Study of the Kinetic Hydrate Inhibitor Performance of a Series of Poly

Nov 18, 2010 - University of Stavanger, 4036 Stavanger, Norway. Received August 18, 2010. Revised Manuscript Received October 30, 2010. Kinetic hydrat...
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Energy Fuels 2010, 24, 6400–6410 Published on Web 11/18/2010

: DOI:10.1021/ef101107r

Study of the Kinetic Hydrate Inhibitor Performance of a Series of Poly(N-alkyl-N-vinylacetamide)s Hiroharu Ajiro,† Yukie Takemoto,† Mitsuru Akashi,† Pei Cheng Chua,‡ and Malcolm A. Kelland*,‡ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan, and ‡Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway Received August 18, 2010. Revised Manuscript Received October 30, 2010

Kinetic hydrate inhibitors (KHIs) have been used successfully for about the last 16 years to prevent gas hydrate formation mostly in gas and oilfield production lines. They work by delaying the nucleation and often also the growth of gas hydrate crystals for periods of time dependent mostly on the subcooling in the system. Poly(N-alkyl-N-vinylamide)s have been briefly investigated previously but no work has previously been published detailing a systematic study of structure versus performance. In this paper we report the KHI performance of a series of poly(N-alkyl-N-vinylacetamide)s polymers with alkyl side groups up to five carbon atoms. The study includes hydrate crystal growth tests on structure II tetrahydrofuran hydrate crystals as well as high pressure nucleation and crystal growth studies on a synthetic natural gas mixture giving structure II hydrates. The SII gas hydrate inhibition results correlate well with the THF hydrate crystal growth inhibition results since the polymers with the larger isopentyl or isobutyl groups performed best in both equipments but not significantly better than a commercial low molecular weight Nvinyl caprolactam-N-vinyl pyrrolidone 1:1 copolymer.

the Middle East.8-14 Polymers of N-isopropylmethacrylamide have recently been commercialized, and a polyester pyroglutamate KHI is also commercially available, which although it has good biodegradability, has limited performance.15-17 Other biodegradable KHIs have recently been developed.18-22 All these classes of KHI polymers contain amide groups which are particularly useful for hydrogen bonding from both the amino nitrogen atom and the carbonyl oxygen.23 Poly(N-alkyl-N-vinyl amides) have been patented as KHIs (see Figure 1).24 However, only one polymer in this class, poly(N-methyl-N-vinyl acetamide) (PolyVIMA) was investigated and was shown to have poor performance. Besides the

Introduction Kinetic hydrate inhibitors (KHIs) are a class of low dosage hydrate inhibitor (LDHI) that have been in commercial use in the oil and gas industry for about 14 years.1-3 KHIs are watersoluble 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 on the subcooling (ΔT ) in the system, the higher the subcooling the lower the induction time. The absolute pressure is also an important factor.4-7 There are currently only two main classes of polymers that are used in KHI formulations in oil and gas field operations, which are (a) homopolymers and copolymers of N-vinyl caprolactam and (b) hyperbranched poly(ester amide)s. These polymers are being successfully used to prevent gas hydrate formation in pipelines in locations such as the U.K. sector of the North Sea, the Gulf of Mexico, South America, and

(8) Argo, C. B.; Blaine, R. A.; Osborne, C. G.; Priestly, I. C. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 1997; SPE 37255. (9) Talley, L. D.; Mitchell, G. F. In Proceedings of the 30th Annual Offshore Technology Conference, Houston TX, May 3-6, 1998; OTC 11036. (10) Fu, S. B.; Cenegy, L. M.; Neff, C. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 13-16, 2001; SPE 65022. (11) Phillips, N. J.; Grainger, M. In Proceedings of the Annual Gas Technology Symposium, Calgary, Alberta, Canada, March 15-18, 1998; SPE 40030. (12) Leporcher, E. M.; Fourest, J. P.; Labes-Carrier, C.; Lompre, M. In Proceedings of the SPE European Petroleum Conference, The Hague, The Netherlands, October 20-22, 1998; SPE 50683. (13) MacDonald A. W. R.; Petrie M.; Wylde J. J.; Chalmers, A. J.; Arjmandi, M. In Proceedings of the SPE Gas Technology Symposium, Calgary, Alberta, Canada, May 15-17, 2006; SPE 99388. (14) Glenat, P.; Peytavy, J. L.; Holland-Jones, N.; Grainger, M. In Proceedings of the 11th Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, U.A.E., October 10-13, 2004; SPE 88751. (15) Leinweber, D.; Feustel, M. World Patent Application WO/2006/ 084613, 2006. (16) Leinweber, D.; Feustel, M. World Patent Application WO/2007/ 054226, 2007. (17) Arjmandi, M.; Leinweber, D.; Allan, K. Development of A New Class of Green Kinetic Hydrate Inhibitors. 19th International Oilfield Chemical Symposium, Geilo, Norway, March 2008.

*To whom correspondence should be addressed. Telephone: þ47 51831823. Fax: þ47 51831750. E-mail: [email protected]. (1) Kelland, M. A. Energy Fuels 2006, 20, 825. (2) Kelland, M. A. Production Chemicals for the Oil and Gas Industry; CRC Press, Taylor & Francis, Boca Raton, FL, 2009; Chapter 9. (3) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008. (4) Kelland, M. A.; Svartaas, T. M.; Dybvik, L. Experiments Related to the Performance of Gas Hydrate Kinetic Inhibitors. Ann. N.Y. Acad. Sci. 2000, 912, 744–752. (5) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Sci. 2005, 60, 1313–1321. (6) Peytavy, J.-L.; Glenat, P.; Bourg, P. In Proceedings of the International Petroleum Technology Conference, Dubai, U.A.E., December 4-6, 2007; IPTC 11233. (7) 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. In Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, July 6-10, 2008. r 2010 American Chemical Society

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Figure 1. Poly(N-alkyl-N-vinyl amide)s.

Figure 2. Poly(N-alkyl-N-vinyl acetamide)s.

