Cryogenic Distillation NRU Fundamentals

1. Cryogenic Distillation for N₂ Rejection

Cryogenic distillation is the industry standard technology for nitrogen rejection from natural gas at high flow rates and elevated nitrogen concentrations. The process exploits the significant boiling point difference between nitrogen (−320°F / −196°C) and methane (−259°F / −161°C) to separate the two components through fractional distillation at cryogenic temperatures. This 61°F boiling point spread provides excellent relative volatility, making distillation highly efficient for N₂–CH₄ separation.

Cryogenic NRU technology dominates the nitrogen rejection market for applications exceeding approximately 50 MMSCFD inlet gas flow with nitrogen concentrations above 10 mol%. Below these thresholds, competing technologies such as pressure swing adsorption (PSA) and membrane separation may offer lower capital cost, but cryogenic distillation delivers superior methane recovery (95–98%) and can handle the widest range of nitrogen concentrations (5–70 mol%) of any available technology.

Thermodynamic Basis

The N₂–CH₄ binary system exhibits nearly ideal vapor-liquid equilibrium behavior at cryogenic conditions, with a relative volatility (α) of approximately 3.5–5.0 depending on pressure and temperature. This favorable thermodynamic characteristic means that relatively few theoretical stages are needed for high-purity separation. The absence of an azeotrope in the N₂–CH₄ system allows complete separation to any desired purity with sufficient stages and reflux.

Pre-Treatment Requirements

Cryogenic distillation requires thorough feed gas pre-treatment to prevent freeze-out of contaminants on cold heat exchange surfaces and in the distillation column. At cryogenic operating temperatures, even trace quantities of certain components will form solids that can plug equipment and cause operational failures. Required pre-treatment steps include:

• Dehydration: Water content must be reduced to less than 0.1 ppmv (typically <1 lb H₂O/MMSCF) using molecular sieve dehydration. Any residual moisture will freeze as ice in the cold box exchangers, progressively blocking heat transfer surfaces and flow passages
• CO₂ removal: Carbon dioxide must be reduced below 50–100 ppmv to prevent formation of solid CO₂ (dry ice), which freezes at −109.3°F at atmospheric pressure. Amine treating or molecular sieve adsorption provides the required removal
• Heavy hydrocarbon removal: C₅+ and aromatic hydrocarbons (particularly benzene, which freezes at 41.9°F) must be removed to prevent freezing and wax formation. A front-end NGL recovery unit, charcoal adsorption, or a dedicated heavy-ends removal system addresses this requirement
• Mercury removal: Mercury attacks aluminum heat exchangers used in cryogenic service through liquid metal embrittlement. Activated carbon or metal sulfide adsorbent beds reduce mercury to below 0.01 µg/Nm³

Process Selection Criteria

The decision to use cryogenic distillation over competing nitrogen rejection technologies is driven by the combination of flow rate, nitrogen content, methane recovery requirement, and product specifications. Cryogenic NRU is strongly preferred when:

2. Single-Column NRU

The single-column NRU is the simpler of the two cryogenic distillation configurations and is widely used for moderate nitrogen concentrations (10–30 mol% N₂) where maximum methane recovery is not the primary objective. The process uses a single distillation column with a condenser-reboiler to achieve the N₂–CH₄ separation.

Process Description

Feed gas, after thorough pre-treatment, enters the cold box where it is progressively cooled against returning cold product and waste streams in the brazed aluminum plate-fin heat exchangers. The feed is cooled to near its dew point or partially condensed, depending on composition and pressure, and then fed to the distillation column at an intermediate tray location.

Inside the column, the lighter nitrogen component concentrates in the overhead vapor, while the heavier methane concentrates in the bottoms liquid. The overhead nitrogen-rich vapor passes through the condenser-reboiler, where it is partially condensed by exchanging heat against the boiling column bottoms liquid. The resulting nitrogen-rich condensate provides reflux to the column, while the uncondensed nitrogen vapor is vented or used as fuel gas. The methane-rich bottoms product is subcooled, letdown in pressure, and routed through the cold box heat exchangers to recover refrigeration before delivery as sales gas.

The Condenser-Reboiler

The condenser-reboiler is the single most critical heat exchanger in the single-column NRU. It thermally links the column overhead and bottoms by condensing overhead nitrogen vapor against boiling bottoms liquid. This dual function simultaneously provides reflux for the rectifying section and vapor boilup for the stripping section, all within a single exchanger.

