Gas plants

Gas composition

When gas is exported, many gas trains include additional equipment for further gas processing to remove unwanted components such as hydrogen sulfide and carbon dioxide. These gases are called acids and sweetening/acid removal is the process of removing them.

Natural gas sweetening methods include absorption processes, cryogenic processes, adsorption processes (PSA, TSA and iron sponge) and membranes. Often hybrid combinations are used, such as cryogenic and membranes.

Gas treatment may also include calibration. If the delivery specification is for a specific calorific value (BTU per scf or MJ per scm), gas with higher values can be adjusted by adding an inert gas, such as nitrogen. This is often done at a common point such as a pipeline gathering system or a pipeline onshore terminal.

Raw natural gas from the well consists of methane as well, as many other smaller fractions of heavier hydrocarbons and various other components.

Natural gas is characterized in several ways dependent on the content of these components:

·         Wet gas is raw gas with a methane content of less than 85%.

·         Dry gas is raw or treated natural gas that contains less than 15 liters of condensate per 1,000 SM³ . (0.1 gallon per 1000 scf).

·         Sour gas is raw gas with a content of more than 5.7 mg hydrogen sulfide (H₂S) per scm (0.25 grains per 100 scf); this is about 4 ppm.

·         Acid gas has a high content of acidic gases such as carbon dioxide (CO₂) or H₂St. Pipeline natural gas specification is typically less than 2% CO₂. Acid gas fields with up to 90% CO₂ exist, but the normal range for sour raw gas is 20-40%.

·         Condensates are a mixture of hydrocarbons and other components in the above table. These are normally gaseous from the well but condense out as liquid during the production process (see previous chapter). This is a refinery and petrochemical feedstock.

Raw gas is processed into various products or fractions:

Ø  Natural gas in its marketable form has been processed for a specific composition of hydrocarbons, sour and acid components, etc., and energy content. Content is typically 90% methane, with 10% other light alkenes.

Ø  Natural gas liquids (NGL) is a processed purified product consisting of ethane, propane, butane or some higher alkenes separately, or in a blend. It is primarily a raw material for petrochemical industry and is often processed from the condensate.

Ø  Liquefied petroleum gas (LPG) refers to propane or butane or a mixture of these that has been compressed to liquid at room temperature (200 to 900 kPa depending on composition). LPG is filled in bottles for consumer domestic use as fuel, and is also used as aerosol propellant (in spray cans) and refrigerant (e.g., in air conditioners). Energy to volume ratio is 74% of gasoline.

Ø  Liquefied natural gas (LNG) is natural gas that is refrigerated and liquefied at below -162 °C, for storage and transport. It is stored at close to atmospheric pressure, typically less than 125 kPa. As a liquid, LNG takes up 1/600 of the volume of the gas at room temperature. Energy to volume ratio is 66% of gasoline. After transport and storage it is reheated/vaporized and compressed for pipeline transport.

Ø  Compressed natural gas (CNG) is natural gas that is compressed at 2-2,2 MPa to less than 1% of volume at atmospheric pressure. Unlike higher alkenes, methane cannot be kept liquid by high pressure at normal ambient temperatures because of a low critical temperature. CNG is used as a less costly alternative to LNG for lower capacity and medium distance transport. Methane for vehicle fuel is also stored as CNG. Energy to volume ratio is typically 25% of gasoline.