Membrane transport polymer membranes

In some cases, conducting polymer-based ion-selective membranes can substitute the commonly used plasticized PVC-based ion-selective (liquid) membranes. However, there are fundamental differences between the two types of membranes. Conducting polymer membranes are most often non-plasticized and the charge is transported by both... 

In these devices polymer materials containing specific ingredients constitute the backbone of the film covering the electrochemical transducer. Here we deal with a liquid membrane, because the organic solvent provides the medium in which the ions permeate across the membrane. The polymer membrane ion-selective electrodes (ISE) and their ion transport across the membrane function similarly as the ion transport across the membranes of living cells (Figure 8.29). We follow the presentation given by Widmer (1993). 

Invention of several types of membranes—those for mediating redox catalyst systems for chemical synthesis, chemically stable anion exchange membranes, and polymer membranes with gradually changing structural properties along the direction of transport... 

The maximum HaS facilitated flux value corresponds to an HaS permeability of 332 x 10" cm STP)cm/ cm s kPa). Matson (8) reported a HaS permeability range of 2250-3000 x 10 cm cm/(cm s kPa) for an ILM containing aqueous solution of Ka(X), for a temperature range of 363-HO3 K. The feed gas HaS partial pressure In Matson s studies was approximately 20 kPa with a total feed pressure of 2.17 x 10 kPa. Robb (22) reported an ambient temperature HaS permeability of 638 x 10 cm cm/(cm s kPa) for a silicone rubber membrane. However, polymer membranes such as silicone rubber have much lower selectlvltles than facilitated transport membranes. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Liquid Membranes. A number of reviews summarize the considerable research effort ia the 1970s and 1980s on Hquid membranes containing carriers to faciUtate selective transport of gases or ions (58,59). Although stiU being explored ia a number of laboratories, the mote recent development of much mote selective conventional polymer membranes has diminished interest ia processes using Hquid membranes. 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

The presence of -S02(OH) groups reduced the carbon dioxide permeability by a factor of three. This can be explained (15) by the decrease in local segmental mobility of the polymer chains due to the interactions arising from hydrogen bonding. However, the overall transport process for this polymer membrane is more complicated and involves a more pronounced discrimination against methane molecules due to the highly polar nature of the polymer. 

Note that in the component mass balance the kinetic rate laws relating reaction rate to species concentrations become important and must be specified. As with the total mass balance, the specific form of each term will vary from one mass transfer problem to the next. A complete description of the behavior of a system with n components includes a total mass balance and n - 1 component mass balances, since the total mass balance is the sum of the individual component mass balances. The solution of this set of equations provides relationships between the dependent variables (usually masses or concentrations) and the independent variables (usually time and/or spatial position) in the particular problem. Further manipulation of the results may also be necessary, since the natural dependent variable in the problem is not always of the greatest interest. For example, in describing drug diffusion in polymer membranes, the concentration of the drug within the membrane is the natural dependent variable, while the cumulative mass transported across the membrane is often of greater interest and can be derived from the concentration. 

HW Osterhoudt. Transport properties of hydrophilic polymer membranes. The influence of volume fraction polymer and tortuosity on permeability. J Phys Chem 78 408-411, 1974. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

A fuel cell that has desirable features for transportation and portable power is the polymer electrolyte membrane (PEM) system. The core of this technology is a polymer membrane that conducts... 

This review has highlighted the important effects that should be modeled. These include two-phase flow of liquid water and gas in the fuel-cell sandwich, a robust membrane model that accounts for the different membrane transport modes, nonisothermal effects, especially in the directions perpendicular to the sandwich, and multidimensional effects such as changing gas composition along the channel, among others. For any model, a balance must be struck between the complexity required to describe the physical reality and the additional costs of such complexity. In other words, while more complex models more accurately describe the physics of the transport processes, they are more computationally costly and may have so many unknown parameters that their results are not as meaningful. Hopefully, this review has shown and broken down for the reader the vast complexities of transport within polymer-electrolyte fuel cells and the various ways they have been and can be modeled. 

The most important electrolyte property is ionic conductivity. For the PEFC system, water and proton transport in the polymer electrolyte occurs concurrently. Springer et al. correlated the proton conductivity (in S/cm) in the polymer membrane with its water content as follows... 

O2 consumption rate becomes smaller under 0.7 V, the O2 concentration at the reaction surface recovers, thus leading to an increase in the cell current density. The current rise time corresponds well with the characteristic time scale of gas phase transport as analyzed above. The rise in the cell current, however, experiences an overshoot because the polymer membrane still maintains a higher water content corresponding to 0.6 V. It then takes about 15 s for the membrane to adjust its water content at the steady state corresponding to 0.7 V. This numerical example clearly illustrates the profound impact of water management on transient dynamics of low humidity PEFC engines where the polymer membrane relies on reaction water for hydration or dehydration. 

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

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