For the condenser-reboiler to function, a sufficient temperature difference must exist between the condensing overhead nitrogen and the boiling bottoms methane. This temperature driving force is established through the column pressure profile—the overhead operates at a higher pressure than the bottoms, ensuring the nitrogen condenses at a temperature above the methane boiling temperature. Typical approach temperatures in the condenser-reboiler are 3–8°F.

Column Sizing and Stage Requirements

The single-column NRU typically requires 15–25 theoretical stages for effective N₂–CH₄ separation, with the exact number depending on feed nitrogen content, desired methane recovery, and product purity specifications. Trayed columns with sieve or valve trays are the standard construction, although structured packing is increasingly used in smaller installations for its lower pressure drop and higher efficiency per unit height.

Limitations

The single-column configuration has several inherent limitations that restrict its application range:

• Limited methane purity control: Because a single column must perform both stripping and rectification with one condenser-reboiler, the operating flexibility to independently adjust overhead and bottoms purity is constrained. Trade-offs between methane recovery and nitrogen rejection purity are more severe than in the double-column design
• Moderate nitrogen range: The single column works best for feeds with 10–30 mol% nitrogen. At lower nitrogen concentrations (<10%), the column becomes oversized relative to the nitrogen removal requirement. At higher concentrations (>30%), the condenser-reboiler duty becomes excessive and methane recovery declines
• Nitrogen vent losses: The overhead nitrogen stream typically contains 5–30 mol% methane, representing lost product. Reducing this methane slip requires more stages and reflux, which increases both capital and operating cost
• Single pressure level: Operating at a single pressure limits the available temperature driving forces in the condenser-reboiler and constrains the achievable separation. The double-column design overcomes this limitation by operating at two distinct pressure levels

3. Double-Column NRU

The double-column NRU is the more sophisticated cryogenic distillation configuration, using two thermally linked distillation columns operating at different pressures to achieve superior methane recovery and product purity compared to the single-column design. This configuration is the preferred choice for large-scale nitrogen rejection applications (typically >75 MMSCFD) and situations requiring high methane recovery (>97%) or handling wide nitrogen concentration ranges (5–70 mol%).

Process Description

In the double-column arrangement, the high-pressure (HP) column and low-pressure (LP) column are thermally linked through a shared condenser-reboiler. Pre-treated feed gas is cooled in the cold box and fed to the HP column, which operates at 400–500 psig. The HP column performs a preliminary separation, producing a nitrogen-rich overhead and a methane-rich bottoms stream.

The HP column overhead (nitrogen-rich vapor) is condensed in the LP column reboiler, providing heat for boilup in the LP column while simultaneously generating liquid nitrogen reflux. A portion of the HP column bottoms liquid (methane-rich) is subcooled, letdown in pressure, and fed to the LP column as an intermediate feed. The LP column, operating at 15–50 psig, performs the final separation to produce high-purity methane bottoms product and nitrogen overhead for venting.

Thermal Linking: HP Condenser / LP Reboiler

The thermal integration between the two columns is the defining feature of the double-column NRU. The HP column operates at sufficient pressure that its overhead nitrogen condenses at a temperature higher than the LP column bottoms boiling temperature. This pressure-driven temperature difference allows the HP column condenser and the LP column reboiler to be combined into a single heat exchanger, analogous to the Linde double-column concept used extensively in air separation.

Where T_cond,HP is the condensing temperature of the HP column overhead nitrogen at its operating pressure, T_reb,LP is the boiling temperature of the LP column bottoms methane at its operating pressure, and ΔT_min is the minimum approach temperature in the condenser-reboiler. The HP column pressure is selected to ensure this temperature driving force is maintained under all operating conditions.

Advantages Over Single-Column Design

The double-column NRU offers several significant advantages that justify its higher capital cost and complexity for demanding applications:

• Higher methane recovery: The two-stage separation achieves 97–99% methane recovery, compared to 90–97% for the single-column design. The LP column provides additional rectification that captures methane that would otherwise be lost in the nitrogen vent
• Superior purity control: Independent control of the HP and LP columns allows simultaneous optimization of methane product purity and nitrogen rejection purity. The two degrees of freedom (HP reflux ratio, LP reflux ratio) provide greater operational flexibility
• Wider nitrogen range: The double-column design effectively handles nitrogen concentrations from 5 to 70 mol%, a much wider range than the single column. This flexibility is particularly valuable for fields with declining or variable nitrogen content over the production life
• Better energy integration: The two-pressure-level operation creates more opportunities for heat integration within the cold box, reducing the net refrigeration requirement and improving overall energy efficiency

Operating Conditions

Column Internals

Both HP and LP columns typically use sieve trays or structured packing designed for cryogenic service. Material selection is critical: column shells are fabricated from 9% nickel steel or austenitic stainless steel (304/304L) to maintain adequate toughness at operating temperatures below −250°F. Tray materials are typically 304 stainless steel or aluminum alloy. Structured packing, when used, is fabricated from aluminum for its excellent thermal conductivity and low weight at cryogenic temperatures.

