Solute, back transport mechanisms

Diffusive back-transport is the most common back-transport mechanism [12], and is represented by the mass transfer coefficient, k = D/S where D is the solute diffusivity and S is the boundary-layer thickness. The possible effect ofshp flow velocity on diffusive back-transport [13, 14] is discussed in Chapter 6. Steady-state conditions are reached when the convective transport of the solutes to the membrane is equal to the sum of the permeate flow and the diffusive back-transport of the solutes. This steady-state is... 

The concentration boundary layer forms because of the convective transport of solutes toward the membrane due to the viscous drag exerted by the flux. A diffusive back-transport is produced by the concentration gradient between the membranes surface and the bulk. At equiUbrium the two transport mechanisms are equal to each other. Solving the equations leads to an expression of the flux ... 

The simplest practicable approach considers the membrane as a continuous, nonporous phase in which water of hydration is dissolved.In such a scenario, which is based on concentrated solution theory, the sole thermodynamic variable for specifying the local state of the membrane is the water activity the relevant mechanism of water back-transport is diffusion in an activity gradient. However, pure diffusion models provide an incomplete description of the membrane response to changing external operation conditions, as explained in Section 6.6.2. They cannot predict the net water flux across a saturated membrane that results from applying a difference in total gas pressures between cathodic and anodic gas compartments. 

An idealized schematic diagram of alkali metal cation transport across a liquid surfactant (emulsion) membrane by an lonizable crown ether is shown in Figure 7. Thus a metal cation is transported from an external aqueous source phase across the liquid surfactant membrane which forms the outer surface of the emulsion droplet into an interior aqueous receiving phase. Metal ion transport is driven by a pH gradient and back transport of protons from the internal to the external aqueous solution according to the mechanism illustrated earlier in Figure 1. In this system, transport is rapid due to the thin organic membrane. 

Polymer inclusion membrane systems for transport experiments The study of PIMs almost always involves characterization of the extraction of the target solute from a source solution (sometimes called the feed ) into a PIM this is frequently combined with simultaneous back-extraction on the other side of the membrane into a receiving solution (sometimes called the strip ). Where extraction and back-extraction occur simultaneously, the solute is transported across the membrane. These experiments are often carried out in a system consisting of two compartments (cells) with mechanically stirred solutions and a membrane sandwiched between them (Fig. 10.5). 

For the anions, the two transport mechanisms act in opposite directions (Figure 3.2c). 50 ions migrate toward the cathode by diffusion, as do the copper ions, but they are repelled there and driven back into solution by the... 

The anion exchange and solvation mechanisms make it possible to transport metal salts across flowsheets. Stripping a loaded chlorometallate solution of an anion exchanger back into an aqueous phase which contains a low chloride concentration liberates a metal chloride, e.g. 

II) transport of salt beyond the double layer, through the solution around the particle, and back on the other side. Range a mechanism diffusion. 

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