Ethane Recovery Process Fundamentals

1. Cryogenic NGL Recovery Overview

Ethane recovery is the process of extracting ethane (C₂H₆) from raw natural gas streams for sale as a petrochemical feedstock, primarily for ethylene production in steam crackers. As the lightest NGL component, ethane requires the deepest cooling and the most thermodynamically efficient process configurations to achieve high recovery levels. The economic justification for ethane recovery depends on the spread between ethane product value and the alternative value of leaving the ethane in the residue gas (sales gas) stream.

Cryogenic processing using turboexpander technology is the dominant method for deep ethane recovery. The turboexpander replaced the Joule-Thomson (JT) valve as the primary expansion device beginning in the 1970s because it provides substantially higher thermodynamic efficiency. When gas expands through a turboexpander, the process approaches isentropic expansion rather than the isenthalpic expansion of a JT valve. This produces colder temperatures at a given pressure drop and recovers a portion of the expansion energy as shaft work, which is used to drive a recompressor on the residue gas. The net result is 15–25% higher ethane recovery at equivalent compression power compared to JT-based plants.

Feed Gas Richness and Economic Threshold

The richness of the feed gas, expressed in gallons of liquid per thousand standard cubic feet (GPM), determines whether ethane recovery is economically justified. Feed gas richness is the sum of the theoretical liquid content of each NGL component (C₂+) in the gas stream.

As a general guideline, feed gas with a total C₂+ GPM above approximately 2.5–3.0 is considered a candidate for ethane recovery, although the actual economic threshold depends on current ethane pricing, natural gas pricing, capital cost of the recovery facility, and pipeline infrastructure availability. When the ethane-to-gas price ratio (frac spread) is unfavorable, operators may choose to reject ethane into the residue gas stream. Modern plants are designed with ethane rejection flexibility, allowing the demethanizer to operate in either C₂ recovery or C₂ rejection mode by adjusting operating temperatures and reflux rates.

Ethane Rejection vs. Recovery Economics

The decision to recover or reject ethane is driven by the frac spread — the difference between the value of ethane as a purity product and its heating value when left in the residue gas. When ethane prices are low relative to natural gas, the incremental revenue from ethane recovery does not justify the additional compression power, refrigeration, and capital cost. Key factors in the recovery/rejection decision include:

• Ethane product price — Typically quoted in cents per gallon; must exceed the fuel-equivalent value in the residue gas
• Incremental compression cost — Higher C₂ recovery requires deeper cooling, higher expansion ratios, and more recompression horsepower
• Pipeline BTU specifications — Rejecting ethane increases residue gas heating value; some pipelines have maximum BTU limits that constrain how much ethane can be rejected
• Plant design flexibility — Modern GSP and CRR plants include ethane rejection capability with minimal operational changes

Recovery Comparison by Process Type

The progression from JT valve to CRR represents increasing thermodynamic sophistication, capital investment, and ethane recovery. The process selection depends on the required recovery level, feed gas composition, available compression, and project economics. For most new grassroots cryogenic plants targeting deep ethane recovery, the GSP or CRR configuration is the standard design basis.

2. Process Configurations

All modern cryogenic ethane recovery processes share a common framework: the inlet gas is cooled against cold process streams in a cold box, expanded to generate cryogenic temperatures, and fed to a demethanizer column that separates methane (overhead residue gas) from C₂+ NGL (bottoms product). The key differences between process configurations lie in how they generate reflux for the demethanizer and how effectively they utilize heat integration.

Gas Subcooled Process (GSP)

The GSP, developed in the 1980s, is the most widely used cryogenic NGL recovery process in the midstream industry. It achieves 85–90% ethane recovery by splitting the cold separator liquid and vapor into separate streams, each processed differently before entering the demethanizer.

In the GSP configuration, the inlet gas is first cooled in the cold box against returning cold residue gas and demethanizer side-draw streams. The cooled gas enters a cold separator where vapor and liquid phases are separated. The cold separator liquid is subcooled by heat exchange with cold demethanizer overhead vapor, then flashed across a valve and fed to the top of the demethanizer as reflux. This subcooled liquid provides the key reflux that enables high ethane recovery. The cold separator vapor is expanded through the turboexpander and fed to the demethanizer at an intermediate point.

