Nitrogen Rejection Process for Natural Gas

A Nitrogen Rejection Unit is used to remove nitrogen from natural gas to increase the heating value of the gas. This process is generally involved cryogenic distillation to separate nitrogen gas from liquefied natural gas. The below process flow diagram illustrates the nitrogen rejection process. The design may vary from one application to another due to different operating conditions and product requirements. Cooling may be carried out by using external refrigeration cycles or using the excess pressure of the gas stream.

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First natural gas is compressed up to appropriate pressure (375 psig in the below application) and pre-cooled in a multipass exchanger (LNG exchanger). The pressure of the NG stream slightly dropped through the valve in such a way as to maintain the appropriate flow in stream R7.

The flash separator (V-100) separates the vapors from the liquid stream. Liquid from V-100 further cools down in LNG exchanger and its pressure further drops up to 10 psig before entering to the distillation column. Vapors from V-100 further cool down by exchanging their heat from column top product stream and entering the column as a reflux stream.

The distillation column has 30 stages and feeds NG8 enters at stage 10. The top product stream leaves the column around -170°C at the pressure of 7 psig. This stream mainly consists of nitrogen (65%) and methane (35%). The bottom stream consists of liquefied natural gas with a very small amount of nitrogen (< 0.5%). The bottom stream again vaporizes after a couple of LNG exchangers. The detailed operating conditions can be seen below video.