Gas Hydrate Prevention and Flow Assurance by Using Mixtures of

Energy Fuels , 2016, 30 (4), pp 3541–3548. DOI: 10.1021/acs.energyfuels.5b03001. Publication Date (Web): March 1, 2016. Copyright © 2016 American ...
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Gas Hydrate Prevention and Flow Assurance by Using Mixtures of Ionic Liquids and Synergent Compounds: Combined Kinetics and Thermodynamic Approach M. Fahed Qureshi,†,∥ Mert Atilhan,*,† Tausif Altamash,†,∥ Mohamad Tariq,† Majeda Khraisheh,† Santiago Aparicio,‡ and Bahman Tohidi§ †

Department of Chemical Engineering, Qatar University, Doha, Qatar Department of Chemistry, University of Burgos, 09001 Burgos, Spain § Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom ‡

ABSTRACT: The thermodynamic and kinetic hydrates inhibition effects of addition of synergents poly(ethylene oxide) (PEO) and vinyl caprolactum (VCAP) with ionic liquids 1-methyl-1-propylpyrrolidinium chloride [PMPy][Cl] and 1-methyl-1propylpyrrolidinium triflate [PMPy][triflate] were studied on a synthetic quaternary gas mixture (methane, C1 = 84.20%; ethane, C2 = 9.90%; n-hexane, C6+ = 0.015%; CO2 = 2.46%; N2 = 2.19%). The results show that the addition of synergents with ionic liquids helps to improve their thermodynamic and kinetic hydrate inhibition effectiveness simultaneously.

1. INTRODUCTION Gas hydrates are crystalline compounds that are made up of gas and water molecules.1 The formation of gas hydrates mainly takes place in offshore subsea pipelines at high pressure and low temperature.2 Hydrates are found in three typical structures: structure I (sI), structure II, and structure H (sH).3 The surface of the hydrate is inherently hydrophilic, and a thin layer of liquid water film always exists on the hydrate surface, which leads to a capillary bridge between hydrate particles.4 It is this capillary force that holds the particles together and causes agglomeration, which leads to blockages in offshore subsea pipelines.5 Thus, hydrates are a major flow assurance concern to oil and gas industries.6 The formation of hydrates is constantly encountered in the Caspian Sea, North Sea, and permafrost regions such as Alaska where high-pressure and low-temperature conditions are common, causing pipeline blockages.7 In the Gulf of Mexico, hydrate formation mainly occurs during drilling operations, because at the water depth of 3000 m, the pressure goes up to 30 MPa and the temperature drops to 2−4 °C which is suitable for hydrate formation in pipelines.8 In the Middle East, hydrate formation becomes a problem during the winter months.9 Therefore, annually the industry spends over a billion dollars on measures preventing hydrate formation.10 Despite being a threat to flow assurance in the offshore industry, gas hydrates also offer many potential benefits and can be used for carbon capture and sequestration,11 natural gas storage,12 coal mine gas separation,13 desalination,14 and separating undesirable (toxic or incombustible) species from biogases.15 Generally, hydrate formation is mitigated by shifting hydrate equilibrium conditions using thermodynamic hydrate inhibitors (THI) or by delaying the hydrate crystal growth using kinetic hydrate inhibitor (KHI).16 These inhibitors are different from antiagglomerants (AG) which prevent hydrate crystals from © 2016 American Chemical Society

