Optimizing the Kinetic Hydrate Inhibition Performance of N-Alkyl-N

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Optimizing the Kinetic Hydrate Inhibition Performance of N-Alkyl-N-vinylamide Copolymers Qian Zhang, Ryo Kawatani, Hiroharu Ajiro, and Malcolm A. Kelland Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00251 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Optimizing the Kinetic Hydrate Inhibition Performance of NAlkyl-N-vinylamide Copolymers Qian Zhang,1 Ryo Kawatani,2 Hiroharu Ajiro,2 Malcolm A. Kelland1* 1

Department of Chemistry, Bioscience and Environmental Engineering, Faculty of Science

and Technology, University of Stavanger, N-4036 Stavanger, Norway 2

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5

Takayama-cho, Ikoma, Nara 630-0192 * Corresponding author: Tel.: +47 51831823; fax +47 51831750 E-mail address: [email protected] (Malcolm Kelland)

Keywords: petroleum, gas hydrates, kinetic hydrate inhibitors, polymers, polyvinylamide

Abstract The generation of gas hydrates in gas and multiphase flowlines can cause blockages, leading to downtime, economic losses and even potential accidents. Injecting kinetic hydrate inhibitors (KHIs) is an effective way to prevent gas hydrate formation. Most KHI formulations are built around water-soluble polymers contain amides groups. Based on past work on N-alkyl-N-vinylamide polymers from our groups, we have now been able to get much closer to designing the optimum KHI for this class of polymers. In this study we have synthesized four N-alkyl-N-vinylamide monomers where the alkyl group is n-propyl, iso-

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propyl, n-butyl and iso-butyl. These have been copolymerised successfully with N-methyl-Nvinylacetamide and N-vinylformamide to form a series of copolymers with low molecular weights. We have investigated their KHI performance by using a slow constant cooling method with synthetic natural gas mixture in high-pressure rocker cells at 76 bar. All of the new N-vinylamide copolymers shown good KHI performance. The average onset temperature of the best copolymer, N-vinylformamide: N-isobutyl-N-vinylformamide copolymer, at 2500 ppm concentration in deionized water was 8.2 oC. This decreased considerably to 4.7 oC (ca. 15.3

o

C subcooling) when 10000 ppm of n-butyl glycol ether solvent was added,

demonstrating good synergy between the polymer and solvent. Two of the best copolymers were further investigated at varying concentrations in the range of 1000-7500 ppm, and showed increased performance as the dosage increased.

INTRODUCTION Under low temperature and high pressure conditions, gas hydrates tend to form and even cause flow assurance problems in the process of gas and oil production and transportation. Gas hydrates are ice-like non-metrological compounds, whose main constituents are gas and water. From the gas hydrates microstructure point of view, their periphery are water "cages" formed by the connection of van der Waals forces, and light gases such as methane, ethane, nitrogen, carbon dioxide and so on are trapped in these water cages.1 In order to prevent the formation of gas hydrate during gas and oil operation and transportation, different inhibition methods have been proposed.2-3 Using hydrate inhibitors especially low-dosage hydrate inhibitors (LDHIs) is one much more economic and efficient option compared with other gas hydrate management methods. Kinetic hydrate inhibitor (KHI) is one of the two main branches of LDHIs, which has been widely used in this gas


