Multicomponent transport in nanoporous networks: theory and simulation

We present a new theory to estimate fluxes and effective transport conductances of binary mixtures through a membrane comprising a nonuniform porous medium with both pore size and pore length distributions, using the Onsager formulation at the single pore level. The theory defines a conductance of each species that is dependent on the concentration gradients of the various species, and on using effective medium theory determines the fluxes and concentration profiles self-consistently in the porous medium. The transport of CH/H mixtures in a silica membrane having a known pore size distribution is examined using this theory, and the results compared with those from rigorous simulations, showing good agreement. It is found that an optimal network coordination number exists at which species fluxes are a maximum, due to the opposing effects of increasing porosity and mean pore length with increase in coordination number. Further, network fluxes decrease with increase in pore dispersion, indicating that uniform pore size is optimal. A species and pressure-dependent optimal temperature is also predicted, due to the competing effects of increase in diffusivity and decrease in adsorption on increasing temperature. It is seen that the CH selectivity is very sensitive to temperature, and undergoes a cross-over, with the membrane being more selective to CH at low temperature and to H at high temperature. In general, the selectivity is very sensitive to the distribution of pore volume, and for bimodal pore networks, undergoes a sharp transition at the percolation threshold, when the smaller pore size is impermeable to the larger species, CH. The approach offers a convenient adaption of effective medium theory to multicomponent systems with nonlinear isotherms, overcoming drawbacks of existing theory

Atomistic Investigation of Mixed-Gas Separation in a Fluorinated Polyimide Membrane

We have used equilibrium molecular dynamics (EMD) simulations to investigate the temperature dependence of Maxwell–Stefan (MS) diffusivities of a pure component as well as an equimolar mixture of CO2 and CH4 in a fluorinated polyimide polymer membrane. The morphology of the polymer membrane is characterized, and gas adsorption isotherms of the pure as well as an equimolar mixture of CO2 and CH4 are extracted considering the polymer swelling upon gas adsorption, using a combination of EMD in the constant pressure ensemble and grand canonical Monte Carlo simulation. Significant swelling of the polymer in the presence of CO2 is found, as a result of which, the predictions of traditional models, such as ideal adsorption solution theory and dual mode sorption for mixed gases in mixed-gas conditions, are inaccurate, particularly for CH4. Our results show that plasticization behavior of the polymer leads to increase in CO2 permeability with increase in pressure. The Onsager coefficients indicate that, in mixed-gas conditions, finite correlations exist between the diffusing species in the polymer membrane. Further, the swollen membrane is kinetically selective for CH4 at high pressures in mixtures due to availability of large pores, in contrast to pure gas conditions where the membrane is kinetically selective for CO2 over CH4 at all pressures. Analysis of membrane behavior under practical conditions using EMD-based transport coefficients shows that, while the CO2/CH4 perm-selectivity increases with an increase in pressure based on pure component data, the trend is opposite for mixture data. Thus, the commonly used approach of screening membrane materials based on pure component data can be misleading, as it overlooks the correlation effects arising from the presence of other species in the mixture

Modeling mixture transport at the nanoscale: Departure from existing paradigms

We present a novel theory of mixture transport in nanopores, which represents wall effects via a species-specific friction coefficient determined by its low density diffusion coefficient. Onsager coefficients from the theory are in good agreement with those from molecular dynamics simulation, when the nonuniformity of the density distribution is included. It is found that the commonly used assumption of a uniform density in the momentum balance is in serious error, as is also the traditional use of a mixture center of mass based frame of reference

Tractable molecular theory of transport of Lennard-Jones fluids in nanopores

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed. (C) 2004 American Institute of Physics

Influence of in-plane Stone-Thrower-Wales defects and edge functionalisation on the adsorption of CO2 and H2O on graphene

There is now increasing recognition of the potential of graphene membranes for gas separation, with the application to CO2 capture being one of specific interest; however, the co-adsorption of H2O which saturates flue-gas remains a major impediment. Towards enhancing hydrophobic characteristics of graphene while increasing specificity to CO2, we investigate here the adsorption characteristics of CO2 and H2O on four different kinds of graphene sheet - namely, hydrogen-terminated and fluorine-terminated pristine sheets, and the corresponding Stone-Thrower-Wales (STW) defect-incorporated sheets using density functional theory methods. Our results reveal that fluorine termination enhances hydrophobicity and favours the adsorption of CO2, while reducing that of H2O, in comparison to hydrogen termination. On the other hand, H2O adsorption affinity is increased on introducing the Stone-Thrower-Wales defect in both H-terminated and F-terminated sheets, while for CO2 the affinity change is more marginal, evidenced from the change in height of the adsorbed molecule above the surface, and of the adsorption energy. The Henry law constant for H2O is reduced by 54% on F-termination, for both pristine and defective H-terminated graphene sheets, while for CO2 it is increased by 12% and reduced by 18% respectively, on F-termination of the two sheets; indicating the pristine F-terminated sheet as the preferred option. From the density of states analysis, the Fermi level shows a 0.7 eV shift towards the valence band for fluorine termination in both pristine and STW sheets, but is not influenced by CO2 and H2O adsorption. Fluorine termination is shown to have a significant effect on the valence band, and offers a convenient route for tuning the electronic structure of graphene

Comparison of hollow fiber and flat mixed-matrix membranes: theory and simulation