accumulating into large masses but do not inhibit the hydrate formation.17 Recently, the research interest in finding a potential hydrate inhibitor has been shifted toward dual function inhibitors known as ionic liquids (ILs).18 The ILs are known as dual function inhibitors because they can act as both thermodynamic and kinetic inhibitors at the same time.19 These ILs are formulated by combination of bulky, N-containing organic cations (like imidazolium, pyrrolidinium, pyridinium) with simple inorganic ions (like chloride, bromide, etc.).20 In some cases, more complex organic species (like triflate, acetate, etc.) can be combined with bulky cations.20 Simple changes in the cation and anion combinations allow ILs to be designed for specific purpose.21 The IL solutions behave as electrolytes, and electrolytes have shown thermodynamic inhibition effect for hydrate formation.22 Xiao et al.23 reported that imidazolium-based ILs having halides as anion exhibited dual function behavior on methane hydrate formation. According to Xiao et al.,23 ILs must have strong electrolyte charges and the capability to form hydrogen bonds with water molecule that helps to shift the hydrates’ hydrate liquid−vapor equilibrium (HLVE) curve to lower temperature at particular pressure and also slow hydrate nucleation growth rates. Kim et al.24 synthesized pyrrolidinium cation-based ILs with tetrafluoroborate [BF4]− anion and investigated their inhibition effects on methane hydrate formation. It was reported that these ILs shifted the hydrate equilibrium curve to lower temperature and also delayed gas hydrate formation. Shin et al.25 also investigated the inhibition effect of pyrrolidinium cation-based ILs on CO2 hydrates and reported that anions mostly contributed to the thermodynamic inhibition. Received: December 24, 2015 Revised: March 1, 2016 Published: March 1, 2016 3541

DOI: 10.1021/acs.energyfuels.5b03001 Energy Fuels 2016, 30, 3541−3548

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Energy & Fuels

forth inside the cell to agitate the solution and provide necessary turbulence within the measurement cell.10 A diluted solution of ionic liquid with volume of 15 cm3 is added to each cell, and the cells are carefully tightened to prevent any leakages. Then they are placed on a rocking skid, which is submerged in a cooling bath. The mixing inside the cells is carried out by swinging the cells back and forth with a hold angle of 30° and at rocking frequency rate fixed at 10 rocks/min. Once the cells are loaded into the cooling bath, each cell is pressurized separately with the QM mixture up to the desired pressure. In a typical measurement routine, cells are pressurized as 40, 60, 80, 100, and 120 bar in to scan a wide range of the hydrate equilibrium curve. The temperature and pressure of the cells is monitored by the help of the data acquisition system throughout the experiment. The cooling bath is connected to an external cooling circulator Huber Ministat 125w that is capable of operating between the temperatures of −25 and 150 °C. Apparatus temperature sensors have an accuracy of ±0.25 °C, and pressure sensors have an accuracy of 0.1% of the pressure full scale. 2.3. Experimental Procedure. Before initiating the experiment, the rocking cell system is stabilized at the temperature of 20 °C for 1 h. Mass transfer30 and heat transfer play essential roles in hydrate formation process.31 Therefore, once the system is stabilized at 20 °C, the experiment process is carried out using an isochoric pressure search method.32 It has been implemented by Ohmura et al.,33 and the method is mainly based on cooling and heating cycles at a constant volume.34 The loaded cells in the bath are gradually cooled from 20 to 2 °C in the period of 9 h with the cooling rate adjusted in the bath to 1.8 °C/h, followed by an isothermal step of 24 h at fixed temperature of 2 °C and then slowly heating the cells at the rate of 0.1 °C per hour until the temperature returns to 20 °C (Figure 1). In a recent study,35

To enhance the inhibition performance of hydrate inhibitors, the effects of mixing synergents like polyvinylpyrrolidone (PVP), polyvinylcaprolactam (PVCap), and antifreezing polymers with hydrate inhibitors have been studied in different literature contributions.26 Kelland et al.27 studied the synergistic properties of PVCap with hexaalkylguanidinium salts and phosphonium bromide salts. Yang and Tohidi28 reported delay in hydrate nucleation using PVCap with glycol ether. Synergistic performance of poly(ethylene oxide) (PEO) has also been studied by different researchers.29 The aim of this work is to further investigate the kinetic inhibition (KI) and thermodynamic inhibition (TI) effect of mixtures containing pyrrolidinium cation-based ILs and synergent compounds (Syn) on a synthetic quaternary gas mixture (QM). The KI and TI effects of high concentration of IL (10 wt %) on QM mixture is also investigated and compared with that of commercial THI methanol as well.

2. EXPERIMENTAL SECTION 2.1. Materials. The ionic liquids and synergents used for this work are shown in Table 1. All of these ILs and Syn are soluble in water.