The GSP's advantage over a simple expander plant is the generation of a subcooled reflux stream that absorbs C₂ from the rising vapor in the demethanizer rectifying section. Without this reflux, the demethanizer overhead vapor would carry significant ethane, limiting recovery to 70–80%.

Cold Residue Recycle (CRR)

The CRR process achieves the highest ethane recovery levels (93–97%) by recycling a portion of the cold residue gas back through the cold box and into the top of the demethanizer as additional reflux. This recycled stream, being predominantly methane at very low temperatures, provides superior absorption of ethane from the demethanizer overhead vapor.

In the CRR configuration, a fraction (typically 15–30%) of the high-pressure residue gas is diverted before entering the sales gas pipeline. This recycle stream is cooled in the cold box against the main process streams, expanded through a JT valve or a second expander, and introduced at or near the top of the demethanizer. The extremely cold, methane-rich recycle liquid acts as a powerful absorbing agent, scrubbing ethane from the overhead vapor and driving C₂ recovery above 95%.

The penalty for CRR's higher recovery is increased recompression power. The recycled gas must be recompressed from the demethanizer operating pressure back to the residue gas pipeline pressure, which adds 10–20% to the total compression requirement compared to a GSP plant at the same throughput.

Improved Single-Split (ISS)

The ISS process is an intermediate configuration between GSP and CRR that achieves 88–92% ethane recovery. It uses a single split of the inlet gas (rather than the vapor-liquid split of the cold separator in GSP) combined with enhanced heat integration to generate effective demethanizer reflux.

In the ISS configuration, a portion of the partially cooled inlet gas is diverted, further cooled in the cold box, and used as a subcooled reflux feed to the demethanizer top. The remaining inlet gas follows the conventional path through the cold separator and turboexpander. The ISS achieves higher recovery than GSP by providing a colder and more effective reflux stream, but without the additional recompression penalty of full residue gas recycle.

Conventional JT Process

The simplest cryogenic NGL recovery process uses a JT valve for gas expansion instead of a turboexpander. The inlet gas is cooled in the cold box, then expanded across a JT valve to achieve cryogenic temperatures. The two-phase mixture enters the demethanizer directly. Without a turboexpander, there is no power recovery, and the isenthalpic expansion of the JT valve produces less cooling than the isentropic expansion of an expander at the same pressure ratio.

JT plants are limited to approximately 40–50% ethane recovery but can achieve 85–90% propane recovery. They remain in service at smaller facilities or older plants where the capital cost of turboexpander retrofits is not justified by the incremental ethane recovery revenue.

Process Trade-offs Summary

3. Turboexpander Technology

The turboexpander is the central piece of rotating equipment in a cryogenic NGL recovery plant. It converts the pressure energy of the high-pressure inlet gas into shaft work while simultaneously producing deep cryogenic cooling through near-isentropic expansion. The recovered shaft work drives a directly coupled recompressor (booster compressor) that partially recompresses the residue gas, reducing the external compression requirement.

Operating Principle

The turboexpander operates as a radial-inflow turbine. High-pressure gas enters the expander through adjustable inlet guide vanes (nozzles), accelerates through the radial impeller, and exits at reduced pressure and temperature. The expansion process follows a path between isentropic (ideal) and isenthalpic (JT valve) on the enthalpy-entropy diagram, with the actual path determined by the expander's isentropic efficiency.

The temperature drop across the expander is substantially greater than across a JT valve at the same pressure ratio because the isentropic process removes energy from the gas as shaft work. For a typical inlet condition of 900 psig and 0°F expanding to 300 psig, a turboexpander produces outlet temperatures of approximately −130°F to −150°F, compared to −80°F to −100°F for a JT valve at the same conditions.