VIMA monomer, the only other commercially available N-vinyl amide monomers are N-vinyl acetamide and N-vinyl formamide. We were interested in studying polymers of N-vinyl amide monomers with somewhat larger alkyl groups since it seems clear from KHI studies on other classes of watersoluble amide polymers that polymers with adjacent alkyl groups of 3-5 carbon atoms are probably optimal for KHI performance. This includes studies on polyaspartamides, poly(2-alkyl-2-oxazoline)s, polyalkylacrylamides, and hyperbranched poly(ester amide)s.2 These alkyl groups are of optimum size to interact with structure II hydrate 51264 cages. N-Methyl-N-vinyl acetamide-N-vinyl caprolactam 1:1 copolymer (VIMA-VCap 1:1) copolymer has been shown to perform better than poly-N-vinylcaprolactam of similar molecular weight and was used commercially for several years.25,26 There are several possible routes to make poly(N-alkylN-vinylamide)s. They include (a) polymerization or copolymerization of N-alkyl-N-vinylamides monomers;27,28 (b) amidation and then alkylation of polyvinylamine or a reverse of these two steps; (c) transamidation of easily available poly(Nalkyl-N-vinylamide)s such as poly(N-methyl-N-vinyl acetamide); and (d) alkylation of N-vinyl amides followed by polymerization or a reverse of these two steps. Methods b-d have not been reported previously for small N-alkyl groups, so we used method a. A couple of attempts to specifically prepare KHIs within the poly(N-alkyl-N-vinyl amide) class using route a have been attempted previously.1 N-Vinyl-N-propyl propanamide monomer was successfully synthesized but it could not be polymerized, probably due to steric reasons. It also could not be copolymerized with N-vinyl pyrrolidone. N-Vinyl-N-isobutyl acetamide was also synthesized by another research group, but this also refused to polymerize. Because of these steric problems during attempted

polymerization we chose to stick to R2 as a small methyl group in the N-alkyl-N-vinyl amide and to vary only the size of the alkyl group on the nitrogen atom (see Figure 2). The synthesis of some monomers within this class of N-alkyl-N-vinyl acetamide is known and is discussed in the next section. The polymers we synthesized were investigated for their KHI performance in a high-pressure titanium autoclave as well as for their ability to modify and inhibit the growth of tetrahydrofuran (THF) hydrate crystals which form structure II hydrate crystals. Two different high-pressure autoclave tests were carried out. The first method we will call the “constant cooling” method, the second method we will call the “precursor constant cooling” method. The constant cooling method is conducted by cooling with agitation down to a very low temperature (high subcooling). The time and temperature for the onset of hydrate formation as well as rapid hydrate formation are recorded. Details are given in the experimental section. The “precursor constant cooling” method is a relatively new development first investigated by workers at TOTAL and the University of Pau, France.29-31 This method entails forming gas hydrates, then melting the hydrates to a maximum of a few degrees Celsius above the equilibrium temperature for a short time so as to preserve hydrate “precursor” structures in the water phase (which is one theory for the so-called hydrate “memory effect” discussed later), and then cooling again into the hydrate-forming region to form hydrates for a second time. The reason we used this method is because these workers claim that it gives more repeatable results than standard nonprecursor methods in which the stochastic nature of hydrate formation often gives quite wide variations in results in small “clean” cells compared with large production pipelines. In fact prior to this project, we carried out a separate project to investigate, among other things, the repeatability of the precursor test method. Our findings confirmed the work of TOTAL and University of Pau which will be submitted in a separate publication. However, we found it was necessary to use a liquid hydrocarbon phase (in our experiments, decane was used) to avoid the hydrates from being thrown up into the upper part of the cell which on warming would cause the hydrates to be melted at separate stages. It is important to avoid separate hydrate phases in order to ensure simultaneous melting of all the hydrates. Further, in a second paper just published by the University of Pau research team,31 and after this work was carried out, they recommend a longer test procedure to rank KHIs using precursor test methods. The revised method

(18) Musa, O. M.; Cuiyue, L. World Patent Application WO 2010/ 117660, 2010. (19) Musa, O. M.; Cuiyue, L. World Patent Application WO 2010/ 114761, 2010. (20) Rodrı´ guez Gonzales, R.; Djuve, J. World Patent Application WO 2010/101477, 2010. (21) Frenzel, S.; Assmann, A.; Reichenbach-Klinke, R. “Green” Polymers For The North Sea - Biodegradability As Key Towards Environmentally Friendly Chemistry For The Oilfield Industry, Royal Society of Chemistry. Chemistry In The Oil Industry XI, Manchester, U. K., November 2-4, 2009. (22) Musa, O. M.; Cuiyue, L.; Zheng, J.; Alexandre, M. Advances In Kinetic Gas Hydrate Inhibitors. Royal Society Of Chemistry, Chemistry in the Oil Industry XI, Manchester, U.K., November 2-4, 2009. (23) Ziao, N.; Laurence, C.; Le Questel, J.-Y. CrystEngComm 2002, 4 (59), 326. (24) Colle, K. S.; Talley, L. D.; Oelfke, R. H.; Berluche, E. World Patent Application 96/41784, 1996. (25) Colle, K. S.; Oelfke, R. H.; Kelland, M. A. U.S. Patent 5,874,660, 1999. (26) Talley, L. D.; Edwards, M.; Pipeline Gas J., March 1999. (27) Akashi, M.; Yashima, E.; Yamashita, T.; Miyauchi, N.; Sugita, S.; Marumo, K. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 3487. (28) Ajiro, H.; Akashi, M. Macromolecules 2009, 42, 489.

(29) Duchateau C.; Peytavy, J.-L.; Glenat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Laboratory Evaluation of Kinetic Hydrate Inhibitors: A New Procedure for Improving the Reproducibility of Measurements. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, British Columbia, Canada, July 6-10, 2008. (30) Duchateau, C.; Peytavy, J.-L.; Glenat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2009, 23 (2), 962–966. (31) Duchateau, C.; Glenat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2010, 24, 616.

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entails determining the equilibrium temperature, Teq, for each polymer KHI solution by dissociating hydrates that were first formed in the presence of the KHI polymer. Then in the precursor experiment itself, one melts the preformed hydrates at a temperature “Teq þ x” where x is kept constant in all tests but Teq will vary from polymer to polymer. In this way, it is proposed that there are a more equivalent number of precursors in each test than if the true Teq for pure water is used and also is kept constant for all the different polymer solutions. As can be seen from our test results, we used the original precursor test method, warming to the same temperature above the true Teq in all tests. Using this method, we obtained somewhat better reproducibility compared to nonprecursor test methods in our autoclave equipment. The time to hydrate formation is assumed to be more repeatable with this precursor method because hydrate formation is now mainly triggered by heterogeneous germination. Several groups have observed a memory effect from experiments with gas hydrates at temperatures well above the normal melting point of ice.32-35 Others have reported results on tetrahydrofuran hydrate where no memory effect exists.36 Neutron diffraction studies have shown there is no significant difference in the water structure before methane hydrate formation and after melting the hydrate. However, rather than there being partial clusters of hydrates (precursors), molecular modeling work has shown that there may be a persistent higher-than-equilibrium concentration of methane in the water phase after melting methane hydrate, which could cause the memory effect.

