Pores transport

Dead-end Pores Dead-end volumes cause dispersion in unsteady flow (concentration profiles ar> ing) because, as a solute-rich front passes the pore, transport oceurs by molecular diffusion into the pore. After the front has passed, this solute will diffuse back out, thus dispersing. 

The effective diffusivity depends on the statistical distribution of the pore transport coefficients W j. The derivation shows that the semi-empirical volume-averaging method can only be regarded as an approximation to a more complex dynamic behavior which depends non-locally on the history of the system. Under certain circumstances the long-time (t —> oo) diffusivity will not depend on t (for further details, see [191]). In such a case, the usual Pick diffusion scenario applies. The derivation presented above can, with minor revisions, be applied to the problem of flow in porous media. When considering the heat conduction problem, however, some new aspects have to be taken into accoimt, as heat is transported not only inside the pore space, but also inside the solid phase. 

Sarsani, V.R. and Subramaniam, B. (2009) Isobutane/butene alkylation on microporous and mesoporous solid acid catalysts probing the pore transport effeds with liquid and near critical reaction media. Green Chem., 11, 102-108. 

Compared to rivers and lakes, transport in porous media is generally slow, three-dimensional, and spatially variable due to heterogeneities in the medium. The velocity of transport differs by orders of magnitude among the phases of air, water, colloids, and solids. Due to the small size of the pores, transport is seldom turbulent. Molecular diffusion and dispersion along the flow are the main producers of randomness in the mass flux of chemical compounds. 

The diffusion behaviour of Shirakawa polyacetylene is complicated by its fibrillar morphology and high surface area, so that weight changes depend on pore transport and surface adsorption, as well as on diffusion into the fibrils. Chien 6) has reviewed earlier studies of the diffusion of dopant counter-ions in Shirakawa polymer and has emphasised the wide range of values of diffusion coefficient which are reported and which depend a great deal upon the morphological model chosen to interpret experimental data. 

Uitto, O.D., and H.S. White. 2003. Electroosmotic pore transport in human skin. Pharm Res 20 646. 

Burnette, R. R. and Ongpipattanakul, B. Characterization of the pore transport properties and tissue alteration of excised human skin during iontophoresis. J. Pharm. Sci. 77(2) 132-137, 1988. 

Further evidence for pore transport is presented by Yoshida and Roberts [62] in terms of the temperature dependence of iontophoretic flux for solutes of differing size. They showed that the iontophoretic flux for sodium (MW = 23) and cyclosporin (MW = 1203) were relatively temperature insensitive (Fig. 3). The activation energies for iontophoretic transport are similar to activation energies observed for differences of solutes in aqueous solution and indicate that the iontophoretic transport of both solutes is through the pores [62]. 

At present two models are available for description of pore-transport of multicomponent gas mixtures the Mean Transport-Pore Model (MTPM)[4,5] and the Dusty Gas Model (DGM)[6,7]. Both models permit combination of multicomponent transport steps with other rate processes, which proceed simultaneously (catalytic reaction, gas-solid reaction, adsorption, etc). These models are based on the modified Maxwell-Stefan constitutive equation for multicomponent diffusion in pores. One of the experimentally performed transport processes, which can be used for evaluation of transport parameters, is diffusion of simple gases through porous particles packed in a chromatographic column. 

A simulation of trickle-bed hydrogenations was also performed, giving almost the same high selectivity as in the cross-flow case. This shows that the high selectivity is primarily a pore transport effect and not a result of the addition of the arene and hydrogen to opposite sides of the porous plate. 

Diffusive transport in thin porous electrocatalysts can change the above intrinsic selectivities. For example, the specificity of C in two parallel paths (Eq. 68), under slow pore transport of reactant (/i > 3), is given by (67)... 

This ratio can be less than or larger than unity, depending on //, aj, and a. If > a positive potential will enhance B formation. Similarly, B is favored by slow pore transport if kj < 2 and Uj < 02 (6J). One should notice that these conditions are detrimental to product B in the absence of diffusive transport. 

Concentration gradients can exist within the resin pore structure. Ion diffusion is complex since the resin porosity is low and this leads to steric hindrance effects and tortuous diffusion paths. Also, ion diffusion is coupled to the fixed ionic groups and the mobility of each ion within the resin due to charge balance. This coupled diffusion is present in both boundary layer and pore transport. Also, the forward and reverse rates of ion exchange can be affected by the different mobilities of the ions. 

B. Relationship Between Tip Current and Pore Transport Rate... 

Qualitative interpretation of SECM images of a membrane is generally possible without detailed consideration of how the SECM tip influences the diffusion field around a pore opening. However, more quantitative analyses of pore transport generally require that the SECM tip behave as a noninteracting probe. This means that the rate of consumption of electroactive molecules at the SECM tip must be sufficiently small so that the diffusion field... 

Selectivity is defined as the ability of a catalyst to selecttively favor one among various competitive chemical reactions. Intrinsic selectivity is associated with the chemical composition and structure of surface (support). Shape selectivity is associated with pore transport limitations (Figures 4.14 and 4.15). 

Finely Porous - membrane is a dense material punctured by pores, transport is determined by partitioning between bulk and pore fluid. 

Akey, C. W. (1990). Visualization of transport-related configurations of the nuclear pore transporter. Biophys. J. 58, 341-355. 

Finlay, D. R., and Forbes, D. J. (1990). Reconstitution of biochemically altered pores Transport can be eliminated and restored. Cell (Cambridge, Mass.) 60, 17-29. 

Adsorption/desorption and pore-transport are key parameters influencing the activity, effectiveness factors, and product selectivity in porous catalysts. With conventional reaction media (either gas or liqnid phase), one of these parameters is generally favorable while the other is not. For instance, while desorption of heavy hydrocarbons from the catalyst is nsnally the rate-limiting step (and therefore detrimental to catalyst performance) in gas-phase reactions, transport of the reactants/products is the limiting step in liquid-phase reaction media. Furthermore, with conventional media, it is usually difficult to achieve the desired combination of flnid properties for optimum system performance. In contrast, density and transport properties can be continuously pressure-tuned in the near-critical region to obtain unique fluid properties (eg, gas-like transport properties yet liquid-like solvent power and heat capacities). 

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