Effect of Reactor Alloy Composition on Coke Formation during Butane and Ethane Steam Cracking

The use of highly specialized alloys in ethane steam cracking environments can lead to reduced coking and alloy aging. However, the fouling resistance of these materials when exposed to heavier feedstocks, such as butane, especially at higher temperatures (950 °C) and toward the end of a cracking run, remained unknown. To address this knowledge gap, this study investigated the coke resistances of presulfided three alloys, Al-Boosted, Spinel-Coated, and 35/45 Cr–Ni, during the steam cracking of both ethane and butane using a quartz Electrobalance setup with a tubular reactor. Under experimental conditions of T_gas = 950 °C, P = 1.1 bar, χ_Butane ≈ 79%, χ_Ethane ≈ 68%, dilution δ = 0.4 kg_H2O/kg_HC, and DMDS = 48 ppmw S/HC, the results revealed that butane cracking exhibited higher coking compared to ethane for the Al-Boosted and 35/45 Cr–Ni. This difference was attributed to the generation of larger quantities of coke precursors, such as aromatics, naphthenes, and butadiene, during butane cracking. Notably, the Al-containing alloy displayed exceptional resistance to aging, in contrast to the non-Al-containing alloys, which experienced severe carburization and aging effects after multiple cracking cycles. Detailed scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses indicated that the presence of a continuous and robust Al₂O₃ layer on the surface of the Al-containing alloy provided significant protection to the catalytic sites within the material. The average catalytic coking rate for the Al-containing alloy during butane cracking was measured at 5.83 × 10^–6 kg m^–2 s^–1, while for the non-Al-containing alloys, it ranged from 7.26 × 10^–6 to 15.25 × 10^–6 kg m^–2 s^–1. Furthermore, this study explored the relative influence of the feedstock and reactor material, revealing that for the Al-Boosted alloy, the feedstock had a greater impact on coking behavior. Conversely, for the 35/45 Cr–Ni alloy, aging had a more pronounced effect compared to that of the specific feedstock used. This research provides valuable insights into the fouling behavior of specialized alloys in the steam cracking of ethane and butane. The study highlights the superior resistance to aging and reduced coking rate exhibited by the Al-containing alloy, underscoring the protective role of the Al₂O₃ layer. Additionally, it emphasizes the relative significance of both feedstock composition and reactor material on coking tendencies, thereby contributing to a better understanding of fouling mechanisms in the steam cracking process.