GPSA Section 16 · GPA RR-76 · Cryogenic Distillation
Cryogenic N₂ removal, BAHX design, column sizing, and NRU vs membrane economics
An NRU removes nitrogen from natural gas to meet pipeline specifications, using cryogenic distillation, PSA, or membrane separation technologies.
It calculates N₂ removal efficiency, methane recovery, refrigeration duty, and column sizing for cryogenic NRU design using GPSA methods.
The calculator covers cryogenic NRU, pressure swing adsorption (PSA), and membrane separation for nitrogen rejection.
An NRU is needed when nitrogen content exceeds pipeline specifications, typically when N2 is above 4 mol%. NRUs also improve gas heating value by removing the inert nitrogen. They are most common in fields with 5-25% nitrogen content.
Cryogenic distillation achieves greater than 95% methane recovery and is preferred for high-volume, high-N2 applications above 20 MMSCFD. Pressure swing adsorption (PSA) provides 90-93% recovery with simpler operation. Membrane separation is another option for moderate applications.
Cryogenic NRU columns operate at -220 to -260°F with relative volatility of 2.0-3.0. They require 30-60 theoretical stages, feed purity of less than 0.1 ppm H2O and less than 50 ppm CO2, and brazed aluminum heat exchanger approach temperatures of 3-5°F at the cold end.