PSA Nitrogen Rejection Fundamentals

1. Nitrogen in Natural Gas

Nitrogen is one of the most common non-hydrocarbon contaminants found in natural gas streams. Unlike acid gases (H₂S, CO₂) that can be removed by chemical reaction with amines or physical solvents, nitrogen is chemically inert and must be separated from methane by exploiting differences in physical properties—molecular size, adsorption affinity, boiling point, or membrane permeability. When nitrogen concentrations exceed pipeline tariff limits, a nitrogen rejection unit (NRU) is required to bring the gas into specification.

Sources of Nitrogen Contamination

Nitrogen enters the natural gas production system through several mechanisms, each contributing varying concentrations depending on reservoir characteristics and operating practices:

• Reservoir nitrogen: Many formations contain naturally occurring nitrogen at concentrations ranging from trace levels to over 50 mol%. Hugoton field in Kansas, for example, produces gas with 15–25% N₂. Other nitrogen-rich basins include portions of the San Juan, Permian, and Michigan basins. Reservoir nitrogen originates from atmospheric trapping during burial, radiogenic decay of potassium-bearing minerals, and thermal decomposition of organic nitrogen compounds in the source rock
• Nitrogen injection for enhanced oil recovery (EOR): Nitrogen is widely used as an injection gas for miscible and immiscible displacement in EOR projects. As the flood front advances, breakthrough nitrogen mixes with produced gas, creating a steadily increasing nitrogen content in the production stream. N₂ concentrations in EOR-affected gas can range from 5% to 70% depending on flood maturity
• Well stimulation and workover operations: Nitrogen-energized fracturing fluids, nitrogen lifts, and nitrogen displacement during well completions introduce N₂ into the produced gas stream. These contributions are typically transient, producing elevated nitrogen levels for days to weeks following the operation

Impact on Gas Quality

Nitrogen is an inert diluent that reduces the heating value of natural gas without contributing any combustion energy. The primary impacts on gas quality and commercial value include:

Nitrogen Rejection Technology Options

Three commercial technologies are available for removing nitrogen from natural gas, each suited to different flow rates, nitrogen concentrations, and recovery requirements:

Selection Criteria

The choice among these technologies depends on a combination of technical and economic factors. Key selection criteria include:

• Flow rate: Cryogenic NRUs dominate at high throughput (>50 MMSCFD) where economies of scale favor the capital-intensive distillation equipment. PSA and membranes are competitive at lower flow rates where the simpler equipment and faster installation offset lower per-unit efficiency
• Nitrogen content: PSA and membranes handle moderate N₂ levels (5–30%) most efficiently. Cryogenic units can process gas with very high nitrogen content (up to 70%) where the other technologies become impractical
• Required methane recovery: When methane losses must be minimized (recovery >95%), cryogenic distillation is typically the only option. PSA achieves 85–95% recovery, while membranes are limited to 80–92%
• Site constraints: PSA units are modular, skid-mounted, and can be installed in remote locations without cryogenic infrastructure. Cryogenic NRUs require significant plot space, specialized materials, and longer construction timelines
• Economics: Capital cost, operating cost, methane value, and nitrogen disposal options all factor into the selection. PSA offers the lowest capital cost for small-to-medium applications but has higher methane losses than cryogenic units

2. PSA Process Principles

Pressure swing adsorption exploits the difference in adsorption affinity between methane and nitrogen on selective adsorbent materials. The process operates by alternating between high-pressure adsorption (where methane is preferentially retained on the adsorbent) and low-pressure desorption (where the adsorbed methane is released as product). Nitrogen, being less strongly adsorbed, passes through the bed at high pressure and is vented as the reject stream.

