Cryogenic Absorption NRU Fundamentals

1. Cryogenic NRU Overview

Nitrogen rejection is required when natural gas contains sufficient nitrogen to reduce the gross heating value below pipeline specifications, typically 950–1,000 BTU/scf. Nitrogen is an inert diluent that contributes no heating value but occupies pipeline capacity, making its removal essential for gas marketability. The nitrogen rejection unit (NRU) separates nitrogen from the hydrocarbon stream, producing a pipeline-quality sales gas and a nitrogen-rich vent or product stream.

Cryogenic processes are the dominant technology for nitrogen rejection because the boiling point difference between nitrogen (−320°F at atmospheric pressure) and methane (−259°F) is large enough to achieve effective separation at temperatures in the range of −150 to −250°F. Two fundamentally different cryogenic approaches exist: distillation-based NRU and absorption-based NRU. Each has distinct advantages depending on feed gas nitrogen content, flow rate, and plant economics.

Distillation vs. Absorption NRU

Cryogenic distillation NRUs use one or two distillation columns to separate nitrogen from methane based on vapor-liquid equilibrium. These systems are well-suited for high nitrogen content (>20 mol%) and large throughput applications. Absorption-based NRUs, by contrast, use a cold solvent to preferentially absorb methane from the nitrogen-rich gas stream, leaving the nitrogen in the vapor phase for rejection. The absorption approach offers advantages in specific operating windows:

When Absorption NRU Is Selected

The absorption-based NRU is typically the preferred technology when the following conditions are met:

• Moderate nitrogen content (5–20 mol%): At these nitrogen levels, the absorption process achieves adequate methane recovery without the complexity of multi-column distillation. Below 5% nitrogen, membrane or pressure swing adsorption may be more cost-effective; above 20%, distillation becomes more efficient
• Moderate flow rates (5–100 MMSCFD): The absorption process scales well within this range, with competitive capital and operating costs compared to distillation alternatives
• Lower methane recovery requirement: When 90–97% methane recovery is acceptable (compared to 97–99% for double-column distillation), the simpler absorption process can deliver the required performance at lower cost
• Space-constrained sites: Absorption NRU systems generally have a smaller footprint than equivalent-capacity distillation NRU installations

Pre-Treatment Requirements

Cryogenic processing at temperatures below −150°F imposes stringent feed gas quality requirements. Contaminants that are liquid or solid at ambient conditions will freeze and plug cryogenic heat exchangers and column internals. Feed gas must be treated to remove:

• Water: Dehydration to less than 0.1 ppm (approximately −150°F dewpoint) using molecular sieve adsorption beds. Glycol dehydration alone is insufficient for cryogenic service
• Carbon dioxide: Removal to less than 50 ppm, typically less than 25 ppm for conservative design. CO₂ freezes at −109°F and will deposit as solid in cryogenic equipment. Amine treating is the standard removal method
• Heavy hydrocarbons: C₆+ components must be removed or controlled to prevent freezing and wax formation in cryogenic exchangers. Upstream NGL recovery or inlet scrubbing with chilling is typically required
• Mercury: Removal to less than 0.01 µg/Nm³ to prevent amalgamation damage to aluminum brazed heat exchangers. Activated carbon or metal sulfide beds are used for mercury guard service

2. Cold Solvent Absorption Process

The absorption-based NRU operates on the principle that a cold solvent can preferentially absorb methane from a nitrogen-methane gas mixture while leaving the nitrogen largely in the vapor phase. The solubility of methane in the cold solvent is significantly greater than that of nitrogen, providing the selectivity needed for effective separation. The process operates at cryogenic temperatures where the solvent's absorption capacity for methane is maximized.

Process Description

The feed gas, after pre-treatment to remove water, CO₂, and heavy hydrocarbons, is cooled to cryogenic temperatures in a series of heat exchangers (typically brazed aluminum plate-fin units housed in an insulated cold box). The chilled feed enters the bottom of the absorption column, where it contacts a stream of cold solvent flowing downward from the column top. The solvent preferentially absorbs methane and heavier hydrocarbons from the gas, while nitrogen passes through the column as a vapor and exits from the top.

