Article pubs.acs.org/EF
Poly(N-vinyl azacyclooctanone): A More Powerful Structure II Kinetic Hydrate Inhibitor than Poly(N-vinyl caprolactam) Pei Cheng Chua and Malcolm A. Kelland* Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: Poly(N-vinyl azacyclooctanone) (PVACO) has been synthesized for the first time. Dependent upon the method of polymerization and polymer molecular weight, the cloud point of a 1.0 wt % solution in water can be varied between approximately 14 and 22 °C. Using identical polymerization conditions for the four N-vinyl lactams with 5−8-membered rings, the polymer molecular weight decreases as the ring size increases. This is probably due to the relative steric effect of the monomers in the polymerization process. In high-pressure rocking cell experiments with a structure-II-forming hydrocarbon gas mixture, PVACO was shown to be a more powerful kinetic hydrate inhibitor (KHI) than the other 5−7-ring poly(N-vinyl lactam)s of similar molecular weight made using an otherwise identical method to PVACO. The synergistic effect of mono-nbutyl glycol ether with PVACO and the effect of the polymer molecular weight on KHI performance are also discussed.
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INTRODUCTION Kinetic hydrate inhibitors (KHIs) are now a well-known technology for preventing gas hydrate plugs in upstream oilfield operations.1−4 KHIs are water-soluble polymers, often with added synergists that improve their performance. KHIs delay the nucleation and usually also the crystal growth of gas hydrates. The nucleation delay time (induction time), which is the most critical factor for field operations, is dependent upon the subcooling (ΔT) in the system: the higher the subcooling, the lower the induction time. The absolute pressure is also an important factor.5−8 Probably the commonest class of polymers used in commercial KHI formulations are homo-polymers and copolymers of the N-vinyl lactams N-vinyl pyrrolidone (VP) and N-vinyl caprolactam (VCap).9−16 Recently, we showed that the homo-polymer of the 6-ring N-vinyl lactam monomer N-vinyl piperidone (VPip) had an intermediate KHI gas hydrate performance between that of poly(N-vinyl pyrrolidone) and poly(N-vinyl caprolactam).17,18 Thus, the KHI performance increases with an increasing lactam ring size. The structures of the homo-polymers of VP, VPip, and VCap are given in Figure 1. It was therefore of great interest to investigate whether the homo-polymer of an even larger ring N-vinyl lactam would perform better than homo-polymers with the smaller lactam rings. The 8-ring N-vinyl lactam monomer N-vinyl azacyclooctanone (VACO) has not been reported previously nor have any polymers from this monomer. We were also uncertain if the homo-polymer poly(N-vinyl azacyclooctanone) (PVACO) was even water-soluble and, therefore, could be tested as a KHI, because the cloud points of the poly(N-vinyl lactam)s decrease with lactam ring size. For example, PVCap as a 1.0 wt % solution in water has a cloud point of about 30−40 °C depending upon the polymerization method and molecular weight, whereas cloud points for PVPip are generally in the range of 60−80 °C.17,18 In this paper, we report the synthesis and structure II (SII) gas hydrate KHI performance of PVACO for the first time and © 2012 American Chemical Society
Figure 1. Structures of poly(N-vinyl pyrrolidone) (PVP) (top left), poly(N-vinyl piperidone) (PVPip) (top right), poly(N-vinyl caprolactam) (PVCap) (bottom left), and poly(N-vinyl azacyclooctanone) (PVACO) (bottom right).
compare the results to other poly(N-vinyl lactam) homopolymers. The effect of varying the polymerizing solvent, including the synergistic effect of mono-n-butyl glycol ether, and the polymer molecular weight is also reported.
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EXPERIMENTAL SECTION
Polymer Synthesis. The VACO monomer was generously made by BASF, Germany, by vinylation of azacyclooctanone with ethyne under high pressure. VPip was also supplied by BASF, as previously reported.17,18 All other chemicals were obtained from commercial sources. Homo-polymers of the four N-vinyl lactams were synthesized using the following general procedure: The monomer was mixed with Received: April 24, 2012 Revised: June 14, 2012 Published: June 18, 2012 4481
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The constant cooling test procedure was as follows: (1) Each cell was filled with 20 mL of distilled water in which the polymer was dissolved. To avoid precipitation of polymer above the cloud point, solutions were made up at 16 °C and kept at this temperature when added to the rocking cells. (2) Air in the cells was removed with a combination of vacuum pumping and filling with synthetic natural gas (SNG) to 2 bar and then repeating the procedure. (3) The cell was pressurized to 76 bar (except for 74 bar for PVACO; see point 4 below) and rocked at 20 rocks per minute at an angle of 40°. (4) For experiments with PVP, PVPip, and PVCap, the cells were cooled from 20.5 °C at a rate of 1 °C/h to 2 °C over 18.5 h. To avoid polymer precipitation problems, the PVACO was cooled from 17 °C and 74 bar but at the same rate of 1 °C/h to 2 °C. If rapid hydrate formation had not occurred during this time, as judged by a large fast pressure drop, the temperature was held at 2 °C until it had occurred. (5) The pressure and temperature for each individual cell, as well as the cooling bath, was logged on a computer.
