Propane Recovery Process Fundamentals

1. C₃+ Recovery Overview

Propane recovery, commonly referred to as C₃+ recovery, is a gas processing strategy that maximizes the extraction of propane (C₃), butanes (C₄), and natural gasoline (C₅+) from the inlet gas stream while rejecting ethane (C₂) to the residue gas. This approach is distinct from deep ethane recovery (C₂+ recovery) in that the plant is designed and operated to keep ethane in the sales gas rather than recovering it as a separate NGL product.

The primary purpose of C₃+ recovery is to capture the higher-value liquid hydrocarbons—propane, butanes, and natural gasoline—while avoiding the additional capital cost, operating complexity, and cryogenic temperatures required for ethane recovery. The resulting NGL product stream is routed to a downstream fractionation train (depropanizer, debutanizer, and optionally a butane splitter) for separation into individual purity products.

When to Reject Ethane

Operating in ethane rejection mode (C₃+ recovery) is the preferred strategy under several market and operational conditions:

• Unfavorable ethane pricing: When the ethane price ($/gal) is below the equivalent BTU value in the residue gas, recovering ethane destroys economic value. The ethane breakeven price is the point at which recovery becomes profitable after accounting for shrinkage, compression, and fractionation costs.
• No ethane pipeline access: Many gas processing plants lack access to an ethane pipeline or petrochemical market. Without a physical outlet for ethane product, rejection is the only option.
• Low petrochemical demand: During periods of reduced ethylene cracker demand or ethane storage overhang, spot ethane prices can fall below fuel-equivalent value, making rejection economically rational.
• Pipeline heating value specifications: Some residue gas pipelines have minimum heating value requirements (typically 950–1,050 BTU/scf). Rejecting ethane increases the heating value of the residue gas, helping meet pipeline tariff specifications.
• Reduced capital investment: New grassroots plants targeting C₃+ recovery can be built with significantly lower capital cost than deep-cut C₂ plants, since they operate at warmer temperatures and require less cryogenic equipment.

Feed Gas Richness Requirements

The economic viability of C₃+ recovery depends heavily on the richness of the inlet gas, measured in gallons per thousand standard cubic feet (GPM). The C₃+ GPM represents the total liquid content of propane and heavier components in the feed gas. As a general guideline:

C₃+ Recovery vs. Deep C₂ Recovery

C₃+ recovery plants offer several inherent advantages over deep ethane recovery facilities, particularly for operators who do not have access to ethane markets or who prefer a simpler, lower-risk investment profile:

Recovery Efficiency by Process Type

Different process technologies achieve varying levels of C₃+ recovery efficiency. The choice of process depends on inlet conditions, target recovery, available utilities, and project economics:

2. Process Configurations for C₃+ Recovery

Several distinct process configurations are used for C₃+ recovery, each with different trade-offs in capital cost, operating cost, recovery efficiency, and operational flexibility. The selection of the optimal process depends on inlet gas pressure, gas richness (C₃+ GPM), target propane recovery, available utilities, and the operator's tolerance for complexity.

Turboexpander in C₃+ Mode

The turboexpander process is the most widely used technology for high-efficiency C₃+ recovery in modern gas processing plants. When operated in C₃+ mode (as opposed to deep C₂ recovery mode), the expander produces a warmer cold separator temperature, typically −40°F to −80°F versus −130°F to −160°F for ethane recovery. This warmer operating point results in several design advantages:

• Reduced expander work: Less isentropic expansion is required, meaning the expander produces less power but also requires less recompression energy. The expander-compressor unit is smaller.
• Smaller cold box: The gas-gas heat exchanger (cold box) requires less surface area because the temperature approach to the cold separator is less severe.
• Carbon steel construction: At cold separator temperatures above −50°F, impact-tested carbon steel can be used instead of stainless steel, significantly reducing fabrication cost.
• Higher propane recovery: Propane recovery of 95–99% is routinely achieved in expander-based C₃+ plants.

The basic process flow consists of inlet gas dehydration (molecular sieve or TEG), gas-gas heat exchange in the cold box, isentropic expansion through the turboexpander, cold separator for gas/liquid separation, and a deethanizer column to strip ethane from the NGL product. The expander discharge gas is reheated in the cold box against the inlet gas and then recompressed by the expander-driven compressor before entering the residue gas pipeline.

JT Valve Process

The Joule-Thomson (JT) valve process is the simplest and lowest-capital approach to C₃+ recovery. The inlet gas is cooled by gas-gas heat exchange and then expanded through an isenthalpic throttling valve (JT valve) to reduce the temperature and condense heavier hydrocarbons. The two-phase mixture enters a cold separator where condensed NGL is separated from the gas phase.

