Kinetic Studies on Methane Hydrate Formation in the Presence of

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Kinetic Studies on Methane Hydrate Formation in the Presence of Kinetic Inhibitor via in Situ Raman Spectroscopy Sang Yeon Hong, Jae Il Lim, Joung Ha Kim, and Ju Dong Lee* Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, 1274 Jisa-dong, Gangseo-gu, Busan 618-230, Republic of Korea ABSTRACT: To fully understand the influence of kinetic inhibitor, poly-N-vinylcaprolactam (PVCap), on methane hydrate formation, gas uptake measurement (macroscopic point of view) and in situ Raman spectroscopic analysis (microscopic point of view) were carried out simultaneously in a semibatch stirred tank reactor at constant temperature (T) and pressure (P). The capturing the behavior of guest molecules (large to small cavity ratio or cage variation) is one of the most important properties of gas hydrate studies. However the real-time properties of the cage variation under constant T and P conditions in an agitation system have not been reported previously. In this study, we measured this property (i.e., the large to small cavity ratio) from the in situ Raman spectra during hydrate formation in an agitation system instead of a static system, which provided valuable information on the time-dependent hydrate kinetic behavior. The study reveals that the presence of PVCap prevents the rate of large cavity encapsulation at an early stage of hydrate formation. The influence of PVCap from microscopic and macroscopic points of view is also presented. varied, depending on fields conditions, between 0.5 and 3.0 wt %.13 It is generally suggested that the kinetic inhibitors hinder hydrate formation by adsorbing to the surface of hydrate crystals, and also by sterically blocking guest molecules from entering and completing a hydrate cavity.14,15 Adsorption of kinetic inhibitors to the surface of hydrate has been studied extensively by Makogon and Carver et al.16,17 They found that kinetic inhibitors with lactam pendant groups adsorb to the hydrate surface with the lactam ring sterically stabilized in the large cavity. However, there is still a significant knowledge gap to fully understand the inhibition mechanism of kinetic inhibitors.14,16 In this study, the influence of a kinetic inhibitor, poly-Nvinylcaprolactam (PVCap), on methane hydrate formation was investigated. To get vital information for cage occupancy and capturing kinetics of guest molecules, in situ Raman analysis and gas uptake measurement (kinetic experiment) were carried out simultaneously. We present the molecular level of structure I (sI) hydrate cavity behaviors as well as traditional hydrate kinetic data, which have not been reported previously.

I. INTRODUCTION Gas hydrates are nonstoichiometric ice-like solid compounds consisting of small gas molecules and water molecules.1 Generally, gas hydrates have three basic crystal structures (structure I, structure II, and structure H) where gas molecules (guest molecules) are enclathrated in cavity structures that are formed by hydrogen-bonded water molecules (host molecules).2,3 They have attracted attention because of their great potential to be used as a gas storage and capturing medium. However, natural gas hydrate formation can cause blockages in subsea gas and oil flow lines, which can lead to catastrophic economic loss and ecological risks.2,4 Therefore, many researchers have tried to apply various inhibitors to delay the formation rate of gas hydrates because the prevention and removal of hydrates during natural gas and oil subsea production and transportation are major concerns to the energy industries.5,6 There are three classes of hydrate inhibitors; thermodynamic, antiagglomerant, and kinetic inhibitors. The thermodynamic inhibitors such as methanol and methyl ethylene glycol are extensively used.7 Thermodynamic inhibitors are additives that work by changing the hydrates' thermodynamic equilibrium conditions.7 A commonly used thermodynamic inhibitor is methanol.8,9 Thermodynamic inhibitors may not be practical in some cases where it would be costly to use and maintain them such as in offshore or remote production facilities. One of their major drawbacks is the fact that they must be used at high concentrations (10−50 wt %) in order to be effective.10 Kinetic hydrate inhibitors (KHIs) are a class of low dosage hydrate inhibitor (LDHI) and are commercial materials often used in the upstream oil and gas industry.11 They are usually watersoluble polymers and are effective at concentrations typically 10 to 100 times less than ethylene glycol or methanol concentrations.12 KHIs are effective at concentrations as low as 0.5 wt %; however, the optimum concentration of KHIs are © 2012 American Chemical Society

