Liquid membranes applications

Eyal A and Bressler El. Industrial separation of carboxylic and amino acids by liquid membranes Applicability, process considerations, and potential advantages. Biotechnol Bioeng, 1993 41 2S1-293. 

Water reclamation, the treatment of wastewater to meet the water quality standards of various applications economically, is becoming increasingly important in view of the increasing world population and scarcity of fresh water sources. The major technology used for water reclamation is membrane technology. This entry gives an overview of the major membrane types used for water reclamation reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and liquid membranes. Applications of these membranes in municipal and industrial wastewater reclamation have been described. 

The porosity of the support refers to the percentage of the total volume which is void space. The porosity determines the total volume of the liquid membrane which can be immobilized in the pore volume. The volume of liquid membrane solvent and the carrier solubility determine the maximum amount of carrier which can be immobilized in the membrane. Increasing the amount of carrier in the membrane will increase the solute fluxes. The strength of the functional dependence of the solute flux on carrier concentration will depend on whether the facilitated transport system is reaction or diffusion limited. Consequently, a high porosity support is desirable for liquid membrane applications. 

Permeation is a general term for the concentration-driven membrane-based mass transport process. Differences in the permeabiUty produce a separation between solutes at constant driving force. Because the diffusion coefficients in liquids are typically orders of magnitude higher than in polymers, a larger flux can be obtained with liquid membranes. Application of a pressure difference, an electric held, or temperature considerably intensifies the process, but these special methods are beyond the scope of this book. 

The apparatus used for this study produced low transport rates for C02, as well as the previously discussed 02, with and without liquid membranes compared with developed oxygenators. The reason for this slow transport is the very large (approximately 0.4 cm) liquid membrane encapsulated bubbles contrasted with the small bubbles of developed oxygenator. A means is needed to produce small fluorocarbon liquid membranes in blood so that the rapid transport achieved in other liquid membrane applications using small diameter liquid membranes can be achieved for transferring gases to and from blood. 

The specific rate of oxygen transfer per unit of liquid membrane area seems to be quite reasonable. However, methods to form and utilize effectively much smaller diameter liquid membranes, perhaps similar to those used in other liquid membrane applications, would be required to obtain enough membrane area per unit blood volume for a practical blood oxygenator. The stability of the liquid membranes does not seem to be a major problem however, more definitive liquid membrane stability information would be required before the blood oxygenator application. 

This volume Is divided Into three sections theory, carrier chemistry, and applications. The theory section Includes chapters which thoroughly describe the theory and analysis of various liquid membrane types and configurations (107-110) The carrier chemistry section contains two articles on the use of macrocycles for cation separations (111-112). The applications section begins with a survey article which thoroughly reviews the liquid membrane applications In the literature and discusses both potential and commercial aspects of liquid membrane technology. The remaining articles discuss both gas phase (113-115) and liquid phase transport (116-117). 

Seminar on "Liquid membrane applications in waste water treatment and metals recovery." UMIST, England (1980). 

Li, N.N., Proc. of Conf. on Liquid Membrane Applications in Waste Water Treatment and Metals Recovery. UMIST (1980) 9. 

Apart from the work conducted by Li et al. at Exxon which is fully reported in the proceedings of a seminar on Liquid Membrane Applications in Waste Water Treatment and Metals Recovery held at UMIST in May 1980, investigations on applications of liquid membranes to metals recovery are conducted at a number of other places. Thus, extraction of copper is studied at UMIST (11, 12), Graz (13-15) and Bend Research (16). The last place is also active in the use of membranes for the extraction of uranium (17), whereas work on the extraction of copper, zinc, cadmium and lead is conducted by Boyadzhiev et al. (18). Extraction of different metals has also been studied by Stelmaszek (19) and Strzelbicki (20, 21) and of zinc alone at Imperial College (7). All the accumulated data point towards membrane extraction as a promising process for the solution of separation problems, particularly in dilute solutions. 