Adsorption Fundamentals

The separation between methane and nitrogen in a PSA system can be achieved through two distinct mechanisms, depending on the adsorbent type selected:

• Kinetic separation (carbon molecular sieve): Carbon molecular sieve (CMS) materials have precisely controlled micropore openings (3.7–4.0 Å) that allow the smaller nitrogen molecule (kinetic diameter 3.64 Å) to diffuse into the micropores more rapidly than the larger methane molecule (kinetic diameter 3.80 Å). Both molecules will eventually reach equilibrium adsorption, but the difference in diffusion rate allows separation within the controlled cycle time. CMS-based PSA adsorbs nitrogen preferentially and produces methane-enriched gas as the raffinate product at high pressure
• Equilibrium separation (zeolite molecular sieve): Certain zeolite adsorbents (particularly clinoptilolite and modified 4A zeolites) adsorb methane more strongly than nitrogen at equilibrium. The separation is driven by the difference in adsorption isotherms rather than diffusion rates. Zeolite-based PSA adsorbs methane preferentially and produces nitrogen-enriched gas at high pressure, with methane recovered during the depressurization and purge steps

The Skarstrom Cycle

The basic PSA cycle is derived from the Skarstrom cycle, which consists of four sequential steps in each adsorption bed. In practice, commercial PSA units use modified versions of this cycle with additional steps to improve recovery and efficiency, but the fundamental four-step cycle illustrates the core operating principle:

Pressure Ratio and Recovery

The pressure ratio (adsorption pressure divided by desorption pressure) is the primary thermodynamic driving force for PSA separation. A higher pressure ratio provides a greater difference in adsorbent loading between the adsorption and desorption steps, which directly translates to higher working capacity per cycle and better separation performance.

Typical PSA systems for nitrogen rejection operate with adsorption pressures of 100–300 psig and desorption pressures of 1–15 psig, yielding pressure ratios in the range of 7:1 to 30:1. Higher pressure ratios generally improve methane recovery but require more compression energy for repressurization. The economic optimum balances recovery improvement against compression power cost.

Where P_ads is the adsorption (high) pressure and P_des is the desorption (low) pressure. Increasing the pressure ratio from 5:1 to 15:1 can improve methane recovery by 5–10 percentage points, but the incremental benefit diminishes at very high ratios as the adsorption isotherm approaches saturation.

Cycle Time and Throughput

Total cycle time for a PSA nitrogen rejection unit typically ranges from 2 to 10 minutes, depending on the adsorbent type, bed size, and separation difficulty. Shorter cycle times increase the number of adsorption/desorption cycles per hour, which raises the effective throughput per unit of adsorbent but also increases valve cycling frequency and mechanical wear. The optimal cycle time represents a balance between adsorbent utilization and equipment longevity.

For CMS-based kinetic separation, cycle time is particularly critical because the separation depends on the difference in diffusion rates between N₂ and CH₄. If the adsorption step is too long, both components reach equilibrium and the kinetic selectivity advantage is lost. Typical CMS adsorption step times are 30–120 seconds.

3. Bed Design and Adsorbent Selection

The performance of a PSA nitrogen rejection unit depends critically on the adsorbent material, the bed geometry, and the number of beds arranged in the cycle. Proper bed design ensures adequate throughput, acceptable pressure drop, efficient adsorbent utilization, and long adsorbent life while meeting the required product purity and recovery targets.

Carbon Molecular Sieve (CMS)

Carbon molecular sieve is the most widely used adsorbent for PSA nitrogen rejection from natural gas. CMS pellets are manufactured from carbonaceous precursors (coal, coconut shell, polymers) through controlled carbonization and carbon vapor deposition to create uniform micropore openings in the 3.7–4.0 Å range. Key characteristics include:

• Kinetic selectivity: N₂ diffuses into CMS micropores 10–30 times faster than CH₄ under typical operating conditions. This kinetic selectivity enables effective separation even though the equilibrium adsorption capacities for both gases are similar
• Operating capacity: Typical N₂ working capacity of 0.3–0.8 mmol/g per cycle, depending on pressure ratio, temperature, and cycle time
• Temperature sensitivity: CMS separation performance is temperature-dependent. Diffusion rates increase with temperature, but selectivity decreases. Optimal operating temperature is typically 60–110°F
• Contaminant sensitivity: CMS micropores can be blocked by heavy hydrocarbons (C₆+), water, H₂S, and other condensable contaminants, requiring upstream guard beds for protection

