Influence of the transport direction on gas permeation in two-layer ceramic membranes

Experimental and theoretical results of studying gas permeation through porous membranes are presented. In order to mimic an asymmetric membrane two porous ceramic disks with different pore radii were arranged in series. Besides the possibility to perform conventional permeation measurements, the applied experimental setup permits the determination of the pressure at the interface between the two discs. To predict the performance of the asymmetric structure, in preliminary experiments structure parameters were determined for both membranes separately. For the same total pressure difference across the two-disk arrangement, different interlayer pressures and fluxes were predicted and detected experimentally depending on the flow direction.     

A molecular dynamics simulation study of surface effects on gas permeation in free-standing polyimide membranes

A 141100-atom model of a glassy ODPA–ODA polyimide free-standing membrane, corresponding to a thickness of two average radii of gyration for the 40-mers chains, has been studied using molecular dynamics (MD) simulations. Due to the large-scale of the fully atomistic model, a parallelized particle-mesh technique using an iterative solution of the Poisson equation had to be implemented for the efficient evaluation of the electrostatic interactions. With flattened-chain configurations, the density was found to adjust itself naturally in the middle of the membrane to 95% of the ODPA–ODA experimental value. At the free-standing surfaces, the density profile became sigmoïdal, indicating surface roughness. For comparison, two isotropic bulk models, one at the “normal” density as obtained for ODPA–ODA under ambient conditions and the other one at 95% of the normal-density, were built. Small gas probes were inserted into all three models in order to investigate whether the interfacial structure of the glassy free-standing membrane can influence penetrant transport. Gas diffusion in the low-density part of the interface was found to be very fast. The limiting value for the gas diffusion coefficient D_membrane is only attained when the probes enter more dense regions in the membrane. Indeed, D_membrane compares well with D_bulk obtained for the 95%-density bulk system, i.e. about twice that in the normal-density bulk. Solubility is larger in the membrane than in both bulk models, thus suggesting an effect of chain flattening in addition to the density. Adsorption is particularly high at the free-standing interfaces.     

Differential permeation — Part I: A method for the study of solvent diffusion through membranes

We describe the differential permeation method for the study of the diffusion of solvents from a liquid (or liquid mixture) through flat or tubular membranes. This method consists of measuring the transient permeation rates through the membrane when one of its faces is suddenly put into contact with the liquid medium. The change in the transient rate with time is analyzed by numerical best fitting methods to determine the Fickian diffusion coefficient. A simplified equation is proposed for the fitting of the response of a tubular membrane. Deviations from the Fickian transport mechanism with concentration-independent diffusion coefficient can be evidenced and eventually analyzed by using other mechanistic models.     

Application of capillary gas chromatography to automated on-line quantitative analysis of solvent vapors

Solvent vapors in air may be measured with capillary gas chromatographic columns. By using large diameter columns and sample loops of approximately the same internal diameter, the column may be connected directly to the gas sampling valve. This approach eliminates the use of a splitter or cryogenic trapping and allows low levels to be measured. By operating the column at high velocity, column efficiency is sacrificed for increase in speed.     

Effect of poly(ethylene glycol) 200 on the formation of a polyetherimide asymmetric membrane and its performance in aqueous solvent mixture permeation

To investigate the effect of poly(ethylene glycol) (PEG) 200 on membrane performance, asymmetric polyetherimide (PEI) membranes with a small pore size were prepared by dry/wet-phase inversion from the casting solution containing N-methyl-2-pyrrolidone as a solvent and poly(ethylene glycol) 200 as an additive. Our experiment revealed that the addition of PEG 200 has an influence on the casting solution properties, permeation properties, and resulting membrane structures. Moreover, a drying process also affects the formation of a dense skin layer. Increasing the amount of PEG 200 drastically improved the solute rejection rate. The drying process improved the rejection rate. We also observed the effect of the mixed solvent (water/ethanol) on permeation through the membranes with various pore sizes. In the case of the membrane with a dense skin layer, the solvent permeation showed relationships with solution viscosity, surface tension, and membrane-solvent interaction.     

