Introduction
It is estimated that 25% of the United States natural gas reserves contain unacceptably large 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. Thus, nitrogen needs to be rejected from the gas through a Nitrogen Rejection Unit NRU. Typically, the feed to nitrogen rejection unit comes from the top of the DeMethanizer column of a cryogenic NGL recovery unit.Process Description
The feed gas is first cooled against the HP column reboiler and further cooled in the main cryogenic heat exchanger before introducing to the HP column. The bottom methane rich liquid is subcooled and sent to an intermediate location of LP column. The top Nitrogen rich liquid is sent to the top of the LP column after being subcooled against nitrogen vapor. Nitrogen vapor out of the top of LP column first subcools the nitrogen rich liquid and then the methane rich liquid, and ultimately the cold is fully recovered in the main cryogenic heat exchanger.
The LP column reboiler and HP column condenser is physically one heat exchanger. On the LP column side, methane liquid is reboiled and on the HP column side, the nitrogen rich vapor is condensed. The bottom methane liquid from the LP column is pumped to the subcooler 1 and finally sent to the main cryogenic heat exchanger after the cold being recovered.
Result
In the feed, the nitrogen content is 30%, after the first HP column, it becomes 22.5% and finally in the LP column, N2 is reduced to 4%, which is generally well accepted pipeline specification in the United States.
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Reference: Muhammad Tahir Ashraf, PARAMETRIC OPTIMIZATION OF DOUBLE COLUMN NITROGEN REJECTION UNIT
