Carbamate Polymers as Kinetic Hydrate Inhibitors - Energy & Fuels

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CARBAMATE POLYMERS AS KINETIC HYDRATE INHIBITORS Eirin Abrahamsen, and Malcolm A. Kelland Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01349 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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CARBAMATE POLYMERS AS KINETIC HYDRATE INHIBITORS Eirin Abrahamsen, *‡, Malcolm A. Kelland‡ ‡

Department of Mathematics and Natural Sciences, Faculty of Science and Technology,

University of Stavanger, N-4036 Stavanger, Norway. * Corresponding author: [email protected]

ABSTRACT

Formation of gas hydrates is a problem in the petroleum industry where the gas hydrates can cause blockage of the flowlines. Kinetic hydrate inhibitors (KHIs) are water-soluble polymers that are used to prevent gas hydrate blockages and they have been used in the field successfully. In this paper we present the first KHI performance results of a series of polymers containing pendant carbamate groups, poly(hydroxyl-N-alkylcarbamate)s. Similar polymers have been investigated as KHIs before, some of which has been commercialized. Hydroxyalkylcarbamates with varying alkyl pendant groups from methyl to iso-butyl are reported. It was found that increasing the pendant alkyl chain and branching gave increasing KHI performance, however the polymer also became significantly less soluble in water or had a very low cloud point temperature (TCl). Both solubility and TCl was slightly improved by copolymerization, we found that the copolymer with pendant iso-butyl- and methylcarbamate 2:1 and 3:1 gave the best results of average To = 8.5 °C and 8.4 °C respectively.

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A copolymer of 2.5:1 with pendant iso-butyl- and methylcarbamate was also investigated at concentrations ranging from 1000 ppm to 7000 ppm, where the increased polymer concentration showed increasing KHI performance.

INTRODUCTION When water is together with gas under certain pressure and temperature conditions, the water molecules can start to form cage-like structures that trap the gas molecules inside, causing a stable clathrate structure taking form as an ice-like solid. These natural gas hydrates are known to cause blockage of the flow in the upstream and midstream petroleum industry.1-3 Typical gas molecules include light hydrocarbons (methane, propane, iso-butane etc.) as well as carbon dioxide and hydrogen sulfide. Low dosage hydrate inhibitors (LDHIs) have been used successfully for hydrate control in the petroleum industry, in addition their use has been proven to give significant CAPEX and OPEX advantages compared to other hydrate control methods, for example the use of traditional thermodynamic inhibitors (THIs) such as methanol or MEG.1, 4, 5 Kinetic hydrate inhibitors (KHIs) falls under the LDHI category. They delay hydrate nucleation and/or crystal growth when the pressure and temperature conditions enter the hydrate stable region. The mechanisms for inhibition are not fully understood. However, there are some hypotheses suggested, such as perturbation of the water structure cause the inhibiting effect because of interference with the hydrogen bonds between the water molecules.6 Another hypothesis suggests that the KHI adsorbs to the hydrate surface thus inhibiting nucleation and 2 ACS Paragon Plus Environment

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growth for a period of time.7-9 It is also suggested that certain chemical groups like the amide groups in the inhibitor give good H-bonding sites to the growing hydrate crystal.8-10 However it has also been claimed that the amide group may be insignificant according to results of a molecular

dynamics

study

of

the

free

energy

profile

of

the

commercial

KHI

polyvinylcaprolactam (PVCap).11 It was found that the amide group can form hydrogen bonds with the water on the hydrate surface, but also hydrogen bond just as well with the liquid water. Therefore, there is no rationale that the hydrogen bonding is stronger to the hydrate surface than to liquid water. However, the results indicate that amide groups are advantageous for polymer solubility in the water phase. In a recent paper, we investigated the KHI performance of poly-N-(n-propyl-N-vinylformamide), poly-N-(iso-propyl-N-vinylformamide) (Figure 1) and a range of copolymers of these propylated monomers with N-vinylformamide (NVF) to give higher cloud points.12 It was found that the poly-N-(n-propyl-N-vinylformamide) and its NVF copolymers performed better than the equivalent poly-N-(iso-propyl-N-vinylformamide) series. This is perhaps because of the difference in cloud point and the ability of the n-propyl group to perturb the bulk water better than iso-propyl and so giving a better prevention of hydrate nucleation.