The methane-rich (loaded) solvent exits the column bottom and is routed to the regeneration system, where pressure reduction, heating, or stripping liberates the absorbed methane. The regenerated lean solvent is recooled and recycled to the absorption column. The recovered methane is recompressed and combined with the sales gas stream.

Solvent Selection

The choice of absorption solvent is critical to process performance. The ideal solvent must have high methane solubility at cryogenic temperatures, low nitrogen solubility (for selectivity), a low freezing point, acceptable viscosity at operating temperatures, low vapor pressure (to minimize solvent losses), and chemical stability under process conditions. Common solvent options include:

Absorption Column Design

The absorption column is the central piece of equipment in the process. It operates at cryogenic temperatures (−150 to −250°F) and moderate to high pressures (300–800 psig). Key design parameters include:

• Operating temperature: Column temperature is determined by the solvent type and desired absorption efficiency. Lower temperatures increase methane solubility and improve recovery but require more refrigeration energy. Typical column operating temperatures range from −150 to −220°F for methanol-based systems
• Operating pressure: Higher pressure increases the driving force for methane absorption into the solvent. Column pressure is typically set by the feed gas pressure after pre-treatment and cooling, usually 300–800 psig
• Solvent circulation rate: The lean solvent flow rate is determined by the required methane absorption capacity, which depends on feed gas composition, column temperature, and the desired methane recovery. Solvent-to-gas ratios typically range from 0.5 to 2.0 gal/Mscf of feed gas
• Number of theoretical stages: Typically 5–15 theoretical stages, depending on the required nitrogen rejection and methane recovery. Higher stage counts improve separation but increase column height and cost

Rich Solvent Regeneration

The methane-laden rich solvent must be regenerated to recover the absorbed methane and prepare the solvent for recirculation. Three regeneration methods are used, either individually or in combination:

• Flash regeneration: The rich solvent is depressurized through one or more flash stages, releasing dissolved methane as gas. Multi-stage flash (typically 2–3 stages) at progressively lower pressures maximizes methane recovery while producing gas at different pressure levels for efficient recompression. Flash regeneration alone can recover 70–85% of the absorbed methane
• Stripping regeneration: After flash stages, the partially regenerated solvent enters a stripping column where it contacts a countercurrent stream of heated vapor (typically recycled methane or nitrogen) to remove residual dissolved methane. Stripping can increase overall methane recovery to 90–97%
• Thermal regeneration: The solvent is heated in a reboiler to drive off dissolved gases. This is used primarily when the solvent has high methane retention after flash and stripping, or when NGL components must also be removed from the solvent

Refrigeration Requirements

Maintaining the absorption column at cryogenic temperatures requires significant refrigeration input. The refrigeration system must overcome heat leak through insulation, cool the incoming feed gas and solvent, and compensate for the heat of absorption. Total refrigeration duty depends on feed gas flow rate, column operating temperature, and the degree of heat recovery in the cold box. Typical specific refrigeration requirements range from 50 to 150 BTU/Mscf of feed gas processed.

3. Process Design Considerations

Designing a cryogenic absorption NRU requires careful attention to feed gas preparation, heat exchanger design, material selection, and insulation systems. The extreme operating temperatures introduce engineering challenges that are not encountered in conventional gas processing equipment, and the consequences of design errors—frozen equipment, brittle fracture, or loss of containment—can be severe.

Feed Gas Preparation

The feed gas quality requirements for cryogenic absorption NRU service are among the most stringent in the gas processing industry. Failure to meet these requirements results in freezing and plugging of heat exchangers, column internals, and control valves:

Cold Box Design

The cold box is the insulated enclosure that houses the cryogenic heat exchangers, phase separators, and associated piping. Brazed aluminum plate-fin heat exchangers (BAHXs) are the standard choice for cryogenic NRU service because of their exceptionally high surface area density (up to 1,500 ft²/ft³), low weight, and ability to handle multiple process streams simultaneously in a single core.

