Coupled transport membranes

In this section, we will discuss the major characteristics of coupled transport systems. The discussion will be illustrated with results obtained with supported liquid membranes. However, the same principles apply to emulsion membranes. We use supported liquid membrane results because the geometry of supported membranes is well-defined and it is possible to maintain the conditions of the feed and product solutions constant. This allows parametric studies to be per- 

These reagents are described in more detail in a later section of this chapter. 

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R.W. Baker and I. Blume, Coupled Transport Membranes, in Handbook of Industrial Membrane Technology, M.C. Porter (ed.), Noyes Publications, Park Ridge, NJ, pp. 511-558 (1990). 

Figure 11.6 An illustration of the carrier agent concentration gradients that form in coupled transport membranes...

Equations (11.1)—(11.10) provide a basis for rationalizing the principal features of coupled transport membranes. It follows from Equation (11.8) that coupled transport membranes can move metal ions from a dilute to a concentrated solution against the metal ion concentration gradient, provided the gradient in the second coupled ion concentration is sufficient. A typical experimental result demonstrating this unique feature of coupled transport is shown in Figure 11.7. The process is counter-transport of copper driven by hydrogen ions, as described in Equation (11.1). In this particular experiment, a pH difference of 1.5 units is... 

A second characteristic of coupled transport membranes is that the membrane flux usually increases with increasing metal concentration in the feed solution, but is usually independent of the metal concentration in the product solution. This behavior follows from the flux Equations (11.6) and (11.8). In typical coupled... 

W.C. Babcock, R.W. Baker, D.J. Kelly and E.D. LaChapelle, Coupled Transport Membranes for Uranium Recovery, in Proceedings of ISEC 80, University of Liege, Liege, Belgium (1980). 

Baker R and Blume I. Coupled transport membranes. In Porter MC, Ed. Handbook of Industrial Membrane Technology. Noyes Publications Park Ridge, NJ 1990 pp. 511-558. 

Figure 9.2 Historical development of coupled transport membranes.

A second characteristic of coupled transport membranes is that the membrane flux usually increases with increasing metal concentration in the feed solution but is usually independent of the metal concentration in the product solution. This behavior follows from the flux Equations 13 and 15. In typical coupled transport experiments, the concentration of the driving ion A in the product solution is very high. For example, in coupled transport of copper, the driving ions are hydrogen ions and 100 g/ sulfuric acid is used as the product solution. As a result, the term [MRn] g is very small compared to [MRn] 0 and Equation 15 reduces to ... 

The metal extraction curves described above mirror the behavior of these complexing agents when incorporated into coupled transport membranes. Thus, when the product solution pH is maintained at a very low level, the term [MRn] g in Equation 13 is reduced to zero and the flux is determined by the concentration of complexed metal in equilibrium with the feed solution [MRn] q. As a result, the flux vs pH behavior follows the metal extraction curves. This is shown in Figure 9.14, which shows the flux pH (curves) for copper with each of the three complexing agents whose extraction curves are shown in Figure 9.12. The magnitude of the coupled transport flux depends on the liquid viscosity and complexed metal diffusion coefficient, but the pH dependence of the flux is very similar to the metal ion extraction curves. 

Complexing agents used in coupled transport membranes are usually diluted with a carrier solvent, typically a mixed aliphatic-aromatic hydrocarbon. We would generally expect the amount of metal extracted by the complexing agent solution to increase with increasing agent concentration, and this is usually the... 

Figure 9.16 The effect of small concentrations of copper in the feed on iron permeation through a coupled transport membrane.19 (Membrane Celgard2400/ LIX 64N. Feed pH 2.5. Product pH 1.0).

Figure 9.21 The effect of agent concentration on the uranium flux through a coupled transport membrane calculated from Equations 13 and 24. The measured flux data are shown for comparison.17 (Membrane Celgard 2400/Alamine 336 dissolved in Aromatic 150. Feed 0.2% Uranium, pH 1.0. Product pH 4.5).

Figure 9.23 The effect of membrane thickness on flux through a coupled transport membrane.20 (Membrane Laminated Celgard 2400/various reagents. Feed 0.2% copper, pH 2.5. Product 100 g/C H2SO4).

Figure 9,26 Schematic representation of the buffer cycle and pH gradient within the boundary layer of a coupled transport membrane.20...

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