Inhibition Effect Study of Carboxyl-Terminated Polyvinyl Caprolactam

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Inhibition Effect Study of Carboxyl-terminated Polyvinyl Caprolactam on Methane Hydrate Formation Qian Zhang, Xiaodong Shen, Xuebing Zhou, and De-Qing Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02603 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Inhibition Effect Study of Carboxyl-terminated Polyvinyl Caprolactam on Methane Hydrate Formation Qian Zhang, a, b, c, d Xiaodong Shen, a, b, c, d Xuebing Zhou, a, b, c Deqing Liang a, b, c* a

b

Key Laboratory of Natural Gas Hydrate, Chinese Academy of Sciences, Guangzhou 510640, China Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China c

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China d

Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Injecting kinetic hydrate inhibitors (KHIs) is an efficient option to prevent gas hydrate blockages in equipment and pipelines which widely exist in petroleum production and transportation operations. Polyvinyl caprolactam (PVCap) is a kind of effective and commercialized KHIs. In this paper, one kind of carboxyl acid group modified PVCap named carboxyl-terminated polyvinyl caprolactam (PVCSCOOH) has been synthesized, and the KHI performance has also been evaluated.

The microscopic properties for hydrate samples were detected with Cryo-scanning electron microscope, Raman spectroscopy and powder X-ray diffraction, respectively. It was found that the maximum subcooling degree of 2 wt % PVCSCOOH that not generating methane hydrate was 15 °C, while this figure for PVCap was only 12 °C, PVCSCOOH had a better inhibition performance than PVCap of similar molecular weight. In addition, we also noticed that PVCSCOOH could change the hydrate appearance and lower the large to small cavity ratio (L/S) to 2.00, but it had no impact on hydrate structure. 1

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Keywords: Kinetic hydrate inhibitor，Polyvinyl caprolactam，Methane hydrate 1. INTRODUCTION Gas hydrates are ice-like non-stoichiometric crystalline compounds. Lightweight gases such as methane, ethane, carbon dioxide, etc. can be encapsulated by water lattice at high pressures and low temperatures.1-3 Naturally occurring gas hydrates can be divided into structure I (sI), structure II (sII), and structure H (sH) according to the different size and types of water lattice.4 Due to a growing demand of gas fuel production, the pipeline transportation technology has been widely applied. Then, the potential pipeline blockage caused by a build-up of gas hydrates should be inevitably taken into consideration because the gas pipelines often provide favorable pressure and temperature conditions for gas hydrate formation. To guarantee the flow assurance of fuel gas, methods to prevent the gas hydrate crystallization should be investigated.5 The process of hydrate formation has two important steps, nucleation and growth.6 Studying gas hydrate nucleation and growth is of great importance in the field of gas hydrate inhibition. According to the traditional methods, a gas hydrate blockage in pipelines is usually prevented by lowering the pressure, raising the temperature or removing the water.7 These methods may either pull down the gas transport efficiency or require huge energy investment. Additionally, these methods are often very costly or space-limited when the pipelines are laid in colder and deeper places.8 One promising technology to relief this problem is to add hydrate inhibitors into the pipelines. In general, hydrate inhibitors can be divided into two categories. Soluble alcohols, like methanol, ethanol and ethylene glycol, and several kinds of inorganic salts are defined as thermodynamic hydrate inhibitors (THIs) which shift hydrate equilibrium conditions to higher pressure or lower temperature.9 The other kind of inhibitors is called kinetic hydrate inhibitors (KHIs) which delay hydrate nucleation or avoid rapid

