Transport of gaseous mixtures through membranes

Thermoosmosis of ternary mixtures of oxygen, nitrogen, and carbon dioxide across a porous unglazed membrane has been studied, The thermoosmotic pressure difference, ΔP, created by a temperature difference, ΔT, has been measured at various mean temperatures and pressures. Experimental data have been interpreted in the light of non-equilibrium thermodynamics of irreversible processes and the dusty gas model of Mason. Heats of transport for the mixtures, Q^★, have been estimated from the measured values of pressure difference and temperature difference. It is found that the heat of transport of mixtures is independent of the mean temperature and temperature difference as was found in earlier studies on multicomponent mixtures.     

The influence of the porous support layer of composite membranes on the separation of binary gas mixtures

Thin film composite (TFC) membranes exhibit a high flux for gas and vapor permeation and are viable for a wide range of applications. The high flux may also increase the importance of the resistance of the porous support structure depending on the application and process conditions. A comprehensive modeling approach for TFC membranes is introduced, which considers boundary layer resistances near the membrane surface, solution-diffusion through the coating, and the influence of the porous sublayer. Permeation through the support structure is described by the dusty gas model (DGM) with the support treated as a two-layered structure with a dense but porous skin and a more open substructure.The model accurately describes experimental data on TCE/nitrogen separation using a sweep gas on the permeate side very well. The main resistance towards TCE permeation through two different membranes tested is the porous support. It is shown that changes in the support morphology can greatly enhance the performance of the composite membranes. Model calculations were also performed for vacuum assisted permeation. The pressure drop across the support is considerable depending on the coating thickness. The TCE permeation is again dominated by the resistance of the support layer, which can be reduced by altering the morphological parameters of the structure.The proposed model is able to describe the performance of the composite membrane and to identify optimum process conditions for given performance characteristics. It can be used to aid in the development of membrane structures for enhanced performance.     

On the gas-separation properties of two-layer porous membranes

The peculiarities of the flows of binary gaseous mixtures through two-layer membranes are investigated at different orientations of the membranes with respect to the flows. The separation properties of the membranes are shown to depend on the membrane orientation and to dramatically decrease when supporting coarse layers are located before the finely disperse active layer with respect to the direction of a gaseous mixture flow through the membrane. The consecutive location of the layers with increasing average pore sizes along the flow direction appears to be more advantageous. The magnitude of the asymmetry effect significantly depends on the parameters that characterize the Knudsen and bulk diffusion and the viscous transfer of a gaseous mixture.     

On the nonequilibrium thermodynamics of gas flow in nanoporous membranes

Gas flow has been considered in a porous membrane with nanosized channels, for which the influence of surface forces and relevant effects must be taken into account. The problem of entropy production in such systems has been discussed. It has been shown that, when calculating the total entropy production in such a system, it is necessary to take into account the entropy production at the inlets and outlets of the channels of the porous body. The entropy production has been determined, and the phenomenological equations describing the heat and mass transfer through the porous body-gas interface have been found.     

Spinodal phase separation of unstable solid-state binary n-alkane mixtures

Two mixtures of unequal chain length n-alkanes in which one component is deuterated have been investigated by infrared spectroscopy as they demixed. The measurements followed the band shapes of the scissors vibrations as a function of time. The band envelopes are analyzed as composites of a number of reference mixtures of known concentration. The unequal-chain mixtures separate into phases that slowly change their composition toward pure alkane phases. The method of analysis, which reveals local concentrations, should be generally applicable to polymethylene systems.     

Performance and pore characterization of nanoporous carbon membranes for gas separation

The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated.     

Gas separation in nanoporous membranes formed by etching ion irradiated polymer foils

Polymer membranes with pores with radii in the range of several 10–100 nm were formed by irradiating polyimide foil with highly energetic heavy ions and etching the latent ion tracks with hypochlorite. The aerial density of the pores could be chosen up to an upper limit of 10⁸ pores cm^?2, at which too many pores start to overlap. The straight cylindrical pores were tested for their gas permeation and gas separation performance. With a gas mixture of CO and CO₂ as model system, gas chromatographic measurements showed that CO penetrates faster through the membrane than CO₂, leading to gas separation. This is possible because the mean free path of the molecules is in the order of the pore radius, which is in the transition flow region close to molecular flow conditions.     

The role of binary phase diagrams in separation of stereoisomeric mixtures

Crystallization from the melt was applied to separate components of stereoisomeric mixtures. Binary phase diagrams were determined by DSC and used in the design of the crystallization process. The method is illustrated by the separation of diastereoisomeric (cis-trans) permethrinic acid and by that of the enantiomeric excess and racemic fraction of the Corey-lacton.