4. Cold Box and Heat Exchanger Design

The cold box is the heart of the cryogenic NRU, containing the multi-stream heat exchangers, phase separators, and interconnecting piping that achieve the cryogenic temperatures required for distillation. Efficient cold box design is essential for minimizing refrigeration power consumption and overall plant operating cost, as the heat exchangers must recover nearly all of the refrigeration content from the cold product and waste streams to pre-cool the incoming feed gas.

Brazed Aluminum Plate-Fin Heat Exchangers (BAHX)

Brazed aluminum plate-fin heat exchangers are the standard choice for cryogenic NRU service, dominating this application for several compelling reasons:

• High surface area density: BAHX units provide 300–800 ft²/ft³ of heat transfer surface, approximately 5–10 times the density of conventional shell-and-tube exchangers. This compactness is critical for fitting the large heat transfer area required for cryogenic service into a manageable cold box envelope
• Multi-stream capability: A single BAHX core can accommodate 4–12 separate process streams in a single exchanger block, enabling simultaneous heat exchange between feed gas, sales gas product, nitrogen vent stream, and intermediate process streams. This eliminates the need for multiple individual exchangers and reduces piping complexity
• Close approach temperatures: BAHX units routinely achieve approach temperatures of 1–3°F, with heat transfer effectiveness exceeding 95%. This tight thermal approach minimizes the external refrigeration requirement by maximizing internal heat recovery
• Aluminum properties at cryogenic temperatures: Aluminum alloy (typically 3003 or 5083) becomes stronger and more ductile at cryogenic temperatures, unlike carbon steel which becomes brittle. Aluminum also has excellent thermal conductivity (approximately 90 BTU/hr·ft·°F at −260°F), enhancing heat transfer performance

BAHX Design Considerations

Cold Box Arrangement

The cold box is a large rectangular or cylindrical steel vessel that encloses the BAHX cores, distillation column(s), phase separators, and all cryogenic piping. The vessel is filled with perlite insulation (expanded volcanic glass) to minimize heat leak from the ambient environment into the cryogenic equipment. Typical cold box design features include:

• Perlite insulation: Loose-fill expanded perlite provides excellent thermal insulation at a density of 4–8 lb/ft³. The cold box is maintained under a slight positive nitrogen pressure (0.5–2 in. w.c.) to prevent moisture ingress into the perlite, which would degrade its insulating properties
• Steel enclosure: The cold box shell is fabricated from carbon steel (the outer shell operates near ambient temperature) with penetrations for all process piping, instrument connections, and relief devices. Expansion bellows or loops accommodate thermal contraction of internal piping (aluminum contracts approximately 0.4% from ambient to −300°F)
• Internal supports: Cryogenic equipment is supported on low-conductivity hangers or pads (typically stainless steel or fiberglass) to minimize heat conduction from the warm shell to the cold internals
• Pressure relief: Multiple relief devices protect against overpressure from blocked-in cryogenic liquids that warm and vaporize. Relief valve discharge is routed to a safe location (typically a vent stack)

Piping Materials

All piping and equipment operating below −150°F must be fabricated from materials that maintain adequate toughness at cryogenic temperatures. Standard piping material selections for NRU service include:

Turboexpander for Supplemental Refrigeration

Most cryogenic NRU installations use a turboexpander to provide supplemental refrigeration and recover energy from the pressure letdown of process streams. The turboexpander isentropically expands a portion of the high-pressure feed gas (or an intermediate process stream), producing refrigeration through the Joule-Thomson effect combined with work extraction. Key features include:

• Isentropic efficiency: Modern turboexpanders achieve 80–88% isentropic efficiency, converting pressure energy into shaft work that drives a booster compressor (expander-compressor) or generates electrical power
• Temperature drop: A single-stage expansion from 500 psig to 100 psig can produce a temperature drop of 80–120°F, providing substantial refrigeration for the cold box
• Two-phase operation: Some NRU turboexpanders operate with partial liquid formation at the outlet (up to 15–20 wt% liquid), which is separated in a downstream phase separator before the liquid fraction feeds the column
• Variable capacity: Inlet guide vanes or variable-speed drives allow the turboexpander to adjust refrigeration output as feed gas conditions change, providing operational flexibility across varying flow rates and compositions

5. Helium Recovery and Economic Considerations

Cryogenic distillation NRU facilities represent substantial capital investment but offer unique advantages for large-scale nitrogen rejection, including the potential for helium recovery as a high-value byproduct. The economic viability of a cryogenic NRU project depends on the interplay of gas composition, flow rate, product specifications, energy costs, and the availability of alternative nitrogen rejection technologies.