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Figure 1. Poly-N-(n-propyl-N-vinylformamide) (left) and poly-N-(iso-propyl-N-vinylformamide) (right).12

We have also previously investigated N-alkyl derivatives of polyvinylacetamides, both homopolymers and copolymers (Figure 2).13 In general, we found that the performance increased with the larger alkyl groups until water-solubility was lost. The larger n-alkyl groups, in addition to branching alkyl groups, were also difficult to polymerize. For example the derivative of isopropyl monomer could not be homopolymerized, probably due to steric hindrance from the isopropyl group in addition to the methyl group on the acetamide moiety.

Figure 2. Structures (from left to right) for poly N-(n-propyl-N-vinylacetamide), poly(N-vinyliso-butyramide), and poly(N-iso-propylmethacrylamide). 4 ACS Paragon Plus Environment

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For another study we were able to make poly(vinylalkanamide)s with pendant n-propyl, isopropyl and iso-butyramide groups.14 This study also showed that the increased alkyl chain length and branch length gave increasing KHI performance as long as it was soluble in water. Other similar KHI polymers that have been investigated and that are used in field operations are methacrylamide–based polymers, particularly with iso-propyl groups.15-19 We were interested in testing N-vinyl carbamate polymers, as they have closely related structures to N-vinyl amide polymers (Figure 3). The carbamate group is also known as the urethane group, which is extremely common in a range of polyurethane plastics.20 Although much less research has been done on the polarity of the carbamate group relative to the amide group, theoretical studies that are published suggest that carbamates are less polar and give less H-bonding to water than amide groups.21, 22

Figure 3. General structure of alkyl N-vinyl carbamate polymers (left) and N-alkyl vinyl carbamate (right) with the carbamate group reversed.


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As far as we are aware, easy synthetic routes to N-vinyl carbamate monomers and polymers are not available. Routes to alkyl vinylcarbamate monomers have been reported by the pyrolysis of dialkyl ethylidenebiscarbamates or the catalytic cracking of alkyl N-(α-alkoxyethyl)carbamates or alkyl N-(α-alkoxyethyl)-N-alkylcarbamates.23,

24

However, we came across a facile route to a

related class of N-alkyl vinyl carbamate polymers, in which the carbamate group is attached to a polyvinyl backbone via the oxygen atom, together with a varying size pendant R group from the nitrogen atom and a hydroxyl group on the neighboring backbone carbon atom (Figure 4). These poly(hydroxyl-N-alkylcarbamate)s can be made in one step from poly(vinylene carbonate) (PVCa) by ring-opening with organic amines.25

Figure 4. Modification of poly(vinylene carbonate) to poly(hydroxyl-N-alkylcarbamate).

As mentioned previously, the carbamate group in poly(hydroxyl-N-alkylcarbamate)s is less polar than an amide group, which might significantly limit the water-solubility of alkyl derivatives in this class. However, we reasoned that the neighboring polar hydroxyl group would counteract this


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effect, increasing the overall polarity of the polymers, allowing small alkyl groups to be placed on the carbamate nitrogen whilst still keeping water-solubility. This paper documents for the first time the KHI performance of a series of polymers containing pendant carbamate groups, using poly(hydroxyl-N-alkylcarbamate)s. Experiments where the alkyl group varied in size and shape from methyl to iso-butyl are reported. All tests were carried out in high pressure rocking cells with a Structure II-forming synthetic natural gas.

EXPERIMENTAL METHODS: KINETIC HYDRATE INHIBITOR PERFORMANCE TESTS

Chemicals Poly(N-iso-propylacrylamide) with a molecular weight of approximately 6000 g/mole was synthesized at the University of Stavanger.15 Polyvinylcaprolactam (PVCap) with a molecular weight of approximately 2000−4000 g/mole was obtained from BASF as a 41.1 wt % solution in monoethylene glycol. All chemicals for synthesis were purchased from VWR and Sigma-Aldrich and all solvents were used without further purification. Synthesis of the poly(vinylene carbonate) (PVCa) was done according to the reported procedure by Ding et al.25 P(VCa) was prepared by Masami Kamigaito (Mn = 9900 g/mole, PDI = 1.96) and was modified according to the procedure given in the next section.


