Catalyst deactivation rate, impact

Figure 4 Impact of catalyst deactivation rate on predicted MAT and FFB conversions.

We have shown that additive coke (Cat]j) has much less impact on catalyst activity than catalytic coke (Ccal) at the same coke-on-catalyst level, but the initial catalyst deactivation rate during ARO cracking is greaier than ihat of VGO cracking because of the fast deposition of additional coke on the caialyst surface. The general correlations developed in this paper can be conveniently u sod in the modeling of catalytic cracking reaction kinetics. 

Coke selectivity directly influences the rate of catalyst deactivation as seen by comparing coke selectivities in Tables VI and VII with observed rate constants in Table V. Our data indicate calcined AFS zeolites show higher coke selectivities than USY zeolites when compared at similar unit cell sizes. This result suggests that distribution of framework acid sites(as reflected by the distribution of framework silicon) has a strong impact on coke selectivity. In addition, coke selectivity has been shown to correlate with the density of strong acid sites in the framework(20). Our data confirm this and show that steaming decreases the density of such sites which, in turn, leads to decreased coke selectivities. 

For many catalytic processes involving hydrocarbons the major deactivation pathway comes from the formation and deposition of carbonaceous species. Often referred to as coke, on many catalytic materials its deposition leads to transport resistances for reactants and products and blocks access to active sites. U nderstanding its rate of formation and impact on catalyst deactivation is of great importance in developing and modeling new catalytic processes. Developing a fundamental understanding of coke deposition enables the proper selection of catalytic materials and process conditions to minimize the impact of deactivation on a process. 

Once a catalyst that meets the minimum required performance standards is identified, the research effort is then shifted from discovery toward a development type of activity as shown in the middle cycle. Hence, more detailed evaluation of the catalyst candidates is conducted using a more sophisticated reactor system. This system should be designed so it can provide experimental data that can be used as the basis for discrimination between various proposed kinetic mechanisms and the associated kinetic rate parameters. It should also be crqrable of providing information on catalyst activity versus time-on-stream for quantifrcation of catalyst deactivation. Since the cost of periodic catalyst replacement or regeneration to maintain plant productivity can have a significant impact on process economics, information on catalyst activity and the catalyst performance parameters over a range of activities is critical for identifying more precise catalyst research milestones. If the minimum required level of catalyst performance versus time-on-stream is not attained, it may be necessary for additional discovery work to be undertaken. 

The concentration of metals in the feedstock can also have a major impact on catalyst life. Figure 53 compares the relative catalyst lifetimes for a typical HDS catalyst processing the high-metals Maya residuum and an Arabian Heavy residuum. As is evident, a higher concentration of metals in the feedstock increases the rate of deactivation of both the intermediate period and the final pore-plugging phase. New catalyst systems are required to handle heavy feeds that have metal concentrations of this magnitude. 

The catalyst support can have a significant impact on the rate of a catalyzed reaction, the reaction pathway (quantities and species of intermediates and products) and the resistance of the catalyst to deactivation. This section summarizes some observed effects of the catalyst support on these parameters. 

The catalyst support impacts the rate of a catalyzed reaction, the reaction pathway (quantities and species of intermediates and products) and the resistance of the catalyst to deactivation. In DBCP reactions, powders had higher rate constants than beads, presumably due to reduced mass transfer limitations alumina yielded a faster rate than C, which had a faster rate than PEI/silica. Sorptive capabilities of the supports may also play an important role Kovenklioglu found that supports which sorbed 1,1,2-TCA more strongly had higher reaction rates, and Farrell concluded that TCE sorption to Fe cause higher reaction rates on Pd/Fe electrodes than on pure Pd electrodes. It is also clear that supports influence reaction products, but the correlation between a given support and pathways/products it promotes is not yet understood. The choice of support can also affect its resistance to deactivation this implies that catalyst supports may be tailored to maximize activity over the long term. 

MS analysis was conducted on the deactivated catalysts from the MAT reactors using a VG instrument in which the probe was heated from ambient to 500 C at a rate of 20 C min1 and spectra over the mass range 50-600 were recorded every 5s. Spectra were recorded in both electron impact (El) and chemical ionisation (Cl, with ammonia) modes. A number of deactivated samples have also been analysed after extraction in chloroform to remove physically-trapped molecular species. 

In our water effect experiment, a primarily reversible decrease in CO conversion was observed when up to 30 vol% of water was added to the feed for this catalyst [9], With the 0.5% Pt-15%Co/Al203 catalyst, a reversible water effect was obtained at a lower volume percent of water addition but irreversible deactivation occurred at > 25% vol. water addition [7], One possibility for the effect of water is that the amount of catalytic active sites (i.e., surface cobalt metal atoms) available for the FT reaction changes with partial pressure of water, perhaps by a temporary oxidation process for cobalt [9], Alternatively, competitive adsorption of water may decrease the surface concentration of CO and/or H2 [9], Thus, the following equation is proposed to described the reversible impact of water on the CO reaction rate ... 

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