posted on 2024-01-23, 17:05authored byHamed Mohamadzadeh Shirazi, Lucas dos Santos Vargette, Marie-Françoise Reyniers, Kevin M. Van Geem
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 Tgas = 950 °C, P = 1.1
bar, χButane ≈ 79%, χEthane ≈ 68%, dilution δ = 0.4 kgH2O/kgHC, 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.