II. EXPERIMENTAL SECTION A. Materials. Analytical grade methane (99.9%), obtained from PS CHEM Co., LTD (Korea), was used as a guest substance for hydrate formation. PVCap solution (40 wt % in ethanol) was supplied by BASF and applied as a kinetic inhibitor. The molecular weight of the PVCap (Mw = 23 399) was measured by gel permeation chromatography (GPC, Waters e2695, U.S.A.) and a Waters 2414 refractive index detector. Distilled deionized water was used to prepare all solutions. B. Experimental Apparatus and Procedure. The experimental apparatus, as shown Figure 1, mainly consists of a stainless-steel Received: August 18, 2012 Revised: October 22, 2012 Published: October 22, 2012 7045

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Figure 1. Schematic diagram of experimental apparatus. reactor (372.5 mL), a supply vessel (566.5 mL), a refrigeration system, a water bath, a data acquisition system, and a real-time Raman spectroscopy instrument. Prior to each hydrate kinetic experimental run, the reactor was filled with 135 cm3 of aqueous test sample (pure water or liquid solution of inhibitor), and then, the reactor was flushed at least three times with the methane gas to remove any residual air. Subsequently, the whole system was kept at a desired temperature, and the reactor was filled with methane gas until the experimental pressure was obtained. All experiments were conducted at 5.0 MPa, 274.15 K. Once the temperature was stabilized, the solution was agitated using a magnetically driven impeller with a rotational speed of 300 rpm, which was determined to be suitable for our experiment and applied to every experimental run. As the gas in the reactor is consumed during the hydrate formation, additional methane gas is automatically supplied from a supply vessel, and the moles of gas uptake over time are calculated from the pressure drop profile in the supply vessel (>5.0 MPa) with respect to the initial pressure. A detailed description of the kinetic experimental procedure was given by Lee et al.9,18 A PID (proportional integral derivative) controller and metering and control valves were employed for maintaining constant pressure of the system. The reactor and the supply vessel sat in a water bath and water was circulated for temperature control. Two copper-constantan thermocouples were used to measure the liquid and gas temperatures in the reactor with an accuracy of 0.1 K. Pressure is being monitored by a pressure transducer with an accuracy of 0.5%. C. In Situ Raman Measurement. Laser Raman spectroscopy is one of the most powerful tools for the in situ study of time-dependent hydrate formation. As described in Figure 1, we conducted in situ Raman spectroscopic analysis during the experiments of hydrate formation. A Sentinel Raman spectrometer (Bruker Optics Ltd.) with a Unilab II probe (fiber optic) and a CCD (charge coupled device) detector was used in this work. In situ Raman analysis was carried out using optical fibers, allowing a Raman probe tip to be coupled to a spectrometer. A single cable is used to transmit the laser to the sample; at the same time, another fiber is then used to transfer the Raman signal from the sample to a standard spectrometer and detection

system. These two cables are connected to a compact, rugged Raman probe tip that focuses the laser onto the sample and collects the Raman signals. As shown in Figure 1, the probe tip was especially designed and inserted into the hydrate reactor for in situ Raman analysis. A 9.5 mm diameter cylindrical stainless-steel probe tip with an 8 mm ball lens (N-BK7, Edmund optics) was inserted through a fitting in the wall of the reactor. The front focal length (FFD = 3.7 mm), which is the distance from the front focal point of the system to the vertex of the first optical surface of the lens, was determined by the “lens-maker’s formula” using the refractive index and radius of curvature of the surfaces of the lens.19 A 532 nm Nd:Yag laser source was used for excitation with an incident laser power of 100 mW. The spectral range was 500 cm−1 to 4400 cm−1 and the spectral resolution was 4 cm−1 to 6 cm−1. During the hydrate formation experiment, the Raman spectra were collected with 3 s of spectral integration time and three accumulations at that integration time that are coadded to yield the spectrum. Therefore, the Raman spectra were collected at every 9 s (3 × 3 s). The probe tip with experimental setup was designed for use at high pressure and slurry or suspended particle system. It can operate in an immersion mode where the analysis probe tip was dipped within the reactor. Based on our experimental results, the spectra obtained from the moving hydrate particles can be used to express the gas uptake measurement pattern.