JT valve expansion is inherently less efficient than isentropic turboexpander expansion because the JT process is isenthalpic (constant enthalpy) rather than isentropic (constant entropy). For a given pressure drop, the JT valve produces a smaller temperature reduction than a turboexpander, resulting in lower C₃ recovery (typically 80–88%). However, the JT process has several advantages for smaller or simpler installations:

• No rotating equipment (lower maintenance and higher reliability)
• Lower capital cost (no expander-compressor unit)
• Simpler control and operation
• Suitable for remote or unmanned locations

Mechanical Refrigeration

Mechanical refrigeration uses a propane (or mixed refrigerant) chiller to cool the inlet gas before the cold separator. The refrigeration compressor provides the cooling duty by evaporating liquid propane refrigerant at a controlled temperature, typically −20°F to −40°F. This process achieves C₃ recoveries of 88–92% and is particularly well suited for lean gas processing where the gas richness is moderate and steady.

The main advantage of mechanical refrigeration over JT expansion is the ability to achieve lower and more precisely controlled temperatures independent of inlet gas pressure. The main disadvantage is the capital and operating cost of the refrigeration compressor, which typically requires 500–2,000 HP per 100 MMSCFD of gas processed, depending on the cooling duty.

Lean Oil Absorption

Lean oil absorption is an older technology that uses a circulating absorption oil (typically a kerosene-range or heavier hydrocarbon) to physically absorb C₃+ components from the gas stream in a contactor tower. The rich oil is then heated and stripped in a still column to release the absorbed NGL, and the lean oil is cooled and recirculated. Lean oil plants achieve C₃ recoveries of 85–94% depending on circulation rate, lean oil molecular weight, and contactor temperature.

While largely superseded by cryogenic processes for new construction, lean oil absorption remains viable for retrofitting existing plants, processing gas at higher pressures (above 800 psig) where expansion-based processes lose efficiency, and situations where the operator prefers a non-cryogenic solution.

Hybrid Systems

Hybrid process configurations combine two or more technologies to optimize recovery and efficiency. Common hybrid arrangements include:

• Refrigeration + JT: A propane chiller precools the gas before JT expansion, achieving 90–95% C₃ recovery at moderate capital cost.
• Refrigeration + Turboexpander: Mechanical refrigeration supplements the expander cooling, enabling very high C₃ recovery (97–99+%) even in lean gas or at lower inlet pressures where the expander alone cannot achieve sufficient cooling.
• Gas Subcooled Process (GSP): A portion of the cold separator liquid is subcooled and used as reflux at the top of the deethanizer, improving C₃ recovery beyond what a conventional expander plant can achieve.

Process Selection Criteria

3. Ethane Rejection Strategies

Ethane rejection is the deliberate operating strategy of allowing ethane to remain in the residue gas rather than recovering it as a liquid NGL product. This is the defining characteristic of C₃+ recovery operations and requires specific process design to achieve clean separation of ethane from propane while maintaining high C₃+ recovery efficiency.

Why Reject Ethane

The decision to reject ethane is driven by a combination of economic, logistical, and operational factors:

• Economics: When ethane is worth less as a liquid product (typically $0.10–0.30/gal) than its BTU value in the residue gas, rejection maximizes plant revenue. The crossover point depends on natural gas prices, ethane contract terms, and fractionation/transportation costs.
• Pipeline specifications: Residue gas pipelines often have heating value specifications (minimum 950–1,050 BTU/scf) and hydrocarbon dewpoint limits. Rejecting ethane increases the residue gas heating value by approximately 65–75 BTU/scf per GPM of ethane rejected.
• Processing flexibility: Plants designed for dual-mode operation (C₂ recovery and C₃+ recovery) can switch between modes based on market conditions. This flexibility is a significant competitive advantage but adds design complexity.

Deethanizer Column

The deethanizer is the key piece of equipment in the ethane rejection process. This distillation column separates ethane (C₂) from propane and heavier components (C₃+). The overhead product (ethane and lighter components) is returned to the residue gas stream, while the bottoms product (C₃+ NGL) is sent to downstream fractionation.

In C₃+ recovery mode, the deethanizer performs the critical C₂/C₃ separation. The key design parameters are:

Reflux Strategies

The deethanizer overhead must be partially condensed to generate reflux for the column. Because the overhead is predominantly ethane at pressures of 350–500 psig, the condensing temperature is very low (−30°F to −80°F), requiring cryogenic cooling. Several reflux strategies are employed:

• Cold feed reflux: The cold separator liquid from the turboexpander or JT process provides internal reflux when fed to the upper section of the deethanizer. This is the simplest and most common approach in expander plants.
• Subcooled process (GSP) reflux: A portion of the cold separator liquid is subcooled in a heat exchanger and fed as reflux to the top of the deethanizer, significantly improving C₃ recovery. This approach typically adds 3–5 percentage points of C₃ recovery compared to conventional cold feed reflux.
• External refrigeration reflux: A mechanical refrigeration system provides the condensing duty for generating deethanizer reflux. This is common in refrigeration-based plants where no expander is present.

