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HPV Product Categories
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Natural Gas Processing |
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Natural gas refers to a number of individual hydrocarbon gases including methane, ethane, propane, butane, and isobutane, or mixtures of these individual gases like natural gasoline. Most natural gas originates from oil and gas production wells, but methane can also be recovered from coal bed mining operations. Natural gas is used not only as clean-burning fuel for homes, cars, appliances, and industrial plants, but also for petrochemical feedstock and refinery blending to help make common products such as plastics and motor fuel. Because natural gas comes from the ground in a mixture, its individual components must be separated and processed in order to be used. The first step is to treat it to remove substances that make it difficult to process, such as water and carbon dioxide. Methane is then separated from the rest of the mixture and sent to market to be used as heating and cooking fuel. Methane is by far the most common and least expensive component in natural gas; a typical American natural gas well is approximately 85% methane, 9% ethane, and 5% propane. The components other than methane, including ethane, propane, normal butane, isobutane, and a very small percentage of heavier components, are often referred to as natural gas liquids. How are natural gas components separated? Heavier hydrocarbon molecules usually occur in smaller percentages than lighter hydrocarbon molecules in natural gas mixtures. The number of carbon atoms primarily determines a hydrocarbon molecule’s weight—a carbon atom weighs almost 12 times that of the molecule’s other component, hydrogen. The boiling point is the temperature at which a liquid can turn to gas. Generally, the heavier the hydrocarbon, the higher the temperature at which it changes phase from a liquid to a gas. Butane, with four carbon atoms, is usually a gas at outside temperature and pressure, but can remain a liquid in colder climates (boiling point 31ºF). Methane, the lightest with one carbon atom, has a boiling point of –258ºF, so it is not seen as a liquid. These thermodynamic properties of natural gas components allow them to be separated. They are defined by their phase envelope, which is the temperature and pressure at which one substance can change phase from a gas to a liquid. Process engineers use the differences in these properties to separate each individual gas from the mixture. Subjecting the mixture to certain temperature and pressure conditions allows one substance to remain a gas while the rest become liquids. Vessels are built so that heavier components can be removed from the bottom, while lighter components rise to the top. This way, each lighter component is separated from the mixture and can be stored and sold individually. The process is repeated until a small portion of a heavier mixture is left (natural gas liquids) that can be sold as refinery feedstock. The separation process is referred to as fractionation. For more information on EPA’s TSCA Inventory of Chemical Substances, visit http://www.epa.gov/opptintr/newchems/invntory.htm.
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