Emergent Properties of Antiagglomerant Films Control Methane Transport: Implications for Hydrate Management

Abstract

The relationship between collective properties and performance of antiagglomerants (AAs) used in hydrate management is handled using molecular dynamics simulations and enhanced sampling techniques. A thin film of AAs adsorbed at the interface between one flat sII methane hydrate substrate and a fluid hydrocarbon mixture containing methane and n-dodecane is studied. The AA considered is a surface-active compound with a complex hydrophilic head that contains both amide and tertiary ammonium cation groups and hydrophobic tails. At a sufficiently high AA density, the interplay between the surfactant layer and the liquid hydrocarbon excludes methane from the interfacial region. In this scenario, we combine metadynamics and umbrella sampling frameworks to study accurately the free-energy landscape and the equilibrium rates associated with the transport of one methane molecule across the AA film. We observe that the local configurational changes of the liquid hydrocarbon packed within the AA film are associated with high free-energy barriers for methane transport. The time scales estimated for the transport of methane across the AA film can be, in some cases, comparable to those reported in the literature for the growth of hydrates, suggesting that one possible mechanism by which AAs delay the formation of hydrate plugs could be providing a barrier to methane transport. Considering the interplay between the structural design and collective properties of AAs might be of relevance to improve their performance in flow assurance.

Synergistic and Antagonistic Effects of Aromatics on the Agglomeration of Gas Hydrates

“…To quantify synergistic or antagonistic effects, we monitored changes in the structure of the AAs within the interfacial film due to the presence of aromatics. It should be noted that the order of the interfacial film was found to be correlated with the AAs performance, as well as with the ability of methane to diffuse from the hydrocarbon phase to the hydrate substrate in our prior studies 8,27 . To quantify the order of the AAs films, we calculated the deuterium order parameter, S CD , for carbon atoms in the long alkyl tails of the AAs.…”

“…1a. To construct the initial configurations, we followed the procedure described in our previous study 27 . Each simulated system contains a sII methane hydrate substrate of size 5.193 × 5.193 × 3.462 nm.…”

“…7, right panel, is drilled within the hydrate substrate. However, building on arguments that relate emulsions stability to interfacial mechanic barriers or the film strength as measured by interfacial elasticity 62,63 , ordered and rigid interfacial films should yield strong repulsion between the two approaching hydrates 8,27 .…”

Clathrate hydrates: recent advances on CH4 and CO2 hydrates, and possible new frontiers

“…184 Some hydrocarbon chains were found to penetrate the AAs film, in qualitative agreement with experimental data by Tokiwa et al 185 These results also provide a new hypothesis for the mechanism by which AAs function: when the AAs form an ordered film, they could prevent methane transport across the interface. 186…”

Molecular properties of interfaces relevant for clathrate hydrate agglomeration

“…Because the density of methane within such interfacial films was negligible, Bui et al [47] suggested that the AAs they considered could prevent hydrate plug formation, in part, causing an interfacial barrier that prevent both water and methane from reaching the hydrate particle. Sicard et al [48] quantified the strong free energy barrier encountered by methane when it travels across such interfaces. For these calculations, enhanced sampling techniques including metadynamics, umbrella sampling, and adiabatic biased MD were implemented.…”