Article pubs.acs.org/EF
Influence on Hydrate Dissociation for Methane Hydrates Formed in the Presence of PolyVinylCaprolactam versus PolyVinylCaprolactam + Butyl Glycol Ether Ann Cecilie Gulbrandsen*,† and Thor Martin Svartaas Department of Petroleum Engineering, Faculty of Science and Technology, University of Stavanger, Stavanger 4035, Norway S Supporting Information *
ABSTRACT: It has previously been shown that hydrates formed in the presence of efficient kinetic hydrate inhibitors (KHI) such as PolyVinylCaprolactam (PVCap) dissociate at temperatures above the equilibrium dissociation temperature of the uninhibited hydrates. The question regarding how hydrates form and dissociate as a function of time is an important issue for the industry. As KHI, commercial PVCap is commonly supplied in a solvent, e.g., butyl glycol ether (BGE), and the solvent acts as a synergist and improves the KHI performance of PVCap. In the present paper, the effect of BGE on dissociation of methane hydrates formed in the presence of PVCap has been examined. The pure BGE solvent showed no effect on the dissociation temperature at the applied concentrations. Two different molecular weight fractions of PVCap (Mw = 2k and 6k) have been tested either as pure substance or in mixture with BGE. The PVCap polymers have been synthesized in different solvents and stored for different periods of time prior to testing. The results show that both the production method and the time of storage influence their effect on the hydrate dissociation temperature. Hydrates formed in the presence of a “fresh” 1:1 solution of PVCap-2k and BGE (Inhibex 101) showed a reduction in dissociation temperature compared to hydrates formed in the presence of PVCap alone. On the other hand, hydrates formed in the presence of PVCap-6k + BGE showed a slightly increased dissociation temperature as compared to hydrates formed with PVCap alone. It appears that both production method and aging of PVCap and BGE can affect the influence on the hydrate dissociation temperature.
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INTRODUCTION Gas hydrates are nonstoichiometric and ice like crystal compounds that are formed from water and small sized nonpolar hydrocarbon gas molecules, such as methane, ethane, propane, and iso-butane, in addition to inorganic gases, such as CO2, nitrogen and others.1 In contrast to ice, hydrates may form at temperatures above the freezing point of water and formation temperature is pressure dependent. The potential for gas hydrate formation in hydrocarbon transportation lines is a major issue that needs to be addressed when considering possible solutions for field production. The issue concerning hydrate formation is particularly important for situations like long tie-backs, deep water, and shut-in situations in general, since the fluids then have time to cool down into the hydrate region. Hydrate inhibition and control is often the design basis for deepwater field development.2 Injection of inhibiting chemicals known as thermodynamic hydrate inhibitors (THIs) has been the conventional method applied by the oil and gas industry when dealing with the issue of unwanted hydrate formation in pipelines. THIs are strong hydrogen bonding chemicals that shift the equilibrium curve toward lower temperatures or higher pressures. Typical THI concentrations to obtain the required inhibiting effect may vary from 10 to 60 wt % of the water phase present. In recent years, environmental restrictions and economic aspects have facilitated the use of low dosage hydrate inhibitors / kinetic hydrate inhibitors (LDHIs/KHIs).2−10 Polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) are chemical additives that have been employed as KHIs. It is believed that KHI polymers delay the hydrate nucleation and/or hydrate © 2017 American Chemical Society