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hydrates inhibition field since the early 1990s. In hydrate-stable temperature-pressure phase equilibrium area, KHIs can kinetically delay hydrate nucleation and growth, thus guarantee the oil and gas in pipeline flow smoothly before reaching destination. Usually, synergists are added to improve the performance of KHIs.4 Almost all of the effective KHIs are water-soluble polymers that contain certain repeating amide groups, such as poly(N-vinylpyrrolidone) (PVP), poly(N-vinylcaprolactam) (PVCap), poly(N-isopropylmethacrylamide), and so forth.5-6 However, at present, there is no unanimous conclusion about the mechanism of hydrate inhibition.7-8 Different research teams have proposed different suppressive mechanism hypotheses. One convincing theory is the adsorption mechanism. This theory suggests that kinetic inhibitors can adsorb on the surface of hydrate particles by hydrogen bonds, thereby limiting the hydrate nuclei and crystal growth.9-12 Another generally believed mechanism is the perturbation theory: Hydrophobic groups present in kinetic inhibitors are able to intercalate into water structures, disrupt hydrate particles so that hydrate nuclei cannot grow to the critical size for further crystal growth.13-16 Previous studies show that poly(N-alkyl-N-vinylacetamides) derivatives have good inhibition effect for gas hydrates, especially when the N-alkyl is large isobutyl group and isopentyl group.17 However, maybe due to the steric hindrance from the methyl group on the carbon moiety of the amide group and/or the N-alkyl group, none of isopropyl or n-butyl monomer derivatives

could

be

homopolymerized

or copolymerized

vinylacetamides).


with

poly(N-methyl-N-

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Figure 1. Structure of N-methyl- N-vinylacetamide: N-alkyl- N-vinylacetamide copolymers. R1 = methyl, isobutyl, or isopentyl.

In a later study, a series of poly(N-vinylalkanamide)s were synthesized, in which the alkyl groups on the carbonyl group were expanded from methyl to ethyl, propyl, and isobutyl, whereas there is no N-alkyl groups, compared with poly(N-alkyl-N-vinylacetamides) derivatives.18 Thus, monomers with pendant isopropyl and n-propyl could polymerize probably due to the less steric hindrance of NH group. Data show that the polymers containing 3 carbon atoms in the main chain of the pendant alkyl groups, such as n-propyl, isobutyl, perform the best as KHIs.

Figure 2. Structure of poly(N-vinylalkanamide)s. R` = methyl, ethyl, n-propyl, isopropyl, or isobutyl.


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Afterwards, poly(N-(n-propyl)-N-vinylformamide) and poly(N-isopropyl-N-vinylformamide) were successful synthesized, and the polymer with n-propyl group was shown to give a superior KHI performance to that with isopropyl group.19 As both of N-(n-propyl)-Nvinylformamide and N-isopropyl- N-vinylformamide homopolymers have a quite low cloud points, which is not good for industry usage, so each of their copolymers with Nvinylformamide were synthesized. However, the addition of N-vinylformamide monomers increase the cloud point of polymers, but also decrease their inhibition performance. In addition, all of the mentioned above polymers with isobutyl groups are copolymers, because although some of these homopolymers with isobutyl groups were obtained, they were not water-soluble due to the large hydrophobic alkyl.

n N

O

N

O

m

H

H

H

Figure 3. Structure of N-vinylformamide: N-n-propyl-N-vinylformamide copolymers (right) and N-vinylformamide: N-(isopropyl)-N-vinylformamide copolymers (left).

In

this

study,

we

synthesized

N-methyl-N-vinylacetamide

with

N-(n-propyl)-N-

vinylformamide copolymers, as well as N-methyl-N-vinylacetamide with N-isopropyl-N-


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vinylformamide copolymers to investigate whether N-methyl- N-vinylacetamide would affect the inhibition performance of polymers or not, when it improves the solubility and cloud point of the polymers. In addition, by synthesizing the copolymers with N-vinylformamide we further investigated the potential of poly(N-alkyl-N-vinylformamides) derivatives with isobutyl groups, as well as compared them to these with n-butyl groups. The synergistic effect with different synergists was also reported in this study.

(a)

(b)

(c)

(d)

Figure

4.

Structure

of

N-methyl-N-vinylacetamide:

N-isopropyl-N-vinylformamide

copolymers (a), N-methyl-N-inylacetamide: N-n-propyl-N-vinylformamide copolymers (b), N-vinylformamide: N-isobutyl-N-vinylformamide copolymers (c), N-vinylformamide: N-nbutyl-N-vinylformamide copolymers (d).