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hydrate growth at low dosage.10-16 Due to economic and environmental concerns, KHIs have received growing interest as alternatives. Several kinds of water-soluble polymers that can be used as KHIs are reported since Kuliev published their first experience of successfully preventing hydrate blocking by using a low dosage chemical. Heterocyclic polyamides are a series of powerful kinetic inhibitors.17,18 Poly vinyl pyrrolidone (PVP) is a five-ring member of polyvinyllactams, which has good performance in preventing hydrate plugging at a relatively high temperature, usually higher than 5 °C.19 The homopolymer polyvinyl caprolactam (PVCap) in high molecular weight can delay hydrate nucleation to approximately 24 h at the subcooling degree of 8 ~ 9 °C at the dosage of 0.5 wt %.20 However, PVCap becomes useless at a high degree of subcooling (above 12 °C). On the other hand, the low cloud point (TCl) or critical solution temperature (LCST) of PVCap is another limitation for its applications.21 To gain KHIs with higher solubility and better inhibition performance, some modified versions of them have been studied.22,23 Usually, hydrophobic tails as well as hydrophilic groups are expected to alter the performance of KHIs.24 Functional groups including heterocycles, chain or cyclic amides, betaines, amine oxides were grafted into water-soluble polymers to make better LDHIs through laboratory experiments or numerical simulations;25-28 The molecular modeling result of Bjørn Kvamme shows that the modified PVCap with a hydroxyl group adding to the ring is expected to have a better inhibition performance than PVCap.29 In this paper, we reported the synthesis carboxyl-terminated polvinyl caprolactam (PVCSCOOH) and compared the KHI performance with polvinyl caprolactam (PVCap). To further elucidate the inhibition mechanism, Raman spectroscopy, powder X-ray diffraction (PXRD) and scanning electron

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microscope (SEM) have been used to detect the cage occupancy, crystal structure and topographical microstructure of CH4 hydrate formed in the presence of PVCSCOOH respectively. 2. EXPERIMENTAL SECTION 2.1. Materials. Detail information of the chemicals used in the experiment was shown in Table 1. All of these chemicals were used without further purification. Deionized water was made in the laboratory with a resistivity of 18.25 mΩ·cm−1. An electronic analytical balance with an uncertainty of ± 0.001 g was used to weigh the mass of the materials. 2.2. Synthesis Method of Polyvinyl Caprolactam (PVCap) A 250 ml, three-necked, round-bottomed flask provided with a magnetic stirrer, a temperature sensor and a condenser pipe was placed with 352 mg (2 mmol) of AIBN and 20.0 g (144 mmol) of N-vinyl caprolactam monomer (NVCap). Then the glass flask was washed by nitrogen gas and evacuated three times. After 100 ml of DMF was introduced, the glass flask was washed by nitrogen gas and evacuated three times again, and then the mixture was heated to 80 °C for 7 h under the atmosphere of nitrogen. After natural cooling, the product was transferred into the rotary evaporator and evaporated away most of the solvent. After that, the product was dropped gradually into 1 L of cold ethyl ether and then filtrated. The deposited white solid was dried in the vacuum drying box for 48 h at the temperature of 45 °C, then increased the temperature of the vacuum drying box to 105 °C and kept for one hour to get rid of moisture. 2.3. Synthesis Method of Carboxyl-terminated Polyvinyl Caprolactam （PVCSCOOH））. A 250 ml, three-necked, round-bottomed flask provided with a magnetic stirrer, a temperature sensor and a condenser pipe was placed with 352 mg (2 mmol) of AIBN and 20.0 g (144 mmol) of NVCap.

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Then the glass flask was washed by nitrogen gas and evacuated three times. Next, 0.56 mL (8 mmol) mercaptoacetic acid was added by using a syringe. After 100 ml of DMF was introduced, the glass flask was washed by nitrogen gas and evacuated three times again, and then the mixture was heated to 80 °C for 7 h under the atmosphere of nitrogen. After natural cooling, the product was transferred into the rotary evaporator and evaporated away most of the solvent. After that, the product was dropped gradually into 1 L of cold ethyl ether and then filtrated. The deposited light yellow solid was dried in the vacuum drying box for 48 h at the temperature of 45 °C, then increased the temperature of the vacuum drying box to 105 °C and kept for one hour to get rid of moisture. 2.4. Measurements. C nuclear magnetic resonance ( 13CNMR) spectrograms of polymers were determined by a nuclear magnetic resonance instrument (Brooke, AVANCE Ⅲ). The Mw values of polymers were measured by a gel permeation chromatograph (Waters, GPC-1515) at 28 °C, and DMF was used as the eluent. 2.5. Cloud Point (TCl) Measurement. Knowing the cloud point (TCl) for KHIs is important because the application of inhibitors should be below the cloud point in order to avoid sediment.27 According to the procedure used in previous research,23 the cloud point measurement steps were as follows: First, 0.5 g of polymer was dissolved in 100 mL of deionized water. Second, 10 mL of this solution was measured into a test tube and heated it till there was haze in the tube. Then, the solution was cooled slowly and observed carefully throughout the measurement. The cloud point (TCl) was gained at the temperature at which the first vision of the haze totally disappeared in the solution. Every test was repeated for 3 times. 2.6. Experimental Apparatus. Experiment apparatus mainly included the Sapphire reaction cell which had an maximum effective

