Benchtop Euler Wheel for Kinetic Hydrate Inhibitor Screening

Oct 17, 2017 - Benchtop Euler Wheel for Kinetic Hydrate Inhibitor Screening: Comparison to Rocking Cells. Malcolm A. Kelland† , Anders Grinrød‡, ...
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Benchtop Euler Wheel for Kinetic Hydrate Inhibitor Screening: Comparison to Rocking Cells Malcolm A. Kelland,*,† Anders Grinrød,‡ and Jovana Milanovic† †

Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ M-I Swaco (Schlumberger), Koppholen 23, 4313 Sandnes, Norway ABSTRACT: A new type of high-pressure instrument dubbed the Euler wheel has been used for the testing of kinetic hydrate inhibitors (KHIs). The acrylic top of the wheel allows for observations of the fluids and hydrate formation through the entire loop. A series of four to six experiments on each KHI was carried out at an active dosage of 2500 ppm and an initial pressure of 40 bar using the constant-cooling method. The results were compared to KHI tests carried out in steel rocking cells. The rankings between the two types of instruments were found to be very similar. However, the best KHIs did not cause a pressure drop due to hydrates in the wheel, even at the minimum experimental temperature (0.7 °C), which represents a fairly high subcooling (ΔT = 14.3 °C). Further tests were therefore conducted at a lower concentration of 1000 ppm KHI, which helped to determine a clearer ranking of the best KHIs. The fluid movement in the wheel is closer to laminar flow, whereas the flow in rocking cells is more turbulent. This suggests that the lower turbulence in the wheel is the reason for the lower onset temperatures observed in the wheel. Thus, the type of fluid flow encountered in the field can impact the performance of KHIs.

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INTRODUCTION The plugging of oil and gas production flowlines with gas hydrates is a major concern to operator companies. This problem is exacerbated in deeper water (because of higher pressures and lower temperatures) and colder climates.1,2 In addition to the traditional use of thermodynamic inhibitors (THIs) to alleviate this problem, several low-dosage chemical methods are now available. They include the use of kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).3,4 Collectively, such chemicals are known as low-dosage hydrate inhibitors (LDHIs). LDHIs are relatively expensive oilfield chemicals, particularly in comparison to THIs, and it is therefore important to determine the required dosages in laboratory- and pilot-scale experiments. A number of types of test equipment can be used for first-time screening of LDHIs, the most popular being rocking cells and stirred autoclaves; for larger-scale testing, a range of flow loops of various sizes and pipe wheels are available.5 Recently, we reported on a new type of high-pressure benchtop wheel, which we call the Euler wheel.5 This wheel has a thick acrylic glass top, allowing for excellent visual observation of the whole loop at pressures of up to 40 bar. The motion of the wheel is like that of an Euler disk, which is a way of tipping the wheel at constantly different positions around a central axis, similarly to a spinning coin that has almost come to rest.6 This motion causes the fluids to move around the wheel without having to rotate the wheel. We previously used the Euler wheel to investigate the performance of AAs and compared the results to tests in high-pressure sapphire rocking cells using the same gas composition and liquids.7 We concluded that the two different types of instruments gave the same ranking of AAs, but, in general, we found that the wheel tests required a higher concentration of AA to achieve the same good AA effect under the same test conditions, especially in saline solutions. A stationary benchtop wheel for LDHI testing in which fluids are © XXXX American Chemical Society

pushed around using a metal slug and an outside magnet has been described.8 Currently, our Euler wheel has limited speed of movement, meaning that there is limited turbulence of the fluids or even the possibility of laminar flow. Autoclaves with the correct stirrer and stirrer speed could potentially give similar flow patterns. However, the main benefit of the Euler wheel is the ability to make visual observations of all of the fluids in the whole loop. In this study, we turn our attention to KHIs. A range of polymer KHIs, sometimes with added nonpolymeric synergist, have been tested in the Euler wheel and compared to results obtained in steel rocking cells.

