First Investigation of the Kinetic Hydrate Inhibitor Performance of Poly

Oct 11, 2014 - We report for the first time the synthesis and kinetic gas hydrate inhibition of a series of pseudo-polypeptides, poly(N-alkylglycine)s...
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First Investigation of the Kinetic Hydrate Inhibitor Performance of Poly(N‑alkylglycine)s Fernando T. Reyes,† Li Guo,‡ John W. Hedgepeth,‡ Donghui Zhang,‡ and Malcolm A. Kelland*,† †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States ABSTRACT: We report for the first time the synthesis and kinetic gas hydrate inhibition of a series of pseudo-polypeptides, poly(N-alkylglycine)s, with varying length alkyl side chains. The polymers have similar molecular weights for performance comparison. Tests were carried out in a multi-cell rocking rig at high pressure using a structure-II-forming natural gas blend. The best polymer tested was poly(N-propylglycine), with the performance being dependent upon the polymer molecular weight and water solubility.

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platforms and large injection facilities and flow lines. This leads to high THI costs, especially offshore. Moreover, THIs can lead to other problematic situations, such as methanol toxicity, the need for THI regeneration facilities, downstream hydrocarbon pollution, enhanced scaling issues, etc. The other chemical approach to gas hydrate management in the upstream oil industry is the use of LDHIs. LDHIs are divided in two different categories, kinetic hydrate inhibitors (KHIs) and anti-agglomerants (AAs). KHIs delay hydrate formation, whereas AAs disperse hydrate particles and prevent them for forming plugs. All current commercial KHIs are based on specific amide-containing water-soluble polymers as the main active ingredient, and nearly all of them have limited biodegradability in seawater. Examples of commercial KHI polymers include polymers and copolymers of N-vinylcaprolactam (VCap), N-vinylpyrrolidone (VP), N-isopropylmethacrylamide, and hyperbranched poly(ester amide)s.2,17,18 Polymers with 6 and 8 rings, poly(N-vinylpiperidone) (PVPip) and poly(N-vinylazacyclooctanone) (PVACO), have also recently been investigated, with the 8-ring polymer showing superior KHI performance compared to the smaller rings.19−21 These polymers are shown in Figure 1. Understanding of the structure−activity relationship (SAR) is a key factor in the development of novel KHIs. This is of the upmost importance, and for that, a brief explanation of the major trends in KHIs will be given. There are several important structural factors that a KHI appears to need to show good

INTRODUCTION Gas hydrates and their crystal structure characteristics, nucleation, and crystal growth are very well-described in the literature.1−11 Gas hydrates and their plugging of conduits is a major problem in the oil and gas industry, including offshore and onshore production plants. The severity of the gas hydrate problem depends upon many factors but especially the temperature profile within the pressure−temperature gas hydrate stable region. There are two different overall methods for gas hydrate control. The first is control of the physical properties. This method can be achieved by keeping the pressure low or controlling the temperature in the pipeline through insulation or heating methods. Alternatively, the gas phase can be modified with another gas, e.g., nitrogen or carbon dioxide, to bring the system out of the hydrate-stable region. In the last few decades, there has been a paradigm change in flow assurance control, from gas hydrate prevention to more risk management control methods.12 For example, it is well-known from several studies that some crude oils present non-plugging effects, although very little is known about the inherent characteristics of the oil that produces non-plugging effects. A plugging index is sometimes referenced, where certain crude oils in accordance with their emulsifying properties would produce a non-plugging effect.13−16 Another method that is currently under development has been termed “cold flow”. This method is proposed to produce non-agglomerating, small gas hydrate particles that can be transported along pipelines in the carrier fluids.17 An important method widely used today is chemical control, which consists of the use of gas hydrate inhibitor chemicals. This can be achieved with thermodynamic inhibitors (THIs) or low-dosage hydrate inhibitors (LDHIs).1,2 Thermodynamic inhibitors shift the thermodynamic properties, lowering the temperature at which hydrates will form. The most potent thermodynamic inhibitor is methanol. Ethanol and various glycols, such as monoethylene glycol, diethylene glycol, and triethylene glycol, are also used. Nevertheless, large amounts of THIs are required (up to one barrel of THI for barrel of water produced), meaning big storage places are needed on the © XXXX American Chemical Society

