In the production of hydrogen or synthesis gas by steam reforming or autothermal reforming, it is an option to have a prereformer prior to the main steam reformer / autothermal reformer. The prereformer has the ability to convert higher hydrocarbons by steam reforming with steam and subsequent methanation to methane following the below given reactions: CxHy + x H2O =&gt; x CO + (y/2+x) H2 CO + 3 H2 ó CH4 + H2O Prereforming catalysts are generally nickel based catalysts on various supports. Besides reforming the higher hydrocarbons, the prereformer will also capture traces of sulfur coming with the feed reacting with the nickel crystals and forming surface NiS. This is beneficial since it protects downstream processes from sulfur poisoning and subsequent deactivation. Industrial start-up, operation, and shut-down conditions of the prereformer catalyst can vary, and the present work has the objective to study the effect of the sulfur in the catalytic bed of a prereformer at conditions where the catalyst is being fed with a feed of steam without the presence of hydrogen. The tests were performed in a pilot plant 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, alumina-magnesia spinel or calcium aluminate), the sulfur will migrate down and through the catalyst bed. Although this leads to a re-activation of the catalyst in the upper part of the catalyst bed due to the elimination of sulfur, the overall activity will drop due to the more significant deactivation of the downstream catalyst. The migration of the sulfur is accompanied by drastic nickel particle sintering resulting in larger nickel particle sizes in the part of the bed where sulfur has been present, further reducing catalyst activity. The rate of the migration is temperature dependent in the tested temperature interval from 350°C to 550°C. The experimental investigations have further been verified by industrial observation, where a high steam to hydrogen ratio in prereformers during operation has lead to prereformer re-activation in the upper part and observed sulfur poisoning of downstream catalysts.

Migration of sulfur in prereforming catalysts during steaming conditions

249th National Meeting (2015)

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