Anomalous Surface Conformation for Polymeric Gas-Hydrate-Crystal Inhibitors
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Anomalous Surface Conformation for Polymeric Gas-Hydrate-Crystal Inhibitors H.E. King, Jr.; Jeffrey L. Hutter1; Min Y. Lin2; and Thomas Sun3 ExxonMobil Research and Engineering Company Annandale, NJ 08801 1 Present address: Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7 2 Also at the National Institute of Standards and Technology, Gaithersburg, MD 20899. 3 Present address: Exxon Chemical Company, Baytown, TX 77522. ABSTRACT We have used both conventional small-angle neutron scattering and contrast variation techniques to characterize the polymer conformations of two non-ionic water soluble polymers: poly(ethylene oxide) and poly(N-vinyl-2-pyrollidone). The second of these is able to kinetically suppress hydrate crystallization, and an objective of these studies is to obtain a understanding of this inhibition mechanism. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows only a small difference between the polymers. The singlechain characteristics are unperturbed, but the hydrate inhibitor polymer seems to show an enhanced tendency to form aggregates in solution. This is evidenced by excess low-q scattering following a q-2.5 power law. Much more evident is the strong perturbation in the conformation of the inhibitor polymer upon crystallization of the hydrate. We utilize contrast variation methods to resolve the scattering of the polymer on the hydrate surface. Unlike expectations from polymer scaling laws, the resulting layer is considerably thicker (550Å) than the singlechain radius of gyration, 80Å. Also surprising, only 2 percent of the available crystal surface is covered. Within the covered areas, the polymer concentration is significantly enhanced over that in the surrounding solution, about 2.5c* (where c* is the overlap concentration). This suggests a coverage of clumps of polymer widely separated from one another. Consideration of the concentration and implied spacing of such clumps on the hydrate surface suggests that they can effectively inhibit crystal growth. INTRODUCTION Gas hydrates are ice-like crystals consisting of water cages surrounding small molecules such as propane or methane.[1] Enclathration of the small molecules is key to the crystals' stability, and depending upon the gas pressure their melting point can extend to temperatures well above that of ice. Because of this, they crystallize in many settings where ordinary ice is not stable. For example there are many natural-gas hydrate deposits on the sea floor associated with gas seepage into cold ocean water[2]. Similar conditions can be found in oil and gas transport pipelines. Under deep-sea conditions, inside such pipelines one finds a ready supply of water and natural gas at temperatures of ~5Û&DQGSUHVVXUHVRIa03D7KXVK\GUDWH formation within pipelines is a significant long-standing problem for the oil industry.[3] The consequences of hydrate formation can be very serious. Blockage of the pipeline, with consequent loss of production is one scen
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