Properties of Antiagglomerant for Gas Hydrate management
The understanding of the relation between collective properties and performance of antiagglomerants used in gas hydrate management has been a major issue in a variety of scientific and industrial contexts, including climate change modeling, hydrocarbon extraction, natural gas storage, and planetary surface chemistry. Gas hydrates are ice-like inclusion compounds consisting of polyhedral hydrogen-bonded water cages stabilized by guest gas molecules. They are not chemical compounds because no strong chemical bonds exist between water and gas molecules. They are formed under high pressure and low temperature conditions such as those found in deep oceans and pipelines. The gas molecules able to be trapped into the water cages are usually small. Of particular interest are the hydrocarbon hydrates that can form blockages in oil and gas pipelines.
There are three major stages associated with hydrate plug formation in pipelines: nucleation, growth, and agglomeration. To manage hydrates in pipelines, hydrate inhibitors are used. They are differentiated depending on their mode of action: thermodynamic hydrate inhibitors (TIs), such as methanol and monoethylene glycol, shift the stability conditions of hydrates to lower temperatures and higher pressures, but require large amounts to be effective. Low dosage hydrate inhibitors (LDHIs) instead are effective at low concentrations. Kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs) are the two main LDHIs classes. AAs, mostly surface-active surfactants, are usually amphiphilic chemicals with complex hydrophobic tails and hydrophilic headgroups. They allow the hydrate particles to form, but keep them dispersed yielding transportable slurries. When AAs adsorb at the oil-hydrate interface, the hydrophobic tails preferably point toward the hydrocarbon phase, possibly inducing an effective repulsion when two hydrates approach each other. When the AAs polar headgroups are adsorbed on the hydrate surface, they could interfere with the hydrate growth. While the use of AAs is increasing in subsea projects across the industry, their mechanisms of action remain poorly understood. Such understanding is necessary to improve their cost-effectiveness and expand the range of conditions over which their use is safe and convenient.
Sequence of simulation snapshots representing the transport mechanism of the methane molecule (red sphere) across a flat interfacial layer composed of a mixture of AA and dodecane (blue molecules). The bulk hydrocarbon phase and the sII hydrate are not shown for clarity. The AA layer is only shown in the left panel (Sicard et al., 2018).
Because classical molecular dynamics (MD) simulations can follow the trajectories of individual molecules, MD has been the preferred technique to investigate the formation of hydrates with and without the presence of KHIs. Recent numerical studies have concentrated on the relation between structure and performance of model AAs, with the emergent molecular-level characterization of the surface adsorption mechanisms of surfactants to hydrates considered as a signature of microscopic performance. Recently, the macroscopic performance of a class of AAs in flow-assurance applications has been related to the molecular level properties of the surfactant interfacial film. Those simulations, compared to experiments, suggested that effective AAs could provide energy barriers in methane transport.
We have recently quantified such energy barriers as experienced by one methane molecule diffusing from the hydrocarbon phase to the growing hydrate. We considered AAs effective at excluding methane from the film of AAs formed at the water-hydrocarbon interface. We combined Metadynamics and Umbrella sampling frameworks to study accurately the free-energy (FE) landscape and the equilibrium rates associated with the transport mechanisms of one free methane molecule accross a densely packed interfacial layer. At sufficiently high antiagglomerant density, we showed that the FE barrier is due to local configurational changes of the liquid hydrocarbon molecules packed within the antiagglomerant film. The flexibility of the oil molecules is impacted by the rigidity of the antiagglomerant molecules. We observed that the interaction of the methane molecule with the AA short tail can increase the stability of the system and invert its thermodynamic stability. Increasing the AAs surface density leads to an increase in the heigh of the free-energy barrier, improving eventually the efficiency of the interfacial layer in limiting the transport of methane (Sicard et al., 2018).
- F. Sicard, T.Bui, D. Monteiro, Q. Lan, M. Ceglio, C. Burress and A. Striolo, Emergent properties of antiagglomerant films control methane transport: implications for hydrate management, Langmuir 34 (33), 9701-9710 (2018)
- T. Bui, F. Sicard, D. Monteiro, Q. Lan, M. Ceglio, C. Burress and A. Striolo, Anti-agglomerants affect Gas Hydrate Growth, J. Phys. Chem. Lett. 9, 3491-3496 (2018)
last update: June 2018