Table 1. Ionic Liquids (ILs) and Synergents (Syn) Studied in This Work

The IL 1-methyl-1-propylpyrrolidinium chloride [PMPy][Cl] and 1methyl-1-propylpyrrolidinium triflate [PMPy][triflate] were purchased from Iolitec ionic liquid technologies GmbH (purity ≥98%), and Syn poly(ethylene oxide) (PEO) and vinyl caprolactum were purchased from Sigma-Aldrich Korea (purity ≥98%). The ILs and Syn mixtures were diluted with deionized water to prepare sample solutions for the rocking cell. The QM mixture used for these experiments was purchased from Buzwair Industrial Gas Factories (Doha, Qatar), and the purchased mixture composition was confirmed using in-house gas chromatography (GC) analysis (Table 2).

Table 2. Mol Composition of the QM Mixture as per GC Analysis component

composition (Mol %)

C1 C2 C6+ CO2 N2

84.20 9.90 0.015 2.46 2.19

Figure 1. Experimental loop for the QM mixture with the three experimental steps and the hydrate dissociation point. it has been reported that a constant heating rate up to 0.5 °C/h does not affect the equilibrium point measurements. In our case, this rate is much slower (0.1 °C/h). Also, the extraction of accurate equilibrium point has been done using the method proposed earlier.36 The hydrate formation initiates during subcooling, and the crystal growth is identified from the sharp pressure drop that takes place during isothermal step when the temperature is kept constant at constant volume.34 A consistent protocol throughout the experiment with a constant subcooling of 18 °C is used. 2.4. Extracting of Hydrate Dissociation Point. The hydrate dissociation starts when the H−V (hydrate−vapor) and the H−Lw−V (hydrate−liquid−vapor) equilibrium lines intersect,37 and then the hydrate phase completely disappears as the temperature is increased

2.2. Experimental Apparatus. All experiments were conducted using a hydrate rocking cell purchased from PSL SystemtechniK GmbH. The rocking cell apparatus has 5 stainless steel test cells that can work in parallel to each other, and they are attached on to same skid that rocks the cells. These cells are capable of operating at pressures up to 200 bar (2900 psi) and have temperature rating of −10 °C to 60 °C. Each cell has a volume of 40.13 cm3 and contains a stainless steel ball with the diameter of 17 mm, which moves back and 3542

DOI: 10.1021/acs.energyfuels.5b03001 Energy Fuels 2016, 30, 3541−3548

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Energy & Fuels further.37 This location on the H−Lw−V equilibrium line is considered as a hydrate dissociation point (Figure 1). The combined standard uncertainty of the experiments (including the uncertainties resulting from temperature and pressure measurements and the composition uncertainty of the gas mixture) was found to be 5.66%. The combined uncertainty for the experiments was calculated using uncertainty measurement guidelines provided by Stephanie Bell38 and the spreadsheet model provided by Mahrenholtz + Partner (m+p) International (U.K.). 2.5. Induction Time Measurements. The induction time is the time that indicates the first sign of hydrate crystal formation through an abrupt decrease in pressure from the initial values.27a The induction times in the absence and presence of inhibitors were calculated from the pressure−time (P−t) curves obtained for a specific sample solution. The detailed calculation procedure is reported elsewhere.10 The uncertainty in the reported induction times is ±0.5 h.

3. RESULTS AND DISCUSSION 3.1. Methane HLVE and Validation of Results. Before the rocking cell was used, for validation purposes, initial tests were conducted using only a pure methane and water system. The data obtained was compared with the literature.39 Apparatus calibration results with methane hydrates were found to be in agreement with the reported literature (Figure 2).39 However, no results have been published in the literature

Figure 3. Comparison of the QM mixture experimental HLVE curve with the HLVE curve obtained using WatGas V2011 simulation software.

Table 3. Hydrate Dissociation Points for the QM Mixture P (bar)

T (°C)

40.79 66.10 80.02 99.63 117.95

11.21 14.21 15.51 17.18 18.05

However, there was no significant TI effect observed at 1 wt % measurements. Therefore, higher dosages of 5 wt % ILs were studied in this work. When using 5 wt % [PMPy][Cl] and 5 wt % [PMPy][triflate], a noticeable increase in TI effect was observed (Figure 4). The most prominent shift of temperature, 1.8 ± 0.2 °C, was observed at 40 bar for [PMPy][Cl]. It is clear from the literature23,32 that during hydrate formation the water molecules form loosely ordered clusters around gas molecule,

