Natural gas permeation in polyimide membranes

Polyimide membranes derived from 6FDA-DAM:DABA and 6FDA-6FpDA:DABA copolymers have been used to separate 50/50 CO₂/CH₄ mixtures and multicomponent synthetic natural gas mixtures at 35 °C and feed pressures up to 55 atm. For 6FDA-DAM:DABA 2:1 membranes the effects of thermal annealing and covalent crosslinking are decoupled with respect to effects on permeabilities and selectivity. Crosslinking at 295 °C with 1,4-butylene glycol and 1,4-cyclohexanedimethanol increases CO₂ permeabilities by factors of 4.1 and 2.4, respectively, at 20 atm feed pressure, without a loss in selectivity, relative to crosslinking at 220 °C. Thermal annealing and crosslinking also reduce CO₂ plasticization effects. Crosslinking of DABA-containing copolymers, therefore, can produce membranes with tunable transport properties that offer significantly higher performance with better plasticization-resistance than that reported in the literature for the commercial polymers Matrimid^® and cellulose acetate for CO₂ removal from natural gas mixtures. Separation of complex mixtures containing CO₂, CH₄, C₂H₆, C₃H₈, and C₄H₁₀ or toluene results in a significant decrease of the CO₂ permeability, but only a moderate decrease in the CO₂/CH₄ selectivity.     

Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.     

Optical nuclear polarization in the gas phase

Optical nuclear polarization (absorption) has been detected experimentally for the first time in benzaldehyde under UV irradiation in a magnetic field of 4.7 T. The phenomenon has been interpreted theoretically under the assumption of a non-equilibrium electron (optical) polarization transfer to the nuclear subsystem by a scalar cross relaxation mechanism of the second kind.     

Concentration polarization and nonequilibrium electroosmotic slip in hierarchical monolithic structures

This article illustrates the appearance and electrohydrodynamic consequences of concentration polarization (CP) in hierarchically structured monolithic fixed beds used as stationary phases in CEC and related electrical-field-assisted separation techniques. Subject of the investigation are silica-based monoliths in capillary format with a bimodal pore size distribution. Ion-permselectivity in the intraskeleton pore space together with diffusive and electrokinetic transport induces depleted and enriched CP zones at the anodic and cathodic interfaces, respectively, of the cation-selective mesoporous skeleton. The extent of electrical-field-induced CP is shown to be governed by the fluid phase ionic strength, which tunes the ion-permselectivity of the mesoporous monolith skeleton via local electrical double layer overlap, and by the applied electrical field strength, which determines local transport. The analysis of quantitative confocal laser scanning microscopy data, resolving CP on the local skeleton scale, indicates that at sufficiently high field strength a transition from intraskeleton to interskeleton boundary-layer-dominated transport of charged species occurs. This transition is correlated to the onset of macroscopically measured, nonlinear EOF velocities, whose occurrence is explained in the framework of a nonequilibrium electroosmotic slip. It is shown that the onset of nonlinear electrokinetics in the system can be tuned by properties of the BGE, particularly buffer pH, which modulates the pH-dependent surface charge density and consequently the ion-permselective skeleton's charge selectivity. Finally, the CP dynamics of monolithic and particulate fixed beds are compared, and the observed differences are related to the specific morphologies of the two hierarchical fixed bed structures.     

Dual-membrane separation II: Concentration polarization reduction in dual cells

The reduction in concentration polarization in a thin layer dual-hyperfiltration cell with laminar flow, due to interactions of concentration boundary     

Concentration polarization and nonequilibrium electroosmotic slip in dense multiparticle systems

