Reaction-controlled regime

In the reaction controlled regime the overall rate of layer formation is only limited by the rate of chemical transformations (chemical reaction as such). Therefore, the ApBq layer grows at the highest rate possible under given conditions ... 

If the ApBq layer grows in the reaction controlled regime with regard to component B, then... 

In the reaction controlled regime the layer of each of two chemical compounds ApBq and ArBs grows at the expense of two partial chemical reactions taking place at its interfaces with adjacent phases. 

Though there are no restrictions on the number of compound layers growing simultaneously in the reaction controlled regimes, from a physicochemical viewpoint their formation in the A-B reaction couple of a multiphase binary system must be sequential and not simultaneous. 

Growth kinetics of nanocrystals in the presence of capping agents is determined by several complex factors, and signatures of either the diffusion or the reaction-controlled regimes are... 

According to Equation (5.7) the increase in processing temperature and preform thickness leads to larger values of Valid poor density uniformity. As stated previously, CVI processes preferably operate in a chemical-reaction-controlled regime where the ratio of k/D is small. For Fick diffusion, discussed in Section2.3.1, the diffusivity D is inversely proportional to the pressure and thus operates at lower pressures. Furthermore, coarser pore structures correspond to more uniform deposition. Figure 5.5 shows the microstructures of C/SiC composites prepared at different 9 numbers. 

Accoring to the processing conditions of a CVD process, supersaturation can be divided into two categories, bulk supersaturation and local supersaturation [13], If a CVD process occurs in the chemical reaction control regime discussed in Section 4.3.4, it is reasonable to assume that the compositions of a gas on the substrate surface equal that in the bulk gas. Accordingly, the expression of supersaturation in Equation (6.2) is used for calculation. However, if a CVD process occurs in the... 

If ( ) > 1, absorption occurs between regimes 2 and 3, and the specific rate is given in Table 11.18. Under these conditions, the impeller speed (and hence kj) is reduced so that ( ) becomes greater than 3 and the absorption operation becomes fast, i.e., reaction-controlled (regime 3). The other condition <6< ( ) (Table 11.18), is also checked. The rate of absorption per unit liquid volume is given by... 

In a temperature scanning experiment, where both temperature and concentrations vary in each run, one must be sure that the range of conditions that will be covered does not include a transition from a chemical-reaction-controlled regime to one controlled by an intervening process, say diffusion. Validation of temperature scanning data therefore requires careful examination of the conditions present at the site of reaction. 

Effectiveness factor calculations summarized in Tables 19-1 to 19-5 are consistent with Langmuir-Hinshelwood kinetics, as discussed in this chapter. E is larger and approaches 1 asymptotically in the reaction-controlled regime where the intrapellet Damkohler number is small, and E decreases in the diffusion-controlled regime at large values of A a- These trends are verified by simulations provided in Table 19-1. 

The experimental data are shown In Figure 11a, together with a dotted line, extrapolating the reaction controlled regime to zero time. On the other hand some model calculations have been performed under the assumption that, at the beginning of the experiment, the reaction rate is in the diffusion controlled regime. Then only a restricted spherical shell close to the bead surface Is participating In the substrate conversion. If superimposed on the reaction a time dependent catalyst deactivation occurs, the reactive shell should move to the center of the catalyst particle where active cells are available to substitute for the deactivated cells closer to the surface. 

In a first case we will model the chemical reaction controlled regime and discuss the effects of temperature, pressure, catalyst loading and presence of adsorbable additives on the selectivity. 

How the regimes are identified in an Arrhenius plot is indicated in Figure 6.12 for deposition of silicon using various precursors having different reactivities (but similar slopes in the reaction-controlled regime). The reaction-limited (or catalytic)... 

Question Calculate the crossover temperature between the surface-reaction-controlled regime and the diffusion-controlled regime for the active gas corrosion of Ti(s) by HCl(g). In other words, calculate the temperature at which the rates of these two processes are equal. Use the same values provided in Examples 5.2 and 5.3 and assume that Z>hci is independent of temperature. 

With the exception of the presence of methane product in the feed, the high-tenperature operation, the excess hydrogen, and the elevated pressure all support an increase in reaction rate and a reduction in reactor volume. This suggests that the catalyst is not hot —that is, the catalyst is still operating in the reaction-controlled regime and mass transfer effects have not started to intrude. For these conditions, the manipulation of tenperatures and pressures is essential to limit the reactor size. 

Figure 6, Combustion rates various coals near chemical reaction controlling regime (2, 22,23,24,25 ...

An empirical equation in the chemical reaction controlling regime developed by Wen ( ) based on the volume reaction model has the form ... 

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