Tail-End Acetylene Hydrogenation

Acetylene is hydrogenated in the separated C2 stream, which means that sufficient hydrogen must be added to the gas before the reactor. The theoretical amount for complete removal is 1-mol volume of hydrogen per mole volume of acetylene, giving 100% conversion to ethylene, but this has always been impossible to achieve. There is usually an ethylene loss associated with complete 

In some of the early large ethylene plants, high ethylene losses led to the formation of several tonnes of green oil per day. This resulted in the need for extremely frequent catalyst regeneration. Good control of the catalyst temperature and the volume of added hydrogen was essential to give optimum and stable operation. 

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  

The hydrogen ratio recommended is usually 1.5-2.0 in single or final beds, where the acetylene content is lowest and maximum conversion is needed, but only 0.8-1.3 in the first or second beds. Temperature rise can be roughly calculated as 65°C or 35°C for the conversion of 1% acetylene to ethane or ethylene, respectively. 

With very high acetylene levels in the C2 strearrr, the rrse of trrbular reactors can provide good temperature corrtrol. An iderrtical spare reactor shordd be avadable to allow continuous plant operation when the catalyst has to be regene- 

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