Electrical field-induced concentration polarization (CP) and CP-based nonequilibrium electroosmotic slip are studied in fixed beds of strong cation-exchange particles using confocal laser scanning microscopy (CLSM) and the macroscopic electroosmotic flow (EOF) dynamics. A key property of the investigated fixed beds is the coexistence of quasi-electroneutral macroporous regions between the micrometer-sized particles and the ion-permselective (here, cation-selective) intraparticle mesopores with a mean size of 10 nm. The application of an external electrical field to the particles induces depleted and enriched CP zones along their anodic and cathodic interfaces, respectively, by the local interplay of diffusive and electrokinetic transport. The intensity and dimension of the CP zones depend on the applied electrical field strength and the fluid-phase ionic strength. With increasing field strength a limiting current density through a particle is approached, meaning that charge transport locally through a particle becomes controlled by the dynamics in the adjoining extraparticle convective-diffusion boundary layer (depleted CP zone). In this regime a nonequilibrium electrical double layer can be induced electrokinetically in the depleted CP zone and intraparticle pore space, resulting in nonlinear EOF in the interparticle macropore space. The local CP dynamics analyzed by CLSM is successfully correlated with the onset of nonlinearity in the macroscopic EOF dynamics. We further demonstrate that multiparticle effects arising in fixed beds (random close packings) of ion-permselective particles modulate significantly the local pattern of CP and intensity of the nonequilibrium electroosmotic slip with respect to the undisturbed single-particle picture.     

Concentration and temperature polarization effects during osmotic membrane distillation

Pineapple juice is one of the popular fruit juice due to its pleasant aroma and flavor. Concentration of clarified pineapple juice was carried out by osmotic membrane distillation in a plate and frame membrane module. Concentration and temperature polarization effects are found to have significant role on flux reduction during osmotic membrane distillation process. The contribution of these polarization effects on reduction of the driving force (in turn the flux) at various process conditions such as osmotic agent concentration (2–10 mol/kg (1 molality = 1 mol/kg)), flow rate (25–100 ml/min) of feed and osmotic agent are studied. Concentration polarization has more significant effect on flux reduction when compared to temperature polarization. The experimental fluxes were in good agreement with theoretical fluxes when calculated by considering both concentration and temperature polarization effects. The pineapple juice was concentrated up to a total soluble solids content of 62°Brix at ambient temperature.     

Concentration polarization of ion-exchange membranes in electrodialysis: An interferometric study

The three-frequency laser interferometry method is used for studying the concentration polarization of ion-exchange membranes in a wide range of current densities. The concentration and temperature fields of solutions involved in electrodialysis of sodium chloride are measured simultaneously. The proposed method makes it possible to measure absolute values of local concentrations of acids and bases that form during irreversible dissociation of water molecules at the solution-membrane interfaces following an increase in the current density above the limiting value. According to an analysis of the concentration and temperature distributions in an electrodialyzer channel, maximum variations in the measured quantities occur near the interface. Dependences of the near-membrane concentrations and temperatures on the current density are compared with the voltammogram. The comparison shows that reaching the limiting current density sharply accelerates the nonequilibrium dissociation of water molecules and increases the temperature     

Concentration polarization in a reverse osmosis/nanofiltration plate-and-frame membrane module

The flow structure and solute concentration distribution in a nanofiltration/reverse osmosis plate-and-frame module with radial thin feed channels that have considerable entrance and outlet effects was determined by computational fluid dynamics (CFD). Simulations were performed for binary aqueous solutions, Reynolds (Re) numbers in the range of 64–570 (based on the channel height) and Schmidt (Sc) numbers between 450 and 8900.     

Polymer membranes with selective gas and vapor permeation properties

Since many years synthetic membranes have been used in reverse osmosis or ultrafiltration for the separation of aqueous mixtures. More recently the separation of gases and vapors by selective membrane permeation has gained significant technical and commercial interest. The recovery of hydrogen from petrochemical purge gases and ammonia production processes or the removal of CO₂ from natural gas by selective membrane permeation are today state of the art procedures. The recovery of organic solvents from waste air streams is another very promising application of synthetic membranes. In this paper the main parameters determining the performance of a membrane in gas and vapor separation are described. The requested intrinsic properties of the polymer to be useful as a barrier for a selective gas and vapor transport are discussed. The preparation of appropriate membranes is described. Their performance in practicle applications is illustrated in selected examples.     