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Zusammenfassung Zur Abtrennung von Komponenten aus stereoisomeren Mischungen wurde die Kristallisation aus der Schmelze verwendet. Die binären Schmelzdiagramme wurden mittels DSC untersucht und zur Festlegund der Parameter des Kristallisations-prozesses genutzt. Die Methode wird-erläutert an den Beispielen (1) Trennung der Diastereomeren (cis/trans) Permethrinsäuren, (2) Abtrennung des Enantiomeren-überschusses von racemischem Corey-Lacton.

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Speciation fingerprints of binary mixtures by optimized repeated two-phase separation

A method for determination of the composition of binary mixtures of a metal or radionuclide species by optimized repeated two-phase separations (SORTS) was proposed and theoretically substantiated. Its principle consists in repeated equilibration of two immiscible phases, one being the original liquid or solid matrix with minimal adjustment of its composition and varying the phase ratio (separation stage cut) as the optimized parameter. The batch separation technique may consist in the repeated solvent extraction or aqueous biphasic distribution, or in the replicate equilibration with solvent or leaching solution. Results of SORTS can be presented e.g. by Tukey box diagrams as the characteristic fingerprints of original species composition.     

Spontaneous "phase separation" of patterned binary alkanethiol mixtures

This article describes novel phase-separation behavior by a binary mixture of alkanethiols when deposited onto a gold surface using micro- and nanodeposition tools, such as microcontact printing (muCP) and dip-pen nanolithography (DPN). This behavior is significantly different than that observed in the bulk. We demonstrate this behavior using three model compounds: 16-mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), and CF3(CF2)11(CH2)2SH (PFT). The identity of the resulting segregated structure is confirmed by lateral force microscopy (LFM) and by selective metal-organic coordination chemistry. Importantly, this phenomenon can be exploited to print sub-100 nm wide alkanethiol lines via conventional muCP and to form sub-15 nm features using DPN, which is below the ultimate resolution of both these techniques. We also demonstrate that these nano-patterned materials can serve as templates for constructing more complex architectures.     

On the ideality of binary mixtures of ionic liquids

In this work, structural and dynamical properties of the binary mixture of 1-ethyl-3-methyl-imidazolium chloride and 1-ethyl-3-methyl-imidazolium thiocyanate are investigated from ab initio molecular dynamics simulations and compared to the pure ionic liquids. Furthermore, the binary mixture is simulated with two different densities to gain insight into how the selected density affects the different properties. In addition, a simple NMR experiment is carried out to investigate the changes of the chemical shifts of the hydrogen atoms due to the composition of the mixture.     

Modelling of the pervaporation of binary mixtures through moderately swelling,non-reacting membranes

The pervaporation of binary mixtures through moderately swelling membranes is analyzed as a solution—diffusion process, on the assumption that the diffusion coefficient of each permeant is an exponential function of both concentrations. A model is derived in which changes in selectivity and fluxes are related to major external conditions: the upstream mole fraction in the feed and the downstream total pressure of the pervaporate. This model leads to the computation of the pervaporation selectivity and the fluxes as a function of the parameters of the model. Numerical examples are given, relative to different shapes of curves for selectivity and fluxes. A flowsheet is given and allows, through the comparison with experimental results, the determination of the parameters of the model. Internal concentration profiles in the membrane may also be computed. In order to test the validity of this model, it has been applied to a set of experimental data for the pervaporation of hydrocarbon binary mixtures through nitrile—butadiene and styrene—butadiene copolymers.     

Parallel factor analysis of HPLC-DAD data for binary mixtures of lidocaine and prilocaine with different levels of chromatographic separation

A set of 17 samples containing a constant amount of lidocaine (667 μM) and a decreasing amount of prilocaine (667-0.3 μM) was analysed by LC-DAD at three different levels of separation, followed by parallel factor analysis (PARAFAC) of the data obtained. In Case 1 no column was connected, the chromatographic resolution (R_s) therefore being zero, while Cases 2 and 3 had partly separated peaks (R_s=0.7 and 1.0). The results showed that in Case 1, analysed without any separation, the PARAFAC decomposition with a model consisting of two components gave a good estimate of the spectral and concentration profiles of the two compounds. In Cases 2 and 3, the use of PARAFAC models with two components resolved the underlying chromatographic, spectral and concentration profiles. The loadings related to the concentration profile of prilocaine were used for regression and prediction of the prilocaine content. The results showed that prediction of prilocaine content was possible with satisfactory prediction (RMSEP<0.01). This study shows that PARAFAC is a powerful technique for resolving partly separated peaks into their pure chromatographic, spectral and concentration profiles, even with completely overlapping spectra and the absence or very low levels of separation.     