III. RESULTS AND DISCUSSION Table 1 gives a summary of experimental results for methane hydrate formation along with induction time, gas consumption (Δn), water to hydrate conversion (x), and large to small cavity ratio (L/S). Each experimental run was conducted in the absence or the presence of PVCap (0.04 wt %) under constant temperature and pressure conditions. The Δn, x, and L/S were measured after 700 min after the beginning of experiment, and the results are listed in Table 1. The hydrate conversion was calculated by assuming that the cage occupancy of CH4 is 100% 7046

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Table 1. Summary of Experimental Results for Methane Hydrate Formation with or without PVCap at 274.15 K and 5.0 MPa

system

no.

induction time (min)

pure water

1 2 3 4 5 6

5 6 4 174 134 153

0.04 wt % PVCap

gas consumption after 700 min, Δn (mol)

water to hydrate conversion after 700 min, x (%)

large to small cavity ratio after 700 min, L/S

1.006 1.002 0.998 0.671 0.664 0.654

77.1 76.8 76.5 51.4 50.9 50.1

3.29 2.95 3.13 2.90 2.88 2.85

in both the small and large cages. In this paper, 5.75 was referred to as the hydration number. All experiments showed similar patterns on the gas uptake measurement, and therefore, only two experimental runs (experiments 1 and 4 as listed in Table 1 and Figure 2) were

Figure 3. Comparison of kinetic and Raman peak intensity of CH4 hydrate formation with 0.04 wt % PVCap.

In all experiments, sufficient experimental time was given in order to perform full conversion of hydrate formation. Theoretically, 1.3 mol of CH4 should be required for the full conversion of hydrate formation at the given water amount (135 mL). As shown in Figure 2, the system with pure water reached around 77% of the theoretical value of conversion. However the system with 0.04 wt % PVCap showed a maximum of 51% hydrate conversion even though sufficient time (700 min) was given. This indicates that the PVCap significantly delays hydrate nucleation and also prevents further gas hydrate growth. Various researchers have studied hydrate formation kinetics using KHIs, and they also suggested that the KHIs prevent hydrate formation by delaying hydrate nucleation and growth.1,15 However, no molecular-scale information could be obtained from these macroscopic experiments, such as the formation rate of the hydrate cavity type (512, 51262, and 51264) and hydrate structure information. Raman and NMR spectroscopy have been used to study the molecular-scale hydrate formation processes.20−22 Most Raman spectroscopic studies to date have off-line (i.e., ex situ) measurements where hydrate samples are measured after system cooling and depressurization; thus, the sample status does not represent the real conditions. Recently, the in situ Raman technique was performed using an optical cell such as diamond anvil cell, in which real-time analysis was possible through a window under high pressure conditions.23−25 Nevertheless, this method is not appropriate for real industrial situations because it is typically used for measuring the sample without any agitation.26 In this study, in situ Raman analysis with the help of an inserted probe tip was performed under agitation, which would represent real industrial situations more closely. Furthermore, to get highly detailed information at the microscopic (molecular scale) as well as the macroscopic point of view, gas consumption data together with in situ Raman spectra were obtained at the same time. As shown in Figure 2, when the amount of gas consumption reached specific levels (0.08, 0.1, 0.3, and 0.6 mol consumption), each in situ Raman spectrum was obtained, which are plotted in Figure 4. The large to small cavity ratios, determined by Raman spectra, are also shown in Figure 4. The

