Grand Canonical Molecular Dynamics Research Articles

Exploration of Capturing CO2 from Flue Gas by Calcite Slit‐Nanopores: A Computational Investigation

AbstractCapturing CO2 from flue gas is recognized as a key process to improve the environment, therefore, developing a new cost‐effective sorbent is crucial. In this study, the calcite slit‐nanopores with various pore sizes ranging from ∼5 Å to ∼30 Å were constructed, the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to examine the microscopic behaviors of CO2 and N2 in calcite slit‐nanopores. Diverse adsorption layer structures for CO2 and N2 molecules as single component in calcite slit‐nanopores are found as the pore size changing, of which the CO2 represents a transformation from single adsorption layer to five adsorption layers, while N2 represents a variation from single adsorption layer to three adsorption layers. Meantime, different adsorption orientations of CO2 and N2 molecules adsorbed close onto the calcite pore surface are also found. Moreover, strong competitive adsorption of CO2 over N2 is found due to the different interactions between CO2 and N2 molecules with the calcite pore surface, the capacity of competitive adsorption decreases with the pore size increasing. The self‐diffusion and isosteric heat of CO2 and N2 molecules in calcite slit‐nanopores are also examined to demonstrate the different adsorption intensities of gases in calcite slit‐nanopores. This work demonstrates the detailed microscopic behaviors of CO2 and N2 molecules in calcite slit‐nanopores, which not only enriches the theoretical knowledge about gases adsorption in calcite slit‐nanopores, but also gives out the feasibility of capturing CO2 from flue gas by calcite‐based substances.

Molecular-scale simulations of organic compounds on ice: application to atmospheric and interstellar sciences

ABSTRACTIn this paper, we present a brief review of what has been learned about the adsorption characteristics of various organic molecules at the surface of ice, from more than 15 years of computer simulation studies at the molecular scale. In particular, grand canonical Monte Carlo and molecular dynamics calculations were performed to determine the adsorption isotherms, the saturation coverage of the first molecular layer at the ice surface, the preferred orientations of the molecules in their adsorption sites, and the corresponding adsorption energies. The results of the simulations indicated that the main driving force for trace gas adsorption on ice is hydrogen bonding not only between the adsorbate and the water molecules of the ice surface, but also within the adsorbate. When possible, the comparison with available experimental data showed a close agreement, supporting thus the methodology used in the modelling. Finally, the present review demonstrates how computer simulation can nicely complement experimental approaches for studying interactions between trace gases and ice under tropospheric and interstellar condition

A comparison of dynamic mean field theory and grand canonical molecular dynamics for the dynamics of pore filling and capillary condensation of fluids in mesopores.

We use results from grand canonical molecular dynamics (GCMD) to test the predictions from dynamic mean field theory (DMFT) for the pore filling and capillary condensation mechanisms of a fluid confined in slit shaped mesopores. The theory predicts that capillary condensation occurs by a nucleation process in which a liquid bridge forms between the two walls, and the pore is filled via the growth of this bridge. For longer pores, multiple bridging is seen. These mechanisms are confirmed by the molecular dynamics simulations. The primary difference between the theory and simulations lies in the role of fluctuations. DMFT predicts a single nucleation time and location, while in GCMD (and in nature) a distribution of nucleation times and locations is seen.

Computational Screening of Metal-Organic Frameworks for Membrane-Based CO2/N2/H2O Separations: Best Materials for Flue Gas Separation.