Effect of substituents on the permeation properties of polyamide membranes

Aromatic polyamides, designed for evaluation as gas separation membranes, were processed into dense films, whose properties were measured with special emphasis on their mechanical and thermal properties. The polymers had been synthesized from monomers bearing side substituents, such as methyl, iso-propyl or tert-butyl, and various hinge-like connecting linkages of p-phenylene moieties, which yielded amorphous aromatic polyamides, with improved solubility, high glass transition temperatures (over 250 °C) and excellent mechanical properties (tensile strength about 100 MPa, and moduli about 2.0 GPa). The permeability of the polymer films were investigated using helium, oxygen, nitrogen, carbon dioxide and methane. Gas permeability typically increased with increasing free volume, and, in general, free volume could be related to the chemical structure. The analysis of the transport parameters (permeability, diffusivity and solubility coefficients) as a function of the chemical structure, confirmed the predominant role of the side substituents and of the linking groups connecting phenylene units on the permeation properties. Besides, a molecular modelling study carried out via computational chemistry, made it clear that an acceptable theoretical explanation can be given of how the nature of hinge groups between aromatic rings can affect torsional mobility and gas diffusion of aromatic polyamides.

The experimental aromatic polyamides of this report, as a whole, showed a favourable combination of permeability–selectivity, better than that of conventional polyamides and that of most engineering thermoplastics, confirming the hypothesis that the incorporation of side bulky substituents is a convenient approach to hinder the inherently efficient chain packing of polyamides.     

Morphology influence on the permeation properties of styrene and polystyrene modified polyethylene membranes

Influence of the structure of styrene and polystyrene modified PE membranes on their permeation properties is presented. It is demonstrated that the temperature of styrene treatment (N membranes) and the presence of polystyrene (P membranes) results in high differentiation of the permeation properties. SALS studies have shown the structural reorganization depending on modification extent and diffusion of the penetrant. It is also shown that the morphology of the dried films remains unchanged. This work proves that the appropriate modification of PE films results in membranes with controlled permeation behavior.     

On the unusual solvent retention and the effect on the gas transport in perfluorinated Hyflon AD membranes

Dense membranes of Hyflon AD 60X were prepared by the solvent evaporation method and by melt pressing. The diffusion coefficient, solubility and permeability of the membranes were measured for six permanent gases using time lag and steady state permeation measurements. The thermal properties were determined by Differential Scanning Calorimetry (DSC) and the solvent content was measured gravimetrically and was estimated by the Fox equation. It was found that unusually strong solvent retention in the solution-cast membrane leads to considerable plasticization of the polymer, to possible foam formation upon drying and, most important, to significant changes in the permeation properties. The residual solvent increases the diffusion coefficient and permeability of the larger gas species up to almost one order of magnitude, and it reduces the permselectivity. For most gas species the solubility is about two times higher in the solvent-free melt-pressed film than in the solution-cast film. The relation between the residual solvent and the membrane properties is discussed.     

Gas permeation properties of ethylene vinyl acetate–silica nanocomposite membranes

The effect of silica nanoparticles on the gas separation properties of ethylene vinyl acetate (EVA) copolymer containing 28% vinyl acetate has been investigated. The EVA and hybrid EVA–silica membranes were prepared via thermal phase inversion method. Silica nanoparticles prepared by hydrolysis of tetraethylorthosilicate (TEOS), through the sol–gel mechanism. The prepared membranes were characterized using FT-IR, SEM, DSC and XRD methods. FT-IR and SEM results indicated the nanoscale dispersion of silica particles in polymer matrix. As confirmed by XRD and DSC analyses, increasing the silica content enhances the amorphous regions significantly. Gas permeation of EVA–silica nanocomposite membranes with silica contents of 5, 6 and 10 wt.% was studied for N₂, O₂, CO₂ and CH₄ single gases at pressures of 4, 6 and 8 bar. The obtained results suggest a significant increase in permeability of all gases and an increase in CO₂/N₂ and CO₂/CH₄ gases selectivities upon increasing the silica content. The possible reasons for such behavior were stated and discussed. The pressure dependence of the gas permeabilities of the membranes was also investigated.     