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volume of 38 mL and could withstand high pressures up to 12 MPa, pressure transducers (CYB-20S) with an uncertainty of ± 0.025 MPa, resistance thermometers (PT100) with a maximum uncertainty of ± 0.1 °C, constant temperature air bath, data acquisition and display module, and camera. The detailed schematic of the experimental apparatus was shown in Figure 3. Raman spectrograms of gas hydrate were measured by a Raman spectrometer (Horiba, LabRAM HR) at -100 °C in atmospheric pressure, and a 523 nm Ar+ laser was used. XRD spectrograms of gas hydrate were determined by a PXRD spectrometer (PANalytical, X’Pert Pro MPD) at -100 °C in atmospheric pressure, and the 2θ range was from 5° to 60°. The topographical microstructures of gas hydrate were obtained by a Cryo-SEM (Hitachi, S-4800) at -100 °C in vacuum environment. 2.7. Experimental Methods of Testing the Inhibitors Effect. The constant cooling method can measure the maximum degree of subcooling that the inhibitor can withstand.30-35 According to the literature, the constant cooling method had 4 main steps: (1) 12.0 mL of distilled water, in which the inhibitor was dissolved, was loaded in the reactor; (2) The reactor was evacuated by a vacuum pump and then was purged with CH4 at least three times; (3) Under a stirring speed of 800 rpm, the reactor was pressurized up to approximately 8.0 MPa at the temperature of 20 °C，then closed the intake valve; (4) The reactor was cooled from 20 °C to -8 °C at a constant cooling rate of approximately 1 °C /h (-8 °C was below the methane hydrate formation temperature in this pressure condition). Pressure and temperature sensors were used to collect the data during the process of the gas hydrate formation. A typical graph of the data obtained according to this procedure is shown in Figure 4. In this experiment, the concentration of kinetic inhibitor was 0.5 wt % (5000 ppm). Before the hydrate generated, the experimental pressure dropped in a constant rate during cooling because of the system

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being closed. The deviation from the pressure drop turned up as soon as the first onset of hydrate formation occurred. The temperature at which this happens is named the onset temperature (To). The To value of 0.5 wt % PVCSCOOH was -2.2 °C. Constant temperature method can measure hydrate induction time.36,37 This method is widely used to evaluate the inhibition performance of KHIs, although not as precise as visual observation.38 According to the literatures, the constant temperature method had 4 main steps: (1) 12.0 mL of distilled water, in which the inhibitor was dissolved, was loaded in the reactor; (2) The reactor was evacuated by a vacuum pump and then was purged with CH4 at least three times; (3) Maintained the temperature inside the reactor to a certain value (for example, 2 °C), then turned off the stirrer, after that, the reactor was pressurized up to approximately 8.0 MPa; (4) After the temperature and pressure maintained stable, stirrer was turned on at a speed of 800 rpm. Hydrate inhibition time (also known as the induction time) was the time from the moment of starting stirring to the moment of the pressure began to drop immediately. Typical pressure graphs of the data obtained according to the constant temperature procedure are shown in Figure 5. In this experiment, the concentrations of kinetic inhibitor were 0.5 wt % (5000 ppm) and 2.0 wt % (20000 ppm) respectively at the constant temperature of 0 °C. The 2.0 wt % PVCSCOOH t-P graph shows that when the stirring started, the pressure dropped a little because methane gas dissolved in water. Then, the temperature and pressure in this system remained stable till the hydrate generated. The pressure drop turned up because of the first observed onset of hydrate formation. The induction time of 2.0 wt % PVCSCOOH was 348 min. However, the 0.5 wt % PVCSCOOH t-P graph illustrates that gas hydrate formed as soon as the stirring started at the constant temperature of 0 °C.

