Thermodynamically Consistent Methodology for Estimation of Diffusivities of Mixtures of Guest Molecules in Microporous Materials

Krishna, Rajamani

doi:10.1021/acsomega.9b01873

Cited by 13 publications

(6 citation statements)

References 73 publications

(188 reference statements)

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“…Molecular dynamics simulation data for mixture diffusion have shown that cluster formation tends to reduce the diffusivities of both water and alcohol molecules; ,,,,− this mutual slowing-down phenomena is a corollary to the moderation of entropy effects in mixture adsorption.…”

Section: Resultsmentioning

confidence: 99%

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Krishna

Baten

2020

ACS Omega

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Microporous crystalline porous materials such as zeolites, metal−organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water−alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.

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Section: Resultsmentioning

confidence: 99%

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Krishna

Baten

2020

ACS Omega

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“…Published MD data for mixture diffusion have shown that the occurrence of molecular clustering, because of say hydrogen bonding, causes the component diffusivities in mixtures to deviate significantly from the values for the corresponding unaries. ,,,− …”

Section: Results and Discussionmentioning

confidence: 99%

Using Molecular Simulations to Unravel the Benefits of Characterizing Mixture Permeation in Microporous Membranes in Terms of the Spreading Pressure

Krishna

Baten

2020

ACS Omega

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The separation performance of microporous crystalline materials in membrane constructs is dictated by a combination of mixture adsorption and intracrystalline diffusion characteristics; the permeation selectivity S perm is a product of the adsorption selectivity S ads and the diffusion selectivity, S diff . The primary objective of this article is to gain fundamental insights into S ads and S diff by use of molecular simulations. We performed configurational-bias Monte Carlo (CBMC) simulations of mixture adsorption equilibrium and molecular dynamics (MD) simulations of guest self-diffusivities of a number of binary mixtures of light gaseous molecules (CO 2 , CH 4 , N 2 , H 2 , and C 2 H 6 ) in a variety of microporous hosts of different pore dimensions and topologies. Irrespective of the bulk gas compositions and bulk gas fugacities, the adsorption selectivity, S ads , is found to be uniquely determined by the adsorption potential, Φ, a convenient and practical proxy for the spreading pressure π that is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The adsorption potential Φ is also a proxy for the pore occupancy and is the thermodynamically appropriate yardstick to determine the loading and composition dependences of intracrystalline diffusivities and diffusion selectivities, S diff . When compared at the same Φ, the component permeabilities, Π i for CO 2 , CH 4 , and N 2 , determinable from CBMC/MD data, are found to be independent of the partners in the various mixtures investigated and have practically the same values as the values for the corresponding unary permeabilities. In all investigated systems, the H 2 permeability in a mixture is significantly lower than the corresponding unary value. These reported results have important practical consequences in process development and are also useful for screening of materials for use as membrane devices.

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“…The M-S diffusivity Đ i equals the corresponding diffusivity for a unary system, determined at the same pore occupancy; the diffusivity for any species i in a mixture remains invariant to the choice of the partner species. ,− Other variables in eqs and , are described in the Nomenclature section.…”

Section: Transient Mixture Uptakementioning

confidence: 99%

Fundamental Insights into Intracrystalline Diffusional Influences on Mixture Separations in Fixed Bed Adsorbers

Krishna

2024

Chem Bio Eng.

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This article has the objective of elucidating the variety of factors that quantify influences of intracrystalline diffusion on mixture separations in fixed bed devices packed with microporous crystalline adsorbents such as metal-organic frameworks (MOFs) and zeolites. Such diffusional influences may act either synergistically or anti-synergistically to the mixture adsorption equilibrium, providing the ratio of the diffusivities Đ 1 /Đ 2 ≫1. Experimental data on transient mixture uptake inside single crystals display overshoots in the loadings of the more mobile guest species; this overshoot can be quantitatively captured by use of the Maxwell−Stefan (M-S) diffusion formulation that takes proper account of thermodynamic coupling influences; if such thermodynamic influences are ignored, as is done in the Linear Driving Force (LDF) model, overshoots are not realizable. The use of the M-S formulation to simulate transient breakthroughs in fixed bed adsorbers provides a quantitative match with experiments; the match is significantly poorer if thermodynamic coupling effects are ignored. For a fixed bed of length L, packed with adsorbent particles of radius r c and operating with an interstitial gas velocity, v, the diffusional influences are quantified by two separate parameters: (a) diffusional time constant, Đ 1 /r c 2 , and gas−particle contact time L/v. The transient breakthroughs are uniquely dependent on the product (Đ 1 /r c 2 )(L/v); this result is of practical importance for scaling up from small scale laboratory units to large scale industrial units that use different particle sizes, bed dimensions, and gas velocities.

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Thermodynamically Consistent Methodology for Estimation of Diffusivities of Mixtures of Guest Molecules in Microporous Materials

Cited by 13 publications

References 73 publications

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Using Molecular Simulations to Unravel the Benefits of Characterizing Mixture Permeation in Microporous Membranes in Terms of the Spreading Pressure

Fundamental Insights into Intracrystalline Diffusional Influences on Mixture Separations in Fixed Bed Adsorbers

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