Article pubs.acs.org/EF
Kinetic Hydrate Inhibitors: Structure−Activity Relationship Studies on a Series of Branched Poly(ethylene citramide)s with Varying Lipophilic Groups Fernando T. Reyes,*,† Malcolm A. Kelland,† Li Sun,‡,§ and Jian Dong‡ †
Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ School of Chemistry and Chemical Engineering, Shaoxing University Shaoxing, Zhejiang 312000, People’s Republic of China § School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, People’s Republic of China ABSTRACT: Novel citric-acid-based polyamides were synthesized by polycondensation reactions using tributyl citrate (TBC) and ethylene diamine (EDA), producing hyperbranched poly(ethylene citramide)s with NH2 termini, which were functionalized with isopropyl, n-butyl, and cyclohexyl groups via the urea group. Characterization was carried out by 1H and 13C nuclear magnetic resonance (NMR) spectroscopic techniques. Molecular weights in this polymer family varied from 3000 to 10 000 Da. The activity of this polymer family as gas hydrate kinetic inhibitors (KHIs) was evaluated for the first time. Constant cooling (1 °C/h) experiments were carried out in high-pressure steel rocking cells using a synthetic natural gas that preferentially forms structure II gas hydrates. The best KHI performance was given by poly(ethylene citramide) CONHCyHe with pendant Ncyclohexyl groups (Mn = 5.65 × 103, and Mw = 1.14 × 104). A structure−activity relationship (SAR) analysis of the KHI test results supports the hypothesis that increasing the size of the lipophilic groups increases the KHI performance as long as water solubility is maintained through hydrogen bonding via the amide and/or urea functional groups.
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INTRODUCTION Gas hydrates are clathrates formed from natural gas and water at high pressure and low temperature. Their crystal structure, nucleation, and crystal growth are described in the literature, with structure I (SI) and structure II (SII) being the most problematic isoforms encountered in the oil industry.1−11 Gas hydrates are a major challenge in this industry because hydrate formation and deposition can reduce the flow in production lines and even block a line completely. Most of the onshore and offshore platforms are designed to reduce the conditions that can cause gas hydrate deposition during production. However, gas hydrates can still be a major problem for planned and unexpected shut-in situations. There are two common approaches that are used to avoid gas hydrate deposition and conduct line blockage. These options are physical (engineering) or chemical management. There are several ways to control gas hydrate formation with physical methods. Physical treatment is intended to maintain the flow lines outside the gas hydrate stable region. This can be achieved by keeping the pressure low or through insulation or heating methods. However, these options can be uneconomical, especially if needed for continuous production. In recent years, there has been a paradigm change in flow assurance, going from gas hydrate avoidance to gas hydrate management.12 Several studies have been carried out for water−oil systems, showing that some crude oils contain natural anti-agglomerants (AAs) with emulsifier properties that lead to a non-plugging effect. Nevertheless, little is known about the exact chemical structures and proportions needed in these components to produce a non-plugging effect.13−16 Cold flow is another methodology under development.2,17−20 It is © 2015 American Chemical Society
designed to produce gas hydrates very quickly and at an early stage as small particles avoiding the “sticky” agglomerating phase of hydrate formation. With this methodology, gas hydrates are meant to be transported as a slurry through the conduct lines without the use of chemicals. Gas hydrate chemical treatment is divided into thermodynamic hydrate inhibition (THI) and low-dosage hydrate inhibition (LDHI). THI consists of a series of low-molecularweight additives, which shift the gas hydrate formation pressure−temperature equilibrium curve to lower temperatures. Examples of common THI are methanol, ethanol, and glycols, such as monoethylene glycol (MEG). However, large quantities of THI are needed to keep the flow lines outside the gas hydrate stable region. This leads to a high cost and other problematic situations, such as large storage facilities, downstream hydrocarbon pollution, regeneration facilities, etc.1−11 LDHI by chemicals is divided into two subclasses, AAs and kinetic hydrate inhibitors (KHIs). Current commercial AAs are usually based on quaternary ammonium or phosphonium salt surfactants.2,17 These inhibitors allow for controlled gas hydrate formation and keep gas hydrates as dispersed particles, avoiding agglomeration and deposition. KHIs are based on certain classes of water-soluble polymers and have been used in the field earlier than AAs, since the mid-1990s. Since then, it has been a challenge to develop new and more efficient KHIs. The stricter environmental regulations in the North Sea have also led to intensive research efforts into the development of more Received: March 25, 2015 Revised: July 17, 2015 Published: July 22, 2015 4774