Figure 3. Synthesis of poly(N-alkyl-N-vinylacetamide)s. Table 1. Attempted Polymerizations and the Results Obtained sample HA7-025 HA7-026 HA7-027 HA7-028 HA7-029 HA7-038 HA7-030 HA7-031 HA7-032 HA7-033 HA7-034 HA7-035 HA7-039 HA7-036 HA7-037

R1, monomer 1 R1, monomer 2 mole ratio methyl isopentyl isobutyl n-butyl n-propyl isopropyl n-ethyl isopentyl isobutyl n-butyl n-propyl ethyl isopropyl isopentyl isobutyl

methyl methyl methyl methyl methyl methyl methyl methyl

100:0 100:0 100:0 100:0 100:0 100:0 100:0 50:50 50:50 50:50 50:50 50:50 50:50 20:80 20:80

result polymer gelated some polymer gelated polymer no polymer gelated gelated polymer gelated gelated gelated no polymer some polymer some polymer

nitrogen and washed with anhydrous THF three times. After 20 mL of anhydrous DMF was introduced, NVA solution in DMF (82.3 mmol) was slowly added at 0 °C, then the mixture was heat up to 40 °C to combine with 1-bromo-3-methylbutane (12.4 mL, 99 mmol) for 12 h. The reaction mixture was washed with aqueous NaCl and dried over anhydrous MgSO4. Silica gel column chromatography with hexane/ ethyl acetate (2/1) was carried out to obtain the liquid monomer (10.2 g, 65.8 mmol, 66% yield). 1 H NMR (CD3CN, 400 MHz) δ: 0.94 (m, 6H, CH3), 1.37 (m, 1H, CH), 2.15 (m, 2H, CH2;CH2;CH), 3.51 (m, 2H  0.3, N; CH2, trans), 3.59 (m, 2H  0.7, N;CH2, cis), 4.28 (d, J = 8 Hz, 1H  0.7, CH2dCH, cis), 4.34 (d, J=8 Hz, 1H  0.3, CH2dCH, trans), 4.45 (d, J=16 Hz, 1H  0.7, CH2dCH, cis), 4.48 (d, J=20 Hz, 1H  0.3, CH2dCH, trans), 6.85 (dd, J = 16 Hz and 8 Hz, 1H  0.7, CH2dCH, cis), 7.34 (dd, J= 16 Hz and 8 Hz, 1H  0.3, CH2dCH, trans). FT-IR (cm-1): 2956, 1670, 1618, 1385, 1346, 1210, 1024, 976, 837, 591. Polymerization (See Figure 3). A typical experimental procedure of the polymerization of N-alkyl-N-vinylacetamides is described as follow: AIBN (32 mg, 0.195 mmol) was placed in a 25 mL glass flask under nitrogen. N-Methyl-N-vinylacetamide (4.03 g, 40.7 mmol) was then introduced and heated up to 50 °C for 12 days. The reaction mixture was dissolved in CHCl 3 and then poured into a large amount of hexane (>99% yield) to precipitate the polymer, which was filtered off and dried. All synthesized polymers are given in Table 1. Measurements. 1H NMR and 13C NMR were measured by JEOL JNM-GSX400 system. Attenuated total reflection infrared (ATR IR) spectra were obtained with a Spectrum 100 FT-IR spectrometer (Perkin-Elmer). The interferograms were coadded 4 times and Fourier-transformed at a resolution of 4 cm-1. N-Propyl-N-vinylacetamide monomer has been reported previously and its homopolymer which has a lower critical solution temperature (LCST) of about 40 °C as a 1 wt % solution in water. N-Isobutyl-N-vinylacetamide monomer

Synthesis of Poly(N-alkyl-N-vinylacetamide)s Materials. N-Vinylacetamide (NVA) (Showa Denko K. K., Japan) was recrystallized from cyclohexane/toluene (3/1, w/w) and dried in vacuum at room temperature. N-MethylN-vinylacetamide (Aldrich, Co., NJ) was distilled from CaH2 under reduced pressure. 1-Bromopropane (Tokyo Chemical Industry Co. Ltd., Japan), 1-bromo-2-methylpropane (Tokyo Chemical Industry Co. Ltd., Japan), 1-bromo3-methylbutane (Tokyo Chemical Industry Co. Ltd., Japan), sodium hydride (NaH) in oil, 60% (Tokyo Chemical Industry, Japan), anhydrous DMF (Aldrich Co., NJ), and 2,20 azobisisobutyronitrile (AIBN) (Tokyo Chemical Industry Co. Ltd., Japan) were used without further purification. N-Substituted N-vinylacetamides, such as N-(n-propylN-vinylacetamide), N-(isobutyl-N-vinylacetamide), and N-(isopentyl-N-vinylacetamide), were synthesized by nucleophilic substitution reaction with NVA and the corresponding alkylbromides. Only the synthesis of the last monomer has not been reported previously and therefore is given here. Synthesis of N-(Isopentyl-N-vinylacetamide). In the glass flask, activated molecular sieve 4A, anhydrous DMF (28 mL), and NVA (7.0 g, 82.3 mmol) were combined. To the threenecked glass flask, NaH (3.95 g, 99 mmol) was placed under (32) Takeya, S.; Hori, A.; Hondoh, T.; Uchida, T. J. Chem. Phys. B 2000, 104, 4164. (33) Parent, J. S.; Bishnoi, P. R. Chem. Eng. Commun. 1996, 144, 51. (34) Lee, J. D.; Englezos, P. Unusual kinetic inhibition effects on gas hydrate formation. Chem. Eng. Sci. 2006, 61, 1368–1376. (35) Ohmura, R.; Ogawa, M.; Yasuja, K.; Mori, Y. H. J. Phys. Chem. B. 2003, 107, 5289. (36) Wilson, P. W.; Haymet, A. D. J.; Kozielski, K. A.; Hartley, P.; Becker, N. C. Nucleation of Clathrates from Supercooled THF/Water Mixtures Shows that No Memory Effect Exists. Proceedings of the 6th International Conference on Gas Hydrates (ICGH6), Vancouver, British Columbia, Canada, July 6-10, 2008.

