Supported liquid membranes advantage

A simulation of the hybrid fermentation-pertraction process for production of butyric acid shows that the pH of fermentation and pertraction should be optimized independently [198]. It is advantageous to have the pH of the feed into pertraction at about 4.0 for both IL and TOA carriers. Choosing a proper carrier in the supported liquid membrane between IL and TOA should be made according to actual operation conditions, because of the different transport properties of these carriers in respect to the concentration of undisociated form of BA. While at lower BA concentrations the IL is better, at higher concentrations of above 20kgm 3 and pH equal to 4.0, the membrane area needed is lower for TOA. An important factor will be the toxicity of the carrier to biomass. TOA is not very good in this respect and data for IL used are not available, but it is hoped that IL will be less toxic. 

A supported liquid membrane (SLM) process has been considered, among other possible options, for the reiiK>val of contaminants from groundwater, because of the following advantages of SLM s over competing techniques (solvent extraction, ion exchange, polymeric membrane processes, etc.) ... 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

This architecture (Fig. 8.11) was dominant during the nineteen seventies and eighties, and was also exploited when LLE was implemented in flow injection analysis. Nowadays, it is still widely used, as emphasised in recent comprehensive reviews [153,161]. Further innovations involving the use of a supported liquid membrane between two aqueous phases [162] and /or taking advantage of the high versatility of the unsegmented flow systems have also been proposed. 

The use of two types of liquid membranes is described in [302] liquid emulsion membranes (LEMs), and supported liquid membranes (SLMs), where isoparaffin or kerosene and their mixtures were used as organic phases. A surfactant of the type of Span 80 served as emulsifier. LEMs are used, for example, for selective separation of L-phenylalanine from a racemic mixture of L-leucine biosynthesis as well as conversion of penicillins to 6-APA (6-aminopenicillanic acid). SLMs have a higher stability. A number of their commercial applications have been studied, e.g. in separation of penicillin from fermentation broth, as well as in the recovery of citric acid, lactic acid and some aminoacids. Compared with other separation methods (ultrafiltration, ultracentrifugation and ion exchange), LEMs and SLMs are advantageous in the separation of stereospecific isomers in racemic mixtures. 

The removal of actinides from reprocessing acidic waste solutions is advantageous in terms of minimizing the radioactive discharge to the natural environment. The separation of plutonium using supported liquid membranes was extensively studied, as well as U(V1) and Pu(lV) selective transport over fission products and minor actinide contaminants (Lakshmi et al. 2004 Sriram et al. 2000 Kedari et al. 1999). 

These have the advantage of high membrane flux, which results from the very small thickness of the organic membrane. However, there are a number of operational difficulties. The first of these concerns the osmotic transport of water across the membrane as a result of different ionic concentrations in the two aqueous phases. This causes the membrane drops to swell and ultimately to break down, mixing the strip and feed solutions. Another difficulty arises with the ultimate breaking of the emulsion and separation of the two phases that can give rise to entrainment problems. In addition, the overall process is much more complex than that of the supported liquid membrane. 

Other techniques have been developed to improve upon SX among them of particular interest are liquid membranes with the main t)q)es of these membranes being bulk liquid membranes (BLMs), emulsion hquid membranes (ELMs) and supported liquid membranes (SLMs) (Kolev, 2005) (Fig. 10.1). While these all have advantages compared to SX systems, they have not yet achieved wide eommercial acceptance. The following paragraphs present a brief deseription of the principles utilized by BLMs, ELMs and SLMs. For more information about liquid membranes please refer also to Chapters 7 and 8 of this volume. 

A side-by-side comparison of coarse and micro- emulsions for use as liquid membranes indicates that each possesses unique advantages. The microemulsion displays faster rates of separation yet more difficult demulsification than the coarse or macro- emulsion system. Both systems suffer from swell. A new method of contacting emulsion liquid membranes with the feed solution minimizes swell while maintaining high separation flux. The key advantage of the HFC contactor lies in its ability to stabilize the liquid membrane from leal ge. Thus, surfactant concentration can be minimized and swell essentially eliminated. The system is much like a supported liquid membrane but will not produce short circuits due to solvent loss since the solvent is continuously supplied on the emulsion side of the membrane. Our lab is currently characterizing such systems. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Coupled transport with supported and emulsion liquid membranes has made very little real progress towards commercialization in the last 15 years. In addition, it is now apparent that only a few important separation problems exist for which coupled transport offers clear technical and economic advantages over conventional technology. Unless some completely unexpected breakthrough occurs, it is difficult to imagine that coupled transport will be used on a significant commercial scale within the next 10-20 years. The future prospects for coupled transport are, therefore, dim. 

Facilitated transport has been briefly described in Chapter 1. In facilitated transport, the selective transport medium is a liquid or molten salt contained or immobilized in a porous support. The liquid membrane is held tightly in the support pores by capillary forces. The liquid or molten salt selectively reacts with a gas or vapor species and the reacting species diffuses across the liquid or salt and desorbed on the other side of the facilitated transport membrane. The major advantage of the facilitated transpoa is that diffusion is generally several orders of magnitude faster than diffusion through solid membranes. The support is, therefore, not a membrane by definition. Comprehensive... 

By judicious choice of the membrane liquid, complexation agent and support, immobilized liquid membranes (ILM) can have both high selectivity and high permeant fluxes. Liquid membranes have the additional advantage that diffusion coefficients in liquids are several orders of magnitude larger than in polymeric membranes. Previously reported ILM research in the literature includes purification and recovery processes in both gas and liquid phases ( ). This variety of applications creates different requirements for supports for ILMs. This paper discusses criteria which influence selection of ILM support materials. 

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