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Energy & Fuels environmentally friendly or readily biodegradable KHIs.2,17 In recent years, interest in the field application of KHIs has made a quantum leap, becoming much more acceptable by operators for use in the field. New projects are currently underway in several countries that have never previously employed the use of KHIs. Even so, current KHIs in the market have limited subcooling performance, and it would be helpful to develop novel KHIs that can be used at more extreme field conditions.21 One theory for KHIs is inhibition at the nucleation stage, increasing the activation energy by disrupting the tertiary water structure, a process that also slows the crystal growth process. Polymers that have been shown to have weak hydrate crystal growth inhibition properties, such as poly(N-isopropylacrylamide), may possibly be inhibited by this mechanism.22 Nucleation can also be arrested by adsorption of the KHI polymer onto subcritical nuclei. Also, crystal growth can be inhibited by direct adsorption of the KHI on the hydrate crystal surface. For either process, specific chemical structural groups are required. Most of the current commercially available KHIs are hydrophilic linear polymers and copolymers. These polymer families contain functional groups, including lactam rings and amides with medium-size aliphatic carbon chains. Some of the commercially available linear KHIs are polymers and copolymers made from N-vinylcaprolactam, N-vinylpyrrolidone, and N-isopropylmethacrylamide.2,17,23−25 Another major class of KHIs already commercially available are hyperbranched polyesteramides.2,17,26−32 An example is shown in Figure 1. For
polymers presenting low TCl can potentially precipitate in the flow lines at elevated temperatures. It has been suggested that polymers with a low cloud point might present better kinetic inhibition. However, the TCl values should be used with caution as a prediction tool for novel KHIs. The cloud point will vary according to the molecular weight, hydrophobicity, intramolecular structure in polymer pendant groups, among other characteristics, as well as intra- and intermolecular hydrogen bonding.2,17,33−36 We have carried out structure−activity relationship (SAR) studies on novel polymer families acting as KHIs. This evaluation can lead to the design of more powerful KHIs with better biodegradability. Moreover, functional group modification is a key factor to understand SAR analysis, and it provides insight into the kinetic inhibition mechanisms involved in nucleation and crystal growth. We have shown in our previous reports that, by increasing lipophilicity through the carbon chain length in active amide-based functional groups, the performance of KHIs can be improved.33−36 In this study, we report a SAR evaluation for a novel class of branched poly(ethylene citramide)s. This polymer family contains amide groups in the backbone that help to maintain water solubility. However, they differ from other polymer families previously reported in the type of functional group linkage to the polymer backbone and the branched structure. Instead of amide-based active functional groups, we have introduced the functional groups through urea bonds on the ethyleneamine termini of the polymer. These functional groups are isopropyl (CONHi-Pr), n-butyl (CONHn-Bu), and cyclohexyl (CONHCyHe). Unmodified and modified poly(ethylene citramide)s are shown in Figure 2. These series of polymers have molecular weights ranging from 3000 to 10 000 Da and are soluble in deionized water at least up to a concentration of 10 000 ppm and up to a temperature of 95 °C.
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MATERIALS AND SYNTHESIS
Tributyl citrate (TBC), ethylene diamine (EDA), isopropyl isocyanate, n-butyl isocyanate, cyclohexyl isocyanate, N,N′-dimethylacetamide (DMAc), and 4-dimethylaminopyridine (DMAP) were all purchased in analytical grade from Aladdin Industrial Corporation (Shanghai, China). A total of 14.0 mL of TBC and 4.0 mL of EDA were mixed in 20.0 mL of freshly distilled DMAc in a 100 mL round-bottom flask. The mixture was refluxed at 70 °C for 6 h in a silicone oil bath. A yellowish precipitate was gradually formed. The reactants were transferred to a 250 mL beaker and precipitated with 150 mL of acetone. The precipitate was washed with acetone for 30 min, filtered, and washed twice with acetone. The solid powder was ground and vacuum-dried at 35 °C for 15 h, yielding poly(ethylene citramide) (Figure 3). A total of 1 g of poly(ethylene citramide) was mixed with 15 mL of DMAc in a 100 mL round-bottom flask. A mixture of DMAP (1 mg) and isopropyl isocyanate was prepared and added to the polymer solution. The reactants were heated at 85 °C for 6 h. The product was poured into 200 mL of acetone, precipitated, washed with acetone 3 times, filtered, and dried in a vacuum oven at 35 °C for 15 h. The polymers modified by isopropyl isocyanate, n-butyl isocyanate, or cyclohexyl isocyanate were prepared with a molar ratio of the polymer repeating unit to the corresponding isocyanate of 1:3, 1:1, and 1:0.8, respectively. The reaction scheme is shown in Figure 3.
