Nitrogen Rejection Units

Nitrogen Removal Technology

 

Cryogenic Nitrogen removal consists of the chilling and liquefaction of the feed natural gas to such a level that the material can be distilled to achieve high recovery of the desired methane product and a Nitrogen waste stream that can be safely discharged to atmosphere. Such chilling and liquefaction entails the consumption of power, minimisation of which is the main aim of Costain's process selection, whilst developing an operable and maintainable plant that will provide robust long-term service.

The type of process applicable is defined by the Nitrogen content of the natural gas.

Costain has recently filed two patent applications for new nitrogen rejection processes. These low-energy processes give superior performance for nitrogen removal from feed gas containing up to about 35% nitrogen.

Gas High in N₂

Costain would typically propose their double column process (fig.1) or similar where the feed gas contains in excess of 25% Nitrogen.

 

 

Feed natural gas is first treated for the removal of materials that would otherwise freeze at low temperatures and then enters the cryogenic section. Within a special multi-stream heat exchanger, conventional in low temperature gas processing, the feed gas is cooled and then passed to the first of two distillation columns. The columns operate at different pressures such that the condenser for the high-pressure column is also the reboiler for the low-pressure column. This integrated processing scheme has a long pedigree in the low temperature gas separation industry where it is used for the separation of air and production of Oxygen.

 

In the high-pressure distillation column, the feed gas is separated into a pure Nitrogen stream overhead and a Methane-rich stream from the bottom. The Nitrogen is extracted and condensed to form liquid Nitrogen by reboiling the low-pressure column. A portion of this liquid Nitrogen is used as reflux for the high-pressure column. The remainder, together with the Methane-rich product from the bottom of the high pressure column, is cooled and expanded into the low pressure column.

 

The low pressure column produces a high purity Methane product while rejecting Nitrogen with minimal Methane losses,  suitable for atmospheric venting. The Methane product is pumped to minimise recompression power requirements, and is then vapourised and rewarmed to provide refrigeration for feed cooling.

 

 

 

Gas Low in N₂

Where the Nitrogen content is below 20 - 25%, the amount of Nitrogen in the feed is insufficient to meet reflux requirements for the double column process used for high Nitrogen-containing gases (Fig. 1). Alternative methods of separation have been developed by Costain to address this situation: the Single Column Process (Fig 2) and the Pre-separation Process.

 

The Single Column process uses a classic compression expansion heat pump process to provide refrigeration at the temperature levels required. This process has the benefit of being able to produce high-pressure Nitrogen in addition to high-pressure Methane that can be of considerable value where the Nitrogen is to be further compressed for re-injection into the gas reservoir for pressure maintenance or Enhanced Oil Recovery purposes.

 

The process consists of conventional distillation of the cooled natural gas mixture with interstage reflux added by condensation against cold refrigerant. The flowscheme shown uses expansion of the Nitrogen from the top of the column to provide additional refrigeration by expanding the high-pressure Nitrogen to low pressure through a turbo-expander, reducing refrigeration requirements. Methane is withdrawn at appropriate pressures for evaporation to provide feed cooling and is then compressed.

 

The Pre-Separation approach provides a more efficient and flexible solution and is particularly well suited to removal of Nitrogen from natural gas at a range of feed gas compositions. The process essentially uses an initial distillation column operating at high pressure to perform an initial separation of a pure Methane stream and an overhead stream enriched in Nitrogen which is suitable for processing in the conventional double column process described for high Nitrogen containing gases. The advantage of this process is that a high proportion of the Methane product is withdrawn at high pressure, thus minimising recompression power consumption. 

Enhanced Oil Recovery

Where Nitrogen is used to provide a sweep gas for the enhancement of oil production, Nitrogen will, at some point in time, start to break through into the associated gas. The Nitrogen content of the gas will tend to rise over time, making the design of the cryogenic unit even more challenging. In such cases the Front End Engineering Design will rigorously review the expected break-through profile and develop the design to allow a phased addition of compression equipment to meet the varying refrigeration demands. 

 

Depending on the range of Nitrogen content being considered, either the Double-Column process or the Pre-Separation process could be appropriate. Costain has the skills to select and recommend the best solution for the application, taking into account the cryogenic unit and associated compression requirements.

 

 

LNG Flash Gas

 

Within LNG production trains, Methane is withdrawn as a liquefied product for storage and transportation. Where the feed gas contains Nitrogen, this must be flashed/stripped from the LNG, resulting in high levels of Nitrogen in the flash gas. Generally this flash gas is used to provide fuel for gas turbines on the LNG facility, but if the amount of Nitrogen is high, the burners in such equipment may not be able to operate effectively. In this case, processing flash gas for Nitrogen removal may be required.

 

Costain's approach is to develop a simple facility separate from the LNG plant, integrated with the flash gas compression machinery requirements, to provide a suitable fuel for the gas turbines and a waste Nitrogen stream that can be vented to atmosphere with minimum environmental impact.