Isentropic Efficiency

Turboexpander isentropic efficiency (η_s) is defined as the ratio of actual enthalpy drop to the ideal isentropic enthalpy drop:

Where h₁ is the inlet enthalpy, h_2,actual is the actual outlet enthalpy, and h_2,isentropic is the outlet enthalpy for an ideal isentropic expansion to the same outlet pressure. Typical isentropic efficiencies for modern turboexpanders range from 78–86%, with the highest efficiencies achieved at design-point operation on machines with optimized impeller geometry.

Expansion Ratio and Cooling

The expansion ratio (inlet pressure / outlet pressure) directly affects the degree of cooling and the amount of liquid formed in the expander outlet stream. Higher expansion ratios produce colder temperatures and more liquid formation, which improves C₂ recovery but also increases the recompression requirement for the residue gas.

Expander Wheel Design

The expander wheel (impeller) is a radial-inflow design, typically machined from a single forging of high-strength aluminum alloy (AA 7075-T6) or titanium (Ti-6Al-4V) for cryogenic service. The wheel features backward-curved blades that decelerate the gas relative to the rotating frame, converting kinetic energy to shaft work. Wheel tip speeds range from 700–1,200 ft/s, with rotational speeds of 10,000–60,000 RPM depending on wheel diameter and gas molecular weight.

The expander and booster compressor share a common shaft supported on active magnetic bearings or tilting-pad oil bearings. Magnetic bearings are increasingly preferred for cryogenic service because they eliminate the risk of lube oil contamination in the cold process gas and require less maintenance.

Supplemental Compression

The shaft-coupled booster compressor typically recovers 60–75% of the recompression work needed to bring the residue gas from demethanizer overhead pressure back to sales gas pipeline pressure. The remaining compression is provided by an external residue gas compressor, usually a centrifugal machine driven by a gas turbine or electric motor. For a plant operating at 300 psig demethanizer pressure and delivering residue gas at 1,000 psig pipeline pressure, the booster typically provides 1.3–1.6:1 compression ratio, with the external compressor providing the balance.

Expander Surge and Operating Envelope

Turboexpanders have a defined operating envelope bounded by surge, choke (stonewall), and maximum speed limits. Unlike centrifugal compressors, turboexpanders are inherently stable and do not experience classical surge. However, they do have a minimum flow limit below which the inlet guide vanes cannot maintain proper flow angles, leading to reduced efficiency and potential vibration. The expander control system modulates the inlet guide vane position to maintain the desired expansion ratio and outlet temperature across varying throughput conditions.

Turndown capability is typically 40–100% of design flow, with efficiency degradation of 5–10 percentage points at the low end. Below the minimum stable flow, the gas can be bypassed around the expander through a JT valve, although this significantly reduces ethane recovery due to the less efficient isenthalpic expansion.

4. Demethanizer Column Design

The demethanizer (DeC1) is the primary separation column in a cryogenic ethane recovery plant. It separates methane (C₁) as the overhead product (residue gas) from ethane and heavier hydrocarbons (C₂+) as the bottoms product (NGL or raw make). The demethanizer operates at cryogenic temperatures, with the overhead temperature ranging from −130°F to −165°F depending on the ethane recovery target and process configuration.

Key Separation: C₁/C₂

The key separation in the demethanizer is between methane (light key) and ethane (heavy key). The relative volatility of methane to ethane at demethanizer conditions is relatively favorable (α = 3.5–5.0), making the separation thermodynamically straightforward. The challenge lies in achieving the required cryogenic temperatures and providing sufficient reflux to drive ethane recovery above 90%.