It has become a significant challenge to select the best metal–organic frameworks (MOFs) for membrane-based gas separations because the number of synthesized MOFs is growing exceptionally fast. In this work, we used high-throughput computational screening to identify the top MOF membranes for flue gas separation. Grand canonical Monte Carlo and molecular dynamics simulations were performed to assess adsorption and diffusion properties of CO2 and N2 in 3806 different MOFs. Using these data, selectivities and permeabilities of MOF membranes were predicted and compared with those of conventional membranes, polymers, and zeolites. The best performing MOF membranes offering CO2/N2 selectivity > 350 and CO2 permeability > 106 Barrer were identified. Ternary CO2/N2/H2O mixture simulations were then performed for the top MOFs to unlock their potential under industrial operating conditions, and results showed that the presence of water decreases CO2/N2 selectivity and CO2 permeability of some MOF membranes. As a result of this stepwise screening procedure, the number of promising MOF membranes to be investigated for flue gas separation in future experimental studies was narrowed down from thousands to tens. We finally examined the structure–performance relations of MOFs to understand which properties lead to the greatest promise for flue gas separation and concluded that lanthanide-based MOFs with narrow pore openings (<4.5 Å), low porosities (<0.75), and low surface areas (<1000 m2/g) are the best materials for membrane-based CO2/N2 separations.

Adsorption and diffusion of CO2 and CH4 in covalent organic frameworks: an MC/MD simulation study

ABSTRACTCovalent organic frameworks (COFs) are a promising gas separation material which have been developed recently. In this work, we have used grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations to investigate the adsorption and diffusion properties of CO2 and CH4 in five recent synthesised COF materials. We have also considered the properties of amino-modified COFs by adding –NH2 group to the five COFs. The adsorption isotherm, adsorption/diffusion selectivity, self/transport diffusion coefficients have been examined and discussed. All of the five COFs exhibit promising adsorption selectivity which is higher than common nanoporous materials. An S-shaped adsorption isotherm can be found for CO2 instead of CH4 adsorption. The introduction of –NH2 group is effective at low pressure region (<200 kPa). The diffusion coefficients are similar for TS-COFs but increase with the pore size for PI-COFs, and the diffusion coefficients seem less dependent on the –NH2 groups.

Grand Canonical Investigation of the Quasi Liquid Layer of Ice: Is It Liquid?

In this study, the solid-vapor equilibrium and the quasi liquid layer (QLL) of ice Ih exposing the basal and primary prismatic faces were explored by means of grand canonical molecular dynamics simulations with the monatomic mW potential. For this model, the solid-vapor equilibrium was found to follow the Clausius-Clapeyron relation in the range examined, from 250 to 270 K, with a Δ Hsub of 50 kJ/mol in excellent agreement with the experimental value. The phase diagram of the mW model was constructed for the low pressure region around the triple point. The analysis of the crystallization dynamics during condensation and evaporation revealed that, for the basal face, both processes are highly activated, and in particular cubic ice is formed during condensation, producing stacking-disordered ice. The basal and primary prismatic surfaces of ice Ih were investigated at different temperatures and at their corresponding equilibrium vapor pressures. Our results show that the region known as QLL can be interpreted as the outermost layers of the solid where a partial melting takes place. Solid islands in the nanometer length scale are surrounded by interconnected liquid areas, generating a bidimensional nanophase segregation that spans throughout the entire width of the outermost layer even at 250 K. Two approaches were adopted to quantify the QLL and discussed in light of their ability to reflect this nanophase segregation phenomena. Our results in the μVT ensemble were compared with NPT and NVT simulations for two system sizes. No significant differences were found between the results as a consequence of model system size or of the working ensemble. Nevertheless, certain advantages of performing μVT simulations in order to reproduce the experimental situation are highlighted. On the one hand, the QLL thickness measured out of equilibrium might be affected because of crystallization being slower than condensation. On the other, preliminary simulations of AFM indentation experiments show that the tip can induce capillary condensation over the ice surface, enlarging the apparent QLL.

