Natural gas refinement, or processing, transforms raw gas extracted from underground reservoirs into a usable and safe commodity. The raw gas arriving at a processing plant is a varied mixture containing methane, heavier hydrocarbons, and numerous contaminants. The primary objective is to produce “pipeline-quality” dry gas, which is overwhelmingly pure methane, while recovering valuable non-methane components. This purification ensures the gas is non-corrosive, will not freeze, and meets the safety and energy content standards required for long-distance transport and end-user consumption.
Initial Separation and Liquid Handling
The refinement process begins with the separation of bulk liquids and solid debris immediately upon the gas stream’s arrival at the facility. Raw natural gas often carries water, crude oil, and hydrocarbon condensates from the reservoir. Equipment like slug catchers and inlet separators are the first line of defense, slowing the gas flow to allow gravity to pull these heavier materials out of the stream.
A slug catcher is a large pressure vessel or a series of pipes designed to handle sudden, large influxes of liquid, known as “slugs,” that accumulate in pipelines. Separating these liquids early protects downstream equipment from damage and ensures a steady gas flow into the chemical purification units. The removed liquids, particularly the hydrocarbon condensates, are often sent to refineries for further processing.
Removing Chemical Impurities (Sweetening and Dehydration)
Once the bulk liquids are removed, the gas stream must be purified to remove corrosive and hydrate-forming compounds. This stage focuses on removing “acid gases,” primarily hydrogen sulfide (\(\text{H}_2\text{S}\)) and carbon dioxide (\(\text{CO}_2\)), through a process known as sweetening. These gases are called acidic because they form corrosive acids when combined with water, which can damage pipelines and equipment.
Sweetening is commonly achieved using an amine scrubbing system. This involves passing the sour gas through a contactor tower filled with a circulating aqueous solution of alkanolamines. The amine solution chemically absorbs the \(\text{H}_2\text{S}\) and \(\text{CO}_2\), transforming “sour gas” into “sweet gas.” The amine solution, now rich with acid gases, is then regenerated with heat and recycled back into the absorber, making the process continuous.
The next step is dehydration, which removes water vapor to prevent the formation of hydrates—ice-like solids that can plug pipelines—and to mitigate corrosion. Triethylene glycol (TEG) is the most widely used liquid desiccant for this purpose due to its high affinity for water. The wet gas flows upward through an absorber tower, making contact with the lean TEG flowing downward, which absorbs the moisture. This glycol dehydration process reduces the water content to meet pipeline specifications.
Extracting Natural Gas Liquids
After removing contaminants, the gas still contains valuable hydrocarbons called Natural Gas Liquids (NGLs), such as ethane, propane, butane, and natural gasoline. These NGLs must be separated from the methane for two reasons: they command a higher market value as petrochemical feedstocks and fuels, and their removal prevents the final gas product from exceeding the specified heating value.
One common method for NGL extraction is the absorption method, which uses an absorption oil to capture the NGLs from the gas stream. The oil acts similarly to the glycol in dehydration, selectively absorbing the heavier hydrocarbons. For higher recovery rates, especially of lighter NGLs like ethane, cryogenic expansion is the preferred technique.
Cryogenic processing involves rapidly cooling the gas stream using external refrigerants and turboexpander technology. At these low temperatures, the NGLs condense into a liquid state while the methane remains a gas, allowing for efficient separation. This process recovers a high percentage of the ethane and other NGLs, which are then further separated into pure products like propane and butane through fractionation.
Preparing the Gas for Transport
The final stage ensures the purified methane meets all regulatory and commercial standards for safe and efficient transmission. A requirement is meeting the British Thermal Unit (BTU) specification, which measures the gas’s heating value. This value is maintained by the earlier removal of NGLs.
The most noticeable addition to the gas at this stage is the mandatory process of odorization. Since pure methane is colorless and odorless, a chemical odorant, typically a sulfur compound like mercaptan, is injected into the gas stream. This measure is enforced by federal regulations to ensure that any gas leak can be detected by smell at concentrations far below the explosive limit, enhancing public safety.
Finally, the purified, odorized gas must be pressurized using compressors to overcome the friction and distance of the pipeline journey. Compression stations are strategically placed along the pipeline route to maintain the necessary high pressure for continuous flow. This high-pressure transport ensures the gas is delivered efficiently from the processing plant to local distribution systems and end-users.