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

Abstract

Surfactants are often used to stabilize aqueous dispersions. For example, surfactants can be used to prevent hydrate particles from forming large plugs that can clog, and sometimes rupture pipelines. Changes in oil composition, however dramatically affect the performance of said surfactants. In this work we demonstrate that aromatic compounds, dissolved in the hydrocarbon phase, can have both synergistic and antagonistic effects, depending on their molecular structure, with respect to surfactants developed to prevent hydrate agglomerations. While monocyclic aromatics such as benzene were found to disrupt the structure of surfactant films at low surfactant density, they are expelled from the interfacial film at high surfactant density. On the other hand, polycyclic aromatics, in particular pyrene, are found to induce order and stabilize the surfactant films both at low and high surfactant density. Based on our simulation results, polycyclic aromatics could behave as natural anti-agglomerants and enhance the performance of the specific surfactants considered here, while monocyclic aromatics could, in some cases, negatively affect performance. Although limited to the conditions chosen for the present simulations, the results, explained in terms of molecular features, could be valuable for better understanding synergistic and antagonistic effects relevant for stabilizing aqueous dispersions used in diverse applications, ranging from foodstuff to processing of nanomaterials and advanced manufacturing. Dispersions are found in a variety of applications, from foodstuff to minerals processing, from biotechnology to nanotechnology, from 3D printing to advanced manufacturing. One relevant application in the energy sector concerns flow assurance. In oil and gas pipelines, the simultaneous presence of natural gas and water can lead to the formation of hydrate plugs, which could clog, and sometimes rupture the pipeline. In addition, gas hydrates can form in many offshore energy processes 1. The unintended formation of hydrate plugs can cause disruptions in oil and gas production, as well as large negative environmental consequences 1-3. Among other approaches to manage gas hydrates is the stabilization of hydrate particles in hydrocarbon dispersions. Specifically designed surfactants, known as anti-agglomerants (AAs), are optimized to prevent hydrate plug formation in flow assurance 4,5. AAs are believed to adsorb on hydrate particles by their hydrophilic head groups, while the AAs tail groups are soluble in the hydrocarbon phase. The hydrate particles, as well as water droplets, are expected to be covered by a film of AAs and oil, making them repel each other and disperse 5-9. This rich system offers an ideal platform to test our fundamental understanding regarding the stabilization of dispersions using surfactants. In fact, laboratory and field observations alike show that many phenomena determine the AAs' performance. Small changes in the molecular structure of the surfactants, changes in salt content, and in gas/oil and i...

Role of structural rigidity and collective behaviour in the molecular design of gas hydrate anti-agglomerants

“…The headgroup including both the amide and tertiary ammonium cation group is known for its ability to adsorb on the hydrate surface. [20][21][22][23] The two long R 1 tails are composed of a linear hydrocarbon of twelve carbon atoms with an aromatic ring (benzene) positioned either at the bottom (Fig. 2b) or in the middle (Fig.…”

“…For instance, small changes in the molecular structure of the surfactants can strongly affect their ability to prevent hydrate plug formation. 23 Phan et al showed that the adhesion of AAs on hydrate particles is strongly dependent on the molecular features of the AA headgroups, thus explaining the experimental observations. 19 Trout and his group showed that the adhesion between monomers and hydrate surfaces is related to the performance of kinetic hydrate inhibitors, 24 and recent results show that the adsorption of AAs at the hydrate-water interface is strongly affected by the presence of salts in the system.…”

“…The hydrate particles are expected to be covered by a film of AAs and oil, making them repel each other and remain dispersed. [19][20][21][22][23] Laboratory and field observations alike show that many phenomena determine the performance of AAs. For instance, small changes in the molecular structure of the surfactants can strongly affect their ability to prevent hydrate plug formation.…”

“…27 Of particular interest in the present work is the fact that the stability of water-in-oil emulsions is dependent on the rigidity of the interfacial film, which in turn can be determined by the chemical structure and the collective effects of the AAs. 20,22,23 Here, we employ molecular dynamics (MD) simulations and enhanced sampling techniques to study the molecularlevel properties along with the thermodynamic and kinetic information of AAs specifically designed to prevent hydrate agglomerations under the conditions encountered in rocking cell experiments. The AAs considered are based on a compound, recently developed, which has shown good laboratory performance in preventing hydrate formation in light oils.…”

“…The AAs considered are based on a compound, recently developed, which has shown good laboratory performance in preventing hydrate formation in light oils. 20,22,23 The molecular structure of the AA tail groups was computationally modified to improve the rigidity of the compound, while maintaining the ability of the polar head group to adsorb on the hydrate surface. We analyse the behaviour of the newly designed AAs at the hydrate-oil interfaces (Fig.…”