Commercial applications, liquid membrane

Liquid membranes are a specialty, either adsorbed in capillaries or erniilsiFied. Thev are much studied, but little commercial application is Found. 

Liquid membrane processes for removing H2S from process gases are potentially attractive because they may require less energy than conventional techniques. Research is now going on to develop these technologies, but they have not yet achieved commercial application. 

Five liquid membrane electrodes (Table 13.3) are now commercially available and have found wide application in the testing of electrolytes in biological and technological systems. All five electrodes perform well in the concentration range over which the Nernstian slope is maintained, i.e., from 10 -10 moldm . These electrodes to a certain extent have replaced in both chemical and clinical laboratories the more traditional instrumental methods of analysis, such as flame photometry and atomic absorption spectrometry. There are, of course, many more liquid membrane electrodes, but the availability of satisfactory solid electrodes has greatly restricted their development and practical application. 

The process design principles of SLM, non-dispersive extraction, and hybrid hquid membrane systems need to be understood through bench scale experiments using feed solution of practical relevance. While the economic analysis of an ELM process can be performed from small scale experiments, such an analysis is difficult for other LM systems. In particular, availability and cost of hollow fiber membranes for commercial application are not known apriori. A simple rule of thumb for cost scale-up may not be apphcable in the case of an HE membrane. Yet we feel that the pilot plant tests would be adequate to make realistic cost benefit analysis of a liquid membrane process, since the volume of production in )8-lactam antibiotic industries is usually low. 

Because the carrier facilitated transport process employs a reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far larger than the selectivities achieved by other membrane processes. This one fact has maintained interest in facilitated transport for the past 30 years, but no commercial applications have developed. The principal problem is the physical instability of the liquid membrane and the chemical instability of the carrier agent. In recent years a number of potential solutions to this problem have been developed, which may yet make carrier facilitated transport a viable process. 

Until quite recently, most of me facilitated transport results reported in me literature were obtained with supported liquid membranes held by capillarity in microporous films. The instability of these membranes has inhibited commercial application of me process. Three factors contribute to mis instability and me consequent loss of membrane performance over time ... 

Their historical developments and various membrane preparation methods will be discussed in Chapters 2 and 3, respectively. Chapter 4 reviews the general separation and non-separation properties of the membranes and the methods by which they are measured. Chapter 5 presents commercial membrane elements and modules and their application features which are followed by discussions of liquid-phase separation applications in Chapter 6. Many of those applications are commercially practiced. Potential gas separation and other applications (such as sensors and supports for liquid membranes) will be discussed in Chapter 7. 

A historical perspective on aqueous-organic extraction using membrane contactor technology is available in Refs. [1,6,83]. The mechanism of phase interface immobilization was first explored in Ref. [84], while application of membrane solvent extraction for a commercial process was first explored in Ref. [85]. Two aspects of liquid-liquid contact in membrane contactors that are different from typical gas-liquid contact are (1) the membrane used could be hydrophobic, hydrophdic, or a composite of both and (2) the membrane mass transfer resistance is not always negligible. Ensuring that the right fluid occupies the membrane pores vis-a-vis the affinity of the solute in the two phases can minimize membrane resistance. These aspects have been discussed in detail in Refs. [6,86,87]. 

Cahn RP and Li NN. Commercial applications of emulsion liquid membranes. In Li NN, Calo JM, eds. Separation and Purification Technology, New York Marcel Dekker, 1992 195-212. 

The trend for market penetration will probably follow a path as shown schematically in Fig. 1.2. The figure does not pretend to give quantitative information but merely shows the relative importance of different application fields in time and illustrates the increasing complexity. Gas separation with microporous membranes will probably only start on a commercial scale if membrane business for liquid filtration has become sufficiently profitable to bear the developments necessary to produce commercial gas separation membranes. Commercial availability should therefore be improved for applications not directly making use of liquid filtration membranes. 

Other commercialized major applications for liquid membranes are the recovery of zinc from rayon plant... 

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... 

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