Natural Gas Liquids (NGL) include ethane, propane, butane, and heavier hydrocarbons referred to as condensate. Recovery of liquids from produced gas is done for several reasons; to dew point the gas, to meet heating value specifications, and most importantly, because the recovered liquids have a higher value than natural gas.
Rich gas from Inlet Separation may first go through Gas Sweetening, Mercury Removal, and Gas Dehydration. NGL recovery is accomplished by either refrigeration or cryogenic turbo-expander. Some factors that are considered include gas arrival pressure, phase envelope, and desired recovery.
Refrigeration-based systems which most often use propane as refrigerant have proven to be economical and reliable. Their operating temperature, however, typically about –40 °C, limits recovery to propane and heavier.
For higher recovery, a turbo-expander is required. Gas temperatures reach -75 °C, thus condensing both ethane and propane. Braised Aluminum Heat Exchangers are used to withstand the cryogenic temperatures and to get approach temperatures as close as 1 °C. This process allows for the recovery of upto 90 to 95 percent of the ethane from the gas stream
The resulting liquid C2+ stream can be fractionated into ethane, propane, butane and condensate. The fractionation section can consist of a demethanizer, de-ethanizer, depropanizer, and debutanizer. This depends on the desired products. If ethane recovery is not required, then a demethanizer is not used. A depropanizer is used for recovery of propane. For LPG product, which is a mixture of propane and butane, the debutanizer is used to recover LPG overhead and condensate bottoms.
After extraction of the heavier hydrocarbons, then lean gas is warmed up to pipeline temperature against feed gas and is recompressed to the required pipeline pressure.
The extraction of NGLs from the natural gas stream produces both cleaner, purer natural gas, as well as the valuable hydrocarbons that are the NGLs themselves.