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Modification of P(VCa) P(VCa) was modified with addition of different amine groups (Figure 4), 25 a typical example for transformation of P(VCa) to a poly(hydroxyl-N-alkylcarbamate) is given here for preparing the npropylamine derivative. P(VCa) (250.0 mg, 2.9 mmol) was dissolved in acetone (5.0 ml). To this solution, n-propylamine (3.4g, 58.1 mmol) was added dropwise at room temperature. The reaction mixture was left at room temperature overnight (approximately 20 hours) with good stirring. Some excess amine and solvent was removed in vacuo at 40°C. The product was precipitated with diethyl ether (Et2O), excess solvent was decanted off and the product was dried in vacuo at 40 °C. The yield ranged from 65-95% for the different modifications of the polymers. The precipitated polymers are assumed to have 100% conversion of vinylenecarbonate ring groups to hydroxyalkylcarbamates. The use of a large excess of amine ensures the completion of the reaction. For the iso-butyl:methyl carbamate copolymers, the ratio of the alkyl groups is assumed to be the same as the ratio of alkylamines added. NMR spectroscopy could not determine the ratio in the final copolymer as the lines were so broad. A summary of the poly(hydroxyl-N-alkylcarbamate)s synthesized is given in Table 2.

Cloud Point (TCl) Measurement A solution of deionised water at room temperature, containing 2500 ppm of polymer was carefully heated at about 2 °C/min and closely observed throughout the measurement. If the solution was already cloudy at room temperature, then it was cooled in a refrigerator and heated


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slowly to room temperature.The temperature at the first signs of cloudiness in the solution gives the cloud point (TCl). The test was repeated for reproducibility.26

High Pressure Rocker Rig Tests with Synthetic Natural Gas The KHI performance tests were performed according to the procedure used in previous research from our group.27 A rocker rig supplied by PSL Systemtechnik, Germany was used. The rig can hold up to five high-pressure steel rocking cells with a volume of 40 ml per cell. For agitation of the test liquid, a steel ball was placed in each of the cells. 20 ml sample was added to each cell with distilled water as the aqueous phase, then the system was pressurized using a synthetic natural gas (SNG) mixture. The SNG mixture used in this experiment will theoretically form the structure II hydrate as the most thermodynamically stable hydrate. The components of this SNG is listed in Table 1. Table 1. Synthetic natural gas (SNG) mixture used in the KHI performance tests. Component Methane Ethane Propane Iso-butane n-butane N2 CO2

mol % 80.67 10.20 4.90 1.53 0.76 0.10 1.84

The constant cooling KHI test procedure was as follows: 1. For KHI test purposes, deionized water was added to give the desired test concentration at least one day before testing on the high pressure rocking cell equipment. The additive


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was dissolved in distilled water at room temperature and 20 ml sample was loaded in each cell at 20.5 °C. 2. To remove air in the system, vacuum was applied then the cells were purged with 3 – 5 bars of SNG mixture. Vacuum was applied once more after the system was depressurized. 3. The cells rocked at a rate of 20 rocks per minute with a rocking angle of 40° after being pressurized to approximately 76 bars. 4. The cooling rate was approximately 1 °C/hour, starting at a temperature of 20.5 °C and decreasing to 2 °C. 5. Pressure and temperature sensors are connected to each cell, together with a temperature sensor in the cooling bath. This enables data logging on a local computer.

The pressure decreases about 2 bars just as the test starts up due to the gas dissolving in the aqueous phase. As the temperature decreases at a constant rate from 20.5 °C to 2 °C, the pressure will also decrease at a constant rate, since it is a closed system. The first detectable formation of hydrates are found by plotting the pressure and temperature against time as shown in Figures 5 and 6. This temperature, the onset temperature (To), is the result of most importance for field applications as this temperature indicates when the first macroscopic formation of hydrates is observed to occur (nucleation may have taken place earlier but could not be detected in these graphs). From the graphs in Figures 5 and 6, the temperature for rapid hydrate formation can also be found, (Ta). This value indicates the time when the hydrate formation starts to go out of control because the KHI is no longer able to contain the hydrate growth.


