Influence of Contact Time

As revealed earlier in this chapter, one important aspect of the WGS catalyst is that it should be able to operate at high flow rates to reduce the total reactor size for the production of hydrogen. To examine the impact of the contact time, we increased the space velocity over the catalysts for the feed mixture of 0.5% CO and 1.5% H2O in He from 5000 h-1 to 30,000 h-1, as shown in Figure 8.16. All three of the different

FIGURE 8.16 The effect of increasing the flow rate and decreasing the contact time for WGS reaction over the G-66 A, Cu0.2Ce0.8O2-y, and Cu01Ce0.9O2-y catalysts. Empty symbols illustrate low flow rate, S.V. = 5000 h-1, and filled symbols high flow rate, S.V. = 30,000 h-1, and. The dotted line represents the equilibrium curve for a feed-gas composition of 0.5% CO and 1.5% H2O in He. The solid lines are model fits assuming first-order reversible kinetics.

FIGURE 8.16 The effect of increasing the flow rate and decreasing the contact time for WGS reaction over the G-66 A, Cu0.2Ce0.8O2-y, and Cu01Ce0.9O2-y catalysts. Empty symbols illustrate low flow rate, S.V. = 5000 h-1, and filled symbols high flow rate, S.V. = 30,000 h-1, and. The dotted line represents the equilibrium curve for a feed-gas composition of 0.5% CO and 1.5% H2O in He. The solid lines are model fits assuming first-order reversible kinetics.

catalysts respond very similarly, with the CO conversion decreasing upon increasing the space velocity, i.e., decreasing the contact time. To achieve the same CO conversion at the higher space velocity, temperatures approximately 50 to 100 K higher are required.

In conclusion to this section, we can state that the copper-ceria catalysts are nonpyrophoric and stable, showing little or no deactivation during the experiments and shutdown-start-up cycling. The Cu0.2Ce08O2-y catalyst prepared by a coprecip-itation method showed good catalytic activity for the WGS reaction. The Cu0.1Ce0.9O2-y catalyst prepared by sol-gel method was found to be less active, which could be due to a lower amount of active copper sites or to different CuO crystallite size and structure. The copper-ceria catalysts were shown to be selective for the WGS reaction, and no methanation reaction were observed over any catalyst under the experimental conditions used.

Model fits of the experimental data show that the simplified first-order elementary reaction kinetics for these catalysts can be used to approximate the WGS reaction as a single reversible surface reaction. This model reaction-rate expression could be used in all concentration ranges of H2O present in the feed due to a possibly different rate-limiting step not including H2O.

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