growth process by surface adsorption on nuclei, resulting in an increased induction time.11−15 If the KHI-induced induction time is longer than the pipeline fluid residence time at a given condition, then the KHI is able to prevent hydrate formation. Some polymers act as synergists, increasing the performance of the polyvinyllactams, particularly PVCap. KHIs are often used with synergists that can significantly enhance the kinetic inhibition performance of the KHIs.16 ISP (International Specialty Products, now part of Ashland) tested several polymers synthesized in their laboratories and a patent concerning the use of BGE as synergist for vinyl caprolactam polymers.17 The main synergist example was BGE which not only was a good synergist for PVCap but could also be used as the solvent. Glycol ethers have surfactant-like properties.18 The addition of KHIs have been reported to reduce the hydrate growth rate up to a certain point, where after it is followed by an accelerated growth referred to as catastrophic growth.19−23 The reason behind this is not understood but it has been proposed that the hydrate crystals formed in the presence of KHIs might have a morphology that facilitates capillary movement of water molecules to the gas/liquid interface.24,25 Adding synergists to the system complicates the behavior. Investigations of the majority of hydrate related industrial and academic problems, mainly boils down to the most challenging and intriguing question; how gas hydrates form and dissociate against time. Although the structures and equilibrium Received: December 30, 2016 Revised: May 8, 2017 Published: May 9, 2017 6352
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Energy & Fuels thermodynamics of gas hydrates are well characterized, the time dependent processes like formation and decomposition kinetics for hydrates are not well understood.26 The mechanisms behind both formation and dissociation kinetics still need to be elucidated at multiple scales.27 Few molecular descriptions are available for natural gas hydrates formed in the presence of inhibitors.9,28−31 These suggest that inhibitors can have an impact on the structure and composition of mixed hydrates. Previously, reports have stated that the presence of kinetic inhibitors can lead to increased order at the surface, and consequently the water molecules will be more efficiently packed.30 Theories for the interaction of polymers with the clathrate hydrate surface have been proposed, suggesting the interaction takes place by adsorption of pendant groups (i.e., caprolactam) into incomplete large cages on the hydrate surfaces.32,33 Until now, the inhibition mechanisms of KHIs have not been very clear. The research community is very interested in understanding the interaction between KHIs and the hydrate constituents. Studies have indicated that hydrates formed in the presence of KHIs are harder to dissociate than hydrates formed in systems without KHIs present.24,34−40 All these studies showed that the conditions for final hydrate dissolution were displaced toward higher temperatures or lower pressures. This behavior has implications for hydrocarbon transport pipelines as hydrate plugs from water containing KHI may be more difficult to decompose. Furthermore, KHIs are often used together with synergists, and the synergistic mechanisms are not clearly understood. Sloan et al.41 indicated that the performance of PVCap is related to the molecular weight of the polymer. Polymers with an average molecular weight, Mw, of 900 were more effective than polymers with Mw of 1300, 2100, 9200, and 18 000. It is assumed that low-molecular-weight PVCap-based products with added synergists are probably the best KHIs for natural gas hydrate inhibition on the market today. Various synergists, including high molecular weight poly(ethylene oxide) (PEO), can improve the inhibition performance of PVCap.42 Cohen et al.43 have proposed an explanation of the improved performance of kinetic inhibitors in the presence of glycol ethers through effects on the conformation of the polymer in solution. An extended polymer would presumably have more of its length available for interaction with the hydrate crystal and perhaps an alteration regarding the adsorption of the polymer onto hydrate embryos or crystal structures. Although PEO itself is not a kinetic inhibitor, the addition of PEO to a kinetic inhibitor solution was found to enhance the performance of the inhibitor by an order of magnitude, in some cases. It is therefore believed that incorporating PEO into polymer chains may improve the inhibition performance. Research efforts often involve the study of the onset of hydrate formation, nucleation, and growth. Few publications address the dissociation phenomena, especially for hydrates formed in the presence of KHIs in addition to synergists. Qin et al.44 reported that a lower interfacial tension corresponds to a longer onset time, i.e., better kinetic inhibition performance. They therefore suggested a lowest gas/liquid interfacial tension rule for developing KHI or screening KHI synergists as well as for determining the most suitable dosage applied.
