Catalysts uniform poisoning

Find the residence time for 50°/. conversion, (a) with fresh catalyst, (b) with catalyst that is 50% uniformly poisoned. 

Acres et al. (22) have speculated on the modes of phosphorus and lead poisoning in monolithic catalysts, based on data obtained in simulated aging. Conversion vs. time-of-exposure curves for catalysts poisoned by either lead or phosphorus show quite different shapes, which the authors attribute to pore-mouth poisoning for phosphorus, and uniform poisoning... 

For uniform poisoning, the interior of the catalyst acts as an internal guard-bed, removing poison which might otherwise inhibit reaction near the pellet surface. This effect is most pronounced at high values of h, where reaction is confined to the periphery of the catalyst. 

CX)NVECnON, DIFFUSION AND FIRST ORDER REACTION IN POISONED CATALYSTS Uniform model... 

Recognition that only part of the surface need be active has dispelled much of the mystery of poisoning, that is, the ability of small traces of certain impurities to destroy the efficiency of a catalyst. Today, poisons are regarded as substances that are preferentially adsorbed by the active portions of the surface when the active area is small, it can be completely blanketed by small amounts of a poison. The active area is regarded as consisting of a number of patches distributed more or less uniformly over the surface.2,10 The active patches are also termed active points, or more often active centers, and much study has been devoted to learn the probable nature of these areas. 

Figure 10.6 Reactor profiles of A and fraction of catalyst deactivated for the model system of Table 10.5 uniform poisoning and t = 17s.

For the reactor system in Table 10.5, values of J versus r have been calculated for the case of uniform poisoning. Constraints are the maximum allowable catalyst temperature (or the maximum reactor inlet temperature) and the minimum allowable conversion. For the adiabatic reactor being considered, the reactor inlet tem-... 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

This relation is plotted as curve Bin Figure 12.11. Smith (66) has shown that the same limiting forms for are observed using the concept of effective dififusivities and spherical catalyst pellets. Curve B indicates that, for fast reactions on catalyst surfaces where the poisoned sites are uniformly distributed over the pore surface, the apparent activity of the catalyst declines much less rapidly than for the case where catalyst effectiveness factors approach unity. Under these circumstances, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as in the earlier case for small hr. With poisoning, the Thiele modulus hp decreases, and the reaction merely penetrates deeper into the pore. 

Testing of catalyst poisoning is best done in CSTRs since then all of the catalyst is exposed to the same concentration of impurity and the temperature is uniform. 

Two limiting cases of the behavior of catalyst poisons have been recognized. In one, the poison is distributed uniformly throughout the pellet and degrades it gradually. In the other, the poison is so effective that it kills completely as it enters the pore and is simultaneously removed from the stream. Complete deactivation begins at the mouth and moves gradually inward. 

Finally, consider side-by-side deactivation. Whatever the concentration of reactants and products may be, the rate at which the poison from the feed reacts with the surface determines where it deposits. For a small poison rate constant the poison penetrates the pellet uniformly and deactivates all elements of the catalyst surface in the same way. For a large rate constant poisoning occurs at the pellet exterior, as soon as the poison reaches the surface. 

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