Exploring Kinetic Hydrate Inhibitor Test Methods and Conditions Using

Apr 10, 2013 - Comparison of Kinetic Hydrate Inhibitor Performance on Structure I and Structure II ... High-Throughput Testing of Kinetic Hydrate Inhi...
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Exploring Kinetic Hydrate Inhibitor Test Methods and Conditions Using a Multicell Steel Rocker Rig Astrid Lone†,‡ and Malcolm A. Kelland*,† †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ MI-Swaco Production Chemicals (a Schlumberger Company), Koppholen 23, Post Box 403, 4067 Stavanger, Norway ABSTRACT: High pressure rocking cells have overtaken the use of stirred autoclaves as the standard industry method for the first stage laboratory screening of kinetic hydrate inhibitors (KHIs). This study investigates the effect on KHI performance by varying different parameters in the rocking cells. This includes the rocking rate, rocking angle, gas−water ratio (GOR), glass versus steel balls, cooling rate in constant cooling experiments, and KHI concentration. A gas composition containing C1−4 hydrocarbons, CO2, and N2 was used, which was predicted to give Structure II hydrate. Isochoric experiments using the constant cooling method or isothermal test method were used on N-vinyl lactam polymers.

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and mixing of the fluids. The key factor with this equipment is that several rocking cells can be placed in one cooling bath which means the researcher can run several experiments at the same time. Depending on the test method it is usually sufficient to carry out about 5−10 individual experiments on each KHI to screen a series of KHIs and determine their relative performance ranking.15−18 This could be by determining the average hold time or onset temperature using methods discussed earlier for autoclaves and checking for statistically significance results. However, in order to determine the absolute field performance of a KHI it has been argued that an infinite number of tests at the field conditions would need to be carried out and the worst result taken as the expected field performance.13,19,20 Although rocking cells have been in used for a number of years, to the best of our knowledge there are no reports of a detailed study in which the different test conditions are used. This paper discusses KHI performance results using a 5-cell rocker rig in which the following parameters are investigated: • Rocking angle • Rocking speed • Aqueous volume added to the cell • Steel versus glass balls • Isothermal versus constant cooling test • Cooling rate • KHI concentration We have also compared the performance of a range of KHIs in steel rocking cells with tests in different size stirred autoclaves. This study will be reported separately.21

INTRODUCTION Kinetic hydrate inhibitors (KHIs) were first developed in the 1990s and are now used to prevent gas hydrate plugging of flow lines worldwide.1−4 They work by delaying the formation of gas hydrate both at the nucleation and usually also the crystal growth stages. The hydrate formation rate is reduced sufficiently by the KHI such that no hydrates form during the time that the fluids are resident in a pipeline at temperatures below the hydrate formation temperature. This delay time is dependent on the driving force in the system which is generally defined by the subcooling (ΔT) in the system, although the absolute pressure also affects the performance. Commercial KHI formulations are made up of water-soluble polymers often with added synergists which may be the solvent in which they are injected. For many years the commonest screening method to rank the performance of KHIs used to be the stirred, high pressure autoclave.1−4 This apparatus comes in many sizes and materials, ranging from steel, titanium, or sapphire. In small equipment it is important to carry out several identical experiments as the hydrate formation process is stochastic and therefore there will be some variation in the results. For example, the result could be a hold time (“induction time”) before hydrate formation in an isothermal test or an onset temperature for hydrate formation in a temperature-ramped test or “constant rate of cooling” test. However, each autoclave is usually placed in its own cooling bath which limits the number of experiments that can be carried out in a given time period. Small stirred multicell reactors have been developed by one research group.5 Fast screening methods using autoclaves and other equipment have also been developed which usually require melting the hydrates and reusing the same fluids in a series of cycles.6−12 Although stirred autoclaves are still very much in use, the oil industry has moved more and more to using a different type of equipment, the high pressure rocking cell.4,13,14 The cells are usually steel or sapphire (for visual observations) and generally vary in volume from about 10−50 mL. The cell contains a steel or glass ball and is rocked back and forth to create turbulence © 2013 American Chemical Society

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HIGH PRESSURE GAS HYDRATE ROCKER RIG EQUIPMENT TEST METHODS Kinetic hydrate inhibition experiments were conducted in a rocker rig containing five 40 mL steel rocking cells each Received: February 24, 2013 Revised: April 8, 2013 Published: April 10, 2013 2536

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Figure 1. The rocker rig showing the five steel cells in upright and 40° angle position.

