Hydrate Nucleus Research Articles

Comparative study of inhibition mechanism and performance of kinetic hydrate inhibitors: Varying amide group orientation

In this work, the inhibition mechanisms of two kinetic hydrate inhibitors, poly(N-isopropyl acrylamide) (PNIPAM) and poly(N-vinyl isobutyramide) (PNVIBA), were investigated at the molecular level, and the influence of the orientation of the amide group on the inhibition performance was evaluated. The results show that two inhibitors have the similar inhibition behavior, and should have the comparable inhibition performance. However, PNIPAM is slightly better at decreasing the stability of gas clusters to inhibit hydrate nucleation, and PNVIBA is slightly better at disrupting liquid water structure and preventing hydrate nucleus from contacting with the surroundings.

Resolving Temperature-Dependent Hydrate Nucleation Pathway: The Role of "Transition Layer".

Understanding the nucleation of natural gas hydrate (NGH) at different conditions has important implications to NGH recovery and other industrial applications, such as gas storage and separation. Herein, vast numbers of hydrate nucleation events are traced via molecular dynamics (MD) simulations at different degrees of supercooling (or driving forces). Specifically, to precisely characterize a hydrate nucleus from an aqueous system during the MD simulation, we develop an evolutionary order parameter (OP) to recognize the nucleus size and shape. Subsequently, the free energy landscapes of hydrate during nucleation are explored by using the newly developed OP. The results suggest that at 270 K (or 0.92 Tm supercooling, where Tm is the melting point), the near-rounded nucleus prevails during the nucleation, as described from the classical nucleation theory. In contrast, at relatively strong driving forces of 0.85 and 0.88 Tm, nonclassical nucleation events arise. Specifically, the pathway toward an elongated nucleus becomes as important as the pathway toward a near-rounded nucleus. To explain the distinct nucleation phenomena at different supercoolings, a notion of a "transition layer" (or liquid-blob-like layer) is proposed. Here, the transition layer is to describe the interfacial region between the nucleus and aqueous solution, and this layer entails two functionalities: (1) it tends to retain CH4 depending on the degrees of supercooling and (2) it facilitates collision among CH4, which thus promote the incorporation of CH4 into nucleus. Our simulation indicates that compared to the near-rounded nucleus, the transition layer surrounding the elongated nucleus is more evident with the higher collision rate among CH4 molecules. As such, the transition layer tends to promote the elongated nucleus pathway, while offsetting the cost of larger surface free energy associated with the elongated nucleus. At 0.92 Tm, however, the transition layer gradually disappears, and classical nucleation events dominate. Overall, the notion of "transition layer" offers deeper insight into the NGH nucleation at different degrees of supercooling and could be extended to describe other types of hydrate nucleation.

Competitive and synergistic mechanisms of adsorption-hydration during methane storage in the wet ZIF-8 fixed bed

The natural gas storage technology based on wet ZIF-8 fixed bed has been proved to have excellent methane storage performance. To reveal the internal competition and synergism mechanism of adsorption-hydration, molecular dynamics technique was employed to simulate the microscopic dynamics process of methane hydrate formation in the ZIF-8 fixed bed with saturated water. Our results indicated that when the diameter of ZIF-8 particles were small, the low water content of the bed would induce the strong adsorption-hydration competition, and apparently inhibit the formation of methane hydrates. When the water content was sufficient, the gas adsorption performance of ZIF-8 solid particles could significantly accelerate the hydrate nucleation, exhibiting the adsorption - hydration synergy. In this case, the hydrate nucleus always took shape outside the region affected by ZIF-8 (within the range of about 10 Å from the surface of ZIF-8) and continuously grew away from the solid surface. Moreover, the simulation results also showed that the particle sizes of ZIF-8 played a vital role on hydrate nucleation/growth velocities and the types of clathrate cages formed, but had little impact on the conversion ratio of water and methane to hydrates.

