Selective acetylene hydrogenation ethylene loss

For all the samples and for conversions lower than 70%, a selective hydrogenation of acetylene towards ethylene and an additional ethylene production is obtained (negative losses of ethylene), see fig. 3. At high conversions (industrial conditions) the acetylene is hydrogenated totally towards ethane and losses of ethylene are observed by ethylene hydrogenation towards ethane. 

Coke formation during acetylene hydrogenation over palladium catalyst could promote hydrogen transfer to acetylene and ethylene adsorbed molecules leading to a decrease in ethane selectivity and an increase in ethylene loss. 

Significant hydrogenation of ethylene can occm- if the gas is not efficiently cooled or the catalyst is not very selective. This is referred to as ethylene loss. Catalyst selectivity is also important to minimize the formation of green oil polymers, which wastes ethylene and causes operating problems. Some process designs have included tube-cooled adiabatic catalyst reactors to cope with high acetylene coneentrations, but they have not been very popular. 

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. 

Much better catalysts now provide improved operation. Hydrogenation can be controlled by adding traces of carbon monoxide to the hydrogen. Adsorbed carbon monoxide modifies the relative adsorption of acetylene and ethylene on the palladium and minimizes ethylene loss. The catalyst itself can also be made more selective by alloying the palladium with a further metal such as copper or silver. This also affects pallacfium dispersion and the relative adsorption of acetylene and ethylene on the catalyst surface to improve selectivity. To minimize temperature rise catalyst suppliers recommend that one or more catalyst beds with intercoolers be used in each reactor, depending on the acetylene content of the C2 stream ... 

KLP [Dow K Catalyst liquid phase] A selective hydrogenation process for removing acetylenes from cmde C4 hydrocarbons from ethylene cracking, with no loss of butadiene. The catalyst is based on either copper metal or alumina. Developed by Dow Chemical Company and first commercialized at its plant in Temeuzen, The Netherlands. The KLP licensing business was sold to UOP in 1991. 

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