Characterization and gas permeation of polycarbonateliquid crystal composite membrane

Polymer/liquid crystal composite membranes were cast from a 1,2-dichloroethane solution of polycarbonate (PC) and N-(4-ethoxybenzylidene-4'-n- butylaniline) (EBBA). The mixing state of the polymer/liquid crystal composite membrane was investigated on the basis of differential scanning calorimetry, x-ray, density, sorption isotherm and sorption—desorption studies and also by electron microscopic observations. EBBA molecules in the composite membrane exist in an almost molecularly dispersed state up to an EBBA fraction of 30 wt%, and in the case of EBBA fractions above 30 wt% form a crystal domain as the mutual continuous phase among the network of polycarbonate fibrils. The composite membrane containing EBBA of 60 wt% can be handled as a homogeneous medium when considering gas permeation.The diffusive permeability coefficient to water reveals a distinct jump in the vicinity of the crystal—liquid crystal phase transition temperature of EBBA. The permeability coefficients, P, to hydrocarbon gases increases 100-200 times over several degrees in the phase transition temperature range. P for hydrocarbon gases decreases with increasing number of carbon atoms below the phase transition temperature, but increases with increasing number of carbon atoms above it. These results suggest that the permeation process is predominantly controlled by diffusion mechanism below the transition temperature of EBBA, while the solubility factor significantly affects gas permeation above it.     

Estimation of gas transport coefficients from differential permeation, integral permeation, and sorption rate data

Experimental methods for studying the transport of gases in polymers may be divided into three categories: integral permeation rate measurement, in which the cumulative amount of a penetrant that has passed through a membrane is determined; differential permeation rate measurement, in which the rate of penetration through a membrane is measured directly; and sorption rate measurement, or determination of the cumulative amount of a penetrant absorbed in a polymer sample. This paper reviews commonly used techniques for estimating diffusion coefficients from transport data of all three types. Several new estimation formulas are presented, and the relative merits of different measurement and estimation methods are discussed. A general relationship between the traditional time lag method for integral rate data analysis and a recently developed moment method for differential rate data analysis is established, extending the applicability of the moment approach to the analysis of non-ideal transport in membranes of arbitrary geometry and composition.     

Concentration effects in gel permeation chromatography : V. Differential elution and viscosity phenomena

The viscosity phenomena that take place in the interstitial volume of a chromatographic column and are connected with changes in the concentration and specific viscosity of an injected polymer solution have been studied. These phenomena were also studied under conditions of differential elution when, instead of a pure solvent, solutions of the same polymer at concentrations both higher and lower than those of the injected sample were used as the mobile phase. Linear relationships were found between the elution volume and specific viscosity and between the efficiency and specific viscosity. It was also ascertained that these dependences hold under conditions of differential elution; accordingly, the absolute viscosity of the mobile phase is of minor importance for the phenomena investigated.     

Concentration polarization effect at the deposition of charged Langmuir monolayers

The review summarizes the results of the recent studies of the electrokinetic relaxation process within the meniscus region during the deposition of charged Langmuir monolayers. Such electrokinetic relaxation is the consequence of the initial misbalance of partial ion fluxes within a small region near the contact line, where the diffuse parts of electric double layers, formed at the monolayer and the substrate surface, overlap. The concentration polarization within the solution near the three-phase contact line should lead to long-term relaxations of the meniscus after beginning and stopping the deposition process, to changes of the ionic composition within the deposited films, to change of the interaction of the monolayer with the substrate, and to dependence of the maximum deposition rate on the subphase composition.     

Concentration polarization,membrane fouling and cleaning in ultrafiltration of soluble oil

Concentration polarization in ultrafiltration of soluble oil was investigated, and the characteristics of the gel obtained at the end of the experiments were analyzed. It was found that the oil content of the gel was approximately 40 vol.% and was almost independent of pressure. The gel was a concentrated oil/water emulsion. p]Membrane fouling was also analyzed in terms of critical surface tension, wettability of the membrane and capillary pressure. Fouling was mainly due to adsorption of oil on the membrane structure, which modified the critical surface tension and the wettability, as well as the effective pore diameter, resulting in reduced membrane permeability. p]The regeneration of fouled membrane was examined with micellar solutions of the system sodium dodecyl sulfate (NaDS)—n-pentanol—water, which was found to be very efficient and useful with respect to both economics and performance.     

