Two immobilized phase interfaces

This technique has now been successfully adopted to adsorption of heavy metals from contaminated waters with or without disposable membranes (Hestekin et al., 2001). 

The phenomenon of a critical pressure or a breakthrough pressme APbr [Eqs. (26.4) and (26.1), respectively] needed to destroy immobilized interfaces between two fluid phases at the mouth of a membrane pore led to the possibility of one fluid phase being contained within the pores of a porous solid membrane and contacting two different fluid phases on two sides of the solid membrane (Fig. 26.3). The fluid phase inside the pores is identified as a membrane since it acts as a membrane with two phase interfaces. In case the fluid phase is liquid, it is identified as either an immobilized liquid membrane (ILM) or a supported liquid membrane (SLM). The following phase contacting configurations have been extensively studied using an SLM/ILM  

SLM Flowing liquid phase 1 SLM Flowing liquid phase 2 

When a gaseous phase exists inside the membrane pore and two different liquids flow on two sides of the membrane, we have a supported gas membrane (SGM) inside the membrane pores  

SGM Flowing hquid phase 1 SGM Flowing hquid phase 2 

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The membrane system has two immobilized phase interfaces each interface is between two immiscible fluid phases, for example, gas-hquid (1) and hquid-gas (2), liquid-liquid (1) and hquid-liquid (2) (multiple-phase-interface-based membrane contactor) (Fig. 26.3). 

Membrane contactors provide a novel approach to the solution of many such problems (especially of the second and third kind) of contacting two different phases, one of which must be a fluid. Essentially, a porous membrane, most often in hollow-fiber form, is the basic element in such a device. Any membrane in flat or spiral-wound or hollow-fiber or any other form has two interfaces since it has two sides. However, conventional separation processes involve usually one interface in a two-phase system, for example, gas-liquid, vapor-liquid, liquid-liquid, hquid-supercritical fluid, gas-solid, liquid-solid, and the like. Membrane contactors allow the creation of one immobilized phase interface between two phases participating in separation via the porous membrane. Three types of immobilized phase interfaces in two-phase configurations are relevant ... 

Membrane contactors provide a continuous process for contacting two different phases in which one of the phases must be a fluid. Whether using a flat-sheet, hollow-fiber, or spiral-wound type, the membrane acts as a separator for two interfaces as it has two sides compared to conventional separation processes, which involve only one interface in a two-phase system. Therefore, it allows the formation of an immobilized phase interface between the two phases participating in the separation process [9]. Generally, there are five different classes of contacting operations gas-liquid, liquid-liquid, supercritical fluid-liquid, liquid-solid, and contactors as reactors [10]. The most commonly used operation in industry are gas-liquid also known as vapor-liquid, liquid-liquid, and supercritical fluid-liquid. Each class of system has its own modes of operation but in this study, emphasis will be focused on the gas-Uquid contacting systans. Table 9.1 describes the membrane contactor in summary. 

In Chapter 3 we described the structure of interfaces and in the previous section we described their thermodynamic properties. In the following, we will discuss the kinetics of interfaces. However, kinetic effects due to interface energies (eg., Ostwald ripening) are treated in Chapter 12 on phase transformations, whereas Chapter 14 is devoted to the influence of elasticity on the kinetics. As such, we will concentrate here on the basic kinetics of interface reactions. Stationary, immobile phase boundaries in solids (e.g., A/B, A/AX, AX/AY, etc.) may be compared to two-phase heterogeneous systems of which one phase is a liquid. Their kinetics have been extensively studied in electrochemistry and we shall make use of the concepts developed in that subject. For electrodes in dynamic equilibrium, we know that charged atomic particles are continuously crossing the boundary in both directions. This transfer is thermally activated. At the stationary equilibrium boundary, the opposite fluxes of both electrons and ions are necessarily equal. Figure 10-7 shows this situation schematically for two different crystals bounded by the (b) interface. This was already presented in Section 4.5 and we continue that preliminary discussion now in more detail. 

Membrane contactors are systems in which the membrane function is to facilitate diffusive mass transfer between two contacting phases (liquid-liquid, liquid-gas, etc.) without dispersion of one phase within another [12]. The membrane does not act as a selective barrier, but creates and sustains the interfaces immobilized at the... 

Separations in two-phase systems with one immobilized interface(s) are much newer. The first paper on membrane-based solvent extraction (MBSE) published Kim [4] in 1984. However, the inventions of new methods of contacting two and three liquid phases and new types of liquid membranes have led to a significant progress in the last forty years. Separations in systems with immobilized interfaces have begun to be employed in industry. New separation processes in two- and three-phase systems with one or two immobilized L/L interfaces realized with the help of microporous hydrophobic wall(s) (support) are alternatives to classical L/L extraction and are schematically shown in Figure 23.1. Membrane-based solvent extraction (MBSE) in a two-phase system with one immobilized interface feed/solvent at the mouth of microspores of hydrophobic support is depicted in Figure 23.1a and will be discussed... 

PT through SLM 3 One immobilizing wall for two immobilized interfaces - Volume ratio of phases can be varied without limitations - Very small volume of membrane phase - Limited stability of SLM... 

