216
Energy & Fuels 1998, 12, 216-218
Enhanced Hydrate Inhibitors: Powerful Synergism with Glycol Ethers Jeffrey M. Cohen, Philip F. Wolf, and William D. Young* International Specialty Products, 1361 Alps Rd., Wayne, New Jersey 07470 Received September 8, 1997. Revised Manuscript Received December 23, 1997
Laboratory tests in which gas hydrates were formed at 4 °C and 1000 psig in a 300 mL stirred cell show that the performance of kinetic hydrate inhibitors, including VC-713, PVCL, and VCL/ VP copolymers, is greatly improved by the addition of less than 1 wt % of certain glycol ethers in the water or saltwater phases. For example, the hydrate induction time with 0.5 wt % VC713 in seawater increased from 40 min to over 1200 min with the addition of 0.75 wt % 2-butoxyethanol. The most effective glycol ethers tested had either three or four carbon atoms in the alkoxy group. Lower homologs did not seem to have an effect, and higher homologs were insoluble in saltwater. The glycol ethers did not inhibit hydrates (at these low concentrations) without the polymer present.
Introduction Since natural gas hydrates frequently plug oil and gas production lines, various chemical and thermal methods have been developed to prevent hydrate formation. Conventional chemical treatment involves injecting 2050 wt % methanol in the water phase at the wellhead or downhole to depress the freezing point of hydrates below the minimum fluid temperature in the line. However, high methanol injection rates can be impractical and may exacerbate pipeline corrosion. Alternative chemical treatment methods are needed. Recently, Lederhos et al.1 reported that certain watersoluble polymers effectively inhibit hydrates at treatment levels of 0.1-1.0 wt % in the water phase, far less than required by methanol. At typical flowline conditions, these polymers slow the rates of hydrate nucleation and growth to such an extent that virtually no hydrates form in the wellstream during transport to processing facilities. Since the polymers slow hydrate formation rather than depress the freezing point, they are called “kinetic inhibitors”. Numerous industry field tests have demonstrated the viability of this technology.2-4 The most effective kinetic inhibitors are vinylcaprolactam-based polymers, for example poly(vinylcaprolactam) and Gaffix VC-713, a terpolymer of vinylcaprolactam, vinylpyrrolidone, and (dimethylamino)ethyl methacrylate. Poly(vinylpyrrolidone), although not as effective as poly(vinylcaprolactam), has been widely used * Corresponding author: Tel. 201-628-3345; Fax 201-628-3886; E-mail
[email protected]. (1) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R. L.; Sloan, E. D., Jr. Chem. Eng. Sci. 1996, 51, 1221-1229. (2) Notz, K.; Bumgartner, S. B.; Schaneman, B. D.; Todd, J. L. Proceedings of the 27th Annual Offshore Technology Conference; Offshore Technology Conference: Houston, TX, 1995; pp 719-730. (3) Bloys, B.; Lacey, C.; Lynch, P. Proceedings of the 27th Annual Offshore Technology Conference; 1995; pp 691-700. (4) Corrigan, A.; Duncum, S. N.; Edwards, A. R.; Osborne, C. G. Proceedings of the SPE Annual Technical Conference; Society of Petroleum Engineers, Inc.: Richardson, TX, 1995; pp 539-547.
because it costs less and provides adequate protection in less severe applications. Similarly, the vinylcaprolactam/vinylpyrrolidone copolymers (including Gaffix VC-713) provide better solubility and brine compatibility than poly(vinylcaprolactam).1,5 The hydrate inhibitors are normally tested in a highpressure stirred cell at typical pipeline conditions. We have observed that adding a small amount of glycol ether (2-butoxyethanol, for example) substantially improves the performance of the polymeric hydrate inhibitors. This paper presents the results of our experimental study of kinetic hydrate inhibitors containing glycol ether solvents. Experimental Section The tests were conducted in a 300 mL, stainless steel, stirred reactor at high pressure and low temperature. A diagram of the apparatus is shown in Figure 1. The reactor was immersed in a refrigerated bath which normally maintains bath temperature to within 0.1 °C. Pressure in the reactor was controlled to within 5 psi by a programmable syringe pump. The pump displaces hydraulic oil into a piston cylinder which contains the hydrate-forming gas on one side and hydraulic oil on the other. The volume of oil displaced by the syringe pump to maintain constant pressure is equal to the volume change of the reactor. Gas consumption was calculated from the measured volume change. The inhibitors were tested at 0.5 wt % dry polymer and a specified amount of glycol ether in salt solution. In a typical experiment, 0.6 g of dry polymer and 0.9 g of pure glycol ether liquid were added to 120 g of a 3.5 wt %, filtered, synthetic sea salt solution and mixed for at least 1 h. In some experiments, 120 g of deionized water was used instead of saltwater. The resulting solution was transferred to the 300 mL reactor, sealed, and immersed in the temperature bath at 4 °C. Vacuum was pulled on the reactor for a few minutes to remove air. When the reactor contents reached 4 °C, the pressure was increased to 1000 psig with a natural gas mixture (5) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R.; Sloan, E. D. Proceedings of the Seventy-Third GPA Annual Convention; Gas Processors Association, Tulsa, OK, 1994; pp 85-93.