Zeolite Molecular Sieves

Zeolite-based PSA systems use the equilibrium adsorption difference between methane and nitrogen rather than kinetic selectivity. Clinoptilolite (a natural zeolite) and modified synthetic zeolites (Ba-exchanged or Sr-exchanged 4A) preferentially adsorb methane over nitrogen, with equilibrium selectivities of 2–5 depending on pressure and temperature:

Bed Sizing

Bed sizing for PSA nitrogen rejection is determined by the feed gas flow rate, adsorbent working capacity, and the number of cycles per hour. The total adsorbent inventory must be sufficient to process the design feed rate while maintaining the required cycle time and allowing for pressure equalization and purge steps:

Where M_ads is the adsorbent mass per bed (kg), F is the feed molar flow rate (mol/min), y is the mole fraction of the adsorbed component, t_ads is the adsorption step time (seconds), and q_w is the adsorbent working capacity (mol/kg). This calculation determines the minimum adsorbent volume required per bed; design practice applies a 15–25% safety factor to account for non-ideal flow distribution and capacity degradation over the adsorbent's operating life.

Number of Beds

Commercial PSA nitrogen rejection units use 4 to 12 adsorber vessels arranged in a staggered cycle sequence to provide continuous feed processing and product flow. The number of beds is determined by the number of cycle steps and the requirement for uninterrupted production:

Bed Geometry

Adsorber vessels are typically vertical cylindrical pressure vessels with height-to-diameter (L/D) ratios of 2:1 to 5:1. The bed geometry must balance several competing requirements:

• Pressure drop: Pressure drop through the packed bed is proportional to bed height and gas velocity (Ergun equation). Excessive pressure drop wastes compression energy and reduces the effective pressure ratio for separation. Design pressure drop is typically 1–5 psi per bed
• Fluidization velocity: The superficial gas velocity must remain below the minimum fluidization velocity to prevent bed lifting and adsorbent attrition. For CMS pellets (1.5–3 mm diameter, bulk density 650–750 kg/m³), the maximum allowable velocity is typically 0.15–0.25 m/s
• Mass transfer zone: The bed must be long enough to contain the mass transfer zone (MTZ) without breakthrough during the adsorption step. Longer beds provide better separation but increase pressure drop. The MTZ length depends on gas velocity, adsorbent particle size, and diffusion characteristics
• Gas distribution: Uniform gas distribution across the bed cross-section is essential for efficient adsorbent utilization. Inlet distributors and support grids are designed to achieve flow uniformity within ±5% across the bed diameter

Guard Beds for Contaminant Protection

The primary adsorbent (CMS or zeolite) is sensitive to contamination by heavy hydrocarbons, water, H₂S, and other condensable species that can permanently or temporarily reduce adsorption capacity. Guard beds containing activated alumina, silica gel, or activated carbon are installed upstream of the main adsorbent bed to remove these contaminants before they reach the N₂/CH₄ separation zone:

• Water removal: Activated alumina or silica gel layer (6–12 inches) at the bed inlet adsorbs residual water vapor to protect the main adsorbent from moisture-induced capacity loss
• Heavy hydrocarbon removal: Activated carbon layer (6–12 inches) adsorbs C₆+ hydrocarbons that would otherwise accumulate in CMS micropores and block nitrogen access
• H₂S removal: If the feed gas contains H₂S (even at ppm levels), an impregnated activated carbon or zinc oxide layer is required to prevent sulfur poisoning of the main adsorbent

Guard bed materials are regenerated during the blowdown and purge steps of the normal PSA cycle, provided the contaminant loading does not exceed the guard bed capacity within a single adsorption step. Guard beds are typically sized for 5–10 years of continuous operation before replacement is required.

