Thiele modulus poisoned catalyst

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. 

In order to demonstrate the selective effect of pore-mouth poisoning, it is instructive to consider the two limiting cases of reaction conditions corresponding to large and small values of the Thiele modulus for the poisoned reaction. For the case of active catalysts with small pores, the arguments of all the hyperbolic tangent terms in equation 12.3.124 will become unity and... 

This equation indicates that a small amount of poisoned surface can lead to a sharp decline in apparent activity. For example, if only 10% of the catalyst surface has been deactivated in the case where the Thiele modulus for the unpoisoned reaction is 40, 3F = 0.200 so that the... 

For situations where the reaction is very slow relative to diffusion, the effectiveness factor for the poisoned catalyst will be unity, and the apparent activation energy of the reaction will be the true activation energy for the intrinsic chemical reaction. As the temperature increases, however, the reaction rate increases much faster than the diffusion rate and one may enter a regime where hT( 1 — a) is larger than 2, so the apparent activation energy will drop to that given by equation 12.3.85 (approximately half the value for the intrinsic reaction). As the temperature increases further, the Thiele modulus [hT( 1 — a)] continues to increase with a concomitant decrease in the effectiveness with which the catalyst surface area is used and in the depth to which the reactants are capable of... 

For large values of the Thiele modulus the fraction 0(1 - ) will usually be sufficiently large that F= (1 + 0 )". Curve 3 in Fig. 3.12 depicts selective poisoning of active catalysts near the particle exterior and is the function represented by equation 3.59. Curve 4 describes the effect of selective poisoning for large values of the Thiele modulus. For the latter case the activity decreases drastically, after only a small amount of poison has been added. 

On the other hand, uniform or homogeneous catalyst poisoning presumes that the poison precursor species has full access to the catalyst interior before deactivation begins. There is no dif-fusional resistance for this species. This will be more likely to occur when the pores are large, the catalyst pellets small, and the intrinsic deactivation rate is low. In addition smaller poison precursor molecules will be able to diffuse more rapidly into the catalyst interior. Here the Thiele modulus for poison laydown h will be small, and in the limit, zero. 

Another criterion linked to the diffusion hypothesis, suggested by Sing and Merril , provides for MWD narrowing through partial poisoning of the catalyst, since it causes a lowering of Thiele modulus (lower C values). It was found, on the contrary that, by treating TiClj with various poisons, it was possible to obtain even... 

The Wheeler-Robell analysis envisions the main reaction to be dilfusion-con-trolled, but not the poisoning reaction. Whether or not this is so is a question of relative dimensions of molecules, but dual diffusion control would seem more typical. The analysis has been extended to diffusion-controlled poisoning by Haynes [H.W. Haynes, Jr., Chem. Eng. Sci., 25, 1615 (1970)], who used a shell model as an approximation for rapid poisoning in a Type I system. The Thiele modulus for the poisoning reaction is and shell model assumption to be valid. For a spherical catalyst particle the fraction of original activity, s, is related to the radius of the poison-free zone by... 

Some metallic compounds present in trace level (ppm) in petroleum feed, adsorbed to the active site of the catalyst, act and change the selectivity of the reaction by producing more and more unwanted products. When water vapour is present in the sulphur dioxide-air mixture supplied to a platinum-alumina catalyst, a decrease in oxidation activity occurs. This type of poisoning is due to the effect of wafer on fhe sfrucfure of the alumina carrier and is known as stability poisoning. The resulting increase in diffusional resistance may dramatically increase the Thiele modulus, and reduce the effectiveness factor for the reaction. In extreme cases, the pressure drop through a catalyst bed may also increase dramatically. 

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