2. Isothermal test method in which the pressurized cells are cooled without rocking to a set temperature and then rocking is started. At the start of every experiment the pressure was approximately 76 bar. The equilibrium temperature (Teq) at this pressure was determined to be 20.2 °C by five identical standard laboratory dissociation experiments warming at 0.025 °C/h for the last 3−4 °C as reported previously.22−25 This value agrees well with a calculated Teq value of 20.5 °C at 76 bar using Calsep’s PVTSim software. For the isothermal tests, the pressure dropped to around 68 bar before rocking was started due to dissolution of gas in the aqueous phase as well as due to the lower temperature in an isochoric system. The general test procedure was as follows: Each rocking cell was filled with a specified volume of freshly deionized water in which the KHI had been dissolved 16−24 h earlier. All ppm concentrations given refer to active concentrations on polymer. Air in the rocking cells was removed by vacuum and replaced with SNG to 2 bar. This procedure was then repeated before pressurizing with SNG to 76 bar. Another air-flushing method that was used was to pressurize to 30 bar with SNG and release the pressure back down to 1 bar and then repeat the procedure again. This method used up more SNG than use of a vacuum pump. Since a range of KHI test results were found to be statistically insignificantly different using both methods, we adopted the vacuum pump method to save on the use of SNG. For the isothermal tests, cooling was carried out to a predetermined temperature and then rocking started. The experiment was deemed to start when rocking began. For the constant cooling tests the cells were cooled from 20 °C down to 2 °C while rocking. The standard cooling rate was 1 °C/h although we investigated the effect of other cooling rates also (see the section on Discussion of Results). The pressure and temperature for each individual cell, as well as the temperature in the cooling bath, was logged on a computer. Unless otherwise stated, the standard test conditions used were as follows: • Rocking angle = 40° • Rocking rate = 20 rocks/min • Volume of aqueous fluid in the cell = 20 mL • Steel balls used, one per cell • Cooling rate of 1 °C/h for the constant cooling experiments • Pressurization of the cells before cooling in isothermal experiments A typical graph of pressure and temperature data versus time from all five cells using the constant cooling method is shown in Figure 3. The pressure drops about 1.5 bar due to gas being

containing a single steel or glass ball. The equipment was manufactured by PSL Systemtechnik, Germany. The five cells in the temperature controlled bath are shown in Figures 1 and 2 at an upright angle and at a 40° angle. The gas composition

Figure 2. A view inside of a rocking cell with steel and glass balls.

used was a synthetic natural gas (SNG) mixture given in Table 1. In all the experiments in this study a N-vinyl pyrrolidone:NTable 1. Composition of Synthetic Natural Gas (SNG) component

mole %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

vinyl caprolactam copolymer was used. Hereafter we refer to this KHI as VP:VCap1. This copolymer was kindly supplied as a 53.8 wt.% solution in water by BASF, Germany. In addition, for the test comparing steel and glass balls we also used three other polymers. These were a low molecular weight poly(Nvinyl caprolactam) homopolymer (PVCap1) and another Nvinyl pyrrolidone:N-vinyl caprolactam copolymer (VP:VCap2) both supplied in 2-butoxyethanol kindly by Ashland Chemical and a second poly(N-vinyl caprolactam) homopolymer (PVCap2) in monoethylene glycol supplied by BASF, Germany. All test concentrations referred to in this study are active polymer concentrations in the aqueous phase. Two types of isochoric test procedures were used: 1. Constant cooling test method in which the cells are cooled at a constant rate with rocking. 2537

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Figure 3. Graphical presentation of five identical constant cooling KHI experiments.

Figure 4. Determination of To and Ta values from a constant cooling KHI experiment.

to hydrate formation. The first deviation from the pressure drop due the temperature drop is taken as the time for onset of hydrate formation, although nucleation on an undetectable scale may have occurred earlier. The first deviation is

dissolved in the aqueous phase. The temperature drops at a constant rate until the minimum of 2 °C after 1120 min. During this time the pressure also drops at a constant rate, as it is a closed system, until the rate of pressure drop increases due 2538

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Figure 5. Graphical presentation of five identical isothermal KHI experiments.

Figure 6. Determination of to and ta values from an isothermal KHI experiment.

