Diffusion transport mechanisms

We may conclude that our findings support independent water and salt permeation processes, and suggest that the salt permeation is governed by a solution-diffusion transport mechanism. 

The most significant effect of a convective-diffusive transport mechanism is to counteract the tendency of the electronation-current density to reduce the interfacial concentration of electron acceptors to zero. Further, since the interfacial concentration of electron acceptors then remains at a value above that given by the diffusion-based equations, a transition time, indicated by a rapid potential variation, need not be attained. 

Salt and water permeate reverse osmosis membranes according to the solution-diffusion transport mechanism are described in Chapter 2. The water flux,. /, is linked to the pressure and concentration gradients across the membrane by the equation... 

Modelling. Transport and reaction models consider advective, dispersive and diffusive transport mechanisms as well as ad- and desorption processes (Fritsche et al., 2007 Jacoub and Westrich, 2007 Kamahl and Westrich, 2007). 

The removal of water from aqueous salt notations by reveres osmosis, as in seawater desaiiontion with cellulose acetate menibmues or aylon hollow fibers, is believed to occur primarily by a diffusive transport mechanism for both water and solutes. On the other hand, in the use of membranes for die removal of water from aqueous solutions containing higher molecular weight solutes, such as die ultrafiltration of protein solutions, die solvent is believed transported by a viscous How mechanism within the pores of the membrane and nolute molecules are corrected with the solvent in die larger pores.66-66... 

Solution-diffusion transport mechanism When the feed contacts the membrane, the solutes (denoted i in Fig. 3.6-10) adsorb on and subsequently absorb in the membrane surface by solute-polymer interactions (Fig. 3.6-lOA). Preferential solute-polymer interactions imply that the solvating power of the polymer is higher for the solutes than for the bulk solvent. 

Cmob denotes the concentration in the mobile phase, exchc is an exchange coefficient controlling the velocity of dissolution, Umob stands for the porosity of the aqueous phase. The dissolved contaminant is transported downgradient via advective and diffusive transport mechanisms (Fig. 9.4). The numerical model TBC uses a node-centred finite volume approach for simulation of transport therefore concentrations are displayed above and below every layer. 

Then Uemiya et al. [2] reported another Pd membrane reactor for WGS reaction using Fe-Cr oxide catalysts. The model of flow in the palladium membrane reactor is illustrated in Figure 6.3. They proposed that hydrogen is permeated through palladium membrane via a solution diffusion transport mechanism, and the rate of hydrogen permeation, /, per unit area of membrane, is written in terms of Fick s first law as follows ... 

The membranes used in GS can be distinguished in two main categories, polymeric and inorganic. Polymeric membranes, specifically used for GS, are generally asymmetric or composite and based on a solution-diffusion transport mechanism. These membranes, made as flat sheet or hollow fibres, have a thin, dense skin layer on a micro-porous support that provides mechanical strength [7]. Typically, polymeric membranes show high, but finite, selectivities with respect to porous inorganic materials due to their low free-volume... 

Agitation had no effect on the infection rate. Multiplicity of infection had a positive effect on the rate of infection. The rate of infection increased with multiplicity of infection regardless of the species used to induce infection of the cells. This observation was explained using a Brownian diffusion transport mechanism of viral particles to the cell. 

The dimensionless substrate concentration profile in a porous membrane, where the biocatalyst is entrapped, depends on the Thiele modulus heterogeneous catalyst, since it allows for the estimation of the penetration depth within the support and can also be used to identify the mechanism that controls the process rate. The Thiele modulus can be interpreted as a ratio between a diffusion time (SVD if.s) and a kinetic time (A a/ max) or, equivalently, as a ratio between a characteristic kinetic rate Vmax/ m AS = AS and a merely diffusive transport mechanism, characterized by (D s s/S) x (A5/5). When (/) 1 kinetics is the limiting step, the process rate coincides with the reaction rate and the concentration profile is uniform, completely penetrating the support. 

For separations comparable with particle dimensions, the most significant transport mechanism becomes diffusion, the rate of which increases with decreased size of particles and increased temperature. The diffusion transport mechanism was the dominating one in various experimental studies on protein and colloid adsorption [2,8-10]. However, the disadvantage of the diffusion-controlled transport is its inherently unsteady character, leading to considerable decrease in adsorption rate with time. 

The method of transport from one phase to another is clearly defined as a simple atomic shift of distance much smaller than any cell parameter and usually no more than a few 0.1 A. The resistance to this shift is reasonably open to calculation and observation. Thus there exist no problems of volume, surface, or grain boundary diffusion transport mechanisms nor of heat diffusion, problems which beset most systems studied. 

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