OTHER MEMBRANE PROCESSES

Any book must leave something out, and this one has left out a good deal it does not cover membranes used in packaging materials, sensors, ion-selective electrodes, fuel cells, battery separators, electrophoresis and thermal diffusion. In this final chapter, five processes that come under the general title of other are covered briefly. 

Dialysis was also used in the laboratory in the 1950s and 1960s, mainly to purify biological solutions or to fractionate macromolecules. A drawing of the 

Membrane Technology and Applications R. W. Baker 2004 John Wiley Sons, Ltd ISBN 0-470-85445-6 

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. 

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Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

Electrodialysis has advantages and disadvantages. For instance, the process requires very little energy and can recover highly concentrated solutions. On the other hand, similarly to other membrane processes, electrodialysis membranes are susceptible to fouling and must be regularly replaced. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Semipermeable Membrane Devices and Other Membrane Processes. 57... 

Other membrane processes such as microfiltration, ultrafiltration, reverse osmosis, and colloid-enhanced ultrafiltration have been applied to the separation of beta-cypermethrin from wastewater samples [27]. In this study, a separation of above 92% was performed by reverse osmosis by the use of composite membranes and above 80% by colloid-enhanced ultrafiltration by the use of nonionic surfactants. 

Depending on the enrichment term (E0) of the membrane, the modulus can be larger or smaller than 1.0. For reverse osmosis E0 is less than 1.0, and the concentration polarization modulus is normally between 1.1 and 1.5 that is, the concentration of salt at the membrane surface is 1.1 to 1.5 times larger than it would be in the absence of concentration polarization. The salt leakage through the membrane and the osmotic pressure that must be overcome to produce a flow of water are increased proportionately. Fortunately, modem reverse osmosis membranes are extremely selective and permeable, and can still produce useful desalted water under these conditions. In other membrane processes, such as pervaporation or ultrafiltration, the concentration polarization modulus may be as large as 5 to 10 or as small as 0.2 to 0.1, and may seriously affect the performance of the membrane. 

The formation of concentration gradients caused by the flow of ions through a single cationic membrane is shown in Figure 10.8. As in the treatment of concentration polarization in other membrane processes, the resistance of the aqueous solution is modeled as a thin boundary layer of unstirred solution separating the... 

The first widespread use of polymeric membranes for separation applications dates back to the 1960-70S when cellulose acetate was cast for desalination of sea and brackish waters. Since then many new polymeric membranes came to the market for applications extended to ultrafiltration, miciofiltration, dialysis, electrodialysis and gas separations. So far ultrafiltration has been used in more diverse applications than any other membrane processes. The choice of membrane materials is dictated by the application environments, the separation mechanisms by which they operate and economic considerations. Table 1.4 lists some of the common organic polymeric materials for various membrane processes. They include, in addition to cellulose acetate, polyamides. 

Selective separation of hquids by pervaporation is a result of selective sorption and diffusion of a component through the membrane. PV process differs from other membrane processes in the fact that there is a phase change of the permeating molecules on the downstream face of the membrane. PV mechanism can be described by the solution-diffusion mechanism proposed by Binning et al. [3]. According to this model, selective sorption of the component of a hquid mixture takes place at the upstream face of the membrane followed by diffusion through the membrane and desorption on the permeate side. 

Membrane processes, in general, are very attractive for their simplicity and flexibility. They are capable of achieving separations at a molecular level. Membrane modules are often compact and easily scaleable. For clarification and concentration, microfiltration, ultrahltration, and reverse osmosis are the current methods of choice. RO has been widely used in the food industries as an attractive alternative to classical evaporahon the only hmitahon being its dependence on osmotic pressure, which practically limits concentration of fluid streams to 25°Bx-30°Bx. Hence, currently it is used more as a preconcentration step. In recent years, membrane processes, notably pervaporahon, membrane dishUahon and osmotic membrane distillation (OMD) [21], have been used either by themselves or in combinahon with other membrane processes to overcome the problems associated with thermal processes. 

Though MD has provoked intense research activity that has led to numerous publications and patents, it finds relatively fewer commercial applications as a stand-alone process, as compared with other membrane processes. 

Although CFD has been widely used to model blood pumps and other membrane processes, its application in membrane BOs is very limited. Development of a CFD model requires simplifying assumptions. In their model. Baker et al. assumed that blood was statistically homogeneous the reaction between oxygen and hemoglobin was fast axial diffusion of gas was neglected ... 

To overcome the problems with SLM stability, the idea of their integration with other membrane processes was also investigated. Two approaches can be distinguished. Both could lead to significant increase ot the hquid membrane lifetime. One approach is to separate the hquid membrane from the feed and receiving phases. It can be achieved by placing hquid... 

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