Permeation through a heterogeneous membrane: the effect of the dispersed phase

By the incorporation of filler materials in a membrane one may change the permeability of the overall, dense membrane towards various gases, vapours and/or liquids. The influence of solid fillers dispersed in a polymeric matrix on permeability, diffusivity and solubility has been studied extensively in literature. In this paper the influence of the filler on overall membrane permeability resulting in an upper and a lower limit of the permeability given the filler content and the permeability of the continuous phase will be studied theoretically. From this analysis the change in membrane selectivity towards a gas mixture by incorporation of a filler, relative to the selectivity of the pure matrix membrane, can be obtained. The maximum change in membrane selectivity is a function of the filler content only, i.e. irrespective of matrix and filler permeabilities. The difficulties often encountered in estimating the filler permeability from experimental permeability data using membranes with varying amounts of the filler will be discussed, and possible solutions will be given using theoretical and experimental data.     

Permeation of complexed metal ions through a hydrophobic membrane

Selective concentration of a heavy metal complex with acetylacetone (acac) through a hydrophobic polystyrene membrane was carried out under a pressure gradient. A chelate forming heavy metal was selectively concentrated about 2 fold by this method. When using a coating membrane with a high water flux, the permeabilities increased with increasing complex fraction in the aqueous solution, while using a membrane with a low water flux, a bulky complex was not highly concentrated because of steric hindrance. The complex partitioned on the membrane surface was transported and concentrated under a pressure gradient and a linear relationship was found to exist between permeabilities and partition coefficients. It will be possible to concentrate hydrophobic organic solutes by this method, for acac was concentrated when the Cu—acac complex was formed. As the permeabilities increased with decreasing pressure and membrane compaction was strong for a coating membrane, it seems effective to permeate at a low pressure.     

Permeation of electrolyte water-methanol solutions through a Nafion membrane

The volume flux through a cation-exchange membrane (Nafion 117) separating two equal electrolyte water-methanol solutions as a function of the pressure difference was determined under different experimental conditions. The results show that permeation rates through the membrane are strongly dependent on the methanol content of the solutions, thus the value of the flux increases when the methanol percentage increases. The effect of the electrolyte concentration of the solution on the membrane permeability is less important, although its influence becomes significant at low electrolyte concentration and high methanol content on solvent. This behavior is explained in terms of the amount of solvent sorbed by the membrane. Typical flux behaviors observed with pressure difference are linear at low pressures, exhibiting a positive deviation at higher pressure difference values.     

On-line monitoring of cyanide concentration via a gas membrane system in extractive metallurgical processes

The principle of molecular diffusion of gaseous HCN across a tubular microporous hydrophobic PTFE membrane directly immersed in an alkaline solution or in an aqueous mineral suspension has been applied to the design of an on-line sensing system for cyanide. It offers an efficient detection of “available” cyanide and does not require acidification and solution sampling in the reactor. Gaseous HCN diffused through the membrane is dissolved by a sodium hydroxide carder solution which is then submitted to a spectrophotometric analysis by the pyridine-barbituric acid method. The problems related to filtering of sludges are therefore overcome, and many chemical interferences can be eliminated. Complexation reactions of cyanide by metallic cations and the cyanide-consuming properties of sulphide minerals and ores were studied. This method allows the determination of concentrations down to ca. 6 × 10^?7 mol l^?1 HCN + CN^? in the reactor at Ph 10 with a 120-cm membrane tube and a carrier solution flow-rate of 8 cm³ min^?1.     

水分子簇在聚丙烯酰胺膜中的渗透研究

在不同蒸气活度下,通过吸附动力学实验,测定了蒸气状态的水分子簇在聚丙烯酰胺(PAM)膜中的动态吸附曲线,改进了ENSIC模型并用于计算水分子簇在膜内的扩散系数。结果显示,随着水蒸气活度的增加,膜内水分子簇尺寸增大,并在膜的微孔内产生多层吸附甚至毛细管冷凝,导致扩散系数迅速降低。     

Nonsteady-state processes of water transfer in solid-polymer membrane: a mathematical model

The regularities of electroosmotic and diffusion water transport are shown in the case of hydrogen ionization in the porous anode layer applied on the proton-exchange membrane surface. The possibility of critical phenomena due to drying of a membrane region adjacent to the anode is shown.     

Permeation flux of organic molecules through silica-surfactant nanochannels in a porous alumina membrane.