Figure 2. Comparison of kinetics of CH4 hydrate formation with pure water and 0.04 wt % PVCap.

selected for further Raman analysis. Figure 2 shows the formation rate of CH4 gas hydrate with pure water and 0.04 wt % PVCap at 274.15 K and 5.0 MPa. (subcooling = 5.7 K). The formation rate of CH4 hydrate with PVCap was significantly less than that of the pure water system. The hydrate nucleation was also significantly delayed in the presence of PVCap. The induction time of the system with PVCap was 174 min, whereas that of the pure water system was 5 min at given experimental conditions. In the early stage of hydrate formation after the nucleation point, the system with PVCap showed very slow hydrate formation. Subsequently, 250 min after the beginning of experiment, the rate of gas consumption was increased, which is shown in Figure 2. As presented in the Figure 3, the gas uptake measurement and the Raman intensity data shows quite similar trend as function of time. In addition, the first appearance of Raman signals (at 2905 and 2915 cm−1) indicates exact induction time, which was monitored from gas uptake measurement. Therefore, it was concluded that the Raman spectra obtained from the moving hydrate particles in a stirred tank reactor can be used to describe the state of kinetic behavior. 7047

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Figure 4. Raman spectra for CH4 hydrate at the same gas consumption; (a) 0.08, (b) 0.1, and (c) 0.3 mol, (d) 0.6 mol.

Raman bands at 2905 cm−1 and 2915 cm−1 stand for the C−H stretching mode from CH4 encapsulated in large and small cages, respectively. It is also known that structure I (sI) has two different cages that consist of six large cavities (51262) and two small cavities (512) per unit cell. Hence, the ratio of large to small cavities theoretically should be 3.7 When the system reached 0.08 mol consumption (Figure 4a), the large to small cavity ratio (L/S) was 1.59 and 0.98 for pure water and PVCap systems, respectively. The value of L/S did not follow the theoretical capturing trend (L/S = 3) for both systems. This implies that, in the initial stage of hydrate formation, small cages were excessively formed, and in turn, the amount of large cages was not sufficient to form perfect unit cells of sI hydrate. A similar observation was reported by Subramanian et al., who suggested that the formation of the large 51262 cavity is the rate-limiting step for hydrate formation.26 As shown in Figure 4a and b, the L/S value of the PVCap system was less than that of pure water system. This indicates that the PVCap inhibits large cage encapsulation more; thus, the Raman band at 2905 cm−1 (corresponding to the large cavities, 51262) shows smaller intensity. As gas hydrate formation proceeded (shown in Figure 4c and d), the L/S discrepancy away from the theoretical value was reduced, and ultimately, the L/S values approached the theoretical trend.

Figure 5 shows a direct comparison of in situ Raman spectra as functions of time, recorded every 5 or 10 min after the first hydrate signals were observed. In the case of the pure water system, large cavities (51262) and small cavities (512) were gradually developed with time and approached the theoretical ratio of large to small cavities within several 10 minutes, while PVCap retarded the growth of large cavities much more as function of time. The L/S values of the PVCap system were also biased from the theoretical ratio trend. The L/S discrepancy between experimental and the theoretical values continued until 120 min (at this point the water to converted hydrate was around 50%). It was clearly observed that the Raman intensity of the PVCap system at 2905 cm −1 corresponding to the large cavities was very much weaker than that of pure water system for a certain period of time. This observation indicates that the presence of PVCap in the system affects the rate of large cavity encapsulation at an early stage of hydrate formation. As further hydrate formation progressed, the inhibiting effect by PVCap was found to be gradually reduced and the L/S value also reached the theoretical trend. It is possible that the PVCap in bulk solution is gradually reduced by adsorbing to the surface of hydrate crystals as hydrate formation progressed. Therefore, the inhibiting effect might be reduced correspondingly. 7048

