviation (%) 146.0

2176.3

29.6

1834.5 150.3

PVP K90

1

6

277.15

2038 1611 >2880

10


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[EMIm]-

1

6

277.15

BF4

>2880

1447.8

114.2

2279.7

52.7

1550

99.4

1518.6

103.7

18.8 >2880 12.2

[BMIm]-

1

6

277.15

BF4

>2880 >2880 478.8 >2880

[BMIm]-

1

6

277.15

>2880 78

I

>2880 362 [Py14]-B

1

6

277.15

281.8 32.7

F4

>2880 >2880 “>” represented that the induction time exceeded two days and we technically treated it to be 2880 minutes for data processing. Actually, some of the experiments run beyond 4 days even there wasn’t the formation of hydrate. a

The induction time of methane hydrate with or without the presence of ILs was listed in Table 3. In this work, the time from the start of magnetic stirrer to the point of the second rapid pressure drop stage was usually taken as the induction time.63 It could be seen that all the relative standard deviations of the experiments were very big and the repeatability the results was poor. As we know, the formation of gas hydrate was a stochastic process. Regardless of the poor repeatability of the experiments, we could see that the evaluation results of inhibition performance from induction time almost matched with the results from the formation temperature. Almost all the ILs showed kinetic inhibition effects on the formation of methane hydrates. This might be caused by the randomness of the experimental results. In the same initial conditions of temperature and pressure, hydrate formation took place immediately in some experiments while some run beyond four days even there wasn’t the formation of hydrates. The induction times of methane hydrate from an initial pressure of 8 MPa and at experimental temperature of 277.15 K with the presence of ILs were also investigated. The results were not shown here. When the pressure was increased, the subcooling was increased which stand for the driving force of methane hydrate formation. As a result, all the mean induction time decreased. However, the results still showed poor repeatability. If we want to get more reliable results, every experiment must repeat several times. 3.3. Effects of ILs on the growth rate of methane hydrate in the high pressure cell. 11


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We used the cumulative consumption of methane gas during the formation process to demonstrate the growth rate of methane hydrate. The initial pressure was 6 MPa and the experimental temperature was 277.15 K. The experimental results of ILs were shown in Figure 7. From Figure 7 we could see that the cumulative consumption of methane with time of ILs at the start of hydrate formation were almost the same as that of pure water. PVP could slow the formation rate of methane hydrate as there was less methane consumed with time among all the chemicals. As hydrate continued to form, the formation rates of methane hydrate in the solution of ILs were higher than that of pure water except that of [BMIm]-BF4 and [BMIm]-I. This demonstrated that two of the ILs ([EMIm]-BF4, [Py14]-BF4) couldn’t inhibit the growth of methane hydrate. As hydrate formation continued, more methane gas was consumed. In an isothermal process of hydrate formation, this meant that gas pressure was dropping and the driving force continued to decrease. Eventually, the system reached its three-phase equilibrium condition or no free gas and water could meet and there wouldn’t be any more hydrate formed. The inhibition performance of [BMIm]-BF4 and [BMIm]-I showed a similar and complicated relationship with hydrate growth time. The growth rate in system containing these two chemicals was first very fast, then became slow, next turned to fast again, at last became slow and stopped. As we could see from Figure 7, hydrate formation of different solutions of inhibitors stopped at different eventual level. With more methane consumed, there would be more methane hydrate formed. So it could be concluded that all the inhibitors enhanced the final production of methane hydrate except for [EMIm]-BF4 which had similar production level to pure water. The reason was that a more compact form of hydrate was formed in pure water and [EMIm]-BF4 solution which was found from the results of cryo-SEM measurement in the later section and they hindered further formation of hydrate. From a global view ILs couldn’t slow the growth rate of methane hydrate as single KHIs.

