Coke formation procedure

The limitation to low conversion is the major disadvantage of differential operation. This is not critical if the influence of the catalyst properties on deactivation is studied. If, on the other hand, one is interested in the mechanism and the kinetics of coke formation and in the deactivation of the main reactions, it is necessary to reach higher conversions. A solution to this problem is to combine the electrobalance with a recycle reactor. The recycle reactor is operated under complete mixing, so that the reactor is gradientless. Since in a completely mixed reactor the reactions occur at effluent conditions and not at feed conditions, a specific experimental procedure is necessary to obtain the deactivation effect of coke. 

The same procedure as outlined in Figures 4 to 6 was followed for the determination of the separate influence of coke and of n-hexane conversion on the product select vities, which are ratios of reaction rates. The results are given in Tables 3a and b. Clearly, both the hexane conversion and coke formation induce selectivity changes. The selectivity for coke formation was determined as the ratio of the moles of hexane converted to coke to the total amount of n-hexane consumed. 

The catalytic performance and the effect of coke deposition on the activity and selectivity, were found to be strongly dependent on the preparation procedure, on the reduction temperature, and on the type of support. At low reduction temperatures, the deposition of coke on Ni-Al, (Ni-Al-Ti)sg, and Ni-Ti samples decreased the selectivity to ethylene whereas on (Ni-Al-Ti)imp, deposition of coke increased selectivity. At high reduction temperatures, with only the exception of the Ni-Ti catalyst, coke deposition increased the selectivity to the desired product. The pattern of deactivation by coke was also different for the different samples. In Ni-Ti and (Ni-Al-Ti)sg samples, coke formation strongly diminished their activity and simultaneously increased methane production. With Ni-Al and (Ni-Al-Ti)imp samples, coke did not cause an significant deactivation or an increase in methane yield. Finally, catalysts... 

In recent years, attention has tended to be focused on coke deposition in zeolites (6, 7) in order to characterise the coke formed. In one specific study Groten et al (8) carried out a study of coke formation using zeolite USHY with n-hexane as reactant, but in this case, as in others (6, 7) it was necessary to deposit excessive amounts of coke (> 5%) to enable characterisation of the coke deposits to be achieved. However, if demineralisation of the catalyst is used to concentrate the coke as in the present work the inherently quantitative single pulse excitation (SPE) NMR procedure may be used to characterise coke deposits on FCC catalysts at realistic levels of ca 1% by weight. 

If for the new product a different solvent is used, first the old solvent is washed out over the top of the catalyst bed by the new solvent and the solvent mixture sent to the solvent recovery system of the plant. After that the same start-up procedure as before is followed and the reactor pressure adjusted if necessary. The catalyst is kept wet under liquid all the time in order to prevent decomposition reactions or coke formation at the catalyst surface if exposed to gas and/or air. Catalyst life and reactivation... 

During the 1950s and 1960s the performance of platinum reforming catalysts was considerably improved as operating procedures were optimized. More economic catalysts became available as the platinum content was reduced from more than 0.6 wt% to about 0.35 wt% and the sulfur content of feed naphtha was considerably reduced. Higher purity alumina supports, with better activity and lower levels of sodium and iron could be synthesized reproducibly. An optimum amount of chloride added to the support limited coke formation. 

Coke content of Pt(h,500)AVZ run at 250 °C and of WZ run at 200 °C (both with hydrogen) are 0.49% and 0.32 %, respectively, but the latter is much more condensed. Consequently there is not a direct relation between the amount of coke and its maximum burning temperature. This result is also an indication of the importance ofPt in the generation of a less polymerized coke its hydrogen dissociation capacity reduces polymerization reactions that favor the formation of a more condensed coke. From the point of view of regeneration procedures, the degree of coke polymerization is sometimes more important than the amount of coke. 

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