Industrial production of ethylene mainly involves cracking of ethane in the presence of steam at elevated temperatures. This process occurs primarily by gas phase homogeneous radical chain reactions, although heterogeneous wall effects concurrently exist. However, along with the desired ethylene, undesired carbon monoxide (CO) and coke are also formed during the ethane-steam cracking process. Coke deposits on the inner walls of reactor tubes, and results in undesirable bimolecular reactions while CO elevates carbon activity in addition to playing as a poison for downstream catalysts. Carbon formation in ethylene generating furnaces is either catalytic or pyrolytic. In current ethylene generating plants, sulfur-containing species have been predominantly used to lower coke formation in pyrolysis furnaces. Unfortunately, in the reaction tubes of an ethane-cracking furnace, the role of sulfur compounds on coke formations is still not clear. Contradicting reports regarding the sulfur have been presented where some researchers have reported a suppressing effects and others a promoting effect. Therefore, there is need for further study and comprehend on the effects of sulfur compounds on the formation of coke since it is crucial in ethane−steam-cracking processes.
Recently, a team of researchers led by Professor Venkataraman Thangadurai from the Department of Chemistry at University of Calgary in Canada investigated on the effect of presulfidation and continuous addition of hydrogen sulphide gas (H2S) to ethane feed on catalytic and pyrolytic carbon formation on stainless steel-304H coupons, at conditions prevailing in the convection section of an ethane-cracking furnace. Their goal was to clear the air on matters regarding sulfur in the ethane cracking process. Their work is currently published in the research journal, Industrial and Engineering Chemical Research
The research team began their studies by setting up a laboratory-scale quartz reactor heated by a horizontal tube furnace. They exposed the stainless steel-304H coupons and stainless steel-304L powder samples to ethane−steam or dry ethane in varying H2S content at 700 oC after which the stainless steel-304 samples were characterized by scanning electron microscopy.
The authors observed that the H2S co-feeding decreased catalytic carbon formation; while it increased the pyrolytic carbon formation during ethane−steam cracking process. The scholars also noted that preoxidation, presulfidation and addition of steam to ethane feed reduced the amount of catalytic carbon formed on the stainless steel-304H surface in short-term experiments. Additionally, the research team also observed that presulfidation and addition of H2S to ethane feed considerably influenced the shape and size of the carbon formed on the surfaces of the examined metal alloys.
The study conducted by Professor Thangadurai and his team presents a thorough cross-examination of the effects of sulphur in the ethane-cracking process for ethylene production. The researchers observed that preoxidation of the stainless steel-304L powder did not provide sufficient protection against catalytic carbon formation, while as the presulfidation of H2S to ethane−steam considerably reduced the amount of catalytic carbon formed. Altogether, professor Thangadurai and his fellow researchers (Anand Singh, Scott Paulson, Hany Farag, and Viola Birss) demonstrated the importance to maintain optimized pO2 and pS2 levels in order to reduce catalytic and pyrolytic carbon formation and corrosion of stainless steel-304H tubes by sulfidation.
Anand Singh, Scott Paulson, Hany Farag, Viola Birss, Venkataraman Thangadurai. Role of Presulfidation and H2S Co-feeding on Carbon Formation on SS304 Alloy during the Ethane−Steam Cracking Process at 700 °C.. Industrial and Engineering Chemical Research, volume 57(2018) pages 1146−1158