S0887-0624(97)00166-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/18/1998
Enhanced Hydrate Inhibitors
Energy & Fuels, Vol. 12, No. 2, 1998 217
Figure 1. Gas hydrate test apparatus. The 300 mL reactor was charged with 120 g of 3.5% sea salt solution containing 0.5 wt % dissolved inhibitor. Tests were conducted at 4 °C, 1000 psig, and 1000 rpm for up to 20 h. Table 1. Green Canyon Gas Composition component
mol %
component
mol %
nitrogen methane ethane propane
0.4 87.2 7.6 3.1
isobutane n-butane isopentane n-pentane
0.5 0.8 0.2 0.2
(green canyon gas) and pressure control was initiated. The gas was allowed to cool for exactly 5 min, and then the reactor stirrer was turned on to 1000 rpm. Time zero is the time at which the reactor stirrer was started. The gas volume, as measured by the syringe pump, and the reactor pressure and temperature were recorded electronically at 1 min intervals throughout the experiment.
Materials Gaffix VC-713 is a terpolymer of vinylcaprolactam, vinylpyrrolidone, and (dimethylamino)ethyl methacrylate manufactured by ISP. For consistency, all experiments reported here were conducted with the same manufacturing lot of VC713. GHI-1 is the commercial name for a gas hydrate inhibitor formulation composed of 37 wt % VC-713, 50.5 wt % 2-butoxyethanol, and 12.5 wt % water. Ethylene glycol monobutyl ether, or 2-butoxyethanol, has the formula n-C4H9OC2H4OH. It is an industrial solvent with a boiling point of 171 °C and a flash point of 71 °C.6 It is also known by its trade name, butyl Cellosolve (from Union Carbide). The glycol ethers used in this study were purchased from Aldrich Chemical Co. The synthetic sea salt conforms to ASTM “Standard Specification for Substitute Ocean Water” and was purchased from Marine Enterprises of Baltimore, MD. Green canyon gas is a typical natural gas mixture. It has the composition listed in Table 1.
Results Figure 2 shows gas consumption versus time curves for four different hydrate experiments in the stirred cell. The tests were performed at 4 °C and 1000 psig with green canyon gas and 120 g of 3.5% seawater. In the experiment with 0.5 wt % VC-713 and 0.75 wt % ethanol in seawater, rapid hydrate formation occurred after 40 min. By comparison, the experiment with 0.5 wt % VC713 plus 0.75 wt % 2-butoxyethanol showed no hydrate (6) Sloan, E. D. Clathrate Hydrates of Natural Gases; Marcel Dekker: New York, 1990.
Figure 2. Gas consumption in the hydrate cell at 4 °C and 1000 psig in 3.5% sea saltwater (subcooling ) 12.4 °C) showing the effect of 2-butoxyethanol on hydrate inhibition. Starting from the top, the first curve is 0.75% 2-butoxyethanol (no polymer), the second curve is 0.5% VC-713 plus 0.75% ethanol, the third curve is sea saltwater (no additives), and the fourth curve is 0.5% VC-713 plus 0.75% 2-butoxyethanol. Concentrations are expressed as weight percent in the brine. Low gas consumption indicates good hydrate inhibition. Table 2. Induction Times for 0.5 wt % VC-713 plus 0.75 wt % Glycol Ether Additives in Sea Salt Water at 39.2 °F and 1000 psig additive (0.75 wt %)
induction time (min)
2-hexyloxyethanol 2-butoxyethanol 2-propoxyethanol 2-isopropoxyethanol 2-ethoxyethanol 2-methoxyethanol ethanol 1-butoxy-2-propanol 1-propoxy-2-propanol 3-ethoxy-1-propanol 1-methoxy-2-propanol 2-(2-butoxyethoxy)ethanol 2-(2-ethoxyethoxy)ethanol 2-(2-methoxyethoxy)ethanol none (seawater only)
10 >1200 350-600 700-1000 10 0 40 450-1000 800-1200 5 10 440 0 5 0
formation during the full 20 h test. This result has been reproduced with several lots of VC-713 and 2-butoxyethanol. Seawater with no inhibitor formed hydrates instantly at these conditions, as shown by the “seawater” curve in Figure 2. Similarly, 0.75 wt % 2-butoxyethanol in seawater with no polymer added also formed hydrate instantly. In fact, more hydrate formed with 2-butoxyethanol in seawater than with seawater alone. Subcooling, defined here as the difference between the equilibrium dissociation temperature for the hydrates and the operating temperature, has been used to measure the level of protection provided by a gas hydrate inhibitor.7,8 At 1000 psig, hydrates of green canyon gas have an equilibrium dissociation temperature of 17.9 °C in deionized water. Adding 3.5% salt depresses the dissociation temperature approximately 1.5 °C at this pressure. Therefore, at 1000 psig, 4 °C, and 3.5% salt, the subcooling is 12.4 °C. Combining 2-butoxyethanol with VC-713 significantly improved the performance of the hydrate inhibitor. Following up on this result, other glycol ethers with similar structures were tested at the same conditions. Table 2 lists the observed induction times for three (7) Young, W. D. International Conference on Natural Gas Hydrates. Ann. N.Y. Acad. Sci. 1994, 715, 341-343. (8) Lederhos, J. P.; Sloan, E. D. SPE Annu. Tech. Conf. 1996, 173179.