Figure 2. Comparison between rocking cell experimental results and the literature results for the pure methane HVLE data. The results are in good agreement, showing that the equipment was well-calibrated for the experiments.

for the QM mixture used in this work; therefore, the results obtained for the QM mixture were compared with the results of simulation software Wat Gas v2011 (Figure 3 and Table 3). The program is designed and produced by Nor Craft Software Company (Canada). It is capable of performing hydrate formation calculations and uses specific gravity method40 to calculate the hydrate formation conditions using the Berge correlation.41 The experimental results obtained for the QM mixture are in agreement with the simulation results of the Wat Gas v2011 simulation software (Figure 3). Details of the obtained results are explained below. 3.2. IL and IL−Syn Thermodynamic Hydrate Inhibition. The first set of experiments was performed to evaluate the hydrate inhibition effect for QM using 1 wt % of ILs.

Figure 4. Thermodynamic inhibition (TI) effect shown by pure 5 wt % ionic liquid mixtures (ILs) on QM gas mixture. 3543

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Energy & Fuels and these clusters increase in size and become more structured at low temperatures. The introduction of an IL in fluid disrupts the hydrogen bonding between the water molecules within these clusters. Moreover, concentration and structure of ILs hydrate inhibitors also play an important role in thermodynamic inhibition performance. Keshavarz et al.,39a Richard and Adidharma,18 and Partoon et. al42 have all received better TI effect on increasing the concentration of [BMIm][BF4−] and [BMIm][Cl−] in experiments. Recently, our group published a review32 in which the TI effect of imidazolium-based ILs in cation/anion is summarized. It was found that the ILs containing [Cl−] ion tend to give better TI effect than the ILs containing [BF4−] ions on the methane gas. This is the first time we are reporting the comparison of inhibition quality between [PMPy][Cl] and [PMPy][triflate] ILs on the QM gas mixture. It is worthwhile to compare the results with the common electrolyte NaCl in order to access the thermodynamic inhibition ability of tested ILs at the same concentration. Because there are no published results reported for the QM mixture with NaCl in the literature, the simulation package Hydraflash43 was used to obtain hydrate suppression temperatures44 for the QM mixture in the presence of 5 wt % NaCl. The addition of 5 wt % NaCl with the QM mixture showed a suppression temperature of 2 °C at 60 bar and 2.5 °C at 100 bar. Comparatively, the addition of 5 wt % IL with the QM mixture showed the hydrate suppression temperature of 0.5 ± 0.2 °C at 60 bar and 0.7 ± 0.2 °C at 100 bar. This clearly indicates that the tested ILs are not as effective inhibitors as classical THIs at the same concentration. Figure 4 also shows that both the [PMPy][Cl] and [PMPy][triflate] ILs did not have significant effect on the temperature shift of the hydrate equilibrium curve for the QM mixture. Therefore, to get the better temperature shift, synergistic compounds caprolactum (VCAP) and poly(ethylene oxide) (PEO) were added with ILs [PMPy][Cl] and [PMPy][triflate]. The synergents were added with ILs in molar ratio 1:1. There is no relevant published work reported on the selected mixtures of ILs and synergistic compounds. Figure 5 shows the effect of adding synergistic compounds VCAP and PEO with the IL [PMPy][Cl]. The Syn VCAP showed considerable shift in hydrate formation temperature at all pressures. At 40 bar, a temperature shift of 2.2 ± 0.2 °C was noted, which is obviously better than the temperature shift provided by using IL 5 wt % [PMPy][Cl] by itself. The Syn PEO did not provide the similar effect. Therefore, in terms of effectiveness, the VCAP was found to be more effective than PEO.

Figure 5. Thermodynamic inhibition (KI) effect of mixtures of IL [PMPy][Cl] with the Syn PEO and VCAP. The VCAP showed a thermodynamic inhibition effect better than that of PEO. The addition of VCAP into the mixture shifted the original hydrate formation temperature (QM) by 2.2 ± 0.2 °C at 40 bar and 1.3 ± 0.2 °C at 100 bar. The hydrate formation temperature at different pressures was determined by using the graphical grid lines.