Figure 1. General active structure of commercial poly(ester amide)s.
polymers with large hydrophobic groups, such as with the hydrophobic cyclohexyl group, the polymer can contain peripheral hydrophilic groups, such as N,N-dimethylaminopropylamide, to maintain polymer water solubility but maybe also to act as functional groups for hydrate inhibition. These polymers can be used for field applications up to about 10 °C subcooling and can also be used in the presence of high-salinity water and/or to perform better with other synergists.26−32 The molecular weight of these polymers is low, often between 1500 and 3000 Da. Despite many chemicals having been screened as KHIs, very few polymer families have shown the right combination of efficacy and biodegradation. Our research group has investigated polymer families, considering different factors that can modify efficacy and biodegradation. Some of these factors are molecular weight (length), tacticity, backbone type, size of peripheral functional groups, and polymer cloud point (TCl). TCl is an important measurement for field applications, because
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CHARACTERIZATION OF POLYMERS Nuclear magnetic resonance (NMR) experiments were carried out on Bruker AVANCE III 400 MHz NMR spectrometer, using D2O as the solvent. The 1H and 13C NMR spectra were recorded as described earlier.37 Gel permeation chromatog4775
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Figure 2. General structures of poly(ethylene citramide) (R = H) and its N-alkyl urea derivatives, poly(ethylene citramide)CONHi-Pr, poly(ethylene citramide)CONHn-Bu, and poly(ethylene citramide)CONHCyHe.
Figure 3. Step 1 shows unmodified poly(ethylene citramide) preparation by condensation polymerization of TBC and EDA. Step 2 shows poly(ethylene citramide) functionalization with N-alkyl urea groups, CONHi-Pr, CONHn-Bu, and CONHCyHe.
shows the unlabeled general monomeric repeating unit as well as the labeled carbons. Figure 5 represents 1H NMR for unmodified poly(ethylene citramide). Two broad signals at 2.51 and 2.69 ppm correspond to protons C2 and C2′ directly attached to the citric moiety. Signals at 3.16, 3.20, and 3.24 ppm represent ethylene groups forming amide bonds that correspond to C4, C4′, C5, C5′, C7, and C8. Signals at 2.81 and 2.90 ppm correspond to methylene protons bound to the amine. Figure 6 shows 13C NMR for unmodified poly(ethylene citramide). Signals at 174 and 178 ppm correspond to amide carbonyl groups for C3 and C3′. Signal at 181 and 77 ppm correspond to C6 and C1, respectively. Aliphatic signals at 45 and 48 ppm represents methylene groups in the citric moiety. Signals at 40 and 43 ppm contain C4, C4′, C5, C5′, C7, and C8 assigned to ethylene carbons in the amide bonds.
raphy (GPC) analysis was performed using Agilent 1100 GPC with a PL Aquagel-OH MIXED column (8 μm), with deionized water as the mobile phase, at a flow rate of 1.0 mL/min. Figure 4 shows the monomeric repeating unit for the unmodified poly(ethylene citramide). This representation
Figure 4. (Left) Unmodified poly(ethylene citramide) repeating unit. (Right) Carbon-atom-labeled monomeric repeating unit. 4776
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Figure 5. 1H NMR spectrum of poly(ethylene citramide).
Figure 7. 1H NMR spectra of (a) poly(ethylene citramide), (b) poly(ethylene citramide)CONHi-Pr, (c) poly(ethylene citramide)CONHn-Bu, and (d) poly(ethylene citramide)CONHCyHe. The asterisks denote peaks as a result of residual solvent DMAc or precipitating solvent acetone.
Figure 6. 13C NMR spectrum of poly(ethylene citramide).
