Carburization of high-temperature alloys during steam cracking : the impact of alloy composition and temperature

Prior studies found that the carburization tendency is negligible for technically developed materials at temperatures below 900 degrees C, especially for Al-containing alloys compared to non-Al-containing alloys applied in steam cracking reactors. However, it is unclear whether this benefit remains at higher temperatures. This work compared coke deposition tendencies of recently developed alloys to each other and to a conventional material when cracking ethane at a higher temperature (950 degrees C) than previously reported. Under the studied experimental conditions, coking rates vary in the range of 1.22 x 10-6 to 20.5 x 10-6 kg/(m2 center dot s) for Al-containing alloys, while for non-Al-containing alloys, coke deposition rates fall in a range of 4.95 x 10-6 to 33 x 10-6 kg/ (m2 center dot s). In general, aluminum-containing alloys showed less coke formation and improved stability against aging after five cracking cycles compared to the non-aluminum-containing materials. The results also showed that substantially less, if any, carburization could be detected in developed alloys even at high temperatures, in contrast to the results from a reference conventional alloy of 27/ 34 Cr-Ni. At higher temperatures, the protective oxide surface layer of the Al-containing alloys remained noticeably durable and integrated with negligible spallation compared to the ones of non-Al-containing alloys. In fact, the 27/34 Cr-Ni alloy showed the highest coking rate and carburization, implying the most vulnerable oxide surface layer relative to the other alloys. However, energy dispersive X-ray (EDX) results showed no signs of carburization or deterioration of the surface of 27/34 Cr-Ni after experiments at 880 degrees C, indicating that both the temperature and matrix composition can play significant roles in determining the suitability of an alloy for application in steam crackers.