Abstract. The specifications for contaminant concentrations in high purity ethylene and propylene have become increasingly stringent. The utilization of high activity polymerization catalysts by customers has been the principle driving force for the reduction in inhibitor concentration by the light olefin producers. Ethylene purity requirements have increased from 98.5% not too many years ago to better than 99.5% today. Total nonhydrocarbon contaminants have been reduced to much less than 1 ppm. Many technological developments have been required to allow a material of this high purity to be produced.
Acetylene is a particularly interesting contaminant in ethylene. The purity specifications for this species has fallen from <5 ppm in 1980 to < 1 ppm for many customers today. Many production facilities produce an ethylene product that typically contains <0.1 ppm acetylene. A modern olefin plant meets this challenge by the selective hydrogenation of acetylene in the ethylene stream. This paper will discuss recent catalyst developments that allow the complete conversion of acetylene to ethylene with near 100% selectivity. The use of these catalysts eliminate the risk of reactor run-away and allow efficient new process design alternatives to be implemented.
INTRO D U CTION Acetylene removal is a crucial purification step in the production of ethylene. It is a product of steam cracking of hydrocarbon feed stocks. As is the case for most contaminants in olefin streams, the acetylene concentration that is acceptable to most ethylene customers has decreased significantly over the last several years. This has put substantial demands on processing technology designed to control the undesired components. Several processing schemes are available for controlling acetylene. At Phillips, acetylene is removed in front end hydrogenation reactors. This paper will focus on the process requirements of that technology.
In plants of front end de-ethanizer design, the cracked gas from the furnace is compressed to about 200 psig, washed with caustic, and introduced into a de-ethanizer. This is a distillation tower which is operated to obtain a C2 and lighter overhead stream and a C3 and heavier bottoms stream. The C2 and lighter stream is compressed further to about 500 psig and fed to the acetylene hydrogenation reactor. Palladium supported on alumina is the most commonly used catalyst for this conversion.
The feed to the acetylene reactor contains not only ethylene, ethane, and acetylene; but also hydrogen, methane, and carbon monoxide. Hydrogen is present in great surplus relative to the acetylene present, therefore excessive hydrogenation of ethylene is 798 always a concern. There is only a narrow window for operation in which acetylene removal can be accompli