Figure 1. From left to right: structures of poly(N-vinylpyrrolidone) (PVP), poly(N-vinylpiperidone) (PVPip), poly(N-vinylcaprolactam) (PVCap), and poly(N-vinylazacyclooctanone) (PVACO). Received: August 8, 2014 Revised: October 10, 2014

A

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and growth in fish, insects, and other organisms. Although some of these proteins have been shown to have good activity as KHIs, the research in this area has been limited by the small amount of proteins that can be isolated. Polyaspartamides are closely related polymers to primary protein structures and have been investigated as KHIs (Figure 3).27,28 These polymers can be considered as polypeptides and have shown good performance as KHIs; studies were carried out with different molecular weight polymers. The polyaspartamide backbone consists of a ratio of seven units of 4alkyl-amide-4-amino acid residue and three units of 5-alkylamide-5-oxopentanoic residue. Polyaspartamides contain alkylamide functional groups in their structure that are pendant to the backbone. These functional groups are those that give the activity as KHIs, but the backbone gives the polymers good biodegradability. We have also investigated a series of polymers of pseudo-peptides, ring-opened poly(alkyloxazolines) as well as polymers of ring-closed alkylated isopropenyloxazoline.29,30 To continue developing the SAR study, we now report a KHI investigation on a novel class of polymers, poly(Nalkylglycine)s [or poly(α-peptoid) polymers] (Figure 3).31−34 These polymers differ from proteins only in the position where functional groups are attached to the polymer backbone. In poly(N-alkylglycine)s, the functional groups are attached directly to a nitrogen next to the α carbon in the backbone, instead of the carbon atom in common amino acid residues.

activity (performance): the polymer needs functional groups that can form hydrogen bonding with water molecules as well as a hydrophobic component that causes affinity to gas hydrate cavities and/or perturbs the bulk water structure. Thus, functional groups that have shown some of the best performances are often amide-based because of good hydrogen-bonding, e.g., cyclic and acyclic amides, imides, carbonylpyrrolidine, and lactams already discussed. Examples of polymers with these groups are shown in Figure 2. Polymer

Figure 2. From left to right: structures of poly(N-alkyl(meth)acrylamide)s, poly(acryloylpyrrolidine)s, and poly(N-alkyl-Nvinylalkanamide)s.

molecular weight is important because low-molecular-weight polymers show better performance than high-molecular-weight polymers, often reaching a maximum KHI activity in the range of 1500−3000 Da.2 The cloud and deposition points of KHI polymers are also important because polymers with low cloud and deposition points drop out of solution if injected into hot well streams, causing their own plug (fouling) in the system. For certain polymer classes, it has been found that polymers with low cloud points often show better activity as KHIs. Nevertheless, this is not a general rule, because certain molecular interactions can lead to a higher kinetic inhibition even for polymers with high cloud points and, therefore, have to be considered with caution in a SAR study.18 Proteins and peptides have been investigated as alternative KHIs because they are expected to be readily biodegradable and, therefore, have low environmental impact (Figure 3).18 All

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EXPERIMENTAL SECTION

Synthesis and Details of Poly(N-alkylglycine)s Investigated. Poly(N-alkylglycine)s were synthesized by ring-opening polymerization of N-substituted N-carboxyanhydride monomers (NCAs). NCA Monomer Synthesis. Me-NCA, Et-NCA, Pr-NCA, iPr-NCA, Bu-NCA, and iBu-NCA were synthesized by adapting a reported procedure (Figure 4).31,32,35

Figure 5. General structure of synthesized (left) poly(N-alkylglycine) homopolymers and (right) random copolymers.

Figure 3. From left to right: general polypeptide structure, structure of polyaspartamides, and general structure of poly(N-alkylglycine)s.

Synthesis of Poly(N-alkylglycine) Homopolymers (Figures 5−8 and Table 1). Polymerization of Et-NCA or Pr-NCA with Benzylamine (BnNH2). Inside a glovebox, R-NCA (Et-NCA or Pr-NCA) was dissolved in tetrahydrofuran (THF) and a predetermined amount of BnNH2/THF stock solution was added by syringe. The reaction mixture was stirred at 50 °C overnight under a nitrogen atmosphere and quenched by precipitating the polymers in an excess of hexane.

proteins are based on amino acid residues covalently bound through peptide bonds, forming polypeptides. Some work has been carried out to find more biodegradable KHIs using antifreeze proteins (AFPs), also known as ice-structuring proteins (ISPs).22−26 These proteins can prevent ice nucleation

Figure 4. NCA monomer synthesis. B

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Figure 6. 1H NMR spectrum of poly(N-methylglycine) in CD3OD.

