Commercial applications, liquid membrane technology

This volume Is divided Into three sections theory, carrier chemistry, and applications. The theory section Includes chapters which thoroughly describe the theory and analysis of various liquid membrane types and configurations (107-110) The carrier chemistry section contains two articles on the use of macrocycles for cation separations (111-112). The applications section begins with a survey article which thoroughly reviews the liquid membrane applications In the literature and discusses both potential and commercial aspects of liquid membrane technology. The remaining articles discuss both gas phase (113-115) and liquid phase transport (116-117). 

Commercial eind laboratory applications of liquid membrane technology are discussed including gas transport, sensor development, metal ion recovery, waste treatment, biotechnology and biomedical engineering. Immobilized liquid membranes, emulsion or liquid surfactant membranes, and membrane reactors are discussed. Economic data from the literature for liquid membrane processes are presented and compared with existing processes such as solvent extraction and cryogenic distillation of air. 

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. 

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 author discusses application of ELM, SLM, and polymer inclusion membrane techniques in separation of metal ions (precious metals, Cu, Ni, Zn, Pb, Cd, Cr(VI), Pu, Am, etc.) and organic pollutants (phenols and its derivatives, carboxylic acids, antibiotics, etc.) from wastewaters using laboratory, pilot, and industrial scale modules. Effects of experimental variables upon the solute flux for the various types of liquid membranes are analyzed. The author discusses potential and commercial aspects of hquid membrane technology in wastewater treatment. 

The extraction capabilities of liquid membranes have been used successfully in many areas. Since 1968 efforts have been made for successfirl industrial application of hquid membrane technology. Emphasis has been on facilitated transport of LSMs. Some of the possible commercial apphcations are discussed below. 

The time required to reach equilibrium is much reduced through the use of novel technology in commercially available automatic membrane osmometers. If, for example, the capillary height in the solution chamber increases because solvent permeates from the solvent chamber, this is immediately compensated by the application, via a servomechanism, of a pressure on the solution chamber, such that the capillary heights above solvent and solution remain the same. Since this method involves the transport of only very small amounts of liquid, equilibrium is reached after only 10-30 min. 

Ultrafiltration membranes are commercially fabricated in sheet, capillary and tubular forms. The liquid to be filtered is forced into the assemblage and dilute permeate passes perpendicularly through the membrane while concentrate passes out the end of the media. This technology is useful for the recovery and recycle of suspended solids and macromolecules. Excellent results have been achieved in textile finishing applications and other situations where neither entrained solids that could clog the filter nor dissolved ions that would pass through are present. Membrane life can be affected by temperature, pH, and fouling. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

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