Catalytic reactions kinetic regimes

Modem catalysts have to be very active and very (100%) selective, that is, they have to catalyze the desired reaction in the temperature window, where the equilibrium conversion is the highest possible and the reaction rate is high enough to permit suitable process economics. To engineer the reaction, one has to obtain first the intrinsic reaction rate, free of heat- and mass-transfer limitations. In many cases this is very difficult, because in the core of the catalytic process there are several physical and chemical steps that must occur and which may preclude the reaction running in the kinetic regime. These steps are as follows ... 

To further ensure there will be no contribution of transport within the washcoat, calculations done based on a method of transient analysis for catalytic ignition by Balakotaiah should be done. Fig. 15 shows the results of the calculation in comparison to where washcoat diffusion occurs. By maintaining a nominal washcoat thickness of 7 micron, the catalyst will operate in the kinetic regime for all reaction conditions considered. 

Figure 3. Transition from the kinetic regime to the diffusion-controlled regime of a heterogeneous catalytic fluid-solid reaction carried out on a porous catalyst.

Here tm is the mass-transfer time. Only under slow reaction kinetic control regime can intrinsic kinetics be derived directly from lab data. Otherwise the intrinsic kinetics have to be extracted from the observed rate by using the mass-transfer and diffusion-reaction equations, in a manner similar to those defined for catalytic gas-solid reactions. For instance, in the slow reaction regime,... 

H has the dimensions of a length and has been called the height of a reactor unit, by analogy with heights of transfer units and equivalent theoretical plates. Interpret Eq. (6.5.4) to show that H is the sum of a height for external mass transfer HTU) and a term dependent on the reaction, the so-called height of a catalytic unit (HCU). Examine the contribution of these terms when the mass transfer, diffusion, and kinetic regimes are dominant. 

As can be easily derived from the concentration pattern, the reaction takes place either mainly in the bulk of the well-mixed liquid phase or in the liquid-phase boundary layer. In reactions which occur in the bulk of the liquid phase, the concentration of gaseous educts decreases only within the interfacial layer (thickness d) to the concentration cAj by physical diffusion processes. Only in the case of mass transport processes that are fast relative to the reaction rate is the latter proportional to the cAl j in the liquid phase. If the catalytic reaction is fast enough a reaction surface may develop within the boundary layer which may even move into the interface itself and, thus, neither the bulk of the liquid nor the liquid-phase boundary layer is utilized any more for the reaction. A simple approach in order to determine the regime of the overall reaction rate can be performed by comparison of the intrinsic kinetics with the rate of mass transfer according to Table 2 [22],... 

Kinetic regime - Reaction velocity is the slowest step of the catalytic process. The surface of the catalysts is used fully and most efficiently in this type of catalysis because the pore surface area and the external surface areas are used equally ... 

A fundamental basis for cyclic optimization of catalytic reactors has been developed. It is based on detailed knowledge of reaction kinetics and fundamental process of mass and energy transport. Power of mathematical modeling and computer simulation has been demonstrated for several reaction systems. It is recommended to invest in fundamental investigations of reacting systems and development of adequate reactor models that could fiirther employ continuously decreasing cost of computer simulation to achieve optimal regimes of chemical reactor performance. 

The important stimuli for the development of the non-linear kinetic theory of steady-state catalytic reactions are 1) necessity to explain the critical phenomena that are experimentally observed in the steady-state kinetic experiments and 2) needs of chemical technology to understand and to apply the advantages of non-linear regimes. 

Recall now that Uq = k/oX and that a L) is the characteristic parameter (similar to the Thiele modulus for catalytic reactions), indicating the magnitude of diffusional effects. Here we wish to define the values of this parameter for F > 0.95 and < 0.05 corresponding to the kinetic regimes indicated in the problem statement. If we let cosh(noT) = M, then... 

Using Equation (23.27)-Equation (23.33), the mathematical simulation of steady-state and dynamic experiments was carried out. The results of calculation are represented in Figures 23.9, 23.10, 23.14, and 23.15 (lines — calculation, points — experiment). It is easily seen that the model demonstrates a good agreement with experiments in both steady-state and dynamic regimes. This confirms the interpretation of the experiments as the influence of capillary condensation on kinetics and dynamics of catalytic reaction. 

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