Stability of liquid membranes

AU the techniques used to increase the stabihty of the SLM, such as the geUed SLM techniques [10, 11] (Fig. 7.5B) and the addition of thin top-layer by interfacial polymerization reaction on the SLM (Fig. 7.5C) [12], are essentiaUy applied in the removal of (metal) ions from solution. The stability of liquid membranes used for the separation of gases is more comphcated. Here, the addition of a top-layer on the macroporous support can negatively influence the permeabihty of gases through the membrane. Therefore, a careful choice of the layer material is important because it has to be impermeable to the solvent and should posses a high permeabihty for the gas molecules considered. In addition, the thickness of the top-layer as weU as that of the whole liquid membrane has to be minimized. 

In the supported liquid membrane process, the liquid membrane phase impregnates a microporous solid support placed between the two bulk phases (Figure 15.1c). The liquid membrane is stabilized by capillary forces making unnecessary the addition of stabilizers to the membrane phase. Two types of support configurations are used hollow fiber or flat sheet membrane modules. These two types of liquid membrane configuration will be discussed in the following sections. 

The emulsion liquid membrane (Fig. 15.1b) is a modification of the single drop membrane configuration presented by Li [2] in order to improve the stability of the membrane and to increase the interfacial area. The membrane phase contains surfactants or other additives that stabilize the emulsion. 

More recently the use of membrane contactors to solve the stability problem of liquid membranes has been proposed [19-21], The concept is illustrated in Figure 11.4. Two membrane contactors are used, one to separate the organic carrier phase from the feed and the other to separate the organic carrier phase from the permeate. In the first contactor metal ions in the feed solution diffuse across the microporous membrane and react with the carrier, liberating hydrogen counter ions. The organic carrier solution is then pumped from the first to the second membrane contactor, where the reaction is reversed. The metal ions are... 

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]. 

Danesi, P.R., Reichley-Yinger, L. and Rickert, P.G. (1987) lifetime of supported liquid membranes The influence of interfacial properties, chemical composition and water transport on the long term stability of the membranes. Journal of Membrane Science, 31, 117. 

Yang, X.J., Fane, A.G. and Soldenhoff, K. (2003) Comparison of liquid membrane processes for metal separations Permeability, stability, and selectivity. Industrialsl Engineering Chemistry Research, 42, 392. 

In SLM extraction, the most widely applied type of three-phase membrane extraction, the membrane consists of an organic solvent, which is held by capillary forces in the pores of a hydrophobic porous membrane supporting the membrane liquid. Such membrane support can be either flat porous PTFE or polypropylene membrane sheet or porous polypropylene hollow fibers. Typical solvents are long-chain hydrocarbons like n-undecane or kerosene and more polar compounds like dihexyl ether, dioctyl phosphate, and others. Various additives can increase the efficiency of extraction considerably. The stability of the membrane depends on the solubility and volatility of the organic liquids, and it is generally possible to obtain membrane preparations that are stable up to several weeks. 

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. 

Despite the many advantages of liquid membranes, which include selectivity and ligand economy, they are not yet applied in industrial scale that is ascribed to their low stability [206]. The common reasons for the instability of liquid membranes are... 

In the biomedical applications outlined by Ward et al. (7 ), more so than in any other separation application of synthetic polymeric membranes, the goal is to mimic natural membranes. Similarly, the development of liquid membranes and biofunctional membranes represent attempts by man to imitate nature. Liquid membranes were first proposed for liquid separation applications by Li (46-48). These liquid membranes were comprised of a thin liquid film stabilized by a surfactant in an emulsion-type mixture. Wtille these membranes never attained widespread commercial success, the concept did lead to immobilized or supported liquid membranes. In... 

In the future, novel developments of liquid membranes for biochemical processes should arise. There are several opportunities in the area of fermentation or cell culture, for the in situ recovery of inhibitory products, for example. Another exciting research direction is the use of liquid membrane for enzyme encapsulation so that enzymatic reaction and separation can be combined in a single step. Chapter 6 by Simmons ial- (49) is devoted to this technique. The elucidation of fundamental mechanisms behind the liquid membrane stability is essential, and models should be developed for the leakage rate in various flow conditions. Such models will be useful to address the effect of parameters such as flow regime, agitation rate, and microdroplet volume... 

Additionally, the salt content and the type of counterions in the aqueous phases influence the emulsion stability and consequently SLM degradation. It was found that the amount of liquid membrane removed from SLM increases with a decrease in the salt concentration of the aqueous phases and with an increase in their flow velocities [83]. 

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. 

The good stability of PIMs over the various types of liquid membranes, including SLMs, and the adequate, but lower, permeability and selectivity show the potential practical applications. The main problem is the low mechanical strength of PIMs. 

Kinugasa, T., Watanabe, K., Takeuchi, H. (1989). Effect of organic solvents on stability of liquid surfactant membranes. Journal of Chemical Engineering of Japan 22 593-597. 

In iheir studies of (he effect of shear rate, Stroeve and coworkers established that the breakup of liquid-membrane droplets can he correlated with the ratio of the apparent viscosity of the disperse (membrane) phase to that of ibe conlinouus (external or feed) phase. They also observed internal circulation of subdroplets within the internal phnte of the liquid-membrane droplets22 21 (Fig. 19.2-1). The effects of this circulation on extraction rates and membrane stability are unkanwn as yet. 

The specific rate of oxygen transfer per unit of liquid membrane area seems to be quite reasonable. However, methods to form and utilize effectively much smaller diameter liquid membranes, perhaps similar to those used in other liquid membrane applications, would be required to obtain enough membrane area per unit blood volume for a practical blood oxygenator. The stability of the liquid membranes does not seem to be a major problem however, more definitive liquid membrane stability information would be required before the blood oxygenator application. 

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. 

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