Specific diffusion control, influence

The retarding influence of the product barrier in many solid—solid interactions is a rate-controlling factor that is not usually apparent in the decompositions of single solids. However, even where diffusion control operates, this is often in addition to, and in conjunction with, geometric factors (i.e. changes in reaction interfacial area with a) and kinetic equations based on contributions from both sources are discussed in Chap. 3, Sect. 3.3. As in the decompositions of single solids, reaction rate coefficients (and the shapes of a—time curves) for solid + solid reactions are sensitive to sizes, shapes and, here, also on the relative dispositions of the components of the reactant mixture. Inevitably as the number of different crystalline components present initially is increased, the number of variables requiring specification to define the reactant completely rises the parameters concerned are mentioned in Table 17. 

In assessing whether a reactor is influenced by intraparticle mass transfer effects WeiSZ and Prater 24 developed a criterion for isothermal reactions based upon the observation that the effectiveness factor approaches unity when the generalised Thiele modulus is of the order of unity. It has been showneffectiveness factor for all catalyst geometries and reaction orders (except zero order) tends to unity when the generalised Thiele modulus falls below a value of one. Since tj is about unity when 0 < ll for zero-order reactions, a quite general criterion for diffusion control of simple isothermal reactions not affected by product inhibition is < 1. Since the Thiele modulus (see equation 3.19) contains the specific rate constant for chemical reaction, which is often unknown, a more useful criterion is obtained by substituting l v/CAm (for a first-order reaction) for k to give ... 

Influence of Temperature. The reaction temperature necessarily plays a major role in grafting, where at least two reaction steps (chain propagation and termination) may become diffusion controlled. Indeed, grafting occurs faster at elevated temperatures, but here again each system exhibits a specific behavior. 

Considering the above results, the specific structural properties of HD AM are accounted for the loss of reaction rates experienced for Pd-montmorillonites. In this respect, diffusion control may have an influence on the progress of hydrogenation. 

Temperature In case of a reaction controlled process, changes of reaction temperature would influence the rate of the process [34]. However, experiments with a reagent inflow rate of 0.3 mol/h and a specific energy input of 0.16 W/kg showed no distinct influence of temperature for either of the two solution types tested (see Figure 4 (c)). Based on the evidence presented above the crystal growth of HH is concluded to be a diffusion controlled linear process under the investigated conditions. A more detailed discussion can be found in a previous publication [28]. 

Important factors influencing the analyte volatilisation process are related to diffusion, porosity, and surface area (for solids). To obtain reproducible results it is necessary to control storage temperature and time strictly. The temperature of the sample is very important because of the specific boiling points of the various analytes. The partition coefficient, K, at equilibrium, is... 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

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