Shell Progressive Poisoning

A similar model that specifically considers the poison deposition in a catalyst pellet was presented by Olson [1968] and by Carberry and Gorring [1966], Here, the poison deposition leads to a moving boundary of a poisoned shell surrounding an unpoisoned core. Models of this type are also often used for non catalytic heterogeneous reactions, as discussed in detail in Chapter 4. The pseudo steady-state assumption is made that the boundary moves rather slowly compared to the poison diffusion or reaction rates. Then, steady-state diffusion results can be used for the shell, and the total mass transfer resistance consists of the usual external interfacial, pore diffusion, and boundary chemical reaction steps in series. 

Shp = kgp RIDeP = modified Sherwood number for poison Da = ksP RIDeP = Damkbhler number  

The reference time t ef and the concentration Cp -gf are chosen for a specific application (e.g., in a flow reactor, the mean residence time and feed concentration, respectively). Equation (5.2.3-6) now permits a solution for the amount of poison, (Cpy)/Cpy , to be obtained as a function of the bulk concentration Cp and the physicochemical parameters. In a packed bed tubular reactor, Cp varies along the longitudinal direction. Equation (5.2.3-6) would then be a partial differential equation coupled to the flowing fluid-phase mass balance equation—these applications are considered in Chapter 11. 

This is an implicit equation for Cpj)/Cpj, which is represented in Fig. 5.2.3-1. These results can be used to predict the poison deposition as a function of time and the physiochemical parameters. 

Now that the poison concentration is known, its effect on the chemical reaction must be derived. Again, this is based on the assumption that the poisoned shell is completely inactive so that, for a first-order reaction occurring only in the unpoisoned core of the catalyst, the mathematical model becomes  

The mathematical statement of the rate of poison deposition is as follows  

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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. 

Pavlou, S., and Costas, G. V, Optimal Catalyst Activity Profile in Pellets with Shell-Progressive Poisoning The Case of Fast Linear Kinetics, Chem. Eng. Sci. 45 (3) (1990) 695-703. 

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. 

S. 1 For shell progressive poisoning, the shrinking core model of Chapter 4 was utilized to derive the time rate of change td poison deposition. Eq. S.2c-6 complete the steps leading to this result. 

The effect on the reaction rate for shell progressive poisoning is based on Eqs. S.2c-8,9, and 10. Use these to derive the effectiveness factor relation, Eq. S.2.C-II. 

Effect of Shell-Progressive Poisoning on the Selectivity of Simultaneous Reactions... 

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... 

Table 5.2 Parameters Shell-Progressive Poisoning in the Hydrogenation of Benzene on Ni/Kieselguhr by Thiophene (Lee et al. 1978)...

Figure 5.3 Shell-progressive poisoning, slab-like pellet.

Figure 5.4 Activity factor for uniform and shell-progressive poisoning. (Wheeler 1955)...

Diffusion-affected reaction and shell-progressive poisoning... 

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