Membrane contactors mass transfer resistance

In gas-liquid, liquid-liquid, or hquid-gas-liquid contactors there is no convective flow of any phase across the membrane. Mass transfer occurs only by diffusion across the immobilized phase in the pores. The direction of mass transfer of any molecular species depends on the concentration driving force maintained across the membrane for that species. The presence of the stationary phase in the membrane pore creates an extra diffusional mass transfer resistance. However, it can be shown that in many cases the membrane resistance is negligible, and that in most cases the high active mass transfer area created inside a membrane contactor more than compensates for any additional mass transfer resistance [4—5]. 

Mass transfer resistance in a continuous-contact separation device is the inverse of the mass transfer coefficient. In membrane contactors, the total resistance could be expressed as three resistances in series. These include the individual resistances in each flowing phase and the membrane resistance (Figure 2.4). For a liquid-gas contact system Equation 2.2 could be written for each diffusing species ... 

FIGURE 2.4 Mass transfer resistances in membrane contactor. 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

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]. 

When hydrophobic membranes are used (olefins are preferred because of their low cost), the aqueous absorbent cannot penetrate through the pores and the membrane is gas filled whereas if hydrophilic membranes are employed, the membrane is liquid filled (Figures 38.1 and 38.2). Latter situation is preferred only if the reaction between the gaseous species and the absorbent solution is fast or instantaneous if not, it is better to work with a gas-filled membrane, to reduce mass-transfer resistances. The module design and flow configuration also play an important role in defining the membrane contactors efficiency. This aspect is discussed in detail in Section 38.5. 

Hollow fiber contactors use membranes to separate two phases and transport is due to diffusion, chemical reaction, or chemical potential rather than pressure. The main examples of hollow fiber contactors are found in dialysis, gas adsorption/deadsorption, and solvent extraction. Use of hydrophilic and hydrophobic fiber materials controls the wetting of the pores. Typically, the phase that has higher mass transfer is allowed to wet the pores in order to minimize overall mass transfer resistance. 

A microchannel contactor has been developed and tested with water and cyclohexane streams extracting cyclohexanol [199]. Using this device, the relative importance of mass transfer resistance in the flow channels versus the contactor plate was explored. Both micromachined contactor plates and commercial polymeric membranes were configured with various channel heights both on the feed and solvent sides. Data indicate that contactor plate mass transfer becomes... 

In Chapter 2 we discussed a number of studies with three-phase catalytic membrane reactors. In these reactors the catalyst is impregnated within the membrane, which serves as a contactor between the gas phase (B) and liquid phase reactants (A), and the catalyst that resides within the membrane pores. When gas/liquid reactions occur in conventional (packed, -trickle or fluidized-bed) multiphase catalytic reactors the solid catalyst is wetted by a liquid film as a result, the gas, before reaching the catalyst particle surface or pore, has to diffuse through the liquid layer, which acts as an additional mass transfer resistance between the gas and the solid. In the case of a catalytic membrane reactor, as shown schematically in Fig. 5.16, the active membrane pores are filled simultaneously with the liquid and gas reactants, ensuring an effective contact between the three phases (gas/ liquid, and catalyst). One of the earliest studies of this type of reactor was reported by Akyurtlu et al [5.58], who developed a semi-analytical model coupling analytical results with a numerical solution for this type of reactor. Harold and coworkers (Harold and Ng... 

Various parameters in Equation 31.17 have been defined earlier. Danesi et al. [92] described a simple correlation between permeability coefficient in FSSLM and HFSLM configuration. At very large values of ( ) (as compared to 1), Equation 31.17 is transformed into the one used for FSSLM by Danesi et al. [92]. Hence, the smaller the value of ( ), the higher will be the negative value of the left-hand side of Equation 31.17, which suggests the higher rate of mass transfer. Later on, D Elia et al. [93] considered the resistance in series model where they have studied the mass transport across hollow-fiber contactors in NDSX mode. They showed that the overall mass transfer resistance is equal to the sum of individual mass transfer resistances across the aqueous boundary layer and membrane phase. Mathematically, it can be written as follows ... 