Multiple Feed Points

A distinguishing feature of cryogenic demethanizer design is the use of multiple feed points at different elevations and thermal conditions. Depending on the process configuration, the demethanizer may receive three to five separate feed streams:

• Subcooled liquid reflux (top feed) — Coldest stream; provides the primary reflux for C₂ recovery in the rectifying section. In GSP, this is the subcooled cold separator liquid; in CRR, this is the recycled residue gas condensate
• Expander outlet (upper-mid feed) — Two-phase mixture from the turboexpander; enters at or near the transition between rectifying and stripping sections
• Cold separator liquid (mid feed) — In some configurations, a portion of the cold separator liquid bypasses the subcooler and enters the column at an intermediate tray
• Side reboiler return streams (lower feeds) — Partially vaporized liquid returned from side reboilers located in the cold box; these provide intermediate heat input to the stripping section

Operating Conditions

Cryogenic Internals

Demethanizer internals must be designed for the extreme temperature conditions and the presence of two-phase flow at multiple feed points. Both trays and structured packing are used in demethanizer service:

• Sieve trays — Traditional choice; robust, well-understood hydraulics, tolerant of fouling. Tray spacing of 24 inches is standard. Typical tray efficiency is 60–75% for the C₁/C₂ system at cryogenic conditions
• Structured packing — Increasingly used in the rectifying section (above the expander feed) where pressure drop must be minimized to maintain low overhead temperatures. Packing offers 30–50% lower pressure drop per theoretical stage compared to trays, which directly translates to colder overhead temperatures and higher C₂ recovery
• Hybrid designs — Some demethanizers use structured packing in the rectifying section (where low pressure drop is critical) and trays in the stripping section (where liquid loads are higher and fouling from heavier hydrocarbons is a concern)

Side Reboilers

The demethanizer typically employs two to three side reboilers that withdraw liquid from the stripping section, heat it against the incoming inlet gas in the cold box, and return the partially vaporized stream to the column. Side reboilers serve a dual purpose: they provide stripping vapor to the lower section of the column (reducing the main reboiler duty) and they precool the inlet gas (reducing the cold box refrigeration load). This heat integration is critical to the thermodynamic efficiency of the cryogenic process.

The side reboiler arrangement typically includes a lower side reboiler (at or near the column bottoms) and an upper side reboiler (several trays above). The temperature levels of these side reboilers are matched to the cooling curve of the inlet gas in the cold box, creating a thermodynamically efficient counter-current heat exchange between the warm inlet gas and the cold column liquid.

Reflux Mechanisms

Providing adequate reflux to the top of the demethanizer is the primary challenge in achieving high ethane recovery. The reflux mechanism differs by process configuration:

• GSP: Reflux is generated by subcooling the cold separator liquid against cold overhead vapor, then flashing it to the demethanizer top tray. The subcooled liquid absorbs ethane as it descends through the rectifying section
• CRR: A portion of the residue gas is recycled, cooled to cryogenic temperatures, condensed, and fed to the demethanizer top. The very cold, methane-rich recycle liquid provides the most effective reflux for deep C₂ recovery
• ISS: A split stream of partially cooled inlet gas is further cooled and subcooled, then flashed to the column top as reflux, providing an intermediate level of effectiveness between GSP and CRR

5. Cold-Box Heat Exchange and Metallurgy

The cold box is the heart of the cryogenic NGL recovery plant, containing the brazed aluminum plate-fin heat exchangers (BAHX) that perform the critical heat integration between warm inlet gas streams and cold process return streams. The cold box is so named because the heat exchangers are housed in an insulated enclosure (the "box") filled with perlite or mineral wool insulation to minimize ambient heat leak at the extreme operating temperatures.

Plate-Fin Heat Exchanger Design

Brazed aluminum plate-fin exchangers are the standard technology for cryogenic NGL service. They offer extremely high heat transfer surface area per unit volume (up to 1,500 ft²/ft³), allowing compact designs that handle the multiple simultaneous heat exchange duties required in cryogenic plants. A single BAHX core can contain 4–12 separate stream passages, enabling the close temperature approaches (2–5°F) necessary for thermodynamically efficient cryogenic processing.

The plate-fin construction consists of corrugated aluminum fins brazed between flat aluminum parting sheets. Different fin geometries (serrated, perforated, plain) are selected for each stream passage based on the required heat transfer coefficient and allowable pressure drop. The brazed assembly forms a monolithic structure with no gaskets or seals, providing excellent leak integrity at cryogenic temperatures.