Table 1. Summary of Poly(N-alkylglycine) Homopolymer Syntheses monomer

[M]0/[BnNH2]0

conversion (%)a

DPb

Mn (kg mol−1)c

Me-NCA Et-NCA iPr-NCA Pr-NCA Pr-NCA Pr-NCA

20:1 20:1

100 100 100 100 100 100

16 20 30 16 22 75

1.2 1.9 3.0 1.7 2.2 7.5

15:1 25:1 100:1

a

Conversion is calculated from the 1H NMR spectrum of an aliquot of reaction mixtures. bDegree of polymerization (DP) is calculated from the integration ratio of the polymer backbone to the end group from the 1H NMR spectrum. cMn is calculated from DP and mass of repeating unit. mixture was stirred at room temperature overnight under a nitrogen atmosphere and quenched by precipitating the polymers in an excess of THF. The white polymers were isolated by centrifugation and decantation and dried under vacuum. Polymerization of iPr-NCA under Melting Conditions. iPr-NCA was stirred at 150 °C overnight under a nitrogen atmosphere. The resulting solid was dissolved in CH2Cl2, and polymer was precipitated out by adding excess hexane. The polymer product was isolated by centrifugation and decantation and dried under vacuum. NMR spectroscopic data have been reported.34 Synthesis of Poly(N-alkylglycine) Random Copolymers (Figures 5, 9, and 10 and Table 2). Inside a glovebox, Et-NCA and Bu-NCA (or iBu-NCA) was dissolved in THF and a predetermined amount of BnNH2/THF stock solution was added by syringe. The reaction mixture was stirred at 50 °C overnight under a nitrogen atmosphere and quenched by precipitating the polymers in an excess of hexane. The white polymers were isolated by centrifugation and decantation and dried under vacuum. Figure 11 shows the individual chemical structures of homopolymers, and Figure 12 shows the chemical structures of copolymers that have been evaluated as KHIs. Highlighted in blue is the active center of the polymer. Table 3 gives the names and acronyms for the polymers investigated in this study, and from now, we will refer to the polymers by their acronyms. High-Pressure Gas Hydrate Rocker Rig Equipment and Test Methods. In this study, we have performed a series of constant cooling experiments to evaluate a novel series of polymers and copolymers as KHIs. We carried out all tests in high-pressure stainless-

Figure 7. 1H NMR spectrum of poly(N-ethylglycine) in CD2Cl2.

Figure 8. 1H NMR spectrum of poly(N-propylglycine) in CD2Cl2. The white polymers were isolated by centrifugation and decantation and dried under vacuum. Polymerization of Me-NCA with BnNH2. Inside a glovebox, MeNCA was dissolved in acetonitrile and a predetermined amount of BnNH2/THF stock solution was added by syringe. The reaction C

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Figure 11. From top left to top right: poly(N-methylglycine) (PNMG16), poly(N-ethylglycine) (PNEG20), and poly(N-propylglycine) (PNPG22). From bottom left to bottom right: poly(Nisopropylglicine) (PNiPGn), poly(N-propylglycine) (PNPG16), and poly(N-propylglycine) (PNPG75). Figure 9. 1H NMR spectrum of poly(N-ethylglycine)-ran-poly(Nbutylglycine) (PNEG-r-PNBG) in CD2Cl2.

Figure 12. From left to right: poly(N-ethylglycine)-ran-poly(Nbutylglycine) (PNEG16-r-PNBG6) and poly(N-ethylglycine)-ran-poly(N-isobutylglycine) (PNEG17-r-PNiBG5).

Table 3. Names and Acronyms of Polymers Tested as KHIs poly(N-methylglycine) poly(N-ethylglycine) poly(N-isopropylglycine) poly(N-propylglycine) poly(N-propylglycine) poly(N-propylglycine) poly(N-ethylglycine)-ran-poly(N-butylglycine) poly(N-ethylglycine)-ran-poly(N-isobutylglycine)