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Figure 1. Outline of the experimental setup of the cells. stainless steel cylinder with a top and bottom end piece. A stirrer blade is connected to a magnet house in the bottom end piece via an axle. An outer rotating magnetic field created by a laboratory stirrer bar drive was used to regulate the stirrer speed. The stirrer motor can be regulated to maintain speed in the range 0 to 1200 rpm. The free volume between the top and end pieces is 197 mL for the steel cell and 141.3 mL for the titanium cell. The free volume (dead volume) around the stirrer magnet inside the magnet house is 8 mL (for both cells). The cells are equipped with a cooling/heating jacket connected to a JULABO F34 HL refrigerated circulator (“High Tech” series with integrated programmer) and the internal bath temperatures could be logged via an RS232 interface connection. The cell temperatures were measured using 1/10 DIN Pt100 elements (DIN = German Industry Norm, accuracy ±0.03 °C). Rosemount 3051TA absolute pressure transmitters were used to collect pressure data. The accuracy of the logged temperature and pressure signals were ±0.1 °C and ±0.25 bar, respectively. Pressure is measured in the inlet tubing and the temperature is measured inside the cell (in the vapor phase). Data were sampled on a computer using the LabView data acquisition program. The experimental progress was continuously monitored on the computer screen during the experiments. At the end of the experiment, data were analyzed for graphical presentation. Hydrates were produced using pure methane (scientific grade 5.5, i.e.,99.9995 mole% purity). KHI chemicals used were as follows: (i) PolyVinylCaprolactam (PVCap), pure; dry powder of the polymer, PVCap 6k (Mw = 6000). The PVCap batch used in the experiments was originally supplied as 50% solution in isopropanol and the isopropanol was removed through a two stage process with vacuum distillation at 20 °C followed by a final vacuum treatment at approximately 10−5 Pa overnight to produce dry PVCap powder. (ii) Inhibex 101 from ISP: mixture of 50 wt % PVCap 2k (Mw = 2000) and 50 wt % butyl glycol ether. PVCap monomers were polymerized in BGE. (iii) Pure PVCap ∼2k precipitated from “aged” Inhibex 101 (precipitation process described below). The following procedure was followed in order to obtain precipitation of PVCap contained in Inhibex 101. (i) Pentane was added to Inhibex 101 to precipitate PVCap from the BGE solution. (ii) Precipitated PVCap was separated from the liquid solution by decantation. (iii) New pentane was added and decantation was repeated. (iv) The precipitated product was left to dry overnight. (v) NMR was applied to analyze the precipitated product and the results showed traces of pentane and BGE. (vi) The product was put in a heating chamber at 60 °C and left to dry overnight.
EXPERIMENTAL PROCEDURE AND DESCRIPTION
Two different cells have been used in the experiments, titanium and stainless steel, and a calibration has been performed. Figure 1 shows the experimental setup of a cell consisting of a titanium cylinder/ 6353
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Energy & Fuels (vii) Pentane was again added to the finished product, and the mixture was thereafter decanted. (viii) The precipitated product was put in a heating chamber at 60 °C and left to dry overnight. (ix) NMR was performed on the precipitated product, and the results still showed traces of pentane and BGE. The result was fine white powder. The procedure for preparation of the various experimental mixtures and filling of the cell is described below. (1) The required amount of PVCap or Inhibex 101 was weighed into a volumetric flask (where half was filled with distilled water (DIW)). At the applied concentrations both pure PVCap powder and Inhibex 101 dissolved completely in DIW within a few minutes at room temperature. The in-house prepared 50:50 blend of PVCap + BGE solution was stirred at 250 rpm for 48 h, prior to weighing the required amount of this composition into a volumetric flask. (2) The magnet house was filled with 10−15 mL of the aqueous solution, and any air residue was squeezed out of the bottom section during mounting of the magnet house. (3) The gas−liquid volume ratio was maintained constant in both cell types adding 50 and 70 mL of the aqueous solution into the titanium cell and steel cell, respectively, and the top end piece was mounted. (4) The heating/cooling unit was adjusted to room temperature or a temperature outside the hydrate region prior to filling of the gas blend to be used. (5) To remove (dilute) any residues of air in the cell, it was purged twice with the actual gas to be used, to a pressure of 60 bara. (6) The cell was then loaded to the desired pressure (at a temperature outside the hydrate region). (7) The system was cooled down. (8) Hydrate formation was induced by starting the magnetic stirring. Hydrate growth took place over a temperature region stretching from T1 (start of hydrate formation) to T2 (end of hydrate formation) corresponding to points B and C in Figure 2. In the figure,