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CHEMICALS AND EXPERIMENTAL METHODS

Low-molecular-weight poly(N-vinyl caprolactam) (PVCap low Mw) as a 41.1 wt % solution in monoethylene glycol (MEG) and N-vinyl caprolactam/N-vinylpyrrolidone copolymer (VP/VCap 1:1) as a 53.8 wt % solution in water were used as received from BASF (Figure 1). Poly(N-vinylpyrrolidone) (PVP) powders were obtained from Ashland Specialty Chemical Company. PVP 12k has an average molecular weight (Mw) of 4000 g/mol, and PVP 30k has and average molecular weight of about 58000 g/mol (Figure 1). Ashland also supplied samples of a low-molecular-weight poly(N-vinyl caprolactam) in butyl glycol ether (PVCap in 50 wt % BGE) and an N-vinyl caprolactam/ N-vinylpyrrolidone copolymer in BGE (VP/VCap in 50 wt % BGE). Pure poly(N-isopropylacrylamide) (PNIPAM) with a Mw of approximately 6000 g/mol, butylated tetraethylenepentamine oxide (TEPABu-AO) as a 31 wt % solution in propan-2-ol, and hexabutylguanidinium chloride (Bu6GuanCl) as a 70 wt % solution in water were all synthesized at the University of Stavanger (Figures 2 and 3).9−11 KHI experiments were carried out in the Euler wheel and compared to previously obtained results from rocking-cell experiments. Received: August 18, 2017 Revised: October 13, 2017 Published: October 17, 2017 A

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Figure 1. Structures of (from left to right) poly(N-vinyl caprolactam) (PVCap), N-vinyl caprolactam/N-vinylpyrrolidone copolymer (VP/VCap 1:1), and poly(N-vinylpyrrolidone) (PVP).

Figure 2. Structures of the main component in butylated tetraethylenepentamine oxide (TEPA-Bu-AO).

Figure 4. Euler wheel placed in the cooling bath. Figure 3. Structures of (left) poly(N-isopropylacrylamide) (PNIPAM) and (right) hexabutylguanidinium chloride (Bu6GuanCl). Full details of the rocking-cell test method are given elsewhere.12,13 The Euler wheel is 325 mm in diameter and is made of a steel disk with a groove cut into it (Figure 4).7 Its small size enables it to fit easily on a standard laboratory bench alongside a heater/cooler unit (Figure 5). The acrylic top disk is flat on both sides and, when mounted, gives a test-groove volume of 200 mL. The transparent top allows one to view any part of the flow channel in the wheel loop while conducting experiments up to 40−45 bar. Fittings in the top disk allow for addition or removal of fluids and for pressurization or degassing. The gas mixture used was standard synthetic North Sea gas (SNG) with the composition reported in Table 1. The wheel is submerged in a glycol/water bath, and pressure and temperature sensors in the wheel are connected to a computer (Figure 6). The wheel is programmed to tip using the Euler motion from an axle at the center of the wheel, also connected to the computer. No pumps or internal equipment for propelling the liquid are needed. The KHI experimental method was as follows: • An aqueous solution (100 mL) containing KHI dissolved at the require concentration was added to the wheel. This is one-half of the total volume of the wheel. • The wheel was flushed with SNG and pressurized to approximately 41.5 bar (±0.1 bar). The pressure dropped over 15−20 min down to approximately 40 bar as some gas dissolved in the aqueous phase. • The Euler motion of the wheel was started. • The cooling bath was set to the starting test temperature (16 °C). • Both the temperature of the water heater/cooler and the internal temperature in the water bath and the pressure in the wheel were recorded using the software. • When the temperature in the water heater/cooler and the water bath had stabilized, the pressure was checked to see if it was 40 bar.

