Deactivation, uniform

Uniform Distribution of Poison Suppose the rate of the adsorption (or reaction) process which poisons the catalytic site is slow with respect to intrapellet diffusion. Then the surface will be deactivated uniformly through the pellet. If a is the fraction of the surface so poisoned, the rate constant will become A i(l — a). The rate per pellet, according to Eq. (11-44), is... 

Figure 10.9 Reactor profiles of the fraction of catalyst deactivated uniform deactivation and T — 20s.

Figure 10.8 Reactor profiles of the concentration of the species causing deactivation uniform deactivation and r = 20. ...

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

The regenerator design, either single-stage or two-stage, should provide uniform catalyst regeneration, increase flexibility for processing a variety of feedstocks, and minimize thermal and hydrothermal deactivation of the catalyst. 

Example 11.15 Coke formation is a major cause of catalyst deactivation. Decoking is accomplished by periodic oxidations in air. Consider a micro-porous catalyst that has its internal surface covered with a uniform layer of coke. Suppose that the decoking reaction is stopped short of completion. What is the distribution of residual coke under the following circumstances ... 

Thin sections cut with a diamond knife microtome can be of great advantage in locating regions of catalyst where important chemical or structural changes take place during reaction. Comparison of equivalent areas of fresh and deactivated catalyst can be a difficult problem if the catalyst support does not have a uniform microstructure as in carbon supports produced from plant materials. Even when specimen selection and preparation are adequate, it may be difficult to know upon which image features to place the electron beam to solve the problem at hand. 

Studies of octylsilane (OS) phases, deactivated by end-capping, have shown that such stationary phases lead to a discrimination between compounds according to their H-bond donor capacity, as the stationary phase presents strong accessible H-bond acceptor groups (-Si-O-Si-) [22, 23]. For OS phases with a uniform matrix of cross-Hnked polysiloxane alkyl groups, relatively low correlations between log few and log Poet were found. 

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. 

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. 

P7.06.08. /3 is the fraction poisoned. The ratio, with the clean effectiveness, r/c. For a given mouth deactivation is more serious than uniform. 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

Very litde is reported regarding the effect of ultrasound on Ziegler-Natta polymerisation. The first report was by Mertes [97] who obtained a more uniform poly(ethene) in the presence of ultrasound. It was suggested this was as the result of a better dispersion of the catalyst and the prevention of catalyst deactivation (sweeping clean) in the presence of ultrasound. 

Deactivation may also be uniform for all sites, or it may be selective, in which case the more active sites, those which supply most of the catalyst activity, are preferentially attacked and deactivated. 

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. 

Three obvious models which could describe the observed reaction rate are (a) concentration equilibrium between all parts of the intracrystalline pore structure and the exterior gas phase (reaction rate limiting), (b) equilibrium between the gas phase and the surface of the zeolite crystallites but diffusional limitations within the intracrystalline pore structure, and (c) concentration uniformity within the intracrystalline pore structure but a large difference from equilibrium at the interface between the zeolite crystal (pore mouth) and the gas phase (product desorption limitation). Combinations of the above may occur, and all models must include catalyst deactivation. 

It is found in this study that an adjustment of pH value of solution by acid (HF or HC1) to 10.5 is very important for the effective formation of uniform mesopores. However, the acid should be added into the mixture solution after the addition of surfactant otherwise, the formation of the ordered mesoporous structure would be affected. The explanation is that when acid is added to a mixture solution without surfactant, the pH value of system will reduce and subsequently influence the interaction between cationic surfactant and anionic silicate species in the mixture, leading to the poor polymerization of inorganic silicate species. In addition, when HF is used prior to the addition of surfactant, the formation of stable NajSiFg can deactivate the polymerization of silicate species, further terminating the growth of mesoporous framework. 

Rajagopalan and Luss (1979) developed a theoretical model to predict the influence of pore properties on the demetallation activity and on the deactivation behavior. In this model the change in restricted diffusion with decreasing pore size was included. Catalysts with slab and spherical geometry composed of nonintersecting pores with uniform radius but variable pore lengths were assumed. The conservation equation for diffusion and first-order reaction in a single pore of radius rp is... 

Agrawal (1980) computed the deposit concentration in the microspheres as a function of position in the pellet and time. Initially, the deposit concentrates in the microspheres near the outer region of the pellet. With the outer region deactivated, deposit formation progresses inward because of access to the inner microspheres through the unobstructed macropores. The profiles in the microspheres are more or less uniform because of the low Thiele modulus. In the case of the unimodal catalyst without macropores, the buildup of deposit in the outer region of the pellet would seal the outer pores and prevent access to internal sites. 

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