low heating rate is required to provide uniform temperature equilibrium throughout the hydrate mass and the cell volume during the melting process. (11) The pressure and temperature conditions in the cell were frequently sampled during the experiment. Details of the cooling− formation−dissociation process is illustrated in Figure 2 and explained below. Hydrate formation was indicated by a sudden pressure drop in the cell due to gas leaving the vapor phase and entering empty cages in the hydrate structure. This can be seen along the path B−C in Figure 2, which describes a typical pressure−temperature plot of sampled data during a run. The experiment is initiated at point A in Figure 2, from where the sample is cooled down and stirring is initiated to induce hydrate formation. Hydrate formation is indicated by an increase in temperature and a decrease in pressure along path B−C. This happens as gas molecules get encaged in the hydrate lattice while energy is released due to exothermic reaction. After liquid was transformed into hydrate (at point C), the temperature was increased in a rapid manner up to point E in Figure 2. From this point and upward a slow heating rate of 0.2 °C/h was applied. The final hydrate dissociation point (Teq,Peq) occurs at point D where the dissociation curve intersects the cooling curve (A−B baseline). The region to the right of point D represents pressure and temperature conditions for system without hydrates present. At a heating gradient lower than 0.2 to 0.3 °C/h the dissociation point in pure system without inhibitor present is reproducible around ±0.1 °C and ±0.25 bara of measured values. The initial pressure and temperature of hydrate formation is expressed P1 and T1, respectively (cf. point B in Figure 2). The final pressure and temperature of hydrate formation is expressed P2 and T2, respectively (cf. point C in Figure 2). The conditions T2, P2 at point C is commonly just inside the hydrate stability region but close to and just above the hydrate equilibrium curve. The displacement of the hydrate dissociation temperature was expressed as ΔTdissoc* = Teq,exp − Teq,sim
(1)
where Teq,exp is the experimentally measured hydrate dissociation temperature of the systems containing kinetic hydrate inhibitor (PVCap or Inhibex 101), and Teq,sim is the simulated hydrate equilibrium temperature for hydrates without inhibitor at the same pressure conditions using CSMHYD.45 For systems with efficient KHI present, the final dissociation temperature may be affected by reduced kinetic rates. Thus, measured differences in dissociation temperatures between systems with and without KHI present may be due to kinetic effects or thermodynamic effects or a combination of both. Simulated dissociation temperatures were compared with experimental dissociation temperatures for hydrates formed in the presence of PVCap versus Inhibex 101. For the stainless steel cell used in the present study, the cell temperature is measured in the vapor phase. This is assumed sufficient to detect system temperature during dissociation at slow heating rates of 0.2 °C/h and below. This has been confirmed by similar studies in our titanium cells which are of similar size and geometry but contains temperature sensors in both the vapor phase and liquid phase. Discrepancy between temperatures in vapor and liquid phases occur during formation and growth due to rapid heat release from the forming hydrates. The dissociation process proceeds at far slower rates and using heating rates of 0.2 °C/h or below we are not able to detect any difference in measured temperatures in vapor phase versus water phase. Figure 3 shows temperatures in vapor phase and liquid phase in the titanium cell during dissociation of sII hydrate in system with Inhibex 101 around the final dissociation point of this system. The differences between the vapor and liquid temperatures during dissociation of the hydrates were negligible and we trust the measurements in the vapor phase to be accurate.