If we consider the typical example of a catalytic porous asymmetric membrane constituted by a thin catalytic layer supported by a macroporous substrate and a wetting liquid phase on the support side and a gas phase on the small pore catalytic side, the liquid will easily fill the pores of both the support and the porous catalytic layer. In order to move the gas-liquid interface from the support towards the catalytic porous layer, a pressure difference of the gas phase has to overcome the capillary pressure of the support. For the same reason, the position of the interface between the two fluid phases inside the porous catalytic layer will depend on the quaUty of the catalytic layer and on the strict control of the pressure difference between the two membrane compartments. In membrane contactors, usually the condition of gas-phase filled pore is preferred in order to reduce the overall mass transfer resistance across the membrane. In a catalytic membrane, both reactants in the two fluid phases need to reach the catalytic sites in the pore and therefore an ideal situation wherein the interface between the phases is very close to the catalytic sites is to be preferred in order to achieve the maximum reactant concentration in the reaction zone. This situation can be approximated by using a wetting liquid, a thin catalytic layer and fluid-fluid... 

Mass Transfer in Gas-Liquid Systems As in conventional contactors, mass transfer rates in membrane contactors for gas-liquid systems are generally described by means of an overall mass transfer coefficient, K, and the gas-liquid interfacial area per unit device volume, a. The overall mass transfer coefficient based on the liquid phase for any species i, Ku, is usually described via the principle of the following resistances in series liquid film resistance (1 /fe,/), membrane resistance (//,/, > ), and the gas film resistance (//,/ kig) for the gas-filled membrane pore case in series leading to the overall resistance (1 /Ku) ... 

The mass transfer resistance theory in membrane contactors can be presented as a resistance in electric circuit. In Figure 9.2, the mass transfer resistance in the membrane contactor consists of resistance from the gas phase, membrane, and the liquid phase. The gas and liquid phases contribute to the overall resistance because of the formation of boundary layers close to the membrane surface. This imphes that the concentration of the bulk of the two phases is different from its concentration at the membrane surfaces. 

S.-H. Lin, C.-F. Hsieh, M.-H. Li, K.-L. Tung, Determination of mass transfer resistance during absorption of carbon dioxide by mixed absorbents in PVDF and PP membrane contactor. Desalination 249 (2009) 647-653. 

The membrane contactor type of reactor, fabricated from a porous carbon artifact, was used for solving the difficulties associated with the supported liquid phase (SLP) hydration catalyst, without introducing deleterious mass-transfer resistance. In fact, the porous contactor is able to... 

Among the various advantage of this hybrid configuration (e.g., the rates of alcohol production are satisfactory the retention of catalyst and separation of product from the reaction mixture are important for industrial-scale operation the carbon membranes used for the contactor are robust and are unaffected by the presence of a strong acid, such as phosphoric acid catalyst), also must be added that the membrane preparation and fabrication techniques can be optimized for reducing the mass-transfer resistance in the liquid-filled pores and consequently increase the yield of desired product. 

When a species is transferred from a phase to another phase by means of a membrane contactor, the mass-transport resistances involved are those offered by the two phases and that of the membrane (see Figure 20.5). The overall mass-transfer coefficient will, therefore, depend on the mass-transfer coefficient of the two phases and of the membrane. 

Useful simplifications are often made in Equation 2.2. We will use gas-liquid contact as an example, and assume gas-filled homogeneous membrane of high porosity, thin wall, and low tortuosity. Since diffusion in gas phase is generally of three orders of magnitude faster than in liquid phase, one can show that and ka are quite high in this case compared to ki, and so the controlling resistance to mass transfer is in the liquid phase. This means A total is essentially the same as If is constant within the contactor the total mass transfer rate in Equation 2.4 can be approximated for the entire contactor as... 

Table 2.7 Values for mass transfer and resistance coefficients estimated for a typical G-L membrane contactor device equipped by a composite membrane with dense top layer...

Profiles in which this latter profile can be found are electrodialysis, per/aporation, gas separation, dialysis, diffusion dialysis, facilitated transport or carrier mediated transport and membrane contactors. The extent of the boundary layer resistance varies from process to process and even for a specific process it is quite a lot dependent on application. Table Vn.2 summarises the causes and consequences of concentration polarisation in various membrane processes. The effect of concentration polarisation is very severe in microfiltration and ultrafiltration both because the fluxes (J) are high and the mass transfer coefficients k (= EV8) are low as a result of the low diffusion coefficients of macromolecuiar solutes and of small particles, colloids and emulsions. Thus, the diffusion coefficients of macromolecules are of the order of lO ° to 10 m /s or less. The effect is less severe in reverse osmosis both because the flux is lower and the mass transfer coefficient is higher. The diffusion coefficients of low molecular weight solutes are roughly of the order of 10 m /s. In gas separation and pervaporation the effect of concentration polarisation is low or can be neglected. The flux is low and the mass transfer coefficient high in gas separation (the diffusion coefficients of gas molecules are of the... 

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