BAHX design for NRU service requires close temperature approaches to maximize thermodynamic efficiency and minimize refrigeration requirements. Typical approach temperatures range from 2 to 5°F, with some exchangers designed for approaches as low as 1°F. These tight approaches necessitate very accurate heat and mass balance calculations and precise flow distribution within the exchanger cores.

Where ε is the heat exchanger effectiveness (typically 0.90–0.98 for cryogenic BAHX service), Q_actual is the actual heat duty transferred, and Q_max is the maximum possible heat duty based on the inlet temperature difference. Effectiveness values above 0.95 are common in cryogenic NRU cold box exchangers.

Material Selection for Cryogenic Service

Carbon steel, the workhorse material of conventional gas processing, becomes brittle at temperatures below approximately −50°F and is unsuitable for cryogenic NRU service. Materials must be selected based on their ductile-to-brittle transition temperature (DBTT) and fracture toughness at the minimum design metal temperature (MDMT):

Aluminum alloys are the preferred material for brazed plate-fin heat exchangers due to their excellent thermal conductivity, low density, and retention of ductility at all cryogenic temperatures. Austenitic stainless steels (304L, 316L) are used for pressure vessels, columns, and piping where higher strength or corrosion resistance is required. The 9% nickel steel is reserved for larger vessels where the cost advantage over stainless steel is significant.

Insulation Systems

Effective thermal insulation is essential to minimize heat leak into the cryogenic equipment and reduce refrigeration requirements. Two primary insulation approaches are used:

• Perlite-filled cold boxes: The most common insulation method for NRU cold boxes. The cold box is a carbon steel enclosure filled with expanded perlite granules (thermal conductivity approximately 0.02 BTU/hr·ft·°F). The perlite space is purged with dry nitrogen to exclude moisture and prevent ice buildup. Typical perlite thickness provides an overall heat transfer coefficient of 0.01–0.03 BTU/hr·ft²·°F
• Vacuum-jacketed piping: Used for transfer piping between cold box equipment and for piping that runs outside the cold box enclosure. The annular space between the inner process pipe and the outer jacket is evacuated to less than 1 millitorr and may contain multi-layer insulation (MLI) or perlite fill. Vacuum-jacketed piping provides heat leak rates of 0.5–2.0 BTU/hr per linear foot, significantly better than conventional insulation

Turboexpander Integration

Turboexpanders are frequently integrated into cryogenic absorption NRU designs to provide a portion of the required refrigeration. By expanding the high-pressure feed gas (or a portion of it) across a turboexpander, the gas undergoes near-isentropic cooling that generates both refrigeration and shaft power. The shaft power can drive a compressor (booster or recompressor), offsetting some of the external compression requirements.

Turboexpander integration reduces the external refrigeration requirement by 30–60%, depending on feed gas pressure and the expansion ratio. The expander is typically located upstream of the absorption column, where the expanded and cooled gas enters the column as feed. Expander isentropic efficiency typically ranges from 80 to 88% for cryogenic gas service.

4. Equipment Design

The major equipment items in a cryogenic absorption NRU include the absorption column, solvent regeneration vessels, cryogenic heat exchangers, refrigeration system, and product gas compression. Each piece of equipment must be designed for reliable operation at cryogenic temperatures while meeting ASME, API, and project-specific requirements.

Absorption Column Internals

The absorption column operates at cryogenic temperatures where liquid viscosity is elevated and surface tension is reduced compared to ambient-temperature service. These conditions favor the use of structured packing over trays for several reasons:

• Lower pressure drop: Structured packing produces 5–10 times less pressure drop per theoretical stage than trays, which is important for maintaining column operating pressure and reducing compression costs
• Higher capacity: The open structure of packing allows higher vapor and liquid throughput per unit cross-sectional area, enabling smaller column diameters
• Better efficiency at low liquid rates: Structured packing maintains good mass transfer performance at the relatively low solvent flow rates typical of absorption NRU service, where tray weeping could be problematic
• Cryogenic compatibility: Stainless steel or aluminum structured packing retains its mechanical properties at cryogenic temperatures, whereas tray components with many bolted connections can develop leaks due to differential thermal contraction

Structured packing with specific surface areas of 250–500 m²/m³ is typical for NRU absorber service. The HETP (height equivalent to a theoretical plate) ranges from 1.5 to 3.0 ft, depending on packing type, operating conditions, and liquid distribution quality. Proper liquid distribution is critical—liquid distributors must provide uniform solvent distribution across the column cross-section with a drip point density of at least 4–6 points per square foot.