Extractant Name Structure Liquid membrane application (Ref.)... 

F igure 8.3. Schematic representation of a hollow fiber contactor set up used for supported liquid membrane applications. Flow directions in a hollow fiber module (a), single fiber (b), and hollow fiber set up for simultaneous extraction and stripping. (Reproduced with permission from Ansari etal, 2011a). 

Carrier-facilitated transport of actinides across bulk, supported, and emulsion liquid membranes, as well as plasticized membranes and recently developed emulsion-free liquid membranes, are reviewed. The discussion includes the effects of important experimental variables upon the solute flux for various types of liquid membranes. Applications of liquid membranes in the recovery and removal of radiotoxic actinides from the nitric acid wastes generated during reprocessing of spent fuel by the PUREX process and wastes produced by other radiochemical operations are surveyed. 

Liquid membranes are a specialty, either adsorbed in capillaries or erniilsiFied. Thev are much studied, but little commercial application is Found. 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

L. J. Brice, W. H. Pirkle, Enantioselective transport through liquid membranes in Chiral separations, applications and technology, S. Ahuja (Ed.), American Chemical Society, Washington... 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

In general, high selectivities can be obtained in liquid membrane systems. However, one disadvantage of this technique is that the enantiomer ratio in the permeate decreases rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed would be an approach to tackle this problem or, alternatively, using a system containing the two opposite selectors, so that the feed stream remains virtually racemic [21]. Another potential drawback of supported enantioselective liquid membranes is the application on an industrial scale. Often a complex multistage process is required in order to achieve the desired purity of the product. This leads to a relatively complicated flow scheme and expensive process equipment for large-scale separations. 

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

As described above, the application of classical liquid- liquid extractions often results in extreme flow ratios. To avoid this, a completely symmetrical system has been developed at Akzo Nobel in the early 1990s [64, 65]. In this system, a supported liquid-membrane separates two miscible chiral liquids containing opposite chiral selectors (Fig. 5-13). When the two liquids flow countercurrently, any desired degree of separation can be achieved. As a result of the system being symmetrical, the racemic mixture to be separated must be added in the middle. Due to the fact that enantioselectivity usually is more pronounced in a nonaqueous environment, organic liquids are used as the chiral liquids and the membrane liquid is aqueous. In this case the chiral selector molecules are lipophilic in order to avoid transport across the liquid membrane. 

In the short term, we do not expect chiral membranes to find large-scale application. Therefore, membrane-assisted enantioselective processes are more likely to be applied. The two processes described in more detail (liquid-membrane fractionation and micellar-enhanced ultrafiltration) rely on established membrane processes and make use of chiral interactions outside the membrane. The major advantages of these... 

Two-phase liquid systems or liquid membranes have found applications in various separation technologies. Liquid membranes have an advantage over traditional... 

One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

Liquid Membranes Several types of liquid membranes exist molten salt, emulsion, immobilized/supported, and hollow-fiber-contained liquid membranes. Araki and Tsukube (Liquid Membranes Chemical Applications, CRC Press, 1990) and Sec. IX and Chap. 42 in Ho and Sirkar (eds.) (op. cit., pp. 724, 764-808) contain detailed information and extensive bibliographies. 

The voltammetric method and concept are expected to be applicable also to the analysis of the oscillation at a very thin membrane such as a bilayer lipid membrane, since even the ion transfer through a bilayer lipid membrane can be observed as a vol-tammogram and interpreted by the way similar to that for a liquid membrane [20,21]. 

Improved ISEs. In 1980, Ammann et al.154 reported on clinical and biological applications of liquid membrane electrodes based on an ion-selective component and a suitable plasticizer in a PVC matrix for the determination of Na +, K, Ca2, Cl and H + in blood serum or whole blood and Na+ and K4 in urine. They gave extensive information on membrane compositions and selectivity... 

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