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

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 Đ1/Đ2 ≫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 rc and operating with an interstitial gas velocity, v, the diffusional influences are quantified by two separate parameters: (a) diffusional time constant, Đ1/rc2, and gas–particle contact time L/v. The transient breakthroughs are uniquely dependent on the product (Đ1/rc2)­(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

Investigating the Validity of the Knudsen Diffusivity Prescription for Mesoporous and Macroporous Materials

The primary objective of this article is to investigate the validity of the Knudsen prescription for pore diffusivity. Published experimental data on transient permeation of He–Ar, He–N₂, He–CO₂, He–C₃H₈, and CO₂–C₃H₈ mixtures across mesoporous and macroporous membranes are analyzed using the Maxwell-Stefan (M-S) formulation, combining molecule–wall and molecule–molecule interactions. For He–Ar and He–N₂ mixtures, both components are poorly adsorbed within the pores, and the experimental permeation data can be modeled adequately taking M-S diffusivity for molecule–wall interactions, <i><i>Đ</i></i>_<i>i</i> = <i>D</i>_{<i>i</i>,<i>Kn</i>}, the corresponding Knudsen diffusivity. For He–CO₂ and He–C₃H₈ mixture permeation, the equality <i><i>Đ</i></i>_<i>i</i> = <i>D</i>_{<i>i</i>,<i>Kn</i>} holds only for He. For either CO₂ or C₃H₈, <i><i>Đ</i></i>_<i>i</i> is lower than <i>D</i>_{<i>i</i>,<i>Kn</i>} by a factor ranging from 0.55 to 0.98, depending on the species and operating temperature. The stronger the adsorption strength, the lower the ratio <i><i>Đ</i></i>_<i>i</i>/<i>D</i>_{<i>i</i>,<i>Kn</i>}. The observed lowering in the M-S diffusivity below the Knudsen value, <i>D</i>_{<i>i</i>,<i>Kn</i>}, is in line with the published Molecular Dynamics (MD) data for cylindrical mesopores. The Knudsen prescription is based on the requirement that a molecule experiences diffuse reflection on collision with the pore wall, i.e., the angle of reflection bears no relation to the angle of incidence. Adsorption at the pore wall introduces a bias that makes a molecule hop to a neighboring site on the surface rather than return to the bulk; this bias increases with increasing adsorption strength and has the effect of reducing the pore diffusivity

Highlighting Diffusional Coupling Effects in Ternary Liquid Extraction and Comparisons with Distillation

Liquid extraction processes involve the separation of mixtures containing three or more species whose compositions are close to the binodal curve; this proximity causes the diffusion equilibration process to be strongly influenced by phase equilibrium thermodynamics. Due to thermodynamic factors, the interphase transfer flux of any component is influenced by the driving force of all the constituent species in the mixture, i.e. the diffusion process is strongly coupled. The transient diffusion equilibration process within spherical droplets dispersed within a continuous liquid phase is quantified by the classic Geddes model, used in combination with the Maxwell–Stefan diffusion formulation. For 13 different partially miscible ternary liquid mixtures, the equilibration trajectories in composition space are found to be curvilinear in shape. In all cases, the component Murphree efficiencies, <i>E</i>_<i>i</i>, are unequal to one another. The separations achieved are significantly different from those predicted by a simpler model that ignores coupling effects. In ternary distillation, the existence of azeotropes creates boundaries in composition space, whose crossings are disallowed in equilibrium-stage calculations. The application of the Geddes model for transient diffusion inside vapor bubbles yields curvilinear trajectories that demonstrate the possibility of boundary crossing; such crossings are in conformity with published experimental data

Investigating the Relative Influences of Molecular Dimensions and Binding Energies on Diffusivities of Guest Species Inside Nanoporous Crystalline Materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and metal–organic frameworks (MOFs). The investigations are based on molecular dynamics (MD) simulations of unary diffusivities, along with configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−<i>Q</i>_st) of a wide variety of guest molecules (CO₂, H₂, N₂, He, Ne, Ar, Kr, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and <i>n</i>C₄H₁₀) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2–4.2 Å size range, separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8), the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g., AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and “open” frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by −<i>Q</i>_st. The stronger the binding energy, the higher the “sticking tendency”, and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application

Investigating the Relative Influences of Molecular Dimensions and Binding Energies on Diffusivities of Guest Species Inside Nanoporous Crystalline Materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and metal–organic frameworks (MOFs). The investigations are based on molecular dynamics (MD) simulations of unary diffusivities, along with configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−<i>Q</i>_st) of a wide variety of guest molecules (CO₂, H₂, N₂, He, Ne, Ar, Kr, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and <i>n</i>C₄H₁₀) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2–4.2 Å size range, separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8), the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g., AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and “open” frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by −<i>Q</i>_st. The stronger the binding energy, the higher the “sticking tendency”, and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application