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the distance between the interacting molecules is smaller than the van der Waals bond length, then the interactions become repulsive. The van der Waals bond length corresponds to the distance between the interacting molecules where the potential is at a minimum. Consequently, the favorable interactions between the lactam ring and the large cavity (steric fit) are highly dependent on the size of the lactam ring. Dependence like this was observed by Freer and Sloan.28 As many studies (by molecular simulation) have elucidated the adsorption of inhibitors on the surface of gas hydrates, it is believed from our experimental results that PVCap has more strong interactions with large cavities on the sI hydrate surface. These interactions effectively inhibited the large cavity formation and prevented further gas molecular encapsulation (shown in Figure 5).29−31 Based on our experimental results, it was also concluded that the inhibition effect of PVCap more successfully acts at an early stage of hydrate formation. As further hydrate formation progressed, the inhibiting effect was gradually reduced, and the cavity ratio also reached the theoretical trend, which was possibly related to the reduced PVCap concentration, by adsorbing on hydrate crystals as a function of time. These results appear to be closely related to the interaction between the lactam ring of PVCap and the large cavities, as many molecular-level studies suggested. Therefore, further studies are required to elucidate this unique behavior.

IV. CONCLUSIONS In this study, we presented an experimental technique to collect microscopic (molecular scale) and macroscopic information on the hydrate formation process. As described in the Experimental Section, the kinetics of CH4 hydrate formation and in situ Raman analysis were simultaneously carried out in a semibatch stirred tank reactor at constant temperature and pressure. From the result of hydrate kinetics, the presence of PVCap in the system significantly delayed hydrate nucleation and also prevented further gas hydrate formation. During the hydrate formation experiment, in situ Raman analysis provided timedependent capturing behavior of guest molecules. In the case of the pure water system, large cavities (51262) and small cavities (512) gradually developed with time and approached the theoretical ratio of large to small cavities within several 10 minutes. Meanwhile, the system with PVCap showed unstable behavior, and the L/S (large to small cavity ratio) values were also biased from the theoretical cavity ratio. The L/S discrepancy between the experimental and the theoretical values continued until 120 min. It was clearly observed that the Raman intensity of the PVCap system at 2905 cm −1 corresponding to the large cavities was very much weaker than that of the pure water system for a certain period of time. This observation indicates that the presence of PVCap in the system affects the rate of large cavity encapsulation at an early stage of hydrate formation. As further hydrate formation progressed, the inhibiting effect was gradually reduced, and the cavity ratio also reached the theoretical trend. The present data and methodologies on the microscopic and macroscopic observations may contribute to better understanding of inhibition mechanisms and practical applications for flow assurance in gas and oil pipelines.

Figure 5. Comparison of Raman spectra between pure water and PVCap system. The spectrum was displayed every 5 or 10 min after nucleation points, as shown in Figure 2.

Lederhos et al. suggested that the lactam rings of PVCap adsorb on the hydrate crystal and sterically block the hydrate growth.27 Furthermore, they hypothesized that a polymer network extends between small stabilized hydrate particles, possibly providing an inhibiting structure to the surrounding water while blocking the most active growth sites of hydrate crystals. Adsorption of kinetic inhibitors to the surface of methane hydrate (sI) has been studied extensively by Carver et al17 using Monte Carlo techniques. They found that kinetic inhibitors with lactam pendant groups adsorb to the hydrate surface with the lactam ring sterically stabilized in the large cavity. These results on the sII surface were also later confirmed by Freer and Sloan,28 making use of molecular dynamics simulations to predict the performance of kinetic inhibitor structures by simulating adsorption on the sII {111} hydrate growth plane. PVP (poly-N-vinylpyrrolidone), PVCap (poly N-vinylcaprolactam), and PVVam (poly-N-vinylvalerolactam) inhibitor systems were compared by them. These kinetic inhibitors differ from each other only in the size of the lactam ring. The van der Waals forces as calculated from these simulations increased with increasing lactam ring size, a result that is consistent with the Lennard-Jones potential in that the potential energy increases due to attractive forces as the distance between the interacting molecules decreases. The distance between the carbon atoms that constitute the lactam ring and the water molecules representing the partially built cage will reduce as the ring size increases. More favorable interactions are obtained when this distance decreases, up to an optimal size. However, if 7049

