Immobilized phase interfaces

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 ... 

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 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. 

Reactions take place at the water film - organic phase interface and are catalyzed by the phase-immobilized complex catalyst which is usually [HRh(CO)(TPPTS)3]. Other... 

Instead of applying a pressure difference, one can apply a potential difference to the electrolyte in a direction parallel to the interface (Fig. 6.138). Once again a layer of charge qd at a distance Kf1 from the solid, immobile phase will be assumed to represent the diffuse-charge region of the interface. 

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. 

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]. 

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... 

Recent developments of optical-fibre chemical sensors are based on the use of immobilized chemical reagents (reagent phase) interfaced with the optical fibres (3,7). The reagent phase provides the selective chemistry by which chemical information pertaining to the analyte is converted into spectroscopic information. The fibre optic transducer converts this spectroscopic information into an electrical signal. 

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). 

Reactions take place at the water/film organic phase interface and are catalyzed by the phase-immobilized complex catalyst which consists also of HRh(CO)(TPPTS)3. Other hydrophilic solvents, e.g. glycols or suitable liquids, adjusted to the requirements of the respective reaction, can be used instead of water for the formation of the immobilized liquid layer. As in all SPC variants, the catalytic process proceeds homogeneously in the supported film (SLPC) or in the interface (SAPC), thus avoiding the problem of the separation of the reaction products from the catalyst. It is believed that the hydrophilicity of the ligands and the support creates interaction energies suffient to maintain the immobilization. 

For surfactant concentrations close to or above the CMC, the Gibbs elasticity and the interfacial viscosity are large enough to immobilize the interface, = 0 (see Refs. 5, 58, 267, and 420). A similar effect is observed when the droplet dynamic viscosity is much larger than the dynamic viscosity of the continuous phase (e.g., in the bitumen emulsions, T d, is about 150,000 p at room temperature). For the case of fllm with inunobile surfaces, from Eq. (251) one can deduce the well-known Reynolds formula [466], in which the disjoining pressure, II, can be also included ... 

Lequeux et al. [74] and other authors [43,52] have shown that the nanocomposite contains a phase at the filler-polymer interface exhibiting dynamics substantially slower in comparison to the neat polymer above Tg. Davis and coworkers [43] have shown that the time constant Tj (determined by the NMR method) belonging to the immobilized phase is comparable to the T2 value of the neat matrix below its Tg. Thus, it seems necessary to extend the Langevin effect [71] by the dynamics effects, which are constrained to the diffuse shell of nanometer thickness surrounding each particle. 

Consider a microporous hydrophilic membrane with an aqueous solution on one side and an organic solvent on the other side. Let the pores of the hydrophilic membrane contain the aqueous solution. If the organic-phase pressure is higher than that of the aqueous phase (but does not exceed a critical pressure difference), the aqueous-organic phase interface will be immobilized at each pore mouth on the organic side of the membrane. 

Conventionally, solvent extraction with chemical reaction is implemented in a dispersive system where one liquid phase is dispersed as drops in the other immiscible liquid phase. Such a separation is also implemented using a porous membrane as a phase barrier (see Figure 3.4.11) the immiscible phase interface is immobilized at the pore mouth of, for example, a solvent-resistant hydrophobic microporous/porous membrane. Solvent extraction or back extraction through such an interface is easiiy implemented without dispersing one phase in the other. Applications of solvent extraction with chemical reaction in such a nondispersive format have been studied for a variety of systems. 

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. 

Figure 26.3 Membrane contactor system with multiple immobilized fluid phase interfaces.

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 ... 

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