Figure 5. Euler wheel in the cooling bath with cooler/heater, all placed on a benchtop. If not, the pressure was adjusted to 40 bar at 16 °C. The equilibrium temperature (Teq) at this pressure is approximately 15.5 °C at 40 bar, as determined using Calsep’s PVTSim software. • The cells were cooled from 16 °C to approximately 0.7 °C at a rate of 2 °C/h. If rapid hydrate formation did not occur during this time, the temperature was further held at 0.7 °C for 2 h. At 0.7 °C, the pressure is approximately 35 bar, assuming that no hydrate formation has taken place. Teq at this pressure is now only 15.0 °C. B

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this occurs at about 132 min (Ta = 13.1 °C), and in Figure 8, it occurs at about 428 min (Ta = 2.2 °C). Sometimes, if very rapid hydrate formation occurs, one can observe an exothermic spike for the temperature in the wheel on the graphical temperature plot. The difference between To and Ta can provide an indication of the ability of the KHI to slow crystal growth. However, this kind of comparison between KHIs is valid only if they have similar To values.

• Hydrate formation was observed by detecting a pressure drop, but could also be observed by visual inspection.

Table 1. Composition of Synthetic Natural Gas (SNG) component

content (mol %)

methane ethane propane isobutane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

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RESULTS AND DISCUSSION More than 80 constant-cooling experiments were conducted in the Euler wheel in this study. Table 2 summarizes the results obtained using 2500 ppm active KHI and provides a comparison to the same KHIs (from the same batch) tested using our standard constant-cooling rocking-cell experiments at 78 bar. Results in rocking cells for two KHIs at a concentration of 2500 ppm were not available, as detailed footnotes c and d to Table 2. The average To and Ta values from four to six (but usually five ) experiments are also included, as well as the subcooling.10,11,28 Subcoolings at To are calculated compensating for the gradual drop in Teq due to the pressure decreasing when cooling the closed system. Calsep’s PVTSim program was used to calculate Teq as 15.5 °C for the Euler wheel tests at 40 bar and as 20.5 °C for the rocking-cell tests at 76 bar. Numbers in parentheses are absolute values of maximum errors in degrees Celsius. Cooling was applied to 0.7 °C with a temperature error in the wheel of ±0.1 °C. We always carried out experiments above 0 °C to avoid the possibility of ice formation in the wheel. Some MEG (maximum of 10%) was added to the cooling fluid to avoid ice formation in the cooling unit, but we found that

Once an experiment was finished, data recording was terminated, and the water bath was heated to 25 °C for 2 h to fully melt any hydrates that might have formed. The movement of the wheel was terminated, and the apparatus was depressurized back to atmospheric pressure. The fluids in the wheel were removed, and the wheel was washed thoroughly three times with distilled water and hydrocarbon liquids. A typical graph of pressure and temperature in the wheel is provided in Figure 7. In addition to the initial pressure drop due to gases dissolving in the aqueous phase, a further constant pressure drop can be seen due to the constant cooling in a closed system. The first deviation from this constant pressure drop, however, indicates gas consumption due to hydrate formation. The tank temperature at which this occurs is called the hydrate onset temperature, To. This can be seen in Figure 7 at about 128 min (To = 13.2 °C) and in Figure 8 at about 330 min (To = 5.4 °C). Hydrate nucleation might possibly have been initiated earlier but gone undetected. When the rate of hydrate formation is at its highest, we record the temperature Ta. In Figure 7,

Figure 6. Schematic of the setup of the whole apparatus (from left to right): gas, gas booster/distributer, water heater/cooler, table-top wheel in the water bath as seen from above, two electronic boxes, computer, and monitor with the software interface.

Figure 7. Typical plot of pressure and temperature versus time for a constant-cooling test with water only, no additive. C

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Figure 8. Typical plot of temperature and pressure versus time for a constant-cooling KHI experiment, shown here for VP/VCap 1:1 copolymer.