218 Energy & Fuels, Vol. 12, No. 2, 1998
Figure 3. Hydrate induction times at 4 °C and 1000 psig for a series of 2-butoxyethanol concentrations. The VC-713 concentration was 0.5 wt % in each experiment. The induction time peaks around 0.5-0.75 wt % 2-butoxyethanol.
Figure 4. Gas consumption in the hydrate cell at 4 °C and 1000 psig in deionized water (subcooling ) 13.9 °C). The top curve is deionized water with no additives. The middle curve is 1.35 wt % GHI-1 in deionized water, and the bottom curve is 2.0 wt % GHI-1 in deionized water. GHI-1 is composed of 37 wt % VC-713, 50.5% 2-butoxyethanol, and 12.5% water; therefore, the VC-713 concentration was 0.5 and 0.75 wt %, respectively.
different series of glycol ethers: ethylene glycols, propylene glycols, and diethylene glycols. The induction time is the time at which gas consumption was first detected, even if hydrate growth was slow beyond that point. An induction time listed as a range (e.g., 350600 min) indicates that hydrate growth started, for example, at 350 min, proceeded very slowly from 350 to 600 min, and accelerated to more rapid growth after 600 min. The slow growth period from 350 to 600 min suggests that the inhibitor was still inhibiting hydrate growth although not completely preventing it. 2-Hexyloxyethanol produced a cloudy solution when mixed with the brine. However, the hydrate test was perfomed, and the result is shown in Table 2. In another series of experiments, the concentration of 2-butoxyethanol was varied while holding VC-713 constant at 0.5 wt %. Figure 3 shows the observed induction times at 4 °C and 1000 psig in 3.5% seawater. The induction time has a maximum around 0.5-0.75 wt % 2-butoxyethanol. At higher concentrations the induction time drops off sharply, although it continues to show better performance than VC-713 with no glycol ether. A few experiments were performed in deionized water rather than saltwater. Figure 4 shows the results of GHI-1 tests in deionized water at 4 °C and 1000 psig (subcooling ) 13.9 °C). GHI-1 is 37% VC-713, 50.5% 2-butoxyethanol, and 12.5% water. At 0.5 wt % VC-
Cohen et al.
Figure 5. Gas consumption in the hydrate cell at 4 °C and 1000 psig in 3.5% sea saltwater (subcooling ) 12.4 °C). The top curve is 0.5 wt % poly(vinylcaprolactam) (PVCL) plus 0.75 wt % ethanol. The bottom curve is 0.5 wt % PVCL plus 0.3 wt % 2-butoxyethanol.
713 () 1.35 wt % GHI-1) the induction time was about 100 min. At 0.75% VC-713 () 2% GHI-1) no hydrates formed during the 20 h test. Figure 4 also shows the gas consumption for deionized water with no inhibitor. The synergistic effect of 2-butoxyethanol was observed with other lactam polymers in addition to VC-713. Figure 5 shows the result of adding 0.3 wt % 2-butoxyethanol to 0.5 wt % poly(vinylcaprolactam) in seawater at 4 °C and 1000 psig, as compared to poly(vinylcaprolactam) in ethanol. As with VC-713, no hydrate formed in the 20 h test with poly(vinylcaprolactam) + 2-butoxyethanol, whereas hydrate formed immediately with poly(vinylcaprolactam) + ethanol. We also tested a 50/ 50 vinylcaprolactam/vinylpyrrolidone copolymer at these conditions and found that the induction time increased from 0 at 0.5 wt % polymer + 0.75% methanol to 350 min with 0.5 wt % polymer + 0.75 wt % 2-butoxyethanol. At low salt concentrations, the 50/50 copolymer is not as efficacious as VC-713 or poly(vinylcaprolactam), either with or without glycol ether present. Nonetheless, the pattern of improved inhibitor performance with glycol ether additive was observed with the copolymer as well as the other inhibitors. Discussion The data in Table 2 show that glycol ethers with either three or four carbons in the alkoxy group significantly enhanced hydrate inhibition when combined with VC-713. Lower homologs did not appear to affect inhibition when combined with VC-713. Higher homologs were insoluble in salt solution. The synergistic effect of 2-butoxyethanol was observed with other lactam polymers and copolymers, including poly(vinylcaprolactam) and 50/50 vinylcaprolactam/vinylpyrrolidone. Glycol ethers, especially the higher homologs, are known to have surfactant-like properties.9 The hydrophobicity of the alkoxy group may cause the molecules to associate with the dissolved polymer, which could alter the polymer’s conformation in solution. An extended polymer would presumably have more of its length available for interaction with the hydrate crystal, which might explain the improved performance of the hydrate inhibitors. EF970166U (9) Union Carbide Glycol Ethers 1993.
