(214g) Confinement and Adsorption of Fluids in Nanoporous Materials From Molecular Dynamics Simulations: Influence On Membrane Separation Performance
AIChE Annual Meeting
2013
2013 AIChE Annual Meeting
Computational Molecular Science and Engineering Forum
Poster Session: Computational Molecular Science and Engineering Forum (CoMSEF)
Monday, November 4, 2013 - 6:00pm to 8:00pm
Separation performance is expressed in terms of either mass throughput or selectivity, and it is the trade-off between these two paradigms that poses the primary challenge for membrane development. While selectivity is crucially determined by the energetic and geometric characteristics of the porous materials, mass throughput can be limited by the mass transfer rate through the selective barrier, the dynamics of the adsorbed layer, entrance effects and mainly by the transport dynamics of the confined fluid inside the pore [1].
The focus of this research lies on the influence of the latter aspects on membrane separations, namely on the effect that nanoscale confinement has on fluid transport and on separation behaviour of ultrathin-film membranes, where adsorption dynamics and entrance effects are prominent [2]. Under nanoscale confinement, fluids exhibit a highly complex behaviour, markedly distinct from their bulk conterparts. The distinction between diffusive and convective mass transport as well as a competition between the phenomenological description and a fully molecular understanding of mass transport [3] give rise to several unanswered questions with respect to fluid transport in porous media. Transport processes in porous materials are influenced by structural and thermodynamic effects and the effects are often superimposed which makes a clear distinction very difficult [4, 5].
It is common to characterise transport properties of confined fluids via the self-diffusion coefficient based on the Einstein diffusion model. The validity of this approach in the context of nano-confined fluids is very limited and it fails to capture important factors [6]. We showcase these limits for a popular reference case, a Lennard-Jones fluid confined in structured slit pores. The influence of the repulsive and attractive fluid-pore interactions, pore loading as well as steric confinement effects on transport is presented. Self-diffusion and collective diffusion coefficients from Equilibrium Molecular Dynamics (EMD), based on the Green-Kubo relations [7], and the thermodynamically related Fickian diffusivities, were calculated.
Figure 1: For the NEMD simulations, a LJ fluid in contact with a slit pore was perturbed by an
external force at the boundary of the simulation box, inducing a density gradient and fluid flow. The steady state flux (in reduced units) as a function of bulk fluid pressure and magnitude of the external field is plotted for a narrow slit pore on the left (H=2.5) and a wide pore on the right (H=5.0) in the depiction above. Simulations were performed at a temperature of T=1.5.
We propose a boundary-driven Non-Equilibrium Molecular Dynamics (NEMD) algorithm in an attempt to calculate corresponding effective diffusion coefficients [8]. The non-equilibrium nature of the methodology allows us to study diffusion under steady state conditions and to be able to discriminate between the transport coefficients inherent of the porous matrices and the entrance and exit effects.
In an attempt to paint a holistic picture of single-component diffusion under confinement, we draw a direct comparison between transport diffusion coefficients calculated from EMD and effective diffusivities from NEMD for a range of pore loading and sizes, as well as varying fluid-pore interactions (c.f. Fig. 1). These results are put in contrast to theoretical models of mass transport in porous materials such as the Knudsen diffusion approach and predictions from hydrodynamics on the molecular level [9], where limitations of these theories become apparent. It stems from our results that a description of transport through porous membranes without due consideration of the either enhanced or restricted transport at the solid- bulk fluid interfaces leads to significant deviations from the expected overall transport.
With considerable effort going into fabricating membranes of an atomistic thickness [10], the limits on maximising mass throughput and selectivity simultaneously are furthermore explored. The selective behaviour of these ultra-thin membrane layers is seen to be dominated by the dynamics of the adsorbed layers which can , however, have significantly negative influence on the separation performance.
References
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