Liquid membrane separation configurations

An attempt to unify the mass transport phenomena of liquid membrane separation underlying the basic LM configurations was presented in this chapter. The basic theory was developed in a simple physical-chemical-mathematical form and applied to the principal techniques in such a way to obtain comparable methods. Of course it is prehminary work once we start forging links between different methods there wiU be spiUover to further possibilities of integration. 

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). 

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. 

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. 

In the next section, four chapters describe three main configurations of liquid membranes supported, emulsion, and bulk LM. Each chapter is subdivided into theory and transport mechanisms, module design and experimental techniques, and applications in different fields of chemical, biochemical, environmental, and pharmaceutical separations. 

Table 7.3 shows a classification of the liquid membranes on the basis of the configuration and module types employed in gas separation. The liquid membranes can be divided in three main classes (i) supported liquid membrane (SLM), (ii) bulk liquid membrane (BLM), and (iii) supported ionic liquid membrane (SILM). 

Table 7.3 Classification of the liquid membranes in the field of gas separation according to the main configurations employed...

A new type of configuration, the flowing liquid membrane (FLM) was studied by Teramoto et al. [20]. In this case, the membrane liquid phase is in motion as the feed and strip phase. In this type of system a plate-and-frame and spiral-wound configuration with flat membrane was used. The scheme of the FLM configuration is drawn in Fig. 7.3A. The hquid phase flows (FLM) between two hydrophobic microporous membranes. The two membranes separate the hquid membrane phase from feed and strip phases. In Fig. 7.3B, it is reported the classical plate-and-frame module employed for the separation of ethylene from ethane [20]. The liquid membrane convection increased the membrane transport coefficient in gas separation. However, the membrane surface packing density (membrane surface area/ equipment volume) is much lower in spiral-wound system than in hollow fiber. 

Recently, Bao et al. [68] compared the efficiency of facilitated transport of CO2 across a liquid membrane by different carriers (diethanolamine (DEA) and carbonic anhydrase (CA) + bicarbonate (NaHCO3) in a polypropylene HFCLM configuration. The hollow fibers used are made of polypropylene, pore size 0.04 pm. In all the experiments, the measured CO2 permeance and selectivity (CO2/O2) using CA bicarbonate as carrier was higher than in the case of DEA. The separation factor (CO2/O2) using DEA was about 152 which are 65% lower than the selectivity calculated with CA bicarbonate. 

If solvent extraction may be considered a source technique, derived liquid-liquid separation techniques include configurations in which an extraction solvent is physically immobilized by a coating or impregnation process onto a solid support such as silica, porous resin beads, or foam [13,84—87]. Other derived techniques include membranes of various configurations bulk liquid membranes, supported liquid membranes, emulsion membranes, and polymer-impregnated membranes [88]. Many derived liquid-liquid techniques have been developed, especially for use in analytical applications [13,60,62,64,75,84,85,87]. In each of these derived techniques, the... 

Immobilized Liquid Membranes. Facilitated transport liquid membranes for gas separations can be prepared In several configurations. The complexatlon agent solution can be held between two nonporous polymer films (2j1), Impregnated Into the pore structure of a micro-porous polymer film (25), or the carrier can be exchanged for the counterion In an Ion exchange membrane (it). 

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). 

To the best of my knowledge, Xenakis and Tondre [12] were the first to use the term microemulsion liquid membrane with reference to a system using an oil-in-water microemulsion to separate oil-soluble components in a U-tube configuration. In a closely related publication [13], the same authors showed the generality of the idea by reversing the transport, using water-in-oil microemulsions to separate and to concentrate water-soluble solutes. 

Compared with liquid membranes, the relatively small volume of solvent in the porous membrane offers the advantages of possible usage of expensive carriers with high separation factors. The advantages of SLMs also include the easy scale-up, low energy requirements, low capital and operating costs, simpler configuration and process, known interface area, and predictable separation performance. 

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