The term “gas processing” is used to refer to the removing of ethane, propane, butane, and heavier components from a gas stream. They may be fractionated and sold as “pure” components, or they may be combined and sold as a natural gas liquids mix, or NGL.
The first step in a gas processing plant is to separate the components that are to be recovered from the gas into an NGL stream. It may then be desirable to fractionate the NGL stream into various liquefied petroleum gas (LPG) components of ethane, propane, iso-butane, or normal-butane. The LPG products are defined by their vapor pressure and must meet certain criteria as shown in Table 9-1. The unfractionated natural gas liquids product (NGL) is defined by the properties in Table 9-2. NGL is made up principally of pentanes and heavier hydrocarbons although it may contain some butanes and very small amounts of propane. It cannot contain heavy components that boil at more than 375°F.
In most instances gas processing plants are installed because it is more economical to extract and sell the liquid products even though this lowers the heating value of gas. The value of the increased volume of liquids sales may be significantly higher than the loss in gas sales revenue because of a decrease in heating value of the gas.
In deciding whether it is economical to remove liquids from a natural gas stream, it is necessary to evaluate the decrease in gas value after extraction of the liquid. Table 9-3 shows the break-even value for various liquids. Below these values the molecules will be more valuable as gas.
The difference between the actual sales price of the liquid and the break-even price of the liquid in Table 9-3 provides the income to pay out the capital cost, fuel cost, and other operating and maintenance expenses necessary to make the recovery of the gas economically attractive.
Another objective of gas processing is to lower the Btu content of the gas by extracting heavier components to meet a maximum allowable heating limit set by a gas sales contract. If the gas is too rich in heavier components, the gas will not work properly in burners that are designed for lower heating values. A common maximum limit is 1100 Btu per SCE Thus, if the gas is rich in propane and heavier components it may have to be processed to lower the heating value, even in cases where it may not be economical to do so.
This chapter briefly describes the basic processes used to separate LPG and NGL liquids from the gas and to fractionate them into their various components. It is beyond the scope of this text to discuss detailed design of gas processing plants.