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has also been reported previously but not its homopolymer or any copolymers.37,38 We attempted the homopolymerization of N-isobutylN-vinylacetamide but did obtain a significant yield of polymer. The failure is probably due to steric effects of the large isobutyl group.39 In addition, it is likely that the homopolymer is insoluble in water at room temperature given that the less hydrophobic poly(N-propyl-N-vinylacetamide) has an LCST of 40 °C. Therefore, we synthesized copolymers of N-isobutyl-N-vinylacetamide monomer with N-methyl-N-vinylacetamide. These are copolymers HA7-032 and HA7-037 with 50 and 20 mol % N-isobutyl-N-vinylacetamide, respectively. However, N-isopentyl-N-vinylacetamide could be copolymerized with VIMA, but to keep the LCST above room temperature it was necessary to use 80% VIMA monomer. This gave the product HA7-036: poly(N-isopentyl-N-vinylacetamide)-co-poly(N-methyl-N-vinylacetamide) (1:4). N-Isopropyl-N-vinylacetamide proved a difficult monomer to make. However, it also did not polymerize by itself or with N-vinylacetamide monomer, even with TEMPO (radical inhibitor) at 80 °C. This behavior is probably due to the steric bulkiness of the isopropyl group. N-IsopropylN-vinylformamide may have been easier to polymerize, but formamide monomers were not investigated. Polymerization of N-ethyl-N-vinylacetamide unfortunately led to cross-linking and subsequent gelation, so no water-soluble polymer could be isolated. Several other attempted polymerizations also led to gelation as indicated in Table 1. The gelation is due to some radical transfer in the monomer compound which is a cross-linking reaction. Although a small amount of poly(N-isobutyl-N-vinylacetamide) was obtained, it was not water-soluble. However, five water-soluble homo- and copolymers could be made in gram quantities and thus tested as KHIs. These were (a) HA7-025, poly(N-methyl-N-vinylacetamide); (b) HA7-029, poly(N-npropyl-N-vinylacetamide); (c) HA7-032, poly(N-isobutyl-Nvinylacetamide)-co-poly(N-methyl-N-vinylacetamide) (1:1); (d) HA7-036, poly(N-isopentyl-N-vinylacetamide)-co-poly(N-methyl-N-vinylacetamide) (1:4); and (e) HA7-037, poly(N-isobutyl-N-vinylacetamide)-co-poly(N-methyl-N-vinylacetamide) (1:4). To compare performances of KHI polymers of different chemical structures, it is important that they are of roughly similar molecular weight since this can affect the performance. For example, it has been shown for polyvinyllactams and polyacryloylpyrrolidones on SII hydrate that the best performance is at a molecular weight of roughly 1500-2000. The KHI performance drops off rapidly below this range and decreases slowly above this range.1 However, a bimodal range of polymer molecular weights has been claimed to give even better performance than a single low molecular range, for example, for poly(N-isopropylmethacrylamide)s.40 The synthesis of our poly(N-vinyl-N-acetamide)s was carried out to obtain polymers with Mn (number average molecular weight) values of roughly 10 000 Da, fairly low molecular

weight, so their structure versus KHI performance could be compared. Besides these five poly(N-vinyl-N-acetamide)s, we also investigated Luvicap 55W, a low molecular weight N-vinylcaprolactam-N-vinylpyrrolidone 1:1 copolymer supplied in water, and Luvicap EG, a low molecular weight poly-N-vinylcaprolactam supplied in monoethylene glycol, both supplied by BASF and used commercially in KHI formulations. These polymers were used as benchmarks by which to judge the KHI performance of the poly(N-alkyl-Nvinylacetamide)s. Polymer concentrations were always 5000 ppm active polymer regardless of the amount of solvent. Luvicap EG is known to be a better KHI than Luvicap 55W.1,2 Poly-N-vinylcaprolactam is known to be a more powerful KHI than N-vinylcaprolactam-N-vinylpyrrolidone 1:1 copolymer at similar molecular weights. Our own studies indicate that the monoethyleneglycol solvent in Luvicap EG does not significantly enhance the polymer KHI performance. The cloud points, Tcl (or lower critical solution temperatures, LCSTs), of 1 wt % solutions of the poly(N-alkylN-vinylacetamide)s in distilled water were determined. The results are given as follows: (a) HA7-025, Tcl > 100 °C; (b) HA7-029, Tcl = 39-40 °C; (c) HA7-032, Tcl = 4445 °C; (d) HA7-036, Tcl=94-95 °C; and (e) HA7-037, Tcl > 100 °C. These data fit the trend that the polymers with the most and the largest hydrophobic groups in the side-chains have the lowest cloud points. Thus, HA7-029 with 100% n-propyl groups in all monomers has the lowest cloud point. HA7032, with a random sequence of 50% isobutyl groups and 50% methyl groups in the side-chains also gave a fairly low cloud point, not much higher than HA7-029. HA7-036 has the largest hydrophobic groups, but as the copolymer contains only 20% of these isopentyl groups and 80% methyl groups, the cloud point is high. Kinetic Hydrate Inhibitor Performance Test Procedures High-Pressure Autoclave Tests with Synthetic Natural Gas. KHI performance tests were carried out in the high-pressure autoclave apparatus shown in Figure 4 and discussed previously.41,42 The autoclave consists of a titanium cell with a volume of 23 mL. The temperature was measured to an accuracy of (0.l °C and the pressure was measured with an accuracy of (0.2 bar. Two kinds of test methods were carried out. The first method we will call the “constant cooling” method, and the second method we will call the “precursor constant cooling” method. Both test methods are described in more detail below. In all the experiments, we used the same standard natural gas (SNG) mixture that gives structure II hydrates (see Table 2). The aqueous phase was distilled water. No liquid hydrocarbon phases were used. At the onset of each “constant cooling” experiment, the pressure was 78 bar. The equilibrium temperature at this pressure was calculated using Calsep’s PVTSim software to be 20.1 °C for distilled water and 90 bar, which was approximately 0.3 °C lower than our laboratory experiments in a larger 200 mL titanium cell to determine the equilibrium temperature by standard slow hydrate dissociation (see Figure 5).35,36 The cooling rate near the equilibrium temperature was 0.14 °C/h. From our

(37) Pavlov, G. M.; Korneeva, E. V.; Zhumel, K.; Kharding, S. E.; Ershov, A. Y.; Gavrilova, I. I.; Panarin, E. F. Zh. Prikl. Khim. (S.-Peterburg, Russ. Fed.) 2000, 73 (6), 992. (38) Ershov, A. Yu.; Gavrilova, I. I.; Panarin, E. F. Zh. Prikl. Khim. (S.-Peterburg, Russ. Fed.) 1998, 71 (11), 1852. (39) Iwamura, T.; Nakagawa, T.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2714. (40) Colle, K.; Talley, L. D.; Longo, J. M. World Patent Application WO 2005/005567, 2005.

(41) Del Villano, L.; Kommedal, R.; Kelland, M. A. Energy Fuels 2008, 22, 3143. (42) Del Villano, L.; Kommedal, R.; Hoogenboom, R.; Fijten, M. W. M.; Kelland, M. A. Energy Fuels. 2009, 23, 3665.

