Effect Of Pore Width Research Articles

Effect of competitive adsorption on the deformation behavior of nanoslit-confined carbon dioxide and methane mixtures

Understanding the behaviors of nanoslit-confined carbon dioxide and methane mixtures is of great importance in geological storage of carbon dioxide (CO2) and extraction of methane (CH4) from shale/coalbeds. Herein, the effects of pore width, bulk mixture composition, and geographic depth on the mixture adsorption and pore deformation tendency are studied by using the classical density functional theory. The solvation pressure against pore width shows an oscillating distribution, indicating a deformation transition from expansion to contraction in nanopores. A high recovery ratio of CH4 can be found in small pores at low temperatures and under high bulk pressure. For the 0.68 nm pore, a remarkably high optimal recovery ratio is identified at the geographic depth of about 0.2 km, while for the pores with pore width of 0.9–1.2 nm a modest optimal recovery ratio locates at the depth of about 0.5–1.0 km. This work provides a thorough understanding of the adsorption and deformation behavior of slit-confined mixtures.

Nonnegligible nano-confinement effect on solvent-mediated interactions between nanoparticles

Separating nanoparticles (NPs) by means of microfluidics represents a promising method. One of the associate challenges is how to regulate the interactions between NPs in confined solvents for avoiding particle aggregation. To tackle this challenge, herein we conduct a theoretical study to interpret the confinement effects on the potential of mean force between two NPs in nanochannels. By using the classical density functional theory, the effects of pore width, wall wettability, NP size, and solvent-NP interactions are unraveled. It is found that the nano-confinement can noticeably enhance the short-ranged attraction between NPs, and the attraction strength can be moderated by enlarging pore size, making wall solvophobic, shrinking NP size, and widening the range of solvent-NP interaction. This study provides a theoretical guidance towards the development of efficient particle separation method in nanochannel-based devices.

Isochoric heat capacity of confined fluids: The effect of pore width

The confinement in nanospaces alters the thermophysical properties of the confined fluid due to the interaction forces between the solid wall and the fluid particles. In this work, we present a detailed study of the confinement effects on the molar isochoric heat capacity by means of Monte Carlo simulations of the following systems: confined ideal gas, confined hard-sphere fluids, and confined methane. These systems are confined within micro- and mesopores of graphite, represented by a Steele potential. We show that the behavior of the molar isochoric heat capacity changes nonmonotonically with the pore width, and that the single-particle distribution has an intrinsic relation with the value of the heat capacity.

Effects of nitrogen and oxygen functional groups and pore width of activated carbon on carbon dioxide capture: Temperature dependence

In this work, activated carbon (AC) was prepared from bamboo waste using heat treatment. The AC was then modified to be nitrogen- and oxygen-enriched ACs by integration with urea, air oxidation, and KOH activation. At 25 °C, the nitrogen-enriched sample showed the highest CO2 adsorption affinity (uptake in pore at low pressures) and capacity (uptake in pore at moderate pressures, i.e., 1 bar). Whereas even at 0 °C, it still gave the highest affinity, but its capacity was reduced to be slightly lower than other samples. Consequently, a Grand Canonical Monte Carlo simulation was performed to macroscopically and microscopically investigate the CO2 adsorption behavior occurring in the experiments. The graphitic slit pore, in the pore width range of 0.7–1.5 nm, without a surface functional group (SFG), with pyridine (N-6), and with hydroxyl (OH) functional groups were modeled. (1) Adsorption affinity: the active site of SFG is dominant where CO2 molecules have the strongest interaction with N-functional group for all studied temperatures. The simulated results are consistent with the experimental data. (2) Adsorption capacity: the effect of pore width is more relevant. A sample with a more effective pore size gives higher capacity. However, the effective pore widths oscillated with temperature. High capacity at a suitable pore width resulted from the balance between the energy of motion and the packing of adsorbed molecules in order to optimize the energy. The energy of motion is more distinctive at high temperature; whereas, the commensurate packing is essential at low temperature.