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EXPERIMENTAL SECTION Materials PVP K15 (poly(N-vinylpyrrolidone), Mw = 8-9000 g/mole) and Luvicap 55W (a low molecular weight N-vinylcaprolactam: N-vinyl pyrrolidone copolymer, 53.8 wt.% in water) were obtained from Ashland and BASF respectively. N-Methyl-N-vinylacetamide (Merck) was purified from CaH2 through vacuum distillation. N-vinylformamide (Merck) was distilled under vacuum at 70 oC. Anhydrous N,N-dimethylformamide (Merck) was used without further purification. 1-Bromopropane, 2-bromopropane, 1-bromo-2-methylpropane, 1-bromobutane, sodium hydride in oil, 60% and 2, 2`-azobisisobutyronitrile were obtained from Tokyo Chemical Industry Co. Ltd., Japan, and were used without further purification. Deionized water was made in our laboratory, with a resistivity of 12 MΩ·cm.

Measurements Proton nuclear magnetic resonance (1H NMR) spectrograms were determined by a JEOL JNM-GSX400 system. The Mn and polydispersity (PDI) values of copolymers were measured by an A JASCO Chem NAV size-exclusion chromatography (SEC) system, equipped with PU-2080, AS-2055, CO-2065, RI-2031 and two commercial columns (TSKgel SuperH4000 and TSKgel GMHXL), at 40 oC and the eluent was DMF.

Synthesis of N-n-butyl-N-vinylformamide


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All N-alkyl-N-vinylformamide monomers, such as N-isopropyl-N-vinylformamide, N-npropyl-N-vinylformamide, N-isobutyl-N-vinylformamide and N-n-butyl-N-vinylformamide, were synthesized by nucleophilic substitution reaction by using NVF and the corresponding alkylbromides. The synthesis of the first two monomers has been reported previously.19 The butylated monomers were made in a similar way. Here, we take the synthesis procedure of Nn-butyl-N-vinylformamide monomer as an example to briefly describe the nucleophilic substitution reaction process used in this study: Firstly, NVF solution in DMF was prepared in a glass flask by combining Activated molecular sieve 4A, anhydrous DMF (28 mL) and NVF (15.5 mL, 205 mmol). Secondly, in a three-necked glass flask, 60% NaH in oil (9.0 g, 225 mmol) was introduced under nitrogen and washed with anhydrous tetrahydrofuran (THF) for 3 times. After 20 ml anhydrous DMF was placed, the temperature was set to 0 °C, and then NVF solution in DMF was slowly added. After that, 1-bromobutane (25 g, 183 mmol) was introduced. Then, the mixture was heated to 40 °C for over 12 h. The product was washed with NaCl solution and then was dried over anhydrous MgSO4. Finally, the pure liquid N-n-butyl-N-vinylformamide monomer was obtained in 87.6% yield through silica gel column chromatography with hexane/ ethyl acetate (2/1) as eluent. The full synthesis details of the monomers and all their 1H NMR spectroscopic data are given in the supplementary information.

Co-polymerization All of the copolymers were synthesized by using typical radical polymerization method as introduced in our previous studies.17, 19-20 Here, we take the synthesising procedure of poly (NVF-co-nBNF) as an example briefly introduce the co-polymerization process used in this study: To a 25 mL glass flask, AIBN (72.4 mg, 0.442 mmol) was placed. Then, NVF (0.97


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mL, 14.1 mmol), N-n-butyl-N-vinylformamide (7.16 mL, 56.4 mmol) and 2 M toluene were introduced under nitrogen. The mixture was heated to 60 °C, then kept for 24 h. After 24 h, the reaction mixture was poured into a large amount of acetone to precipitate the polymer. The analytical data of co-polymerization were listed in Table 1. Although the feed ratio of all monomers was 20:80, the obtained monomer incorporation rates were not the same, which might be due to steric hindrance during polymerization.

