Antiagglomerants Affect Gas Hydrate Growth

Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6 nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000 bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400 bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9 nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4 K depending on the system and the pressure.

Understanding the effect of moderate concentration SDS on CO2 hydrates growth in the presence of THF

Hypothesis Additives like Tetrahydrofuran (THF) and Sodium Dodecylsulfate (SDS) improve Carbon Dioxide (CO2) hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. Simulations and Experiments We exploit atomistic molecular dynamics simulations to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Findings Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation of SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments in the presence of THF showed that only small amounts of SDS are sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. Overall, our results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, useful for determining the optimal conditions for hydrate growth.

Chemical Promoter Performance for CO2 Hydrate Growth: A Molecular Perspective

Carbon dioxide (CO₂) hydrates, which contain a relatively large amount of captured CO₂ (almost 30 wt % of CO₂ with the balance being water), represent a promising CO₂ sequestration option for climate change mitigation. To facilitate CO₂ storage via hydrates, using chemical additives during hydrate formation might help to expedite formation/growth rates, provided the additives do not reduce the storage capacity. Implementing atomistic molecular dynamics, we study the impact of aziridine, pyrrolidine, and tetrahydrofuran (THF) on the kinetics of CO₂ hydrate growth/dissociation. Our simulations are validated via reproducing experimental data for CO₂ and CO₂ + THF hydrates at selected operating conditions. The simulated results show that both aziridine and pyrrolidine could perform as competent thermodynamic and kinetic promoters. Furthermore, aziridine seems to exceed pyrrolidine and THF in expediting the CO₂ hydrate growth rates under the same conditions. Our analysis unveils direct correlations between the kinetics of CO₂ hydrate growth and a combination of the free energy barrier for desorption of CO₂ from the hydrate surface and the binding free energy of chemical additives adsorbed at the growing hydrate substrate. The detailed thermodynamic analysis conducted in both hydrate and aqueous phases reveals molecular-level mechanisms by which CO₂ hydrate promoters are active, which could help to enable the implementation of CO₂ sequestration in hydrate-bearing reservoirs.

Assessing the effect of a liquid water layer on the adsorption of hydrate anti-agglomerants using molecular simulations

We have performed Molecular Dynamics simulations to study the adsorption of ten hydrate anti-agglomerants onto a mixed methane-propane sII hydrate surface covered by layers of liquid water of various thickness. As a general trend, we found that the more liquid water is present on the hydrate surface the less favorable the adsorption becomes, even though there are considerable differences between the individual molecules, indicating that the presence and thickness of this liquid water layer is a crucial parameter for anti-agglomerant adsorption studies. Additionally, we found that there exists an optimal thickness of the liquid water layer favoring hydrate growth due to the presence of both liquid water and hydrate-forming guest molecules. For all other cases of liquid water layer thickness, hydrate growth is slower due to the limited availability of hydrate-forming guests close to the hydrate formation front. Finally, we investigated the connection between the thickness of the liquid water layer and the degree of subcooling, and found a very good agreement between our Molecular Dynamics simulations and theoretical predictions.

Surface morphology effects on clathrate hydrate wettability

HYPOTHESIS

Clathrate hydrates preferentially form at interfaces; hence, wetting properties play an important role in their formation, growth, and agglomeration. Experimental evidence suggests that the hydrate preparation process can strongly affect contact angle measurements, leading to the different results reported in the literature. These differences hamper technological progress. We hypothesize that changes in hydrate surface morphologies are responsible for the wide variation of contact angles reported in the literature.

EXPERIMENTS

Experimental testing of our hypothesis is problematic due to the preparation history of hydrates on their surface properties, and the difficulties in advanced surface characterization. Thus, we employ molecular dynamics simulations, which allow us to systematically change the interfacial features and the system composition. Implementing advanced algorithms, we quantify fundamental thermodynamic properties to validate our observations.

FINDINGS

We achieve excellent agreement with experimental observations for both atomically smooth and rough hydrate surfaces. Our results suggest that contact line pinning forces, enhanced by surface heterogeneity, are accountable for altering water contact angles, thus explaining the differences among reported experimental data. Our analysis and molecular level insights help interpret adhesion force measurements and yield a better understanding of the agglomeration between hydrate particles, providing a microscopic tool for advancing flow assurance applications.