Table 2. Comparison of Constant-Cooling Results Using 2500 ppma Active KHI in the Euler Wheel and Rocking Cellb Euler wheel

a

rocking cell

chemical

To (°C)

ΔTo (°C)

Ta (°C)

To (°C)

ΔTo (°C)

Ta (°C)

no additive PVP 30k PVP 12k PVCap low Mw VP/VCap 1:1 PNIPAM TEPA-AO VP/VCap in BGE PVCap in BGE PVCap low Mw + Bu6GuanCl 1:1 PVCap low Mw + Bu6Guan 1:29

13.4 (0.3) 8.9 (0.3) 8.6 (0.4) 5.1 (0.2) 2.6 (0.5) 2.6 (0.7) 2.2 (0.4) 14.3

13.4 (0.3) 4.6 (0.4) 6.4 (0.7) 2.4 (0.2) >0.7 1.3 (0.3) 1.2 (0.3) − − − −

17.8 12.2 11.3 9.5 8.1 8.5 6.7c 6.9 6.1 4.9d 3.8d

2.7 8.2 9.2 10.9 12.2 11.8 13.5 13.3 14.1 15.3 16.3

17.5 11.7 10.5 8.4 5.9 7.9 5.7c 4.9 3.3 4.1d 2.9d

Unless otherwise stated. bPressure: Euler wheel, 40 bar; rocking cell, 76 bar. c5000 ppm. d3000 ppm.

lower-molecular-weight PVP gave the better performance.14,15 It has also been reported that the first stages of hydrate formation in the presence of a sufficient amount of PVP lead not to plugs or deposits but rather to the formation of a dispersion or slurry of hydrate particles in water. This indicates crystal growth inhibition, as reported by others.16−22 As more water is converted, the particles do agglomerate, and deposits are formed. Thus, this is not true antiagglomerant (AA) behavior, which we reported previously for a range of surfactants.7 We also observed this hydrate-dispersing behavior of PVP in the Euler wheel. Figure 10 shows the slurry state of hydrate formation before the formation of deposits. The situation after rapid hydrate formation giving deposits is very similar to that shown in Figure 9 for the use of no additive. None of the other KHIs appeared to show a significant visual antiagglomerant effect. Some of the VCap-based polymers did show some dispersion of hydrate particles before rapid hydrate formation occurred, but this was always at a lower water conversion (smaller pressure drop due to hydrate formation) than with the PVPs. However, as expected from studies in other equipment using structure II-forming gas mixtures, the VCapbased polymers performed considerably better than PVP.23−25

too much MEG in the aqueous cooling liquid led to many small bubbles and less clarity in the wheel bath, which made it harder to make observations of hydrate formation inside the wheel. The first observation to note is that, in the wheel with no additive, we obtained gas hydrate formation at a small subcooling of 2.1 °C (±0.3 °C). The cooling rate was 2 °C/h. In the rocking cells at almost twice the pressure, the subcooling at the first sign of hydrate formation was 2.7 °C (±0.5 °C). This indicates that there is sufficient agitation of the fluids to produce hydrate formation fairly close to the equilibrium temperature. This is crucial for the testing of a range of KHIs, as the maximum subcooling in the system, while avoiding ice formation, is not very high because of the limited pressure allowance in the wheel. We were also able to observe the whole process of hydrate formation leading to large deposits and sometimes a plug, preventing movement of the fluids around the whole wheel. A photograph of typical deposits observed when no additive was used is shown in Figure 9. Regarding the KHIs, the PVP polymers gave the highest To values (or the lowest ΔTo values) and therefore the worst performance. As previously observed by several groups, the D