Optimization of asymmetric permeation in heterogeneous composite membranes

Asymmetric permeation in two-phase composite membranes with heterogeneous structures represented by a one-dimensional distribution of composition is treated theoretically on the basis of an irreversible thermodynamic transport equation. It is assumed that the permeability of one of the component phases is a monotone function of the activity of permeant while that of the other phase is constant, and that the permeability of the composite membrane is given by the volume average of the resistance coefficient, which is the inverse of permeability. Under these assumptions, it is shown that the optimal membrane which maximizes the degree of asymmetric permeation reduces to a binary laminate membrane. The condition for constructing the optimal laminate membrane is obtained explicitly. Conversely a condition on a desirable membrane component which realizes an arbitrary degree of asymmetric permeation is presented. These results can be applied to the optimal design of a membrane valve which is a chemical analog of a diode. © 1993 John Wiley & Sons, Inc.     

Permeation of water as a tool for characterizing the effect of solvent, film thickness and water solubility in cellulose acetate membranes

Cellulose acetate membranes have been used in many applications; of particular interest are reverse osmosis systems, and as a neutral matrix for incorporation of different polymers (e.g., conducting polymers), inorganic ions (e.g., lanthanides) and organic (e.g., pharmaceutical) compounds. The properties of the new polymers derived from cellulose acetate or blends depend on those of cellulose acetate. This work presents an attempt to find links between thermodynamic and kinetic properties of cellulose acetate membranes in equilibrium with water. Water diffusion coefficients in cellulose acetate membranes are reported, measured with a simple water permeation technique. The comparison of these values with the percentage of water uptake and polymer thickness leads to interesting conclusions related with different polymer properties.     

Correlations between gas permeation and free-volume hole properties of polyurethane membranes

A series of polyurethane films based on hard segments consisting of toluene diisocyanate and 1,4-butanediol and different soft segments consisting of hydroxyl terminated polybutadiene, hydroxyl terminated polybutadiene/styrene and hydroxyl terminated polybutadiene/acrylonitrile were synthesized by solution polymerization separately. Positron annihilation lifetimes were measured at room temperature for all samples studied. We found that both the free volume size and fractional free-volume decreased with the increase of hard segment content. On the other hand, direct relationship between the gas permeability and the free-volume has been established based on the free-volume parameters and gas diffusivity measured. Experimental results revealed that the free-volume plays an important role in determining the gas permeability.     

Electrospun cellulose acetate fibers: effect of solvent system on morphology and fiber diameter

This paper reports an investigation of the effects of solvent system, solution concentration, and applied electrostatic field strength (EFS) on the morphological appearance and/or size of as-spun cellulose acetate (CA) products. The single-solvent systems were acetone, chloroform, N,N -dimethylformamide (DMF), dichloromethane (DCM), methanol (MeOH), formic acid, and pyridine. The mixed-solvent systems were acetone–DMAc, chloroform–MeOH, and DCM–MeOH. Chloroform, DMF, DCM, MeOH, formic acid, and pyridine were able to dissolve CA, forming clear solutions (at 5% w/v), but electrospinning of these solutions produced mainly discrete beads. In contrast, electrospinning of the solution of CA in acetone produced short and beaded fibers. At the same solution concentration of 5% (w/v) electrospinning of the CA solutions was improved by addition of MeOH to either chloroform or DCM. For all the solvent systems investigated smooth fibers were obtained from 16% (w/v) CA solutions in 1:1, 2:1, and 3:1 (v/v) acetone–DMAc, 14–20% (w/v) CA solutions in 2:1 (v/v) acetone–DMAc, and 8–12% (w/v) CA solutions in 4:1 (v/v) DCM–MeOH. For the as-spun fibers from CA solutions in acetone–DMAc the average diameter ranged between 0.14 and 0.37 μm whereas for the fibers from solutions in DCM–MeOH it ranged between 0.48 and 1.58 μm. After submersion in distilled water for 24 h the as-spun CA fibers swelled appreciably (i.e. from 620 to 1110%) but the physical integrity of the fibrous structure remained intact.     