Molecular Simulations of Methane Adsorption Behavior in Illite Nanopores Considering Basal and Edge Surfaces

The adsorption properties of methane (CH4) have a great influence on shale gas exploration and development. The surface chemistry characteristics of nanopores are key factors in adsorption phenomena. The clay pores in shale formations exhibit basal surface and edge surfaces (mainly as A and C chain and B chain surfaces in illite). Little research regarding CH4 adsorption on clay edge surfaces has been carried out despite their distinct surface chemistries. In this work, the adsorption of CH4 confined in nanoscale illite slit pores with basal and edge surfaces was investigated by grand canonical Monte Carlo and molecular dynamics simulations. The adsorbed phase density, adsorption capacity, adsorption energy, isosteric heat of adsorption, and adsorption sites were calculated and analyzed. The simulated adsorption capacity compares favorably with the available experimental data. The results show that the edge surfaces have van der Waals interactions that are weaker than those of the basal surfaces. The adsorption capacity follows the order basal surface > B chain surface > A and C chain surface. However, the differences of adsorption capacity between these surfaces are small; thus, edge surfaces cannot be ignored in shale formation. Additionally, we confirmed that the adsorbed phase has a thickness of approximately 0.9 nm. The pore size determines the interaction overlap strength on the gas molecules, and the threshold value of the pore size is about 2 nm. The preferential adsorption sites locate differently on edge and basal surfaces. These findings could provide deep insights into CH4 adsorption behavior in natural illite-bearing shales.

Molecular investigation of gas adsorption, separation, and transport on carbon nanoscrolls: A combined grand canonical Monte Carlo and molecular dynamics study

Carbon nanoscrolls (CNSs), with controllable interlayer distances, have been systematically studied with respect to noble gas adsorption, separation, and transport properties. Grand canonical Monte Carlo simulations were performed on pure noble gases and noble gas mixtures interacting with CNSs at different temperatures. There is separation of gases across a series of CNS interlayer distances at different pressures. The optimal interlayer distance for each noble gas was determined. Selective adsorption was characterized for noble gas mixtures, and the corresponding selectivity was explored. A molecular sieving effect was observed for a certain interlayer gap range, in which the energetically unfavorable smaller gas atoms were better stored in the CNS. In addition, molecular dynamics simulations were conducted to understand the transport properties of noble gases in carbon nanoscrolls in both the cross-sectional and axial directions of the CNS. Gas molecules are found to be captured most strongly when the CNS has the optimal interlayer distance for adsorption.

Clay Swelling in Dry Supercritical Carbon Dioxide: Effects of Interlayer Cations on the Structure, Dynamics, and Energetics of CO2 Intercalation Probed by XRD, NMR, and GCMD Simulations

In situ X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) experiments combined with molecular dynamics simulations using the grand canonical ensemble [grand canonical molecular dynamics (GCMD)] show that the cation size, charge, and solvation energy play critical roles in determining the interlayer expansion of smectite clay minerals when exposed to dry supercritical scCO2 (scCO2) under conditions relevant to petroleum reservoirs and geological CO2 sequestration conditions. The GCMD results show that the smectite mineral, hectorite, containing interlayer alkali and alkaline earth cations with small ionic radii and high solvation energies (e.g., Na+ and Ca2+) does not intercalate CO2 and that the fully collapsed interlayer structure is the most energetically stable configuration. With Cs+ and Ba2+, the monolayer structure is the stable configuration, and CO2 should spontaneously enter the interlayer. With Cs+, there is not even an energy barrier for CO2 intercalation, in agreement with the XRD a...

Probing spatial locality in ionic liquids with the grand canonical adaptive resolution molecular dynamics technique

We employ the Grand Canonical Adaptive Resolution Simulation (GC-AdResS) molecular dynamics technique to test the spatial locality of the 1-ethyl 3-methyl imidazolium chloride liquid. In GC-AdResS, atomistic details are kept only in an open sub-region of the system while the environment is treated at coarse-grained level; thus, if spatial quantities calculated in such a sub-region agree with the equivalent quantities calculated in a full atomistic simulation, then the atomistic degrees of freedom outside the sub-region play a negligible role. The size of the sub-region fixes the degree of spatial locality of a certain quantity. We show that even for sub-regions whose radius corresponds to the size of a few molecules, spatial properties are reasonably reproduced thus suggesting a higher degree of spatial locality, a hypothesis put forward also by other researchers and that seems to play an important role for the characterization of fundamental properties of a large class of ionic liquids.

