Effect of surface resistances on the diffusion of binary mixtures in the silicalite single crystal membrane

Diffusion of methane and argon mixtures through the silicalite single-crystal membrane is studied using the dual-control volume-grand canonical molecular dynamics method to understand how surface resistances alter selectivity and permeance. Comparison of results from intracrystalline transport and entrance simulations for binary mixtures of CH4 and Ar shows that the selectivity of silicalite membranes toward Ar is enhanced in the presence of the surface resistances. In both cases, however, diffusion of faster Ar molecules was inhibited by slower diffusing CH4 molecules, whereas diffusion of the latter remained unaffected. This behavior was explained by the difference between the magnitudes of surface resistances for two molecules, which is much smaller for Ar because of its smaller permeant-crystal interaction size. We find that selectivity of the membrane at the surface depends strongly on total feed pressure and temperature, whereas this dependence is weak for intracrystalline diffusion. Furthermore, we show that the selectivity at the surface diminishes with crystal thickness until a certain thickness is reached, whereas the intracrystalline selectivity remains constant with increasing thickness. Finally, a study of diffusion of C2H6 and CF4 mixtures shows that the diatomic ethane molecules diffuse faster inside the zeolite channels, but their desorption is hindered to a larger extent than that of a spherical molecule with larger diameter and lower heat of adsorption. This observation indicates that the difference in molecular geometry is also a significant factor to explain the exit effect.     

Diffusion of aromatic hydrocarbons in silicalite/HZSM-5

Diffusion of benzene and the xylene isomers in silicalite/HZSM-5 has been studied by a wide range of different experimental techniques. The available data are reviewed in an attempt to draw general conclusions concerning the intracrystalline diffusion process. The results for benzene are remarkably consistent, and the conformity between transport and tracer diffusion and between single crystal membrane and ZLC and frequency response data suggests that diffusion is essentially isotropic with no significant difference between the self and “corrected” transport diffusivity. The situation is more complex for p-xylene which shows clear evidence of non-isotropic behavior and a significant difference between tracer and transport diffusivities.     

Transport Properties of Ethane, Butanes and Their Binary Mixtures in MFI-Type Zeolite and Zeolite-Membrane Samples

Transport properties of ethane, butane and their binary mixtures in large crystals of silicalite-1, ZSM-5, and an MFI-zeolite membrane as well as agglomerates (pellets) of silicalite zeolites have been investigated by the Zero Length Column (ZLC) method. It was found that in the large crystals of silicalite-1 and ZSM-5, and in the membrane sample desorption of iso-butane was controlled by micropore diffusion, while in the case of pelleted silicalite sample it was controlled by macropore diffusion. The effective thickness of the zeolite membrane can be reasonably evaluated by comparing the diffusivity data obtained from the ZLC and gas permeation measurements. Desorption of ethane and n-butane in the large crystals of silicalite-1 and ZSM-5 and the membrane sample is attributed to both equilibrium effects and micropore diffusion. The diffusivity of ethane is significantly reduced in the presence of iso-butane giving rise to a micropore diffusion-controlled process. Furthermore, diffusion of iso-butane in the zeolite samples is affected by the counter flow of ethane.     

NaA型分子筛膜的合成及分离性能的研究

在自制的片状多孔陶瓷载体上,通过多次原位水热晶化合成出ＮａＡ型分子筛膜,通过扫描电子显微镜观测,发现在某些区域,小颗粒的ＮａＡ型分子筛以非常紧密的形式畸晶孪生在一起,其致密度远好于由分子筛晶粒松散无规律堆积而形成的膜排列经类膜生长形式可能是获得取致密无缺陷型分子筛膜的一种途径,单组分及双组分气体渗透测试结果表明,在所合成的分子筛膜上,晶粒间隙孔可能是主要的膜扩散通道,可凝聚气体异丁烷因发生毛细管凝     

Measurement of molecular transport in poly(vinyl chloride) by the step response technique

Sorption and diffusion of methane and 1-butene in poly(vinyl chloride) (PVC) powder have been investigated by the step-response method using gas chromatography. The sorption equilibrium constant of 1-butene was lower than that of chlorine, despite the higher critical temperature of 1-butene, while the sorption of methane was negligibly small at temperatures much higher than critical. The diffusivity of 1-butene in PVC showed a discontinuous dependence on temperature at the glass transition point, suggesting that the micro-Brownian motion of the polymer chain enhances the diffusivity at the higher temperature. Below the glass transition temperature, the diffusivity of 1-butene in PVC showed a lower value than has been measured for other smaller molecules such as chlorine and hydrogen chloride.     

