(66d) Multistage Cryogenic Expander Applications

The cryogenic turboexpander was introduced into the ethylene process design in the 1970's. The expander is generally utilized in ethylene plants to expand hydrogen offgas from the demethanizer feed separator in the LP demethanizer system or to expand methane offgas from the HP demethanizer overhead. Since the offgas is high on hydrogen content and hence has low molecular weight, expansion is normally achieved in multiple stages, mostly in two stages and in some process applications up to four stages.

This paper will discuss the advantages of four stages vs. tradition two stages off expansion.