The permeation fluxes of phenol, benzene sulfonate (BS) and benzene disulfonate (BDS) through a porous anodic alumina membrane with the perpendicularly oriented silica-surfactant nanochannel assembly membrane (NAM) were measured in water-ethanol mixture media. The permeation flux depended on solute charges and on solvent composition. As the ethanol ratio increased, the fluxes of BS and BDS increased and the flux of phenol decreased. The results of extraction/elution experiments also depended on the solute charges and the solvent composition. Chromatographic experiments in n-hexane showed that dipole and hydrophobic interactions affect the retention of solutes. Permeation of the solute across the NAM in water-ethanol mixture is likely to be determined by various factors such as dipole interaction, hydrophobic interaction, solvation, and anion-exchange efficiencies.     

Permeation of nitrogen and water vapor through sulfonated polyetherethersulfone membrane

The novel polyetherethersulfone (PES-C) prepared from phenol-phthalein in our institute is an amorphous, rigid, tough material with good mechanical properties over a wide temperature range. To improve its water vapor permeability for the application of gas drying, the PES-C was sulfonated with concentrated sulfuric acid and transferred in sodium, cupric, and ferric salt forms. The sulfonation degree can be regulated by controlling the temperature and reaction time. Characterization of sulfonated PES-C in sodium form was made by IR. Some properties of the sulfonated PES-C, such as solubility, glass transition temperature, thermal stability, mechanical properties, and transport properties to nitrogen and water vapor have also been discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2133–2140, 1997     

Anesthetic molecules embedded in a lipid membrane: a computer simulation study

The effect of four general anesthetic molecules, i.e., chloroform, halothane, diethyl ether and enflurane, on the properties of a fully hydrated dipalmitoylphosphatidylcholine (DPPC) membrane is studied in detail by long molecular dynamics simulations. Furthermore, to address the problem of pressure reversal, the effect of pressure on the anesthetic containing membranes is also investigated. In order to ensure sufficient equilibration and adequate sampling, the simulations performed have been at least an order of magnitude longer than the studies reported previously in the literature on general anesthetics. The results obtained can help in resolving several long-standing contradictions concerning the effect of anesthetics, some of which were the consequence of too short simulation time used in several previous studies. More importantly, a number of seeming contradictions are found to originate from the fact that different anesthetic molecules affect the membrane structure differently in several respects. In particular, halothane, being able to weakly hydrogen bound to the ester group of the lipid tails, is found to behave in a markedly different way than the other three molecules considered. Besides, we also found that two changes, namely lateral expansion of the membrane and increasing local disorder in the lipid tails next to the anesthetic molecules, are clearly induced by all four anesthetic molecules tested here in the same way, and both of these effects are reverted by the increase in pressure.     

Thermodynamic description of sorption processes in swelling systems

The thermodynamics of sorptive moistening of various types of swelling sorbents have been analyzed. The energy and entropy characteristics of the sorbed substance have been calculated.Institute of Physical Chemistry, Russian Academy of Sciences, Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 29–33, January, 1992.     

Dean vortices: comparison of numerical simulation of shear stress and improvement of mass transfer in membrane processes at low permeation fluxes

In order to rationalize the effect of Dean vortices on mass transfer improvement during membrane filtration, we present preliminary calculations of the wall shear stress in curved tube with non-porous walls. Previous experimental work has already shown strong positive effect of Dean vortices on mass transfer. In this paper, a numerical simulation of shear stress is proposed in order to determine the influence of the geometric parameters in four different tubes: straight, torus, helical and woven tube. The simulation results are tested against the analytical solutions which are available for velocity and pressure distributions in straight tubes. The simulation gives local values from which the location of Dean vortices in cross-section can be deduced and which depends on geometry and Reynolds number. Moreover, published results dealing with oxygenation of water by a membrane process and pervaporation of organic volatile compounds are considered using the present simulation results.     

Modelling and simulation of integrated membrane processes for recovery of Cr(VI) with Aliquat 336

Extraction processes which employ porous membranes as a barrier between aqueous and organic phases provide a versatile means for selective removal and enrichment of solutes from aqueous streams. They require relatively little maintenance, their energy consumption is very low and the organic liquid losses are negligible if the pressure is controlled properly. The modelling and simulation of a complete plant for the removal and recovery of Cr(VI) with Aliquat 336 using hollow fibre modules have been studied. Both single and dual function membrane modules have been analyzed. Simulated concentration profiles through the module were obtained by solving mass transfer balances corresponding to all the species involved in the process.     