1−20 wt % of the initiator 2,2′-azobis(2-methylpropionitrile) (AIBN) and 4 times its weight of 2-propanol in a Schlenk tube. The resulting solution was degassed on a high vacuum line and sealed under nitrogen. The reaction mixture was then stirred and allowed to polymerize at 80 °C for typically 16 h. Solvents were removed under reduced pressure to leave a white solid. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy indicated greater than 99% monomer conversion. Different homo-polymer molecular weights were synthesized using different monomer/AIBN ratios, as determined by gel permeation chromatography (GPC). The cloud points (Tcl) of the polymers as 1.0 wt % solutions in fresh water were determined.17 The 1 H and 13C(1H) NMR spectroscopic data recorded on a Bruker 500 MHz machine are given here for the VACO monomer and polyVACO. VACO Monomer. 1H NMR (CDCl3) δ: 1.39 (2H, m), 1.50 (2H, m), 1.64 (2H, m), 1.78 (2H, m), 2.53 (2H, virt t), 3.70 (2H, t), 4.38 (2H, dd), 7.36 (1H, dd). 13C(1H) NMR (CDCl3) δ: 24.2, 27.7, 29.2, 34.7, 43.0, 130.9, 173.9. PolyVACO. 1H NMR (DMSO-d6) δ: 1.2−1.7 (broad), 2.2−2.6 (broad), 3.2−3.5 (broad), 4.1−4.6 (broad). High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition experiments were conducted in five high-pressure 40 mL steel rocking cells each containing a steel ball (Figure 2). The equipment was supplied by PSL Systemtechnikk, Germany. The gas composition used was a synthetic natural gas mixture given in Table 1.
Figure 3. Pressure and temperature data obtained from constant cooling KHI tests in the multi-cell rocker rig. Cells 1−3 contain the same KHI, and cells 4 and 5 contain a different KHI. A typical graph of pressure and temperature data versus time from all five cells is shown in Figure 3. Three cells represented in this graph were identical and contained one KHI, and two other cells contained a second KHI. At the start, the pressure drops about 2 bar because of gas being dissolved in the aqueous phase. The temperature drops at a constant rate (the minimum temperature reached of 2 °C is not shown on this graph because all cells had plugged with hydrates at higher temperatures). During the experimental time, the pressure also drops at a constant rate, because it is a closed system, until the rate of pressure drop increases as a result of hydrate formation. The first deviation from the pressure drop as a result of the temperature drop is taken as the time for onset of hydrate formation, although nucleation on an undetectable scale may have occurred earlier. The onset temperature, To, at this time is determined. In Figure 3, which shows data from two different KHI tests, To is in the range of 12.3−12.8 °C for the three cells with the same KHI. This degree of scattering is typical of the range observed in this multi-cell rocker rig, is never more than 15−20%, and reflects the stochastic nature of gas hydrate formation. For the other two cells with a different KHI, the To values are 9.9 and 10.0 °C. A more thorough investigation of the reproducibility at various test conditions in this multi-cell rocker rig has been carried out.21 At some point, rapid hydrate formation ensues, as detected by a rapid pressure drop in the cells. In Figure 4, which shows a single experiment, this occurs after about 680 min. The temperature at which rapid hydrate formation occurs, Ta, is determined for each cell. Ta is determined from when hydrate growth is at its most rapid, i.e., the steepest part of the pressure versus time graph. Generally, we find that there is less scattering in the Ta values (100 78.0 33.0 16.0
average onset temperature, To (°C)
average rapid hydrate formation temperature, Ta (°C)
17.9
17.5
11.3 10.5 10.1 9.7
10.5 9.2 9.6 9.4
with no additive are given for comparison. Cloud points (Tcl) of 1 wt % solutions in fresh water are also given. Figure 6 shows results for To values only for all of the experiments with these polymers.
