Permeability constant, composite membranes

In order to interpret the physicochemical steps of retinal transduction as well as membrane excitability, we analyze macroscopic properties of membranes within biological components. Such membranes separate two aqueous ionic phases the chemical compositions of which are kept constant separately. The total flux through the membrane is directly deduced from the counterbalance quantities in order to maintain the involved thermodynamical affinities constant. From such measurement, we calculate the dynamical membrane permeability. This permeability depends not only on membrane structure but also on internal chemical reactions. 

Discrete and continuum models of transfer of molecules over various sorption sites of a microheterogeneous membrane were considered for systems with weak intermolecular interactions and membranes with constant composition and structure. An equation for estimating size effects on permeability coefficient II of microheterogeneous membranes was derived [188], and the possibility of applying the continuum model to calculate the n value in thin films of thickness L is numerically analyzed. The effect of the composition and structure of a uniformly microheterogeneous membrane on the permeability coefficients II was studied. The dependence of n on the composition is a convex function if the migration between different sorption sites proceeds more quickly than between identical sites and a concave one in the opposite case [189],... 

The combination of two or more membranes in series results in a composite membrane. The permeability constant P for a composite membrane may be expressed (39)... 

The permeability constant of the composite membrane is therefore represented by the harmonic average of the permeability constants of the individual layers, the respective weights being x /Ji., the ratio of layer thickness to the total. Although composite membranes Include layers of dense films or even liquid layers in series with films, in this discussion the term is being limited to those series in which at least one of the members is a phase inversion membrane of either the integrally-skinned or skinless variety. 

The concept of the pH electrode has been extended to include other ions as well. Considerable research has gone into the development of these ion-selective electrodes over the years, especially in studying the composition of the membrane that separates the internal solution from the analyte solution. The internal solution must contain a constant concentration of the analyte ion, as with the pH electrode. Today we utilize electrodes with 1) glass membranes of varying compositions, 2) crystalline membranes, 3) liquid membranes, and 4) gas-permeable membranes. In each case, the interior of the electrode has a silver-silver chloride wire immersed in a solution of the analyte ion. 

The effect of annealing temperatures (65 - 250 °C) and blend composition of Nafion 117, solution-cast Nafion , poly(vinyl alcohol) (PVA) and Nafion /PVAblend membranes for application to the direct methanol fuel cell is reported in [148], These authors have found that a Nafion /PVAblend membrane at 5 wt% PVA (annealed at 230 °C) show a similar proton conductivity of that found to Nafion 117, but with a three times lower methanol permeability compared to Nafion 117. They also found that for Nafion /PVA (50 wt% PVA) blend membranes, the methanol permeability decreases by approximately one order of magnitude, whilst the proton conductivity remained relatively constant, with increasing annealing temperature. The Nafion /PVA blend membrane at 5 wt% PVA and 230 °C annealing temperature had a similar proton conductivity, but three times lower methanol permeability compared to unannealed Nafion 117 (benchmark in PEM fuel cells). 

The ratio of the permeabilities of two cations in a cation exchange membrane is equal to the product of the ion exchange equilibrium constant and their mobility ratio (1). Therefore it is important to characterize the equilibrium ion exchange selectivity of ion exchange polymers in order to understand their dynamic properties when used in membrane form. Nafion (E.I. du Pont de Nemours and Co.) perfluorinated sulfonate membranes have found wide use in a variety of applications, many of which involve exchange of cations across membranes that separate solutions of different ionic composition. The inherent cationic selectivity of the polymer is an important consideration for such applications. Results of ion exchange selectivity studies of Nafion polymers are reviewed in this chapter, and are compared to those of other sulfonate ion exchange polymers. 

Membrane processes, lika other unit operations, should be approached with a systematic design procedure supported by a solid data hese. Because of their neveity, Ibe del a hese for membrane processes is still mnch smaller lhan thei for corresponding older unit operations such as distillation. The current lack of information often necessitates the estimation and use of constant values for component permeabilities although thase coalficients are kaown to be functions of pressure and gas composition in many cases.28,29... 

It is agreed that the permeabilities of the components may not be constant, but rather depend on a number of factors, such as the retentate and/or permeate composition(s), and mass transfer through the membrane can be quite complex. However, for demonstration purposes, a simplified approach will be followed. For this reason, constant relative permeabilities have been assumed, resulting in what is known as a Knudsen membrane [12]. 

The goal of the model for membrane unit for gas separation is to predict the flow rate and composition of retentate and permeate streams, for a given feed stream containing n components, membrane type and area, and permeate pressure. Here, the process boundary and variables are limited to one of the membrane modules shown in Figure 4.5. In this section, the solution-diffusion mechanism is used to predict the separation behavior of the membrane. In the development of a membrane model, it is assumed that the process is at steady state, pressure is constant on feed side, and permeability of a component through the... 

Assumptions used and ideal flow patterns. In deriving theoretical models for gas separation by membranes, isothermal conditions and negligible pressure drop in the feed stream and permeate stream are generally assumed. It is also assumed that the effects of total pressure and/or composition of the gas are negligible and that the permeability of each component is constant (i.e., no interactions between different components). 

---