Nitrogen Rejection Unit

A Nitrogen Rejection Unit (NRU) removes impurities from a gas by liquifying and regasifying the stream.

A Nitrogen Rejection Unit is any system that removes nitrogen from natural gas. For much larger applications, cyrogenic processing is the norm. In this process, a system of compression and heat exchangers brings the temperature of the gas to a point where the methane is liquified and the nitrogen remains in a gaseous state (nitrogen has a lower boiling point than methane).

For smaller volumes of gas, a system utilizing Pressure Swing Adsorption (PSA) is a more typical method of separation. With PSA, the methane is the gas is, essentially, sieved through adsorbent media while the nitrogen passes through unaffected. PSA is especially beneficial because the sytems can be scaled to very small sizes.

An estimated 25% of the US natural gas reserves are contaminated with unacceptable quantities of nitrogen. Nitrogen is inert and lowers the Btu value of natural gas. It also takes up capacity in pipelines that could be used for valuable methane.

Pipeline specifications for nitrogen are extremely variable, though "no more than 4% nitrogen" seems to be the most typical spec.