Pore-scale lattice Boltzmann simulation of micro-gaseous flow considering surface diffusion effect

Recent studies have shown that adsorbed gas and its surface diffusion have profound influence on micro-gaseous flow through organic pores in shale gas reservoirs. In this paper, a multiple-relaxation-time (MRT) LB model is adopted to estimate the apparent permeability of organic shale and a new boundary condition, which combines Langmuir adsorption theory with Maxwellian diffusive reflection boundary condition, is proposed to capture gas slip and surface diffusion of adsorbed gas. The simulation results match well with previous studies carried out using Molecular Dynamics (MD) and show that Maxwell slip boundary condition fails to characterize gas transport in the near wall region under the influence of the adsorbed gas. The total molar flux can be either enhanced or reduced depending on variations in adsorbed gas coverage and surface diffusion velocity. The effects of pore width, pressure as well as Langmuir properties on apparent permeability of methane transport in organic pores are further studied. It is found that the surface transport plays a significant role in determining the apparent permeability, and the variation of apparent permeability with pore size and pressure is affected by the adsorption and surface diffusion.

Segregation Structures and Miscellaneous Diffusions for Ethanol/Water Mixtures in Graphene-Based Nanoscale Pores

Molecular dynamics simulation was conducted to study ethanol–water mixtures and the corresponding pure species, confined within slit-shaped graphene nanopores. Extensive structural and dynamical properties of the confined fluids, including hydrogen-bonding behavior, were investigated. The effects of pore width and mixture composition on the confined behavior were illustrated. It is observed that a layered structure is formed within the confined spaces and the ethanol–water mixtures show segregation at larger pores, with ethanol molecules preferentially adsorbing on graphene surfaces. This microphase demixing behavior stems from the competitive effect of the solid–fluid and fluid–fluid interactions. Moreover, miscellaneous diffusion mechanisms have been revealed for the hydrogen-bonding mixtures within the graphene pores. In the mixtures, water and ethanol generally display analogous diffusion mechanism due to ethanol–water association, converting from short-time subdiffusion to long-time Fickian diffusion...

Anatomy of Adsorption in Open-End and Closed-End Slit Mesopores: Adsorption, Desorption, and Equilibrium Branches of Hysteresis Loop

We present a detailed analysis, using simulated data, of the adsorption and desorption in two finite length slit mesopores, one with both ends open to the surroundings and the other with one end closed, to investigate the microscopic behavior of an adsorbate in a confined space. The mesoscopic analysis of the interface separating the gas-like phase and the condensed phase reveals the mechanisms that lead to hysteresis in these two types of pore. The effects of pore width, temperature, and surface strength on these mechanisms are also studied to probe the detailed behavior of the system just before condensation and just after evaporation. In particular, for closed-end pores, we observe the development of a meniscus and the subsequent movement of the developed meniscus in adsorption and desorption. From these observations we are able to explain why adsorption and desorption branches have two distinct sections, slow and fast variations with pressure (corresponding to the two stages (1) developing meniscus and (2) developed meniscus), which have not been recognized in earlier literature.

Modeling the adsorptive selectivity of carbon nanotubes for effective separation of CO2/N2 mixtures

Canonical Monte Carlo (CMC) simulations were carried out to investigate the behavior of CO(2) and N(2) mixtures upon adsorption on single walled carbon nanotubes (CNTs). In the simulation, all the particle-particle interactions between CO(2), N and C were modeled using Lennard-Jones (LJ) potential. To provide deep insight into the effect of pore width, temperature, pressure and bulk composition on the adsorption behavior of CO(2) /N(2) mixtures, five different CNTs [(6,6), (7,7), (8,8) (9,9) and (10,10) CNT] with diameters ranging from 0.807 to 1.35 nm, three temperatures (300 323 and 343 K), six pressures (0.15, 2, 4, 6, 8 and 10 MPa), and three bulk mole compositions of carbon dioxide (0.3 0.5 and 0.7) were tested. The results from all the simulation conditions investigated in this work show that CNTs preferentially adsorb carbon dioxide relative to nitrogen in a binary mixture. The results are consistent with the hypothesis that stronger interaction of one component with the nanotube surface results in a higher adsorption capacity compared to the other component. An optimized pore size of D = 8.07 nm corresponding to (6, 6) CNT, at T = 300 K and P = 0.15 MPa at a bulk mole composition of y(CO2) =0.3 was identified in which carbon nanotubes demonstrate the greatest selectivity for separation of carbon dioxide relative to nitrogen. In addition, it is worth pointing out that, under similar simulation conditions, CNTs exhibit higher selectivity compared to other carbon-based materials [carbon membrane polyimide (PI) and PI/multi-wall carbon nanotubes (MWCNTs)] for CO(2) adsorption. As a prototype, the selectivity of an equimolar mixture of CO(2) /N(2) for adsorption on (6, 6) CNTs at 300 K and 0.15 MPa reaches 9.68, which is considerably larger than that reported in carbon membrane. Therefore, it can be concluded that carbon nanotubes can act as a capable adsorbent for adsorption/desorption of CO(2) in comparison with other carbon-based materials in the literature.