Figure 5. Average molecular weights of poly(N-vinyl lactam)s made by identical methods.
polymerizing 20 wt % solutions of the N-vinyl lactams in 2propanol at 80 °C using 1% AIBN initiator for 18 h under a nitrogen atmosphere. Using identical polymerization conditions, the polymer molecular weight decreases as the ring size increases, as shown by GPC analysis. This trend is probably due to the relative steric effect of the monomers in the polymerization process. The cloud point (Tcl) of the PVACO sample from Figure 5 was approximately 16 °C as a 1.0 wt % solution in fresh water. We found that Tcl of PVACO could vary between about 15 and 24 °C depending upon the polymerization procedure, including varying the temperature, amount of initiator, solvent, and concentration of monomer in the solvent. Further details of this synthesis work are outside the scope of this paper and will be reported separately. It is well-known that polymer molecular weight affects the performance as a KHI.1−4 Low molecular weights in the order of 1500−2000 have been found to be best for a number of
Figure 6. Hydrate onset temperature values (To) from constant cooling results for 0.25 wt % poly(N-vinyl lactam)s of similar low molecular weight.
The results indicate that all of the polymers perform significantly better than no additive. There is also a clear and statistically significant trend showing an increase in KHI performance as the ring size increases. From t tests on the four polymers, we obtained p values of less than the critical value of 0.05 between each of the sets of 10 To values for the four poly(N-vinyl lactam)s, except between PVCap and PVPip. The p value between these two polymers was 0.061, which is very close to the 95% statistically significant level. We have recently published work on the relative KHI performance of other 4483
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Table 3. Average Results from Five Rocking Cell Tests with Various PVACOs polymer + solvent PVACOa PVACOa PVACO + PVACO + PVACO + PVACO + PVACO + PVACO + a
BGE BGE BGE BGE BGE BGE
polymer Mw/Mn
concentration (wt %)
average onset temperature, To (°C)
average rapid hydrate formation temperature, Ta (°C)
4700/1700 2700/1100 3500/1600 2600/1600 2600/1600 1700/1100 1700/1100 1120/720
0.25 0.25 0.25 + 1.0 0.25 + 1.0 0.50 + 2.0 0.25 + 1.0 0.5 + 2.0 0.25 + 1.0
9.8 6.6 3.9 2.7 2.3 2.0 4.0 4.8
9.4 5.8 3.5 2.3 >200 h in five testsb 1−3 h at 2 °C in five tests >53 h at 2 °C in five tests and >200 h in five testsb 4.1
Made in 2-propanol, with the solvent removed. bMaximum pressure drop of 1−4 bar because of hydrate formation.
Results of rocking cell tests at 0.25 and 0.50 wt % with this polymer are given in Table 3 alongside other results, which are also discussed below. For a 20 wt % polymer solution in BGE, tests with 0.25 wt % polymer will give a BGE test concentration of 1.0 wt %. Likewise, for 0.5 wt % polymer, the concentration of BGE is 2.0 wt %. With 0.25 wt % PVACO with Mw/Mn 2500/1600, we obtained significantly lower average To values than PVACO used alone (but made in 2-propanol solvent), even though the PVACO made in BGE has a slightly higher molecular weight. This shows the strong synergism of BGE with PVACO. At a BGE concentration of 1.0 wt %, the effect on the equilibrium temperature for hydrate formation is insignificant for the comparison of the KHI effect of these mixtures. The final parameter that we investigated was lowering the polymer molecular weight even further using larger amounts of AIBN initiator (up to 10%) in BGE solvent and a higher polymerization temperature. In this way, we hoped to obtain oligomers with molecular weights below the optimum performance value. Chain-transfer agents (CTAs) can also be used, and our work with CTAs will be reported separately. A lower molecular weight PVACO (Mw/Mn = 2600/1100) made in BGE gave average To and Ta values of 2.7 and 2.3 °C when tested at 0.25 wt % polymer with 1.0 wt % BGE. When the experimental concentration (polymer and BGE) was doubled, the To value dropped to 2.3 °C and rapid hydrate formation did not occur for over 200 h at 2.0 °C. This is a subcooling of about 18.2 °C at 68 bar. The very slow pressure drop observed over this time as a result of hydrate formation was about 2−3 bar. With our test method, a typical pressure drop, including the more rapid drop for the rapid hydrate formation phase, is usually about 8 bar. Using 10 wt % AIBN in BGE, we were able to obtain a PVACO