Figure 5. Synthesis of N-vinylformamide: N-n-butyl-N-vinylformamide copolymers.

Table 1. Analytical data of new copolymers in this study.a Monomer in Yield Monomer Sample M1 M2 feed

(%)

copolymer

in Mne

PDIe

RK2-033

NVF

nBuNVF

20: 80

18b

75:25

4200

3.38

RK1-134

NVF

nBuNVF

20: 80

26c

81:19

4500

3.44

RK1-120

NVF

nBuNVF

20: 80

19c

67:33

15400

3.02

RK1-135

NVF

iBuNVF

20: 80

22c

51:49

7400

3.13

RK2-034

NVF

iBuNVF

20: 80

14b

74:26

9700

3.35

RK1-121

NVF

iBuNVF

20: 80

16c

58:42

10500

3.58

RK2-035

MNVA

nPrNVF

20: 80

27b

25:75

2800

2.20

RK1-125

MNVA

nPrNVF

20: 80

50d

56:44

6200

2.08

RK2-037

MNVA

iPrNVF

15: 85

15b

37:63

2000

1.86


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RK2-036

MNVA

iPrNVF

20: 80

17b

42:58

2000

2.08

RK1-136

MNVA

iPrNVF

20: 80

27d

28:72

2300

2.94

RK1-126

MNVA

iPrNVF

20: 80

30d

27:73

4500

1.94

a

Radical polymerization was achieved with AIBN in toluene at 60 °C at 2 M. bDiethyl ether-

insoluble part. cAcetone-insoluble part. dHexane-insoluble part. eDetermined by SEC with polystyrene standard in DMF. Cloud Point (Tcl) Measurement We normally used 5 mL of 2mg/mL aqueous solution of the sample for its transmittance determined by the JASCO V-550 ETC-505S and JASCO V-550 ETC-505T system with 500 nm as wavelength. The LCST, taken as the cloud point (Tcl) was estimated as the temperature at 50% transmittance of the sample.

High-Pressure Rocking Cell Tests with Synthetic Natural Gas (SNG) As in our previous studies on this class of polymer, the performance of these new KHI polymers was tested in rocking cell equipment supplied by PSL Systemtechnik, Germany.2125

Five separated high-pressure steel rocking cells can be held simultaneously in the

equipment. The volume of each cell is 40ml, and normally 20ml solution was loaded to test. A steel ball was placed in each corresponding cell for agitating. Synthetic natural gas (SNG) mixture, listed in Table 2, was used to generate gas hydrates. Theoretically, this SNG mixture forms structure II hydrate as the most thermodynamically stable phase, which also is the most common situation encountered in field applications.

Table 2. Composition of synthetic natural gas (SNG) mixture.


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Component methane ethane propane CO2 isobutane n-butane N2

mole % 80.67 10.20 4.90 1.84 1.53 0.76 0.10

The slow constant cooling method was used to test the performance of KHIs, as this method gives some measure of the lowest temperature that the inhibitor can withstand.6,

26

The

constant cooling method includes five main steps: (1) 20.0 mL of distilled water, in which the inhibitor was dissolved, was loaded in each of the five cells; (2) The air in the system was removed by a vacuum and then approximately 5 bar of SNG mixture was purged into the cells. The system was then depressurized and evacuated once more; (3) The five cells were pressurized up to approximately 76 bar at the temperature of 20.5 oC, then intake valves were closed so that the pressure in each cell was separated; (4) Temperature was set to decrease from 20.5 oC to 2 oC at a constant cooling rate of approximately 1 oC /h for cooling down the cells continuously. (5) Each cell is equipped with pressure and temperature sensors logging on a computer to collect the pressure-time and temperature-time data during the testing process. The hydrate dissociation temperature (HDT) at 76 bar was investigated in previous reports and found to be 20.2 ± 0.05 °C. This is in close agreement with the calculated HDT by Calsep’s PVTSim software.21 A typical graph generated from the high-pressure rocker rig tests with constant cooling testing method is shown in Figure 6, from which, the To value (onset temperature) and Ta value (rapid hydrate formation temperature) for each cell can be found.


