Types emulsion liquid membranes

Liquid Membranes Se eral types of liquid membranes exist molten salt, emulsion, irnmobilized/siipported, and hollow-fiber-contained liquid membranes, Arald and Tsiikiibe (Liquid Membranes Chemical Ajijilieafions, (JR(J Press, 1990) and Sec, IX and (Jhap, 42 in Ho and Sirlcar (eds,) (op, cit, pp, 724, 764-808) contain detailed information and extensi -e bibliographies. 

Fig. 5-2. Three types of the liquid membrane eonfiguration (a) emulsion liquid membrane (b) supported liquid membrane (e) elassieal bulk liquid membrane set-up.

As discussed by Frankemfeld and Li(28) and del Cerro and Boey(29), liquid membrane extraction 28,29) involves the transport of solutes across thin layers of liquid interposed between two otherwise miscible liquid phases. There are two types of liquid membranes, emulsion liquid membranes (ELM) and supported liquid membranes (SLM). They are conceptually similar, but substantially different in their engineering. 

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. 

The emulsion liquid membrane for cephalosporins relies essentially on facilitated transport. There are basically, however, two types of facilitated transport in emulsion liquid membrane system, i. e.. Type I and Type II facilitation. In the first type, the concentration gradient of the membrane soluble solute/permeate... 

Emulsion liquid membrane extraction of cephalosporins conform to the type II facilitated transport. Here the solute transport is either associated with a cotransport or counter-transport of an anionic species depending on whether ion-pair or ion-exchange extraction is exploited in the ELM system. 

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

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. 

Broadly speaking, there are three different types of liquid membranes. Bulk liquid membrane (BLM) is a stirred organic phase of lower density than the aqueous phase positioned under it or vice versa. In emulsion liquid membrane (ELM), the receiver aqueous phase containing oil droplets is dispersed into the feed aqueous phase. The total volume of the receiving phase inside the oil droplets is at least ten times smaller than that of the source phase. The thickness of the membrane (organic film) is very small, while the surface area is enormous resulting in very fast separations. Though the efficiency of mass transfer in the liquid membranes is inversely proportional to the thickness of the membrane phase, too thin a film has poor stability due to low but finite solubility in F and R. It can also be disturbed by pressure differences created by the two aqueous phases. 

Liquid membranes can be of three types—bulk liquid membrane, immobilized on a solid supported hquid membrane, and liquid membrane as double emulsions. Of these three types, ELMs can achieve much higher mass transfer area than the other two membranes. ELMs were first used by Li [1] for separation of hydrocarbons. Since then, considerable work has been done to demonstrate qualitatively the feasibihty of performing separations with specific formulations. 

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. 

Liquid membranes can be prepared in two different configurations (see fig. 1). A liquid can be Impregnated in the pores of a porous solid for mechanical support. This form is commonly known as an immobilized liquid membrane (ILM). In the alternate configuration, the receiving phase is emulsified in an immiscible liquid membrane. This type of liquid membrane is known as a liquid surfactant, or emulsion liquid membrane (ELM). 

Co-anion type and concentration are examined as parameters that can be varied to achieve various metal cation separations in macrocycle-facilitated emulsion liquid membranes. Membrane systems where the metal is present in the source phase as a complex anion or as a neutral complex (cation-anion(s)) are discussed. The experimental separations of Cd(II) from Zn(II) and/or Hg(II), Au(I) from Ag(I), and Au(III) from Pd(II) or Ag(I) are given to illustrate separation design using these membrane systems. The separations are discussed in terms of free energies of hydration, distribution coefficients, and equilibrium constants for the various interactions that occur. 

The liquid membrane (LM) concept combines solvent extraction (SX) and membrane-based technologies, enabling both extraction and back-extraction in a single step with reduced consumption of extractants and diluents. For these reasons, separation based on LMs can be viewed as a promising alternative to traditional SX. The LM separation approach involves mass transfer of a target chemical species between two solutions (i.e., feed and receiver solutions) separated by an immiscible LM [1]. The main types of LMs are bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs), supported liquid membranes (SLMs), and polymer inclusion membranes (PIMs). 

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. 

Figure VI - 31. Preparation of an emulsion type of liquid membrane (ELM).

In describing membrane development, both types of liquid membranes should be distinguished, i.e. the supported liquid membrane (SLM) and the emulsion liquid membrane (ELM). 

Figure 20 Types of liquid membranes (a) bulk liquid membrane (b) emulsion liquid membrane. (Reproduced from Ref. 80. lUPAC, 1986.) (c) supported liquid membrane. (Reproduced from Ref. 80. lUPAC, 1986.) and (d) pol)mer inclusion membrane.

Emulsion pertraction is a combination of the two types of liquid membrane process where an unstabilized water-in-oil emulsion is fed down the lumen of a hollow fiber that is surrounded by the aqueous feed. 

In contrast to osmotic, dialysis, filtration or size-exclusion type membranes, PIMs as other liquid membranes (i.e., bulk liquid membranes, emulsion liquid membranes and supported liquid membranes) rely on the action of a chemical agent to extract the solute of interest from an aqueous phase (Kolev, 2005). The action of this chemical agent is the most important factor in the performance of any PIM and its behavior shares considerable similarities with SX apphcations (e.g., hydrometaUurgy). 

Rgure 8. Frequently Used Membrane Types. A, B, C, and D are bulk liquid membrane, emulsion liquid membrane, supported liquid membrane, and dual module hollow fiber membrane configurations respectively. (Reproduced with permission from ref. 47. Copyright 1990 CRC Press.)... 

Transport Across Emulsion Liquid Membranes. The transport of transition metal cations across emulsion liquid membrane experiments were conducted in the cell shown in Figure 2b. The liquid membrane consisted of the carrier and an emulsifier dissolved in kerosene. The liquid membrane system was a water-in-oil-in-water type of emulsion which was obtained by stirring the aqueous receiving phase solution with the organic phase to form an emulsion which was then mixed with the aqueous source phase solution. 

This review covers recent advances in the theory for emulsion liquid membranes (ELMs) briefly and in ELM applications in more detail. In the theory, the state-of-the-art models for two types of facilitation for ELMs are discussed. In the applications, significant advances have been made recently. Commercial applications include the removal of zinc, phenol, and cyanide fi om wastewaters and in well control fluid. Potential applications include wastewater treatment, biochemical processing, rare earth metal extraction, radioactive material removal, and nickel recovery. The ELM systems for these applications are described. 

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. 

Figure 1. Different Types of Liquid Membranes BLM = Bulk Liquid Membrane ELM = Emulsion Liquid Membrane and SLM = Supported Liquid Membrane with a) Flat Membrane and b) Hollow Fiber Membrane Configurations. (F = feed solution M = membrane phase S = stripping solution)...

Representations of the three general types of liquid membranes, e.g., bulk liquid membranes, emulsion liquid membranes, and supported or immobilized liquid membranes, are presented in Figure 1. 

Carrier-facilitated transport of actinides across bulk, supported, and emulsion liquid membranes, as well as plasticized membranes and recently developed emulsion-free liquid membranes, are reviewed. The discussion includes the effects of important experimental variables upon the solute flux for various types of liquid membranes. Applications of liquid membranes in the recovery and removal of radiotoxic actinides from the nitric acid wastes generated during reprocessing of spent fuel by the PUREX process and wastes produced by other radiochemical operations are surveyed. 

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