Microscopic molecular and experimental insights into multi-stage inhibition mechanisms of alkylated hydrate inhibitor

A deep understanding of the mechanism of hydrate inhibitors on hydrate formation is crucial for the development of efficient natural gas hydrate inhibitors. However, the current understanding of the inhibitory mechanism of hydrate inhibitors in the nucleation and formation processes is still very limited, which greatly hinders the development of new types of hydrate inhibitors. In this study, the solution polymerization method was employed to prepare poly (N-vinyl pyrrolidone-co-N,N-dimethyl acrylamide), as a new kinetic hydrate inhibitor. The inhibition properties were investigated by the constant cooling test and the step cooling test. Meanwhile, the multi-stage inhibition mechanisms of alkylated hydrate inhibitor were also revealed based on molecular simulations. The results of experiments and molecular simulations illustrate that the mechanism of the inhibitors varies at different stages. The inhibitors retard hydrate formation by reducing the solubility of methane molecules in water during the nucleation phase. However, during the formation phase, the hydrate inhibition is achieved by adsorption. In particular, the absorption of poly (vinyl pyrrolidone) on the surface of hydrate can be effectively improved by alkylation, and the interaction energy of poly (N-vinyl pyrrolidone-co-N,N-dimethyl acrylamide) with the hydrate nucleus was increased by 3.22 times compared to poly (vinyl pyrrolidone). It resulted in an increase in subcooling of 8.2 °C. Also, the effective induction time of poly (N-vinyl pyrrolidone-co-N,N-dimethyl acrylamide) at 8 °C was extended to 987 min. A molecular model for evaluating the interaction of inhibitors with hydrates is also successfully developed, which corresponds well to the experimental results. These results contribute to a better understanding of hydrate formation in drilling fluids and implications for developing new materials for high-performance hydrate drilling fluids.

Microscopic insights on clathrate hydrate growth from non-equilibrium molecular dynamics simulations

Clathrate hydrates form and grow at interfaces. Understanding the relevant molecular processes is crucial for developing hydrate-based technologies. Many computational studies focus on hydrate growth within the aqueous phase using the ‘direct coexistence method’, which is limited in its ability to investigate hydrate film growth at hydrocarbon-water interfaces. To overcome this shortcoming, a new simulation setup is presented here, which allows us to study the growth of a methane hydrate nucleus in a system where oil–water, hydrate-water, and hydrate-oil interfaces are all simultaneously present, thereby mimicking experimental setups. Using this setup, hydrate growth is studied here under the influence of two additives, a polyvinylcaprolactam oligomer and sodium dodecyl sulfate, at varying concentrations. Our results confirm that hydrate films grow along the oil–water interface, in general agreement with visual experimental observations; growth, albeit slower, also occurs at the hydrate-water interface, the interface most often interrogated via simulations. The results obtained demonstrate that the additives present within curved interfaces control the solubility of methane in the aqueous phase, which correlates with hydrate growth rate. Building on our simulation insights, we suggest that by combining data for the potential of mean force profile for methane transport across the oil–water interface and for the average free energy required to perturb a flat interface, it is possible to predict the performance of additives used to control hydrate growth. These insights could be helpful to achieve optimal methane storage in hydrates, one of many applications which are attracting significant fundamental and applied interests.

HCFC-141b hydrate formation with amino acid surfactants: Insights into hydrate formation promotion mechanism

Surfactants are good promoters for hydrate nucleation and growth. The hydrate promoting mechanism study by surfactants is very important for hydrate application technology. Amino acid surfactants (AAS) are environmentally friendly and easily degradable surfactants. Five AAS are used to investigate the influence on hydrate nucleation and growth. HCFC-141b is effectively dispersed in water by AAS. AAS with head group glutamic acid and longer aliphatic acyl chain can effectively promote hydrate formation. Sodium cocoyl glutamate and sodium stearoyl glutamate improve and stabilize hydrate nucleation. The hydrate conversion rate and hydrate cooling storage capacity by adding the optimal mass fraction of sodium cocoyl glutamate or sodium stearoyl glutamate is similar. Hydrate nucleation and growth are studied using microscopic imaging techniques. Hydrate nucleus first appear at the surface of HCFC-141b droplets. Hydrophobic HCFC-141b droplets formed under the help of AAS is the key to improve hydrate nucleation and growth.

