Nitrogen rejection unit

A nitrogen rejection unit (NRU) selectively removes nitrogen from a gas. The name can be applied to any system that removes nitrogen from natural gas. A nitrogen rejection unit (NRU) selectively removes nitrogen from a gas. The name can be applied to any system that removes nitrogen from natural gas. For high flow-rate applications, typically >15 MMSCFD, cryogenic processing is the norm. This is a distillation process which utilizes the different volatilities of methane (boiling point of −161.6 °C) and nitrogen (boiling point of −195.69 °C) to achieve separation. In this process, a system of compression and distillation columns drastically reduces the temperature of the gas mixture to a point where methane is liquified and the nitrogen is not. For smaller applications, a series of heat exchangers may be used as an alternative to distillation columns. For smaller volumes of gas, a system utilizing Pressure Swing Adsorption (PSA) is a more typical method of separation. In PSA, methane and nitrogen can be separated by using an adsorbent with an aperture size very close to the molecular diameter of the larger species, in this case methane (3.8 angstroms). This means nitrogen is able to diffuse through the adsorbent, filling adsorption sites, whilst methane is not. This results in a purified natural gas stream that fits pipeline specifications. The adsorbent can then be regenerated, leaving a highly pure nitrogen stream. PSA is a flexible method for nitrogen rejection, being applied to both small and large flow rates.