Investigating the Relative Influences of Molecular Dimensions and Binding Energies on Diffusivities of Guest Species Inside Nanoporous Crystalline Materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and metal–organic frameworks (MOFs). The investigations are based on molecular dynamics (MD) simulations of unary diffusivities, along with configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−<i>Q</i>_st) of a wide variety of guest molecules (CO₂, H₂, N₂, He, Ne, Ar, Kr, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and <i>n</i>C₄H₁₀) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2–4.2 Å size range, separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8), the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g., AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and “open” frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by −<i>Q</i>_st. The stronger the binding energy, the higher the “sticking tendency”, and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application

Investigating the Relative Influences of Molecular Dimensions and Binding Energies on Diffusivities of Guest Species Inside Nanoporous Crystalline Materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and metal–organic frameworks (MOFs). The investigations are based on molecular dynamics (MD) simulations of unary diffusivities, along with configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−<i>Q</i>_st) of a wide variety of guest molecules (CO₂, H₂, N₂, He, Ne, Ar, Kr, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and <i>n</i>C₄H₁₀) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2–4.2 Å size range, separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8), the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g., AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and “open” frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by −<i>Q</i>_st. The stronger the binding energy, the higher the “sticking tendency”, and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application

Natural Gas Purification Using a Porous Coordination Polymer with Water and Chemical Stability

Porous coordination polymers (PCPs), constructed by bridging the metals or clusters and organic linkers, can provide a functional pore environment for gas storage and separation. But the rational design for identifying PCPs with high efficiency and low energy cost remains a challenge. Here, we demonstrate a new PCP, [(Cu₄Cl)­(BTBA)₈·(CH₃)₂NH₂)·(H₂O)₁₂]·<i>x</i>Guest (PCP-33⊃guest), which shows high potential for purification of natural gas, separation of C₂H₂/CO₂ mixtures, and selective removal of C₂H₂ from C₂H₂/C₂H₄ mixtures at ambient temperature. The lower binding energy of the framework toward these light hydrocarbons indicates the reduced net costs for material regeneration, and meanwhile, the good water and chemical stability of it, in particular at pH = 2 and 60 °C, shows high potential usage under some harsh conditions. In addition, the adsorption process and effective site for separation was unravelled by <i>in situ</i> infrared spectroscopy studies

Использование социального сервиса подкастов в обучении иноязычному говорению

The selective removal of water from mixtures with methanol, ethanol, and 1-propanol is an important task in the processing industries. With the aid of configurational-bias Monte Carlo simulations of unary and mixture adsorption, we establish the potential of CuBTC for this separation task. For operations close to pore saturation conditions, the adsorption is selective to water that has a significantly higher saturation capacity compared to that of 1-alcohols. The water-selective separation relies on subtle entropy effects that manifest near pore saturation conditions. A further distinguishing feature is that mixture adsorption is determined to be strongly nonideal, and the activity coefficients of the constituent components deviate strongly from unity as pore saturation is approached. The predictions of the ideal adsorbed solution theory (IAST), though qualitatively correct, do not predict the component loadings for mixture adsorption with adequate accuracy. Consequently, the activity coefficients, after appropriate parametrization, have been incorporated into the real adsorbed solution theory (RAST). Transient breakthrough simulations, using the RAST model as a basis, demonstrate the capability of CuBTC for selective adsorption of water in fixed-bed adsorption devices operating under ambient conditions

Hydroquinone and Quinone-Grafted Porous Carbons for Highly Selective CO₂ Capture from Flue Gases and Natural Gas Upgrading

Hydroquinone and quinone functional groups were grafted onto a hierarchical porous carbon framework via the Friedel–Crafts reaction to develop more efficient adsorbents for the selective capture and removal of carbon dioxide from flue gases and natural gas. The oxygen-doped porous carbons were characterized with scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. CO₂, CH₄, and N₂ adsorption isotherms were measured and correlated with the Langmuir model. An ideal adsorbed solution theory (IAST) selectivity for the CO₂/N₂ separation of 26.5 (298 K, 1 atm) was obtained on the hydroquinone-grafted carbon, which is 58.7% higher than that of the pristine porous carbon, and a CO₂/CH₄ selectivity value of 4.6 (298 K, 1 atm) was obtained on the quinone-grafted carbon (OAC-2), which represents a 28.4% improvement over the pristine porous carbon. The highest CO₂ adsorption capacity on the oxygen-doped carbon adsorbents is 3.46 mmol g^–1 at 298 K and 1 atm. In addition, transient breakthrough simulations for CO₂/CH₄/N₂ mixture separation were conducted to demonstrate the good separation performance of the oxygen-doped carbons in fixed bed adsorbers. Combining excellent adsorption separation properties and low heats of adsorption, the oxygen-doped carbons developed in this work appear to be very promising for flue gas treatment and natural gas upgrading