Diffusion film, pore

Mechanism 1. External film 2. Solid diffusion 3. Pore diffusion 4. Reaction kinetics... 

The basic assumptions made in developing the model should be kept in mind when analyzing the results. Sorption is assumed to be occurring at a rate much faster than pore diffusion, while pore diffusion is taken to be slower than film diffusion. The model assumes radial flux... 

Catalyst selection involves two features productivity and selectivity. The process rate is a subtle combination of four limiting steps adsorption/desorption of reac-tants/product, surface reaction between species, diffusion through pores and diffusion through external film. Pore structure, surface area, nature and distribution of active sites play a crucial role in forming the process rate at the level of catalyst... 

B. Chem, C. W. Hui and G. Mckay, Film-pore diffusion modeling and contact time optimization for the adsorption of dyestuffs on pith, Chem. Eng. J., 84, 77-94,2001. 

To understand the release mechanism, cryomicrotomy was used to slice 10 m-thick sections throughout the matrices. Viewed under an optical microscope, polymer films cast without proteins appeared as nonporous sheets. Matrices cast with proteins and sectioned prior to release displayed areas of either polymer or protein. Matrices initially cast with proteins and released to exhaustion (e.g., greater than 5 months) appeared as porous films. Pores with diameters as large as 100 /xm, the size of the protein particles, were observed. The structures visualized were also confirmed by Nomarski (differential interference contrast microscopy). It appeared that although pure polymer films were impermeable to macromolecules (2), molecules incorporated in the matrix dissolved once water penetrated the matrix and were then able to diffuse to the surface through pores created as the particles of molecules dissolved. Scanning electron microscopy showed that the pores were interconnected (7). 

The cementation reaetion is the electrochemical process in which metal ions from a solution are precipitated onto surface of a more electronegative metal. The process of cathodic deposition is coupled with the anodic dissolution of the substrate metal. The difficulty in ion diffusion through pores of the growing coating is the factor that limits the film thickness. As a rule, this thickness does not exceed... 

Our library contains FRFs for four simple isothermal mechanisms Langmuir kinetics, film resistance model, micropore diffusion, and pore-surface model. For each mechanism, a short description with the model equations is given, together with the expressions for the first-order FRF Fx p(w), and two second-order FRFs, F2,pp(w, w) and F2,pp(w, —[Pg.293]

After passing through the film, solute then diffuses in the pores by normal diffusion (large pores), Knudsen diffusion (small pores), or surface diffusion. In polymer resins where there are no permanent pores, the solute diffuses in the polymer phase. For spherical particles with a radial coordinate r, the diffusion equation in pores is... 

A critical aspect of this type of research is the designing of appropriate reaction cells. To correlate the structure and activity of the catalysts, the flow properties in the EXAFS should ideally be similar to those found in the reactor and for time resolved studies, diffusion (film and pore diffusion) should be faster than the corresponding chemical reaction. Thus several different designs have been reported. Perhaps the three most popular methods are the pelletized catalyst powder disk mounted in a gas cell mounted powder and plug-flow capillary reactor (18,149,162). At this stage, we also draw the attention of the reader to the specially designed hydrothermal vessels that have been used for studying the self-assembly processes for crystalline materials with XAS (163-165). 

In order to minimize contact time, it is necessary to use a contact time model, which may be a kinetic model (first order, second order, Elovich kinetics, etc.) or a mass transport diffusion model (film, pore, surface, pore-surface). The present case study is based on a pseudo-second order kinetic model (Ho and McKay, 1998), and the kinetic equation is ... 

The kinetics data of the liquid-solid catalytic esterification are correlated with various kinetic models, over wide ranges of temperature and feed composition. The activity coefficients calculated using the NTRL model are utilized to represent the non-ideality behavior of the species in the liquid solutions. Meanwhile, the effects of film diffusion and pore diffusion appear to be negligible at the experimental conditions. The results reveal that the Langmuir-Hinshelwood (LH) model yielded the best representation for the kinetic behavior of the liquid-solid catalytic esterification ... 

Fig. 3.9 shows a schematic of such a plot. Drawing a horizontal line through the intercept demarcates film diffusion from pore diffusion and reaction rate. Below that horizontal line, the catalytic process is limited by a combination of pore diffusion rate and reaction rate. We must know which resistance controls product formation before scaling the process. 

Ng J. C. Y, Cheung W. H., and McKay G. (2002). Contact time optimization of two-stage batch adsorber systems using the modified film-pore diffusion model. J. Colloid Interf. Sci. 255(1), 64-74. 

All parameters needed for the calculation are given in Table 6.5.2. The result of such a calculation for a typical blast furnace coke with a particle diameter of 4 cm is given in Figure 6.5.14. At temperatures above about 1000 °C, pore diffusion limits the effective rate, and for T> 1500 °C the rate is increasingly controlled by the external diffusion (film diffusion). 

Mean pore size is equally as important as particle size. Affinity support matrices are all porous materials. This both permits and restricts mass transfer within the particle [35]. Typically, diffusion, film resistance, and (in affinity chromatography) slow reaction kinetics all restrict the transport of large proteins within the pores. 

SURFACE REACTION DESORPTION (D PORE DIFFUSION FILM DIFFUSION... 

Although the model provides a mechanism to measure the total activity of enzyme immobilized on an electrode, it also allows estimation of the combined mass transfer parameter (A/nad+) for the diffusing substrate (in this case, NAD" ). This value, however, is limited somewhat by the fact that the term is cumulative for several variables that may individually contribute to overall electrode mass transfer combined contributions of the molecular diffusion coefficient, effective surface area, diffusion length, pore structure, and mass transfer through the film. Unfortunately, this approach will remain limited unless additional modeling is incorporated that can specifically differentiate these parameters. If more parameters are to be added to the... 

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