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PVCap and on a par with the VP/VCap copolymer if the To values are considered. However, the VP/VCap copolymer showed a strong ability to inhibit further hydrate growth, as rapid hydrate formation did not ensue even after 2 h at the minimum temperature of 0.7 °C. The next issue to consider is whether the ranking of the KHIs is similar to that observed in other equipment. We already mentioned that PVP is not a very powerful KHI, and our test results in rocking cells in Table 2 bear this out. As shown, the two PVPs were also the worst KHIs (of those tested). In addition, just as we found in the Euler wheel, the lower-molecularweight PVP gave a somewhat better performance with an average To value almost 1 °C lower than that of the higher-molecularweight PVP (significant result by t test giving p < 0.05). For the other KHIs, we obtained generally good agreement with the rankings based on To values observed in the rocking cells. The studies in the wheel were not the carried out at the same pressure as in the rocker rig, and it is known that KHI performance is affected by pressure.31−34 However, to the best of our knowledge, the performance varies with pressure for all of the KHI polymers in the studies published to date, and according to these studies, there is no evidence that the ranking changes with pressure for a set of polymers. For the best KHIs that did not form hydrates in the Euler wheel tests, we obviously cannot compare the results to rockingcell tests, although these KHIs also generally gave the lowest To values in the rocking cells, with the exception of TEPA-Bu-AO. Therefore, we also conducted Euler wheel experiments with the best KHIs at 1000 ppm. These results are reported in Table 3.

Figure 9. Deposits of gas hydrates in the Euler wheel instrument from a test with no additive.

Table 3. Constant-Cooling Results Using 1000 ppm Active KHI in the Euler Wheela chemical

To (°C)

ΔTo (°C)

Ta (°C)

no additive PVCap low Mw VP/VCap in BGE PVCap low Mw + Bu6GuanCl 1:29 PVCap low Mw + Bu6GuanCl 1:1 PVCap in BGE

13.4 (0.3) 6.1 (0.3) 5.0 (0.3) 4.6 (0.2) 4.4 (0.5) 3.3 (0.2)

2.1 9.1 10.2 10.6 10.8 11.8

13.4 (0.3) 5.0 (0.3) 2.6 (0.2) 4.1 (0.3) 3.7 (0.3) 1.4 (0.5)

Figure 10. Experiment with PVP 12K showing the gas hydrate slurry formed at partial water conversion.

a

The PVCap low Mw (in MEG) and the VP/VCap copolymer (aqueous) KHIs gave average To values of 5.1 and 2.6 °C, respectively. However, the two polymers that are sold in BGE solvent, PVCap in BGE and VP/VCap in BGE, did not show any signs of hydrate formation (visual or by pressure drop) even after 2 h at the minimum temperature of 0.7 °C. BGE is known to be a good synergist for VCap-based polymers, whereas MEG shows much less significant synergy at these low concentrations.26,27 Two other synergistic mixtures with PVCap also did not show any visual signs or a pressure drop due to hydrate formation down to 0.7 °C. These mixtures contained hexabutylguanidinium chloride (Bu6GuanCl), which has been shown to be a powerful synergist with VCap-based polymers.11,28 This includes a 1:29 PVCap/Bu6GuanCl mixture containing just 83 ppm polymer. Two other KHIs that were tested were PNIPAM and TEPABu-AO. The first polymer has been known for over 20 years to be an effective KHI, whereas the second chemical, an oligomeric amine oxide, was recently shown to also be a useful KHI.29,30 In the Euler wheel, these KHIs performed better than low Mw

The concentration was low enough that all of the KHIs formed hydrates during cooling and also led to rapid hydrate formation before reaching the minimum bath temperature of 0.7 °C. From these tests, one see that PVCap in BGE (a synergist solvent) gave the best performance. It should be noted that this was not the same PVCap as the other PVCaps reported in Tables 2 and 3. PVCap low Mw is available as a 41.1% solution in MEG and was obtained from a different supplier; it was used alone and also in synergistic mixtures with Bu6GuanCl. The difference between To and Ta (i.e., To − Ta value) can sometimes provide an indication of the ability of a KHI to retard the growth process once observed nucleation has begun. This is only true if the To − Ta values to be compared are obtained at similar To values; otherwise, the driving force at the start of nucleation is not the same, which will affect hydrate growth rates. In general, it appears that the To − Ta values are higher for the Euler wheel experiments than for the rocking-cell experiments. This difference might reflect the lower turbulence and mixing in the wheel. The two PVPs tested in the Euler wheel have similar To values. The higher-molecular-weight polymer gave E

Initial pressure = 40 bar.