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each “precursor constant cooling” experiment, the pressure was 98 bar. The equilibrium temperature at this pressure was calculated using Calsep’s PVTSim software to be approximately 18.3 °C (see Figure 5). The same initial procedure for the preparation of the KHI experiment and filling of the cell was followed in all highpressure experiments in the titanium cell: (1) The polymer to be tested was dissolved in distilled water to the desired concentration, usually 0.5 wt %. (2) The magnet housing of the cell was filled with the aqueous solution containing the additive to be tested. For standard constant cooling tests, we used 8 mL of aqueous solution. For the precursor constant cooling tests, we used 3 mL of aqueous solution and 5 mL of decane. The magnet housing was then mounted in the bottom end piece of the cell, which thereafter was attached to the titanium tube and the cell holder. (3) The desired amount of the aqueous solution was filled in the cell (above the cell bottom) using a pipet, the top end piece was mounted, and the cell was placed into the cooling bath (plastic cylinder). (4) The temperature of the cooling bath was adjusted to 1-3 °C above the hydrate equilibrium temperature at the pressure conditions to be used in the experiment. (5) After purging the cell twice with the SNG, the cell was loaded with SNG to the desired pressure while stirring at 600 rpm. Constant Cooling Test Procedure. Following the initial procedure given above, the cell was charged with SNG gas and the aqueous solution at 78 bar and 20.5 °C. The cell was stirred at 600 rpm while cooling at a constant rate to 2 °C over 18 h. Because of it being a closed system, the pressure drops as the temperature decreases. Since the rate of cooling was slow we were able to determine the detectable start of hydrate formation as the first deviation from the constant decrease in the absolute pressure in the cell during cooling. This indicates that gas is being used to form gas hydrates. We also noted the temperature at which fast, catastrophic hydrate formation took place as the point when the pressure drop curve became almost vertical, i.e., when gas consumption for gas hydrate formation was very fast. This led to a hydrate plug forming in the cell and stopping of the stirrer. A typical plot of pressure and temperature versus time is given in Figure 6. In total, 5-6 constant cooling experiments were carried out with each polymer at 0.5 wt % in the aqueous phase. The onset temperatures for the first detectable hydrate formation (To) and the temperatures for catastrophic hydrate formation (Ta) are given in Table 3 along with the averages of these two values. Precursor Constant Cooling Test Procedure. Following the initial procedure for loading of the cell with aquous fluid and decane given above, the precursor constant cooling test procedure we used is as follows. Step 1: From 98 bar and 20.5 °C, cool the cell and fluids at a constant rate to 1 °C over 2 h with stirring to make hydrates. The hydrates are left to grow for 1 h. This usually gave a hydrate plug and stopping of the stirrer. Step 2: Warm up the cell contents fast (approximately 20 °C/h) to 3.4 °C outside the hydrate region and hold at this temperature for 1 h. In our system, the equilibrium temperature for fresh water is 19.6 °C, so we warmed the contents to 23.0 °C. Step 3: The rest of the test procedure is similar to the standard constant cooling experimental procedure. Thus, the cell is cooled at a constant rate to 2 °C over 20.4 h, and To and Ta are measured from the graph of pressure and temperature versus time. Steps 2 and 3 are repeated using the same fluids several times to get several KHI precursor constant cooling test

Figure 4. Titanium autoclave high-pressure test equipment. Table 2. 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

Figure 5. Pressure temperature graphs for the two systems used, calculated using Calsep’s PVTSim software.

experience, the equilibrium temperature could have been up to 0.2 °C lower if we had used a cooling rate of 0.05 °C/h, which would have given very good agreement with the predicted equilibrium temperature, but this makes the experiments considerably more time-consuming. For the rest of this paper we will assume that the equilibrium temperature for our SNG-water system at 78 bar is 20.1 °C. At the onset of 6404

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Figure 6. Constant cooling test with 5000 ppm HA7-025. Hydrate formation is first detected at 528 min at To = 12.2 °C, as a pressure drop deviation from the pressure drop due to the temperature decrease. Fast hydrate formation occurs after 651 min at Ta = 10.5 °C. Table 3. High-Pressure KHI Test Resultsa

tetraalkylammonium salts, where the alkyl group is preferably n-butyl, n-pentyl, or isopentyl, are good inhibitors of THF hydrate crystal growth, yet they are very poor nucleation inhibitors of natural gas hydrates when used alone.1 The method we used for studying the inhibition of THF hydrate crystal growth has been reported previously by us and for work on poly(N-vinyl lactam)s and antifreeze proteins.42,44,48,49 NaCl (26.28 g) and THF (99.9%, 170 g) are mixed, and distilled water is added to give a final volume of 900 mL. This gives a stoichiometrically correct molar composition for making structure II THF hydrate, THF 3 17H2O. The test procedure is as follows: (1) 80 mL of the aqueous THF/NaCl solution is placed in a 100 mL glass beaker. (2) The test chemical is dissolved in this solution to give the desired concentration, for example, 0.32 g of polymer in 80 mL of solution gives a 0.4 wt % (4000 ppm) solution of the polymer. (3) The beaker is placed in a stirred cooling bath preset to a temperature of -0.5 °C ((0.05 °C). (4) The solution is briefly stirred manually with a glass rod every 5 min, without touching the glass beaker walls, while being cooled for 20 min. (5) A hollow glass tube with inner diameter 3 mm was filled at the end with ice crystals kept at -10 °C. The ice crystals are used to initiate THF hydrate formation. (6) The glass tube was placed almost halfway down in the cooled polymer/THF/NaCl solution after the solution had been cooled for 20 min. (7) THF hydrate crystals were allowed to grow at the end of the glass tube for 60 min. (8) After this time, the tube was removed, the THF hydrate crystals weighed, and the crystal growth rate in grams per hour determined. The shape and morphology of the crystals both in the beaker (if any) and on the end of the glass tube were visually analyzed.

normal constant cooling memory effect constant cooling polymer no polymer Luvicap EG Luvicap 55W HA7-025 HA7-029 HA7-032 HA7-036 HA7-037

To (av.)/°C Ta (av.)/°C 12.0 4.2 7.1 10.2 8.9 8.2 7.9 7.6

11.3 2.4b 6.3 8.9 7.1 7.1 7.1 7.1

To (av.)/°C

Ta (av.)/°C

17.1

15.4

b

13.1 15.4 13.4 12.5 13.0 13.6

7.7 13.7 12.1 10.4 11.8 12.8

a To = onset temperature, Ta = catastrophic growth temperature, av. = average. Subcoolings can be determined using the graph in Figure 5. b Hydrates were not fully melted at 23 °C as the pressure did not return to the original level at the maximum temperature before hydrate formation (see Figure 6).

results. A typical cycle of test results is given in Figure 7. From this graph an enlargement of each individual experiment can be made and To and Ta determined more accurately as shown in shown in Figures 8 and 9. In Figure 8 it is not clear when pressure drop due to hydrate formation first occurs because hydrate formation is initially very slow. Therefore an enlargement of part of the graph is made as in Figure 9 to determine To. THF Hydrate Crystal Growth Test Procedure Tetrahydrofuran (THF) forms structure II hydrate crystals at about 4 °C under atmospheric pressure. Polymers that show good inhibition of THF hydrate crystals might also show good kinetic inhibition of gas hydrate crystals if the same mechanism of surface adsorption can be assumed. However, conclusions should be handled with care since this is not always the case. Tetrahydrofuran hydrate ball-stop test results on poly(2-isopropyloxazoline) polymers revealed that these polymers perform no better than the tests with no additive, yet they are reasonably active as KHIs in mini-loop tests with natural gas mixtures.43 Conversely, it is known that nonpolymeric

KHI Performance Test Results and Discussion We chose to evaluate the relative KHI performance of the poly(N-alkyl-N-vinylamide)s using the constant cooling test method because the titanium cell we used had been shown in previous research studies to give somewhat more reproducible results using this method compared to other KHI test methods

(43) Colle, K. S.; Talley, L. D.; Oelfke, R. H.; Berluche, E. World Patent Application 96/08673, 1996.