4. Cycle Design and Optimization

The cycle design is the heart of any PSA system, defining the sequence and timing of pressure and flow changes in each adsorber vessel. For nitrogen rejection applications, the cycle must be carefully optimized to maximize methane recovery while meeting the product purity specification at the design throughput. Commercial PSA cycles for N₂/CH₄ separation are significantly more complex than the basic four-step Skarstrom cycle, incorporating pressure equalization, cocurrent depressurization, and multiple purge steps to improve recovery.

Advanced Cycle Steps

A typical commercial PSA cycle for nitrogen rejection includes six to ten discrete steps per bed, executed in a precise time sequence coordinated across all beds in the system:

Pressure Equalization Steps

Pressure equalization is the single most important cycle modification for improving methane recovery beyond the basic Skarstrom cycle. During equalization, a high-pressure bed that has completed its adsorption step transfers its void-space gas (which is rich in methane) to a low-pressure bed that is being prepared for the next adsorption step. This recovers methane that would otherwise be lost during blowdown and simultaneously reduces the energy required for repressurization:

Each additional equalization step provides diminishing returns while adding complexity (more beds, more valves, more sophisticated control logic). Most commercial PSA nitrogen rejection units use one or two equalization steps as the optimal compromise between recovery and complexity.

Cycle Time Optimization

Cycle time is a critical operating parameter that directly affects throughput, recovery, and equipment life. The optimal cycle time depends on the adsorbent type and the specific separation requirements:

• CMS-based systems: Cycle times of 2–6 minutes are typical. The adsorption step must be short enough to exploit the kinetic selectivity before methane diffuses into the micropores and reduces separation performance. Overly short cycles reduce adsorbent utilization; excessively long cycles lose kinetic selectivity
• Zeolite-based systems: Cycle times of 4–10 minutes are common. Since separation is equilibrium-based, longer adsorption times improve adsorbent utilization without losing selectivity, but they require larger beds
• Valve cycling: Each complete cycle involves 6–12 valve actuations per bed. At 2-minute cycle times, this translates to 250,000–500,000 valve operations per year per valve. High-quality switching valves rated for 1–5 million cycles are essential for reliable operation

Product Purity vs. Recovery Trade-Off

There is a fundamental inverse relationship between product purity and methane recovery in PSA systems. Operating at higher product purity (lower N₂ content in the sales gas) requires rejecting more gas during the blowdown and purge steps, which increases methane losses to the waste stream. Typical performance ranges for PSA nitrogen rejection are:

Turndown and Operational Flexibility

PSA units offer inherent operational flexibility to handle feed rate variations and changing nitrogen concentrations. The primary turndown mechanisms are:

• Cycle time adjustment: Extending the adsorption step time at reduced feed rates maintains product purity while decreasing throughput proportionally. Turndown to 40–50% of design capacity is achievable by cycle time extension alone
• Bed skipping: At very low feed rates, one or more beds can be taken offline (maintained in a pressurized standby state), reducing the number of active beds and the minimum feed rate for stable operation. This extends turndown capability to 25–30% of design
• Feed composition changes: PSA systems can handle moderate fluctuations in feed N₂ content (typically ±5–10 mol% from design) by adjusting cycle timing and purge gas quantities through the control system

5. Comparison with Alternative NRU Technologies

Selecting the appropriate nitrogen rejection technology requires evaluating multiple factors including capacity, recovery, capital cost, operating cost, and operational complexity. While PSA is well-suited for small-to-medium applications, cryogenic distillation and membrane separation each have distinct advantages in specific operating envelopes. This section provides a comprehensive comparison to guide technology selection.

