Supported liquid membranes separation technique

Supported Liquid Membrane Separation Technique—THE Principle... 

Supported liquid membrane extraction techniques employ either two or three phases, with simultaneous forward- and back-extraction in the latter configuration. The aqueous sample phase is separated from the bulk organic or an aqueous receiver phase by a porous polymer membrane, in the form of either a flat sheet or a hollow fiber that has been impregnated with the organic solvent phase. The sample phase is continuously pumped, the receiver phase may be stagnant or pumped, and the organic phase in the membrane pores is stagnant and reusable [8-10]. 

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. 

In order to develop the liquid membrane techniques, i.e., emulsion Hquid membrane (ELM), supported liquid membrane (SLM), non-dispersive extraction in hollow fiber membrane (HFM), etc., for practical processes, it is necessary to generate data on equilibrium and kinetics of reactive extraction. Furthermore, a prior demonstration of the phenomena of facilitated transport in a simple liquid membrane system, the so-called bulk liquid membrane (BLM), is thought to be effective. Since discovery by Li [28], the liquid membrane technique has been extensively studied for the separation of metal ion, amino acid, and carboxyHc acid, etc., from dilute aqueous solutions [29]. 

Rathore, N.S. Sonawane, J.V. Kumar, A. Venugopalan, A.K. Singh, R.K. Bajpai, D.D. Shukla, J.P. Hollow fiber supported liquid membrane A novel technique for separation and recovery of plutonium from aqueous acidic wastes, J. Membr. Sci. 189 (2001) 119-128. 

Schulz, G. (1988) Separation techniques with supported liquid membranes. Desalination, 68, 191. 

Nonporous membrane techniques involve two or three phases separated by distinct phase boundaries. In three-phase membrane systems, a separate membrane phase is surrounded by two different liquid phases (donor and acceptor) forming a system with two phase-boundaries and thus two different extraction (partition) steps. These can be tailored to different types of chemical reactions, leading to a high degree of selectivity. The membrane phase can be a liquid, a polymer, or a gas, and the donor and acceptor phases can be either gas or hquid (aqueous or organic). Liquid membrane phases are often arranged in the pores of a porous hydrophobic membrane support material, which leads to a convenient experimental system, termed supported liquid membrane (SLM). There are several other ways to arrange a hquid membrane phase between two aqueous phases as described below. 

Supported liquid membrane stability and lifetime limit the industrial application of this separation technique. Therefore, the stability of these membranes needs to be enhanced drastically. A proper choice of the operating and membrane composition factors might improve the lifetime of SLM systems. 

Promising results are shown by recently developed integrated SLM-ELM [84, 85] systems. These techniques are known as supported liquid membrane with strip dispersion (SLMSD), pseudo-emulsion-based hollow fiber strip dispersion (PEHFSD), emulsion pertraction technology (EPP), and strip dispersion hybrid Hquid membrane (SDHLM). AH techniques are the same the organic phase (carrier, dissolved in diluent) and back extraction aqueous phase are emulsified before injection into the module and can be separated at the module outlet. The difference is only in the type of the SLM contactors hoUow fiber or flat sheet and in the Hquid membrane (carrier) composition. These techniques have been successfuUy demonstrated for the removal and recovery of metals from wastewaters. Nevertheless, the techniques stiU need to be tested in specific apphcations to evaluate the suitabUity of the technology for commercial use. 

If solvent extraction may be considered a source technique, derived liquid-liquid separation techniques include configurations in which an extraction solvent is physically immobilized by a coating or impregnation process onto a solid support such as silica, porous resin beads, or foam [13,84—87]. Other derived techniques include membranes of various configurations bulk liquid membranes, supported liquid membranes, emulsion membranes, and polymer-impregnated membranes [88]. Many derived liquid-liquid techniques have been developed, especially for use in analytical applications [13,60,62,64,75,84,85,87]. In each of these derived techniques, the... 

Chimuka, L., Cukrowska, E., Soko, L. Naicker, K. (2003) Supported-liquid membrane extraction as a selective sample preparation technique for monitoring uranium in complex matrix samples. Journal of Separation Science, 26, 601-608. 

The utility of a thin layer of liquid as a selective membrane for separations has been explored extensively over the last thirty years. Three techniques have been used traditionally to exploit a thin liquid layer as a membrane emulsion liquid membranes (ELM) for separation of liquid solutions (7) supported liquid membranes (SLM) in the pores of a porous/microporous support membrane for the separation of liquid feeds and immobilized liquid membranes (ILM) in the pores of a porous/microporous support membrane for separating a gas mixture. SLM and ILM are different names for the same liquid membrane technique and have been briefly reviewed by Majumdar et aL (2) and Boyadzhiev and Lazarova (i). 

The enantiomer separation with the use of membranes is a promising technique, more convenient than traditional methods, due to its high processing capacity, continuous operation mode and low energy consumption allowing its use in a large scale enantiomer separation processes. Supported liquid membranes show high chiral selectivity, however they are not stable on the contrary, solid membranes with immobilized chiral carrier polymer are stable and therefore able to a durable enantiomer separation [100]. 

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. 

Another emerging technique which deserves mention is the use of a supported ionic liquid membrane. This involves two liquid phases that both contain an enzyme and are separated by the membrane. Lipase-catalyzed esterification takes place in the feed phase to afford a mixture of the (R)-acid and the (S)-ester (Figure 10.22). The latter diffuses through the membrane and is hydrolyzed in the receiving phase to afford the (S)-acid [151, 152]. The methodology has been applied, for example, to the resolution of ibuprofen [151]. 

---