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Figure 6. Typical graph of pressure and temperature versus time from all five rocking cells, using our standard constant cooling test procedure. This example is for 2500 ppm of RK1136.

In addition, a typical graph from a single rocking cell from a standard constant cooling test is shown in Figure 7. This figure shows temperature (T1) and pressure (P1) versus time for cell 1. The pressure drops a little in the beginning of the test due to the SNG is slightly soluble in water. Before the gas hydrate generated, the experimental pressure dropped continuously during cooling because it is a closed system. When the deviation from the linear part of the pressure turns up, the according temperature at that time is analysed to be the To value. This turning point of the pressure-time line indicates the first detectable formation of gas hydrates. The To value is the most important guideline for evaluating a KHI in field applications.19 The steepest point in the pressure-time graph indicates the corresponding temperature of Ta value


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at which the rapid hydrate formation occurs. In this graph, the To value is 9.7 oC, and To value is 8.9 oC.

Figure 7. Typical graph of pressure and temperature versus time from cell 1, showing determination of To and Ta values.

Due to the stochastic nature of gas hydrate formation and in order to obtain statistical significance for ranking the performance of the polymers, the average To and Ta values for each KHI are necessary to obtain from 8-10 standard constant cooling tests. No rocking cell gave consistently better or worse results than another. In other words, none of the cells produces any systematic errors.

RESULTS AND DISCUSSION


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Table 3 summarizes average To and Ta values (based on 8-10 constant cooling tests) for Nsubstituted N-vinylformamide or N-vinylacetamide copolymers at the concentration of 2500ppm in deionized water. Cloud points are also included in this table. For comparison, results of pure water and two kinds of commercialized KHIs, PVP K15 and Luvicap 55W (active copolymer concentration), are also listed.

Table 3. Cloud point and average To and Ta values for KHI polymers at 2500ppm concentration. Sample

Cloud point

Ta(av) (˚C)

To(av) - Ta(av)

16

15.8

(˚C) 0.2

To(av) (˚C)

(˚C) DIW PVP K15

＞95

13.9

10.3

3.6

Luvicap 55W

78

7.3

6.3

1.0

RK2-033

28

9.3

9

0.3

RK1-134

33

9.9

9.5

0.4

RK1-120

28

9.1

8.9

0.2

RK1-135

20

8.2

8

0.2

RK2-034

41

8.6

8.4

0.2

RK1-121

34

9.9

9.4

0.5

RK2-035

66

8.7

7.4

1.3

RK1-125

59

9.2

8.6

0.6

RK2-037

94

9.9

9

0.9

RK2-036

＞95

10.3

9.2

1.1

RK1-136

86

9.4

8.8

0.6


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RK1-126

83

10.5

9.4

1.1

The performances of all KHI polymers were significantly better than no additive. The commercial polymer PVP K15 has an average To value of 13.9 ˚C. Luvicap 55W, which has shown good inhibition performance for many industrial applications, has a To(av) value of 7.3 ˚C here. All new copolymers show good and quite similar KHI performance, with To(av) values only varying from 8.2-10.5 ˚C at 2500ppm. In copolymers composed of two identical monomers, a trend can be see that increasing the percentage of the active monomers (propylated or butylated monomers) improves the KHI performance of the copolymers. For NVF-co-nBuNVF polymers, when the nBuNVF monomers increase from 19% to 33%, the average To value drops from 9.9 to 9.1 ˚C. This is a statistically significant difference as p < 0.05 in a statistical t-test.27 Also statistically significant, the To(av) value decreases from 9.2 to 8.7 ˚C due to the 31% increase of the nPrNVF monomers in MNVA-co-nPrNVF polymers. However, for the NVF-co-iBuNVF copolymers, although the polymer with the highest percentage of iBuNVF monomers perform the best (RK1-135), the performance for RK1121, which contains almost as much percentage iBuNVF monomer (42%, compared to 49%) shows a little worse inhibition performance than both RK1-135 and RK2-034. However, the molecular weight of polymers is also a factor affecting the KHI performance.18, 24 Therefore, we suggest that the higher molecular weight (and P.D.I value) of RK1-121 is responsible for the lower KHI performance. This optimized molecular weight theory can also be applied to explain the slightly lower performance of RK1-126. This iso-propylated copolymer has almost twice the molecular weight of the other three iso-propylated copolymers. Further, the comparison of RK2-033 and RK2-034 with same NVF monomers as well as similar active monomers ratios of BuNVF shows that NVF-co-iBuNVF (To(av) = 8.6 ˚C, Ta = 8.4 ˚C) gives significantly better performance than NVF-co-nBuNVF (To(av) = 9.3 ˚C, Ta ACS Paragon Plus Environment