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Figure 5. Summary of the results from a constant cooling test with only 4 cells used.

Figure 5 shows a typical graph generated from the high-pressure rocker rig tests. This graph shows the results from 5 cells after a constant cooling test, where the pressure and temperature is plotted against time in minutes. From the chart in Figure 5, the graph from each single cell can be analyzed to find the To and Ta as shown in Figure 6. P4 indicates the pressure curve for cell 4 and T4 indicates the temperature for cell 4, the lowest temperature at 2 °C is not shown in this graph. To is found after approximately 552 minutes at a temperature of 11.9 °C and Ta is found after approximately 578 minutes at a temperature of 11.5 °C.


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Figure 6. Determination of To and Ta from cell 4 after a constant cooling test.

A total of 8-10 individual experiments using the standard constant cooling KHI test method were carried out for each polymer, except for the tests at 5000 ppm (3 tests) of the 2.5:1 isobutyl:methylcarbamate copolymer, because of limited amounts of this polymer. Due to the stochastic nature of hydrate formation in the small test cells used for this research, there is often up to 10-15% scattering of the To values in a set of 10 experiments.12, 28, 29 None of the cells showed any trend of systematic error, meaning that none of the cells showed consistently better or worse results than the others.


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RESULTS AND DISCUSSION KHI performance comparison between the different carbamate polymers The results from the high-pressure KHI tests are summarized in Table 2 and illustrated in Figure 7. The table lists the average To and Ta in addition to the cloud point (TCl) from each modification of the P(VCa) with different pendant alkylcarbamate groups. Results with two known KHI polymers, poly(N-iso-propylacrylamide) (PNIPAM) and poly(N-vinyl caprolactam) (PVCap) are also added for comparison.

Table 2. Summary of the constant cooling KHI test results at 2500 ppm.

Polymer

To(av) [°C]

Ta(av) [°C]

To(av) - Ta(av) [°C]

Cloud pt. (TCl) [°C]

No polymer

17.2

16.1

1.1

n/a

Poly(N-iso-propylacrylamide)

8.5

7.9

0.6

32

Poly(N-vinylcaprolactam)

8.1

7.5

0.6

31

Methylcarbamate

15.2

13.7

1.5

>95

Ethylcarbamate

14.4

14.0

0.4

>95

n-propylcarbamate

12.8

12.4

0.4

PS*

iso-propylcarbamate

9.2

8.6

0.6

4-6

iso-butylcarbamate

n/a

n/a

n/a

Not soluble in water

Pyrrolidine

13.8

13.3

0.5

>95

2:1 iso-butyl:methylcarbamate

8.5

8.0

0.5

>95


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2.5:1 iso-butyl:methylcarbamate

9.2

9.0

0.2

4-6

3:1 iso-butylcarbamate:methylcarbamate

8.4

7.9

0.5

PS*

*Partially soluble in water at 4 °C.

Figure 7. To and Ta for each polymer tested.

All the polymers gave an improved performance compared to using deionized water with no additive. In general, it was found that increasing the pendant alkyl group size and the branching of this group gave increased KHI performance as long as the polymer was water-soluble at the test temperature range. However there were some issues regarding low TCl and low solubility in deionized water which are discussed below.


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The hydroxyalkylcarbamate was unfortunately somewhat less polar than predicted. For example, the iso-propyl derivative gave a cloud point of about 4-6oC. Poly(iso-propylacrylamide) and poly(N-vinyliso-butyramide), which also have pendant iso-propyl groups, have cloud points of about 30oC and 41oC respectively.14 We assumed that the pendant hydroxyl (–OH) group would give additional bonding sites to the hydrate surface as well as increase solubility in water. But since the TCl values were relatively low, it appears there is less hydrogen-bonding available to the bulk water molecules. We surmise that this may be due to two factors. Firstly, tautomerisation of the –C(=O)-NH- moiety to the hydroxyimine –C(OH)=N- is less likely when this group is part of a carbamate as the oxygen atom is more electron-withdrawing than the nitrogen atom.30 This reduces the polarity of the carbamate C=O group compared to an amide C=O group. Secondly, internal hydrogen bonding in the polymer between pendant –OH groups and carbamate groups can occur in several ways (Figure 8). Intramolecular hydrogen-bonding may also occur between neighboring carbamate groups if the nitrogen atom is covalently bonded to a hydrogen atom. Increasing the chain length and branching of the pendant alkyl group also reduced the TCl and water-solubility significantly.

