G. Bulvestre

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

Abstract 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.

Sorption and pervaporation of dilute aqueous solutions of organic compounds through polymer membranes

Abstract Pervaporation of dilute aqueous binary mixtures of four organic compounds (benzene, chloroform, acetone and ethanol) through nitrile—butadiene and styrene—butadiene copolymers was investigated. A pervaporation device has been built, which allows measurement of the pervaporation flux and selectivity of a membrane as a function of the upstream composition of the feed and the downstream total pressure of the pervaporate. In order to relate pervaporation results to equilibrium properties of the membranes, the sorption of water and dilute aqueous solutions was mainly investigated. The pervaporation of dilute aqueous solutions of benzene and chloroform has been extensively studied, including the separation of traces of chloroform, and is modelled through a “sixcoefficients exponential model” [1]. This model is derived from a solution—diffusion analysis of the selective transfer, assuming an exponential dependence of both diffusivities on concentrations of both permeants. Semi-quantitative information about the potential interactions existing in the system solute i —solvent j —membrane and about the concentration profiles at steady-state may be derived from these coefficients.

Determination of the diffusion coefficients of ions in cation-exchange membranes, supposed to be homogeneous, from the electrical membrane conductivity and the equilibrium quantity of absorbed electrolyte

Abstract Two ion-exchange membrane characteristics, the quantity of absorbed electrolyte and the electrical membrane conductivity have been measured and correlated for three cation-exchange membranes (CM2, CMx and MK-40), two electrolytes (KCl and LiCl), over a large concentration range of the solution (0.1 M≤ C 0 ≤3.0 M). The membrane conductivity has been measured according to a French standard. However, because of the lack of standards fixing all the operating conditions for good determination of the second parameter, we established an experimental protocol, validated by a statistical study and made a comparison with the theoretical Glueckaufs equation. This method will be proposed to be standardized for this category of measurements. Relationships have been established between the two parameters allowing us to determine both the counter-ion and the co-ion diffusion coefficients in a cation-exchange membrane, considered as homogeneous. The values of these coefficients for the counter-ions are in good agreement with those obtained from other methods. The variations of the computed diffusion coefficients of counter-ions and co-ions for the different studied systems are discussed in terms of internal interactions with the charged polymer matrix and the other diffusing species. The discussion also takes into account the free water content of the polymer.

Separation of benzene—n-heptane mixtures by pervaporation with elastomeric membranes. II. Contribution of sorption to the separation mechanism

Abstract The separation mechanism of pervaporation can be considered as consisting of two steps: sorption at the upper solution—membrane interface and diffusion through the membrane. In the system benzene—n-heptane—poly(butadiene—acrylonitrile) (NBR)membrane, sorption is studied according to the same parameters as pervaporation [1]; it emphasizes the importance of nitrile groups in the separation and reveals, for lower swellings, an equivalent contribution of the two steps in the separation mechanism. This permits sorption selectivity to be taken as a first approximation of pervaporation selectivity.

Determination of the streaming potential in ion-exchange membranes

Abstract The streaming potential in ion-exchange membranes has been experimentally studied. Results obtained with two sulfonic membranes with a different degree of swelling and equilibrated in HCl, NaCl and KCl solutions with concentrations ranging from 10 −3 M to 1 M are given. Concentration polarisation problems are solved by extrapolation of the experimental data to zero diffusion time.

Détermination du coefficient d’affinité d’une membrane échangeuse de cations à différentes forces ioniques

Resume Bien qu’il soit connu que les polymeres echangeurs d’ions possedent une affinite variable pour differents contre-ions, ce probleme est en general neglige dans la modelisation des transferts membranaires. Nous avons determine cette caracteristique pour trois membranes echangeuses d’ions (CM1, CM2 et Nafion® 117) de natures differentes et souvent utilisees dans des applications industrielles. Les solutions d’equilibrage sont des melanges equimolaires de chlorures alcalins (KCl + NaCl, KCl + LiCl et NaCl + LiCl) dont la force ionique (I) varie de 0,1 a 1,5 mol l−1. Les resultats obtenus montrent que (i) le coefficient d’affinite decroit pour les faibles forces ioniques et tend asymptotiquement vers l’unite pour des valeurs de I assez elevees, (ii) ce coefficient est fonction d’une part de la taille des cations a l’etat hydrate et d’autre part de la nature et de la structure de la membrane, (iii) la variation de la pression interne a la membrane est a l’origine des variations du coefficient d’affinite.

Experimental and theoretical studies of the crossed ionic fluxes through a cation-exchange membrane

Abstract This paper presents first a set of experimental data on the measurements of the ionic fluxes through a cation-exchange membrane separating two electrolyte solutions with the same concentration, the same co-ion but different counter-ions (bi-ionic system). Then these results are compared with the theoretical predictions given by the resolution of the corrected Nernst–Planck’s equation in the cases of the homogeneous and heterogeneous models of ion-exchange membranes. A good agreement between the experimental results and the computed values is observed only in the case of the heterogeneous model. This confrontation confirms (i) the existence of three concentration domains: low, medium and high concentrations, corresponding respectively to a complete diffusion boundary layers (DBLs) control, a mixed control and a complete membrane control of the inter-diffusion process; (ii) the homogeneous model, even if it is convenient for a good interpretation of the bi-ionic potential (BIP) results, can not explain quantitatively the trans-membrane ionic fluxes, which are extensive quantities; and (iii) the bi-phasic model gives the best predictions for both fluxes and BIPs.

Studies of the crossed ionic fluxes through a cation-exchange membrane in the case of Donnan dialysis

Abstract This paper first presents a set of experimental data on the measurements of the ionic fluxes through a cation-exchange membrane separating two electrolyte solutions with the same concentration, the same co-ion but different counter-ions (bi-ionic system). Then these results are compared with the theoretical predictions given by the resolution of the corrected Nernst—Plancks equation in the cases of the homogeneous and heterogeneous models of ion-exchange membranes. Good agreement between the experimental results and the computed values is observed only in the case of the heterogeneous model. This confrontation confirms (1) the existence of three concentration domains: low, medium and high concentrations, corresponding, respectively, to a complete diffusion boundary layers (DBLs) control, a mixed control and a complete membrane control of the inter-diffusion process; (2) the homogeneous model, even if it is convenient for a good interpretation of the bi-ionic potential (BIP) results, cannot explain quantitatively the transmembrane ionic fluxes, which are of extensive quantities; and (3) the bi-phasic model gives the best predictions for both fluxes and BIPs.