Liquid membrane configurations

Fig. 5-2. Three types of the liquid membrane configuration (a) emulsion liquid membrane (b) supported liquid membrane (c) classical bulk liquid membrane set-up.

In the emulsion liquid membrane configuration, the liquid membrane is formed by dispersing into the feed (phase 1) an emulsion of the stripping... 

Fig. 15.1 Liquid membrane configurations (a) Single-drop liquid membrane (b) emulsion globule (c) supported liquid membrane.

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. 

A liquid membrane configuration, which is much used for technical applications, is the emulsion liquid membrane (ELM) systems where the acceptor phase is dispersed as a colloid phase, each colloid drop surrounded by a thin organic, surface active phase. This principle does not seem to have been used for analytical sample preparation, probably due to the difficulty of quantitatively recovering the disperse acceptor phase. 

Selectivity parameters, needed for the BOHLM or BAHLM module design and their determination techniques, are analyzed. Selectivity can be controlled by adjusting the concentration, volume, and flow rate of the LM phase. Such control of the selectivity is one of the advantages of the bulk liquid membrane systems in comparison with other liquid membranes configurations and Donnan dialysis techniques. The idea of dynamic selectivity and determination techniques are presented and discussed. 

In summary, this chapter presents the entire breadth of LM technology with the intention of furthering research and industrial applications. The various types of liquid membrane configurations are surveyed and the advantages and disadvantages of each type are described. The tutorial section of this chapter also discusses typical experimental techniques and a survey of theoretical approaches. 

In all forms of liquid membrane configurations, the transport efficiency can be changed based on the nature of the organic extractant, feed and receiver phase compositions and the viscosity of the membrane phase. Also, the nature of the diffusing species is important as the diffusion coefficient is dependent on the molar volume of the diffusing species, as per the Wilke-Chang equation (Wilke et al, 1955) ... 

The 27 chapters of this book describe separations of metal ions, anionic species, organic molecules, and gas mixtures that involve liquid membrane processes. Scientists and engineers in academic, governmental, and industrial laboratories in eight countries contributed to this volume. An overview chapter provides an introduction to the various liquid membrane configurations, transport mechanisms, and experimental techniques. A tribute chapter follows, summarizing the many contributions of Norman N. Li in the field of membrane science. The remainder of the book is divided into sections on theory and mechanism (6 chapters), carrier design, synthesis, and evaluation (6 chapters), and applications in... 

Figure 1. Schematic Representation of Different Liquid Membrane Configurations (a) Bulk Liquid Membrane, (b) emulsion liquid membrane, (c) flat-sheet supported liquid membrane, (d) hollow fiber supported liquid membrane.

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

Two configurations of liquid membranes are mainly used in analytical applications flat sheet liquid membranes that give acceptable extraction efficiencies and enriched sample volumes down to 10-15 pL, and hollow fiber liquid membranes that allow smaller enriched sample volumes. Flat sheet liquid membrane devices consist of two identical blocks, rectangular or circular in shape, made of chemically inert and mechanically rigid material (PTFE, PVDF, titanium) in which channels are machined so that when... 

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. 

Nondispersive solvent extraction is a novel configuration of the conventional solvent extraction process. The term nondispersive solvent extraction arises from the fact that instead of producing a drop dispersion of one phase in the other, the phases are contacted using porous membrane modules. The module membrane separates two of the immiscible phases, one of which impregnates the membrane, thus bringing the liquid-liquid interface to one side of the membrane. This process differs from the supported liquid membrane in that the liquid impregnating the membrane is also the bulk phase at one side of the porous membrane, thus reducing the number of liquid-liquid interfaces between the bulk phases to just one. 

This section aims to explain the unique features of membrane separation methods, their superior performance in contaminant removal, and their operational sensitivities and limitations. We focus particularly on the factors that need to be carefully assessed when the membrane technology to be used in the treatment of liquid radioactive waste is being considered. These include membrane configuration and arrangement, process application, operational experience, data related to key performance parameters, and plant and organizational impacts. 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

The rising need for new separation processes for the biotechnology industry and the increasing attention towards development of new industrial eruyme processes demonstrate a potential for the use of liquid membranes (LMs). This technique is particularly appropriate for multiple enzyme / cofactor systems since any number of enzymes as well as other molecules can be coencapsulated. This paper focuses on the application of LMs for enzyme encapsulation. The formulation and properties of LMs are first introduced for those unfamiliar with the technique. Special attention is paid to carrier-facilitated transport of amino acids in LMs, since this is a central feature involved in the operation of many LM encapsulated enzyme bioreactor systems. Current work in this laboratory with a tyrosinase/ ascorbate system for isolation of reactive intermediate oxidation products related to L-DOPA is discussed. A brief review of previous LM enzyme systems and reactor configurations is included for reference. 

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