(44) Kelland, M. A.; Del Villano, L. Chem. Eng. Sci. 2009, 64, 3197.

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Figure 7. Graphical representation of a cycle of precursor constant cooling experiments with HA7-036.

Figure 8. Graphical enlargement of the first experiment in a series of precursor constant cooling experiments with HA7-036. Hydrate formation is first detected at 863 min at To = 13.9 °C (see also Figure 9), as a pressure drop deviation from the pressure drop due to the temperature decrease. Catastrophic hydrate formation occurs after 984 min at Ta = 11.9 °C.

such as measuring the induction time after start of stirring in isothermal tests. It has been proposed to add impurities to obtain better reproducibility in KHI tests in small equipment.3 However, one cannot be sure, as work from Ripmeester’s group points out, that the KHI performance is not just a function of the KHI interacting with the impurity surfaces preventing a specific heteronucleation. This could lead to erroneous ranking of KHIs compared to true field conditions where they will be applied if this is not the dominant inhibition mechanism of the KHIs. Standard Constant Cooling Experiments in the High-Pressure Titanium Cell. The poly(N-alkyl-N-vinylacetamide)s were first tested using the standard constant cooling method at 0.5 wt % dissolved in distilled water cooling from 78 bar and 20.5 to 2 °C over 18 h with 600 rpm stirring. The average To and Ta values of 6-8 individual experimental results on each polymer as well as with no polymer are listed in Table 3. There was, as expected, some scattering in the To and Ta due to the stochastic nature of hydrate formation in a small cell. The percentage variation in the To and Ta values was about

20-25% for all test series. For example, with no additive To varied from 9.6 to 14.9 °C with an average value of 12.0 °C over 6 experiments. For Luvicap EG, the To value varied between 3.7 and 4.9 °C and for HA7-036 between 6.6 and 9.0 °C. The percentage variations in To or induction times of the same magnitude have been reported previously from our laboratories.42 Independent samples t tests have been used to test for statistical differences in To values between the samples tested in Table 3. P-values smaller than 0.05 are considered to indicate statistically significant differences.45 The results of the standard nonprecursor constant cooling tests will be discussed first. The use of no polymer, just distilled water, clearly gave the highest onset temperatures with a clear statistical difference (P-value < 0.05) to all the polymers tested. The best polymer tested was Luvicap EG, a (45) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability & Statistics for Engineers & Scientists; Pearson Education Int.: Upper Saddle River, NJ, 2007.

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Figure 9. Enlargement of part of the cooling process in Figure 7 illustrating more clearly when slow gas uptake for hydrate formation is first detected at 863 min and To = 13.9 °C.

low molecular weight polyvinylcaprolactam. It gave consistently lower To values than To values in all tests with all other polymers including Luvicap 55W, a 1:1 VP:VCap copolymer, known to be a worse KHI than PVCap. Luvicap 55W gave a lower average To value (7.2 °C) than all the poly(N-alkyl-N-vinylamide)s. However, there is only a clear statistical difference (based on the P-values) between the average To value of Luvicap 55W with HA7-025 and HA7029. Further tests would be needed to determine whether Luvicap 55W had a significantly better KHI performance than HA7-032, HA7-036, and HA7-037 by this test method. This also means it was not possible to determine with statistical significance any difference in the KHI performance of the polymers HA7-032, HA7-036, and HA7-037, which contain isobutyl or isopentyl groups in the sidechains of the polymers. However, HA7-025, poly(N-methylN-vinylacetamide), clearly gave a worse performance than any of the other other poly(N-alkyl-N-vinylamide)s (P-value was less than 0.05 for comparisons to all poly(N-alkylN-vinylamide)s). This was expected as this polymer has the smallest alkyl groups, methyl, on the side-chains of the polymer giving less interaction and perturbation of the water and less interaction with hydrate cavities compared with polymers with larger alkyl groups. HA7-029, poly(N-n-propyl-N-vinylacetamide), gave the second worse performance of the poly(N-alkyl-N-vinylamide)s with a P-value of less than 0.05 between HA7-029 and all four of the other polymers. Thus, placing larger hydrophobic n-propyl groups on the nitrogen atoms compared to HA7-025 with methyl groups on the nitrogen atoms does improve the performance of the polymer. However, slightly larger alkyl groups with branching in some of the alkyl groups in the polymer improves the performance further as exemplified by polymers HA7-032, HA7-036, and HA7-037. It was a pity that polymers with isopropyl groups could not be synthesized for comparison, but all attempts to produce either homopolymers or copolymers failed. Precursor Constant Cooling Experiments in the High-Pressure Titanium Cell. Since the results of the standard nonprecursor constant cooling tests were unable to distinguish a difference in the performance between three of the poly(Nalkyl-N-vinylamide)s, it was decided to carry out so-called

precursor KHI tests as the originators of this test method have claimed that the reproducibility of this test method is less stochastic and therefore better than standard nonprecursor test methods. (We considered using standard isothermal tests in which induction times are measured in order to better compare the performances of the polymers, but as mentioned earlier the titanium cell appeared to give greater scattering of results using this method in earlier projects and three of the poly(N-alkyl-N-vinylamide)s had fairly similar performances, so distinguishing the ranking of these three polymers would be difficult). Our research group has carried out earlier studies using this new precursor test method and showed that it does give better reproducibility in To values or induction times in isothermal tests if the correct test conditions and test methods are used. This work will be published separately in due course. Therefore, we were confident that this method would reduce the spread in the To values for a given poly(N-alkyl-N-vinylamide) and thus allow for a statistically more significant ranking of KHI performances. However, from our earlier work we were not sure that the ranking of the KHI performance of the polymers using a precursor test method would agree with the ranking obtained using nonprecursor test methods. All results are given in Table 3 alongside the results with the nonprecursor method for comparison. We deliberately chose to use a higher pressure of 90 bar at room temperature in the precursor experiments to give the same subcooling while cooling as in the nonprecursor experiments at 70 bar. This is due to the presence of decane in the precursor experiments which lowers the equilibrium temperature for hydrate formation at any given pressure. The first conclusion to be drawn from the precursor test results is that the scattering or deviation of the To and Ta temperatures in a series of tests for a specific polymer is considerable improved compared with the nonprecursor test method. This often gave P-values from t tests of less than 0.05 as discussed later. In general, we obtained deviations of maximum 10% either side of the average for any of the polymers or no additive. The worst scattering in To values in a series of four tests was with HA7-032 in which we obtained To values of 12.1, 12.6, 13.2, and 13.9 °C with an average of 13.0 °C. The scattering in Ta values was even less. For HA7-032 we 6407

