Membranes facilitated transport

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

Novel chiral. separations using enzymes and chiral surfactants as carriers have been realized using facilitated transport membranes. Japanese workers have reported the synthesis of a novel norbornadiene polymeric membrane with optically active pendent groups that show enantio.selectivity, which has shown promi.se in the. separation of propronalol. 

There seems to be an opportunity to extend the electrochemical process to direct membrane transport that is, with electrodes plated on either side of a facilitated-transport membrane similar to that of Johnson [24]. The shuttling action of the carrier (Fig. 9) could then be brought about by electrochemical reduction and oxidation instead of pressure difference. 

Noble, R.D. Koval, C.A. Pellegrino, JJ. Facilitated transport membrane systems, Chem. Eng. Prog. 85 (1989) 58-70. 

To-be-developed industrial membrane separation technologies Carrier facilitated transport Membrane contactors Piezodialysis, etc. Major problems remain to be solved before industrial systems will be installed on a large scale... 

Carrier facilitated transport membranes incorporate a reactive carrier in the membrane. The carrier reacts with and helps to transport one of the components of the feed across the membrane. Much of the work on carrier facilitated transport has employed liquid membranes containing a dissolved carrier agent held by capillary action in the pores of a microporous film. 

Ward et al. at General Electric produce high flux, COyHjS facilitated transport membranes -1967-1980... 

Steigelmann and Hughes develop olefin/paraffin facilitated transport membrane demonstrate process at the Peinemann demonstrates pilot scale - 1980-1982 dispersed solid Ag + salts can facilitate olefins -1992... 

A milestone chart showing the historical development of carrier facilitated transport membranes is given in Figure 11.5. Because of the differences between coupled and facilitated transport applications these processes are described separately. Reviews of carrier facilitated transport have been given by Ho et al. [18], Cussler, Noble and Way [34-37,39], Laciak [38] and Figoli et al. [40],... 

Figure 11.18 Flux through a facilitated transport membrane calculated using Equation (11.20) ([A] 0 and DRA[R](m)tot/f 1)...

Yet another approach to stabilizing facilitated transport membranes is to form multilayer structures in which the supported liquid-selective membrane is encapsulated between thin layers of very permeable but nonselective dense polymer layers. The coating layers must be very permeable to avoid reducing the gas flux through the membrane materials such as silicone rubber or poly(trimethylsilox-ane) are usually used [26],... 

From the late 1960s to the early 1980s, Ward and others at General Electric studied facilitated transport membranes, particularly for separation of the acid gases carbon dioxide and hydrogen sulfide from methane and hydrogen [23-26],... 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Figure 11.26 Performance of a 37 m2 hollow fiber silver-nitrate-impregnated facilitated transport membrane for the separation of propylene/propane mixtures. The feed pressure was 5-13 atm the permeate was a hexane liquid sweep stream. The vertical dotted lines show when the membrane was regenerated with fresh silver nitrate solution [27]. Reprinted with permission from R.D. Hughes, J.A. Mahoney and E.F. Steigelmann, Olefin Separation by Facilitated Transport Membranes, in Recent Developments in Separation Science, N.N. Li and J.M. Calo (eds) (1986). Copyright CRC Press, Boca Raton, FL...

Figure 11.28 The oxygen/nitrogen selectivity plotted against oxygen permeability for polymeric membranes [68] and Co(3-MeOsaltmen)-based facilitated transport membranes [66]...

Carrier facilitated transport membranes have been the subject of serious study for more than 30 years, but no commercial process has resulted. These membranes are a popular topic with academic researchers, because spectacular separations can be achieved with simple laboratory equipment. Unfortunately, converting these laboratory results into practical processes requires the solution of a number of intractable technological problems. 

The prospects for facilitated transport membranes for gas separation are better because these membranes offer clear potential economic and technical advantages for a number of important separation problems. Nevertheless, the technical problems that must be solved to develop these membranes to an industrial scale are daunting. Industrial processes require high-performance membranes able to operate reliably without replacement for at least one and preferably several years. No current facilitated transport membrane approaches this target, although some of the solid polymer electrolyte and bound-carrier membranes show promise. 

Development of industrial-scale facilitated transport membranes and systems... 

J.C. Davis, R.J. Valus, R. Eshraghi and A.E. Velikoff, Facilitated Transport Membrane Hybrid Systems for Olefin Purification, Sep. Sci. Technol. 28, 463 (1993). 

Another type of gas exchange process, developed to the pilot plant stage, is separation of gaseous olefin/paraffin mixtures by absorption of the olefin into silver nitrate solution. This process is related to the separation of olefin/paraffin mixtures by facilitated transport membranes described in Chapter 11. A membrane contactor provides a gas-liquid interface for gas absorption to take place a flow schematic of the process is shown in Figure 13.11 [28,29], The olefin/paraffin gas mixture is circulated on the outside of a hollow fiber membrane contactor, while a 1-5 M silver nitrate solution is circulated countercurrently down the fiber bores. Hydrophilic hollow fiber membranes, which are wetted by the aqueous silver nitrate solution, are used. 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

Facilitated transport has been briefly described in Chapter 1. In facilitated transport, the selective transport medium is a liquid or molten salt contained or immobilized in a porous support. The liquid membrane is held tightly in the support pores by capillary forces. The liquid or molten salt selectively reacts with a gas or vapor species and the reacting species diffuses across the liquid or salt and desorbed on the other side of the facilitated transport membrane. The major advantage of the facilitated transpoa is that diffusion is generally several orders of magnitude faster than diffusion through solid membranes. The support is, therefore, not a membrane by definition. Comprehensive... 

Appleby. J.B.. R. Quinn and G.P. Pez. 1993. New facilitated transport membranes and absorbents for the separtion of hydrogen sulfide, presented at 205th Am. Chem. Soc. National Meeting. Denver. CO. USA. 

Goering RM, Bowman CN, Koval CA, and Noble RD. Mechanisms of olefin transport through facilitated transport membranes. Polym Mater Sci Eng 1997 77 260-261. 

Davis JC, Valus RJ, Eshraghi R, and Vilikoff AE. Facilitated transport membrane hybrid systems for olefin purification. Separation Science and Technology 1993 28(1-3) 463-476. 

Teramoto M, Takeuchi N, Maki T, and Matsuyama H. Ethylene/ethane separation by facilitated transport membrane accompanied by permeation of aqueous silver nitrate solution. Sep Purif Technol, 2002 28(2) 117-124. 

Teramoto M, Kitada S, Ohnishi N, Matsuyama H, and Yonehara N. Separation and concentration of CO2 by capillary-type facilitated transport membrane module with permeation of carrier solution. J Mem Sci, 2004 234(1-2) 83-94. 

Quinn R, Appleby JB, and Pez GP. New facilitated transport membranes for the separation of carbon dioxide from hydrogen and methane. 7 Mem 5d, 1995 104(1-2) 139-146. 

We have also investigated the mechanism of transport in these membranes. In such MR-based facilitated-transport membranes, the permeating species is transported by sequential binding and unbinding events with the MR agent [3,5,44]. 

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