Modem Acetylene Hydrogenation Catalysts

Until 1958 no ethylene plant had used a tail-end palladium catalyst to hydrogenate all of the acetylene formed in the steam cracker. This was an attractive possibility, however, and many of the large new US plants built in the 1960s were designed in this way. The less efficient front-end nickel and iron catalysts were soon obsolete. Several significant changes followed the use of tail-end catalysts  

In Europe fom or five of the more modem 1950s steam crackers, based on naphtha feed, replaced the fused iron or nickel front-end catalysts with a new palladium catalyst using an a-alumina support. Success in meeting the strict new acetylene specifications, while hydrogenating 95% of the methyl acetylene and forming no green oil, led to the use of this catalyst in many new ethylene plant desigrts. 

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Catalysts suitable for selective hydrogenation of acetylenic compounds in cracked gas streams contain elements of group VI and VIII of the periodic table. An early catalyst was molybdenum sulfide supported on activated alumina (Key and Eastwood, 1946). This was followed by the development of cobalt molybdate and nickel based catalysts (Giaro, 1956 Barry, 1950). Modem catalysts for impure (sulfur-bearing) cracked gas streams typically contain nickel, cobalt, and chromium on a silica-alumina base (United Catalysts, 1993). 

A supported cobalt/molybdate catalyst, probably based on the ones developed in the 1930s, was one of the first types to be used in modem ethylene plants. The front-end reactor was located in the compressor train after heavy hydrocarbons were removed but before sulfur removal or gas drying. The catalyst was, therefore, partly sulfided. Careful temperature control was required to limit ethylene loss. About 10% steam was added to cracked gas, which limited the temperature rise and improved selectivity. An unusual feature of operation was that a significant proportion of the acetylene was removed as a polymer. This decreased the potential temperature rise but meant that catalyst regeneration and subsequent reactivation was a routine procedure at intervals of 2-4 weeks and that a spare reactor was needed. To compensate for loss of activity the gas temperature was continuously increased throughout the operating cycle. Acetylene levels were reduced to about 10-20 ppm with 1-3% ethylene loss. Up to 50% of any butadiene present in the gas was also hydrogenated. The catalyst was replaced after 1-2 years. 

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