Ion partition — The partition coefficients of ions between two liquid phases can be determined with electrochemical methods See ion transfer at liquid-liquid interfaces, and - droplets, electrochemistry of immobilized . 

In the above definition the presence of two different phases is stated and consequently there is an interface between them. One of these phases provides the analyte transport and is usually referred to as the mobile phase, and the other phase is immobile and is typically referred to as the stationary phase. A mixture of components, usually called analytes, are dispersed in the mobile phase at the molecular level allowing for their uniform transport and interactions with the mobile and stationary phases. 

Sundararaj and Macosko (1995) and Beck Tan et al. (1996) observed that the addition of a block copolymer to the droplet phase before mixing it with the matrix phase had little effect on the resulting droplet size at low droplet volume fraction. Although a block copolymer should reduce the interfacial tension between the two phases, and thereby lead to smaller droplets, the diffusion time of the block copolymer may be too long for it to saturate the new interfacial area that must form rapidly if a droplet is to fragment. However, block copolymers do seem to suppress coalescence, possibly by immobilizing the interface... 

The membrane in a contactor acts as a passive barrier and as a means of bringing two immiscible fluid phases (such as gas and hquid, or an aqueous hquid and an organic hquid, etc.) in contact with each other without dispersion. The phase interface is immobilized at the membrane pore surface, with the pore volume occupied by one of the two fluid phases that are in contact. Since it enables the phases to come in direct contact, the membrane contactor functions as a continuous-contact mass transfer device, such as a packed tower. However, there is no need to physically disperse one phase into the other, or to separate the phases after separation is completed. Several conventional chemical engineering separation processes that are based on mass exchange between phases (e.g., gas absorption, gas stripping, hquid-hquid extraction, etc.) can therefore be carried out in membrane contactors. 

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]. 

There are other published resolutions of 4-hydroxyphenylglycine, one of which involves the enzyme-catalysed hydrolysis of the ethyl ester (Scheme 6.9). Since the substrate is fully blocked at the amino and the carboxylate functions, it is scarcely soluble in water and must be dissolved in an organic solvent. The specific hydrolysis of the ester catalysed by an enzyme in an aqueous phase in contact with the organic solvent will yield a water-soluble carboxylic acid which transfers to the aqueous phase containing the enzyme. The conditions of the reaction therefore effect both hydrolysis of the ester and facile separation of the product. In fact, the enzyme is immobilized on a hydrophilic membrane at the interface between the two immiscible phases. The membrane reactor (Figure 6.2) comprises a large bundle of hollow fibres, each having an external diameter of about... 

The conditions required to favor esterification can be obtained in different manners. It is possible to add a water-miscible solvent that will lower the water concentration and increase the solubility of organic substrates and products. It is also possible to work in a two-phase system with a non-water-miscible solvent, which will serve as a reservoir for the substrates and products. This can be achieved either with macroscopic phases or with highly dispersed systems such as reversed micelles. In the above-mentioned cases, the enzyme-catalyzed reaction takes place in the aqueous phase or at the phase interface. The enzyme can be dissolved in this phase or immobilized by covalent attachment to a solid carrier... 

Figure 4 Schematic representation of a retention cycie pathway for the conformational interconversion of a globular protein or polypeptide, Pn.m> fi" solution in the mobile phase and in two unfolded states, and Pus> which occur in the presence of a liquid-solid interface involving immobilized nonpolar ligands of an RPC or HIC sorbent in the presence of an aquo-organic solvent, a kosmotropic or a chaotropic mobile phase system. If the globular protein or polypeptide undergoes a two-stage interconversion in the mobile phase and at the surface, the corresponding distribution process between the two chromatographic phases will involve the unfolded intermediates, Pum and PJjs- Also shown are the corresponding rate constants, k, for these interconversions. The subscripts refer to the native and unfolded states, N and U, respectively, and the mobile and stationary phase, M and S, respectively.

Typically, the SC-CO2 and IL system form a biphasic mixture that contains the enzyme in a denser phase, which is the IL phase whereas the lighter phase acts as a carrier for the substrates and products. In enzyme-catalyzed reactions in a biphasic mixture, the immobilized enzymes are suspended in the IL phase, and reaction substrates are dissolved in SC-CO2. The substrates diffuse from the bulk of the SC-CO2 phase into the two-phase interface, followed by partitioning between the two phases and diffusion into the IL phase toward the active site of the enzyme. The products are then released in the IL phase and extracted by SC-CO2 (de los Rios et al. 2007a). 

A most important additional aspect of such devices is that, as long as the phase interfaces are immobilized via appropriate pressure/wetting conditions, one can have a very wide range of flow rate ratios between the two phases. There is no need for any density difference between the phases. The issue of flooding does not arise, emulsification is unlikely to arise, and the need for coalescence is absent However, surfactant impurities, if present, could interfere with interface immobilization. Further, the solvents must not swell the membrane very much. Therefore the compatibility of the membrane with the solvents to be used should be checked. Smaller pore membranes will lead to a broader range of pressure difference between the two phases for nondispersive operation. The value of Kta for such devices can be larger than conventional devices by 5-50 times. 

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