HYDROCARBON RECOVERY

Gas processing covers a broad range of operations to prepare natural gas for market. Processes for removal of contaminants such as H2S, CO2 and water are covered extensively in other sections of the Data Book. This chapter will cover the processes involved in recovering light hydrocarbon liquids for sale. The equipment components included in the processes described are covered in other sections of the Data Book. This section will bring those components together in process configurations used for liquid production.

INTRODUCTION :

The recovery of light hydrocarbon liquids from natural gas streams can range from simple dew point control to deep ethane extraction. The desired degree of liquid recovery has a profound effect on process selection, complexity, and cost of the processing facility.
The term NGL (natural gas liquids ) is a general term which applies to liquids recovered from natural gas and as such refers to ethane and heavier products. The term LPG (liquefied petroleum gas) describes hydrocarbon mixtures in which the main components are propane, iso and normal butane, propene and butenes. Typically in natural gas production olefins are not present in LPG.
Typically, modern gas processing facilities produce a single ethane plus product (normally called Y-grade) which is often sent offsite for further fractionation and processing. Whether accomplished on-site or at another facility, the mixed product will be further fractionated to make products such as purity ethane, ethane-propane (EP), commercial propane, isobutane, normal butane, mixed butanes, butane-gasoline (BG), and gasoline (or stabilized condensate). The degree of fractionation which occurs is market and geographically dependent.
Early efforts in the 20th century for liquid recovery involved compression and cooling of the gas stream and stabilization of a gasoline product. The lean oil absorption process was developed in the 1920s to increase recovery of gasoline and produce  products with increasing quantities of butane. These gasoline
products were, and still are, sold on a Reid vapor pressure (RVP) specification. Vapor pressures such as 10, 12, 14, 20 or 26 psia are common specifications for gasoline products. In order to further increase production of liquids, refrigerated lean oil absorption was developed in the 1950s. By cooling the
oil and the gas with refrigeration, propane product can be recovered. With the production of propane from lean oil plants, a market developed for LPG as a portable liquid fuel.                                                                                                                                                                                                                       In lieu of using lean oil, refrigeration of the gas can be used for propane and heavier component recovery. The use of straight refrigeration typically results in a much more economical processing facility. The refrigeration of the gas can be accomplished with mechanical refrigeration, absorption refrigeration, expansion through a J-T valve, or a combination. In order to achieve still lower processing temperatures, cascade refrigeration, mixed refrigerants, and turboexpander technologies have been developed and applied. With these technologies, recoveries of liquids can be significantly increased to achieve deep ethane recoveries. Early ethane recovery facilities targeted about 50 % ethane recovery. As processes developed, ethane recovery efficiencies have increased to well over 90%.                                          In some instances heavy hydrocarbons are removed to control the hydrocarbon dew point of the gas and prevent liquid from condensing in pipeline transmission and fuel systems. In this case the liquids are a byproduct of the processing and if no market exists for the liquids, they may be used as fuel. Alternatively, the liquids may be stabilized and marketed as condensate.