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Figure 10. Two precursor constant cooling test results with Luvicap EG.

obtained Ta values of 11.7, 12.0, 12.2, and 12.4 °C. The worst scattering for Ta was with HA7-037 where we obtained Ta values of 13.1, 12.8, 12.2, and 13.0 °C in that chronological order with the same solution giving an average Ta of 12.8 °C. Thus, there was no common trend for all polymers that To or Ta values increased from the first to the fourth test, it occurred coincidentally and occasionally. The second conclusion that can be drawn from the results in Table 3 is that the To values are significantly higher for all chemicals, including no chemical, compared with tests without prior melting of gas hydrates. This suggests that some effect is taking place due to melting hydrates first, which may be due to residual structure on the nanoscale in the bulk water phase (i.e., precursors) which initiates macroscopic gas hydrate formation more quickly. For example, without any chemical, the average To in 4 precursor tests was 17.1 °C whereas in nonprecursor tests the average To was 12.0 °C in 5 tests. For Luvicap 55W, the average To was 13.1 °C in the 4 precursor tests whereas it was 7.1 °C in the nonprecursor tests. However, for Luvicap EG we did not obtain meaningful results because the pressure did not return to the original level when warming to temperatures higher than the equilibrium temperature (Teq) of 19.6 °C and up to 23 °C after first making hydrates (see Figure 10). This is because the hydrates are not fully melted in the time period of about 1 h when warmed above Teq. This effect is due to Luvicap EG preventing the melting of hydrates at Teq. This effect has been reported previously for various poly-N-vinylcaprolactams. In general, it was found that with 5000 ppm polyN-vinylcaprolactams it was necessary to warm to at least 5-6 °C above Teq to get the SII gas hydrates to melt in a fairly short time period. In our tests, we warmed the hydrates to only 3.4 °C above Teq, which is apparently not high enough to get full melting of the hydrates in 1 h. The third conclusion to be drawn is that the ranking of the KHI performance of the polymers by the precursor test method, using the To values, is

and HA7-029. The ranking using To values obtained with the nonprecursor method was less clear: Luvicap EG > Luvicap 55W  HA7-037  HA7-036  HA7-032 > HA7-029 > HA7-025 > no additive Thus, the ranking by the two methods is consistent with one possible exception, which is the relative performance of HA7-032. This polymer is close to being significantly worse than Luvicap 55W by the standard method (P-value of 0.09, which is a little higher than the threshold value of 0.05 for a statistically significant difference) but better than Luvicap 55W by the precursor method (P-value of 0.04). To clarify this possible anomaly, further constant cooling tests were carried out on HA7-032 by the nonprecursor method. However, this did not help decide the issue as the new To values gave just as a big range of scattering in results, which did not lower the P-value below 0.05. The precursor test results using the To values (and also the Ta values) suggests that polymer HA7-032 is the best KHI of the poly(N-alkyl-N-vinylacetamide)s. Theory indicates that a higher percentage (50% for HA7-032) of isobutyl groups would give a better KHI than a low percentage such as 20% in HA7-037. HA7-036, which has only 20% isopentyl groups, may have performed better if a higher percentage of isopentyl groups could have been incorporated but it was not synthetically possible. Branching in the alkyl group in the side-chain is also preferable since HA7-037 performs better than HA7-029 despite the latter polymer having 100% npropyl groups in the side-chains. With the use of the Ta values for catastrophic hydrate growth in Table 3, the ranking of KHIs is as follows: nonprecursor method : Luvicap EG > Luvicap 55W > HA7-032  HA7-036  HA7-037  HA7-029 > HA7-025

HA7-032 > HA7-036  Luvicap 55W > HA7-029  HA7-037 > HA7-025 > no additive

precursor method : Luvicap 55W > HA7-032 > HA7-036  HA7-029 > HA7-037 > HA7-025

Statistical P-values of less than 0.05 were calculated from t tests for comparisons of data between all polymers except between Luvicap 55W and HA7-036 and between HA7-037

On the basis of To values in the precursor tests, Luvicap 55W clearly delays catastrophic growth much more than any of the poly(N-alkyl-N-vinylacetamide)s. This suggests that 6408

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Table 4. Results on the Growth Rate and Morphology of THF Hydrate Crystals Grown with Various Polymers concentration [ppm] polymer

4000

HA7-025

plates in beaker, cannot weigh

HA7-029

cannot be weighed; plates in whole beaker after 15 min; some soft plates on end of the tube cannot be weighed; some soft plates on the tube; fine plates of crystals throughout the whole beaker after ∼20 min cannot be weighed; crystals in whole beaker; crystals form later than HA7-032 plates in whole beaker, cannot be weighed

