Gas Processing

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Before natural gas can enter a commercial pipeline, it must undergo a rigorous purification process. In this engineering guide, we break down how processing plants transform raw wellhead gas into high-purity methane while recovering valuable byproducts like NGLs.

Natural gas processing involves purifying raw gas extracted from underground fields to transform it into consumer-grade fuel. While raw natural gas contains various impurities, the final processed version is almost entirely methane. Processing is mandatory to satisfy strict quality requirements set by governments and utility companies. These standards ensure the gas has a consistent heating value—roughly 1035 BTU per standard cubic foot in the United States—and prevent damage to infrastructure. For instance, gas must be dehydrated to prevent methane hydrates from blocking pipes and stripped of mercury to avoid the corrosion of aluminum components.

Raw natural gas is sourced from three main types of wells. Associated gas is found alongside crude oil, while non-associated gas comes from gas or condensate wells with little to no oil. A third source is coalbed gas, which is often called sweet gas because it contains very little hydrogen sulfide.

The basic processing scheme is as follows:

First, liquid water and natural gas condensates are removed at the wellhead. The gas then enters the plant for acid gas removal, where substances like hydrogen sulfide and carbon dioxide are stripped away, usually through amine treating. The captured hydrogen sulfide is converted into elemental sulfur.

Next, the gas undergoes dehydration to remove water vapor, typically using triethylene glycol or molecular sieves. Mercury is then filtered out using activated carbon or similar adsorbents.

If the gas contains high levels of nitrogen, it is removed through cryogenic distillation or absorption processes.

The final major stage is the recovery of natural gas liquids, or NGLs. This is achieved through cryogenic cooling and the use of a turbo-expander. The resulting NGL stream is sent to a fractionation train, which is a series of distillation towers: De-ethanizer, De-propanizer, and De-butanizer. These columns separate the mixture into ethane, propane, butane, and heavier hydrocarbons known as C5+.

Once these steps are complete, the remaining methane is ready for transport to the consumer market as sales gas.

For engineering studies, explore the table below for an overview of commonly used gas processing technologies. For deeper insights and technical calculators, read our expert articles.