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approximately 0.05 bar. The onset temperature, To, at this time is determined as depicted in Figure 4. Here the To value is found to be 6.3 °C. For Figure 3, which shows data from five identical KHI tests run simultaneously, To is in the range 6.5− 6.8 °C. This degree of scattering is typical of the range observed in this multicell rocker rig and is never more than 15−20% (relative error) and reflects the stochastic nature of gas hydrate formation. At some point rapid hydrate formation ensues as detected by a rapid pressure drop in the cells. The temperature at which rapid hydrate formation occurs is called Ta. Ta is determined from when hydrate growth is at its most rapid, i.e. the steepest part of the pressure versus time graph. In Figure 3 this occurs at about 890−920 min in the five cells. In Figure 4, which shows a single experiment, the fastest hydrate formation is at approximately 1030 min giving a Ta value of 3.7 °C. A large difference between To and Ta is an indication of a chemical that can strongly inhibit the growth of gas hydrate crystals. Generally we find that there is less scattering in the Ta values ( 0.15) in the average induction time or rapid hydrate formation times using 5000 ppm VP:VCap1 at 7 °C (Figures 13 and 14). Only one to and ta value was clearly higher

Figure 10. Rapid hydrate formation time (ta) values from isothermal KHI experiments with 5000 ppm VP:VCap1 at varying rocking angles.

atures. We used 2500 ppm or 5000 ppm VP:VCap1. Five parallel tests at 2500 ppm KHI, rocking at 10 rocks per minute, gave an average hydrate onset temperature, To, of 5.6 °C, and rocking at 20 rocks per minute gave an average To value of 6.6 °C (Figure 11). However, the minimum To values (i.e., worst Figure 13. Induction times (to) when rocking at varying rocking rates with 5000 ppm KHI.

Figure 11. To and Ta values when rocking at 10 rocks per minute and 20 rocks per minute, with an inhibitor concentration of 2500 ppm. Figure 14. Rapid hydrate formation times (ta) at varying rocking rates with 5000 ppm KHI.

case results) obtained in the five parallel tests were quite similar at both rocking rates. The rapid hydrate formation temperature, Ta, remained unchanged at 5.8 °C when the rocker rate was reduced. At 5000 ppm the average Ta value at both rocking rates was again the same at 3.6 °C (Figure 12). However, the To values were significantly higher at the faster rocking rate. At 10 rocks/min the average To value was 5.7 °C, whereas at 20 rocks/min the value was 6.8 °C. The lowest To values (worst case results) were also significantly different. This is the opposite result of the series of tests at 2500 ppm. At present

at 20 rocks/min compared to 10 rocks/min, whereas the minimum values (worst case) for both to and ta at both rocking rates are very similar. In conclusion, further tests need to be carried out to determine if the rocking rates used in this study affect KHI performance. However, another group has shown that a very slow rocking rate of 2 rocks per minute does give much longer induction times than 10−20 rocks per minute.26 Effect of Aqueous Volume in the Cell. The steel rocking cells have a total volume of 40 mL. A series of tests was carried out using 10, 20, or 30 mL of aqueous solution in the cells. Both standard constant cooling and isothermal tests at 7 °C were carried out. The constant cooling tests were carried out with 2500 ppm or 5000 ppm VP:VCap1, and the results are given in Figures 15and 16, respectively. For both KHI concentrations there is a clear statistically significant trend to lower To and Ta values as the volume of aqueous solution increases from 10 to 30 mL. The isothermal results were carried out at 5000 ppm only and are summarized in Figures 17 and 18. Again, there is a clear statistically significant trend to longer induction times as the volume of aqueous solution increases. The problem of varying induction time with varying water volume in the rocker cells may be due to at least two factors. The first factor is the varying water-gas interfacial area. If you

Figure 12. To and Ta values when rocking at 10 rocks per minute and 20 rocks per minute, with an inhibitor concentration of 5000 ppm. 2541

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Figure 17. Effect of aqueous volume in the rocking cell on the induction time (to) using 5000 ppm VP:VCap1 in isothermal tests at 7 °C (subcooling = ca. 12.5 °C).

