(629d) Significantly Enhanced Reactor-Scale Efficiency and Catalyst Lifetime by Rational Design of the Hierarchical Catalyst Pore Network – Application to Hydrodemetalation

Authors: 
Rao, S. M., Rensselaer Polytechnic Institute
Coppens, M. O., Rensselaer Polytechnic Institute


Our previous work has shown that the lifetime of nanoporous catalysts may be significantly extended by introducing a broad pore channel network of optimized geometry [1,2]. This is realized by an optimal uniform macroporosity and an optimal constant macropore size; interestingly, broad distributions of pore sizes do not lead to notable improvements. This was demonstrated for the example of hydrodemetalation (HDM) catalysts. However, these results only showed the effects at the catalyst pellet scale. A non-trivial, practically relevant extension that we address in this presentation is the effect of the optimized broad pore network at the reactor scale.

First, we mathematically optimize the uniform macroporosity and uniform broad pore size of a hierarchically structured HDM catalyst, to maximize the total mass of metal sulfides deposited over a given time on stream. Following Macé and Wei, first-order HDM kinetics are assumed [3]. Catalyst deactivation, due to coverage of active sites and pore blockage by metal sulfides, is accounted for through the Random Spheres Model (RSM) [3,4] in both the purely mesoporous regions and the macropores of the hierarchically structured catalyst. The properties of the purely mesoporous regions of the hierarchically structured catalyst are taken from the patent literature [5].For a time on stream of 3 months, optimization of the broad pore network results in an optimum initial macroporosity of 25% and an optimum broad pore size of 192 nm. Both values are within the corresponding range of macroporosity and broad pore sizes reported in the patent. Optimization for longer times on stream did not significantly increase the total reaction yield, or significantly change the optimal macroporosity and broad pore size. The total reaction yield from the optimal hierarchically structured catalyst is 1.7 times larger than the total reaction yield from the purely mesoporous catalyst. The purely mesoporous catalyst already completely deactivates within 40 days on stream, when the optimized catalyst is still very active.

Next, the optimized HDM catalyst is used in an isothermal, one-dimensional plug flow model of an industrial fixed bed reactor.  The purely mesoporous HDM catalyst is chosen as a basis to compare the performance of the optimized HDM catalyst against. The purely mesoporous catalyst bed deactivates completely within 3 months on stream; whereas, the lifetime of the optimized hierarchically structured catalyst bed can be extended to more than 7 months. However, with longer times on stream, the exit conversion from the reactor also decreases, and based on a minimum metals conversion of 60%, a time on stream of 5 months is found to be optimal. When utilizing the reactor for 5 months on stream, the total mass of deposited metal sulfides nearly doubles, compared to the purely mesoporous catalyst.  Sensitivity calculations show that varying the initial macroporosity of the catalyst, between 70-200% of its optimized value, in the reactor simulation, does not decrease the total yield from the reactor by more than 15%.

Our work demonstrates the industrial, macroscale opportunities of optimizing the hierarchical catalyst pore network structure in a rational way.

References

1. Rao, S. M.; Coppens, M.-O. Mitigating Deactivation Effects through Rational Design of Hierarchically Structured Catalysts—Application to Hydrodemetalation. Ind. Eng. Chem. Res. 2010, 49, 11087.

2. Rao, S. M.; Coppens, M.-O. Optimal Broad Pore Networks to Improve Resistance to Catalyst Deactivation—Application to Hydrodemetalation. 102nd AIChE Annual Meeting, Salt Lake City, 2010, paper 48f.

3. Macé, O.; Wei, J. Diffusion in Random Particle Models for Hydrodemetalation Catalysts. Ind. Eng. Chem. Res. 1991, 30, 909.

4. van Eekelen, H. A. M. The Random-Spheres Model for Porous Materials. J. Catal. 1973, 29, 75.

5. Abe, S.; Hino, A.; Shimowake, M.; Fujta, K. High-Macropore Hydroprocessing Catalyst and its Use. U.S. Patent 7,186,329, March 6, 2007.