Design studies for monolithic high temperature shift catalysts: Effect of operational parameters

In this study, the performance of a commercial high temperature shift (HTS) catalyst has been investigated. The catalyst was a wash-coated ceramic monolith type and used to adjust the H₂/CO ratio in a simulated syngas stream. In the experiments, the effects of inlet gas composition, gas hourly space velocity, inlet steam to CO ratio and reaction temperature have been investigated. The results showed that all these parameters have considerable influence on the design of HTS reactor. Precious metal based monolith catalysts enable working under high space velocities and thus reduce the volume of HTS reactor. Specifically defined selectivity, as ratio of the total amount of CO₂ and H₂ to the amount of CO₂ formed during the process, seems to be a good measure of possible unwanted side reactions that might occur. It was found that the selectivity ratio should be ideally about two for HTS process, minimizing the formation of side reactions. Selectivity values below 2.0 indicate the presence of unbalanced H₂ which is incompatible with WGS reaction stoichiometry. Results showed that steam is an effective parameter in determining the probability of side reactions, especially the reactions leading to the catalyst deactivation through coke deposition. Another point is that coking tendency of the catalyst is more severe at lower operating temperatures. The formation of methane, an unwanted by-product, was seen to be favored by lower gas hourly space velocities, possibly via the reaction with H₂. The formation of methane results in hydrogen consumption to some extent and consequently alters the product composition. The optimum operating conditions of the wash-coated monolith type HTS catalyst studied were found to be as follows: Temperature = ~ 375-400 C, GHSV = ~ 50,000 h^{- 1} and the inlet steam to CO ratio = ~ 2.0.