Figure 15. Effect of aqueous volume in the rocking cell for the KHI performance of 2500 ppm VP:VCap1 in five constant cooling tests. NB. All five Ta values at 30 mL are 5.0 °C.

have little water in the cell it will slosh around and cover more metal surface than a fuller cell of water. On average this gives a larger water-gas interfacial area at low water level in a closed vessel. Since hydrate nucleation occurs at the water-gas interface, a larger interfacial area should lead to higher onset temperatures in constant cooling tests or shorter induction times in isothermal tests. Second, the SNG we used contains 1.84 mol % CO2. The amount of CO2 dissolved in the aqueous phase will vary with the amount of water. This will change the thermodynamics of the system as the volume of water changes. This will give higher subcooling at lower water volume and therefore shorter induction times. Dissolution of CO2 may also affect the kinetics of the system. At low CO2 mol % these effects may be negligible, but when CO2 is present in significant amounts it is advisable to check the hydrate equilibrium temperature at the different water cuts, either using suitable software or through laboratory measurements. Another factor that can affect the time to rapid hydrate formation is the change in pressure drop as the volume of aqueous fluid in the cell is varied. For the constant cooling experiments, the pressure drop in the cells (due to gas hydrate formation only) increased with increased aqueous fluid volume as follows:

Figure 18. Effect of aqueous volume in the rocking cell on the rapid hydrate formation time (ta) using 5000 ppm VP:VCap1 in isothermal tests at 7 °C (subcooling = ca. 12.5 °C).

10 mL fluid: 2500 ppm/5000 ppm -3.2/2.5 bar 20 mL fluid: 2500 ppm/5000 ppm -10.3/9.8 bar 30 mL fluid: 2500 ppm/5000 ppm -36.5/26.3 bar If the rate of consumption of the hydrate-forming gases is not the same during the early stage of hydrate formation, then the composition of the remaining SNG will change. This will cause a greater than additional change in the system equilibrium

Figure 16. Effect of aqueous volume in the rocking cell for the KHI performance of 5000 ppm VP:VCap1 in five constant cooling tests. 2542

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temperature and thus the subcooling, beyond that, due to the pressure drop. The Effect of Using Glass versus Steel Balls. In all experiments in other sections of this work we have used a single steel ball in each rocking cell. A set of glass balls of similar diameter and with a matt, nonshiny surface was also available for testing. We used these in identical experiments as with the steel balls to ascertain if using them would make any difference to the performance of the VP:VCap1 KHI (Figure 3). Both constant cooling and isothermal tests were carried out. Results from the constant cooling tests using 2500 ppm or 5000 ppm KHI are shown in Figures 19 and 20. In these figures the x-axis

Figure 21. Induction time, to, in isothermal tests at 7 °C using both balls types and 5000 ppm VP:VCap1.

Figure 19. The relationship between To values and different rocking balls, using VP:VCap1 in constant cooling tests.

Figure 22. Time to rapid hydrate formation, ta, in isothermal tests at 7 °C using both balls types and 5000 ppm VP:VCap1.

formation. Although three of the cells show longer induction times with steel balls than glass balls, the result is opposite for cell 1. Therefore, as with the constant cooling tests, there appears to be no clear difference between the two types of balls regarding KHI performance, and further tests would be needed to make a more solid conclusion. Due to the inconclusive results discussed above, constant cooling tests were also carried out with three other polymers, PVCap1, PVCap2, and VP:VCap2. The average To values and Ta values using glass and steel balls for constant cooling tests are listed in Table 2. Results for VP:VCap1, taken from Figures 19 and 20, are also included. No hydrates were formed using PVCap1 at 5000 ppm down to the minimum test temperature of 2 °C indicating that this is the most powerful Structure II KHI tested in this study. With a broader range of KHIs tested, there now appears to be a clearer trend in which the glass balls generally gives lower To values than the steel balls, the only exception being VP:VCap at 2500 ppm where there is no statistically significant difference between the 5 tests with each type of ball (P > 0.05). However, the Ta values for both sets of balls are much closer indicating that the type of ball does not make a significant difference to the time or temperature when rapid hydrate formation occurs. This can be rationalized as effect of the number of super critical nuclei available during the test. For any particular KHI, if the hydrate onset temperature

Figure 20. The relationship between Ta values and different rocking balls, using VP:VCap1 in constant cooling tests.

represents the five parallel cells in the equipment. Only cells 1− 4 gave results at 2500 ppm with steel balls due to a leak in cell 5. At 5000 ppm, all five cells with glass balls gave lower To values than with steel balls. However, at 2500 ppm there is no clear difference between the two sets of balls. Interestingly, both sets of balls gave similar Ta values at both concentrations, 2500 ppm and 5000 ppm. Nonetheless, taking into consideration both concentrations tested, it is impossible to say there is significant difference in the To or Ta temperatures between the steel and glass balls and no one cell that gives consistently better results than another. Figures 21 and 22 show results using the isothermal test method at 7 °C with 5000 ppm the VP:VCap1. Cell 4 gave a significantly longer induction time than the other cells using the steel ball, underlining the stochastic nature of hydrate 2543