PSA vs. Cryogenic Distillation NRU

Cryogenic distillation is the dominant nitrogen rejection technology for large-scale applications. The process separates nitrogen from methane by exploiting their boiling point difference (−320°F for N₂ vs. −259°F for CH₄) in a distillation column operating at cryogenic temperatures. Key comparisons with PSA:

PSA vs. Membrane Separation

Membrane-based nitrogen rejection uses polymer membranes that are selectively permeable to nitrogen (or methane, depending on membrane type) to separate the two gases. Conventional membranes are more permeable to nitrogen, producing a nitrogen-enriched permeate and a methane-enriched retentate at high pressure. Key comparisons:

PSA Advantages

PSA technology offers several distinct advantages that make it the preferred choice for many nitrogen rejection applications, particularly in the small-to-medium capacity range:

• No cryogenic equipment: PSA operates at ambient temperatures, eliminating the need for specialized cryogenic materials (stainless steel, aluminum), brazed aluminum heat exchangers, and turboexpanders. This simplifies construction, reduces capital cost, and improves reliability
• Simple operation: The PSA process is controlled by a PLC that sequences automatic switching valves on fixed time cycles. No rotating equipment is required in the separation system itself. Operator intervention is minimal during normal operation
• Modular and skid-mounted: Complete PSA units can be factory-fabricated on transportable skids, minimizing field construction time and cost. Units up to 20–30 MMSCFD capacity are routinely skid-mounted. Installation timelines of 3–6 months are typical compared to 12–24 months for cryogenic NRUs
• Fast startup and shutdown: PSA units reach steady-state operation within minutes to hours of startup, making them well-suited for intermittent operation and rapid response to changing feed conditions. Cryogenic NRUs require hours to days for cooldown and stabilization
• No process chemicals: PSA is a physical adsorption process requiring no solvents, refrigerants, or chemical consumables. The only consumable is the adsorbent itself, which lasts 5–10 years under normal operating conditions

PSA Limitations

Despite its advantages, PSA technology has inherent limitations that restrict its applicability in certain scenarios:

• Capacity limitation: PSA units become impractical above approximately 50 MMSCFD due to the very large number of adsorber vessels and switching valves required. Cryogenic distillation is the only proven technology for high-volume nitrogen rejection
• Lower recovery than cryogenic: Maximum methane recovery of 90–95% compares unfavorably with 95–99% recovery achievable with cryogenic distillation. The lost methane in the PSA reject stream represents both revenue loss and potential flaring/emission concerns
• No NGL or helium co-recovery: PSA separates only N₂ from CH₄ and cannot simultaneously recover NGL liquids or helium. If these byproducts have value, a cryogenic NRU with integrated NGL and helium recovery may be more economical despite higher capital cost
• Reject stream quality: The PSA reject stream contains 30–60% methane (depending on recovery setting), which may require flaring, fuel gas use, or recompression and recycling. Disposing of the methane-laden reject stream is both an economic and environmental consideration

Hybrid Systems

In some applications, combining PSA with another nitrogen rejection technology provides performance advantages that neither technology achieves alone:

• PSA + cryogenic: A PSA unit can serve as a bulk nitrogen removal step upstream of a smaller cryogenic polishing unit. The PSA removes the majority of nitrogen from high-N₂ feed gas, reducing the cryogenic unit size and energy consumption. This hybrid approach can be attractive for 20–80 MMSCFD applications with very high nitrogen content
• PSA + membrane: Membrane permeate (nitrogen-enriched but still containing significant methane) can be processed through a PSA unit to recover the residual methane, improving overall system recovery. Alternatively, PSA reject gas can be further processed by membranes to recover additional methane

Process Selection Guidelines

The following guidelines provide a starting point for nitrogen rejection technology selection based on flow rate and nitrogen content. Final selection should consider site-specific factors including NGL and helium value, utility availability, environmental regulations, and project economics:

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
4. GPA Standard 2166 — Obtaining Natural Gas Samples for Analysis by Gas Chromatography