DOI: 10.1021/acs.energyfuels.7b02422 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels a significantly higher To − Ta value, indicating stronger crystal growth inhibition. This was also found by Shell Oil Company in groundbreaking studies in the early 1990s.14 The VCapcontaining polymers generally also gave quite large To − Ta values, albeit at lower To values than for the PVPs. This is expected, as VCap-containing polymers are known to be excellent gas hydrate KHIs and good crystal growth inhibitors of tetrahydrofuran hydrates.35−37 The To − Ta values are also generally larger from tests at 2500 ppm (Table 2) compared to values from tests at 1000 ppm (Table 3).

(11) Magnusson, C. D.; Kelland, M. A. Energy Fuels 2016, 30, 4725− 4732. (12) Chua, P. C.; Kelland, M. A. Energy Fuels 2012, 26, 1160−1168. (13) Lin, H.; Wolf, T.; Wurm, F. R.; Kelland, M. A. Energy Fuels 2017, 31, 3843−3848. (14) Anselme, M. J.; Reijnhout, M. J.; Muijs, H.M.; Klomp, U. C. International Patent Application WO 93/25798, 1993. (15) O’Reilly, R.; Ieong, N. S.; Chua, P. C.; Kelland, M. A. Chem. Eng. Sci. 2011, 66, 6555−6560. (16) Kumar, R.; Lee, J. D.; Song, M.; Englezos, P. J. Cryst. Growth 2008, 310, 1154−1166. (17) Tang, C.; Du, J.; Liang, D.; Fan, S.; Li, X.; Chen, Y.; Huang, N. Xi’an Jiaotong Daxue Xuebao 2008, 42 (3), 333−336 367. (18) Daraboina, N.; Ripmeester, J.; Walker, V. K.; Englezos, P. Energy Fuels 2011, 25, 4392. (19) Talaghat, M. R. Can. J. Chem. Eng. 2012, 90 (2), 429−436. (20) Ivall, J.; Pasieka, J.; Posteraro, D.; Servio, P. Energy Fuels 2015, 29, 2329−2335. (21) Zhao, X.; Qiu, Z.; Zhou, G.; Huang, W. J. Nat. Gas Sci. Eng. 2015, 23, 47−54. (22) Posteraro, D.; Ivall, J.; Maric, M.; Servio, P. Chem. Eng. Sci. 2015, 126, 91−98. (23) Cha, M.; Shin, K.; Seo, Y.; Shin, J. Y.; Kang, S. P. J. Phys. Chem. A 2013, 117, 13988−13995. (24) Zhang, J. S.; Lo, C.; Couzis, A.; Somasundaran, P.; Wu, J.; Lee, J. W. J. Phys. Chem. C 2009, 113 (40), 17418−17420. (25) Chua, P. C.; Kelland, M. A. Energy Fuels 2012, 26, 4481−4485. (26) Cohen, J. M.; Wolf, P. F.; Young, W. D. U.S. Patent Application 5,723,524, 1998. (27) Semenov, A. P.; Medvedev, V. I.; Gushchin, P. A.; Yakushev, V. S.; Vinokurov, V. A. Chem. Technol. Fuels Oils 2016, 52 (1), 43−51. (28) Kelland, M. A.; Moi, N.; Howarth, M. Energy Fuels 2013, 27, 711. (29) Park, J.; da Silveira, K. C.; Sheng, Q.; Wood, C. D.; Seo, Y. Energy Fuels 2017, 31 (3), 2697−2704. (30) Kelland, M. A.; Magnusson, C.; Lin, H.; Abrahamsen, E.; Mady, M. F. Energy Fuels 2016, 30, 5665−5671. (31) Svartaas, T. M.; Kelland, M. A.; Dybvik, L. Ann. N. Y. Acad. Sci. 2000, 912, 744−752. (32) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Sci. 2005, 60, 1313−1321. (33) Peytavy, J.-L.; Glénat, P.; Bourg, P. Kinetic hydrate inhibitors Sensitivity towards pressure and corrosion inhibitors. Presented at the International Petroleum Technology Conference, Dubai, United Arab Emirates, Dec 4−6, 2007; IPTC 11233. (34) Kelland, M. A.; Mønig, K.; Iversen, J. E.; Lekvam, K. A feasibility study for the use of kinetic hydrate inhibitors in deep water drilling fluids. Presented at the 6th International Conference on Gas Hydrates, Vancouver, British Columbia, Canada, July 6−10, 2008. (35) Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilib. 1998, 150−151, 353−360. (36) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R. L.; Sloan, E. D. Presented at the 73rd Annual GPA Convention, New Orleans, LA, Mar 7−9, 1994. (37) Sloan, E. D.; Christiansen, R. L.; Lederhos, J. P.; Long, J. P.; Panchalingam, V.; Du, Y.; Sum, A. K. W. U.S. Patent 5,639,925, 1997.