Flooding Calculations

Column diameter is sized to avoid flooding at the maximum anticipated throughput, with an appropriate design margin. For structured packing, the flooding velocity is determined from vendor-specific capacity correlations or generalized pressure drop correlations. The design capacity factor (F-factor) is typically set at 60–75% of the flooding value to provide operational margin:

Where F_s is the F-factor (typically 0.5–2.0 (ft/s)(lb/ft³)^0.5 for structured packing), V_s is the superficial gas velocity (ft/s), and ρ_V is the vapor density (lb/ft³). Maximum allowable F-factor depends on packing type, liquid loading, and physical properties at the operating temperature.

Regeneration System

The rich solvent regeneration system recovers absorbed methane and returns the lean solvent for recirculation. The system typically consists of:

• High-pressure flash drum: The first stage of pressure reduction, typically releasing 40–60% of the absorbed methane at moderate pressure (100–300 psig). The flash gas is routed to the sales gas stream or to a recompressor
• Low-pressure flash drum: A second flash stage at 15–50 psig recovers an additional 15–25% of the absorbed methane. This lower-pressure gas requires more compression to reach sales gas pressure
• Stripping column: The partially regenerated solvent enters a stripping column where it is contacted with heated strip gas (methane or nitrogen) to drive off the remaining dissolved methane. The stripping column typically contains 3–8 theoretical stages of structured packing and operates at near-atmospheric pressure

Heat Exchangers

Brazed aluminum plate-fin heat exchangers are the standard selection for the cold box exchangers. Design considerations specific to NRU service include:

Refrigeration System Sizing

The refrigeration system must provide cooling from ambient temperature down to the absorption column operating temperature while compensating for heat leak and process inefficiencies. The total refrigeration load consists of:

• Feed gas cooling: Sensible heat removal from the feed gas temperature (typically 80–120°F after pre-treatment) to the column operating temperature. This is the largest refrigeration component, typically 60–75% of total duty
• Heat of absorption: The exothermic heat released when methane dissolves in the solvent, typically 200–400 BTU/lb-mol of methane absorbed
• Heat leak: Thermal energy entering through the cold box insulation and piping supports. Typically 5–15% of total refrigeration duty for a well-insulated cold box
• Solvent cooling: Removing the heat gained by the solvent during regeneration before it returns to the absorber column

Common refrigeration options include mixed refrigerant systems, cascaded propane/ethane or propane/ethylene cycles, and turboexpander-based refrigeration. The choice depends on the required temperature levels, available utilities, and project economics.

Nitrogen Vent and Product Gas Handling

The nitrogen-rich overhead vapor from the absorption column typically contains 70–95 mol% nitrogen with residual methane. This stream can be:

• Vented to atmosphere: The simplest disposal method, permitted when methane content is below regulatory emission thresholds (typically <5 mol% methane) and total vent volume is within permit limits
• Used as fuel gas: If methane content is sufficient (typically >10 mol%), the nitrogen-rich stream can supplement plant fuel gas after pressure reduction
• Reinjected: In some applications, the nitrogen stream is compressed and reinjected into the reservoir for pressure maintenance or enhanced recovery

The product sales gas leaving the NRU typically requires recompression to pipeline delivery pressure. Recompression horsepower is a significant operating cost component and must be factored into the overall process economics.

5. Integration and Economics

The economic viability of a cryogenic absorption NRU depends on the integration with upstream gas processing facilities, the value of recovered methane, capital and operating costs, and regulatory requirements for nitrogen-contaminated gas. Proper integration with existing plant infrastructure can significantly improve project economics by sharing utilities, compression, and treating capacity.