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= 9 ˚C). The higher molecular weight and cloud point of RK2-033 should probably influence the performance negatively compared to RK2-034. Therefore, a reasonable explanation for the improved performance of RK2-33 may lie with the shape of the pendant butyl groups, i.e. the iso-butyl group in the polymer gives better performance than the n-butyl group. It is possible because the optimum carbon backbone length and the branching of isobutyl groups have a greater interaction with the hydrate particles.24 In addition, the inhibiting performance of the copolymers with nPrNVF surpass those with iPrNVF, which is consistent with the conclusions obtained in our previous work.19 Possible reasons why this happens maybe as followings: One is that, the cloud point of the nPrNVF polymers is lower than that of the iPrNVF ones. The other is that, the hydrophobic n-propyl groups in the polymers cause greater water perturbation than the isopropyl groups, sticking further out into the bulk water phase. From our related 2015 paper, we know that among all the NVF-co-PrNVF polymers the best polymer was HA8-014A, which is the homopolymer poly(nPrNVF) (To(av) = 8.8 ˚C, Ta(av) = 7.8 ˚C, Tcl = 20 ˚C, Mn = 3000).19 What is interesting is that RK2-035 has about the same Mn value and gives about the same performance, yet it has a much higher cloud point, which is useful for field applications. This means that the proper percentage addition of MNVA monomers should not reduce the KHI performance of polymers but give the benefit of significantly increase their cloud and deposition points. This effect has been seen previously for N-methyl-N-vinyl acetamide: Nvinyl caprolactam copolymers where the first monomer is the same as MNVA but with an additional methyl group.28

Table 4. Summary of average To and Ta values for RK2-034 and RK2-035 at varying concentrations.


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Sample

Concentration

RK2-034

(ppm) 1000

RK2-035

Ta (°C)

To(av) - Ta(av)

11.4

11.2

(˚C) 0.2

2500

8.6

8.4

0.2

5000

7.5

7.1

0.3

7500

6.7

5.8

0.9

1000

10.7

10.2

0.5

2500

8.7

7.4

1.3

5000

7.8

5.3

2.5

7500

8.0

7.4

0.6

To (°C)

Figure 8. To and Ta values for RK2-034 versus concentration.


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Figure 9. To and Ta values for RK2-035 versus concentration.

Table 4 gives a summary of To(av) and Ta(av) values at 1000, 2500, 5000 and 7500ppm in deionized water for RK2-034 and RK2-035, which rank the second and third best inhibiting performance respectively. We did not use the best polymer RK1-135 because no further sample was available. The data for RK2-034 and RK2-035 are also summarized graphically in Figures 8 and 9 respectively. Table 4 and Figure 9 show that, in the concentration range from 1000 to 7500ppm, the higher the concentration, the lower the To(av) and Ta(av) values. This indicates that the KHI effect of RK2-034 becomes stronger as the concentration increases within this range. For RK2-035, there is an obvious increase trend in KHI performance as increasing the concentration from 1000 to 5000ppm. However, when at the concentration of 7500ppm, the highest concentration does not achieve the best inhibition effect. We repeated this test several times


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more to be sure of the conclusion and we obtained the same result. It would seem that the optimal concentration of RK2-035 for preventing gas hydrates generation is below 7500ppm under these test conditions. In Figure 9, for Ta values for RK2-035 at 2500 and 5000ppm, it looks like less repeating tests were done. However, tests at each concentration were repeated at least 8 times and the values overlapped on the graph.

