Emulsion liquid membranes advantages

All the novel separation techniques discussed in this chapter offer some advantages over conventional solvent extraction for particular types of feed, such as dilute solutions and the separation of biomolecules. Some of them, such as the emulsion liquid membrane and nondispersive solvent extraction, have been investigated at pilot plant scale and have shown good potential for industrial application. However, despite their advantages, many industries are slow to take up novel approaches to solvent extraction unless substantial economic advantages can be gained. Nevertheless, in the future it is probable that some of these techniques will be taken up at full scale in industry. 

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. 

Although microemulsions offer many potential advantages when used as emulsion liquid membranes, their effective usefulness in an industrial setting is questionable. Studies on a variety of systems have shown some disadvantages of microemulsions as opposed to coarse emulsions ... 

This paper reviews the use of emulsions and microemulsions as liquid membranes with sp ial emphasis placed on the separation of mercury, as Hg(N03)2, from water using oleic acid as the extractant Although emulsion (either macro- or micro-) liquid membranes offer advantages in terms of fast rates of separation, new modes of creating a stabilized liquid membrane utilizing hollow fiber contactors offer comparable flux in a more stable format. The paper wiU start with a review of the basic types of liquid membranes as currently used in research. The discussion will then focus on the author s experience with emulsified liquid membrane systems. The last section of the paper will discuss the obvious next step in liquid membrane technology, the use of emulsion liquid membranes in hollow fiber contactors. 

Much effort has been expended in our labs over the last few years investigating the use of emulsion liquid membranes to carry out such wastewater treatment schemes with a special focus on the removal of mercury ions from water. Both coarse or macroemulsions as well as microemulsions were studied and compared. The advantage of emulsion liquid membrane extraction is the large surface area available for mass transfer which results in fast separations. Because the volume ratio of the feed to internal receiving phase is high, the separated metal is concentrated by factors as high as... 

Summary of Significant Shortcomings. The emulsion liquid membranes studied each possess advantages when compared against one another. In specific, the microemulsion systems displayed separation kinetics which were typically an order of magnitude faster than the macroemulsion counterparts. However, tlm increased rate of separation was at the expense of product recovery. The concentrated mercury was easily recovered by electrostatic demulsification for the macroemulsion system, but required the addition of butanol to the microemulsion before mercury recovery was achieved. The requirement of chemical demulsification is a major disadvantage of the microemulsion-based liquid membrane system. 

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. 

The advantages and disadvantages of membrane based processes and pertraction through various types of liquid membranes are summarized in Table 23.5. HF contactors are supposed in these processes with the exception of pertraction into stable emulsions (ELM) where mixed column contactors or mixer-settlers are used. 

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 currently existing difficulties in handling EL membranes, associated mainly with the formation and breakdown of the emulsion itself, have prevented the widespread use of this type of liquid membrane in the analytical practice despite the advantages outlined above. 

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. 

Onium salts, such as tetraethylammonium bromide (TEAB) and tetra-n-butylammonium bromide (TBAB), were also tested as PTCs immobilized on clay. In particular, Montmorillonite KIO modified with TBAB efficiently catalyzed the substitution reaction of a-tosyloxyketones with azide to a-azidoketones, in a biphasic CHCI3/water system (Figure 6.13). ° The transformation is a PTC reaction, where the reagents get transferred from the hquid to the solid phase. The authors dubbed the PTC-modified catalyst system surfactant pillared clay that formed a thin membrane-hke film at the interface of the chloroform in water emulsion, that is, a third liquid phase with a high affinity for the clay. The advantages over traditional nucleophilic substitution conditions were that the product obtained was very pure under these conditions and could be easily recovered without the need for dangerous distillation steps. 

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