(318b) Molecular Dynamics Analysis of Salt Effect on Anti-Agglomerant Surface Adsorption in Natural Gas Hydrates | AIChE

(318b) Molecular Dynamics Analysis of Salt Effect on Anti-Agglomerant Surface Adsorption in Natural Gas Hydrates

Authors 

Trout, B. L., Massachusetts Institute of Technology
The effect of salt on the performance of anti-agglomerant (AA) molecules is AA-dependent, and in most cases salinity improves the performance of AA’s. There is no consensus on salt’s effect on the AA-hydrate interaction, but understanding the influence of salt on AA-hydrate interaction mechanisms and energetics could be a step towards designing better AA’s with reduced experimental trial-and-error in the innovation process. We have used molecular dynamics simulations to examine the surface adsorption of a model anti-agglomerant inhibitor molecule binding to a sII methane-propane hydrate in environments of different salinity. From our simulation data, we were able to identify the preferred binding sites on hydrate and characterize the equilibrium binding configurations. In addition, for a subset of these binding configurations, we calculated the standard binding free energy in different concentrations of brine using the potential of mean force (PMF) based free energy calculations. We demonstrate that in higher salinity environments, the surface adsorption of the anti-agglomerants is enhanced through two distinct mechanisms. First, the salt decreases the solubility of the anti-agglomerant in the solution, which increases the thermodynamic driving force for surface adsorption. Second, the salt ions create a negatively charged interfacial layer close to the hydrate surface that effectively solvates the cationic head of the anti-agglomerant molecule, creating an energetically favorable bound state for the anti-agglomerant molecule. Quantitatively, we find that the presence of 3.5 wt% and 10 wt% NaCl decreases the standard binding free energy of the long hydrocarbon tail binding configuration by 0.8 and 1.4 kcal/mol, and decreases the standard binding free energy of the cationic head binding configuration by 1.5 and 3.3 kcal/mol, respectively. Finally, it decreases the standard binding free energy of simultaneous head and tail binding configuration by 1.9 and 4.3 kcal/mol, respectively.