PNMG16 PNEG20 PNiPGn PNPG16 PNPG22 PNPG75 PNEG16-r-PNBG6 PNEG17-r-PNiBG5

Figure 10. 1H NMR spectrum of poly(N-ethylglycine)-ran-poly(Nisobutylglycine) (PNEG-r-PNiBG) in CD2Cl2. steel 40 mL rocking cells. All cells contain a stainless-steel rocking ball to simulate the turbulent regiment. The supplier of the RC5 rocking equipment is PSL Systemtechnikk, Germany, which is shown in Figure 13. For projects in flow assurance, the most common gas hydrate structure formed is structure II (SII). To reproduce the pipeline conditions for SII, we used a mixture of synthetic natural gas (SNG) in all experiments. The composition of the synthetic natural gas is described in Table 4. This gas mixture represents a similar composition to those found in the North Sea. We tested a series of poly(N-alkylglycine) polymers following a constant cooling procedure, which has been previously reported.20,21,30 To avoid precipitation of some polymers that have low cloud points, we started cooling from 17 °C instead of 20.5 °C. The temperature of 17 °C is approximately 2.5 °C below the equilibrium temperature, reported previously.30,36,37 In every experiment, we filled all five cells with 20 mL of a polymer solution in deionized water at a concentration of 2500 ppm. In a standard experimental procedure in

Figure 13. Rocker rig showing the five stainless-steel cells, temperature sensors, and pressure sensors in a temperature-controlled bath.

Table 2. Summary of Poly(N-alkylglycine) Random Copolymer Syntheses

a b

copolymera

M1

M2

[M1]0/[M2]0/[BnNH2]0

conversion (%)b

Mn (kg mol−1)c

PNEG−PNBG PNEG−PNiBG

Et-NCA Et-NCA

Bu-NCA iBu-NCA

14:6:1 20:5:1

100 100

2.1 2.1

Degree of polymerization is calculated from the integration ratio of the polymer backbone to the end group from the 1H NMR spectrum. Conversion is calculated from the 1H NMR spectrum of an aliquot of reaction mixtures. cMn is calculated from DP and mass of repeating unit. D

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Homopolymer PNPG16 was initially difficult to dissolve at room temperature, even after the sample was stirred for 24 h at 4 °C, followed by sonication for several hours. However, when this procedure was repeated several times, a cloudy solution with no precipitate was achieved. Homopolymer PNPG22 was fully dissolved below its Tcl of approximately 31 °C. The higher molecular weight homopolymer PNPG75 was shown to be almost impossible to dissolve in deionized water by the same treatment. To perform the gas hydrate inhibition tests, PNPG75 was first dissolved in a minimum amount of methanol as possible, then deionized water was added to the mixture, and methanol was removed from the mixture under reduced pressure. After treatment via reduced pressure, the remaining methanol in solution was at a trace level; therefore, its thermodynamic effect was negligible. Then, the polymer was stirred in deionized water at 4 °C for 24 h, followed by sonication for several hours. This procedure was repeated several times until a mildly cloudy solution was obtained and no suspended particles were observed. High-Pressure KHI Experiments. We tested a homologous series of several poly(N-alkylglycine)s to evaluate their potential as KHIs. The KHI testing was carried out using pure polymers in deionized water and a SII-forming natural gas blend. For the SAR evaluation, we performed all comparative tests at 2500 ppm concentration. For further comparative purposes, we tested known KHIs, poly(N-vinylpyrrolidone) (PVP 15k, Mn 4000) and poly(N-vinylcaprolactam) (PVCap, Mn 2600), obtained from BASF with no solvents. Tests with no polymer were also carried out. In addition, we chose PNEG-rPNiBG to perform a series of experiments at different concentrations to confirm the increase in performance with higher polymer concentrations. The gas hydrate formation process is stochastic. Therefore, we tested all samples 10 times, so that the performance ranking of results can be statistically representative. Table 6 shows the summarized results for poly(N-alkylglycine)s and all other polymers, giving the average of 10 test results for each sample. The onset temperature To and fastest crystal growth temperature Ta are given. To is more important for flow assurance applications; nevertheless, the

Table 4. Composition of SNG component

mol %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

a constant cooling experiment, we cooled the system with a temperature ramp of 1 °C/h with a constant rocking (20 rocks/ min) with a maximum rocking angle of 40 °C. A representative graph of the data obtained is shown in Figure 14 (all cells) and Figure 15 (one cell), where the temperature and pressure decrease until the minimum temperature of 2 °C was reached. The first temperature at which there is a deviation in the pressure that is not due to cooling the fluids in a closed system corresponds to the onset temperature called To; this temperature is the first indication of the macroscopic gas hydrate formation. After that, the temperature Ta is recorded. This is the temperature at which hydrates show the fastest crystal growth (most rapid pressure drop), from the autocatalytic and exothermic process. The exothermic behavior is sometimes possible to detect via a slight increase in the temperature. The evaluation of the onset temperature and rapid crystal growth is shown in Figure 15.