Operating Temperature Ranges

Minimum Design Metal Temperature (MDMT)

All pressure-containing components in the cold box must be designed and fabricated from materials suitable for the minimum expected operating temperature, known as the minimum design metal temperature (MDMT). The MDMT is established per ASME Section VIII Division 1, which requires impact testing (Charpy V-notch) for carbon steel components below −20°F and specifies minimum toughness requirements for cryogenic service.

Material Selection for Cryogenic Service

Austenitic stainless steels (304, 304L, 316L) maintain excellent ductility and toughness at cryogenic temperatures because they do not undergo a ductile-to-brittle transition. This makes them the standard material for all piping, vessels, and column shells in the cold section of NGL recovery plants. Carbon steel is acceptable only in the warm section where temperatures remain above −20°F (or the impact-tested MDMT).

CO₂ Freeze-Out Concerns

Carbon dioxide (CO₂) in the feed gas can freeze and deposit as a solid at the cryogenic temperatures encountered in ethane recovery plants. The CO₂ solidification temperature depends on the partial pressure of CO₂ in the gas and the local temperature; for typical demethanizer pressures of 350–450 psig, CO₂ can begin to freeze at temperatures below approximately −100°F to −120°F when the CO₂ content exceeds 0.5–1.0 mol%.

CO₂ freeze-out is prevented by one or more of the following approaches:

• Upstream CO₂ removal — Amine treating or molecular sieve adsorption reduces inlet CO₂ to below 50–100 ppmv, effectively eliminating freeze-out risk at any cryogenic temperature
• Operating temperature management — Maintaining demethanizer overhead temperature above the CO₂ frost point for the given CO₂ concentration; this may limit maximum ethane recovery
• Controlled freeze zone (CFZ) technology — Specialized column designs that allow controlled CO₂ freezing and melting within a designated section of the column; this is a niche technology for high-CO₂ feed gases

For most cryogenic plants processing pipeline-quality gas with CO₂ content below 2%, upstream amine treating followed by molecular sieve dehydration (which also removes a portion of the CO₂) is sufficient to prevent freeze-out. Plants processing rich gas from shale plays or associated gas may encounter higher CO₂ levels that require dedicated CO₂ removal upstream of the cryogenic section.

Process Control and Turndown

Cryogenic NGL recovery plants must operate within tight temperature and pressure envelopes to maintain ethane recovery targets while avoiding CO₂ freeze-out and hydrate formation. Key control loops include:

• Demethanizer overhead temperature — Controlled by adjusting expander guide vane position (expansion ratio) and reflux rate; this is the primary variable for ethane recovery control
• Demethanizer pressure — Maintained by the residue gas compressor suction control; affects both recovery and column hydraulics
• Side reboiler duties — Adjusted to balance heat integration and maintain proper column temperature profile
• Cold box approach temperatures — Monitored to detect fouling, maldistribution, or frost formation in the plate-fin exchangers

Turndown capability is typically 40–100% of design throughput. At reduced rates, the expander guide vanes close to maintain the required expansion ratio, and reflux rates are reduced proportionally. Below approximately 40% of design rate, the expander may be bypassed and a JT valve used for expansion, with a corresponding reduction in ethane recovery. Plants designed for wide throughput variation may include a variable-speed expander or multiple parallel expander trains.

ASME Section VIII Requirements

All pressure vessels in the cryogenic section (cold separator, demethanizer column, reflux drums) must be designed, fabricated, and stamped per ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (or Division 2 for high-pressure applications). Key cryogenic-specific requirements include:

• Impact testing per UG-84 for all materials at or below the MDMT
• Post-weld heat treatment (PWHT) requirements per UCS-56 for carbon steel components
• Material certification and traceability per UG-93
• Hydrostatic testing per UG-99 at temperatures above the MDMT of all components
• Special consideration for thermal cycling and differential thermal expansion between dissimilar materials at cryogenic temperature transitions

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Standard 2145 — Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry
3. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
4. GPA Midstream Association — Technical Publications on Cryogenic Processing