Liquid separation membranes

With regard to the enantioselective transport through the membrane, one advantage of liquid membrane separation is the fact that the diffusion coefficient of a solute in a liquid is orders of magnitude higher as compared to the diffusion coefficient in a solid. The flux through the membrane depends linearly on the diffusion coefficient and concentration of the solute, and inversely on the thickness of the membrane [7]. 

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. 

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. 

Huang, D.S., Huang, K.L., Chen, S.P. et al. (2008) Rapid reaction-diffusion model for the enantioseparation of phenylalanine across hollow fiber supported liquid membrane. Separation Science and Technology, 43 (2), 259-272. 

Dreher, T.M. and Stevens, G.W. (1998) Instability mechanisms of supported liquid membranes. Separation Science and Technology, 33, 835. 

FIGURE 35 Emulsified liquid membrane separation mechanisms (A) selective permeation (B) chemical reaction inside emulsion droplet and (C) chemical conversion in liquid membrane and further conversion inside droplet. Both (B) and (C) provide quasi-infinite sink conditions for extraction from the feed solution. 

A liquid membrane consists of a solvent immiscible with water and a reagent that acts as extractant and complexing agent for an ion. If such a liquid membrane separates two solutions, ion selectivity is achieved through preferential extraction of... 

Basu R and Sirkar KK. Hollow fiber contained liquid membrane separation of citric acid. AIChE J, 1991 37(3) 383-393. 

Borwanker RP, Chan CC, Wasan DT, Kurzeja RM, Gu ZM, and Li NN. Analysis of the effect of internal phase leakage on liquid membrane separations. AIChE J 1988 34 753-762. 

Rautenbach R and Machhammer O. Modeling of liquid membrane separation processes. J Membr Sci 1988 36 425-444. 

Zhang XJ, Fan QJ, Zhang XT, and Liu ZF. New surfactant LMS-2 used for industrial application in liquid membrane separation. In Li NN, Strathmann H, eds. Separation Technology, New York United Engineering Trustees, 1988 215-226. 

Feng ZL, Wang XD, and Zhang XJ. A new high voltage electrostatic coalescer EC-1 applied to liquid membrane separation. Water Treat 1988 3 320-328. 

Sastre, A.M. et al., Improved techniques in liquid membrane separations, Sep. Purif. Methods 27, 213, 1998. 

Christensen, J.J., Lamb, J.D., Brown, P.R., Oscarson, J.L., and Izatt, R.M., Liquid membrane separation of metal cations using... 

Liquid-liquid methods include solvent extraction with immiscible liquid-liquid systems in which a suitable ligand is dissolved in an organic phase and contacted with a metal ion containing an aqueous phase and liquid membranes. Separations can also be achieved with pseudo-phase systems such as micelles, microemulsions, and vesicles. Such separations can be solid-liquid or liquid-liquid and include separations with normal- and reversed-phase silica, and polymeric supports where the mobile phase contains the organized molecular assembly (OMA) of micelles, microemulsions, or vesicles. Separation of metal ions using the pseudo-phase systems is stiU in its infancy and a brief account will be provided here. 

Turning to the other aspect of this discussion, today s membranes perform a myriad of separations utilizing one of three mechanisms that is, mechanical sieving, controlled diffusion, and adsorption, with the first two mechanisms dominating. Driving forces for these separations can be pressure, concentration differential, electrical potential differential or pH differential (a relatively new driving force used in liquid membrane separations). Exemplary applications are ... 

The commonly accepted mechanism for the transport of a solute in LM is solution-diffusion. The solute species dissolve in the liquid membrane and diffuse across the membrane due to an imposed concentration gradient. Different solutes have different solubilities and diffusion coefficients in a LM. The efficiency and selectivity of transport across the LM may be markedly enhanced by the presence of a mobile complexation agent (carrier) in the liquid membrane. Carrier in the membrane phase reacts rapidly and reversibly with the desired solute to form a complex. This process is known as facilitated or carrier-mediated liquid membrane separation. In many cases of LM transport, the facilitated transport is combined with coupling counter- or cotransport of different ions through LM. The coupling effect supplies the energy for uphill transport of the solute. 

Liquid membrane separation is a rate process and the separation occurs due to a chemical potential gradient, not by equilibrium between phases. 

An attempt to unify the mass transport phenomena of liquid membrane separation underlying the basic LM configurations was presented in this chapter. The basic theory was developed in a simple physical-chemical-mathematical form and applied to the principal techniques in such a way to obtain comparable methods. Of course it is prehminary work once we start forging links between different methods there wiU be spiUover to further possibilities of integration. 

Supported Liquid Membrane Separation Technique—THE Principle... 

Liquid membrane separation combines the solvent extraction and stripping processes (re-extraction) in a single step. The great potential for energy saving, low capital and operating cost, and the possibility to use expensive extractants, due to the small amounts of the membrane phase, make SLMs an area deserving special attention. 

Dzygiel, P., Wieczorek, P., Kafarski, P. (2003). Supported liquid membrane separation of amine and amino acid derivatives with chiral esters of phosphoric acids as carriers. J. Sep. Sci., 26, 1050-6. 

Ghosh, A. C., Bora, M. M., Dutta, N. N. (1996). Developments in liquid membrane separation of beta-lactam antibiotics. Bioseparation, 6, 91-105. 

Distributed resistance models for liquid membrane separations... 

Liquid membrane separation processes have received considerable attention because of their potential advantages over other separation processes, both conventional (distillation, solvent extraction) and newly developed (sohd membranes). 

MHS with pervaporation of water from LM (MHS-PV) is presented in Fig. 5.9. Contrary to the simple MHS with an agitated bulk liquid membrane, separated from the feed and strip solutions by flat hydrophobic or hydrophilic or ion-exchange membranes, the MHS-PV system exploits a Hquid membrane continuously flowing between the two flat cation-exchange and two pervaporation membranes. To couple the separation and pervaporation processes, the LM is simultaneously pumped through the MHS and PV modules. The pervaporation membranes are placed on stainless steel porous supports. Aqueous feed and strip solutions are intensively agitated. 

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