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RESULTS AND DISCUSSION Polymer Cloud Points and Water Solubility. For KHIs, the polymer cloud point (Tcl) is important because polymers with very low cloud points (and therefore usually also deposition points) could potentially precipitate in hot sections of the pipeline, leading to a blockage. Therefore, it is important to measure the polymers Tcl value. Tcl values are measured by slowly warming a 1 wt % solution in water until cloudiness is seen because of the polymer phase change. The Tcl values along with the molecular weight of each polymer tested are given in Table 5. This series of polymers showed a typical cloud point trend as the size and shape of the hydrophobic groups in the side groups were varied. Thus, the increase in size of the hydrophobic alkyl groups causes a lowering of the Tcl value.

Figure 14. Pressure and temperature graph in a constant cooling test, showing temperature and pressure conditions in five rocking cells. E

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Figure 15. Evaluation of the onset temperature and fast hydrate crystal growth in a constant cooling rocker rig KHI experiment.

give ideal interaction with the clathrate clusters and crystals nor will they give much perturbation of the water structure preventing hydrate nucleation. However, we noticed that as the alkyl chain increases up to optimal size (while keeping water solubility), the KHI performance increased, i.e., To decreased. This is reflected by the fact that PNiPG (To = 8.0 °C, and Tcl = 49 °C) has better activity than PNMG and PNEG and similar KHI performance results to those of the copolymers PNE16G-r-PNBG6 (To = 8.4 °C, and Tcl = 46 °C) and PNEG17r-PNiBG5 (To = 8.1 °C, and Tcl = 66 °C) with identical molecular weights. It has been suggested that a low cloud point for a polymer produces better kinetic inhibition, but this is not the case here for these two copolymers. It is possible that isobutyl groups fit better in the labile clathrate clusters rather than n-butyl groups. Moreover, the two copolymers tested have lower molecular weight than PNiPG that could give better KHI performance. However, these copolymers contain a random ethyl/butyl distribution of 16:6 for PNEG16-r-PNBG6 and ethyl/isobutyl distribution of 17:5 for PNEG17-r-PNiBG5 that apparently leads to a lower KHI activity, possibly because the majority of alkyl groups are ethyl groups, which have lower affinity to labile clathrate clusters than butyl and isobutyl functional groups. Copolymers with higher molar percentages of isobutyl or n-butyl groups were not water-soluble. The three PNPG homopolymers gave very different performances depending upon not only molecular weight but also water solubility. The best result was given by PNPG22 (Mn of 2.2 kg/mol) with an average To value of 4.7 °C. This was the best performing KHI of all polymers in this study. The molecular weight of this polymer is in a typical range where good performance would be expected. Too high or too low molecular weight represented by PNPG16 (Mn of 1.7 kg/mol) and PNPG75 (Mn of 7.5 kg/mol) gave worse results. The poor water solubility of PNPG16 and PNPG75 (they gave cloudy solutions at best) may lead to reduced KHI activity because part of the polymer might be in equilibrium with the extended ternary structure and the collapsed structure, whereas the collapsed structure would have active centers that can neither interact with hydrate structures or disrupt the bulk water structure, leading to poor activity as a KHI. According to the KHI theory, increasing the KHI concentration within a certain range should lead to better

Table 5. Poly(N-alkylglycine) Molecular Weight and Temperature Cloud Point poly(N-alkylglycine)

Mn (kg/mol)

Tcl (°C)

PNMG16 PNEG20 PNiPGn PNPG16 PNPG22 PNPG75 PNEG16-r-PNBG6 PNEG17-r-PNiBG5

1.2 1.9 3 1.7 2.2 7.5 2.1 2.1

>100 >100 49 31 46 66

Table 6. Average Onset Temperature and Average Crystal Rapid Growth Temperature for Poly(N-alkylglycine)s Tested as Gas Hydrate Kinetic Inhibitors chemical

average To (°C)

average Ta (°C)

no additive PVP 15k PVCap Mn = 2600 PNMG16 PNEG20 PNiPGn PNPG16 PNPG22 PNPG75 PNEG16-r-PNBG6 PNEG17-r-PNiBG5