New differential permeation rate method for determination of membrane transport parameters of gases

A new method for determining the membrane transport parameters (diffusivity, permeability and solubility) of gases through nonporous polymeric membranes is described. The method employs a continuous permeation chamber containing a flat membrane. The most important feature of this method is that, instead of a step concentration change, a rectangular pulse or impulse is sent to the upstream side of the membrane. Consequently, no steady state is approached but a signal peak of typical form can be recorded. The permeability and the diffusivity can be estimated from the height and half-width of the peak, respectively. The method was applied to measure the permeability of hydrocarbons through a polyethylene membrane, the permeation rate being measured by a flame ionization detector. The method and the derived relations are valid for other detectors and gas—membrane combinations as well. The advantages of this novel method are that all the membrane transport parameters can be directly evaluated from data of the response peak, whilst approaching the steady state is not necessary and thus the measuring time can be shortened. Finally, the known and new differential permeation rate methods are compared by generalization of the relationship between the input and output (response) functions.     

4，4′－联苯二甲酸－酚酞／四溴酚酞共聚酯的气体透过性能

合成了一系列单体比例不同的４，４′－联苯二甲酸－酚酞／四溴酚酞共聚酯，并制成柔软透明的均质薄膜。测定了膜对氢气、氧气、氮气、二氧化碳、甲烷的气体透过性能，重点研究单体比例不同的共聚物中分子主链溴含量对气体透过性能的影响，发现随着四溴酚酞单体含量的增加，气体在聚合物中的溶解系数增大；扩散系数下降；气体透过系数在一定范围内存在较大值；分离系数的变化则不明显。结果表明在聚芳酯的芳环上引入适当含量的溴，可以提高聚合物的气体透过系数，而对分离系数的值影响不大。     

Diffusion of methane and carbon dioxide in carbon molecular sieve membranes by multinuclear pulsed field gradient NMR

Carbon molecular sieve (CMS) membranes are promising materials for energy efficient separations of light gases. In this work, we report a detailed microscopic study of carbon dioxide and methane self-diffusion in three CMS membrane derived from 6FDA/BPDA(1:1)-DAM and Matrimid polymers. In addition to diffusion of one-component sorbates, diffusion of a carbon dioxide/methane mixture was investigated. Self-diffusion studies were performed by the multinuclear (i.e., (1)H and (13)C) pulsed field gradient (PFG) NMR technique which combines the advantages of high field (17.6 T) NMR and high magnetic field gradients (up to 30 T/m). Diffusion measurements were carried out at different temperatures and for a broad range of the root-mean-square displacements of gas molecules inside the membranes. The diffusion data obtained from PFG NMR are compared with the corresponding results of membrane permeation measurements reported previously for the same membrane types. The observed differences between the transport diffusivities and self-diffusion coefficients of carbon dioxide and methane are discussed.     

Molecular dynamics simulation of iso- and n-butane permeations through a ZSM-5 type silicalite membrane

Molecular dynamics simulation of the permeation processes of single and mixed gases of iso- and n-butane through a ZSM-5 type silicalite membrane are presented. After 200 ps of simulation time the permeation of n-butane is observed whereas the permeation of iso-butane is not observed. The permeation of n-butane at 373 K takes place after the saturation of the zeolite pores, whereas at higher temperature, 773 K, it occurs without significant pores saturation. The calculated permeability of n-butane is close to experimental data. The permeation of the gas mixture shows that the membrane can separate the two isomers, n-butane permeates whereas iso-butane does not.     