Water/oil interfacial tension reduction - an interfacial entropy driven process

The interfacial tension (IFT) of a fluid-fluid interface plays an important role in a wide range of applications and processes. When low IFT is desired, surface active compounds (e.g. surfactants) can be added to the system. Numerous attempts have been made to relate changes in IFT arising from such compounds to the specific nature of the interface. However, the IFT is controlled by an interplay of factors such as temperature and molecular structure of surface-active compounds, which make it difficult to predict IFT as those conditions change. In this study, we present the results from molecular dynamics simulations revealing the specific role surfactants play in IFT. We find that, in addition to reducing direct contact between the two fluids, surfactants serve to increase the disorder at the interface (related to interfacial entropy) and consequently reduce the water/oil IFT, especially when surfactants are present at high surface density. Our results suggest that surfactants that yield more disordered interfacial films (e.g. with flexible and/or unsaturated tails) reduce the water/oil IFT more effectively than surfactants which yield highly ordered interfacial films. Our results shed light on some of the factors that control IFT and could have important practical implications in industrial applications such as the design of cosmetics, food products, and detergents.

Correlating Antiagglomerant Performance with Gas Hydrate Cohesion

Although inhibiting hydrate formation in hydrocarbon-water systems is paramount in preventing pipe blockage in hydrocarbon transport systems, the molecular mechanisms responsible for antiagglomerant (AA) performance are not completely understood. To better understand why macroscopic performance is affected by apparently small changes in the AA molecular structure, we perform molecular dynamics simulations. We quantify the cohesion energy between two gas hydrate nanoparticles dispersed in liquid hydrocarbons in the presence of different AAs, and we achieve excellent agreement against experimental data obtained at high pressure using the micromechanical force apparatus. This suggests that the proposed simulation approach could provide a screening method for predicting, in silico, the performance of new molecules designed to manage hydrates in flow assurance. Our results suggest that entropy and free energy of solvation of AAs, combined in some cases with the molecular orientation at hydrate-oil interfaces, are descriptors that could be used to predict performance, should the results presented here be reproduced for other systems as well. These insights could help speed up the design of new AAs and guide future experiments.

Inhibition of Methane Hydrates Using Biodegradable Additives

Prevention of hydrate plugs during transportation of oil and natural gas in the pipeline network is challenging. Certain additives are often introduced into the process to eliminate/delay plug formation. Dominantly synthetic inhibitors are deployed in large volumes (∼20 to 30% by volume) to counter the problem and are highly expensive and, in some circumstances, toxic. The search for novel additives that are eco-friendly and act as inhibitors is in demand. The present study reports the thermodynamic inhibition (THI) capacity of some vastly available natural biopowders, such as Azadirachta indica (neem), Piper betel (betel), and Nelumbo nucifera (Indian lotus) in low dosage (0.5 wt %), on methane hydrate (MH) formation. Since the gas flow is dynamic, experiments are conducted in stirred geometry by varying the speed range from 0 to 1000 rotations per minute (rpm). All of the studies are performed in the isochoric method procedure. The biopowders act as efficient thermodynamic hydrate inhibitors. Once the nucleation triggers, they act as kinetic hydrate promoters. Since sodium dodecyl sulfate (SDS) is an excellent kinetic hydrate promoter in both stirred and nonstirred geometries, the obtained results are compared with the SDS system. Hydrate nucleation is triggered at higher subcooling (∼8 to 10 K) in the presence of water-soluble bioextracts. The neem leaf extracts showed a ∼30% lower hydrate conversion than SDS in identical experimental conditions. Two-stage hydrate nucleation occurred at higher stirring speeds, and the hydrate conversion is inferior (∼6%) between the primary and secondary stages. The addition of biopowder extracts is useful in controlling hydrate formation. A small quantity of biopowders provides higher inhibition and reduces synthetic chemicals used in real-time applications.

Structural Effects of Gas Hydrate Antiagglomerant Molecules on Interfacial Interparticle Force Interactions

