Natural Gas Liquids
Posted: May 23, 2016
What are Natural Gas Liquids?
Natural gas liquids (NGLs) are naturally occurring hydrocarbons (carbon and hydrogen compounds) that can be found in natural gas. The components that make up natural gas can vary based on the ‘thermal maturity’ of the gas, which depends on temperature and pressure in the geologic formation over time (Figure 1). Natural gas is known as being dry or wet, with dry gas being more thermally mature and consisting primarily of methane (CH4). Wet gas is less thermally mature and contains condensates or heavier hydrocarbons greater than 0.1 gallon per 1,000 cubic feet of gas.
The NGLs are: ethane, propane, normal butane (and isobutane), and pentane. The chemical formulas of NGLs differ in the number of carbon and hydrogen atoms. This is why they are sometimes referred to as heavier hydrocarbons, and thus have a higher BTU or energy output. NGLs can comprise 5 to 50% of the natural gas stream, depending on its thermal maturity. They are often more valuable separated out to be used in a wide range of commercial and industrial purposes.
How are NGLs separated from the natural gas stream?
NGLs can be condensed from the gaseous state into the liquid state in two ways. They can naturally condense at the wellhead when pressure is reduced, or they can be induced by distillation and refrigeration at gas processing plants. Two methods used to separate the NGLs from methane are the absorption process and the cryogenic expansion process.
In the absorption process, an absorbing oil that has an affinity for the NGLs is used to separate them out as the natural gas is passed through an absorption tower. The oil laden with NGLs is then heated above their boiling points to separate them from the oil.
The cryogenic process drops the gas stream temperatures to about -120 degrees Fahrenheit, and is more effective in extracting ethane and lighter hydrocarbons from the natural gas stream. At this temperature, the NGLs condense and drop out of the methane gas stream.
Separating the NGLs from the natural gas stream makes for a cleaner, purer natural gas that is pipeline ready for commercial, electric power, residential, and industrial use.
What is Fractionation?
Fractionation is used to break down the various NGLs into their useful base components. Basically, the NGLs are boiled off one by one to separate and isolate each hydrocarbon. The name of a particular fractionator identifies what hydrocarbon is being boiled off. (Thus, the deethanizer separates ethane from the NGL stream, the depropanizer separates the propane, the debutanizer boils off the butanes, and the butane splitter or deisobutanizer separates out the iso and normal butanes.) The process starts with the removal of the lighter NGLs, working on up to the heavier components.
With the glut of natural gas on the market, NGLs have provided an incentive to drill more in the liquids rich resource areas rather than in dry gas regions. NGLs may have a higher value, but they also require additional separation and processing. To reach their best and highest use, viable markets and related transportation means have to be in place. Various companies are analyzing the economics for building additional processing facilities in regions closer to where NGLS are produced in quantity. In the meantime, companies are determining if it’s best to transport and market NGLs or, with ethane, the most abundant NGL, to ‘reject’ it back into the gas stream when it is not economical to separate it.
TitleNatural Gas Liquids
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