Fluorophore-encapsulated solid-supported bilayer vesicles: a method for studying membrane permeation processes

This letter describes a new method for studying the interaction of the membrane-lysing enzyme phospholipase A(2) (PLA(2)) with phospholipid bilayers by simultaneous measurements of enzyme binding and vesicle lysis using surface plasmon resonance (SPR) and permeabilization using surface plasmon field-enhanced fluorescence spectroscopy (SPFS). The PLA(2) inhibitor dimethyl-eicosadienoic acid was incorporated into the surface-bound vesicles and support bilayer in order to study its role in preventing PLA(2)-mediated vesicle lysis. This methodology has a generic applicability for the study of a range of membrane-disrupting agents.     

Proton hopping in phosphoric acid solvated nafion membrane: a molecular simulation study

Molecular simulation studies of the microstructure and of the proton transport properties of phosphoric acid solvated Nafion membrane are carried out. The ab initio calculations show that the phosphoric acid is a good solvent to promote the proton ionization of the sulfonic acid group, and only two phosphoric acid molecules are necessary for the dissociation of one sulfonic acid group. A mechanism of proton hopping between phosphoric acid and protonated phosphoric acid cation in the hydrophilic subphase is also elucidated by ab initio calculations. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 25.4% (wt) which is slightly lower than that of phosphoric acid swollen Nafion, show that the phosphoric acid exists in subphases and that it cannot develop into a continuous subphase. Thus, proton-hopping pathways are interrupted, and the conductivity is expected to be lower than that for pure phosphoric acid. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 45.1% (wt) which corresponds to an unstable state, show that the hydrophobic poly(tetrafluoroethylene) backbones trend to gather together forming hydrophobic clusters and that the phosphoric acid forms a continuous subphase with the sulfonic acid groups located at the hydrophobic/hydrophilic interface. Thus, proton-hopping pathways can develop uninterruptedly like the pure phosphoric acid, and high conductivity is expected. The molecular dynamics study also shows that the hydrogen-bonding characteristics of phosphoric acid and sulfonate anion are similar regardless of the factor that the former can move freely while the latter is attached to Nafion backbone.     

Dynamic simulation of pH in anaerobic processes

With the objective of contributing to the buildup of mathematical tools for anaerobic process simulation, an algorithm for the dynamic simulation of pH was developed. The dynamic simulation of the gaseous phase variables was also considered. The pH algorithm was validated for a watery system, obtaining good agreement between predicted and experimental data. The applied methodology provides a differential equation that allows the inclusion of pH as a state variable of the system that can be easily included in a general mathematical model of anaerobic digestion using matrix notation. This methodology also allows a noticeable decrease in computing time in simulations. A dynamic anaerobic digestion model of complex substrates taken from the literature was completed with the developed algorithms, and it was used to predict the response of an anaerobic reactor against overloading and against the presence of pH-dependent inhibitors with satisfactory results.     

Modeling and simulation of stationary catalytic processes

Generalized models for steady state catalytic processes are presented in matrix form. Multistep reaction rate control is assumed. Numerical algorithms for solving of the created linear and nonlinear equation systems are developed and tested. Four examples are considered: an Eley–Rideal-mechanism, a Langmuir–Hinshelwood mechanism, a dual route, dual site mechanism, and a monomolecular decomposition with steady state multiplicity. The overall reaction rates are simulated as a function of the reactant concentrations. A maximum reaction rate is obtained in the case of a Langmuir–Hinshelwood mechanism (example 2), the location of the rate maximum in the concentration domain is shifted towards the concentration of the reactant with the lowest adsorption constants. An Eley–Rideal mechanism (example 1) has always monotonously increasing rate curves. In the case of steady state multiplicity (example 4) all steady states could be simulated with the proposed algorithm. The computation of reaction rate surfaces is important in investigating the behavior of complicated catalytic systems (e.g., systems with multistep rate control and/or steady state multiplicity), in planning of experiments and in chemical reactor simulation.     

Thermodynamics and digital simulation of intercalation processes

The insertion of guest particles into insertion and intercalation compounds is reconsidered from the viewpoint of reversible thermodynamics. A correlation between enthalpy of formation and volume changes is described, the latter being accessible from X-ray data. The phase changes caused by guest insertion are emphasized. Tungsten bronzes are used as model data. Electronic Publication     

Subdiffusion in a system with a thick membrane

We theoretically study subdiffusion in a system, in which homogeneous thick membrane separates two media; in each of them there are different subdiffusion parameters. Subdiffusion is described by the linear differential equations with fractional time derivative and the boundary conditions requiring that the ratio of substance concentrations on both sides of the membrane surface is constant in time. Starting with the Green’s functions derived for the considered system, we discuss the property of the concentrations found in the long time limit for the system where initially the membrane separates pure solvent from homogeneous solution.