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2.8. Sample Preparation of Gas Hydrate. After a dramatic drop, the pressure could maintain stable again. When the second stabilization of the pressure kept for more than 10 h, the ice in the high-pressure cell was considered to be transformed into CH4 hydrate to the largest extent. Then, the CH4 hydrate crystals in the high-pressure cell were taken out and ground (operated in liquid nitrogen vessel) into small partials with an average diameter of approximately 50 um. The ground CH4 hydrate crystals were stored in liquid nitrogen for the later Raman, PXRD and Cryo-SEM experiment. 3. RESULTS AND DISCUSSION Table 4 gives a summary of the onset temperatures (To) for different concentrations of PVCSCOOH. Contrast results with pure water, with PVPK90 and with PVCap are also included. It has been known that molecular weight is one of the important factors that affect the inhibition effect.14,25 Thus, the molecular weight of PVCap used in this experiment was roughly similar with that of PVCSCOOH. Therefore, their inhibition performance could be contrasted. Table 4 illustrates that all of the three polymers gave a better result than polymer-free. A statistically significant and clear trend showing an increase in KHI performance as the concentration of PVCap or PVCSCOOH increased could also be noticed. Interestingly, PVCSCOOH consistently provided lower To value compared to PVCap at the same concentration. For example, the To value of 0.5 wt % PVCSCOOH was -2.2 °C, much lower than the 0.5 wt % known KHI PVCap which had a To value of 1.7 °C. The disparity of To value between PVCSCOOH and PVCap in the concentration of 1.0 wt % and 2.0 wt % could also be observed. Considering the To value difference between the two was approximately 3 ~ 4 °C which was statistically significant from constant cooling test analysis, these

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results confirmed that the inhibition performance of PVCSCOOH is much better than that of the equivalent PVCap. For more intuitive, the To values of PVCSCOOH and PVCap are listed in figure 6. Table 5 shows the induction time for polymers in different degree of subcoolings. There is an obvious trend that the higher the concentration was, the longer the induction time could be. Also, PVCSCOOH consistently gave longer induction time when compared to PVCap at an equivalent concentration. For example, the induction time obtained from the 0.5 wt % PVCSCOOH solution in the subcooling degree of 7 °C was 2091 min，whereas the induction time of PVCap used at the same condition was only 500 min. Furthermore, even under the subcooling degree as high as 11 °C, the 1.0 wt % PVCSCOOH solution could prolong the gas hydrate induction time to 36 min, while the 1.0 wt % PVCap solution at the same subcooling degree almost produced methane hydrate at the moment of starting stirring. This illustrates that PVCSCOOH performed better than PVCap as a KHI to delay the formation of methane hydrate. The reasons why PVCSCOOH has better KHI performance than PVCap may be as follows. First, as shown in table 4 the cloud point of PVCSCOOH was higher than that of PVCap. PVCSCOOH had a TCl value of 39.8 °C, while 38.8 °C for PVCap. This implies that the PVCSCOOH which functionalized with carboxylic acid group caused greater hydrophilicity than the PVCap without the group. This might because the carboxylic acid is a hydrophilic group, so the polymer with a hydrophilic group tail interacted with water molecules via van der Waal interactions more suitably than the polymer without hydrophilic group.39 As a result, it might be difficult for the occupied cages trap guest molecules, and then hydrate nucleation was difficult to generate in this condition.40 Second, the end-capping carboxylic acid group of PVCSCOOH could insert into the gathered water structure, then might cause water perturbation which prevented the hydrate crystal or nuclei from occurring.25

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Additionally, Qin, etc., suggest that the kinetic inhibition performance might be linked to the gas/liquid interfacial tension.24 They reported that one of the effective KHIs: the phenyl-modified PVP, which has the lowest gas/liquid interfacial tension, prolonged the induction time to 82 min (0.5 wt %, subcooling 9.23 °C). This means that the inhibition performance of the phenyl-modified PVP is much worse than that of the equivalent PVCSCOOH (135 min, subcooling 9 °C). However, the phenyl is considered to be a hydrophobic group. The interfacial tension applicability deserves further investigation on PVCSCOOH. Figure 7 shows PXRD spectrum of the CH4 hydrates generated in the solution with 2.0 wt % PVCap and 2.0 wt % PVCSCOOH, compared with that generated without any polymer. On the one hand, the peak locations of the graphs with PVCap and PVCSCOOH are completely overlapped those of the graph of pure CH4 hydrate. This means that although PVCSCOOH, as well as PVCap significantly affected the kinetics performance of hydrate combination, they did not change the hydrates crystal structure.33 On the other hand, the ice intensities in the presence of PVCSCOOH or PVCap are higher than that of pure water. According to the phenomenon that the integral intensities of PXRD peaks are proportional to crystal quantities in solid mixture reported by Nagu Daraboina, et al. apparently, the polymers might impede the ice transform into hydrate.41 This implies that the addition of PVCSCOOH and PVCap could slow down the conversion of ice to hydrate. Figure 8 shows that there are large 51262 cages (corresponding to the Raman shift at 2905-1) and small 512 cages (corresponding to the Raman shift at 2905-1) in each graph, which have been known as the typical characteristics of structure I (sI) methane hydrate.42,43 In the pure water system, the large to small cavity ratio (L/S) is 3.13, which is close to the theoretically sI value of 3.0.44 However, in the experiment with PVCSCOOH or PVCap, all of the L/S values are less than that in pure water.