A study of pore blockage in silicalite zeolite using free energy perturbation calculations

Binary systems consisting of large coadsorbed molecules (n-hexane, cyclohexane, and benzene) with smaller penetrant molecules (methane) were simulated to investigate the mechanisms of pore blockage in the zeolite silicalite. Benzene and cyclohexane trap the methane molecules in the zeolite channels on the time scales of molecular dynamics simulations. Minimum energy paths for methane diffusion past the blocking molecules were determined, and free energy perturbation calculations were carried out along the paths to get the rate constants of methane hopping past coadsorbed benzene and cyclohexane molecules, which adsorb in the channel intersections. Three principal diffusion pathways were found in both the methane/benzene and methane/cyclohexane systems. Minima which were connected by low-energy pathways were grouped together into macrostates. Using the calculated hopping rates between macrostates, kinetic Monte Carlo was then used to obtain the diffusivity of methane with a coadsorbate benzene loading such that all channel intersections are filled by benzene - conditions where molecular dynamics simulations fail. Passage of methane across cyclohexane molecules involved pushing the cyclohexane molecules into the channels from their preferred channel intersection positions.     

Carrier gas flow arrangement based photoacoustic detection method for measuring gas permeability of polymer membranes

This paper describes a membrane permeability measuring set-up based on photoacoustic spectroscopy using continuous carrier gas flow to transport the permeated analyte molecules into a photoacoustic detection cell. The permeability parameters of the sample were determined from the measured permeation curves by using a numerical curve fitting algorithm. The method was applied to different membrane samples for determining methane and carbon-dioxide permeability at various carrier gas flow rates (CFRs). For each sample, a characteristic threshold flow rate (TFR) value can be identified below which a strong dependency of the determined permeation parameters on the CFR was found. For those cases when the CFR value cannot be set to be sufficiently high (i.e. above the TFR value), an extrapolation method was presented giving an accurate estimation of the permeation parameters.     

Xe chemical shift tensor in silicalite and SSZ-24

We report, for the first time, a theoretical prediction of the (129)Xe nuclear magnetic resonance chemical shift tensor of xenon atom in a single crystal of silicalite at near-zero occupancy and the temperature dependence of the Xe NMR chemical shift tensor for the polycrystalline silicalite at maximum occupancy. The former is a measure of the sensitivity of the Xe tensor components to the local structure of the channels without Xe-Xe contributions. The latter is a measure of the sensitivity of the Xe-Xe tensor components to the Xe-Xe distributions, as determined by the Xe-Xe potential function in competition with the Xe-silicalite potential function. Both theoretical predictions can be compared against Xe NMR experiments: the first against the Xe spectra collected as a function of rotation of the single crystal about the three crystalline axes in a magnetic field, and the second against variable temperature Xe NMR studies (below room temperature) of polycrystalline silicalite at maximum Xe occupancy. With the same parameter set (Xe-O potential and shielding functions), we predict the line shapes of Xe in SSZ-24 zeolite under various conditions of occupancy and temperature.     

Membrane reactor performance of steam reforming of methane using hydrogen-permselective catalytic SiO2 membranes

Hydrogen production by steam reforming of methane using catalytic membrane reactors was investigated first by simulation, then by experimentation. The membrane reactor simulation, using an isothermal and plug-flow model with selective permeation from reactant stream to permeate stream, was conducted to evaluate the effect of permselectivity on membrane reactor performance – such as methane conversion and hydrogen yield – at pressures as high as 1000 kPa. The simulation study, with a target for methane conversion of 0.8, showed that hydrogen yield and production rate have approximately the same dependency on operating conditions, such as reaction pressure, if the permeance ratio of hydrogen over nitrogen ((H₂/N₂)) is larger than 100 and of H₂ over H₂O is larger than 15. Catalytic membrane reactors, consisting of a microporous Ni-doped SiO₂ top layer and a catalytic support, were prepared and applied experimentally for steam reforming of methane at 500 °C. A bimodal catalytic support, which allows large diffusivity and high dispersion of the metal catalyst, was prepared for the enhancement of membrane catalytic activity. Catalytic membranes having H₂ permeances in the range of 2–5 × 10⁻⁶ m³ m⁻² s⁻¹ kPa⁻¹, with H₂/N₂ of 25–500 and H₂/H₂O of 6–15, were examined for steam reforming of methane. Increased performance for the production of hydrogen was experimentally obtained with an increase in reaction-side pressure (as high as 500 kPa), which agreed with the theoretical simulation with no fitting parameters.     

