Supported liquid membrane extraction principle

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

The production process for (S)-phenylalanine as an intermediate in aspartame perpetuates the principle of reracemization of the nondesired enantiomer (Figure 4.5) in a hollow fiber/ liquid membrane reactor. Asymmetric hydrolysis of the racemic phenylalanine isopropylester at pH 7.5 leads to enantiopure phenylalanine applying subtilisin Carlsberg. The unconverted enantiomer is continuously extracted via a supported liquid membrane [31] that is immobilized in a microporous membrane into an aqueous solution of pH 3.5. The desired hydrolysis product is charged at high pH and cannot, therefore, be extracted into the acidic solution [32]. 

The principle of a three-phase membrane extraction is illustrated in Figure 1.28. An organic solvent is immobilized in the pores of a porous polymeric support consisting of a flat filter disc or a hollow fiber-shaped material. This supported liquid membrane (SLM) is formed by treating the support material with an organic solvent that diffuses into its pores. The SLM separates an aqueous... 

In this paper an overview of the developments in liquid membrane extraction of cephalosporin antibiotics has been presented. The principle of reactive extraction via the so-called liquid-liquid ion exchange extraction mechanism can be exploited to develop liquid membrane processes for extraction of cephalosporin antibiotics. The mathematical models that have been used to simulate experimental data have been discussed. Emulsion liquid membrane and supported liquid membrane could provide high extraction flux for cephalosporins, but stability problems need to be fully resolved for process application. Non-dispersive extraction in hollow fib er membrane is likely to offer an attractive alternative in this respect. The applicability of the liquid membrane process has been discussed from process engineering and design considerations. 

Classically, flat-sheet porous PTFE or polypropylene membranes are used as support for the membrane liquid and mounted in holders (cells, contactors) permitting one flow channel on each side of the membrane [1,3,6,8,25]. See Figure 12.1. Such membrane units are typically operated in flow systems and in principle apphcable to aU versions of membrane extraction for analytical sample preparation or sampling. Such a setup can be easily interfaced with different analytical instmments, such as HPLC and various spectrometric instmments, and thereby provides good possibdities for automated operation. Drawbacks of this type of devices are relatively large costs and limited availability, as well as some carryover and memory problems as the membrane units are utilized many times, necessitating cleaning between each extraction. 

The separation of the two metals will he primarily determined by the ratio of the two distribution coefficients, kmio/k o. exactly as in solvent extraction. Separation of copper from nickel using L1X65N in the liquid membrane using these principles has been studied by Lee et al. (1978). A general review of metal extraction using liquid membranes supported in microporous polymeric membranes is provided by Danesi (1984-85). 

The same principle of operation as described above is applicable also to liquid-liquid extraction where an aqueous liquid and an organic liquid contact each other inside the contactor for extraction of a solute selectively from one phase to another [6-8]. The critical breakthrough pressure for liquid-liquid system could be calculated by Equation 2.1, except that the term A would now be the interfacial tension between the two liquids. Further variation of membrane contacting technology is called gas membrane or gas-gap membrane where two different liquid phases flow on either side of the membrane, but the membrane pores remain gas filled [9-10]. In this situation two separate gas-hquid contact interfaces are supported on each side of a single membrane. 

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