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

Abstract

Anti-agglomerants (AAs), both natural and commercial, are currently being considered for gas hydrate risk management of petroleum pipelines in offshore operations. However, the molecular mechanisms of the interaction between the AAs and gas hydrate surfaces and the prevention of hydrate agglomeration remain critical and complex questions that need to be addressed to advance this technology. Here, we use molecular dynamics (MD) simulations to investigate the effect of model surfactant molecules (polynuclear aromatic carboxylic acids) on the agglomeration behaviour of gas hydrate particles and disruption of the capillary liquid bridge between hydrate particles. The results show that the anti-agglomeration pathway can be divided into two processes: the spontaneous adsorption effect of surfactant molecules onto the hydrate surface and the weakening effect of the intensity of the liquid bridge between attracted hydrate particles. The MD simulation results also indicate that the anti-agglomeration effectiveness of surfactants is determined by the intrinsic nature of their molecular functional groups. Additionally, we find that surfactant molecules can affect hydrate growth, which decreases hydrate particle size and correspondingly lower the risk of hydrate agglomeration. This study provides molecular-level insights into the anti-agglomeration mechanism of surfactant molecules, which can aid in the ultimate application of natural or commercial AAs with optimal anti-agglomeration properties.

Fig. 1. Main stages of the MD simulation to investigate the effect of surfactants on gas hydrate anti-agglomeration.

Fig. 2. Molecular structures of the three model surfactants: (a) 1-phenylacetic acid, (b) 2-napthylacetic acid, (c) 1-pyreneacetic acid, and (d) the simulation configuration for the surfactant surface adsorption model after 100 ns equilibration simulation (methane and propane molecules are not shown in the figure). Methane–propane binary sII hydrates are used in the system; the hydrocarbon phase is composed of methane, propane and decane (liquid hydrocarbon phase) molecules. During the equilibration process, the cage structure on the hydrate surface collapsed, leading to the presence of guest molecules (methane and propane) in the hydrocarbon phases, as well as the quasi-liquid layer (QLL) at the hydrate surface, and the QLL was also shown in previous simulations. In addition, carboxylic oxygen (O1 and O2) and hydrogen (H) atoms are labelled (e).

Fig. 3. Potential mean force (PMF) profiles obtained for the three surfactants as they travel from the sII hydrate (111) surface to the liquid hydrocarbon phase. Error bars were estimated from bootstrap analysis implemented in GROMACS.

Fig. 4. Initial (a) and final (b) snapshots of the capillary liquid bridge model at the molecular level taken for the system with 1-pyreneacetic acid as an example (1-phenylacetic acid and 2-napthylacetic acid systems are shown in Fig. S6†). The simulation cell is composed of three layers, with bridge water molecules in the middle of the hydrocarbon phase between the hydrate slabs at the top and bottom (in blue), and the hydrocarbon phase in the middle layer is composed of decane molecules with surfactants dispersed within it. The average number of H-bonds in the capillary bridge water molecules (c); the average number of H-bonds between surfactant molecules and bridge water molecules (d).

Fig. 5. Two conformations of 1-pyreneacetic acid: (a) cis and (b) trans, the distribution of the two conformations of the three acids in water (c), and the energy variation with the torsion angle of the carboxyl of 1-pyreneacetic acid (d).

Fig. 6. Electrostatic potential surfaces of the three surfactants: (a) 1-phenylacetic acid, (b) 2-napthylacetic acid, (c) 1-pyreneacetic acid, and (d) H-bond lifetime (ps) formed by carboxylic oxygen or hydrogen atoms for the three acids.

Fig. 7. The count of the carboxylic torsion angles of the 8 1-pyreneacetic acid molecules (labelled no. 1–8) (a) and H-bond number formed by 1-pyreneacetic acid corresponding to the COOH torsion angle in the simulations, including carboxylic oxygen (O1 or O2) and hydrogen (H) with water (WO) and the total number of H-bonds (Total); the red boxes highlight the number of O1-WH H-bonds (b).

Fig. 8. Initial snapshot of the growth configuration (a), methane propane molecules are randomly placed in the bulk phase and not shown in this figure both in the hydrate phase and bulk phase. The bulk phase was divided into twenty layers perpendicular to the x-direction in the figure, which was used to characterize the system behaviour during binary hydrate growth. F₃, Li order parameter for water molecules as a function of time for different layers perpendicular to the x-direction for simulation without (b) or with (c) surfactants.