with Mw/Mn of 1700/ 1100. This is the most powerful KHI reported in this study. With 0.25 wt % of this polymer and 1.0 wt % BGE, the pressure drop as a result of gas hydrate formation was not observed until the minimum temperature of 2.0 °C had been reached in all five tests. A rapid pressure drop occurred 1−3 h later at this temperature. Interestingly, using double the concentration, we obtained an average To value from 10 experiments of 4.0 °C. However, fast hydrate formation did not occur after 53 h in five experiments and after 200 h in another five experiments. Pressure drops of 1−4 bar were obtained. We are not sure of the reason why the To values are higher for the higher concentration of polymer and BGE mixture, although we speculate that it may be due to an increase in polymer−polymer interactions in the bulk water phase. However, these PVACO and BGE mixtures have a very strong effect on the hydrate crystal growth process, as demonstrated by the very slow
samples of PVPip and PVCap and shown that PVCap is, in general, a better KHI than PVPip on SII gas hydrates.18 The observed trends in the To values for the four polymers are the best indication of the change in nucleation temperature for hydrate formation between the polymers under the test conditions. In contrast, there was no clear trend for the Ta values for the four polymers, except that the average Ta value for PVP was significantly higher than for the other three polymers. The difference between the To and Ta values also did not show a trend with increasing ring size. Therefore, for these pure homo-polymers of similar Mw values, we cannot draw any conclusion regarding their relative ability to inhibit SII hydrate crystal growth. Besides the lowering of the cloud point, another possible reason for the improved KHI performance with increasing ring size in the series PVP, PVPip, PVCap, and PVACO may be related to polymer tacticity. Thus, as the steric bulk of the Nvinyl lactam increases, the tendency should be for a more syndiotactic structure when polymerized. Syndiotactic structures maximize the surface/volume ratio of the polymer, which should lead to higher water perturbation than other polymer tacticities and, therefore, higher KHI nucleation inhibition. This has been demonstrated for poly(isopropylmethacrylamide)s, where it was possible to evaluate the polymer tactcity by NMR spectroscopy.23 We are currently investigating methods to determine the tacticity of poly(N-vinyl lactam)s. If the percentage of AIBN initiator used for the polymerization of VACO monomer was increased, the PVACO molecular weight decreased accordingly, as shown by GPC analysis. For example, using 4% AIBN and 2-propanol solvent, we obtained PVACO with Mw/Mn = 2700/1100. This polymer performed significantly better than the higher molecular weight PVACO discussed earlier. Both results are given in Table 3 for comparison. Using 0.25 wt % polymer, the average To dropped from 9.8 to 6.6 °C and the average Ta value dropped from 9.4 to 5.8 °C. These results underline the importance of molecular weight for optimum performance of KHIs. The incorporation of a higher percentage of isobutyronitrile end groups in the polymer when using increasing amounts of AIBN initiator may also be a factor in the increased KHI performance. Mono-n-butyl glycol ether (BGE) has been reported to be a good synergist solvent for PVCap and has been used in commercial KHI formulations.1,4,24,25 Therefore, we were interested to investigate the ability of BGE as a solvent synergist for PVACO. If the polymerization of VACO monomer is conducted in an identical fashion as for polymers in Table 2 at 80 °C with 1% AIBN but as a 20 wt % solution in BGE solvent instead of 2-propanol, we obtained PVACO with Mw/Mn = 3500/1600. Thus, using BGE as the solvent instead of 2-propanol gave a slightly lower polymer molecular weight. 4484
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pressure drop after the first sign of pressure drop as a result of hydrate formation. By increasing the polymerization temperature from 80 to 150 °C, we were able to obtain oligomeric PVACO in BGE with Mw/Mn of 1120/720. This mixture when tested with 0.25 wt % polymer gave an average To value of 4.8 °C and an average Ta value of 4.1 °C. These values are lower than for the PVACOs with Mw/Mn values of 3500/1600, 2700/1600, and 1700/1100. We conclude that this oligomer is below the optimum PVACO molecular weight for the best KHI performance.