Figure 2. Pressure versus temperature plot obtained from an experiment where cooling (A−B) and hydrate formation (B−C) is followed by hydrate dissociation (C−E−D). At point E, the heating gradient is reduced from a fast rate in the region C−E to a slow rate of 0.2 °C/h in the region E−D−A to obtain an accurate equilibrium dissociation point (Teq,Peq) at D. The PT condition at point C (T2,P2) is close to the equilibrium curve. points B and C are located at the same temperature but in the experiments, points B and C were normally separated so that they spanned over a temperature interval. (9) The stirring rate was kept constant at 750 rpm during all experiments. (10) Hydrates were melted by gradually increasing the cell temperature at different preset heating rates dependent on the stage of the process. In the vicinity of the final hydrate equilibrium point, a
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RESULTS AND DISCUSSION For hydrates formed from methane and pure water without PVCap, all measured dissociation temperatures were within 6354
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systems (Inhibex 101 versus BGE + PVCap-6k synthesized in isopropanol). The polymerization process seems to be influential for the manner in which PVCap interacts with the hydrate. It has been reported that the solvent used for the polymerization process is an important factor for the polymer performance.46 Some compounds have the ability to participate in the polymerization reaction, in addition to serving as the inert solvent. Some experiments compared PVCap polymers produced in different solvents. One PVCap was synthesized in ethylene glycol monobutyl ether (EGMBE) solvent and another PVCap was synthesized in isopropanol. The isopropanol component was thereafter removed and replaced with an equal amount of EGMBE as in the first sample. The performance of the PVCap synthesized in EGMBE was clearly superior to the PVCap synthesized in isopropanol.47 It is known that EGMBE has the ability to participate in the polymerization reaction and to incorporate itself into the polymer chain.46 Furthermore, a molecular dynamics study has indicated that that the expansion of a PMMA polymer chain depends mainly on the conformation of solvent molecules instead of the interaction between the polymer chain and the functional group on solvent molecules.48 Tung et al.48 found that the asphericity of polymer chains increased with the increase of interaction force between polymer chain and solvent, i.e., the molecular weight of the solvent. On the other hand, the difference of asphericity of polymer chains in various solvent environments also implies the difference of interaction behavior between polymer chains and solvent molecules. This variation can be examined to elucidate which of attractive or repulsive interactions between polymer chains and solvent molecules is dominant. Since the interaction forces among polymer chains are equal in the simulated solution models, the difference of asphericity of polymer chains in various solvent environments is mainly caused by the difference of interaction forces between polymer chains and solvent molecules. A polymer chain with asphericity away from unity implied a straighter and more aspherical conformation. Also Bishop et al.49 has reported that the repulsive force plays an important role in chain deformation. A study was undertaken where interfacial tension measurements were performed for natural gas hydrates with Inhibex 501 and 2.35 wt % ethanol.44 It was found that Inhibex 501 and 2.35 wt % ethanol decreased the gas/liquid interfacial tension compared to a pure water system with 2.35 wt % ethanol. In addition, stronger inhibition performance was obtained with
Figure 3. Measurement points show no difference between vapor and liquid temperature during dissociation of sII hydrates at a heating gradient of 0.2 °C/h.
±0.15 °C agreement with CSMHYD predicted values. Furthermore, hydrates formed with 750 or 1500 ppm BGE only showed experimental dissociation temperatures in agreement with predicted equilibrium values for the system with pure water. Table 1 shows that hydrates formed with fresh Inhibex 101 give reduced displacement of the hydrate dissociation temperature as compared to hydrates formed with PVCap-6k or PVCap ∼2k only. For hydrates formed with pure PVCap the dissociation temperature increased by approximately 3 °C as compared to uninhibited system, while PVCap in the original BGE solution of Inhibex 101 where it was synthesized, gave hydrate dissociation temperature around 1.5 °C above that of the uninhibited system. This decrease in hydrate dissociation temperature as compared to hydrates formed with PVCap-6k or PVCap ∼2k alone is significant. Furthermore, hydrates formed with BGE + PVCap-6k (synthesized in isopropanol and dried through vacuum distillation) gave higher average dissociation temperature than hydrates formed with PVCap6k only. However, this increase could not be stated significant due to slight overlap in distribution domain of the measured values (3.3 ± 0.2 °C versus 2.9 ± 0.2 °C, respectively). Hydrates formed with PVCap in mixture with the synergist BGE solvent definitely show very different behavior in the two