Concentration polarization,separation factor,and Peclet number in membrane processes

Concentration polarization affects almost all the membrane separation processes and can be the cause of a substantial reduction in the separation factor and flux. A generalized equation relating the modified Peclet number to the concentration polarization occurring in the boundary layer is proposed and shown applicable to the majority of membrane separation processes like gas separations, reverse osmosis, ultrafiltration, pervaporation, and dissolved gas permeation in liquid. The membrane permeability, separation factor (or solute rejection), membrane thickness, boundary layer mass transfer coefficient, and Henry's law coefficient are the factors that determine the extent of polarization. An analysis is presented to offer a clean division of the hydrodynamic effect from the pure membrane property for membrane separation processes of liquid phases. Also the effect of membrane thickness on polarization is discussed. An attempt has been made to reconcile the different approaches taken for different membrane processes in the literature. Experimental data from widely different sources illustrate and confirm the present theory for pervaporative separation of dilute solutions of volatile organic compounds, dissolved gas permeation, and ultrafiltration of proteins and carbowax. Specific suggestions are made to obtain independent experimental measurements of the Peclet number and polarization index in terms of measurable quantities like the actual and intrinsic separation factors.     

Synthesis and gas permeation properties of silicalite-1 zeolite membrane

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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.     

Pure and mixed gas permeation through a composite polydimethylsiloxane membrane

A thin polydimethylsiloxane (PDMS) layer on polyethersulfone (PES) support was synthesized and pure and mixed gas permeation of C₃H₈, CH₄, and H₂ through it was measured. At first, a macroporous PES support was prepared by using the phase inversion method and characterized. Then, a thin layer of PDMS was coated over the support. Finally, permeation behavior of the synthesized composite membrane was investigated by pure and mixed gas experiments under various operating conditions. The synthesized PDMS/PES membrane showed much better gas permeation performance than others reported in the literature. Pure gas experiments showed that increase in the transmembrane pressure increases the permeability coefficient of heavier gases, C₃H₈, while decreases those of lighter ones, CH₄ and H₂. Exactly opposite behavior was observed in mixed gas experiments due to the competitive sorption and diffusion in the plasticized polymer matrix. Temperature was realized to induce similar effects on the permeability of pure and mixed gases. As expected, in rubbery membranes such as PDMS, permeability values of more condensable gases decrease with increasing temperature, whereas those of permanent gases increase. In the case of mixed gas experiments, increase in the C₃H₈ concentration in feed led to increase in the permeabilities of all the components due to the C₃H₈‐induced swelling of the PDMS film. High C₃H₈/H₂ and C₃H₈/CH₄ ideal selectivities of 22.1 and 14.7, respectively, at a transmembrane pressure of 7 atm as well as reasonable C₃H₈ separation factor (SF) values for all mixed gas experiments (in the range of 8.1–16.8) demonstrated the ability of the synthesized PDMS/PES membrane for the separation of organic vapors from permanent gases. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of aryl nitration on gas sorption and permeation in polysulfone

CO_2, CH_4, O_2, and N₂ permeability and solubility of unmodified and aryl-nitrated polysulfone were determined at 35°C and pressures up to 20 atm. The degree of nitration was varied from 0 to 2 nitro groups per repeat unit. The permeability and diffusion coefficients for all gases decreased with increasing degree of nitro substitution. The decrease in gas diffusivity is attributed to a combination of decreased fractional free volume and decreased torsional mobility with increasing degree of substitution. The solubilities of N_2, O_2, and CH₄ do not show a systematic dependence on degree of substitution. However, CO₂ solubility apparently goes through a minimum as the degree of substitution is increased. CO₂ solubility may be influenced by a competition between increases in polymer polarity (favoring higher solubility) and lower free volume (favoring lower solubility) that accompanies increases in the polar nitro substituent concentration. CO₂/CH₄ solubility selectivity increases monotonically as the degree of substitution increases. CO₂/CH₄ permselectivity and diffusivity selectivity increased with increasing degree of substitution. © 1995 John Wiley & Sons, Inc.