Processing of natural gas

Natural gas processing is a complex industrial process designed to clean raw natural gas by separating impurities, various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas. Natural gas processing plants purify raw natural gas by removing common contaminates such as water, carbon dioxide (CO2) and hydrogen sulphide (H2S).

It is interesting to note that some of the substances which contaminate natural gas have economic value, and are further processed or sold. A fully operational processing plant delivers quality lean natural gas that can be used as fuel by residential, commercial and industrial consumers.

Types and major constituents of natural gas Natural-gas processing begins at the well head. The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area. Oil and natural gas are often found together in the same reservoir.

The natural gas produced from oil wells is generally classified as associated-dissolved, meaning that the natural gas is associated with or dissolved in crude oil.

There are various ways to configure the numerous unit processes used in the processing of dense medium natural gas. The block flow diagram below is a generalised and typical configuration for the processing of raw natural gas from non-associated gas wells.

Block diagram of a natural gas processing plant The diagram below shows how raw natural gas is processed into sales gas, and then pipelined to the end user markets. It also shows how processing of the raw natural gas yields the by-products below: • Natural gas condensate • Sulphur • Ethane • Natural gas liquids (NGL): propane, butanes and C5+ (which is the commonly used term for pentanes plus higher molecular weight hydrocarbons) Raw natural gas is commonly gathered from a group of adjacent wells and then processed at that collection point for removal of free liquid water and natural gas condensate. The condensate is usually transported to an oil refinery, while the water is disposed of as waste water.

Initial purification The raw gas is subsequently pipelined to a gas processing plant where the initial purification involves the removal of acid gas precursors (hydrogen sulphide and carbon dioxide). There are many processes that are available for that purpose as shown in the flow diagram. However, amine treatment is the process that was historically used.

Due to a range of performance and environmental constraints of the amine process, a newer technology, based on the use of polymeric membranes to separate the carbon dioxide and hydrogen sulphide from the natural gas stream, has gained increasing acceptance. Membranes are attractive, since no reagents are consumed.

Removal of acid precursor gases The acid precursor gases, if present, are removed by membrane or amine treatment; and then passed into a sulphur recovery unit, which converts the hydrogen sulphide in the acid gas into either elemental sulphur or sulphuric acid. Of the processes available for these conversions, the Claus process is by far the most well known for recovering elemental sulphur, whereas the conventional Contact process and the WSA (Wet sulphuric acid process) are the most used technologies for recovering sulphuric acid.

Subsequently, the residual gas from the Claus process — commonly called tail gas — is then processed in a tail gas treating unit (TGTU) to recover and recycle residual sulphur-containing compounds back into the Claus unit. As shown in the flow diagram, there are a number of processes available for treating the Claus unit tail gas; and for that purpose, a WSA process is also very suitable, since it can work auto thermally on tail gases.

The next step in the gas processing plant is to remove water vapour from the gas, using either the regenerable absorption in liquid triethylene glycol (TEG), commonly referred to as glycol dehydration, deliquescent chloride desiccants, and or a Pressure Swing Adsorption (PSA) unit. Relatively new processes using membranes may also be considered. Next, mercury is removed by using adsorption processes such as activated carbon or regenerable molecular sieves.

Removal of nitrogen While not common, nitrogen is sometimes removed and rejected using one of the three processes indicated on the flow diagram: • Cryogenic process (Nitrogen Rejection Unit), using low temperature distillation. This process can be modified to also recover helium, if desired. • Absorption process, using lean oil or a special solvent as the absorbent. • Adsorption process, using activated carbon or molecular sieves as the adsorbent. This process may have limited applicability because it is said to incur the loss of butanes and heavier hydrocarbons.

Recovery of natural gas liquids The next step is the recovery of the natural gas liquids (NGL) for which most large and state-of-the-art processing plants use another cryogenic low temperature distillation process involving expansion of the gas through a turbo-expander followed by distillation in a demethanising fractionating column.

Some gas processing plants use lean oil absorption process rather than the cryogenic turbo-expander process. The residue gas from the NGL recovery section is the final, purified sales gas, which is pipelined to the end-user markets.

The recovered NGL stream is sometimes processed through a fractionation train consisting of three distillation towers in series: a deethaniser, a depropaniser and a debutaniser. The overhead product from the deethaniser is ethane and the bottoms are fed to the depropaniser. The overhead product from the depropaniser is propane, with the bottoms fed to the debutaniser. The overhead product from the debutaniser is a mixture of normal and iso-butane, and the bottoms product is a C5+ mixture.

The recovered streams of propane, butanes and C5+ may be "sweetened" in a Merox process unit to convert undesirable mercaptans into disulphides, along with the recovered ethane, are the final NGL by-products from the gas processing plant.

Contemporary plants Currently, most cryogenic plants do not include fractionation for economic reasons, and the NGL stream is instead transported as a mixed product to standalone fractionation complexes located near refineries or chemical plants that use the components for feedstock.

In case laying pipeline is not possible for geographical reason, or the distance between source and consumer exceed 3,000 km, natural gas is then transported by ship as LNG (liquefied natural gas) and again converted into its gaseous state in the vicinity of the consumer.