Figure 6. Shows the thermodynamic inhibition (TI) effect of mixtures of IL [PMPy][triflate] with the Syn PEO and VCAP. The VCAP showed better thermodynamic inhibition effect than PEO. The addition of VCAP into the mixture delayed the original hydrate formation temperature (QM) by almost 2.0 ± 0.2 °C at 40 bar and by 0.7 ± 0.2 °C at 100 bar. The hydrate formation temperature at different pressures was determined by using the graphical grid lines.

[PMPy][Cl] + PEO < [PMPy][Cl] + VCAP

Likewise, an identical ratio of synergistic compounds PEO and VCAP was added to IL [PMPy][triflate]. Once again, VCAP was found to provide better TI effect than PEO, which shifted hydrate formation temperature by 2.0 ± 0.2 °C at 40 bar (Figure 6). A close observation of Figure 5 and 6 provides additional information that PEO with [PMPy][Cl] was unable to provide the shift in temperature. Whereas in the case of [PMPy][triflate], a positive temperature shift was observed using PEO at low experimental pressure (40 bar). Previously, a similar weak TI effect was reported by Engeloz and Hall45 for PEO. 3.3. Mixed ILs Effect as Thermodynamic Hydrate Inhibitors. An aqueous solution was prepared by mixing two

ILs (5 wt % [PMPy][Cl] and 5 wt % [PMPy][triflate]) in equal ratio to observe its thermodynamic inhibition performance with QM gas, and the experimental data is listed in Table 4. The thermodynamic results obtained for the two mixed ILs [PMPy][Cl] and [PMPy][triflate] was compared with 10 wt % methanol P−T data (Figure 7) for a comparative thermodynamic inhibition study. The P−T curves for hydrate inhibitor methanol and mixed ILs were found to be very close to each other. The mixed ILs results does not beat methanol 3544

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Energy & Fuels Table 4. QM Mixture Hydrates Dissociation Points Obtained in the Presence of the Ionic Liquids and Synergents mixture QM + 1 wt % [PMPy][Cl]

QM + 5 wt % [PMPy][Cl]

QM + 5 wt % [PMPy][Cl] + 5 wt % PEO

QM + 5 wt % [PMPy][Cl] + 5 wt % VCAP

QM + 5 wt % [PMPy][triflate] + 5 wt % [PMPy][Cl]

P (bar)

T (°C)

38.65 64.83 78.22 98.60 37.77 57.30 77.28 96.88 116.27 38.37 56.88 78.03 97.34 117.21 37.36 56.3 76.1 95.35

10.41 14.04 15.36 17.19 9.25 12.46 14.62 16.32 17.49 9.74 12.81 15.14 16.41 17.32 8.29 11.69 13.92 15.55

QM + 1 wt % [PMPy][triflate]

37.99 55.94 77.20 96.97 116.80

8.15 10.97 13.77 15.33 16.43

QM + 10 wt % methanol

mixture

QM + 5 wt % [PMPy][triflate]

QM + 5 wt % [PMPy][triflate] + 5 wt % PEO

QM + 5 wt % [PMPy][triflate] + 5 wt % VCAP

P (bar)

T (°C)