Table 5. Summary of average To and Ta values for N-substituted-N-vinylformamide copolymers with synergists. Concentration Polymer + solvent

To(av) (°C)

Ta(av)

To(av) - Ta(av)

No additives

(ppm) 0

16.0

(°C) 15.8

(˚C) 0.2

n-BGE

10000

16.1

14.8

1.3

iBGE

10000

16.2

14.2

2.0

RK1-135

2500

8.2

8.0

0.2

RK1-135 + n-BGE

2500 + 10000

4.7

4.1

0.6

RK2-034

2500

8.6

8.4

0.2

RK2-034 + n-BGE

2500 + 10000

7.4

7.0

0.4

RK2-034 + 2-EE

2500 + 10000

9.4

9.2

0.2

RK1-125

2500

9.2

8.6

0.6

RK1-125 + n-BGE

2500 + 10000

8.5

8.0

0.5

RK1-125 + iBGE

2500 + 10000

7.5

5.7

1.8

RK2-035

2500

8.7

7.4

1.3

RK2-035 + n-BGE

2500 + 10000

9.0

8.9

0.1

RK2-035 + 2-EE

2500 + 10000

8.9

7.9

1.0

HA8-014A

2500

8.7

8.2

0.5

HA8-014A + n-BGE

2500 + 10000

7.7

4.4

3.3

HA8-014A + iBGE

2500 + 10000

6.9

4.1

2.8


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2-Ethoxyethanol (2-EE) has been reported to be good synergist solvent for polyacrylamides29, and mono-n-butyl glycol ether (n-BGE) is a well-known synergist solvent in KHI field.21 Therefore, we investigated the ability of 2-EE and n-BGE as solvent synergists for these own the best KHI performance copolymers of this series. Results show that 2-EE is not a synergist for both nPr or iBu copolymers. 2500 ppm RK2-035 with 10000 ppm 2-EE solution gives a To(av) of 8.9 °C, which is quite similar with the To(av) value of the 2500 ppm pure RK2-035 solution. 2500 ppm RK2-034 with 10000 ppm 2-EE solution gives a To(av) of 9.4 °C, which is even worse than the 2500 ppm RK2-034 solution without 2-EE solution. The addition of 10000 ppm n-BGE significantly decreased the To(av) value of the 2500 ppm RK1-135 solution from 8.2 to 4.7 °C, but only changed the 2500 ppm RK2-034 solution from 8.6 to 7.4 °C. The RK1-135 is same copolymer as RK2-034 but with somewhat lower Mn value. The synergistic improvement in performance by adding n-BGE is much greater for RK1-135 than for RK2-034 and is difficult to justify based on the lower Mn value alone. To explain the large increase in performance by adding n-BGE it may be related to the cloud point of the polymers. RK2-34 has Tcl = 41 °C and RK1-135 has Tcl = 20 °C. RK1-135 is cooled in the cold room to make the solution clear before adding to the rocking cells. RK2-34 does not need this procedure, the solution is clear at room temperature. When the RK1-135 solution in the cell is put in the air at room temperature and in the water bath at 20.5 °C it may become cloudy again, but you can’t see the solution in the steel cell. n-BGE appears to be a synergist for both polymers but maybe better for RK1-135 because it helps to resolubilise the RK1-135 cloudy solution which would otherwise not be fully soluble if no nBGE was used. RK1-135 has an iBu group and show good synergy with a straight chain n-BGE. Therefore, we wondered if there was better synergy by having one product with branched alkyl and one with straight chain. Thus, we investigated the reverse situation with straight chain alkyl (nPr)