Diffusion NMR methods applied to xenon gas for materials study

We report initial NMR studies of (i) xenon gas diffusion in model heterogeneous porous media and (ii) continuous flow laser-polarized xenon gas. Both areas utilize the pulsed gradient spin-echo (PGSE) techniques in the gas phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients--a brief overview of this area is provided in the Introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t) (an indicator of mean squared displacement), to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. We find that D(t) of imbibed xenon gas at short diffusion times is similar for the mixed bead pack and a pack of the smaller sized beads alone, hence reflecting the pore surface area to volume ratio of the smaller bead sample. The approach of D(t) to the long-time limit follows that of a pack of the larger sized beads alone, although the limiting D(t) for the mixed bead pack is lower, reflecting the lower porosity of the sample compared to that of a pack of mono-sized glass beads. The Pade approximation is used to interpolate D(t) data between the short- and long-time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20-200 mm s-1). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm s-1 owing to the high diffusivity of gases compared with liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data, namely flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.     

Lysozyme transport in p-HEMA hydrogel contact lenses

Protein binding in hydrogels adversely affects their performance and can interfere with their usage in several biomedical applications including contact lenses. In this study we focus on understanding and modeling the mechanisms of protein transport in hydrogels, specifically focusing on the effect of protein concentration and gel crosslinking on transport. Specifically, we focus on lysozyme, the most abundant protein in tear fluid, and hydrogels of poly-hydroxyethyl methacrylate (p-HEMA), a common contact lens material. Protein uptake experiments with gels of different thicknesses showed a time scale increase as the square of the thickness suggesting diffusion controlled transport. Partition coefficient was found to be dependent on the equilibrium concentration of lysozyme, and also on the degree of crosslinking. Since transport is related to mesh size, gel modulus was obtained for various crosslinkings and utilized to estimate the mesh size. The transport data were fitted to a diffusion model and the fitted diffusivity was compared to diffusivity predicted from a model based on hydrogel mesh size. Both protein absorption and desorption data fitted the diffusion model with the same value of diffusivity, but the experimentally measured diffusivities were significantly smaller than those estimated on the basis of the gel mesh size. Models were modified to take into account protein binding to the polymer but the modified predictions were still larger than the measured values. The results of this study could assist in the development of contact lens materials that exhibit minimal protein binding, in designing cleaning regimens for protein removal from contact lenses, and in applications related to protein binding in several other biomaterials.     

Hydrogen transport through Pd-Ni alloy electrodeposited on Pd substrate

Hydrogen transport through a Pd-Ni alloy electrodeposited on a Pd substrate (Pd-Ni/Pd bilayer symmetric electrode) has been investigated using cyclic voltammetry and a.c. impedance spectroscopy combined with the electrochemical hydrogen permeation method. The permeation build-up current transients and the measured impedance spectra were analyzed using the time-lag method for the bilayer electrode and a complex non-linear least squares data-fitting method based upon the derived Faradaic admittance for the hydrogen absorption into and diffusion through the bilayer electrode under the permeable boundary condition, respectively. The value of the hydrogen diffusivity in the Pd-Ni layer was lower than that in the Pd layer. Furthermore, the values of the charge transfer resistance and equilibrium absorption constant for the Pd-Ni/Pd bilayer electrode were higher than those for the Pd single layer electrode. From the experimental results, the role of the thin Ni(OH)₂ film formed on the Pd-Ni layer surface in the hydrogen transport through the Pd-Ni/Pd bilayer electrode is discussed in terms of its passivating effect and extremely large hydrogen solubility. Received: 22 January 1997 / Accepted: 15 April 1997     

Adsorption of deamidated antibody variants on macroporous and dextran-grafted cation exchangers: II. Adsorption kinetics