Grand canonical Monte Carlo and molecular dynamics simulations of the structural properties, diffusion and adsorption of hydrogen molecules through poly(benzimidazoles)/nanoparticle oxides composites

Comprehensive structural/molecular simulations have been undertaken to study the poly(benzimidazoles) (PBI) membrane combined with four different nano-oxide materials (ZnO, Al2O3, SiO2 and TiO2) for purification and production of hydrogen from natural gases. Composite membranes were built with different amounts of nano-oxide materials to investigate the influence of nano-oxide content on the PBI membrane performance. Several structural characterizations such as FFV, WAXD and also a thermal one (glass transition temperature) were done to study the structural properties of all simulated membrane cells. Moreover, MSD and adsorption isotherms tasks were used to estimate the diffusivity and solubility of hydrogen molecules through the latter mixed matrix membranes (MMMs), respectively. Permeability and permselectivity of H2 penetrate molecules were also carefully calculated using the aforementioned penetrating factors (diffusivity and solubility). Results show a significant improvement in structural and transport properties by increasing the nanomaterials content, which could be due to the growth of penetration pathways through the membranes. Furthermore, membranes with SiO2 yield the best results compared to other three nano-oxide fillers. H2 gas yields the best results that help the storage and separation of this precious gas from other gas molecules, which present in natural gases. Compared to the previous studies and literature results, the current results are accurate and reliable to describe the structural and transport properties of PBI/nano-oxides composites.

Molecular simulations of water adsorption and transport in mesopores with varying hydrophilicity arrangements.

The adsorption and transport of water in an open cylindrical mesopore with two different inner surface arrangements of hydrophilicities were examined by molecular simulations. The first model has a weak hydrophilic surface at both entrances of the pore and a stronger hydrophilic surface in the mid-section. The second pore has stronger hydrophilic surfaces at the entrances and weaker in the middle region. The simulation results show that the water adsorption isotherms obtained from Grand Canonical Monte Carlo simulations and pore filling curves acquired from Grand Canonical Molecular Dynamics simulations change depending on the arrangement of the strong and weak hydrophilic surfaces. In the first model, water condensation focuses on the mid-section forming a liquid bridge or a film, which creates a concave meniscus accelerating subsequent adsorption within the pore. Two bridges form in the entrance regions, where a cavity naturally occurs in between the films, in the second model. The different filling and emptying mechanisms clearly change the adsorption-desorption characteristics for the two pore types, but the second type generally showed faster transitions overall. Flux and meniscus analysis also reveals a circulating flow at the menisci of the interfaces within the pore. The results are expected to be valuable in understanding the effects of interior surface modification of nanopores in future applications.

Fluid-fluid interfacial mobility from random walks

Dual control volume grand canonical molecular dynamics is used to perform the first calculation of fluid-fluid interfacial mobilities. The mobility is calculated from one-dimensional random walks of the interface by relating the diffusion coefficient to the interfacial mobility. Three different calculation methods are employed: one using the interfacial position variance as a function of time, one using the mean-squared interfacial displacement, and one using the time-autocorrelation of the interfacial velocity. The mobility is calculated for two liquid-liquid interfaces and one liquid-vapor interface to examine the robustness of the methods. Excellent agreement between the three calculation methods is shown for all the three interfaces, indicating that any of them could be used to calculate the interfacial mobility.