Figure 8. Possible internal hydrogen bonding in poly(hydroxyl-N-alkylcarbamate)s.


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Polyacryloylpyrrolidine is a well-known KHI.31 Therefore, we also reacted PVCa with pyrrolidine in an attempt to make a polymer with pendant pyrrolidinocarbamate groups. Somewhat surprisingly, the product we obtained did not give good KHI performance, giving an average To of 13.8 °C. Since, pyrrolidine is a secondary amine the nitrogen atom in this polymer lacks a hydrogen atom. This is the only difference between this polymer and all the others which were made from primary amines. Mechanistically, we see no reason why secondary amines should react any differently than primary amines with PVCa. However, we were not able to determine from NMR spectroscopy if the product polymer did indeed have pendant pyrrolidine groups but we did observe that the TCl value was very high, over 95 °C. This may mean there is little intramolecular hydrogen-bonding giving the polymer a high hydrophilicity that can adversely affect its KHI performance. The possible lack of intramolecular hydrogen-bonding may be due to the ring structure causing steric hindrance between potential bond sites. The iso-propylcarbamate derivative gave an average To of 9.2 °C, approximately 3 °C lower than the n-propylcarbamate polymer. Both polymers have identical molecular weights, being made from the same batch of PVCa as all the carbamate derivatives in this study. Both of these polymers also had very low TCl values, which may have some effect on the results. From previous research12-14 the n-propyl- and iso-propyl derivatives of vinylamide polymer give fairly good results, in the same range as these results here. However, had the TCl and solubility of these carbamates been a little higher (i.e. totally clear solutions throughout the test temperature range inside the hydrate region), we suspect the results would have been a better. A polymer with very low TCl and/or poor solubility at the test temperatures will normally give low KHI performance as the polymer does not fully hydrogen-bond to water because it has started to collapse.32 Steric 16 ACS Paragon Plus Environment

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hindrance of the iso-propyl group may cause the polymer to bond internally more than the npropyl derivative, as mentioned previously. This means the n-propyl group is more hydrophobic and therefore has greater influence on the surrounding water and hydrate structures, giving it a superior KHI performance. The iso-butyl derivative was not soluble in water at all test temperatures, so homopolymers with pendant groups of this or larger size were not investigated. To increase both solubility and TCl while still using this alkyl group we made a 2:1 copolymer with iso-butyl:methylcarbamate groups. This copolymer was water-soluble with a high TCl and gave better KHI performance than the all other polymers in this class, on a par with the two known KHI polymers, PNIPAM and PVCap.12 A 3:1 copolymer containing iso-butyl and methylcarbamate units was also prepared to see if the TCl could be lowered and the KHI performance increased. It turned out that this copolymer gave almost the same To results as the 2:1 copolymer (0.1 °C lower in average). However, the 3:1 copolymer was significantly less water-soluble, giving a cloudy solution, but enough was dissolved to give quite good KHI results (average To = 8.4oC). This indicates that it is very effective at a lower concentration compared with the other polymers at 2500 ppm. Following these results, a copolymer of 2.5:1 iso-butyl- and methylcarbamate groups was also made to investigate the solubility and TCl compared with the other copolymers since there was a very big difference in TCl values between the 2:1 copolymer and the 3:1 copolymers. Interestingly, the 2.5:1 iso-butyl- and methylcarbamate copolymer was soluble in water but only below the low TCl value of 4-6 °C (refrigerator temperature). The KHI performance was a little worse than the 2:1 and 3:1 copolymers (average To = 9.2 °C), probably due to the low TCl, below the temperature when hydrated formation first occurred. A statistical t-test between the To values of the 2.5:1 copolymer and either the 2:1 or 3:1 copolymers gave P-values less than 0.05


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indicating a significantly lower performance for the 2.5:1 copolymer.33 The P-value for the t-test between the 2:1 and 3:1 copolymers was greater than 0.05, as was the P-value for the isopropylcarbamate homopolymer vs. the 2.5:1 iso-butyl:methylcarbamate. So, there is no statistically significant difference in the performance of these pairs of polymers. The 2.5:1 isobutyl:methylcarbamate copolymer was further investigated at different concentrations, as discussed below.