Experimental measurement and model prediction on methane hydrate equilibrium conditions in the presence of organic carboxylic sodium salts

Gas hydrate inhibitors are employed to mitigate gas hydrate formation in pipelines. The effects of thermodynamics inhibition were evaluated by measuring the vapor - liquid aqueous solution - solid hydrate (VLH) equilibrium conditions on methane (CH4) hydrate in the presence of three organic carboxylic sodium salts (sodium formate, sodium acetate, sodium propionate) in this study. The experiments were carried out in the temperature range of 273.65 K to 281.65 K. Compared with deionized water, in the electrolyte solutions of sodium formate, sodium acetate and sodium propionate with mole fraction of 0.010 and 0.020, the VLH equilibrium pressure change (ΔP) increased by 12.93, 27.68 %; 18.78, 38.15 %; 23.51, 49.41 %, respectively. Both of the three organic carboxylic sodium salts have obvious thermodynamic inhibition effect on CH4 hydrate. The strength of hydrate inhibition is related to the carbon chain length of organic carboxylate. The dissociation enthalpy (ΔHdiss) of CH4 hydrate in three organic carboxylic sodium salts solutions remains quite uniform with deionized water, which indicates that three organic carboxylic sodium salts have not been involved in the formation of hydrate nucleus. The Chen-Guo hydrate model and N-NRTL-NRF activity model were employed to predict the VLH equilibrium conditions in electrolyte solutions. In the three organic carboxylic sodium salts solutions, the average absolute relative deviations (AARD) of the VLH equilibrium condition data between the predicted and the experimental were 0.81, 2.17, and 3.02 %, respectively. In the sodium chloride (NaCl) and potassium chloride (KCl) solutions, the AARD between the predicted and literature were 1.44, 1.69 %, respectively. The model has good universality and accuracy. Both sodium acetate and sodium propionate are food grade preservatives. Consequently, these are beneficial for the development of environmental-friendly hydrate inhibitors.

Unraveling Amphiphilic Poly(N-vinylcaprolactam)/Water Interface by Nuclear Magnetic Resonance Relaxometry: Control of Clathrate Hydrate Formation Kinetics.

Water-soluble amphiphilic polymers are vital chemicals in the oil and gas industry to retard crystal growth of hydrocarbon hydrate via surface adsorption and suppress nucleation of a pristine hydrate nucleus, thereby preventing formation of hydrate blockages in flow lines during oil and natural gas production. Apart from a few theoretical modeling studies, an experimental method to study the polymer/water interface in the crystal growth is critically needed. Here, water motions in the hydration shells of an exemplary kinetic inhibitor, poly(N-vinylcaprolactam), during hydrate formation from the tetrahydrofuran/water system are revealed via nuclear magnetic resonance relaxometry. Unequivocal experiments show that the pivotal interfacial water in the tightly bound state gradually freezes at rates depending on the polymer molecular weight (MW). This is supported by nonfreezable water analysis, which is correlated to the inhibition time. The polymers tune the kinetics of the hydration process via interaction with and perturbation of the water molecules. The free water component in the polymer solution crystallizes at a very slow rate when in partially restricted mobility, whereas the bound water component increases in the reaction, with the polymer/water interface serving as the reaction sites. The appropriate MW (including average MW and polydispersity values) of the inhibitive polymers can give rise to maximal retardation of the hydrate crystal growth. This work will help control other multiphase crystallization kinetic processes through the design of inhibitors or promoters functioning in the interface.

Microscopic molecular insights into clathrate methane hydrates dissociation in a flowing system

Natural methane hydrates are estimated to be the largest source of unexploited hydrocarbon fuel. Understanding the dissociation of methane hydrates in the flow system is vital for natural gas development, especially for the gas hydrate reservoir protection and the prevention of secondary hydrate generation in the wellbore. In this work, the model of natural gas hydrate in clay mineral pores was established, and the effects of drilling fluid on hydrate stability under different intrusion velocities were investigated by molecular dynamics simulation. The simulation results show that the hydrate nucleus on hydrophilic surfaces will be more susceptible to temperature. As the temperature increases, the hydrate nuclei at the edges and corners collapse preferentially due to the large contact area. Water molecules on hydrophilic surfaces are difficult to participate in the formation of hydrates due to hydrogen bonds. Furthermore, the higher the velocity of the fluid, the greater the degree of hydrate dissociation. And the hydrate nucleus decomposes faster in the vertical flow direction than that in the parallel flow direction. The destruction of hydrate formation and decomposition equilibrium is the main reason for hydrate decomposition in the flow process. These results contribute to a better understanding of hydrate decomposition in a flowing system and implications for developing new materials for high-performance hydrate drilling fluids.