Figure 7 shows 1 H NMR comparison between (a) unmodified poly(ethylene citramide) and poly(ethylene citramide) modified with active functional groups (b) CONHi-Pr, (c) CONHn-Bu, and (d) CONHCyHe. Spectra b, c, and d show new aliphatic signals corresponding to their respective functional groups. In spectrum b, the doublet at 0.96−0.97 ppm corresponds to methylene protons and the multiplet at 3.56 ppm represents CH in the isopropyl group. In spectrum c, the triplet at 0.71 ppm corresponds to methylene protons in the n-butyl functional group. In spectrum d, the multiplets at 1.05 and 1.80 ppm describe CH2 protons in the cyclohexyl functional groups. Figure 8 shows 13C NMR comparison between unmodified and modified polymers: (a) unmodified poly(ethylene citramide), (b) CONHi-Pr, (c) CONHn-Bu, and (d) CONHCy-He. In spectra b, c, and d, new signals appear at 160 ppm; these signals are assigned to the new urea carbonyl group formed. New aliphatic signals in spectra b, c, and d between 10 and 40 ppm correspond to aliphatic carbons in isopropyl, n-butyl, and cyclohexyl groups. Figure 9 shows gel permeation chromatographic analysis results for unmodified and modified poly(ethylene citramide)s. Table 1 lists the values of the number-average and weightaverage molecular weights (Mn and Mw) and polydispersity index (D).
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TEST METHOD FOR KINETIC INHIBITION OF GAS HYDRATES The poly(ethylene citramide) polymer family was evaluated as novel gas hydrate inhibitors. A five rocking cell (RC5) setup was used, with pressure and temperature controllers. The RC5 setup was developed by PSL Systemtechnik (Germany) and has
Figure 8. 13C NMR spectra of (a) poly(ethylene citramide), (b) poly(ethylene citramide)CONHi-Pr, (c) poly(ethylene citramide)CONHn-Bu, and (d) poly(ethylene citramide)CONHCyHe.
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Figure 9. GPC chromatographs of (a) poly(ethylene citramide), (b) poly(ethylene citramide)CONHi-Pr, (c) poly(ethylene citramide)CONHn-Bu, and (d) poly(ethylene citramide)CONHCyHe.
been described in previous reports.38 This equipment is shown in Figure 10. Each stainless-steel cell has a volume of 40 mL and contains a steel rocking ball to improve mixing during gas hydrate formation. All experiments were carried out using deionized water as the water phase. Kinetic inhibition tests were carried out as constant cooling experiments that have been described in previous reports.39 All tests were performed with a temperature ramp from 20.5 to 2 °C with a cooling rate of 1 °C/h. The temperature was maintained at 2 °C for 1 h, and gas hydrates were melted at 25 °C. Then, the water bath was cooled to 20.5 °C. At this stage, the experiment was completed, cells were cleaned, and the polymeric solution was allowed to stabilized at atmospheric pressure for 24 h to avoid any memory effect. The rocking support is located in the central base of the rocking cells. The rocking support moves the cells with a rocking angle of 40° at a maximum rate of 20 rocks per minute. The maximum rocking rate was used in this work. The cells were filled with 20 mL of the desired polymeric solution at the desired concentration in deionized water and pressurized to 76 bar. A blend of synthetic natural gas (composition described in Table 2) was used to make the gas hydrates. 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.40 SAR evaluation was carried out by comparison between unmodified and modified poly(ethylene citramide)s at identical KHI test conditions. For a more significant statistical analysis, 8−10 KHI rocking cell tests were carried out for each sample tested. Figure 11 shows the information collected in one set of experiments showing good reproducibility between the five cells. All experiments were first carried out at an identical polymer concentration of 2500 ppm of the desired polymer in deionized water. One of the polymers was also tested at varying concentrations. The first pressure deviation not a result of the change in the temperature corresponds to the onset temperature (To), which is taken as the first macroscopic indication of gas hydrate formation. The second pressure deviation
Figure 10. Rocker rig (RC5), with five steel rocking cells shown in the water bath with pressure inlets and temperature sensors.
Table 2. Composition of Synthetic Natural Gas (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
represents the autocatalytic crystal growth stage (Ta) at its fastest rate. Figure 12 shows To and Ta evaluation for one individual cell.
