Molecular Dynamics Analysis of Anti-Agglomerant Surface Adsorption in Natural Gas Hydrates

Abstract

We have used molecular dynamics simulations to examine the surface adsorption of a model anti-agglomerant inhibitor (quaternary ammonium salt) binding to a hydrate surface in both aqueous and liquid hydrocarbon phases. From our molecular dynamics simulation data, we were able to identify the preferred binding sites on the (111) crystal face of a methane−propane sII hydrate as well as characterize the equilibrium binding configurations of the inhibitor and their associated binding energies. In the aqueous phase, we observed that the inhibitor proceeds through a two-step surface adsorption mechanism, whereas in the liquid hydrocarbon phase surface adsorption occurs through a single-step mechanism. To characterize the extent of surface adsorption in each liquid phase, we calculated the standard binding free energy using the free energy perturbation method following a double decoupling thermodynamic cycle. We found that the surface adsorption in the liquid hydrocarbon phase is an exergonic process, whereas the surface adsorption in the aqueous phase is an endergonic process. Our results demonstrate that the extent of surface adsorption is much larger in the liquid hydrocarbon phase relative to the aqueous phase and suggest that the inhibitor is less effective in the aqueous phase because the surface adsorption is less favorable. Finally, we examine the effect of the inhibitor on the water structure in the liquid phase and in the hydrate phase, with the results highlighting the difference between the nature of anti-agglomerant/hydrate interactions as compared to kinetic inhibitor/hydrate interactions.

The effect of surfactants on hydrate particle agglomeration in liquid hydrocarbon continuous systems: a molecular dynamics simulation study

“…To identify and quantify the surfactant mechanism at the molecular level, we report single surfactant molecule adsorption onto a hydrate seed and the interaction of the surfactant molecule with the capillary liquid bridge using a classical molecular dynamics (MD) simulation, which has been widely and successfully used in previous investigations of hydrates, including AAs. 15,17,21,24,36 In this paper, we used the general Amber force eld 37 to describe three different surfactant molecules: (1) 1-phenylacetic acid, (2) 2-napthylacetic acid and (3) 1pyreneacetic acid. Using this approach, atom types of each surfactant were taken from the AMBER library.…”

“…Generally, AAs (both natural and commercial) are surfactants with a hydrophilic head and a hydrophobic tail that can adsorb onto the hydrate particle surface, thereby changing the hydrate surface morphology, wettability and interactions to prevent hydrates from agglomerating and forming large plugs. [14][15][16][17][18][19][20][21] In addition, some reports have shown that AAs could also potentially affect hydrate formation and growth processes. [22][23][24][25] To evaluate the effectiveness of AAs, one frequently used method is to measure model (cyclopentane, CyC5) hydrate interparticle cohesive forces in the presence of AAs using an ambient pressure micromechanical force (MMF) apparatus, 26 because the capillary liquid bridge between hydrate particles was proposed to be another essential feature dominating hydrate particle cohesion in oil continuous systems.…”

“…Thus, these surfactants (acid molecules) are considered a new potential type of AA molecule, as the usually commercial AAs tend to be based on quaternary ammonium salts. 1,6,15 This observation was explained by the activity of carboxylic acid molecules, which could change the wettability of the hydrate surface from hydrophilic to lipophilic, leading to a reduction in hydrate agglomeration; 16 however, this explanation lacks theoretical support at the molecular level. The previous experiment yielded another interesting result: the antiagglomeration effectiveness of the carboxylic acid surfactants increases with the number of polynuclear aromatic rings even if these surfactants are homologues.…”

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

“…177 Trout and his group recently reported similar simulations for single AAs. 178 Jimenez-Angeles and Firozaabadi, after studying hydrate nucleation and the hydrate-methane interface, [179][180][181] focused on the adsorption of individual AA molecules on a hydrate surface. 182 On a related footpath, some recent simulations attempted to quantify the collective features of AAs at hydrate-hydrocarbon interfaces.…”

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

“…According to recent MD simulations, the adsorption of a quaternary ammonium surfactant at the surface of sII hydrates is stronger from a liquid hydrocarbon phase than from an aqueous phase. 46.5 …”