17.1 11.6 10.1 16.6 12.6 8.0 6.0 4.7 9.2 8.4 8.1

17.0 11.0 9.6 16.3 10.5 7.3 5.3 3.8 9.0 8.0 7.6

difference between To and Ta is useful to analyze the activity as a hydrate crystal growth inhibitor. To analyze the performance as KHIs, we have compared results of the onset temperature of deionized water to no inhibitors, where the To value is 17.1 °C and the Ta value is 17.0 °C. These temperatures were obtained following the same cooling rate but starting from 20.5 °C.20,21 With the exception of PNMG and PNEG, all of the poly(Nalkylglycine)s tested showed a better KHI performance at 2500 ppm than the commercial products PVP and PVCap. For PNMG and PNEG, it is reasonable to assume that the small hydrophobic methyl and ethyl groups are not large enough to F

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Figure 16. Average onset temperatures (To) and rapid hydrate crystal growth temperatures (Ta) for PNEG17-r-PNiBG5 in a range of concentrations.

molecular weight of 2.2 kg/mol. The performance of this polymer was better than its own analogues of higher and lower molecular weight, which may also be due to part of the polymer being in the ternary collapsed structure. The number and size of the functional groups directly affect the inhibition performance because PNPG22 is significantly better than the two copolymers tested, PNEG16-r-PNBG6 and PNEG17-r-PNiBG5. Despite the copolymers containing larger carbon chains (isobutyl and n-butyl), these functional groups are in minor quantity compared to the “all-propyl” group polymer, PNPG22. These KHI results have given sufficient encouragement for us to develop new modifications within this class of pseudoproteins that will lead to even better performance as poly(Nalkylglycine) KHIs.

kinetic inhibition. To confirm this, we chose one of the copolymers PNEG17-r-PNiBG5 and investigated its KHI performance at a range of concentrations. We did not use the best polymer PNPG22 because the good performance at 2500 ppm may have led to too low onset temperatures at higher concentrations; therefore, the trend could be difficult to see. The graph in Figure 16 and Table 7 show the trend in kinetic Table 7. Average Onset Temperatures (To) and Rapid Hydrate Crystal Growth Temperatures (Ta) for PNEG17-rPNiBG5 in a Range of Concentrations concentration (ppm)

average To (°C)

average Ta (°C)

1000 2500 5000 7000

11.6 8.1 5.5 4.3

11.1 7.6 4.1 2.2

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

hydrate inhibition when the PNEG17-r-PNiBG5 copolymer concentration increases from 1000 to 7000 ppm using the same ramping method discussed earlier. The graph indicates a typical increase in KHI performance as the concentration increases within the range, with the increase beginning to flatten out toward the highest concentration of 7000 ppm.

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2009. (2) Kelland, M. A. Production Chemicals for the Oil and Gas Industry, 2nd ed.; CRC Press: Boca Raton, FL, 2014; Chapter 9. (3) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. J. Pet. Sci. Eng. 2007, 56, 14−31. (4) Makogon, Y. F. J. Nat. Gas Sci. Eng. 2010, 2, 49−59. (5) Demirbas, A. Energy Convers. Manage. 2010, 51, 1547−1561. (6) Sloan, E. D., Jr. Energy Fuels 1998, 12, 191−196. (7) Ripmeester, J. A.; Tse, J. A.; Ratcliffe, C. I.; Powell, B. M. Nature 1987, 325, 135−137. (8) Long, J. P.; Sloan, E. D. Int. J. Thermophys. 1996, 17 (1), 1−13. (9) Kasper, K.; Østergaard, B. T.; Burgass, R. W.; Danesh, A.; Todd, A. C. J. Chem. Eng. Data 2001, 46, 703−708.

CONCLUSION A series of poly(N-alkylglycine)s has been synthesized and tested for gas hydrate kinetic inhibition for the first time using a natural gas mixture to promote SII hydrates (mixture represents similar conditions to the North Sea gas production) in a high-pressure multi-cell rocking rig system. A qualitative analysis in SAR has been carried out, showing that certain modifications to the alkyl carbon chain attached to the polymer (as methyl, ethyl, propyl, isopropyl, n-butyl, and isobutyl) increase kinetic hydrate inhibition as long as water solubility is maintained. The best performance was given by PNPG22 with a G

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dx.doi.org/10.1021/ef501779p | Energy Fuels XXXX, XXX, XXX−XXX