Molecular simulation of shale gas adsorption in organic-matter nanopore

Shale gas is a kind of unconventional oil-gas resource with tremendous potential. For thorough understanding of the methane adsorption and micromechanism in organic-matter nanopores of the shale and better acquaintances of the occurrence form, graphite slit-pores were set up as a representation of organic-matter nanopores by using Material Studio, and the grand canonical Monte Carlo method, molecular mechanics and molecular dynamics were used for the simulation of adsorption and diffusion behaviors in organic-matter pores on CH4 and CO2 at the shale gas common burial depth of 2–4 km in the Upper Yangtze Plate. The results indicated that the adsorptions of CH4 and CO2 were physical and the optimal storage depth was 2 km; The mixed adsorption data showed the rationality of exploit shale gas by injecting CO2 to exchange CH4, and the optimal burial depth was 4 km; The relative density of CH4 and CO2 along the normal direction of the pore inwall showed a trend of symmetric distribution and apparent adsorption stratifications appeared. As a whole, the self-diffusion coefficient of CH4 and CO2 increased with the increase of burial depth, and it's consistent with the reasons for such changes of adsorption amount and adsorption heat.

Competitive Adsorption of CO2 over N2 in Asphaltene Slit Nanopores Studied by Molecular Simulation

CO2 capture and sequestration (CCS) is recognized as one of the most promising alternatives to weaken the greenhouse effect, and nanoporous materials are regarded as the promising candidates; therefore, developing a new cost-effective sorbent to achieve CCS is crucial. In this study, asphaltene-based slit nanopores were used to simulate capturing of CO2 from flue gas. The grand canonical Monte Carlo and molecular dynamics simulation methods were employed to examine the microscopic behaviors of CO2 and N2 in asphaltene slit nanopores. The isosteric heat of CO2 and N2 molecules adsorbed in asphaltene slit nanopores, the adsorption energy of a single molecule of CO2 and N2 adsorbed on the surface of asphaltene fragments, and the self-diffusion of CO2 and N2 molecules adsorbed in asphaltene slit nanopores were examined. Strong competitive adsorption of CO2 over N2 is found in a broad range of temperatures and pressures, and it was found that the temperature plays an important role on the competitive adsorptio...

Effects of Surface Composition on the Microbehaviors of CH4 and CO2 in Slit-Nanopores: A Simulation Exploration.

Molecular dynamics simulation studies were employed to investigate the microscopic behaviors of CH4 and CO2 molecules in slit-nanopores (SNPs) with various surfaces and different compositions. Three kinds of SNPs were constructed by a pair-wise combination of graphene, silica, and the calcite surface. The grand canonical Monte Carlo and molecular dynamics simulation methods were used to investigate the adsorption and self-diffusion of the gases in the nanopores. It is found that in all three cases, the CH4 molecules prefer to adsorb onto the graphene surface, whereas the CO2 molecules prefer to adsorb onto the calcite surface. The adsorption intensity of gases adsorbed onto various surfaces, the adsorption distances, along with the details of adsorption orientations of CH4 and CO2 molecules on various surfaces are calculated. The surface characteristics, such as surface roughness and charge distribution, are analyzed to help understand the microscopic adsorption behaviors of the gases on the specific surface. It was found that competitive adsorptions of CO2 over CH4 broadly occurred, especially in the SNPs containing calcite, because of the strong adsorption interactions between the CO2 molecules and the calcite surface. This work provides the microbehaviors of CH4 and CO2 in SNPs with various surfaces in different compositions to provide useful guidance for better understanding about the microstate of gases in complex nanoporous shale formation and to give out useful guidance for enhancing shale gas recovery by injecting CO2.

Molecular Dynamics Study of CO2 and H2O Intercalation in Smectite Clays: Effect of Temperature and Pressure on Interlayer Structure and Dynamics in Hectorite

Grand Canonical Molecular Dynamics (GCMD) simulations were performed to investigate the intercalation of CO2 and H2O molecules in the interlayers of the smectite clay, Na-hectorite, at temperatures and pressures relevant to petroleum reservoir and geological carbon sequestration conditions and in equilibrium with H2O-saturated CO2. The computed adsorption isotherms indicate that CO2 molecules enter the interlayer space of Na-hectorite only when it is hydrated with approximately three H2O molecules per unit cell. The computed immersion energies show that the bilayer hydrate structure (2WL) contains less CO2 than the monolayer structure (1WL) but that the 2WL hydrate is the most thermodynamically stable state, consistent with experimental results for a similar Na-montmorillonite smectite. Under all T and P conditions examined (323–368 K and 90–150 bar), the CO2 molecules are adsorbed at the midplane of clay interlayers for the 1WL structure and closer to one of the basal surfaces for the 2WL structure. Inte...

