Catalytic Effect of Dimethyl Disulfide on Coke Formation on High-Temperature Alloys: Myth or Reality?

The selection of the reactor material with the lowest coking tendency can result in substantial economic benefits for the steam cracking process. One of the remaining unresolved points of discussion is the determination of the influence of sulfur addition, in particular dimethyl disulfide (DMDS), on various steam cracking reactor alloys with varying Ni–Cr content. To shed some new light on this topic, an extensive thermogravimetric study was performed in a jet-stirred reactor (JSR) setup, evaluating online the coking behavior of four Ni–Cr–Fe alloys under industrially relevant ethane cracking conditions. For each material, the effect of preoxidation/pretreatment with and without the presence of DMDS was evaluated, with an objective to minimize the material coking tendency. The coking rates show that an increased Ni–Cr content of the material improves the coking rates by a factor of 2 or more under the studied process conditions. By continuously feeding DMDS, all non-Al-containing alloys indicate 7 times higher coking rates than the Blank runs, while the carbon oxide(s) formation is suppressed by a factor of 5. In comparison with continuous addition and presulfiding with DMDS, labeled “CA + PreS” experiments, the Al-containing alloy outperforms itself significantly when preoxidized at 1223 K by 50% in terms of coking rates. The results indicate that Al addition to Ni–Cr–Fe alloys improves their anticoking performance, provided that the preoxidation temperature is higher than for materials without Al. The overall results from coking rates and offline scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis for the coked coupons showed an outstanding oxidation homogeneity for the 40/48 Cr–Ni alloy, which was better than that for the Al-containing alloy at lower preoxidation temperatures.