Deactivation shell-progressive

Figure 10.13 Profiles of the concentration of the species causing deactivation shell-progressive deactivation.

Figure 10.14 Profiles of the fraction of catalyst deactivated shell-progressive deactivation.

They considered deactivation to occur by either pore-mouth (shell-progressive) or uniform (homogeneous) poisoning and examined the effect these types of deactivation had on overall activity and production rates for a single catalyst pellet. Analytical solutions were obtained for the production per pore by considering the time dependence of activity. Their results will be used here as the basis for the development of models for deactivation in fixed bed reactors. 

Catalyst deactivation in large-pore slab catalysts, where intrapaiticle convection, diffusion and first order reaction are the competing processes, is analyzed by uniform and shell-progressive models. Analytical solutions arc provid as well as plots of effectiveness factors as a function of model parameters as a basis for steady-state reactor design. 

For unifonnly deactivated catalysts 11 is calcotatcd by multiplying IPdc iven by eq.(8) and ti. given by eq.(lSa), For shell-progressive deactivated catalysts nf is calculated by eq.(M). Results for Da=l and oM). are shown below ... 

In both cases the enhancement of conversion due to iniraparticle convection is higher for some intermedite value of Thiele modulus. The reactor performance is worse in the case of shell-progressive deactivated catalysts for oH). t however, the enhancement due to convection is higher in this case. 

Pore Mouth (or Shell Progressive) Poisoning This mechanism occurs when the poisoning of a pore surface begins at the mouth of the pore and moves gradmuly inward. This is a moving boundary problem, and the pseudo-steady-state assumption is made that the boundary moves slowly compared with diffusion of poison and reactants and reaction on the active surface. P is the fraction of the pore that is deactivated. The poison diffuses through the dead zone and deposits at the interface between the dead and active zones. The reactants diffuse across the dead zone without reaction, followed by diffusion-reaction in the active zone. 

T. Bacaros, S. Bebelis, S. Pavlou and C.G. Vayenas, Optimal catalyst distribution in pellets with shell progressive poisoning the case of linear kinetics, in Catalyst Deactivation 1987 (B. Delmon and G.F. Forment, Eds.), pp. 459-468,1987. 

A little refiection on the results shown in Figure 7.19 should be enough to convince us that when the rate of deactivation is very high, the profiles of activity developed will be very sharp and the deactivated zone in the catalyst particle will proceed from the outside to the center as a growing shell (shell progressive) of deactivated catalyst grows with a well-defined interface between active and deactivated zones. The physical picture is not that much different from that of the zones developed in noncatalytic gas-solid reactions. 

This is just the deactivation function for the shell-progressive model. 

The amount of poison deposited is given as a function of the dimensionless process time by Fig. 5.2a -l. Also, the deactivation function for given poison levels is in Fig. 5.2.C-2. Combine these in a figure for the deactivation function as a function of dimensionless time for the shell progressive mechanism. 

For reactor design, it is important to know how the solutions of Eqs. 5.32, 5.34, and 5.35 affect the intrinsic rate of reaction. Wheeler (1955) treated this deactivation-diffusion problem for two limiting cases uniform and pore-mouth (shell-progressive) poisoning. As described in the previous section for noncatalytic gas-solid reactions, the poison will deposit preferentially on the pore-mouth initially and grow progressively inward with time, if the rate of poisoning is rapid relative... 

It has been pointed out that the exposed surface area is reduced in a shell-progressive manner when the deposition of particles causes the physical deactivation. On the... 

The relationship of q. 6.70 is also applicable to the case of shell-progressive chemical deactivation since Sr/S p = 1 — y, where y is the fraction of catalyst completely deactivated. The value of y is the same under the assumption of uniform sintering whether or not the pellet is also sintered. Here, the time dependence of y can be determined from ... 

For a plug-flow reactor in which the catalyst undergoes either uniform or shell-progressive deactivation, an additional mass balance for the species causing deactivation is required along with Eqs. 10.16 through 10.18 ... 

In the case of shell-progressive deactivation, the global rate is given by the solution (Chapter 5) to ... 

Under the conditions of negligible external mass transfer resistance and an isothermal pellet, Eqs. 10.32, 10.37, and 10.38 can be removed from consideration for the case of uniform deactivation. However, Eq. 10.32 still needs to be retained in a limited form to account for the temperature drop in the deactivated outer shell in the case of shell-progressive deactivation. 

Table 10.6 Analysis Equations for Plug-Flow Reactors Shell-Progressive, Independent Deactivation, and Diffusion Limitation...

Figure 10.15 Profiles of reactor point effectiveness shell-progressive deactivation.

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