How Do Surfactants Control the Agglomeration of Clathrate Hydrates?

Abstract

Clathrate hydrates can spontaneously form under typical conditions found in oil and gas pipelines. The agglomeration of clathrates into large solid masses plugs the pipelines, posing adverse safety, economic, and environmental threats. Surfactants are customarily used to prevent the aggregation of clathrate particles and their coalescence with water droplets. It is generally assumed that a large contact angle between the surfactant-covered clathrate and water is a key predictor of the antiagglomerant performance of the surfactant. Here we use molecular dynamic simulations to investigate the structure and dynamics of surfactant films at the clathrate–oil interface, and their impact on the contact angle and coalescence between water droplets and hydrate particles. In agreement with the experiments, the simulations predict that surfactant-covered clathrate–oil interfaces are oil wet but super-hydrophobic to water. Although the water contact angle determines the driving force for coalescence, we find that a large contact angle is not sufficient to predict good antiagglomerant performance of a surfactant. We conclude that the length of the surfactant molecules, the density of the interfacial film, and the strength of binding of its molecules to the clathrate surface are the main factors in preventing the coalescence and agglomeration of clathrate particles with water droplets in oil. Our analysis provides a molecular foundation to guide the molecular design of effective clathrate antiagglomerants.

Molecular properties of interfaces relevant for clathrate hydrate agglomeration

“…The atomistic simulations by Phan et al [49] also suggested that the surface diffusion of AAs at hydrate-oil interfaces strongly depends on the features of the surfactant headgroup. Also consistent with atomistic simulations [47], Nullage et al [53] showed that dodecane and dodecanol co-adsorb at the hydrate-dodecane interface. For example, at 41% of the full dodecanol monolayer, dodecane intercalation causes the interfacial dodecanoldodecane film to yield an interfacial layer with density ~ 95% that of the crystal.…”

“…These approaches strongly depend on the accuracy of the force fields and on the size of the systems being simulated, with larger systems expected to provide more reliable results because they can account for multiple phenomena occurring simultaneously; benchmarking against both experiments and atomistic simulations is necessary before simulating complex systems using coarse-grained approaches. Naullage et al [53], e.g., recently probed how surfactants could act as AAs using a coarse-grained simulation. The results reinforce the atomistic simulations conclusions, while providing additional insights.…”

“…In agreement with experiments [56], the simulations showed that surfactants adsorption changes the nature of the hydrate interface (Figure 4). By quantifying the simulated contact angles, Naullage et al [53] estimated the surface free energy of the clathrate-dodecane interface as 76.92 mJ m -2 . The snapshots of Figure 4 show the contact angle for water reduces from 180 in dodecane to 882 in vacuum: dispersive interactions between water and hydrocarbons stabilize the water droplet.…”

“…The snapshots of Figure 4 show the contact angle for water reduces from 180 in dodecane to 882 in vacuum: dispersive interactions between water and hydrocarbons stabilize the water droplet. Naullage et al [53] also quantified the free energy landscape encountered by a water droplet as it transports across the interfacial surfactant film. The surfactants only provide a kinetic barrier, and as soon as a channel forms across the surfactant film, coalescence is irreversible.…”

“…In panels (C) and (D) the wetting properties for water and dodecane are quantified at the hydrate covered by surfactants, respectively. Reproduced from Naullage et al [53].…”