Catalyst deactivation uniform

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

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 ... 

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. 

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. 

The local reactor temperature affects the rates of reaction, equilibrium conversion, and catalyst deactivation. As such, the local temperature has to be controlled to maximize reaction rate and to minimize deactivation. In the case of an exothermic (endothermic) reaction, higher (lower) local temperatures can cause suboptimal local concentrations. Heat will have to be removed (added) to maintain more uniform temperature conditions. The mode of heat removal (addition) will depend on the application and on the required heat-transfer rate. 

Catalyst deactivation was first identified as a percolation-type process by Sahimi and Tsotsis (2,3,41). The process studied by these authors was relatively simple. A porous catalyst, inside a differential isothermal reactor (with the catalytic active material uniformly distributed in its pores at an initial concentration CQ) is reacting while simultaneously undergoing slow deactivation. The overall reaction rate r in a single pore of radius R and length L is given by... 

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. 

It is interesting to note that, due to its uniform and nearly straight pore structure, planar anodic alumina membranes have been used as the probes for monitoring catalyst deactivation at the single pore level [Nourbaksh et al, 1989a]. The nearly idealized structure is suitable for various surface analysis techniques such as scanning and transmission election microscopy and associated EDX and XPS to be applied to fresh as well as spent catalysts. In the case of hydrotreating of crude-derived heavy oils, catalyst... 

Due to the non-uniform coking of the two functions, catalyst deactivation leads to a (hange in the products selectivity. This is shown in terms of the initial (I) and final (F) products selectivities given in Table 2. As discussed elsewhere [ref. 8], C1 production is severely reduced by coking of the metal function, since it purportedly requires a site ensemble of more than one contiguous Pt atoms [ref. 9], The selectivity for the other metal catalyzed reactions in parallel such as ring... 

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. 

The predominance of kinetic studies have assumed uniform sites on the catalyst surface. However, it has long been recognized that many catalyst surfaces exhibit non-uniform sites. Boudart and Djega-Mariadassou [3] have discussed the kinetics of non-uniform surfaces and conclude that "a non-uniform surface behaves catalytically. .like a uniform surface..", and that "rate equations are similar for a given mechanism on a uniform or non-uniform surface". This result justifies "the common practice of neglecting non-uniformity of catalytic surfaces in kinetic studies". However, it appears that uniform catalyst sites catmot adequately explain catalyst deactivation phenomena. The objective of the present study was to explain deactivation in terms of a model based on a variable activation energy site distribution on the catalyst. 

There were two major objectives of this study. Firstly, the effects of the addition of potassium and/or silicon on catalyst deactivation rates and changes in catalyst properties with TOS were investigated. Secondly, the possible causes of catalyst deactivation were examined by following aged catalyst properties and reactor conditions as a function of TOS for each catalyst. The FTS was carried out in a continuous-flow stirred slurr) reactor to ensure uniformity in catalyst aging and reactor conditions throughout the reactor. The aged catalyst properties examined as a ftinction of TOS were total surface areas, carbon deposits and phase transformations. 

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