Catalyst deactivation distribution parameter

The foregoing review of the alkylation mechanism and the influence of the catalyst type and reaction conditions show that, in essence, the chemistry is identical with all the examined acid catalysts, liquid and solid. Differences in the importance of individual reaction steps originate from the variety of possible structures and distributions of acid sites of solid catalysts. Changing process parameters induces similar effects with each of the catalysts however, the sensitivity to a particular parameter depends strongly on the catalyst. All the acids deactivate by the formation of unsaturated polymers, which are strongly bound to the acid. 

The severity of the conditions can also influence the catalyst deactivation rate. Higher severity or deeper desulfurization can be obtained by operation at higher temperatures for more catalyst activity. As shown in Fig. 54, however, this higher activity is at the expense of useful catalyst life. At higher temperature the metals distribution parameter is lower and coke formation is more rapid because of increased catalyst activity. 

A model for the riser reactor of commercial fluid catalytic cracking units (FCCU) and pilot plants is developed This model is for real reactors and feedstocks and for commercial FCC catalysts. It is based on hydrodynamic considerations and on the kinetics of cracking and deactivation. The microkinetic model used has five lumps with eight kinetic constants for cracking and two for the catalyst deactivation. These 10 kinetic constants have to be previously determined in laboratory tests for the feedstock-catalyst considered. The model predicts quite well the product distribution at the riser exit. It allows the study of the effect of several operational parameters and of riser revampings. 

Vanadium molecular size distributions in residual oils are measured by size exclusion chromatography with an inductively coupled plasma detector (SEC-ICP). These distributions are then used as input for a reactor model which incorporates reaction and diffusion in cylindrical particles to calculate catalyst activity, product vanadium size distributions, and catalyst deactivation. Both catalytic and non-catalytic reactions are needed to explain the product size distribution of the vanadium-containing molecules. Metal distribution parameters calculated from the model compare well with experimental values determined by electron microprobe analysis, Modelling with feed molecular size distributions instead of an average molecular size results in predictions of shorter catalyst life at high conversion and longer catalyst life at low conversions. 

The deactivation of the catalysts coked by BIT shows a much different relationship with respect to activation energy (Fig. 1). The experimental data are displaced to higher activation energies from those of the AN-coked catalysts. Since the distribution parameter of the fresh catalyst must be the same for the coked catalysts, is it obvious that deactivation by BIT cannot be by SSD. Hence, the BIT-coked catalysts are considered to deactivate by site preference deactivation (SPD). A value of the site preference parameter, g, was selected and a plot generated by varying p. Another g was selected and the process repeated until the best fit to the experiment BIT data of Fig. 1 was obtained. This occurred for a value of g of 2.7, and the fit is shown by the curve on the right-hand side of Fig. 2. 

It is noticed that C7 and Cg aromatics are the major components of the aromatic product. Whereas C9+ aromatics (highers) are relatively in low yields. However the process optimization studies indicated that the product selectivities depend on catalyst characteristics and the various process parameters employed. The data shown in Figure 8, illustrate the effect of process severity on aromatic product distribution. At high severity conditions, increased yield of aromatics accompanied by increased selectivity to the C9+ aromatics. The rate of deactivation of the catalyst was observed to be faster under high severity conditions. It is clear from these observations, that there exists a relationship between C9+ aromatic selectivities and deactivation rates. Both the catalysts and process conditions were optimized for the LPG and aromatic mode of process operations. The activity and stability in activity of these modified catalysts were compared with those of unmodified ZSM-5 catalyst. As noted earlier the aromatic yield is low on HZSM-5 and decreases further with time on stream. In the case of metal modified ZSM-5 catalyst the aromatic selectivity is high, and decrease in activity is comparatively less in this case. In the case of catalysts used for LPG production also similar results were obtained, showing that the acid-modified catalyst is superior to as-synthesized catalyst. It can also be seen that as-synthesized catalyst deactivates much faster than the acid-modified catalyst. 

The performance indexes, which define an optimal catalyst distribution, include effectiveness, selectivity, yield and deactivation rate. The key parameters, affecting the choice of the optimal catalyst profile, are the reaction kinetics, the transport resistances, and the production cost of the catalyst. An extensive review of the theoretical and experimental developments in this area is available [20]. Two typical examples to demonstrate the importance of an appropriate distribution of the active components are now described. 

Simulated data are shown in Figure 1 with solid lines and the parameter values that best fit experimental data in Table 1. The specific reaction rate constant x(l) has the same value for all cases and this is consistent with the fact that each deactivation run was done starting with fi esh catalyst. On the other hand, parameter x(2) reflects some type of equilibrium state reached for free active catalytic sites. It takes very small values for a poison as pyridine, intermediate values for an inhibitor like CS2, and values close to one for pure n-octane and for n-octane mixed with the smallest amount of benzene. In larger amounts, benzene seems to act as coke inhibitor, which is consistent with the fact that it does not change activity in a significant manner but it does change the distribution of reaction products. 

Copper-chromium catalysts employed for CO oxidation were found to be affected by composition and pretreatment parameters. CuCr20i, was more active than CuO only if prereduction was carried out and if metal concentration on alumina support was larger than 12 w t %. The presence of Cr with Cu in the oxide limited the extent of catalyst reduction leading also to less deactivation as compared to Cu on alumina. The presence of Cr also decreased an activity inhibition effected by water. A supported Cu-Cr catalyst used in an automobile ran with leaded petrol was deactivated by lead deposition. Deposits were mainly lead sulphate located on pellet periphery. Also, lead was preferentially distributed on the alumina instead of on the active metal-rich zones of the catalysts. 

Nam et al.l studied the deactivation of a commercial catalyst, 10% V2O5 on alumina, by SO2 in the reduction of NO by NH3. The feed gas was the flue gas from the combustion of No.2 fuel oil in a laboratory furnace, doped with NO and NH3. The physico-chemical properties of the deactivating catalysts were correlated with its activity and accumulating sulfur content, and the deactivation was modeled. The activation energies of fresh and deactivated catalysts were similar. The sulfur content of the catalyst, as well as the surface area, appeared to be a dominant deactivation parameter, analogous to coke-induced deactivation. Pore size distribution changes indicated that... 

The coking and regeneration of a reforming catalyst was studied by physical characterization methods (pore volume, tortuosity, porosity, carbon distribution) as well as by kinetic investigations on the reaction rate of coke bum-off. For temperatures of industrial relevance for the Pt/Re-A Os catalyst, i.e. below 550°C (deactivation), the bum-off rate is determined by the interplay of chemical reaction and pore diffusion limitation by external mass transfer can be excluded. Based on the kinetic parameters, the process of the regeneration of a technical reactor is discussed. 

In addition to these kinetic investigations the catalyst was characterized with respect to the following parameters internal surface area, porosity, pore diameter, radial coke distribution within the particle und the tortuosity. Thereby also the change of these parameters for different carbon loads during deactivation and regeneration were determined. 

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