Clathrate Hydrate Inhibition by Polyisocyanate with Diethylammonium Group.

Polymers containing amide groups have been used as kinetic hydrate inhibitors (KHIs). The amide group has good performance for hydrate nucleus adsorption, resulting in inhibition of hydrate growth. Polyisocyanates composed of an amide backbone can be KHI candidates; however, the use of polyisocyanates as KHIs has not yet been reported. Herein, we prepared water-soluble poly[3-[[2-(diethylamino)ethyl]thio]-1-propyl isocyanate-ran-hexyl isocyanate] (P(DETPIC-ran-HIC)) to investigate the ability of polyisocyanates to inhibit hydrate formation. In the tetrahydrofuran (THF) clathrate hydrate crystal growth inhibition tests, P(DETPIC-ran-HIC) showed better performance than the polyamide, poly(N-vinylpyrrolidone) (PVP).

Effects of waxes and asphaltenes on CO2 hydrate nucleation and decomposition in oil-dominated systems

Hydrate formation is a major flow assurance issue for underwater multiphase pipelines. The heavy fractions in crude oil make it more complicated. However, the impact of a single fraction in oil on hydrate behaviors is still poorly understood. In response, this paper aims to investigate and compare the effects of waxes and asphaltenes on CO2 hydrate nucleation and decomposition, using a self-designed high-pressure autoclave. The results show that both waxes and asphaltenes retard hydrate nucleation, wherein asphaltenes have a more pronounced effect than waxes. Waxes mainly restrict gas diffusion in the bulk phase during hydrate nucleation. Asphaltenes are more effectively adsorbed on the surface of the water droplet and hydrate nucleus to retard the nucleation. In addition, the decomposition rate of hydrate particles is lowered when trapped in the wax gel-like structure, whereas it is enhanced by the increased hydrate surface area with asphaltenes. This work provides insights into the effects of crude oil components on the behaviors of hydrate, which can facilitate flow assurance applications in subsea multiphase pipelines.

Molecular Insights into the Effect of a Solid Surface on the Stability of a Hydrate Nucleus

Understanding and controlling hydrate formation are of vital importance for the storage and transportation of natural gas. In this work, the influence of a solid surface on the stability of a hydrate nucleus is studied by molecular dynamics simulations. The surface with a strong affinity for guest molecules induces the decomposition of the hydrate nucleus because of competitive adsorption on guest molecules. Specifically, the competitive adsorption for guest molecules results in the transformation of interface structures of the hydrate nucleus, and this process can be divided into two steps: transformation from whole cages to semi-cages and aggregation of the isolated semi-cages. While the interaction between the guest molecules and the solid surface decreases to form a weakly affinitive surface, the hydrate nucleus can maintain its structure near the surface. Our research shows that not all surfaces facilitate hydrate nucleation, and the stability of the hydrate nucleus on the surface is closely related to the affinity for guest molecules of the solid surface.

Inhibition Effect of Kinetic Hydrate Inhibitors on the Growth of Methane Hydrate in Gas–Liquid Phase Separation State

The effect of kinetic hydrate inhibitors (KHIs) on the growth of methane hydrate in the gas–liquid phase separation state is studied at the molecular level. The simulation results show that the kinetic inhibitors, named PVP and PVP-A, show good inhibitory effects on the growth of methane hydrate under the gas–liquid phase separation state, and the initial position of the kinetic hydrate inhibitors has a major effect on the growth of methane hydrates. In addition, inhibitors at different locations exhibit different inhibition performances. When the inhibitor molecules are located at the gas–liquid phase interface, increasing the contact area between the groups of the inhibitor molecules and methane is beneficial to enhance the inhibitory performance of the inhibitors. When inhibitor molecules are located at the solid–liquid phase interface, the inhibitor molecules adsorbed on the surface of the hydrate nucleus and decreased the direct contact of hydrate nucleus with the surrounding water and methane molecules, which would delay the growth of hydrate nucleus. Moreover, the increase of hydrate surface curvature and the Gibbs–Thomson effect caused by inhibitors can also reduce the growth rate of methane hydrate.

