(48f) Optimal Broad Pore Networks to Improve Resistance to Catalyst Deactivation--Application to Hydrodemetalation

Nanoporous catalysts possess a very high internal surface area per unit volume, which is why they are well suited for large-scale chemical conversions. However, the availability of this large internal surface area is offset by the slow access to the active sites through the narrow pores of the catalyst. This effect is pronounced when the catalyst deactivates by deposition of solid products of the main and/or side reactions. The solid deposits cover the active sites, causing loss of surface area for reaction. More importantly, the increasing deposition reduces the effective pore size for transport, restricting access to the active sites located deep inside the catalyst particles, and eventually leading to complete loss of access to the active sites over a long period of time.

An optimized broad pore network properly introduced in a purely nanoporous catalyst is responsible for an increased reaction yield for diffusion limited reactions, compared to the yield in a purely nanoporous catalyst [1, 2]. Significant improvements in reaction yield have been demonstrated for both deNOx [3] and autothermal reforming of methane [4], by the introduction of broad pores of a unique size, which separate nanoporous regions of another unique size. It is of interest to determine the nature of the optimal broad pore network for catalysts that undergo deactivation. To this end, hierarchically structured hydrodemetalation catalysts (CoMo/Al₂O₃) are optimized to maximize the reaction yield over a given time on stream. Deactivation due to loss of surface area and pore plugging is modeled using the random spheres model (RSM) [5, 6], and is restricted to the nanoporous regions alone. A two-dimensional continuum approach is used to model the hierarchically structured catalyst pellet. The optimal macroporosity distribution is determined for both uniform (single macroporosity), and non-uniform (distribution in macroporosities) catalyst structures. We show that the optimal uniform and non-uniform structures give very similar reaction yields; moreover, the macroporosities in the optimal non-uniform case fluctuate about the optimal uniform macroporosity. The reaction yield can be increased by a factor of almost eight, over the purely nanoporous catalyst, by employing approximately 21 % less catalytic material in the hierarchically structured catalyst; therefore the effective increase in performance is tenfold.

The effect of deactivation on the optimal broad pore network due to reaction on the broad pore walls is also investigated. In the hierarchically structured catalyst, the reactants can react on the external surface of the nanoporous regions, and, subsequently, the deposits can plug the pore entrances in the nanoporous catalyst, leading to low catalyst utilization, in spite of the presence of a broad pore network. This is particularly true of reactions where coke precursors and coke molecules can deposit on the external surface of the purely nanoporous catalyst [7]. The two dimensional continuum approach is extended to include the reaction on the broad pore walls. The optimization of the broad pore network is carried out to optimize both the macroporosity and the size of the broad pores. This mathematical approach can be applied to other industrially important reactions such as the catalytic reforming of naphtha, alkylation, isomerizations and cracking.

References

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