12


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Figure 7. Methane consumed during the hydrate growth process of ILs in the high pressure cell under an initial pressure 6 MPa and temperature 277.15 K and time 0 represented the start point of hydrate growth (■: pure water; ●: 1wt% PVP; ▲: 1wt% [EMIm]-BF4; ▼: 1wt% [BMIm]-BF4; ◢: 1wt% [BMIm]-I; ◥: 1wt% [Py14]-BF4). 3.4. Effects of ILs on the growth rate of methane hydrate in the high pressure flow loop. The kinetic effects of ILs on the formation of methane hydrate were also investigated in the high pressure flow loop under an isothermal and isobaric conditions. The experimental pressure was 6MPa, and temperature was 273.15K. It should be admitted that throughout all of our experiments there was nearly no induction time during the formation of methane gas hydrate. The longest one was less than five minutes and we think these chemicals had no inhibition effects on the nucleation process in the high pressure flow loop. This might be caused by the difference of the geometry of the experiment equipment. There were more activating sites for hydrate formation in the high pressure loop and the fluids (gas, water even with hydrate) have a different flow conditions. The results of the kinetic effects of ILs on the growth rate of methane gas hydrate were shown in Figure 8. It could be seen that three of the ILs could not inhibit the gas consumption rate during the formation process of methane hydrate compared with pure water system at the same flow rate. On the contrary, they promoted the uptake of methane gas, namely promoting the growth rate of methane hydrate. We could see that PVP could inhibit the growth rate of methane hydrate to some extent at any flow rate. It could be concluded that these ILs investigated were poor inhibitors for methane hydrate (or S Ⅰ hydrate). Their inhibition performance on hydrate of gas mixture (or S Ⅱ hydrate) should be investigated in the future. They cannot be considered as dual function inhibitors for methane hydrate (or S Ⅰ hydrate) and cannot be used in flow loop or industrial pipelines transporting methane gas.

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Figure 8. Methane consumed during the hydrate formation process of ILs in the high pressure flow loop under an isothermal and isobaric condition (■: pure water 0.86m/s; ●: pure water 0.74m/s; ▲: pure water 0.62m/s; ▼: 1wt% chemical 0.86m/s; ◢: 1wt% chemical 0.74m/s; ◥: 1wt% chemical 0.62m/s; a: [EMIm]-BF4; b: [BMIm]-BF4; c: [BMIm]-I; d: [Py14]-BF4). 3.5. Effects of ILs on the crystal structure of methane hydrate. RXRD profiles of methane hydrate samples formed from solutions containing different ILs was shown in Figure 9. RXRD profiles of Ice and methane hydrate formed from pure water system were also included for comparison. As we know, the (100), (002), and (101) crystal planes of hexagonal ice have reflections at about 22.7°, 24.2°, 25.9° 2θ, respectively. While the reflections at about 27.0°, 28.0°, 30.9° and 31.9° 2θ are assigned to be the (320), (321), (400), and (410) crystal planes of sI hydrate.54 The (101) crystal plane of ice and (222) crystal plane of sI hydrate shared the reflection at about 25.9° 2θ. The reflections at about 27.0°, 28.0° 2θ were marked in Figure 9. As compared with pure water system, these ILs could not change the structure of methane hydrate and they were still sI hydrate.50

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Figure 9. RXRD profiles of methane hydrate samples formed from solutions containing different ILs. 3.6. Effects of ILs on the microscopic morphology of methane hydrate surface. Microstructure pictures of methane hydrate containing different kind of ILs was shown in Figure 10. We could see that methane hydrate formed from pure water system had a relatively regular and compact hydrate surface. However, there were many holes in the hydrate surface formed from 1wt% ILs solutions and they were irregular and porous except that of [EMIm]-BF4. It could be seen that hydrate surface containing [Py14]-BF4 were the most irregular. A porous texture couldn’t prevent the growth of methane hydrate effectively, they might even provide some transport channels for methane gas or water molecular. This might be the reason that there was a higher gas consumption rate in solutions containing ILs than that of pure water in the high pressure flow loop. This also caused a higher ultimate methane hydrate production as a compact texture prevent the contact of water and methane molecular. It was also supposed that the interaction of ILs with methane hydrate cage structure account for the porous texture and a lower formation temperature with the presence of ILs. We could find such a result in the experiments in the high pressure cell.

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Figure 10. Microstructure pictures of methane hydrate containing different kind of ILs and the minor length scale was 3μm. 3.7. Effects of ILs on occupation of hydrate cages of methane hydrate crystal.

Figure 11. Raman spectra of methane hydrate formed in pure water system. The Raman spectra of methane hydrate formed in pure water system was shown in Figure 11. There are Raman shift at 2905-1 corresponding to the large 51262 cages and Raman shift at 2915-1 16


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corresponding to the small 512 cages for sI methane hydrate.58,

64

The peak splitting was achieved

through some techniques of Origin.65 The ratio of methane molecule number in the large and small cage could be calculated by the area fitting of the peak. Combined with thermodynamic equation, the absolute occupation of methane molecule in the large and small cages and the hydrate number could be calculated.64 As we know, the ratio of large and small cage number in sI methane hydrate is 3.0. The ratio of methane molecule number in the large and small cage should be closed to this value.

Figure 12. Raman spectra of methane hydrate containing different kind of ILs. Raman spectra of methane hydrate containing different kind of ILs and that of pure water system was shown in Figure 12. Note that the CH4 bands for the hydrates with or without ILs were all split into two peaks, indicating incorporation of CH4 into both cavities of sI hydrate. The Raman shift positions of the peaks were the same. This demonstrated that ILs couldn’t change the structure of methane hydrate which was consistent with that of P-XRD.