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RESULTS AND DISCUSSION As mentioned earlier, TCl is an important parameter in the development of novel KHIs and also of importance in oilfield operations for smooth injection with no polymer precipitation. Medium to large aliphatic groups can lead to low TCl, with increasing potency as KHIs but potentially at the cost of fouling during injection into a hot well stream. We have tested the cloud point of the poly(ethylene citramide)s series in this study, and none of them showed TCl up to 95 °C as 1 wt % solution in deionized water. This means all polymers in this family should be suitable for injection into low-salinity hot well streams. For the KHI testing, for comparative purposes with commercial products, we refer to our previous work in which we carried out tests with no additive, with poly(vinylpyrrolidone) (PVP, Mw ca. 8000) and with poly(vinylcaprolactam) (PVCap, Mn = 4000).33 These benchmark polymer products gave average To values of 11.6 and 10.1 °C and Ta values of 11.0 and 8.6 °C, respectively. Average To, Ta, To − Ta, standard deviation, and coefficient of variation (CV)
Table 1. Number-Average and Weight-Average Molecular Weights (Mn and Mw) and Polydispersity Index (D) polymers poly(ethylene poly(ethylene poly(ethylene poly(ethylene
citramide) citramide)CONHi-Pr citramide)CONHn-Bu citramide)CONHCyHe
Mn 2.89 9.23 5.63 5.65
× × × ×
Mw 3
10 103 103 103
4778
3.67 1.36 1.09 1.14
× × × ×
D = Mw/Mn 3
10 104 104 104
1.27 1.47 1.93 2.02 DOI: 10.1021/acs.energyfuels.5b00628 Energy Fuels 2015, 29, 4774−4782
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Figure 11. Standard constant cooling experimental data showing all five cells.
Figure 12. To and Ta evaluations for a standard constant cooling experiment in one individual cell.
Table 3. Average To, Ta, and To − Ta for 10 Identical Experiments kinetic inhibitor no additive PVP PVCap poly(ethylene poly(ethylene poly(ethylene poly(ethylene
citramide) citramide)CONHi-Pr citramide)CONHn-Bu citramide)CONHCyHe
To (°C)
Ta (°C)
To − Ta (°C)
standard deviation (°C)
CV (%)
17.1 11.6 11.0 17.1 15.7 14.9 10.7
17.0 10.1 8.6 16.3 13.5 13.9 10.1
0.1 1.5 1.4 0.8 2.2 1.0 0.6
0.30 0.78 0.24 0.63 0.98 0.74 0.51
1.8 6.7 2.4 3.7 6.2 5.0 4.8
desired polymer. A comparison between To values for this family of polymers is given in Figure 13. The graphs contain 8−
results for the new poly(ethylene citramide)s are presented in Table 3 as an average of 8−10 identical parallel tests of the 4779
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Figure 13. To values for each sample in the poly(ethylene citramide) series.
Figure 14. To (in blue) and Ta (in red) values as an average of 10 identical experiments at a range of concentrations of 1000, 2500, and 5000 ppm for poly(ethylene citramide)CONHCyHe.
compare To − Ta values for polymers that gave similar subcoolings. Unmodified poly(ethylene citramide) gave a poor result as KHI with To and Ta values of 17.1 and 16.3 °C, respectively, close to the observed gas hydrate formation onset temperature in pure deionized water. This polymer does not contain any pendant lipophilic functional groups that can disrupt tertiary water structure or adsorb to sub- and supercritical particles. To improve the KHI effect, we have added active lipophilic
10 points for each polymer, but there is accidental overlap of some points, making it look like less points. We have found a significant statistical difference between To values for different polymers as well as for Ta values. The difference between To and Ta (To − Ta) is useful to understand how these polymers would behave as crystal growth inhibitors. However, the driving force for gas hydrate formation in these experiments is the subcooling (or temperature). Therefore, it is only possible to 4780
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HCyHe by terminal amino modification with the alkyl urea groups. This polymer series was shown to be soluble in deionized water at least up to a concentration of 10 000 ppm and up to a temperature of 95 °C. The performance of novel poly(ethylene citramide)s was explored for the first time as KHIs. High-pressure standard constant cooling experiments were performed on a five rocking cell system to evaluate gas hydrate inhibition. To mimic realistic problematic situations in on- and offshore platforms, a synthetic natural gas blend was used to produce gas hydrate SII. Unmodified polycitramide showed very poor activity as a KHI. The introduction of the active functional groups CONHi-Pr, CONHn-Bu, and CONHCyHe lead to a better KHI performance. Poly(ethylene citramide)CONHCyHe showed the best performance as a KHI at nucleation inhibition (i.e., lowest average To value) compared to analogues with smaller functional groups. Poly(ethylene citramide)CONHi-Pr presented the best interaction, with formed clusters showing the highest To − Ta value in this series, when compared to polymer analogues that first formed hydrates as similar subcoolings. Further constant cooling experiments at variable concentrations of poly(ethylene citramide)CONHCyHe were carried out. These results showed improved performance as the concentration was increased from 1000 to 5000 ppm. SAR evaluation of this novel family of poly(ethylene citramide)s as KHIs strengthens the assumption that increasing the size and lipophilicity of the active functional groups lead to better performance as KHIs as long as polymer water solubility is maintained in the critical hydrate-forming temperature range.