Table 1. Measured Displacement of the Hydrate Dissociation Temperature (ΔTdissoc*) for sI Methane Hydrates Formed from Water Solutions Containing (a) PVCap-6k, (b) PVCap Precipitated from Inhibex 101, (c) Inhibex 101 (PVCap-2k + BGE), and (d) PVCap-6k + BGE no. of expt
PVCap-6ka (ppm)
PVCap∼2kb (ppm)
PVCap-2kc (ppm)
BGEc (ppm)
BGEd (ppm)
6 6 8 10 5 5
750 0 1500 1500 0 0
0 0 0 0 1500 0
0 750 0 0 0 1500
0 750 0 0 0 1500
0 0 0 1500 0 0
Pexp (bara) 94.9 94.2 95.5 96.0 93.8 93.3
± ± ± ± ± ±
0.2 0.2 0.1 0.3 0.4 0.8
Texp (°C)
Teq ,sim (°C)
± ± ± ± ± ±
13.0 12.9 13.0 13.1 12.9 12.8
15.9 14.4 15.9 16.4 15.8 14.4
0.2 0.2 0.3 0.2 0.4 0.2
ΔTdissoc* e (°C) 2.9 1.5 2.9 3.3 2.9 1.6
± ± ± ± ± ±
0.2 0.2 0.2 0.2 0.4 0.2
a PVCap-6k initially synthesized in isopropanol. Applied as dry powder produced through vacuum distillation and vacuum treatment. bDry powder PVCap ∼2k produced from old Inhibex 101 through precipitation. The length of the polymer chain may be shorter than the original PVCap-2k. c PVCap and BGE as constituents of the original Inhibex 101 solution. dBGE from other supplier. Pure BGE alone (750 and 1500 ppm) showed no effect on the hydrate dissociation temperature. The temperatures coincided with Teq ,sim. eAdditional information: 1500 ppm aged Inhibex 101 showed ΔTdissoc* = 2.9, the same value as 1500 ppm PVCap-6k and 1500 ppm PVCap ∼2k.
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Energy & Fuels Inhibex 501. The KHI tested was a polymer with both hydrophobic and hydrophilic functional groups and the test indicated that KHIs can be absorbed on the surface of an aqueous solution with the hydrophobic groups facing the gas phase. Lower gas/liquid interfacial tension implies stronger liquid/surface adsorption and the KHI molecules have stronger hydrophobic functional groups. On the other hand, with respect to the interface between the aqueous phase and the hydrate, the stronger adsorption of KHI molecules on the surface of the aqueous face or the stronger hydrophobic function groups produces a stronger barrier between liquid water molecules and the hydrate. It was proposed that the synergist may work by enhancing the adsorption of KHI molecules. Tohidi and Yang22 found that glycol ether compounds show significant synergistic effect on the performance of PVCap. The synergistic effect noticeably prolongs the nucleation time of PVCap and extends the delay of catastrophic growth by several times. It was suggested that it is likely that the presence of glycol ether molecules enhances the adsorption of PVCap molecules on the hydrate growth sites more significantly than on hydrate nucleation sites. Our findings related to hydrates formed from BGE + PVCap synthesized in isopropanol indicate that there is a somewhat increased stability for these hydrates. This may support the idea that there is an enhanced adsorption of KHI molecules. However, for the hydrates formed from Inhibex 101 there is a reduced stabilization as compared to PVCap alone in the solution. Different formulations of polymer and synergist could possibly alter the spatial conformation of the PVCap polymer in solution in different ways and consequently alter the interaction characteristics between the hydrate crystal and the polymer. The manner in which the PVCap molecules situate themselves in the hydrate surface channels or become embedded in the hydrate surface could then affect the hydrate morphology or degree of crystal heterogeneity, which again have consequences for the hydrate stability. Formulation can drastically enhance the hydrate dissolution properties of hydrates. A better understanding of hydrate dissociation kinetics can assist in hydrate plug remediation from a flow assurance perspective.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ann Cecilie Gulbrandsen: 0000-0001-6231-4839 Present Address †
A.C.G.: Statoil ASA, Forusbeen 50, 4035 Stavanger, Norway.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank The Norwegian Ministry of Education and Research and the University of Stavanger for their support to this work. The authors also thank StatoilHydro and British Petroleum for their financial support to the hydrate laboratory at the University of Stavanger. Furthermore, the authors thank Eirin Abrahamsen and Professor Malcolm A. Kelland for excellent discussions and experimental assistance.