Table 4. Occupation profiles of methane molecular in the cage of methane hydrate with or without ILs. system

ratio of AL/AS

θL

θS

pure water 1wt% [EMIm]-BF4 1wt% [BMIm]-BF4 1wt% [BMIm]-I 1wt% [Py14]-BF4

3.134±0.05 3.04±0.04 3.042±0.03 3.045±0.06 3.064±0.03

0.970±0.001 0.965±0.002 0.965±0.002 0.966±0.003 0.967±0.001

0.927±0.013 0.953±0.011 0.952±0.008 0.951±0.015 0.946±0.007

Occupation profiles of methane molecular in the cage of methane hydrate with or without ILs were shown in Table 4. Methane molecule had a higher occupation ratio in the lage cages than that of the small cages. With the addition of ILs, the occupation ratio of methane molecule in the large and small 17


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cages would decrease. This demonstrated that ILs had some effects on the cage structure of methane hydrate crystal which disturbing the formation of methane hydrate critical nucleus. This might provide some explanations of the inhibition effects of ILs on the nucleation of methane hydrate as there was a long induction time with the addition of ILs in a high pressure cell. 3.8. Mechanism of kinetic inhibition From the aspects of formation temperature and max subcooling, we knew that all of the ILs had better inhibition performance on methane hydrate than PVP except for [EMIm]-BF4. They could suppress the start of the formation of methane hydrate to a lower formation temperatures or higher subcoolings. On one hand, [EMIm]-BF4 had shorter alkane chain than that of the other three ILs on their large organic cationic heads. From a view of steric effect of inhibitors, inhibitors with longer alkane chain could adhere to the surface of newly formed nucleus of hydrate and effectively prevented it from continuous growth. Thus, inhibitors with longer alkane chains could suppress the formation temperature. On the other hand, all of the ILs had inorganic anions which might pose an electrostatic interaction with water molecules. However, in our experiments, the inorganic anions were [BF4]- and I-. They seemed to have the similar kinetic inhibition effects on methane hydrate formation. [Py14]-BF4 had the best kinetic inhibition performance. This might be caused by the stronger steric effect of its organic cationic head. When the critical nucleus of hydrates was formed and the metastable state of methane hydrate was broken, there would be tremendous formation of hydrates. ILs could not slow the growth rate of hydrate at the beginning of hydrate formation. At the end of the formation, more hydrates were formed in the cases with ILs. This result might be attributed to the fact that ILs were a kind of surfactants with hydrophilic and hydrophobic heads. Surfactants could interact with water and the formed hydrate and reduced the mass transfer resistance of hydrate formation. 4. Conclusions In this work, the kinetic inhibition performance of four kinds of ILs on methane hydrate were first investigated from the aspects of formation temperature, max subcooling, induction time and growth rate in a high pressure cell and flow loop. The cases of pure water and PVP were also included for comparison. It was found that formation temperature or max subcooling could be used as an efficient indicator of kinetic inhibition performance of KHIs. It was time-efficient and had good repeatability. In 18


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contrast, induction time was time-consuming and gave poor repeatability. We propose that the formation temperature or max subcooling could be used as a first step to test the kinetic inhibition performance of inhibitors and fast evaluate the great amount of chemicals. Some of the results could be elaborately verified by the indicator of induction time. Evaluated from the indicator of the formation temperature and max subcooling, the sequence of kinetic inhibition performance of ILs was as following: pure water < 1wt% [EMIm]-BF4 < 1wt% PVP < 1wt% [BMIm]-I < 1wt% [BMIm]-BF4 < 1wt% [Py14]-BF4. From the aspect of growth rate, all the ILs could not slow the growth rate of methane hydrate no matter in the high pressure cell or flow loop. It was analyzed from the results of Raman spectra, cryo-SEM and P-XRD that ILs couldn’t change the structure of methane hydrate and ILs molecules interacted with the cages of methane hydrate. It was proposed that the inhibition mechanism of ILs was attributed to their surfactant nature. We hope our work would be beneficial to the prevention of natural gas hydrates. 

AUTHOR INFORMATION

Corresponding Author *Telephone/Fax: +86-20-8705-7669. E-mail: [email protected] ORCID Deqing Liang: 0000-0001-7534-4578 Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

The authors acknowledge the supports by the National key research and development plan of China(2017YFC0307306), the National Natural Science Foundation of China (51376182，41473063), CAS Program (KGZD-EW-301)

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Kinetic Effects of Ionic Liquids on Methane Hydrate - Energy & Fuels

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