Non-equilibrium molecular dynamics simulation on permeation and separation of H 2/CO in nanoporous carbon membranes

Permeation and separation of H 2/CO binary mixtures in nanoporous carbon membranes are investigated by non-equilibrium molecular dynamics simulations. The carbon membrane pores are modeled as slit-like pores with entrance and exit. The buffer regions between the control volumes and membrane pores are employed to take into account the effects of the entrance and exit of the membrane pores. The effects of pore width, separation temperature, feed gas pressure, the molar fraction of hydrogen, and membrane thickness on flux and dynamic separation factor are discussed. The simulation results indicate that the pore width strongly affects the flux and dynamic separation factor. In addition, molecular sieving dominates the separation of H 2/CO mixtures, when the pore width is smaller by about 0.64 nm, and, in this case, the dynamic separation factor reaches 52.88 at 0.5 MPa and 300 K. The dynamic separation factor increases with the separation temperature and the decrease of feed gas pressure, while changes slightly with the molar fraction of H 2 in the feed gas. Moreover, the dynamic separation factor increases with membrane thickness at the pore width of 0.64 nm, while decreases at the pore width of 1.01 nm due to different separation mechanisms.

A diffusion model for the fluids confined in micropores

The self-diffusion coefficients were calculated by molecular dynamics simulations and the effects of pore width, temperature, and fluid density on diffusion behavior of simple fluid argon and polar fluid water confined in micropores were analyzed and studied. A mathematical model describing diffusion behavior of fluids confined in micropores was proposed from the theories of molecular dynamics and molecular kinematics, and validated on the basis of the simulation results at various conditions. The model indicates that the diffusion coefficient is proportional to the square root of the pore width and to the temperature divided by the density squared. It is applicable to either liquid or gas states and only two parameters are required.

Transport Properties of Fluids in Micropores by Molecular Dynamics Simulation

AbstractThe transport properties of fluid argon in rnicropores. i.e. diffusivity and viscosity. were studied by molecular dynamics simulations. The effects of pore width, temperature and density on diffusivity and viscosity were analyzed in micropores with pore widths from 0.8 to 4.0 run. The results show that the diffusivity in micropores is much lower than the bulk diffusivity, and it decreases as the pore width decreases; but the viscosity in micropores is significantly larger than the bulk one, and it increases sharply in narrow micropores. The diffusivity in channel parallel direction is obviously larger than that in channel perpendicular direction. The temperature and density are important factors that obviously affect diffusivity and viscosity in micropores.

High-temperature treatment effect of microporous carbon on ordered structure of confined SO 2

The X-ray diffraction (XRD) of SO 2 molecules adsorbed in micropores of activated carbon fiber (ACF) was measured at 303 K. Effect of pore width and high-temperature treatment (HTT) of ACF on the XRD of SO 2 in micropores were examined. The HTT gave rise to a more dipole-oriented structure of SO 2 molecules in micropores of 1.1 nm width, whereas the opposite effect was observed in micropores of 0.7 nm. The intensified image potential of an SO 2 molecule for the heat-treated graphitic pore wall should stabilize the dipole-oriented structure in the case of 1.1 nm micropores, while the heat-treatment provides defective structures in the narrow micropore system. The presence of O 2 stabilized the dipole-oriented structure of SO 2 molecules in the micropore.

Effect of Pore Width on Micropore Filling Mechanism of SO2 in Carbon Micropores

The mechanism of SO2 micropore filling in slit-shaped micropores was examined by direct calorimetry and potential calculation, and the effect of the pore width on the SO2 micropore filling mechanism is discussed. Four kinds of activated carbon fibers of different pore widths have been used, and both granular activated carbon and microporous Boehmite aggregates were used for comparison. The differential energy of SO2 adsorption (qd) on ACFs varied with change of the average pore width. An increase of the micropore width leads to a decrease of qd in SO2 micropore filling by decreasing the enhanced micropore field. Molecular potential calculations gave good agreement with the experimental adsorption energy when the image potential due to the SO2 dipole moment was considered. The image potential contributes 25−32% to the total molecular potential, indicating its importance in micropore filling of dipolar SO2.