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Nitrogen Rejection Unit (NRU) Configurations

IP.com Disclosure Number: IPCOM000241297D
Publication Date: 2015-Apr-14
Document File: 3 page(s) / 103K

Publishing Venue

The IP.com Prior Art Database

Abstract

There is often a need to reject nitrogen from a natural gas (NG) stream to produce a nitrogen-lean hydrocarbon stream that can be liquefied or used as fuel, and to reject a pure nitrogen stream that can be vented or otherwise utilized. Such units are often seen at the back end of liquid natural gas (LNG) plants.

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Nitrogen Rejection Unit (NRU) Configurations

There is often a need to reject nitrogen from a natural gas (NG) stream to produce a nitrogen-lean hydrocarbon stream that can be liquefied or used as fuel, and to reject a pure nitrogen stream that can be vented or otherwise utilized. Such units are often seen at the back end of liquid natural gas (LNG) plants.

Fig. 1 shows a conventional single-column nitrogen rejection unit (NRU).

Fig. 1

NG is typically compressed in compressor CMP1, partially liquefied in warm heat exchanger HXW and introduced to the phase separator PS. The vapor portion is liquefied in cold heat exchanger HXC and used as reflux for the distillation column COL.  The liquid portion becomes feed to the column. Nitrogen-rich overhead product (N2) is warmed and vented to atmosphere and nitrogen-lean liquid bottoms product is pumped, vaporized, further warmed, and recompressed. A portion of the vaporized stream provides boil-up for the column. Environmental regulations may require lower methane content of the vent stream. Fig. 2 shows an alternative configuration that results in lower methane content of the vent stream.

Fig. 2

A portion of the vented nitrogen stream (N2) is recycled back to the suction of compressor CMP1. This allows building up of nitrogen in the feed stream and producing a reflux stream with higher nitrogen content. The idea can be applied to other NRU configurations. 

Fluctuations in the feed composition to the NRU make it challenging to effec...