The gas plant takes refinery gas from the distillation and other process units and separates out the gas liquids.

The goal of the gas plant is to separate the more valuable heavy components (propane, butane, etc) for product blending or conversion, and send the lower value methane and ethane to the refinery fuel system.

Often there are two gas plants: a sats gas plant that handles gas streams that contain only saturated hydrocarbons (no olefins) and an unsaturated gas plant that handles streams from cracking units that contain unsaturated olefins such as butylene and propylene.

The gas plant is actually comprised of a number of process units to perform different stages of separation. The typical sequence is as follows:

• de-ethanizer - gas is pressurized and sent through an absorption tower to separate methane and ethane from the heavier components, which are absorbed into a naphtha stream (lean oil) to form fat oil

• sponge absorption - the gas stream (methane and ethane) from the de-ethanizer is fed to another absorber tower where a kerosene stream (sponge oil) is used to extract any lean oil that might have been carried over

• de-butanizer - the fat oil from the de-ethanizer is heated and sent to a fractionation tower that separates the propane and butane from the naphtha based on boiling points

• de-propanizer - the mixed propane/butane stream from the de-butanizer is sent to a second fractionation tower to separate propane from butane based on boiling points

• de-isobutanizer - In some refineries, the butane stream from the de-propanizer is further fractionated in a (very tall) column to separate normal butane from isobutane.