Natural gas processing equipment requires a wide range of engineering and manufacturing capability. Nitrogen rejection units (NRUs) require a different technical approach than liquefied petroleum gas (LPG) or natural gas liquids recovery (NGL).
According to the Gas Research Institute, nitrogen rejection from natural gas is necessary in about 17% of US gas reserves. Most pipeline standards require that natural gas contain less than 4% nitrogen. If there is too much nitrogen present in a pipeline, there is a danger of vapor lock or combustion. Nitrogen also dilutes the heating value of the gas, resulting in a lower BTU and decreased value. High-nitrogen natural gas is essentially stranded, as it cannot feasibly be transported through pipelines to market.
Nitrogen rejection is a difficult technical separation because of the similar molecular size of nitrogen (N2) and methane (CH4) and the lack of a selective reactivity such as there is with carbon dioxide or hydrogen sulfide in an amine plant.
Pressure swing adsorption, cryogenic separation, and lean oil absorption have been employed to remove nitrogen from natural gas.
- Pressure swing adsorption (PSA) uses an adsorbent to selectively separate nitrogen from methane. This adsorbent can be selected to either adsorb the methane or nitrogen, depending on the gas qualities. Once the adsorbent is saturated, the gas is regenerated from the adsorbent with a combination of pressure and thermal changes. PSA is used for gas dehydration, carbon dioxide removal, nitrogen rejection, and hydrocarbon dew point control.
- Cryogenic separation employs low temperature thermodynamic separation based on the comparative boiling points of methane (-259°F) and nitrogen (-320°F) . As the mixture cools, methane will condense before nitrogen, allowing the two to be separated. Cryogenic separation processes are extremely efficient, especially in large installations. Nitrogen concentrations can be reduced to as low as 1% with less power consumption than other methods.
- Lean oil absorption absorbs the methane away from the nitrogen in a higher molecular weight hydrocarbon and then regenerates the absorbed methane either through staged pressure reductions or through thermally-driven distillation.