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Moreover, it is worth noting that the L/S value becomes less as the concentration of the polymers increases. The L/S value of 0.5 wt % PVCap is 3.12, but it turns to be 2.82 when the polymer concentration becomes 1.0 wt %, and finally stands at 2.01 in 2.0 wt % PVCap. The similar trend can also be found in the PVCSCOOH cases. The L/S value of 0.5 wt %, 1.0 wt % and 2.0 wt % PVCSCOOH is 3.10, 2.61 and 2.00 respectively, which means that although both of the two polymers have effects on the L/S value, PVCSCOOH affects slightly stronger. This implies that the presence of PVCSCOOH and PVCap in the system may influence the growth of the large and/or small hydrate cavities. According to the recent studies by Carver et al, the KHIs with lactam ring pendant group is more likely to stabilize in the large cavity.45 The oxygen and sulfur in PVCSCOOH probably form hydrogen bonds with water, which may contribute to a stronger interaction between the polymer and the large cavities.39 Thus, it may conduce to the more powerful function of PVCSCOOH to prevent the large cavity from capturing guest molecular. Figure 9 illustrates microstructure of methane hydrate observed by Cryo-SEM. The microstructure picture of methane hydrate without any additive has a relatively regular and tight hydrate surface. Then, with the additive of 0.5 wt % PVCap, the hydrate surface becomes chaotic and loose with a lot of stomata. Interestingly, there is an obvious trend that the higher of PVCap concentration is the more chaotic and porous the hydrate surface turns up. The microstructure change trend of the methane hydrate generated in the presence of PVCSCOOH at different concentrations is similar with that formed in the presence of PVCap, and the hydrate surface with PVCSCOOH even becomes more scattered than that with PVCap under the same concentration. Based on this, it is reasonable to deduce that although there is hydrate generation in the presence of PVCSCOOH, it may not block the pipeline, because it is easy to scatter the loose and porous solid.

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4. CONCLUSIONS A kind of carboxyl-terminated polyvinyl caprolactam has been synthesized and investigated as methane hydrate kinetic inhibitor. The cloud point of the polymer was higher than that of PVCap, which might imply that PVCSCOOH has better solubility. The inhibition performance of PVCSCOOH was evaluated in a high pressure sapphire reaction kettle. The measurement of To value was through constant cooling experiments at different concentrations and the constant temperature method was carried out to test the induction time. The constant cooling and the constant temperature measurements both confirmed that the inhibitory effect of PVCSCOOH was better than that of PVCap. Microscopic information of methane hydrate generated in the presence of PVCSCOOH was also detected by PXRD spectrometer, Raman spectrometer and Cryo-SEM. PXRD spectrums showed that PVCSCOOH did not change the methane hydrate structure, while Raman spectrums indicated that PVCSCOOH influenced the large to small cavity ratio. Cryo-SEM pictures showed that PVCSCOOH also altered the micromorphology of hydrate. Author information Corresponding author E-mail: [email protected]. Tel.: +86 20 8705 7669 Funding This work was supported by the National Natural Science Foundation of China (51376182), CAS Program (KGZD-EW-301), and Scientific cooperative project by CNPC and CAS（2015A-4813）. Notes The authors declare no competing financial interest.