Comparison

of

the

2.5:1

iso-butyl:methylcarbamate

copolymer

with

different

concentrations. Although 2.5:1 iso-butyl:methylcarbamate copolymer was not the copolymer to give the best result, but we had a sufficient amount to investigate it further. The average To and Ta results for KHI tests for the 2.5:1 iso-butyl:methylcarbamate at different concentrations ranging from 7000 ppm to 1000ppm are listed in Table 3. The results are also presented graphically in Figure 9, where the spread in To and Ta values for each concentration can also be seen.

Table 3. KHI test results for the 2.5:1 iso-butyl:methylcarbamate at different concentrations.

Concentration and KHI 1000 ppm 2500 ppm 5000 ppm 7000 ppm

2.5:1 isobutyl:methylcarbamate 2.5:1 isobutyl:methylcarbamate 2.5:1 isobutyl:methylcarbamate 2.5:1 isobutyl:methylcarbamate

To(av) [°C] Ta(av) [°C]

To(av) - Ta(av) [°C]

11.8

11.5

0.3

9.2

9.0

0.2

6.6

6.2

0.4

4.1

3.4

0.7 18

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Due to foaming during depressurisation, some of the test solution was lost even though the procedure was done very slowly (normally over several hours). Due to the lack of polymer, only 3 results were obtained at 5000 ppm, whilst there were 8-10 results for the other concentrations. New batches of the polymer were made, but these gave slightly different performance, even though the synthetic procedure for attaching the pendant amine was performed identically each time. Residual polymer in the pressure line during depressurization causes by foaming is probably not the cause of the different average To values between batches as the lines were flushed after each experiment. Therefore, we presume the discrepancy is due to small changes in the polymer made, possibly due to differences in the rate of reaction of the two alkylamines used. Instead of using new batches, the initial test batch solution was diluted and used for all tests to ensure comparability. As expected, the results for the constant cooling tests improved as the concentration of KHI increased. Compared to previous studies with other polymer classes, the performance increase is actually quite large, with almost a linear relationship between concentration and average To value as can be seen from Figure 9.28,34,35


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Figure 9. Summary of the To values for the different concentrations of the 2.5:1 isobutylcarbamate : methylcarbamate copolymer.

ACKNOWLEDGEMENTS We thank Professor Masami Kamigaito (Nagoya University, Japan) for the sample of poly(vinylene carbonate).

CONCLUSION A series of polyvinyl polymers with neighboring hydroxyl and carbamate groups with varying size pendant alkyl groups was synthesized. They were investigated as KHIs using high pressure equipment and synthetic natural gas forming structure II hydrates. The increasing hydrocarbon 20 ACS Paragon Plus Environment

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chain length and branching on the pendant amine gave increasing KHI performance. However it also gave decreasing water solubility and/or TCl of some of the polymers, this may be due to intramolecular interactions between a hydroxyl group and the neighboring hydroxyl group and/or the carbamate group, instead of binding to the hydrate clusters or the free water molecules. Iso-propyl- and n-propylcarbamates performed well as KHIs, however the solubility and TCl were too low for practical use. Iso-butylcarbamate was not soluble in water, therefore copolymers with iso-butyl- and methylcarbamate were investigated. Copolymers with different ratios of iso-butyl- and methylcarbamate were made to increase water solubility and TCl. A 2:1 copolymer performed well as a KHI with an average To = 8.5°C and so this mixture was then further investigated. A 3:1 copolymer of iso-butyl:methylcarbamate was only partly soluble in water, even so the result was an average To of 8.4°C, this indicates an effective KHI at lower concentrations compared with the other polymers tested at 2500 ppm. Then a copolymer of 2.5:1 iso-butyl:methylcarbamate was investigated, also at other concentrations ranging from 1000 ppm to 7000 ppm and it was found to have an increasing, approximately linear KHI performance as the concentration increased. We are currently looking at derivatives of P(VCa) for use on Structure I hydrate-forming systems.

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Carbamate Polymers as Kinetic Hydrate Inhibitors - Energy & Fuels

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