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Energy & Fuels

in the polymer and branched alkyl (iBu) in mono-isobutyl glycol ether (isoBGE). So we chose RK1-125 which has nPr groups to inspect this theory. Results show that RK1-125 with iBGE does fit this theory. 2500 ppm RK1-125 with 10000 ppm iBGE solution gives a To(av) of 7.5 °C, with 1.7 °C lower than pure 2500 ppm RK1-125. This means that iBGE is a very good synergist solvent for RK1-125. However, RK1-125 is less good with nBGE. As the To value of 10000 ppm nBGE in DIW is similar with that of iBGE, so it implies that the different performance is made by the interaction between the polymer with different synergist solvents. No further polymer was available to check the theory in more detail with nPr polymer and iBGE or nBGE. So we went back to an older polymer that we had made in greater quantity HA8-014A.19 The KHI performance of the 2500 ppm HA8-014A with 10000 ppm iBGE solution (To(av) = 6.9°C) is better than the 2500 ppm HA8-014A with 10000 ppm iBGE solution (To(av) = 7.7 °C). This indicates that HA8-014A also fits this theory.

Conclusion

In this study, we have synthesized four kinds of N-alkyl-N-vinylamide copolymers, poly(MNVA-co-iPrNVF),

poly(MNVA-co-nPrNVF),

poly(NVF-co-iBuNVF),

and

poly(NVF-co-nBuNVF) and studied them using the high-pressure slow constant cooling KHI test method. We observed that all of the new copolymers give good KHI performance for a predominantly structure II hydrate-forming gas mixture. Copolymers with a high percentage of the active monomers (propylated or butylated monomers) in the copolymer, which usually leads to relatively low cloud points, generally gave the best performances. Lowering the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecular weight of the copolymers gave better gas hydrate kinetic inhibition. When at similar molecular weight and copolymer ratios, poly(MNVA-co-nPrNVF) performed better than poly (MNVA-co-iPrNVF) at 2500 ppm. Poly(NVF-co-iBuNVF) had better KHI performance than poly (NVF-co-nBuNVF) at 2500 ppm at similar molecular weight and copolymer ratios. A certain amount of MNVA in the copolymers significantly increase the cloud point but does not decrease the KHI performance, which is beneficial for oilfield applications where the fluids at the well head are hot. NVF monomer, rather than MNVA, in the copolymers give a greater increase in their solubility and make it possible to test the KHI performance of the polymers contain isobutyl and n-butyl groups. In addition, different concentrations from 1000 to 7500 ppm were tested for two copolymers with good KHI performance. One copolymer showed an increasingly good effect as the concentration increased, however the other had an optimum performance at a concentration of 5000 ppm. Poly(NVF-co-iBuNVF) had a strong synergistic effect with n-butyl glycol ether nBGE solvent. To(av) at 2500 ppm copolymer dropped from 8.2 oC to 4.7 oC (ca. 15.3 oC subcooling) when 10000 ppm of (nBGE) solvent was added. Polymers containing nPrNVF showed stronger synergistic effect with iBGE than nBGE. We suggest that this is because the product with branched alkyl group has better synergy with a second product containing a straight alkyl group. We believe we are close to optimizing the structure of the poly(N-alkyl-N-vinylamide) polymers. However, we will continue to investigate these and other copolymers in this class, such as polymers with larger alkyl groups, e.g. poly (NVF-co-PentylNVF). This is partly to check if the optimum pendant alkyl groups must be of a certain size to fit into open hydrate cavities, which can shed light on the types of KHI mechanisms operating.


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Energy & Fuels

Supporting Information The full synthesis details of the monomers and all their 1H NMR spectroscopic data can be found in the supplementary information. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work is partly supported by TEPCO memorial foundation.

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Optimizing the Kinetic Hydrate Inhibition Performance of N-Alkyl-N

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