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(21) Kelland, M. A.; Moi, N.; Howarth, M. Breakthrough in Synergists for Kinetic Hydrate Inhibitor Polymers, Hexaalkylguanidinium Salts: Tetrahydrofuran Hydrate Crystal Growth Inhibition and Synergism with Polyvinylcaprolactam. Energy Fuels 2013, 27(2), 711-716. (22) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R.L.; Sloan, E. D. Effective kinetic inhibitors for natural gas hydrates. Chem. Eng. Sci. 1996, 51(8), 1221-1229. (23) Kelland, M. A.; Abrahamsen, E.; Ajiro, H.; Akashi, M. Kinetic Hydrate Inhibition with N‑Alkyl‑ N‑vinylformamide Polymers: Comparison of Polymers to n‑Propyl and Isopropyl Groups. Energy Fuels 2015, 29(8), 4941-4946. (24) Qin, H. B.; Sun, C. Y.; Sun, Z. F.; Liu, B.; Chen, G. J. Relationship between the interfacial tension and inhibition performance of hydrate inhibitors. Chemical Engineering Science, 2016, 148, 182-189. (25) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy Fuels 2006, 20, 825−847. (26) Villano, L. D.; Kommedal, R.; Kelland, M. A. Class of Kinetic Hydrate Inhibitors with Good Biodegradability. Energy Fuels 2008, 22 (5), 3143-3149. (27) Reyes, F. T.; Kelland, M. A. First Investigation of the Kinetic Hydrate Inhibitor Performance of Polymers of Alkylated N‑Vinyl Pyrrolidones. Energy Fuels 2013, 27(7), 3730-3735. (28)Lou, X.; Ding, A.; Maeda, N.; Wang, S.; Kozielski, K.; Hartley, P. G. Synthesis of Effective Kinetic Inhibitors for Natural Gas Hydrates. Energy Fuels 2012, 26(2), 1037-1043. (29) Kvamme, B.; Kuznetsova, T.; Aasoldsen, K. Molecular dynamics simulations for selection of kinetic hydrate inhibitors. J. Mol. Graphics Modell. 2005, 23(6), 524-536. (30) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Is subcooling the right driving force for testing low-dosage hydrate inhibitors? Chem. Eng. Sci. 2005, 60(5), 1313-1321.

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(31) O'Reilly, R.; Ieong, N. S.; Chua, P. C.; Kelland, M. A. Missing Poly(N-vinyl lactam) Kinetic Hydrate Inhibitor: High-Pressure Kinetic Hydrate Inhibition of Structure II Gas Hydrates with Poly(N-vinyl piperidone) and Other Poly(N-vinyl lactam) Homopolymers. Energy Fuels 2011, 25(10), 4595-4599. (32) Tohidi, B.; Anderson, R.; Chapoy, A.; Yang, J.; Burgass, R. W. Do We Have New Solutions to the Old Problem of Gas Hydrates? Energy Fuels 2012, 26(7), 4053-4058. (33) Ajiro, H.; Takemoto, Y.; Akashi, M.; Chua, P. C.; Kelland, M.A .Study of the Kinetic Hydrate Inhibitor Performance of a Series of Poly-(N-alkyl-N-vinylacetamide)s. Energy Fuels 2010, 24, 6400-6410. (34) Kelland, M. A.; Kvaestad, A. H.; Astad, E. L. Tetrahydrofuran Hydrate Crystal Growth Inhibition by Trialkylamine Oxides and Synergism with the Gas Kinetic Hydrate Inhibitor Poly(Nvinyl caprolactam). Energy Fuels 2012, 26(7), 4454-4464. (35) Magnusson, C. D.; Liu, D. J.; Chen, E. Y. X.; Kelland, M. A. Non-Amide Kinetic Hydrate Inhibitors: Investigation of the Performance of a Series of Poly(vinylphosphonate) Diesters. Energy Fuels 2015, 29(4), 2336-2341. (36) Qin, H. B.; Sun, Z. F.; Wang, X. Q.; Yang, J. L.; Sun, C. Y.; Liu, B; Yang, L. Y.; Chen, G. J. Synthesis and Evaluation of Two New Kinetic Hydrate Inhibitors. Energy Fuels 2015, 29(11), 7135-7141. (37) Lee, W.; Shin, J. Y.; Kim, K. S.; Kang, S. P. Kinetic Promotion and Inhibition of Methane Hydrate Formation by Morpholinium Ionic Liquids with Chloride and Tetrafluoroborate Anions. Energy Fuels 2016, 30(5), 3879-3885. (38) Chen, L. T.; Sun, C. Y.; Chen, G. J.; Zuo, J. Y.; Ng, H. J. Assessment of hydrate kinetic inhibitors