functional hydrophobic groups to the branched poly(ethylene citramide) series. These polymers showed increased performance compared to the unmodified analogue. The performance of this class of polymers improved in accordance with an increase in the lipophilicity of the N-alkyl groups introduced. Poly(ethylene citramide)CONHi-Pr is the polymer with the smallest lipophilic side groups studied in this series. This polymer gave a slightly better performance in nucleation (average To value) compared to the unmodified analogue. However, poly(ethylene citramide)CONHi-Pr showed the largest difference for To − Ta of all of the polymers, being 2.2 °C. We have compared this value with the CONHn-Bu analogue [average To − Ta = 1 °C (1.0 °C)] and unmodified poly(ethylene citramide) (average To − Ta = 0.8 °C), i.e., polymers that gave fairly similar To values, allowing for a comparison of growth rates. These data suggest that polymers with isopropyl functional groups give better affinity to formed hydrate crystal surfaces than n-butyl groups, inhibiting crystal growth. Poly(ethylene citramide)CONHn-Bu gave average To and Ta values of 14.9 and 13.9 °C, respectively. We suggest that there is a better interaction between n-butyl groups and the tertiary water structure for inhibiting nucleation than with the isopropyl-modified polymer. For this n-butyl analogue, we have increased the lipophilicity by only one methylene unit compared to the isopropyl polymer. Poly(ethylene citramide)CONHCyHe showed a substantial increase in KHI performance compared to the rest of the polymers in this family. This analogue gave average To and Ta values of 10.7 and 10.1 °C. The average value of To − Ta was the lowest of any polymer, being 0.6 °C. However, the rapid crystal growth started at a much lower temperature (higher subcooling) than the rest of the analogues in this polymer family. Therefore, the crystal growth inhibition will not be as effective as at higher temperatures. The increased lipophilicity in poly(ethylene citramide)CONHCyHe makes it the best KHI in this family studied to date. The difference in To values compared to unmodified poly(ethylene citramide) is 6.4 °C. The SAR evaluation in this polymer series strengthens the hypothesis that more lipophilic pendant groups in one polymer family lead to better performance as KHIs. This assumption is supported by the difference presented in the average To and Ta values for this polymer series. The experiments with poly(ethylene citramide)CONHCyHe at 2500 ppm, showing it to be the best KHI in this polymer series, were followed up by evaluating the polymer at different concentrations. This set of experiments was carried out at identical conditions to the standard constant cooling tests. The results are summarized in Figure 14. Experiments at a higher concentration (5000 ppm) gave increased KHI performance, while experiments at 1000 ppm gave lower performance. This trend is in accordance with studies on other polymer classes, in which the difference between the minimum and maximum concentrations can be up to 4 °C.2,17,32 However, poly(ethylene citramide)CONHCyHe showed a small difference in the To values at different concentrations, with 1.1 °C being the maximum difference in the concentration experiments. At this time, we have no good explanation for this difference.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +47-51831866. Fax: +47-51831750. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Zhejiang Province (LY12E03001) and the Department of Science and Technology of Zhejiang Province of China (2012C24003).
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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.; et al. 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) Østergaard, K. K.; Tohidi, B.; Burgass, R. W.; Danesh, A.; Todd, A. C. J. Chem. Eng. Data 2001, 46, 703−708. (10) Kashchiev, D.; Firoozabadi, A. J. Cryst. Growth 2002, 243, 476− 489. (11) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R. L.; Sloan, E. D., Jr. Chem. Eng. Sci. 1996, 51 (8), 1221−1229. (12) Sloan, E. D. Fluid Phase Equilib. 2005, 228−229, 67−74. (13) Høiland, S.; Askvik, K. M.; Fotland, P.; Alagic, E.; Barth, T.; Fadnes, F. J. Colloid Interface Sci. 2005, 287, 217−225.
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CONCLUSION A series of novel poly(ethylene citramide)s have been synthesized by polycondensation reactions of TBC and EDA. The polymers were functionalized with active gas hydrate kinetic inhibitor groups CONHi-Pr, CONHn-Bu, and CON4781
DOI: 10.1021/acs.energyfuels.5b00628 Energy Fuels 2015, 29, 4774−4782
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DOI: 10.1021/acs.energyfuels.5b00628 Energy Fuels 2015, 29, 4774−4782