Experimental and modelling study on the effect of maltose as a green additive on methane hydrate

The thermodynamic effect and kinetic process of maltose on methane hydrate formation were studied in this work. The gas-liquid-hydrate equilibrium (GLHE) conditions were measured using the isothermal pressure search method in deionized water and maltose aqueous solutions, respectively. The experimental results show that maltose has obvious thermodynamic inhibition effect on methane hydrate formation. The dissociation enthalpies (ΔHdiss) of methane hydrate in different solutions remain quite uniform, which indicates that maltose has not been involved in the formation of hydrate nucleus. The kinetic process of maltose on methane hydrate is evaluated by methane consumption, gas storage capacity (GSC) at 5.00 MPa and 276.15 K. The gas-hydrate steady-state time is longer due to the lower driving force of high concentration maltose aqueous solution at same temperature and pressure. Compared with deionized water, the GSC is lower in a high concentration of maltose aqueous solution. The Chen-Guo hydrate thermodynamic model and Wilson activity coefficient model are applied to calculate the GLHE conditions of methane hydrate in the presence of maltose. The calculation results are well correlated with the experimental values. Maltose is a renewable and edible biological product. Consequently, it is beneficial for the development of green biologically-based additive of hydrate.

Molecular dynamics method to simulate the process of hydrate growth in the presence/absence of KHIs

Recently, two derivatives of PVP named PVP-A and PVP-E were synthesized by our group and PVP-A was verified to have good inhibition performance by experiments. PVP-A and PVP-E were obtained by introducing a butyl ester group and a butyl ether group into PVP molecules, respectively. In this work, the inhibition process of hydrate growth at molecular level was simulated to explore the inhibition performance of PVP-A, PVP-E, and PVP and clarified their inhibition mechanism. Our simulation results show that PVP-A has the best inhibition performance among these selected KHIs which was in accordance with experiments. The binding interactions between inhibitors and liquid water molecules play a significant role in inhibition performance of KHIs. The strong binding effect could greatly disrupt the hydrogen-bond structure between the water molecules in the vicinity of hydrate nucleus surface and attract more water molecules to form hydrogen bond with KHIs rather than build hydrate clathrate on the surface of hydrate nucleus and therefore prevents the hydrate nucleus reaching the critical nuclear size for subsequent spontaneous growth.

Aqueous Methane in Slit-Shaped Silica Nanopores: High Solubility and Traces of Hydrates

Equilibrium molecular dynamic simulations were employed to investigate the methane solubility in water confined between two parallel silica surfaces. The solid substrate was obtained from β-cristobalite; all nonbridging oxygen atoms were protonated. The resultant surface density of OH groups was ∼4.54 sites per nm2. The simulations were conducted at constant temperature, 300 K, and at increasing bulk methane pressure for pores of width 1.0 nm. For bulk systems, these thermodynamic conditions are outside the window of methane hydrates stability. Methane solubility in confined water was found to far exceed that in bulk systems. The increase in tangential pressure, observed under confinement, cannot solely explain the marked increase in solubility predicted by our simulations. Most likely, the structure of confined water favors the sequestration of methane. The excess chemical potential for methane was found to significantly decrease within the confined water compared with that in the bulk phase. On the basis of the cage adsorption hypothesis for hydrate nucleation, the predicted solubility of methane in the confined water (up to ∼0.05 mol fraction) is large enough to suggest the possible formation of methane hydrates. Indeed, analysis of simulation data shows the presence of amorphous cages of hydrogen-bonded water that host a single methane molecule. Within the limits of our simulations, these amorphous cages last for only short times. Perhaps the pores considered are too narrow to allow the formation of stable methane hydrates, and perhaps longer simulations would allow us to observe the formation of a hydrate nucleus. The large methane solubility in confined water predicted by our simulations might have consequences for hydraulic fracturing and other technological processes.