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with visual observations. Fluid Phase Equilibria, 2010, 298, 143–149. (39) Sa, J. H.; Kwak, G. H.; Lee, B. R.; Park, D. H.; Han, K.; Lee, K. H. Hydrophobic amino acids as a new class of kinetic inhibitors for gas hydrate formation. Scientific Reports 2013, 3, 2428. (40) Ivall, J.; Pasieka, J.; Posteraro, D.; Servio, P. Profiling the Concentration of the Kinetic Inhibitor Polyvinylpyrrolidone throughout the Methane Hydrate Formation Process. Energy Fuels 2015, 29(4), 2329-2335. (41) Daraboina, N.; Ripmeester, J.; Walker, V. K.; Englezos, P. Natural Gas Hydrate Formation and Decomposition in the Presence of Kinetic Inhibitors. 3. Structural and Compositional Changes. Energy Fuels 2011, 25(10), 4398-4404. (42) Zhou, X. B.; Liang, D. Q.; Liang, S.; Yi, L. Z.; Lin, F. H. Recovering CH4 from Natural Gas Hydrates with the Injection of CO2−N2 Gas Mixtures. Energy Fuels 2015, 29(2), 1099-1106. (43) Hong, S. Y.; Jim, J. I.; Kim, J. H.; Lee, J. D. Kinetic Studies on Methane Hydrate Formation in the Presence of Kinetic Inhibitor via in Situ Raman Spectroscopy. Energy Fuels 2012, 26(11), 7045-7050. (44) Subramanian, S.; Sloan, E. D. Molecular measurements of methane hydrate formation. Fluid Phase Equlibria 1999, 158, 813-820. (45) Carver, T. J.; Drew, M. G. B.; Rodger, P. M. Inhibition of crystal growth in methane hydrate. J. Chem. Soc. 1995, 91(19), 3449-3460.

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Table 1. List of the Chemicals Used for the Experiments chemicals

purity

supplier

2,2′-azodiisobutyronitrile（AIBN）

>99.0%

Ark

Vinylcaprolactam (NVCap)

>98.0%

Tci

N,N-Dimethylformamide (DMF)

>99.0%

Acros

diethyl ether

>99.0%

Guangzhou Kutai Co.

mercaptoacetic acid

>97.0%

Alfa

pure CH 4 gas

99.9%

Guangzhou Yuejia Gases Co.

polyvinyl pyrrolidone

Tci

(PVPK90) deionized water

Laboratory-made

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Energy & Fuels

Table 2. Detail 13C NMR spectrum information of PVCap and PVCSCOOH Structural Formula *

C Atom

13

C NMR (CD3CN) δ

C-1

37.8

C-2

23.3

C-3

49.3

C-4

46.3

C-5

43.7

C-6

27.4

C-7

34.7

C-8

30.5

C-9

176.1

C-1’

37.5

C-2’

23.5

C-3’

49.6

C-4’

46.4

C-5’

43.7

C-6’

27.2

C-7’

34.8

C-8’

30.5

C-9’

176.6

C-10’

23.0

C-11’

171.6

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Page 20 of 30

Table 3. molecular weight data for PVCap and PVCSCOOH entry

Products a（PVCap: PVCSCOOH）

Yield (%)

Mw b

PDI b

(×1000) NVCap

100:0

59.3

12.5

1.42

NVCap+

1:99

48.7

15.9

1.32

mercaptoacetic acid a

Determined by 13C NMR spectra; b Determined by gel permeation chromatograph (GPC)

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Energy & Fuels

Table 4. Summary of To Values and TCl at 0.5 wt %, 1.0 wt % and 2.0 wt % for PVCSCOOH and two kinds of Known KHIs for Comparison. Polymer

Concentration (wt %)

To (°C)

Degree of

TCl (°C)

subcooling (°C) No additive

7.0

4.0

PVPK90

2.0

3.9

7.1

＞90

PVCap

0.5

1.7

9.3

38.8

1.0

0.5

10.5

2.0

-1.3

12.3

0.5

-2.2

13.2

1.0

-2.6

13.6

2.0

-4.0

15

PVCSCOOH

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39.8

Energy & Fuels

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Page 22 of 30

Table 5. Summary of induction time of 0.5 wt %, 1.0 wt % and 2.0 wt % PVCSCOOH at different degree of subcooling. Polymer

Concentration (wt %)

Degree of

Induction time (min)

subcooling (°C) PVCap

0.5

1.0

2.0

PVCSCOOH

0.5

1.0

2.0

7

500

9

25

11

0

7

2591

9

111

11

4

7

4683

9

1813

11

86

7

2091

9

135

11

0

7

3873

9

372

11

36

7

>5000

9

2304

11

348

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Energy & Fuels

*

Figure 1. Synthesis of Polyvinyl Caprolactams.

