Natural gas liquids (NGLs) are a valuable group of hydrocarbons derived from natural gas streams. They are a significant byproduct of natural gas processing, offering diverse applications that influence modern life. These compounds are separated from raw natural gas to improve the quality of the primary gas stream and unlock their economic value. Their importance continues to grow as industries recognize their varied uses beyond energy production.
Defining Natural Gas Liquids
Natural gas liquids are hydrocarbons separated from raw natural gas. Unlike “dry” natural gas, which consists predominantly of methane, NGLs are heavier compounds composed of carbon and hydrogen. While they exist as gases within underground reservoirs alongside methane, NGLs transition into a liquid state under specific conditions after extraction. This differentiates them from the primary natural gas stream and crude oil, positioning NGLs as a valuable category of energy resources.
Key Components of Natural Gas Liquids
NGLs comprise several individual hydrocarbon components. The most common types include ethane, propane, normal butane, isobutane, and pentanes, grouped as natural gasoline. Ethane is the lightest, a two-carbon molecule, and a major component of the NGL stream. Propane, a three-carbon molecule, is frequently combined with butanes to form liquefied petroleum gas (LPG).
Normal butane and isobutane are four-carbon isomers with different molecular structures. These butanes are versatile and find uses both individually and in blends. Pentanes, which are five-carbon molecules, are collectively known as natural gasoline or pentanes plus. This mixture typically contains molecules with five to nine carbon atoms.
Extraction and Processing of Natural Gas Liquids
The journey of NGLs begins at the wellhead where raw natural gas, often containing these liquid components, is extracted. Initial separation occurs to remove gross liquids like water and condensate, preparing the gas for further processing. The primary aim of natural gas processing plants is to purify the natural gas for pipeline specifications while simultaneously recovering valuable NGLs.
At these processing facilities, NGLs are separated from the methane-rich gas stream using specialized techniques. The two predominant methods employed are absorption and cryogenic expansion, which together account for a significant portion of NGL recovery. The absorption method utilizes a lean oil that contacts the natural gas in an absorber tower, soaking up the NGLs due to its affinity for these hydrocarbons. This process is particularly effective for recovering heavier NGLs like butanes and pentanes.
The cryogenic expansion process is employed, especially for lighter NGLs such as ethane. This method involves lowering the temperature of the gas stream using external cooling agents or turbo expanders. The temperature drop causes the NGLs to condense into a liquid state, allowing their separation from the methane. After extraction, the mixed NGL stream is transported to a fractionation plant, where a series of distillation towers separate the individual components based on their unique boiling points.
Diverse Applications of Natural Gas Liquids
Natural gas liquids serve a broad spectrum of industrial and consumer applications.
Ethane is primarily a feedstock for the petrochemical industry, where it is converted into ethylene, a building block for plastics like polyethylene used in products ranging from packaging to water bottles. It also finds limited use as a refrigerant and in some regions for energy generation.
Propane is widely used as a fuel for heating homes, commercial buildings, and agricultural facilities, especially in areas without natural gas pipeline access. It is also a common cooking fuel, powers forklifts, and serves as an auto fuel, often referred to as autogas. Propane is a petrochemical feedstock for producing propylene, which is used in textiles and automotive components.
Normal butane is frequently blended into gasoline, particularly during cooler months, to adjust its volatility and improve engine starting performance. It also functions as a fuel for lighters and portable stoves, an aerosol propellant, and a chemical feedstock for producing synthetic rubber.
Isobutane, an isomer of butane, is valuable in refineries for increasing the octane rating of gasoline. It is also utilized as an environmentally friendly refrigerant, a propellant in aerosol sprays, and a solvent in various chemical processes.
Pentanes and heavier NGLs, known as natural gasoline, are blended into motor gasoline, used as industrial solvents, and serve as a denaturant for fuel ethanol. They also act as a diluent to reduce the viscosity of heavy crude oil for pipeline transport.