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REFERENCES
(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; Taylor & Francis/CRC Press: Boca Raton, FL, 2008. (2) Balson, T.; Craddock, H. A.; Dunlop, J.; Frampton, H.; Payne, G.; Reid, P.; Fu, B. The Development of Advanced Kinetic Hydrate Inhibitors. In Chemistry in the Oil Industry VII: Performance in a Challenging Environment, Royal Society of Chemistry: Cambridge, U.K., 2002; pp 264−276, DOI: 10.1039/9781847550460-00264. (3) Bloys, B.; Lacey, C. Laboratory Testing and Field Trial of a New Kinetic Hydrate Inhibitor. In Offshore Technology Conference, Houston, TX, May 1−4, 1995. (4) Christiansen, R. L.; Sloan, E. D. Mechanisms and Kinetics of Hydrate Formation. Ann. N. Y. Acad. Sci. 1994, 715, 283−305. (5) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R.; Sloan, E. D. Kinetic Inhibitors of Natural Gas Hydrates. In Proceedings of the 73rd Annual Gas Processors Association (GPA) Convention, New Orleans, LA, March 7−9, 1994; p 85. (6) Notz, P. K.; Bumgardner, S. B.; Schaneman, B. D.; Todd, J. L. Application of Kinetic Inhibitors to Gas Hydrate Problems. SPE Prod. Facil. 1996, 11, 256−260. (7) Englezos, P. Nucleation and Growth of Gas Hydrate Crystals in Relation to Kinetic Inhibition. Rev. Inst. Fr. Pet. 1996, 51, 789−795. (8) Sloan, E. D. A Changing Hydrate Paradigm - From Apprehension to Avoidance to Risk Management. Fluid Phase Equilib. 2005, 228, 67−74. (9) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (10) Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. Application of LowDosage Hydrate Inhibitors in Deepwater Operations. SPE Prod. Facil. 2002, 17, 133−137. (11) Freer, E. M.; Sloan, E. D. An engineering approach to kinetic inhibitor design using molecular dynamics simulations. Ann. N. Y. Acad. Sci. 2000, 912, 651−657. (12) Makogon, Y. M.; Sloan, Jr., E. D. Mechanism of kinetic hydrate inhibitors. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19−23, 2002; pp 498−503. (13) Anderson, B. J.; Tester, J. W.; Borghi, G. P.; Trout, B. L. Properties of inhibitors of methane hydrate formation via molecular dynamics simulations. J. Am. Chem. Soc. 2005, 127, 17852−17862.
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CONCLUSION The influence on the dissociation temperature by butyl glycol ether (BGE) as a synergist and solvent for PVCap has been reported for methane hydrates. Two different formulations for BGE + PVCap have been tested: Inhibex 101 (50 wt % PVCap ∼2k and 50 wt % BGE) and a combination of 50 wt % PVCap6k + 50 wt % BGE. The PVCap polymers have been synthesized in different ways and the results show that the effect these have on the hydrate dissociation temperature is very different. Hydrates formed in the presence of Inhibex 101 showed a lower dissociation temperature compared to hydrates formed in the presence of PVCap alone. On the other hand, hydrates formed in the presence of PVCap-6k + BGE showed a somewhat increased dissociation temperature compared to hydrates formed with PVCap alone. The results indicate that the different formulations of PVCap and synergist influence the hydrate stability to different extents.
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Tables of the measured effects of pure PVCap versus PVCap in synergist solvent (BGE) on the displacement of the hydrate dissociation temperature (T) for sI methane hydrates (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03500. 6356
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Energy & Fuels
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DOI: 10.1021/acs.energyfuels.6b03500 Energy Fuels 2017, 31, 6352−6357
