What is the purpose of adding butenes to ethylene in a metathesis unit?

The purpose of adding butenes to ethylene in a metathesis unit is to produce propylene. Metathesis is a catalytic reaction that involves the redistribution of carbon-carbon double bonds between two olefins. When ethylene and butenes (specifically, typically 2-butene) are reacted in a metathesis unit, the double bonds are broken and reformed, resulting in the formation of two molecules of propylene. The overall reaction can be represented as: ethylene (C2H4) + 2-butene (C4H8) <=> 2 propylene (C3H6). This reaction is typically carried out in the presence of a heterogeneous catalyst, such as rhenium oxide or tungsten oxide supported on alumina. Metathesis is an important process for on-purpose propylene production, allowing refiners and petrochemical producers to increase propylene yields from existing feedstocks. By reacting ethylene with butenes, the metathesis unit effectively converts these less valuable olefins into the more valuable propylene.