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(18) Kim, S. M.; Lee, J. D.; Lee, H. J.; Lee, E. K.; Kim, Y. Gas hydrate formation method to capture the carbon dioxide for pre-combustion process in IGCC plant. Int. J. Hydrogen Energy 2011, 36, 1115−1121. (19) Greivenkamp, J. E. Field Guide to Geometrical Optics; SPIE Press: Bellingham, WA, 2004; pp 6−9 (20) Gupta, A.; Dec, S. F.; Koh, C. A.; Sloan, E. D., Jr. NMR investigation of methane hydrate dissociation. J. Phys. Chem. 2007, 2341−2346. (21) Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Evidence of structure II hydrate formation from methane + ethane mixtures. Chem. Eng. Sci. 2000, 55, 1981−1999. (22) Daraboina, N; Ripmeester, J.; Walker, V. K.; Englezon, P. Multiscale assessment of the performance of kinetic hydrate inhibitors. In Proceedings of the 7th International Conference of Gas Hydrate, Edinburgh, Scotland, 2011. (23) Chou, I-M.; Sharma, A.; Burruss, R. C.; Hemley, R. J.; Goncharov, A. F.; Stern, L. A.; Kirby, S. H. Diamond-anvil cell observations of a new methane hydrate phase in the 100-MPa pressure range. J. Phys. Chem. 2001, 4664−4668. (24) Sloan, E. D.; Subramanian, S.; Mattews, P. N.; Lederhos, P; Khokhar., A. A. Quantifying hydrate formation and kinetic inhibition. Ind. Eng. Chem. Res. 1998, 37, 3124−3132. (25) Komail, T.; Kawamura, T.; Kang, S.; Nagashima, K.; Yamamoto, Y. In situ observation of gas hydrate behavior under high pressure by Raman spectroscopy. J. Phys.: Condens. Matter 2002, 14, 11395. (26) Subramanian, S; Sloan, E. D., Jr. Molecular measurements of methane hydrate formation. Fluid Phase Equilib. 1999, 813−820. (27) Lederhos., J. P.; Long., J. P.; Sum., A; Christiansen., R. L.; Sloan, E. D., Jr. Effective kinetic inhibitors for natural gas hydrates. Chem. Eng. Sci. 1996, 1221−1229. (28) Freer, E. M.; Sloan, E. D., Jr. An engineering approach to kinetic inhibitor design using molecular dynamics simulations. Ann. N.Y. Acad. Sci. 2000, 651−657. (29) Kvamme, B.; Kuznetsova, T.; Aasolden, K. Molecular simulations as a tool for selection of kinetic hydrate inhibitors. Mol. Simul. 2005, 31, 1083−1094. (30) Koh, C. A.; Savdge, J. L.; Tang, C. C. Time-resolved in-situ experiments on the crystallization of natural-gas hydrates. J. Phys. Chem. 2006, 100, 6412−6414. (31) King, H. E.; Hutter, J. L; Lin, M. Y.; Sun, T Polymer conformations of gas-hydrate kinetic inhibitors: A small-angle neutron scattering study. J. Chem. Phys. 2000, 112, 2523−2532.

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-51-974-9274. Fax: +82-51-974-9660. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Energy Efficiency and Resources Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and Industrial Infrastructure Program through The Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea government Ministry of Knowledge Economy (2010201010094A, N009700001). This research was also a part of the project titled “Development of key technology in seawater desalination using gas hydrate process” funded by the Ministry of Land, Transport and Maritime Affairs, Korea.

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