58.16 78.30 97.72 118.87 39.07 57.96 77.99 98.80 117.67 38.34 56.18 77.61 97.05

13.08 15.22 16.77 18.67 9.66 12.86 14.97 16.70 17.94 10.73 13.14 15.54 17.16

37.65 56.25 77.10 97.60 115.83 42.69 63.72 78.85 97.70 116.45

8.93 12.06 15.09 16.47 17.17 8.58 11.80 13.32 14.84 15.90

tested ILs in this work have longer alkyl chains compared to methanol; the thermodynamic inhibition effect on the hydrate formation generally decreases with the increase in length of the alkyl chain.39a Another aspect of strong inhibition characteristic of methanol may also be due to strong interaction of the −OH group with the hydrogen bonds in hydrates clusters and additional interaction of the −CH3 group with the C−C or hydrogen bonds within hydrate clusters.46 However, the difference in the TI effect results of methanol and ILs was found to be only 0.5 ± 0.2 °C. This shift is not significant, and further experiments need to be conducted at higher concentration to check if the same ILs can provide better temperature shift than methanol. 3.4. IL and IL−Syn Kinetics of Hydrate Inhibition. Similar to the thermodynamic hydrate inhibition studies, all experimental data presented in this work is also explained from kinetics point of view to check if the selected mixtures exhibit any dual functional behavior.36,37 The experimental data helped to evaluate the kinetic inhibition effect of the selected ILs and their mixtures. The delay in hydrate formation provided by ILs [PMPy][Cl] and [PMPy][triflate] and their mixtures with PEO and VCAP is shown in Figures 8 and 9. Figure 8 shows a gradual increment in hydrate formation time with the addition of PEO and VCAP with IL [PMPy][Cl]. Therefore, the trend of effectiveness for the IL [PMPy][Cl] mixtures as kinetic inhibitors for the QM gas can be written as

Figure 7. Thermodynamic inhibition (TI) effect of using high concentrations of ILs in the mixtures. The comparison is drawn between the results of 10 wt % methanol and the 10 wt % (5 wt % [PMPy][triflate] + 5 wt % [PMPy][Cl]) ILs mixture. The 10 wt % ILs mixture provided the temperature shift of 2.0 ± 0.2 °C at 60 bar and the temperature shift of 1.6 ± 0.2 °C at 100 bar. In contrast, the 10 wt % methanol provided the temperature shift of 2.4 ± 0.2 °C at 60 bar and the temperature shift of 2.2 ± 0.2 °C at 100 bar.

[PMPy][Cl] < [PMPy][Cl] + VCAP < [PMPy][Cl] + PEO

completely, but it does provide interesting shifts in temperature at different pressures. The total 10 wt % ILs mixture provided the temperature shift of 2.0 ± 0.2 °C at 60 bar and the temperature shift of 1.6 ± 0.2 °C at 100 bar, whereas the 10 wt % methanol provided the temperature shift of 2.4 ± 0.2 °C at 60 bar and the temperature shift of 2.2 ± 0.2 °C at 100 bar. The

A similar graphical trend was also noticed for the IL [PMPy][triflate] on the addition of PEO and VCAP (Figure 9). Therefore, the trend of effectiveness for the IL [PMPy][triflate] mixtures as kinetic inhibitors for the QM gas can be written as 3545

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[PMPy][Cl], [PMPy][Cl] and VCAP, and [PMPy][Cl] and PEO at 37.35, 36.93, and 34.78 bar, respectively, whereas the hydrate formation time obtained for [PMPy][triflate], [PMPy][triflate] and VCAP, and [PMPy][triflate] and PEO was 7.27 ± 0.5, 8.49 ± 0.5, and 14.8 ± 0.5 h at 38.64, 37.12, and 35.52 bar, respectively. For the best comparison between pure IL and IL +PEO, addition of PEO with IL [PMPy][Cl] delayed hydrate formation from 6.5 ± 0.5 to 20 ± 0.5 h at low experimental pressure (Figure 8). Similarly, addition of PEO with IL [PMPy][triflate] delayed hydrate formation time from 6.5 ± 0.5 h to 13 ± 0.5 h at low experimental pressure (Figure 9). Kang et al.,14 observed synergistic effect on methane hydrates inhibition using 0.5 wt % PVCap with ILs [EMIM][BF4], [BMP][BF4], and [HEMP][BF4] . The hydrate formation was found to be delayed by 2.0, 1.0, and 3.0 h at 70 bar, respectively. Such enhancement of kinetic inhibition of ILs upon addition of polymeric synergents indicates that they may have nucleation hydrate inhibition effect rather than hydrate crystal growth inhibition effect. There were some other synergistic studies performed in regard to kinetic inhibition enhancement by mixing of salts or MEG with polymeric compounds such as quaternary phosphonium bromide salts with N-vinylcaprolactam (VCap)based kinetic hydrate inhibitor polymers.27b In another approach47 a very little amount of PVCap was added with different concentrations of MEG, which enhanced the kinetic inhibition for natural gas hydrates because the PVCap molecules disrupt the organization of water−gas molecules, increasing the barrier to nucleation. Daraboina et al.10 reported that hydrate nucleation delayed upon addition of PEO with Luvicap. Luvicap is also widely known as a kinetic hydrate inhibitor. Hammouda et al.48 suggested that the presence of a small amount of PEO in water allows PEO to have both hydrophilic and hydrophobic interactions with water. The oxygen atom of PEO develops hydrophilic interactions with water molecule by hydrogen bonding, which can disturb the local structure of water molecules before hydrate nucleation. In contrast, the hydrophobic group keeps away the water molecules, which may associate with inhibitor molecules and change their conformation. 3.5. Mixed ILs Effect as Kinetic Hydrate Inhibitors. The kinetics results obtained for the two mixed ILs [PMPy][Cl] and [PMPy][triflate] was compared with the equal ratio mixtures containing ([PMPy][Cl] and VCAP) and ([PMPy][triflate] and VCAP). As shown in Figure 10, the mixed ILs [PMPy][Cl] and [PMPy][triflate] provided similar kinetic results like the other mixtures containing ([PMPy][Cl] and VCAP) and ([PMPy][triflate] and VCAP). Therefore, the higher concentration of ILs in the mixtures was found to favor better kinetic inhibition effect than the mixtures containing ILs and VCAP in this case. However, the mixtures containing PEO still provide the better kinetic inhibition effect than other mixtures used in this work.

