Transport of sulfur in prereforming catalyst during steaming conditions

Abstract

Industrial start-up, operation, and shutdown conditions of a prereformer catalyst can vary, and the present work has the objective to study the consequence of start-up and shutdown conditions on the sulfur chemisorbed on the nickel surfaces in the catalytic bed. These are conditions where the catalyst is being fed with steam without the presence of hydrogen or a reductive species producing hydrogen. The experimental tests were performed in a pilot unit using aged and presulfided catalyst for the upper part of the catalyst bed and aged but sulfur-free catalyst in the lower part.The results of the experiments showed that, regardless of the support type (alumina, including calcium-carbonate-doped alumina, alumina-magnesia spinel, or calcium aluminate), the sulfur will be released from the nickel surface and transported downstream in the catalyst bed. Although this leads to a re-activation of the catalyst in the upper part of the catalyst bed due to the desorption of sulfur, the overall activity will drop due to the more significant deactivation of the downstream catalyst. The transportation of the sulfur is accompanied by nickel particle sintering, resulting in larger nickel particle sizes in the part of the bed where sulfur has been present consequently reducing the catalyst activity further. The rate of the sulfur transport is highly temperature dependent in the tested temperature interval from 350°C to 550°C, having minor transport at low temperature, but significant transport of sulfur at the higher temperatures.The experimental investigations have been further verified by industrial observation, where a high steam-to-hydrogen ratio in prereformers during operation has led to prereformer re-activation in the upper part and observed sulfur poisoning of catalysts downstream the prereformer.