We extend effective medium theory (EMT) to composite hollow fiber mixed matrix membranes, considering the asymmetric filler volume fraction profile arising from finite system size. This volume fraction profile leads to strong variation of the driving force (i.e. pseudo-bulk concentration gradient) in the regions adjacent to the composite ends, and to sensitivity of the effective permeability of the composite to the geometrical configuration. The new theory is validated against rigorous simulations of the transport in mixed-matrix membranes (MMMs) using both concentration-independent and concentration-dependent diffusivities in the MMM constituent phases. Both theory and simulations show that flat mixed-matrix membranes (F-MMMs) have higher effective overall permeability than hollow fiber mixed-matrix membranes (HF-MMMs), upon comparison of systems having identical operating conditions, filler phase loading and particle size. Furthermore, we show here that the sensitivity to the geometry vanishes with increase of inner radius of the hollow fiber membrane at fixed thickness, and the effective permeability of a HF-MMM is found to asymptotically approach that of a F-MMM

Exceptionally high performance of charged carbon nanotube arrays for CO2 separation from flue gas

We use grand canonical Monte Carlo simulation to investigate the adsorption of a CO/N mixture in neutral and charged (7, 7) carbon nanotube (CNT) arrays. It is found that both the adsorption of CO, and the CO/N selectivity are either enhanced or reduced when the charges are positive or negative. The CO/N selectivity in a CNT bundle carrying +0.05e charge with intertube distance of 0.335 nm exceeds 1000 for pressures up to 15 bar, which is remarkably high. It is seen that strong electrostatic interactions from neighbouring CNTs enhance the adsorption of CO over N, and while the adsorption of CO has complex dependence on intertube distance, the CO/N selectivity decreases with intertube spacing. We propose a quantitative performance coefficient as an aid to assessing the efficiency of CNT bundles to separate CO from flue gas, and show that a +0.05e charged bundle with intertube distance of 0.335 nm provides the best performance. Further, it is found that water vapor in flue gas imposes negligible effect on the adsorption of CO and its selectivity over N in the neutral and positively charged (7, 7) CNT bundles, but dramatically reduces the adsorption of CO and N in negatively charged bundles

Edge functionalised & Li-intercalated 555-777 defective bilayer graphene for the adsorption of CO2 and H2O

The adsorption of CO and HO on divacanacy (DV) defected graphene cluster, and its bilayer counterpart is investigated using first-principles calculations. Both single and bilayer DV graphene cluster, are functionalised with H and F atoms. On these sheets the gas molecules are physisorbed, and the divacancy defect effectively improves the adsorption of CO, while fluorination enhances the hydrophobicity of the graphene cluster. Among the convex and concave curvature regions induced due to the DV defect, the adsorption of the gas molecules on the concave meniscus is more favourable. Fluorine termination induces 73% reduction in Henry law constants for HO, while for the CO molecule it increases by 8%, which indicates the DV defective sheet is a better candidate for CO capture compared to the STW defective sheet. Besides, both AA and AB divacant defect bilayer sheets are equally stable, wherein AA stacking results in a cavity between the sheets, while in AB stacking, the layers slide one over the other. Nevertheless, both these bilayer sheets are comparatively stabler than the monolayer. However, intercalation of lithium decreases the interlayer separation, particularly in AA stacking, which enhances the CO adsorption, but in the Bernal stacking enhances it hydrophobicity

Extending effective medium theory to finite size systems: theory and simulation for permeation in mixed-matrix membranes

We present a novel theory for estimation of the effective permeability of pure gases in flat mixed-matrix membranes (MMMs), in which effective medium theory (EMT) is extended to systems with finite filler size and membrane thickness. We introduce an inhomogeneous filler volume fraction profile, which arises due to depletion of the filler in regions adjacent to the membrane ends, into the MMM permeation model. In this way, the effective medium approach (EMA) can still be applied to systems where the dispersant size is not small in comparison to the membrane thickness, and for which a permeability profiles arises in the MMM that is dependent on both filler size and membrane thickness, besides the filler-polymer equilibrium constant. It is found that increase in particle size reduces the effective membrane permeability at fixed membrane thickness, and that the effective membrane permeability increases with increase of the membrane thickness to asymptotically reach the value predicted by existing models. The present theory is validated against detailed simulations of the transport in MMMs, and theoretical predictions are found to be in agreement with those obtained from the exact calculations. Further, comparison of the exact effective permeability at different filler volume fractions is made for different packing configurations, showing variations in dispersant packing structure to have only a very weak effect on MMM performance

Adsorption of CH4 and CH4/CO2 mixtures in carbon nanotubes and disordered carbons: a molecular simulation study

We report a comparison of the adsorption of CH4 and CO2/CH4 mixtures of different composition in three different types of nanoporous carbons including carbon nanotubes, and activated carbon fibre (ACF-15) and silicon carbide derived carbon (SiC-DC) having distinctly different disordered structures, using Monte Carlo simulation. CO2 is represented as a linear molecule, and both the united-atom and full-atom models are investigated for CH4. It is found that the united-atom model of CH4 overestimates the adsorption of CH4 in all these adsorbents compared to the 5-site model, as a consequence of the enhanced 1-site CH4-adsorbent potential energy. Moreover, the selectivities of the nanoporous carbons for CO2 relative to CH4 calculated using the 1-site CH4 model are underestimated compared to those from the 5-site model, at pressures up to 3.0 MPa. However, differences in the structural disorder of porous carbon models have little impact on CO2 selectivity. Our simulations reveal that the selectivity of an adsorbent for a particular species is strongly dependant on adsorbate-adsorbate interaction effects, comprising the adsorbate-adsorbate potential interactions and an adsorbate sieving effect. As a balance between the confinement and adsorbate-adsorbate effects, it is found that increasing the concentration of CO2 in the gas phase increases the selectivity of (10, 10) CNT dramatically, while having negligible impact on the selectivities in amorphous carbons. Further, it is shown that increasing the temperature reduces the performance of all the carbons in separating CO2, and that an isolated (7,7) CNT has the best performance for CO2/CH4 separation in comparison to the disordered nanoporous carbons investigated. (C) 2014 Elsevier Ltd. All rights reserved