Deactivation, uniform effectiveness

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. 

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. 

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. 

Figure 4. The linear relationship of poisoning might be due to uniform poisoning, i.e., sites of equal activity were deactivated at zero coke content. Figure 4 shows that pyridine and quinoline are more poisonous than aniline. It shows that the higher basicity compounds have greater effectiveness as poisons. Quinoline which has a higher molecular weight and lower basicity than pyridine showed a slightly lower effectiveness than pyridine.

The novel reactor was used to study the deactivation of n-hexane cracking on an US-Y zeolite catalyst. These experiments showed that on a faujasite the coke formation deactivates the main reactions and not the coking reaction itself, in contrast with previous observations on pentasil zeolites. The coke deposition also modifies the product distribution of n-hexane cracking. This effect can be explained by the non-uniform strength of the acid sites in the Y-zeolite and the acid strength requirements of the various reactions. 

As we shall see, pioneers in the use of antipsychotic drugs almost uniformly cited deactivation as the main clinical effect of neuroleptics. Because of this, clinicians often referred to the neuroleptic effect as a chemical lobotomy (Haase, 1959). Bleuler (1978) observed that longterm neuroleptic use also often dampens the vitality and the initiative of the person (p. 301). He concluded, So we see that long-term maintenance with neuroleptics is fraught with some of the same disadvantages that are ascribed to lobotomies (p. 301). Chapter 5 will discuss permanent cognitive impairment and dementia from these drugs. 

Effect of k. For a very low value of kj and the axial profile of activity 0 does not depend strongly on the axial coordinate z. The loweris m the more uniform is 0. Evidently,for low values of k, a standing wave type of deactivation occurs. Here, during the slow deactivation process, the temperature in the hot spot decreases. For k =20.0 (fast deactivation)>a constant pattern profile exist and the temperature increases during the deactivation process. 

In the studies of the synthesis of the ansamycin antibiotic rifamycin S (13S), Corey and Clark [76] found numerous attempts to effect the lactam closure of the linear precursor 132 to 134 uniformly unsuccessful under a variety of experimental conditions, e.g. via activated ester with imidazole and mixed benzoic anhydride. The crux of the problem was associated with the quinone system which so deactivates the amino group to prevent its attachment to mildly activated carboxylic derivatives. Cyclization was achieved after conversion of the quinone system to the hydroquinone system. Thus, as shown in Scheme 45, treatment of 132 with 10 equiv of isobutyl chloroformate and 1 eqtuv of triethylamine at 23 °C produced the corresponding mixed carbonic anhydride in 95% yield. The quinone C=C bond was reduced by hydrogenation with Lindlar catalyst at low temperature. A cold solution of the hydroquinone was added over 2 h to THF at 50 °C and stirred for an additional 12 h at the same temperature. Oxidation with aqueous potassium ferricyanide afforded the cyclic product 134 in 80% yield. Kishi and coworkers [73] gained a similar result by using mixed ethyl carbonic anhydride. 

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. 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Poisoning. Both the membrane and the catalyst in a membrane reactor may become deactivated over time in the application environment. This poisoning arises from some species present in the feed stream or from some product(s) of the reaction. When the poison is present in the feed stream at a relatively high concentration and is weakly adsorbed onto the catalyst or membrane surface or when the poison is formed by reaction, it is uniformly distributed throughout the catalyst. On the other hand, if the poison is present in the feed stream in a relatively low concentration and is strongly adsorbed, the outer pore surfaces can completely lose catalytic activity before the inside pore surfaces do. When significant deactivation of either or both occurs, effective... 

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. 

A similar shape selective effect was observed in the liquid phase. Those catalysts with the smaller pore and channel openings were more selective for para-nitrotoluene. However, in the liquid phase, no induction period was observed. Rather, all catalysts exhibited significant deactivation throughout time on stream and after 5 hrs. little of the original activity remained. As shown in Fig. 3, the para selectivity was found to decrease with time on stream. This would indicate that deactivation occurs within the pore channels effectively reducing the preferential capacity of the catalyst to generate the para isomer. The decrease in para selectivity was not evident on Beta zeolite, which has larger pores and may allow for a more uniform production of... 

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