HA7-032

HA7-036 HA7-037

5000

6000

7000

8000 0.75 g of pyramidal crystals thin plates in whole beaker

0.908 g (soft crystal on tube) some fine plates in the whole beaker no crystals

soft slushy plates encapsulating water; weight variable, 0.4-1.5 g no crystals

no crystals

no crystals

no crystals

no crystals thin plates in whole beaker

Luvicap 55W is superior in inhibiting the growth of SII hydrate crystals. This correlates well with the findings for inhibition of the growth of THF hydrate crystals in the next section. From the Ta values, polymer HA7-032 is now the best of the poly(N-alkyl-N-vinyl acetamide)s. This fits the theory discussed above for correct size and branching of sidechain alkyl groups. The result that a water-soluble amide (co)polymer with pendant isobutyl or isopentyl groups performs better than polymers with smaller alkyl groups is consistent with the conclusion we drew from our investigations on poly(aspartamide) polymers.41 KHIs with isobutyl groups adjacent to amide groups have also been shown to have good performance as KHIs in poly(2-alkyl-2-oxazoline)s and amide derivatives of maleic anhydride copolymers.42,46 These branched alkyl groups are of optimum size to interact via van der Waals forces with structure II 51264 cages, which may explain why they are useful in KHIs. The amide groups in these polymers are strongly hydrogen-bonding, both to water molecules in the bulk aqueous fluids and water molecules in the hydrate nuclei, which probably also plays a role in the kinetic hydrate inhibition mechanism. It would have been interesting to have had polymers or copolymers of N-isopropyl-N-vinylacetamide for comparison of KHI performance but such polymers could not be synthesized as discussed earlier. This may be because the branching of the isopropyl group occurs nearer the nitrogen atom than with isobutyl or isopentyl groups, causing greater steric problems in polymerizing this monomer. Polymers with side-chains with isopropyl groups adjacent to amide groups have been shown to perform well as KHIs.42,46,47 THF Hydrate Crystal Growth Test Results and Discussion. The poly(N-alkyl-N-vinylacetamide)s were tested for their ability to inhibit the growth of THF hydrate crystals during 1 h at -0.5 °C in the THF/water/NaCl mixture. Luvicap 55W and “no additive” were also tested for comparison purposes. Table 4 summarizes the results obtained with the poly(Nalkyl-N-vinylacetamide)s at various concentrations. The THF hydrate crystals, grown under our test conditions with no additive, were octahedral with sharp pyramidal shapes growing away from the end of the glass tubing (see Figure 11). In 1 h, using no additive, the weight of the THF hydrate

Figure 11. Typical pyramidal THF hydrate crystal growth without additives.

crystals growing directly from the end of the glass tube where ice initiates the start of growth is approximately 1.4-2.0 g with an average of about 1.7 g. In general, we observed a scattering in growth rates of about 20-25% either side of the average. This information is in accord with previously reported data.42 Experiments with 0.2 wt % active polymer in Luvicap 55W gave very thin plates, which grew out to the sides of the beaker away from the end of the glass tube (see Figure 12). When the glass tube was removed from the solution, the plates collapsed and it was not possible to weigh them. At higher polymer concentrations, the plates became thinner until at about 5000 ppm we observed no plate growth at all. This general behavior has also been previously observed with N-vinylcaprolactam polymers.41,42 As shown in Table 4, we also observed a threshold concentration for some of the poly(N-alkyl-N-vinylacetamide)s above which no THF hydrate crystals could be detected and thin hydrate plates formed below this concentration. For example, at 4000 ppm HA7-036 (isopentyl-methyl groups in a 1:4 ratio) gave thin plates of crystals in the whole beaker initiated and growing out from the ice at the end of the glass tube. This meant it was impossible to weigh the crystals on the end of the tube, and many of them broke off as the tube was removed from the solution after 1 h. At 5000 ppm and higher concentrations, no crystals were observed at all in the solution or the end of the tube, indicating that total inhibition

(46) Kelland, M. A.; Klug, P. World Patent Application 98/23843, 1998. (47) Talley, L. D.; Oelfke, R. H. World Patent Application 97/07320, 1997.

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Figure 13. Soft “plates” of THF hydrate crystals interspersed with liquid.

These THF hydrate crystal growth results indicate that differences in the size of the alkyl substituents on the nitrogen atom in poly(N-alkyl-N-vinylacetamide)s, and thus the polymer three-dimensional structure, can cause very different effects on the way they interact with SII THF hydrate crystals causing different crystal morphologies and growth rates to be obtained. The results correlate well with the SII gas hydrate inhibition results since the polymers with the larger isopentyl or isobutyl groups performed best in both sets of studies.

Figure 12. Hard plates of THF hydrate crystals throughout the whole beaker.

of THF hydrate crystal growth was occurring at a threshold concentration between 4000 and 5000 ppm. Thus, it appears polymer HA7-036 perturbs the THF hydrate crystal growth on some surfaces to form plates at 4000 ppm, which can be considered to become infinitely thin at the threshold concentration in the range 4000-5000 ppm. HA7-032 also gave a threshold concentration at which total hydrate growth was inhibited but this was at a higher concentration in the range 6000-7000 ppm. However, even below this concentration, the crystals were made up of fine soft, plates (see Figure 13). The term “soft” implies that the crystals are not completely solid but are made up of tiny plates which incorporate liquid and which crush very easily when touched. Thus, it seems that the 20% isopentyl groups in the side-chains of polymer HA7-036 cause a stronger effect on THF hydrate crystal growth than the polymer HA7-032 which has 50% isobutyl groups. Good inhibition of THF hydrate crystals with chemicals with isopentyl groups has only previously been reported for small nonpolymeric quaternary salts.50 For HA7-037 with only 20% isobutyl groups, the effect on the THF hydrate crystal growth is even less than HA7-032. Thin plates formed at 8000 ppm, the maximum concentration tested due to a limited supply of polymer. The other two poly(Nalkyl-N-vinylacetamide)s gave substantial crystal growth even at the maximum concentration tested of 8000 ppm. For HA7-025, with the smallest alkyl group, methyl, on the side-chain nitrogen atoms, the crystal morphology was the same as without an additive but the growth rate somewhat less. For HA7-029 (n-propyl groups) and HA7-037 (isobutyl-methyl groups in a 1:4 ratio), we obtained plates of crystals throughout the whole beaker even at a concentration of 8000 ppm. The plates were thinner with HA7-037 at this concentration.

Conclusions We have synthesized a series of poly(N-alkyl-N-vinylacetamide)s and determined that only certain range of polymers could be obtained due to gelation by cross-linking and due to steric problems in polymerization. The five polymers obtained were tested for their KHI performance in a high-pressure gas hydrate autoclave and as THF hydrate crystal growth inhibitors. The polymers with the largest alkyl groups (isopentyl and isobutyl) on the side-chain nitrogen atoms performed best in both sets of equipment. In nonprecursor constant cooling tests, none of the polymers performed significantly better than a commercial KHI polymer, Luvicap 55W, although further tests would be required to clarify any small performance differences between the commercial polymer and HA7-032, HA7-037. Experiments using the precursor constant cooling test method gave better reproducibility of the To and Ta temperatures which were significantly higher than values obtained with the nonprecursor test method. The ranking of the KHI performance of the polymers, using To values, by the precursor method was in accord with the nonprecursor test method with the possible exception of HA7-032. For Ta values, Luvicap z55W was a better KHI by either high pressure test method. The SII THF hydrate crystal growth inhibition results correlate well with the SII gas hydrate inhibition results since the polymers with the larger isopentyl or isobutyl groups performed best in both sets of equipment. We are also investigating varying the size of the alkyl group on the carbonyl group, rather than this study in which the alkyl group on the nitrogen atom was varied.

(48) Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilib. 1998, 150-151, 353–360. (49) Makogon, T. Y.; Larsen, R.; Knight, C. A.; Sloan, E. D. J. Cryst. Growth 1997, 179, 258–262. (50) Klomp, U. C.; Kruka, V. C.; Reijnhart, R. World Patent Application 95/17579, 1995.

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