Gas hydrate interparticle cohesive forces are important to determine the hydrate crystal particle agglomeration behavior and subsequent hydrate slurry transport that is critical to preventing potentially catastrophic consequences of subsea oil/gas pipeline blockages. A unique high-pressure micromechanical force apparatus has been employed to investigate the effect of the molecular structure of industrially relevant hydrate antiagglomerant (AA) inhibitors on gas hydrate crystal interparticle interactions. Four AA molecules with known detailed structures [quaternary ammonium salts with two long tails (R1) and one short tail (R2)] in which the R1 has 12 carbon (C12) and 8 carbon (C8) and saturated (C-C) versus unsaturated (C═C) bonding are used in this work to investigate their interfacial activity to suppress hydrate crystal interparticle interactions in the presence of two liquid hydrocarbons (n-dodecane and n-heptane). All AAs were able to reduce the interparticle cohesive force from the baseline (23.5 ± 2.5 mN m^-1), but AA-C12 shows superior performance in both liquid hydrocarbons compared to the other AAs. The interfacial measurements indicate that the AA with an R1 longer alkyl chain length can provide a denser barrier, and the AA molecules may have higher packing density when the AA R1 alkyl tail length is comparable to that of the liquid hydrocarbon chain on the gas hydrate crystal surface. Increasing the salinity can promote the effectiveness of an AA molecule and can also eliminate the effect of longer particle contact times, which typically increases the interparticle cohesive force. This work reports the first experimental investigation of high-performance known molecular structure AAs under industrially relevant conditions, showing that these molecules can reduce the interfacial tension and increase the gas hydrate-water contact angle, thereby minimizing the gas hydrate interparticle interactions. The structure-performance relation reported in this work can be used to help in the design of improved AA inhibitor molecules that will be critical to industrial hydrate crystal slurry transport.

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

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.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

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

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.

Schematic of anti-agglomeration effect of surfactants promoting gas hydrate particle dispersion.

Molecular Dynamics Simulation of Methane Hydrate Decomposition in the Presence of Alcohol Additives

The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate are discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at a temperature of 275 K evolve gradually into a long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280 K and pressures between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industrial exploitation of hydrates.

How Do Surfactants Control the Agglomeration of Clathrate Hydrates?

Clathrate hydrates can spontaneously form under typical conditions found in oil and gas pipelines. The agglomeration of clathrates into large solid masses plugs the pipelines, posing adverse safety, economic, and environmental threats. Surfactants are customarily used to prevent the aggregation of clathrate particles and their coalescence with water droplets. It is generally assumed that a large contact angle between the surfactant-covered clathrate and water is a key predictor of the antiagglomerant performance of the surfactant. Here we use molecular dynamic simulations to investigate the structure and dynamics of surfactant films at the clathrate-oil interface, and their impact on the contact angle and coalescence between water droplets and hydrate particles. In agreement with the experiments, the simulations predict that surfactant-covered clathrate-oil interfaces are oil wet but super-hydrophobic to water. Although the water contact angle determines the driving force for coalescence, we find that a large contact angle is not sufficient to predict good antiagglomerant performance of a surfactant. We conclude that the length of the surfactant molecules, the density of the interfacial film, and the strength of binding of its molecules to the clathrate surface are the main factors in preventing the coalescence and agglomeration of clathrate particles with water droplets in oil. Our analysis provides a molecular foundation to guide the molecular design of effective clathrate antiagglomerants.

Hydrophobic hydration affects growth of clathrate hydrate: insight from an NMR relaxometric and calorimetric study

Water tightly bound to the kinetic inhibitors of tetrahydrofuran hydrate is related to the hydrophobic hydration effect of the inhibitors.

Emergent Properties of Antiagglomerant Films Control Methane Transport: Implications for Hydrate Management

The relationship between collective properties and performance of antiagglomerants (AAs) used in hydrate management is handled using molecular dynamics simulations and enhanced sampling techniques. A thin film of AAs adsorbed at the interface between one flat sII methane hydrate substrate and a fluid hydrocarbon mixture containing methane and n-dodecane is studied. The AA considered is a surface-active compound with a complex hydrophilic head that contains both amide and tertiary ammonium cation groups and hydrophobic tails. At a sufficiently high AA density, the interplay between the surfactant layer and the liquid hydrocarbon excludes methane from the interfacial region. In this scenario, we combine metadynamics and umbrella sampling frameworks to study accurately the free-energy landscape and the equilibrium rates associated with the transport of one methane molecule across the AA film. We observe that the local configurational changes of the liquid hydrocarbon packed within the AA film are associated with high free-energy barriers for methane transport. The time scales estimated for the transport of methane across the AA film can be, in some cases, comparable to those reported in the literature for the growth of hydrates, suggesting that one possible mechanism by which AAs delay the formation of hydrate plugs could be providing a barrier to methane transport. Considering the interplay between the structural design and collective properties of AAs might be of relevance to improve their performance in flow assurance.