Section 4.18 Membranes Electrodialysis

FIGURE 26.2 Electrodialysis purifying a feed solution. An alfemation of anion-exchange membranes and cafion-exchange membranes is placed between electrodes in a receiving solution outside the last membranes. Only a central section is shown. 

Fig. 12. Study of the mobility of a-phenylethylammonium ions within a stack of membranes before (i = 0), immediately after (l = 3 hr) electrodialysis as well as after a five day period of restacking the membranes (t = 5 days), c cation concentration in the membrane sections. The size of the circles denotes 95% confidence limits.11 72...

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

Several examples of cells employing ion-exchange membranes have been given in the section on cell construction. Ion-exchange membranes are also used for electrodialysis. 

Figure 5.25 Distribution of sulfur based on sulfonate groups through a cross-section of an anion exchange membrane before electrodialysis (analyzed by EPMA). A commercial anion exchange membrane (NEOSEPTA AM-1 strongly basic anion exchange) was immersed in an aqueous 200ppm anionic polyelectrolyte (polycondensation product of naphthalene sulfonate and formaldehyde, MW ca. 1000) solution for 17 h at 25.0 °C, washed with pure water, and dried.

Synthetic membranes with calibrated pores are used for various operations in the wine industry ultrafiltration, front-end microfiltration, tangential microfiltration and reverse osmosis. Electrodialysis and pervaporation, special separation techniques described elsewhere in this book (Section 12.5.1), also make use of membranes. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

MIP membrane adsorbers for the specific sample enrichment from large volumes by membrane SPE, and for the specific decontamination of large process streams will be among first examples for applications (cf Section V.D). Other promising continous separations are the resolution of enantiomers or the product removal from bioreactors, both feasible by electrodialysis or dialysis (cf. Section V.B). 

Section 4.15 describes membranes and introduces a range of membrane separation options. Molecular geometry is exploited in separations of gases via gas permeation. Section 4.16. Dialysis and electrodialysis are considered in Sections 4.17 and 4.18 respectively. Other methods to separate species in liquids are given in Section 4.19, pervaporation Section 4.20, reverse osmosis Section 4.21, for nanofiltration Section 4.22, for ultrafiltration Section 4.23, for microfil-tration and Section 4.24 for chromatographic separations. Separations of larger sized species are considered heterogeneous systems and are considered in Chapter 5. 

As indicated in the previous section transport across a membrane takes place when a driving force, i.e. a chemical potential difference or an" electrical potential difference, acts on the individual components in the system. The potential difference arises as a result of differences in either pressure, concentration, temperature or electrical potential. Membrane processes involving an electrical potential difference occur in electrodialysis and other related processes. The nature of these processes differs from that of other processes involving a pressure or concentration difference as the driving force, since only charged molecules or ions are affected bv the electrical field. 

Concentration polarisation is not generally severe in dialysis and diffusion dialysis because of the low fluxes involved (lower than in reverse osmosis) and also because the mass transfer coefficient of the low molecular solutes encountered is of the same order of magnitude as in reverse osmosis. In carrier mediated processes and in membrane contactors the effect of concentration polarization may become moderate mainly due to the flux through the membrane. Finally, the effect of concentration polarisation may become ver severe in electrodialysis. In the following sections concentration polarization will be described more in detail. In some module configurations such as plate-and-frame and spiral wound spacer materials are used in the feed compartment (see chapter VIII). These spacers effect the mass transfer coefficient and can be considered as turbulence promoters. 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

A third major class of physical separation is molecular separation, which is often based on membrane processes in which dissolved contaminants or solvents pass through a size-selective membrane under pressure. The products are a relatively pure solvent phase (usually water) and a concentrate rich in the solute impurities. Membrane processes including the special case of reverse osmosis to remove salts from water are discussed for the treatment of water in Chapter 5, Section 5.10. Electrodialysis, employing membranes alternately permeable to cations and to anions and driven by the passage of an electrical current (see Chapter 5, Section 5.10), is sometimes used to concentrate metal plating wastes and to reclaim dissolved metals. 

Figure 8.1.4(c) introduces through the example of dialysis (see Section 4.3.1) a countercurrent flow configuration of a membrane device where two separate feed streams tu-e entering the separator as in the separation system type (2). In the electrodialysis (Section 3.4.2.S) process of selective transport of ions through an ion exchange membrane, the liquid solutions on two sides of any ion exchange membrane are sometimes in countercurrent flow. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

The types of membrane separation technologies include reverse osmosis, hyperfiltration, ultrafiltration, and electrodialysis. At present, reverse osmosis is the only membrane separation technology that has been used as a mobile system and thus is the only such technology discussed in this section. 

A good AEM should fulfill stringent mechanical, thermal, and chemical properties as mentioned in Section 11.2. Historically, the first AEM material was developed by researchers from the Toknyama Soda Company. They introduced quaternary ammonium groups to the divinylbenzene-cross-linked polychloropropene polymer matrix via trimethylamine. Since then, several membrane-associated companies explored various kinds of AEMs and pushed them to commercial market most of them were based on cross-linked polystyrene, polyvinyl alcohol, low (or high)-density polyethylene, and other aliphatic polymers through irradiation-grafting method. The primary objective of developing these materials was for applications in the fields such as electrodialysis, desalination, selective electrode, and waste acid recovery. However, they showed performance in AEM fuel cells far below practical... 

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