Figure 8. Kinetic inhibition effect of mixtures of IL [PMPy][Cl] with the Syn PEO and VCAP. The PEO showed a better kinetic inhibition effect than VCAP. The addition of PEO into the mixture delayed original hydrate formation time (QM) by 13 ± 0.5 h at 40 bar and by 6 ± 0.5 h at 120 bar. The hydrate formation time at different pressures was determined by using the graphical grid lines.

Figure 9. Kinetic inhibition (KI) effect of mixtures of IL [PMPy][triflate] with the Syn PEO and VCAP. The PEO again showed a better kinetic inhibition effect than VCAP. The addition of PEO into the mixture delayed the original hydrate formation time (QM) by almost 7 ± 0.5 h at 40 bar and by 3 ± 0.5 h at 120 bar. The hydrate formation time at different pressures was determined by using the grid lines.

[PMPy][triflate] < [PMPy][triflate] + VCAP

4. CONCLUSION The thermodynamic inhibition (TI) and kinetic inhibition (KI) effects of mixtures containing pyrrolidinium-based ionic liquids (ILs) and synergents PEO and VCAP were studied in this work. The selected mixtures have shown the tendency to exhibit both TI and KI effect simultaneously. The results show that a higher concentration (10 wt %) of pure ILs mixtures provides better TI and KI effect than 5 wt %.

< [PMPy][triflate] + PEO

In all cases, the kinetic hydrate inhibition effect was found to be better in mixtures than the pure ILs. The PEO was found to be the most effective candidate as kinetic synergent for both the ILs. The hydrate formation time obtained at low experimental pressure was 7.67 ± 0.5 h, 8.35 ± 0.5 h, and 22.78 ± 0.5 h for 3546

DOI: 10.1021/acs.energyfuels.5b03001 Energy Fuels 2016, 30, 3541−3548

Article

Energy & Fuels

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Figure 10. Kinetic effect of the addition of higher concentration of ILs on hydrate formation. The results for the mixture of 10 wt % ILs (5 wt % [PMPy][Cl] + 5 wt % of [PMPY][TRIFLATE]) was compared with the equal weight mixture containing 5 wt % IL and 5 wt % VCAP. The results show that the mixture containing higher concentration of IL (10 wt %) provide a kinetic effect that is better than that of the mixtures containing 5 wt % IL and 5 wt % VCAP.

We have also indicated that the use of synergents with ILs can help to improve the effectiveness of ILs in terms of simultaneously shifting hydrate formation temperature and delaying hydrate formation time. This leads us to the search for a new combination of ILs and synergents to formulate mixtures that can shift hydrate formation temperature and delay hydrate formation time simultaneously. Therefore, the IL mixtures used in this work offer a step toward finding a solution to hydrate plugging problems in offshore subsea lines.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

M.F.Q. and T.A. contributed equally to this work.

Notes

Disclosure: The statements made herein are solely the responsibility of the authors. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible by NPRP Grant 6-330-2-140 and GSRA 2-1-0603-14012 from the Qatar National Research Fund (a member of Qatar Foundation).

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