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Table 2. To and Ta Values for Various Polymers at 2500 ppm and 5000 ppm Tested with Steel Balls and Glass balls chemical VP:VCap1 PVCap2 PVCap1 VP:VCap2

concn [ppm] 2500 5000 2500 5000 2500 5000 2500 5000

average values: To steel/To glass [°C]/[°C]

average values: Ta steel/Ta glass [°C]/ [°C]

7.1/7.4 5.8/5.8 6.6/5.7 3.5/3.4 8.7/8.7 8.1/8.0 6.5/5.8 6.3/5.5 7.1/5.0 2.4/2.0 no gas hydrate formation 8.4/6.9 4.9/4.6 7.1/5.0 2.4/2.0

Figure 23. Effect of the cooling rate on To (average) for constant cooling tests with VP:VCap1.

(To) is high, few hydrate nuclei will have reached the critical nuclear size. Therefore it will take a relatively long time (large Ta-To) for many hydrate crystals to form and rapid hydrate growth to occur. At a low onset temperature (To), many more nuclei will have grown to the critical size. In this case, the stage in which macroscopic hydrate crystals slowly grow, before rapid growth ensues, is short (short Ta-To). Another group has shown that glass balls can give shorter induction times in isothermal tests than steel balls. This may be due to using rougher glass balls than those in our own tests, which would increase the number of good heteronucleation sites for hydrate formation.26 Therefore, we conclude that the roughness of the steel or glass balls is probably an important factor regarding the performance of KHIs, just as much as the material that the balls are made of. Rust and other impurities on the steel balls could also play a role as we have seen rust particles accelerate hydrate formation in other systems.27 We recommend keeping to the same type of balls during any comparative KHI performance study, but we do not think it is important to keep the same ball in the same cell each time. The Effect of Cooling Rate on Constant Cooling Experiments. Standard constant cooling experiments require cooling from 20.5 to 2 °C over 18.5 h at a constant rate of 1.0 °C/h. In these experiments the cooling rate from 20.5 to 2 °C was varied to a higher cooling rate (3.0 °C/h) or lower cooling rate (0.33 °C/h). Thus, fast cooling tests take 6 h 10 min to reach 2 °C, while slow cooling tests take 55 h 30 min to reach 2 °C. Experiments with VP:VCap1 at 2500 ppm or 5000 ppm were compared to tests at the standard cooling rate. The results are summarized in Table 3 and shown graphically in Figures 23 and 24.

Figure 24. Effect of the cooling rate on Ta (average) for constant cooling tests with VP:VCap1.

KHI performance. This has been done in autoclaves, loops, wheels, and rocking cells. We have also carried out a small study of this effect using 5 parallel tests with VP:VCap1 in the rocking cells. Results using the constant cooling method are given in Figures 25and 26, and results using the isothermal method at 7 °C are given in Figures 27 and 28. Pressuring before cooling was used as the standard method in the isothermal experiments. Both the constant cooling and isothermal methods show a typical trend of increasing performance as the KHI concentration increases. In the constant cooling tests even 1000 ppm of KHI gave an average To value of 10.3 °C and Ta value of 9.6 °C in the five cells. With no KHI, just deionized water, these values were 17.3 and 17.0 °C, respectively. It should be noted that in the first few weeks after delivery of the equipment, onset temperatures were up to 6% higher in constant cooling tests. This is due to impurities and the roughness of the new equipment. Another factor that can affect reliability is traces of chemicals or surface conditioning that might affect the next KHI test. For example, some corrosion inhibitors are known to affect KHI performance and by design these can be difficult to remove completely from the cell walls.28−30 Therefore, thorough and consistent cleaning of the inside of the rocking cells is also important to avoid this problem. In the isothermal tests there was almost instantaneous nucleation and fast growth in all five tests using 1000 ppm of KHI, but as the KHI concentration increased to 5000 ppm we obtained typical long induction times.

