Nitrogen Rejection Unit

Overview

A nitrogen rejection unit, or NRU, separates nitrogen from natural gas so that the methane meets pipeline specification. Many gas reservoirs, especially older fields under nitrogen injection for enhanced recovery, produce gas with too much nitrogen to sell directly. Pipeline tariffs usually cap inert content at around 4 mol percent because nitrogen lowers the heating value and takes up compression and transport capacity without delivering energy. The NRU shown here uses cryogenic distillation, the most common method once nitrogen content rises above roughly 10 percent, where it is more economical than membrane or adsorption alternatives.

The unit chills the gas until methane and nitrogen separate by their different boiling points. At cryogenic temperatures methane condenses while nitrogen stays largely vapor, and a distillation column sharpens that split into a methane-rich liquid product and a nitrogen-rich vent. Reaching minus 170 C without freezing anything solid is the central engineering challenge, which is why the gas passes through extensive cleanup before it ever enters the Cold Box.

How it works

Feed gas first passes through the Feed Gas Treatment section. Free liquids drop out in the Inlet Separator, carbon dioxide is scrubbed in the Amine Contactor, and the gas is dried to below 0.1 ppmv water in the Molecular Sieve Bed. A Mercury Removal Bed then captures trace mercury that would otherwise attack the aluminum exchangers. These steps matter because water, CO2, and heavy hydrocarbons all freeze solid at cryogenic temperature and would plug the cold box within hours.

Inside the cold box the Main Heat Exchanger, a brazed aluminum plate-fin block, cools the incoming feed against the cold product and reject streams leaving the plant. This counter-current recovery is what makes the process efficient: most of the cold is recycled rather than supplied from outside. Final cooling and the cold needed to overcome losses come from the Turboexpander Package and a supplemental Refrigeration Package loop. The chilled, partly condensed feed enters the Distillation Section section.

Subsystems

The distillation section is the separation heart. A double-column arrangement is common: the High-Pressure Column does the bulk split at elevated pressure, and the Low-Pressure Column polishes the methane to specification at low pressure. The two are thermally linked by a Condenser-Reboiler that condenses nitrogen overhead in one column while reboiling methane in the other, so heat moves between them instead of being wasted. Structured Packing provides the vapor-liquid contact area, and a Cryogenic Reflux Pump returns reflux from the Reflux Drum.

Refrigeration is generated mainly by the Turboexpander Package. Gas expands across the Expander Wheel, doing work that drops its temperature far more efficiently than a simple Joule-Thomson Valve throttle. The work is recovered by the Booster Compressor Wheel on the same shaft, which recompresses gas and reduces the load on the main machines. The methane product leaves the cold box near atmospheric pressure, so the Product Recompression train, built around a Product Compressor, raises it back to pipeline pressure. The whole plant runs under the Control and Safety System, which includes online Gas Analyzer instruments confirming the product stays on spec.

Materials

Cold-end equipment is built almost entirely from aluminum and austenitic stainless steel because ordinary carbon steel becomes brittle below about minus 30 C. The main exchanger and packing are brazed aluminum, light and highly conductive. Cryogenic piping and column shells are stainless. The Expander Wheel is a titanium alloy to handle high tip speed at low temperature. Vessels exposed to ambient temperature in the feed treatment and recompression sections use carbon and low-alloy steel, which is cheaper where embrittlement is not a concern.

Variants

Single-column designs suit lower nitrogen content and lower methane recovery, while double-column designs reach 99 percent recovery at higher inert levels. Plants that also want to capture the nitrogen for reinjection or for liquid nitrogen sale add a purification column on the reject stream. The refrigeration source varies: small units may rely entirely on the turboexpander, while larger units add a closed Refrigeration Package loop using propane or a mixed refrigerant. Where helium is present in the feed, the NRU is often paired with a downstream helium recovery unit, since the cryogenic temperatures already concentrate helium in the nitrogen reject.

Applications

NRUs are installed at gas processing plants treating high-nitrogen reservoirs, at fields under nitrogen flooding for enhanced oil recovery, and at storage caverns where cushion gas mixes with product. They let otherwise unsalable gas reach the pipeline grid. Because the cold box cannot tolerate a freeze, operation centers on protecting the Feed Gas Treatment beds and watching the analyzers; an upset that lets water or CO2 slip through can force a multi-day warm-up and thaw to clear plugging before the unit can restart.