Membrane contactors principles

B.W. Reed, M. J. Semmens and E.L. Cussler, Membrane Contactors, in Membrane Separations Technology Principles and Applications, R.D. Noble and S.A. Stem (eds), Elsevier Science, Amsterdam, pp. 467-498 (1995). 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Figure 9.9 Schematic principle of the nonwetted mode of a membrane contactor.

Reed, B.W., Semmens, M.J. and Cussler, E.L. (1995) Membrane contactors, in Membrane Separation Technology. Principles and Applications (eds R.D. Noble and S.A. Stern), Elsevier Science, Amsterdam, p. 467. 

Reed BW, Semmens MJ, and Cussler EL, Membrane contactors, in Noble RD and Stem SA, eds. Membrane Separation Technology— Principles and Application. Elsevier, Amsterdam 1995, 467-498. 

FIGURE 4.22 Illustration of the principle for removal of CO2 from a gas stream (left) using a membrane contactor with hollow Hbers (right). (From Hoff K.A., Modeling and experimental study of CO2 absorption in a membrane contactor. Thesis NTNU, Trondheim, 2003. With permission.)... 

The mass-transfer efficiencies of various MHF contactors have been studied by many researchers. Dahuron and Cussler [AlChE 34(1), pp. 130-136 (1988)] developed a membrane mass-transfer coefficient model (k ) Yang and Cussler [AIChE /., 32(11), pp. 1910-1916 (1986)] developed a shell-side mass-transfer coefficient model (ks) for flow directed radially into the fibers and Prasad and Sirkar [AIChE /., 34(2), pp. 177-188 (1988)] developed a tube-side mass-transfer coefficient model (k,). Additional studies have been published by Prasad and Sirkar [ Membrane-Based Solvent Extraction, in Membrane Handbook, Ho and Sirkar, eds. (Chapman Hall, 1992)] by Reed, Semmens, and Cussler [ Membrane Contactors, Membrane Separations Technology Principle. and Applications, Noble and Stern, eds. (Elsevier, 1995)] by Qin and Cabral [MChE 43(8), pp. 1975-1988 (1997)] by Baudot, Floury, and Smorenburg [AIChE ]., 47(8), pp. 1780-1793 (2001)] by GonzSlez-Munoz et al. [/. Memhane Sci., 213(1-2), pp. 181-193 (2003) and J. Membrane Sci., 255(1-2), pp. 133-140 (2005)] by Saikia, Dutta, and Dass [/. Membrane Sci., 225(1-2), pp. 1-13 (2003)] by Bocquet et al. [AIChE... 

This chapter presents an overview of different membrane processes and a description of all of the chapters presented in this edition. Chapter 2 focuses on updated information of utility to UF and NF membrane research and development, particularly in the preparation of new types of UF/NF membranes with improved performances. Chapter 3 presents a comprehensive review on RO membrane, the latest developments in the field, important installations demonstrating this technology, and future scope of RO processes. Chapter 4 presents the potential of membrane contactors, especially hollow fiber contactors in the field of chemical and nuclear industry along with their applications, performance, and current challenges faced by indnstry. This chapter also gives an introduction to membrane contactors, their principles of operation and associated mechanisms (where chemical reactions are involved), and fntnre scope of these contactors. 

Many different developments are taking place in the field of membrane contactor technology worldwide. The development of a specific application normally begins with a proof of principle, followed by a feasibility study, further development, pilot-plant tests, and, finally, full-scale demonstration on-site. 

The membrane contactor can be operated in two different ways. The principle of a gas-liquid membrane contactor is given in Figure 1. A gaseous feed, containing CO2 can be fed to the membrane module and the CO2 is selectively removed by absorption in the liquid phase. This is referred to as Membrane Gas Absorption (MGA). It is also possible to use the membrane contactor to regenerate the CO2 rich absorption liquid. In this case the loaded absorption liquid is fed to the membrane module and the CO2 desorbs from the absorption liquid because of a pressure difference across the membrane. This situation is referred to as Membrane Gas Desorption (MGD). 

One of the major advantages of the principle of MGD is the fact that it is possible to adjust the pressure difference across the membrane to obtain CO2 at a pressure higher than atmospheric pressure. Other advantages are, the absorption liquid can be circulated at constant pressure and this means that the setup will require a lower investment in terms of pumps for liquid compression and decompression to overcome the pressure changes, as compared to conventional absorption and desorption units. The use of a membrane contactor will allow for more flexible operation. For a gas-Uquid membrane contactor independence control of the gas and the liquid stream is possible. 

This chapter provides a state-of-the-art review of HFMST including a general review of hollow fiber membrane contactors, operating principles, design consideration, commercial availability of hollow fiber membrane, and module for scale-up and large-scale studies. Application of HFMST in pharmaceutical, biotechnological, gas absorption and stripping, wastewater treatment, and few latest studies of metal ion extraction are described in detail. 

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) ... 

Related to the experimental studies performed in our laboratory, in this review packed-bed membrane reactors were discussed. It should be mentioned that there are significant investigational activities devoted to study catalytically active membranes where the catalyst is deposited in either the membrane pores or on the inner or outer surface of the tubes [11]. Another similarly interesting and promising principle is based on using the Contactor type of membrane reactors, where the reactants are fed from different sides and react within the membrane [79]. Significant efforts have been made to exploit this principle for heterogeneously catalyzed gas-liquid reactions (three-phase membrane reactors) [80, 81]. 

The same principle of operation as described above is applicable also to liquid-liquid extraction where an aqueous liquid and an organic liquid contact each other inside the contactor for extraction of a solute selectively from one phase to another [6-8]. The critical breakthrough pressure for liquid-liquid system could be calculated by Equation 2.1, except that the term A would now be the interfacial tension between the two liquids. Further variation of membrane contacting technology is called gas membrane or gas-gap membrane where two different liquid phases flow on either side of the membrane, but the membrane pores remain gas filled [9-10]. In this situation two separate gas-hquid contact interfaces are supported on each side of a single membrane. 

Classically, flat-sheet porous PTFE or polypropylene membranes are used as support for the membrane liquid and mounted in holders (cells, contactors) permitting one flow channel on each side of the membrane [1,3,6,8,25]. See Figure 12.1. Such membrane units are typically operated in flow systems and in principle apphcable to aU versions of membrane extraction for analytical sample preparation or sampling. Such a setup can be easily interfaced with different analytical instmments, such as HPLC and various spectrometric instmments, and thereby provides good possibdities for automated operation. Drawbacks of this type of devices are relatively large costs and limited availability, as well as some carryover and memory problems as the membrane units are utilized many times, necessitating cleaning between each extraction. 

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