Interrelation between phase state and gas permeation in polysulfone/polyimide blend membranes

The phase state of polysulfone/polyimide (PSF/PI) blends has been studied by differential scanning calorimetry, rheology, and X-ray scattering. The blends rich in PSF form miscible blends when prepared by solution casting from a common solvent. In these PSF-rich blends, the single dynamic process in rheology shifts and broadens, with composition reflecting the change in local friction and the enhancement of concentration fluctuations, respectively. Heating to temperatures above the glass transition temperature results in phase separation into PSF- and PI-rich domains. An apparent phase diagram has been constructed, and helium permeability has been measured in different regimes corresponding to miscible, partially miscible, and completely phase-separated states. We find that one component (PI) controls the permeability values and activation energies for helium permeation in the blends. Gas permeation is found to be very sensitive to local concentration fluctuations and thus can be used as a probe of the phase state in polymer blends. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2788–2798, 1999     

Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Experimental and modeling study of oxygen permeation modes for asymmetric mixed-conducting membranes

An asymmetric mixed-conducting membrane consists of a thin dense layer and a porous support, and its application has drawn considerable attention, because it is expected to have a more promising potential in the practical application compared with the symmetric membrane. However, with the introduction of support in the asymmetric membrane, two possible permeation modes are produced. One mode is that oxygen permeates from the support to the thin dense layer (designated as SD mode). The other is in the direction from the thin dense layer to the support (designated as DS mode). Thus, from the viewpoint of choosing an appropriate oxygen permeation mode to make better use of the membrane, it is necessary to study the oxygen flux in these two modes. In this paper, their effects on the oxygen flux of asymmetric membranes were investigated from the experiment and the model. The modeling results showed a good agreement with the experimental data. Our study demonstrates that when the asymmetric membrane adopts the SD mode, it is beneficial for the membrane to obtain higher oxygen permeation flux.     

Gas and liquid permeation properties of modified interfacial composite reverse osmosis membranes

Gas permeation tests using nitrogen, oxygen, hydrogen, helium and carbon dioxide were performed to assess how membrane modification procedures affect the separating layer morphology of thin-film composite reverse osmosis membranes. Gas selectivity data provided evidence for the presence of nanoscale separating layer defects in dry samples of six commercial membrane types. These defects were eliminated when the membrane surface was coated with a polyether–polyamide block copolymer (PEBAX 1657), as indicated by a 25-fold decrease in gas permeance and at least a 2-fold increase in most selectivity values. Treatment with n-butanol followed by drying reduced water flux and gas flux by 30% and 75%, respectively, suggesting that using n-butanol as a solvent for applying coatings negatively affects membrane performance. The results of this study demonstrate that gas permeation measurements can be used to detect morphological features that impact gas and water membrane flux.     

凝胶渗透色谱-气相色谱-质谱测定花生中乙草胺的残留量

建立了以凝胶渗透色谱(GPC)净化和气相色谱-质谱(GC-MS)技术快速测定花生中乙草胺残留量的方法.经乙腈提取,共提物中的油脂和色素经GPC去除,目标农药采用GC-MS-SIM方式进行定性和定量分析.方法的回收率90%～120%;相对标准偏差2.5%～10%.方法定量限0.005 μg/g.