Integration with NGL Recovery

Many nitrogen-rich gas fields also contain recoverable NGL (C₂+) components. Integrating the NRU with an upstream NGL recovery facility provides several synergies:

• Shared pre-treatment: The molecular sieve dehydration and amine treating systems serve both the NGL recovery and NRU functions, reducing total capital cost compared to separate facilities
• Cascaded refrigeration: The NGL recovery unit operates at moderately cryogenic temperatures (−100 to −160°F), and its cold process streams can provide pre-cooling for the NRU feed, reducing the NRU's dedicated refrigeration requirement
• Heavy hydrocarbon removal: The upstream NGL recovery unit removes C₃+ and most C₂ from the gas, which also satisfies the NRU's requirement for heavy hydrocarbon removal in the feed
• Common compression: Residue gas compression from the NGL plant and product gas recompression from the NRU can potentially share compression equipment or be staged together

Helium Recovery

Some nitrogen-rich natural gas reservoirs also contain economically recoverable concentrations of helium (typically 0.1–2.0 mol%). Because helium has a boiling point (−452°F) even lower than nitrogen, it concentrates in the nitrogen vent stream from the NRU. The NRU nitrogen reject stream can serve as a crude helium feed (5–30 mol% He) for a downstream helium purification unit. Helium recovery adds a high-value revenue stream that can significantly improve overall project economics, particularly given periodic tight helium supply markets.

Capital Cost Drivers

The major capital cost components of a cryogenic absorption NRU include:

Total installed costs for absorption NRU facilities typically range from $15,000 to $40,000 per MMSCFD of feed gas capacity, depending on site conditions, nitrogen content, and the degree of integration with existing facilities. Grassroots installations at the higher end of this range include full pre-treatment, while add-on NRU modules at existing gas plants fall toward the lower end.

Operating Cost Components

Operating costs are dominated by energy consumption for refrigeration and compression:

• Refrigeration power: The largest single operating cost, typically 60–75% of total operating expense. Specific power consumption ranges from 15 to 40 HP per MMSCFD of feed gas, depending on column temperature, feed N₂ content, and the degree of heat integration
• Product gas recompression: Recompressing the recovered methane from regeneration flash pressure to pipeline delivery pressure consumes 10–25 HP per MMSCFD
• Solvent makeup: Solvent losses through entrainment, degradation, and vaporization require periodic makeup. Solvent costs typically represent 2–5% of total operating expense
• Maintenance: Cryogenic equipment requires specialized maintenance procedures and materials, with annual maintenance costs typically 2–4% of installed capital cost

Process Comparison

The following comparison summarizes the trade-offs between the three primary cryogenic nitrogen rejection technologies:

Environmental Considerations

Nitrogen rejection operations must address several environmental concerns:

• Nitrogen venting: The rejected nitrogen stream contains residual methane (a potent greenhouse gas). Minimizing methane content in the vent stream through process optimization reduces the environmental impact and may be required by regulatory permits
• Methane slip: Methane losses in the nitrogen vent typically range from 3–10% of the feed methane for absorption NRU systems. Advanced process designs with optimized regeneration can reduce methane slip to 3–5%
• Noise: Turboexpanders, compressors, and pressure letdown valves generate significant noise. Acoustic enclosures and silencers are typically required to meet occupational and community noise limits
• Flaring: During startup, shutdown, and upset conditions, off-specification gas may require flaring. The flare system must be designed per API 521 for the maximum relief scenario

Regulatory frameworks for NRU emissions vary by jurisdiction, but increasingly stringent methane emission regulations (including EPA Subpart OOOOa and state-level requirements) are driving the adoption of higher-recovery NRU designs and the elimination of routine methane venting where technically feasible.

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Standard 2261 — Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography
3. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Pressure Vessel Design
4. API Standard 521 — Pressure-Relieving and Depressuring Systems
5. ALPEMA Standards — Brazed Aluminum Plate-Fin Heat Exchanger Design