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CONCLUSIONS Experiments in a new type of high-pressure instrument dubbed the Euler wheel have been carried out on a range of kinetic hydrate inhibitors (KHIs). A series of repeated experiments on each KHI was carried out at an active dosage of 2500 ppm and an initial pressure of using the slow constant-cooling method, and the results were compared to similar KHI tests in steel rocking cells. Once hydrate formation began in the wheel, visual observations of the morphology and deposition of the hydrates could also be obtained, as illustrated for PVP. The rankings of the KHIs for the two types of instruments were found to be very similar. However, the best KHIs did not cause a pressure drop due to hydrates in the wheel, even at the minimum experimental temperature (0.7 °C), which represents a fairly high subcooling (ΔT = 14.3 °C), even after this temperature had been held for 2 h. Further tests were therefore conducted at a lower concentration of 1000 ppm KHI, which helped to determine a clearer ranking of the best KHIs. The speed and motion of the Euler wheel, causing less mixing of the fluids compared to small rocking cells, could be the reason for the lower To values (i.e., better performance) observed in the wheel. The fluid movement in the wheel is close to laminar flow, whereas the flow in rocking cells is more turbulent. This suggests that the type of fluid flow encountered in the field can impact the performance of KHIs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Malcolm A. Kelland: 0000-0003-2295-5804 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Carroll, J. Natural Gas Hydrates: A Guide for Engineers; Gulf Professional Publishing/Elsevier Science: Boston, MA, 2003. (3) Kelland, M. A. Production Chemicals for the Oil and Gas Industry, 2nd ed.; CRC Press: Boca Raton, FL, 2014. (4) Kelland, M. A. Energy Fuels 2006, 20, 825−847. (5) Ke, W.; Kelland, M. A. Energy Fuels 2016, 30, 10015−10028. (6) Giant Euler's Disk in slow motion. https://www.youtube.com/ watch?v=VeW2dypZTYQ (accessed Oct. 22, 2017). (7) Kelland, M. A.; Grinrød, A.; Dirdal, E. G. J. Chem. Eng. Data 2015, 60, 252−257. (8) Tian, J.; Littlefield, S. A. Hydrate inhibition test loop. International Patent Application WO 2012/047821, 2012. (9) Magnusson, C. D.; Kelland, M. A. Energy Fuels 2015, 29, 6347− 6354. (10) Abrahamsen, E.; Kelland, M. A. Energy Fuels 2016, 30, 8134− 8140. F

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