Helium Recovery

Natural gas from certain geological formations contains economically significant concentrations of helium, typically 0.1–2.0 mol%. Because helium has the lowest boiling point of any element (−452°F / −269°C), it concentrates in the nitrogen-rich overhead streams of the cryogenic NRU columns. This provides a convenient point for helium extraction without requiring a dedicated separation plant.

The helium recovery process from an NRU overhead stream typically involves two stages:

• Crude helium production: A dedicated crude helium column (or a side draw from the NRU column) concentrates the helium from the nitrogen vent stream to approximately 50–70 mol% helium. The crude helium column operates at the coldest temperatures in the plant, near the nitrogen boiling point, because helium must be separated from the nitrogen carrier. The crude helium product also contains residual nitrogen and trace methane
• Helium purification: The crude helium stream is purified to 99.995–99.999% purity using pressure swing adsorption (PSA) or catalytic oxidation followed by cryogenic final purification. The purified helium is compressed and stored in high-pressure tube trailers or loaded into ISO containers for transport to market

Capital Cost Factors

Cryogenic NRU facilities are capital-intensive due to the specialized materials, equipment, and fabrication required for cryogenic service. The major capital cost components include:

Operating Costs

The dominant operating cost for a cryogenic NRU is the power required for feed gas compression, product recompression, and refrigeration. Because cryogenic processes operate at extremely low temperatures, any heat leak or thermodynamic inefficiency must be compensated by additional refrigeration work. Key operating cost components include:

• Compression power: Feed compression (if inlet pressure is insufficient), sales gas recompression (to pipeline pressure from the low-pressure LP column bottoms), and booster compression driven by the turboexpander. Total power consumption typically ranges from 15–30 HP per MMSCFD of feed gas
• Molecular sieve regeneration: The dehydration system requires periodic regeneration using heated gas, consuming fuel or electric heat. Regeneration gas is typically a slipstream of treated sales gas
• Maintenance: Cryogenic equipment requires specialized maintenance procedures, including periodic warm-up and inspection of BAHX cores, turboexpander overhaul, and molecular sieve bed replacement (3–5 year cycle)
• Oil makeup and chemical costs: Relatively minor but include amine solution makeup (if used for CO₂ removal), mercury adsorbent replacement, and molecular sieve replacement

Process Selection Matrix

Selecting the appropriate nitrogen rejection technology requires evaluation of multiple factors. The following matrix summarizes the competitive positioning of cryogenic distillation against PSA and membrane alternatives:

Project Economics

Cryogenic NRU projects are typically evaluated on a full-lifecycle economic basis, considering the following factors:

• Methane recovery revenue: The primary economic driver. Each 1% improvement in methane recovery can represent significant annual revenue at current gas prices, particularly for large-volume plants. At $3.00/MMBTU and 100 MMSCFD feed with 20% N₂, a 97% vs. 92% methane recovery difference represents approximately $1.5 MM/yr in additional revenue
• Helium revenue: When present at >0.3 mol%, helium recovery can substantially improve project economics. High-purity helium commands premium pricing and can represent 10–30% of total NRU plant revenue in helium-rich gas fields
• Nitrogen credits: In some markets, high-purity nitrogen from the NRU overhead can be sold for enhanced oil recovery (EOR) injection, industrial gas supply, or pipeline purging applications
• Typical payback period: Well-designed cryogenic NRU projects targeting high-nitrogen gas reserves typically achieve payback periods of 3–7 years, depending on gas prices, nitrogen content, and the value of byproduct helium

The economic advantage of cryogenic NRU over competing technologies becomes most pronounced at high flow rates (>100 MMSCFD) where the economies of scale in cryogenic equipment offset the higher unit capital cost, and at high nitrogen concentrations (>15 mol%) where the superior methane recovery of the cryogenic process captures significantly more product compared to PSA or membrane alternatives.

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Midstream Standard 2261 — Analysis for Natural Gas and Similar Gaseous Mixtures
3. API Standard 620 — Design and Construction of Large, Welded, Low-Pressure Storage Tanks
4. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Cryogenic Vessel Design