A mesoscale model for diffusion and permeation of shale gas at geological depth

The demand on energy is rising and shale gas as an important unconventional energy resource has received worldwide attention. It has shown a significant effect on the world's energy structure after the commercial exploitation of shale gas in the United States. Understanding diffusion and permeation of shale gas at geological depths is quite essential, but it cannot be described by traditional Fick or Knudsen diffusion models. In this work, we use dual control volume–grand canonical molecular dynamics method to systematically investigate the permeation process of shale gas in montmorillonite (i.e., a clay mineral of shale) at different geological depths. Results indicate that temperature, pressure, and pore size have an important effect on the permeability, and Knudsen equation cannot describe the permeability of shale gas. Accordingly, on the basis of these simulated data, we propose a new mesoscale model to describe the permeability of shale gas at geological depths. The new mesoscale model shows extensive applicability and can excellently reproduce the extrapolation testing data, and it satisfactorily bridges the gap between Knudsen diffusion and Fick diffusion, which provides important fundamentals for exploitation of shale gas. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1059–1066, 2018

Simulation to Enhance Shale Gas Recovery Using Carbon Dioxide in Silica Nanopores with Different Sizes

AbstractAn understanding of molecular behavior on a microscopic level is always important not only to find out the natural principles but also to decide how to solve problems in applications. In this study, the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were employed to investigate the adsorption and diffusion properties of CH4 and CO2 in silica nanopores with different sizes, and the displacement of residual CH4 in silica nanopores by CO2 at a constant temperature of 323 K and various pressures was studied. The microscopic molecular states of the adsorbed CH4 and CO2 in nanopores of various sizes are different. The competitive adsorption of CO2 over CH4 occurs broadly because of the different intensity of interactions between the gases molecules and the pore surface, of which the degree decreases with the increase of the pore size. An effective displacement process of residual adsorbed CH4 by CO2 was performed, and it is found that the displacement is enhanced with the increase of the CO2 bulk pressure and that the pore size has a significant influence on the displacement. According to the results, CO2 capture and storage (CCS) and the enhancement of CH4 recovery could be achieved at the same time in silica nanopores. This work provides microscopic information on the molecular behavior of CH4 and CO2 in silica nanopores and testifies to the efficiency of the displacement of CH4 by CO2 in silica nanopores with various sizes to provide useful guidance for applications in the enhancement of shale gas recovery by CO2.

Effect of nano-confinement on high pressure methane flow characteristics

High pressure gas flow in shale nanoporous media plays an important role in shale gas recovery. The objective of this work is to study the effect of confinement on high pressure methane flow in nanoscale pores. We used dual control volume grand canonical molecular dynamics for different temperatures and pore sizes and compared our results with conventional hydrodynamic equation. We observed that when pore size is larger than 2 nm, flux increases with pore size. But when pore size is less than 2 nm, flux shows varying behaviors depending on the pore size due to methane molecular configurations and structures under different nano-confinements. We also observed that enhancement over the prediction from Hagen-Poiseuille equation can be more than two orders of magnitude in nanopores of around 1 nm pore size. Similar to previous computer simulation and experiments on liquid flow in nanopores, the dependences of the enhancement factor ε and the calculated slip length Ls on the pore sizes show discontinuity within three distinct regions: R1) pore size less than 0.8 nm; R2) between 0.8 and 1.0 nm; and R3) larger than 1.0 nm. Within R2 and R3, ε and Ls monotonically decrease with pore size. The methane molecular flux, enhancement factor, and calculated slip length increase from 328.15 K to 368.15 K, but remain comparable for 368.15 K and 408.15 K.