Thermodynamic stability of fluid-fluid phase separation in binary athermal mixtures: the role of nonadditivity

We studied the thermodynamic stability of fluid-fluid phase separation in binary nonadditive mixtures of hard-spheres for moderate size ratios. We are interested in elucidating the role played by small amounts of nonadditivity in determining the stability of fluid-fluid phase separation with respect to the fluid-solid phase transition. The demixing curves are built in the framework of the modified-hypernetted chain and of the Rogers-Young integral equation theories through the calculation of the Gibbs free energy. We also evaluated fluid-fluid phase equilibria within a first-order thermodynamic perturbation theory applied to an effective one-component potential obtained by integrating out the degrees of freedom of the small spheres. A qualitative agreement emerges between the two different approaches. We also addressed the determination of the freezing line by applying the first-order thermodynamic perturbation theory to the effective interaction between large spheres. Our results suggest that for intermediate size ratios a modest amount of nonadditivity, smaller than earlier thought, can be sufficient to drive the fluid-fluid critical point into the thermodinamically stable region of the phase diagram. These findings could be significant for rare-gas mixtures in extreme pressure and temperature conditions, where nonadditivity is expected to be rather small.     

On the formation of multilayers in adsorption from binary liquid mixtures

Summary A new excess isotherm is derived for multilayer adsorption from binary liquid mixtures on solid surfaces. This is done by applying the Myers and Sircar's theoretical procedure, and assuming that the adsorption of single components is described by the BET isotherm equation, as modified by Brunauer, Skalny and Bodor. The derived excess equation is next generalized for the case of patchwise heterogeneous solid surfaces. The illustrative calculations presented here include both model investigations and an investigation of two real adsorption systems. It is drawn as a general conclusion that the heterogeneity effects in adsorption from solutions play a greater role than previously supposed.

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Zusammenfassung Eine neue Excessisotherme wurde für vielschichtige Adsorption aus Zweikomponentlösungen an festen Oberflächen abgeleitet. Das wurde mit Anwendung der theoretischen Methode von Myers und Sircar durchgeführt, wobei angenommen wurde, daß die Adsorption der einzelnen Komponenten durch die BET-Gleichung in der Modifizierung von Brunauer, Skalny und Bodor ausgedrückt wird. Die abgeleitete Excessgleichung wurde dann für heterogene feste Oberflächen generalisiert. Die Berechnungen berücksichtigen beide Untersuchungsmodelle und die Untersuchungen der zwei realen Adsorptionssysteme. Es wurde festgestellt, daß der Einfluß der Heterogenität in Adsorption von Lösungen eine größere Rolle spielt, als es vorher angenommen wurde.

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With 9 figures and 1 table     

The permeation of binary gas mixtures through support structures of composite membranes

The dusty gas model (DGM) is used to describe transport of binary gas mixtures through porous membrane supports to quantify the resistance towards permeation. The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary diffusion. Experimental data obtained with the uncoated membrane supports are used to determine the morphological parameters needed in the DGM equations. Flat sheet and hollow fiber membrane supports are characterized by the permeation of a TCE/nitrogen vapor. The DGM shows an excellent fit to experimental data when the asymmetric structure of the membrane supports is taken into account, but the morphological parameters cannot necessarily be related to precise physical structure parameters such as pore size, porosity, and tortuosity. The DGM works well even when the membrane supports are modeled as a single homogenous structure. The membrane supports exhibit different resistances towards the various transport mechanisms that occur within the porous support and the resistances vary with process conditions so that support optimization is not straightforward. With the analysis presented in this paper and transport equations specific to the dense coating and module geometries, the influence of the support layer on gas or vapor separation can be quantified.     

Fabrication and gas separation properties of polybenzimidazole (PBI)/nanoporous silicates hybrid membranes

Composites of polybenzimidazole (PBI) with proton-exchanged AMH-3 and swollen AMH-3 were prepared, characterized by electron microscopy and X-ray scattering and tested for hydrogen/carbon dioxide ideal selectivity. Proton-exchanged AMH-3 was prepared under mild conditions by the ion exchange of Sr and Na cations in the original AMH-3 using aqueous solution of dl-histidine. Swollen AMH-3 was prepared by sequential intercalation of dodecylamine following the ion exchange in the presence of dl-histidine. Both silicate materials were introduced into a continuous phase of PBI as a selective phase. Mixed matrix nanocomposite membranes, prepared under certain casting conditions, with only 3 wt% of swollen AMH-3 present substantial increase of hydrogen/carbon dioxide ideal selectivity at 35 °C, i.e., more than by a factor of 2 compared to pure PBI membranes (40 vs. 15). Similar ideal selectivity was observed using higher loadings (e.g., 14%) of proton-exchanged AMH-3 particles suggesting that transport of hydrogen is faster than carbon dioxide in AMH-3-derived silicates. However, the ideal selectivity of mixed matrix membranes approaches that of pure polymer as the operating temperature increases to 100 °C and 200 °C. The composite membranes with AMH-3-derived materials were compared with MCM-22/PBI membranes. Composite membranes incorporating MCM-22 plate-like crystals show no selectivity enhancements possibly due to the presence of larger pores in MCM-22.