CRE Diffusion and Reaction teacher's.pptx

Diffusion and Reaction CRE-II

Overview This chapter presents the principles of diffusion and reaction.

Overview

Diffusion and Reaction in Spherical Catalyst Pellets

Diffusion and Reaction in Spherical Catalyst Pellets- Effective Diffusivity

Diffusion and Reaction in Spherical Catalyst Pellets-Construction Factor

Derivation of the Differential Equation Describing Diffusion and Reaction

Writing the Equation in Dimensionless Form

Solution to the Differential Equation for a First-Order Reaction

Internal Effectiveness Factor

Internal Effectiveness Factor How can the rate of reaction be increased? Type equation here.

Internal Effectiveness Factor

Internal Effectiveness Factor The above discussion of effectiveness factors is valid only for isothermal conditions. When a reaction is exothermic and nonisothermal , the efeectivness factor can be significantly greater than 1 as shown in figure 12-7. Values of η greater than 1 occur because the external surface temperature of the pellet is less than the temperature inside the pellet where the exothermix reaction is taking place. Therefore, the rate of reaction inside the pellet is greater than the rate at the surface.

Internal Effectiveness Factor Thus, because the effectiveness factor can be greater than 1, depending on the magnitude of the parameters β and γ . The parameter γ is sometimes referred to as the Arrhenius number, and the parameter β represents the maximum temperature difference that could exist in the pellet relative to the surface temperature T S

Internal Effectiveness Factor

Falsified Kinetics There are circumstances under which the measured reaction order and activation energy are not the true values. Consider the case in which we obtain reaction rate data in a differential reactor, where precautions are taken to virtually eliminate external mass transfer resistance. From these data we construct a log-log plot of the rate of reaction as a function of the bulk gas-phase concentration (Figure 12-8). The slope of this plot is the apparent reaction order n ’ and the rate law takes the form

Falsified Kinetics

Overall effectiveness factor For first order reactions we can use an overall effectiveness factor to help us analyze diffusion, flow, and reaction in packed beds. We now consider a situation where external and internal resistance to mass transfer to and within the pellet are of the same order of magnitude (figure 12-9). At steady state, the transport of the reactant(s) from the bulk fluid to the external surface of the catalyst is equal to the net rate of reaction of the reactant within and on the pellet.

Overall effectiveness factor

Estimation of the Diffusion and reaction limited Regimes It is necessary to understand how to identify whether a particular heterogeneous reaction is diffusion limited or reaction limited. We will discuss tools that can be used to diagnose from experimental data if a heterogeneous reaction is external/internal diffusion or reaction limited under the given conditions.

Overall effectiveness factor Let us consider Weisz-Prater criterion often used in industry in order to identify whether the reaction is internal diffusion limiting.

Overall effectiveness factor

Solution

CRE Diffusion and Reaction teacher's.pptx

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CRE Diffusion and Reaction teacher's.pptx