Single and multicomponent batch adsorption kinetics were obtained for deamidated mAb variants on two commercial cation exchangers, one with an open macroporous structure--UNOsphere S--and the other with charged dextran grafts--Capto S. The adsorption kinetics for the macroporous matrix was found to be controlled largely by pore diffusion. The effective diffusivity estimated from single component data was a fraction of the mAb free solution diffusivity, and its value could be used to accurately predict the adsorption kinetics for two- and three-component systems. In this case, when two or more variants were adsorbed simultaneously, both experimental and predicted results showed a temporary overshoot of the amount adsorbed above the equilibrium value for the more deamidated variant followed by a gradual approach to equilibrium. Adsorption rates on the dextran grafted material were much faster than those observed for the macroporous matrix for both single component and simultaneous adsorption cases. In this case, no significant overshoot was observed for the more deamidated forms. The Capto S adsorption kinetics could be described well by a diffusion model with an adsorbed phase driving force for single component adsorption and for the simultaneous adsorption of multiple variants. However, this model failed to predict the adsorption kinetics when more deamidated forms pre-adsorbed on the resin were displaced by less deamidated ones. In this case, the kinetics of the displacement process was much slower indicating that the pre-adsorbed components severely hindered transport of the more strongly bound variants. Overall, the results indicate that despite the lower capacity, the macroporous resin may be more efficient in process applications where displacement of one variant by another takes place as a result of the faster and more predictable kinetics.     

Effect of emulsion breakage on selectivity in the separation of hydrocarbon mixtures using aqueous surfactant membranes

In liquid membrane separation processes emulsion breakage results in non-selective physical mixing of the feed mixture with the receiving solvent phase. In this paper a model is developed for describing the interphase transfer process, which takes emulsion breakage into account. The overall transfer is envisaged as a result of two parallel transfer mechanisms: (i) diffusive transport across the membrane and (ii) non-selective physical mixing of the feed with the receiving phase due to emulsion breakage. For selective removal of aromatics from non-aromatics in a feed mixture the “ideal” selectivity, β, obtained in the absence of non-selective breakage, will he given as the ratio of the products of the distribution coefficients times the diffusivity in the aqueous membrane phase. Experiments were carried out in a batch stirred cell to determine the permeation rates for a benzene-n-heptane mixture. From the experimentally observed selectivities the contribution due to emulsion breakage was estimated. This fractional breakage was in good agreement with values determined independently using a water-insoluble dye tracer technique, lending support to the developed model. Further experiments were carried out with the system 1-methylnaphthalene-dodecane, and breakage-corrected transfer rates were determined. The model developed in this paper, together with the experimental studies, sheds light on the mechanism of liquid membrane permeation and should aid in scaling-up processes for dearomatization of naphtha and kerosine.     

Rubidium impurity diffusion in a poly(ethylene oxide)-sodium iodide polymer electrolyte

Diffusion of the radioisotope (86)Rb in an amorphous polymer-salt complex consisting of poly(ethylene oxide) and sodium iodide was found to be faster at all temperatures investigated than tracer self-diffusion of the smaller alkali metal cation (22)Na. This is the striking result of the first study on impurity diffusion in a polymer electrolyte system and a comparison with ionic self-diffusion and conductivity data previously obtained from the same system. The experimental findings can be rationalized within an ion transport model based on the occurrence of charged single ions and neutral ion pairs. Simultaneous analysis of all data revealed that the diffusivity of Rb(+) is likely to be lower than that of Na(+). Similarly, the diffusivity of RbI(0) pairs was found to be smaller than that of NaI(0) pairs. Surprisingly, the faster overall transport of Rb as measured by radiotracer diffusion appears to be due to a relatively large fraction of RbI pairs, in conjunction with the finding that the ion pair diffusivities exceed the single cation diffusivities by 2 orders of magnitude.     

Modeling support resistance in zeolite membranes

Zeolite membranes are normally grown on an inert support. In most exploratory studies on these membranes, it is hard to achieve ideal experimental conditions in which membrane support resistance and sorbate concentration on the permeate side are negligible. A simple pseudo-binary diffusion model is proposed for the gaseous transport in the support, while the generalized Maxwell–Stefan formulation is used to describe the zeolitic diffusion in the membrane. Based on these models, a simple graphical procedure is presented for analyzing the measured unary permeation flux data to obtain zeolitic diffusivity. Validation of the proposed procedure is demonstrated successfully on published experimental data.