(5) Kelland, M. A.; Svartaas, T. M.; Dybvik, L. Ann. N. Y. Acad. Sci. 2000, 912, 744−752. (6) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Sci. 2005, 60, 1313−1321. (7) Peytavy, J.-L.; Glénat, P.; Bourg, P. Kinetic hydrate inhibitors Sensitivity towards pressure and corrosion inhibitors. Proceedings of the International Petroleum Technology Conference (IPTC); Dubai, United Arab Emirates, Dec 4−6, 2007; IPTC 11233. (8) 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. Proceedings of the 6th International Conference on Gas Hydrates; Vancouver, British Columbia, Canada, July 6−10, 2008. (9) Sloan, E. D. U.S. Patent 5,420,370, 1995. (10) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R. L.; Sloan, E. D. Kinetic inhibitors of natural gas hydrates. Proceedings of the 73rd Annual Gas Processors Association (GPA) Convention; New Orleans, LA, March 7−9, 1994. (11) Sloan, E. D. U.S. Patent 5,432,292, 1995. (12) Sloan, E. D.; Christiansen, R. L.; Lederhos, J.; Panchalingam, V.; Du, Y.; Sum, A. K. W.; Ping, J. U.S. Patent 5,639,925, 1997. (13) Argo, C. B.; Blaine, R. A.; Osborne, C. G.; Priestly, I. C. Commercial deployment of low dosage hydrate inhibitors in a southern North Sea 69 kilometer wet-gas subsea pipeline. Proceedings of the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry; Houston, TX, Feb 18−21, 1997; SPE 37255. (14) Fu, S. B.; Cenegy, L. M.; Neff, C. A summary of successful field applications of a kinetic hydrate inhibitor. Proceedings of the Society of Petroleum Engineers (SPE) International Symposium on Oilfield Chemistry; Houston, TX, Feb 13−16, 2001; SPE 65022. (15) Bakeev, K.; Myers, R.; Chuang, J.-C.; Winkler, T.; Krauss, A. U.S. Patent 6,242,518, 2001. (16) Angel, M.; Stein, S.; Neubecker, K. International Patent Application WO/2001/066602, 2001. (17) Ieong, N. S.; Redhead, M.; Bosquillon, C.; Alexander, C.; Kelland, M. A.; O’Reilly, R. K. Macromolecules 2011, 44, 886. (18) Chua, P. C.; O’Reilly, R.; Ieong, N. S.; Kelland, M. A. Energy Fuels 2011, 25, 4595. (19) Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722−734. (20) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N. Y. Acad. Sci. 2000, 912, 924−931. (21) Lone, A.; Kelland, M. A. Manuscript in preparation. (22) Anselme, M. J.; Reijnhout, M. J.; Klomp, U. C. International Patent Application WO93/25798, 1993. (23) Chua, P. C.; Kelland, M. A.; Hirano, T.; Kamigaito, M. Kinetic hydrate inhibition of poly(N-alkyl(meth)acrylamide)s with different tacticities. Proceedings of the International Conference on Gas Hydrates (ICGH7); Edinburgh, U.K., July 17−21, 2011. (24) Fu, B. The development of advanced kinetic hydrate inhibitors. Proceedings of the Chemistry in the Oil Industry VII; Manchester, U.K., Nov 13−14, 2002; p 264. (25) Cohen, J. M.; Wolf, P. F.; Young, W. D. U.S. Patent Application 5,723,524, 1998.
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CONCLUSION PVACO has been synthesized for the first time. Dependent upon the method of polymerization and polymer molecular weight, the cloud point of the homo-polymer as a 1.0 wt % solution in water can be varied between approximately 14 and 22 °C. Using identical polymerization conditions for the four N-vinyl lactams with 5−8-membered rings, the polymer molecular weight decreases as the ring size increases. This is probably due to the relative steric effect of the monomers in the polymerization process. In high-pressure rocking cell experiments with a SII-forming hydrocarbon gas mixture, PVACO was shown to be a more powerful kinetic hydrate inhibitor than the other 5−7-ring poly(N-vinyl lactam)s of similar molecular weight made in an otherwise identical method to PVACO. BGE has a strong synergistic effect with PVACO. By optimizing the polymerization procedure and the polymer molecular weight, we were able to optimize powerful synergistic blends of PVACO and BGE that could prevent rapid hydrate plugging at about 18.2 °C subcooling and 68 bar in non-saline water for over 200 h. We are continuing to explore the structure I (SI) and SII gas hydrate KHI performance of PVACO and VACO co-polymers made under a variety of polymerization conditions, as well as the effect of PVACO on SII tetrahydrofuran hydrate crystal growth.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +47-51831823. Fax: +47-51831750. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Jorma Kinnari (EWOS Innovation, Dirdal, Norway) for recording the NMR spectra and Nga Sze Ieong (University of Warwick, U.K.) for carrying out GPC analysis on the polymers. We thank Chittawan Nakarit for help with some of the PVP KHI experiments.
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REFERENCES
(1) Kelland, M. A. Energy Fuels 2006, 20, 825. (2) Kelland, M. A. Production Chemicals for the Oil and Gas Industry; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2009; Chapter 9. (3) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press (Taylor and Francis Group): Boca Raton, FL, 2008. (4) Kelland, M. A. A review of kinetic hydrate inhibitorsTailormade water-soluble polymers for oil and gas industry applications. In Advances in Materials Science Research; Wytherst, M. C., Ed.; Nova Science Publishers, Inc.: New York, 2011; Vol. 8, Chapter 5. 4485
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