Figure 2. Synthesis of Carboxyl-terminated Polyvinyl Caprolactams.

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Figure 3. Schematic Diagram of Hydrate Synthesis Device. V1-V3, valves; T1、T2, resistance thermometers; P1、P2, pressure transducers; BC, buffer cell; GC, gas cylinder; C, crystallizer; MS, magnetic stirrer; DAS, data acquisition system.

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Page 25 of 30

8.5 8.0

P (MPa)

7.5 7.0 6.5 6.0

P T

5.5 5.0

0

200

400

600

800

1000

1200

1400

1600

24 22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8 -10

T(°C)

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

Energy & Fuels

t (min)

Figure 4. Typical Time-pressure and Time-temperature Graphs Obtained According to the Constant Cooling Procedure using 0.5 wt % PVCSCOOH.

25


Energy & Fuels

8.5

Hydrate onset 8.0

7.5

P (MPa)

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

Page 26 of 30

7.0

6.5

6.0 -100

— 2.0wt%PVCSCOOH — 0.5wt%PVCSCOOH 0

100

200

300

400

500

600

700

800

t (min)

Figure 5. Typical Time-pressure Graphs Obtained According to the Constant Temperature Procedure at 0 °C.

26


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4 3 2 1 0 -1 To(°C)

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

Energy & Fuels

-2 -3 -4 -5 -6 -7 -8

PVCSCOOH PVCap trend line PVCSCOOH trend line PVCap

0.5

1.0

1.5

2.0

Concentration (wt%)

Figure 6. To Values of PVCSCOOH and PVCap Versus Concentration.

27


*

25

30

35

432

421 332

20

400

15

211

210

10

*

433

* 410 411/330

* *

320

— PVCSCOOH — PVCap — pure water

Intensity(A.U.)

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

Page 28 of 30

321

Energy & Fuels

40

45

50

2θ(Degrees)

Figure 7. PXRD Patterns of CH4 Hydrate Formed in the Presence of 2.0 wt % PVCSCOOH or PVCap. Ice Peaks are Denoted *.

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— pure water( L/S=3.13) — 0.5wt% PVCap(L/S=3.12)

-1

Intensity (A.U.)

2905

(d)

-1

2915

2880

2890

2900

2910

2920

-1

2905

2930

2940

2915

2880

2890

-1

Intensity (A.U.)

— pure water( L/S=3.13) — 1.0wt% PVCap( L/S=2.82)

(e)

-1

2915

2880

2890

2900

2910

2920

-1

2905

— —

2930

2940

2880

2890

2890

2900

2910

2900

2910

2920

2930

2940

-1

Wavenumbers (cm )

(f)

pure water( L/S=3.13) 2.0wt% PVCap( L/S=2.01)

2915

2880

2940

— pure water( L/S=3.13) — 1.0wt% PVCSCOOH(L/S=2.61)

-1

2920

-1

2905


Intensity (A.U.)

-1

2930

-1

-1

2905

2920

2915

Wavenumbers (cm )

(c)

2910

Wavenumbers (cm )

Intensity (A.U.)

-1

2905

2900

-1

-1

Wavenumbers (cm )

(b)


Intensity (A.U.)

(a)

Intensity (A.U.)

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

Energy & Fuels

2930

2940

-1

2915

2880

-1

2890

2900

2910

2920

2930

2940

-1

Wavenumbers (cm )

Wavenumbers (cm )

Figure 8. Raman Patterns of CH4 Hydrate Formed in the Presence of PVCSCOOH or PVCap.

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Page 30 of 30

a

b

e

c

f

d

g

Figure 9. Microstructure Pictures of Methane Hydrate Generated in the Presence of (a) Pure Water; (b) 0.5 wt % PVCap; (c) 1.0 wt % PVCap; (d) 2.0 wt % PVCap; (e) 0.5 wt % PVCSCOOH; (f) 1.0 wt % PVCSCOOH; (g) 2.0 wt % PVCSCOOH. 30


Inhibition Effect Study of Carboxyl-Terminated Polyvinyl Caprolactam

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