Homogeneous Nucleation of Methane Hydrate in Microsecond Molecular Dynamics Simulations.

We report atomistically detailed molecular dynamics simulations of homogeneous nucleation of methane hydrate in bulk aqueous phase in the absence of any interface. Subcritical clusters of water and methane molecules are formed in the initial segment of the simulations, which then aggregate to give the critical hydrate nucleus. This occurs over time scales of several hundred nanoseconds, indicating that the formation and aggregation of subcritical clusters can contribute significantly to the overall rate of hydrate nucleation. The clusters have elements of sI hydrate structure, such as 5(12) and 5(12)6(2) cages as well as other uncommon 5(12)6(3) and 5(12)6(4) cages, but do not possess long-range order. Clusters are dynamic in nature and undergo continuous structural rearrangements.

Can the dodecahedral water cluster naturally form in methane aqueous solutions? A molecular dynamics study on the hydrate nucleation mechanisms

By performing a large scale of molecular dynamics simulations, we analyze 60 x 10(6) hydration shells of methane to examine whether the dodecahedral water cluster (DWC) can naturally form in methane aqueous solutions--a fundamental question relevant to the nucleation mechanisms of methane hydrate. The analyzing method is based on identifying the incomplete cages (ICs) from the hydration shells and quantifying their cagelike degrees (zetaC=0-1). Here, the zetaC is calculated according to the H-bond topological network of IC and reflects how the IC resembles the complete polyhedral cage. In this study, we obtain the zetaC distributions of ICs in methane solutions and find the occurrence probabilities of ICs reduce with zetaC very rapidly. The ICs with zetaC>or=0.65 are studied, which can be regarded as the acceptable cagelike structures in appearance. Both increasing the methane concentration and lowering the temperature can increase their occurrence probabilities through slowing down the water molecules. Their shapes, cage-maker numbers, and average radii are also discussed. About 13-14 of these ICs are face saturated, meaning that every edges are shared by two faces. The face-saturated ICs have the potential to act as precursors of hydrate nucleus because they can prevent the encaged methane from directly contacting other dissolved methane when an event of methane aggregation occurs. The complete cages, i.e., the ICs with zetaC=1, form only in the solutions with high methane concentration, and their occurrence probabilities are about 10(-6). Most of their shapes are different from the known hydrate cages, but we indeed observe a standard 5(12)6(2) hydrate cage. We do not find the expected DWC, and its occurrence probability is estimated to be far less than 10(-7). Additionally, the IC analysis proposed in this work is also very useful in other studies not only on the formation, dissociation, and structural transition of hydrates but also on the hydrophobic hydration of apolar solutes.

The phase-field theory applied to CO 2 and CH 4 hydrate

A phase-field theory is applied to model the growth of carbon dioxide hydrate and methane hydrate from a supersaturated solution in water. Temperature- and pressure-dependent thermodynamics for the two systems are accounted for. Simulations of the growth of a planar hydrate film and a circular hydrate nucleus are presented and the interface velocity has been extrapolated from the results to experimental time scales. We discuss how pressure and temperature affects the growth rate and argue that the governing process for the dynamics is the chemical diffusion of the guest molecule in the aqueous solution. We also present results from anisotropic simulations and outline how this will affect the growth.

The influence of diffusion on hydrate growth

In this study, we examine the rate-limiting process for the formation of hydrates from aqueous solutions of CO2 and the rate-limiting process when CO2 hydrate dissociates toward pure water. A phase field theory is applied to model the growth and dissociation of the gas hydrate in a system consisting of an aqueous CO2 phase, and an initial hydrate nucleus at constant pressure of 150 bar and a temperature of 274.15 K. The diffusion of CO2 in the aqueous phase is shown to be the governing parameter for the growth and dissociation rates. We investigate concentration profiles at the interface and show that diffusion through Fick’s law in the liquid can account for the behavior of the system. We argue that the released heat has little or no effect on the kinetics of growth and dissociation for the systems in this study, although we cannot exclude the potential effect of released heat on the nucleation stage. Finally, we also discuss the effects of anisotropic crystal growth on crystal morphology and kinetic rates of growth.