Table 3. Effect of Cooling Rate on To and Ta Values in Constant Cooling Experiments cooling rate

To/Ta at 2500 ppm

To/Ta at 5000 ppm

3.0 °C/h 01.0 °C/h 0.33 °C/h

6.0/4.9 7.8/5.8 8.6/7.1

4.3/2.6 6.6/3.8 7.5/5.5

The results show a clear trend to higher To and Ta values as the cooling rate is decreased. This is easily rationalized as a straightforward kinetic effect of the KHI. As the cooling rate increases the time to form super critical hydrate nuclei decreases, and so a lower onset temperature is reached than using a faster cooling rate. Effect of KHI Concentration on the KHI Performance. Several studies have previously been conducted by various research groups regarding the effect of KHI concentration on 2544

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Figure 25. Constant cooling tests showing To values at different KHI concentrations.

Figure 26. Constant cooling tests showing Ta values at different KHI concentrations.

Figure 28. Isothermal tests showing ta values at different KHI concentrations.

Figure 27. Isothermal tests showing t o values at different concentrations.

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Brand new equipment can give worse KHI results (shorter induction times or higher onset temperatures) for up to a few weeks, probably because the surfaces inside the cell are less smooth to begin with. Although the initial rough state of the cell surfaces is closer to real field conditions, the smoother cells can still be used for screening and ranking the performance of KHIs but not for determining the ultimate field performance. The worst result in a large series of identical tests is probably closest to the true field performance, as has been pointed out

CONCLUSION We have investigated varying several parameters for testing the performance of KHIs in rocking cells. In most tests we used a VP:VCap copolymer (VP:VCap1) and varied one parameter at a time. These parameters are the rocking angle, rocking speed, volume of aqueous fluid in the cell, type and roughness of rocking ball, cooling rate, and KHI concentration. 2545

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previously.14 However, to screen and rank KHIs in the laboratory a sufficiently large series of tests is normally performed to achieve a statistically significant result. The type of test method is also important to differentiate performances. Thorough and consistent cleaning of the inside of the rocking cells is also important to avoid traces of chemicals or surface conditioning that might affect the next KHI test. In general, we obtained smaller deviations in To values from constant cooling experiments over 18.5 h (average deviation ca. 15−20%) than to values from isothermal tests with induction times of a minimum of several hours (average deviation ca. 20− 25%). For this reason we normally choose to use the constant cooling method to rank different KHIs. A set of five cells tests can be conveniently run in a 24 h period this way in our equipment. The rocking angle did not seem to impact the KHI performance results, as no statistically significant difference was observed. More tests could be run at lower rocking angles to check the validity of this conclusion. The impact of the rocking rate, in the rate interval used (10 or 20 rocks/min), is not clear, and more experiments should be performed to get a better answer on how the rocking rate affects the inhibition of gas hydrates. However, another group has shown that a very slow rocking rate of 2 rocks per minute does give much longer induction times than 10−20 rocks per minute.26 The KHI performance was shown to significantly improve when the aqueous liquid volume in the cell was increased. This was validated using both the constant cooling method and the isothermal method. The reason for this effect is related to the relative amounts of dissolved gas, particularly that of CO2, as well as the change in gas−water interfacial area. Therefore, when comparing KHIs, it is important to fill the cells with the aqueous fluid to the same amount. The type of ball used for the experiments does affect the KHI performance results but was not always clear in every experiment conducted. In general the matt, smooth glass balls we used gave better KHI performance than using steel balls. However, another group has shown that their glass balls give shorter induction times in isothermal tests than steel balls. This may be due to using rougher glass balls than those in our own tests, which would increase the number of good heteronucleation sites for hydrate formation.26 The roughness of the steel or glass balls is probably an important factor regarding the performance of KHIs, just as much as the material that the balls are made of. The cooling rate in constant cooling experiments was shown to have a statistically significant impact on the KHI performance. This is a straightforward kinetic effect of the KHI. As the cooling rate increases the time to form super critical hydrate nuclei decreases and so a lower onset temperature is reached than using a faster cooling rate. As shown by several previous studies there is a clear effect of KHI concentration on KHI performance. This was demonstrated using concentrations of VP:VCap1 between 1000 ppm and 10000 ppm in both constant cooling and isothermal experiments. Both methods show a typical trend of increasing performance as the KHI concentration increases. Above about 10000 ppm (1.0 wt.%) there is only a small gain in KHI performance. Therefore, injection of KHI above this concentration is usually prohibitively expensive for the small increase in performance obtained.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +47 51831823. Fax: +47 51831750. E-mail: malcolm. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Pei Cheng Chua for help with the rocker rig equipment. REFERENCES

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