Switching Between Ethane Recovery and Rejection

Dual-mode plants are designed to switch between C₂ recovery and C₃+ recovery based on market conditions. The key design features for dual-mode capability include:

• Variable-speed or variable-nozzle turboexpander to adjust expansion ratio
• Deethanizer column sized for both operating modes (different reflux ratios and vapor/liquid traffic)
• Bypass piping around the demethanizer (used only in C₂ recovery mode)
• Control system logic to manage the transition between modes smoothly
• Residue gas compressor with adequate capacity for both modes

Switching from C₂ recovery to C₃+ rejection typically involves raising the cold separator temperature, reducing expander work (increasing the JT valve bypass or reducing expander speed), and adjusting deethanizer reflux to allow ethane to exit the overhead with the residue gas. The transition can typically be accomplished within 4–8 hours with gradual adjustments to avoid thermal shock to cryogenic equipment.

Impact on Residue Gas Quality

Rejecting ethane to the residue gas has direct effects on the sales gas composition and properties:

Operators must ensure that the residue gas composition remains within pipeline tariff specifications when switching to ethane rejection mode. Key specifications to monitor include heating value, hydrocarbon dewpoint, Wobbe index, and total inerts content.

4. Equipment Design Considerations

The equipment design for C₃+ recovery plants reflects the moderate cryogenic temperatures involved (warmer than deep C₂ plants) and the emphasis on simplicity, reliability, and capital efficiency. The major equipment items include gas-gas heat exchangers, the cold separator, the deethanizer column, and compression equipment.

Gas-Gas Heat Exchangers (Cold Box)

The gas-gas heat exchanger (commonly called the cold box) is the heart of any cryogenic NGL recovery plant. It precools the inlet gas against the cold residue gas from the cold separator, recovering refrigeration that would otherwise be lost. Two types of heat exchangers are commonly used:

• Brazed aluminum plate-fin exchangers: The standard choice for cryogenic service in gas processing. These exchangers offer very high surface area per unit volume (up to 300 ft²/ft³), close temperature approaches (2–5°F), and the ability to handle multiple streams in a single core. They are limited to clean, non-fouling services and maximum design pressures of approximately 600–900 psig, depending on the manufacturer.
• Shell-and-tube exchangers: Used when the gas contains contaminants that could foul plate-fin surfaces (such as heavy hydrocarbons, glycol carryover, or compressor oil), or when design pressures exceed plate-fin limits. Shell-and-tube exchangers are more robust but significantly larger and heavier for equivalent duty.

Operating Temperature Ranges

C₃+ recovery plants operate at warmer temperatures than deep-cut ethane plants, which has significant implications for material selection and cost:

Material Selection

The warmer operating temperatures of C₃+ recovery plants allow broader use of carbon steel, which is less expensive than stainless steel or nickel alloys. Material selection is based on the minimum design metal temperature (MDMT) of each piece of equipment:

• Carbon steel (SA-516 Gr. 70): Suitable for service temperatures down to approximately −20°F without impact testing. This covers the warm end of the cold box, the deethanizer bottoms section, and all ambient-temperature piping and vessels.
• Impact-tested carbon steel: Carbon steel with Charpy V-notch impact testing at the MDMT can be used down to approximately −50°F. This covers most cold separator and cold piping applications in C₃+ plants operating at moderate cryogenic temperatures.
• Stainless steel (304/304L): Required for service temperatures below −50°F. In C₃+ plants, stainless steel is typically limited to the coldest sections of the cold box, the cold separator internals, and the turboexpander casing. This is a significantly smaller scope of stainless steel than in a deep-cut C₂ plant.
• Aluminum (3003, 5083): Used for brazed plate-fin exchangers in cryogenic service. Aluminum maintains excellent ductility and toughness at temperatures well below −300°F.

Cold Separator Design

The cold separator is a two-phase (or three-phase if free water is present) vessel located downstream of the expansion device. Its function is to separate the condensed NGL liquids from the cold residue gas. Key design considerations include:

• Residence time: 3–5 minutes liquid holdup for gravity separation
• Mist elimination: Wire mesh demister or vane-type mist extractor to remove entrained liquid droplets from the gas stream
• Liquid level control: Level transmitter and control valve on the liquid outlet to the deethanizer feed
• Design pressure: Typically 600–1,000 psig depending on upstream conditions
• Orientation: Vertical separators are common for smaller plants; horizontal separators are used for larger capacities

Side Reboilers and Heat Integration

The deethanizer in a C₃+ recovery plant often includes one or more side reboilers (intermediate reboilers) in addition to the main bottoms reboiler. Side reboilers extract heat from a liquid drawoff at an intermediate tray and return the heated two-phase stream to the column. Benefits of side reboilers include:

• Energy integration: Side reboilers can be integrated with the cold box, using the heat of condensation from the incoming gas to supply reboiler duty. This reduces both the cold box duty and the external heating requirement.
• Reduced bottoms reboiler duty: Transferring 30–50% of the total reboiler duty to side reboilers reduces the bottoms reboiler size and fuel consumption.
• Improved column performance: Distributing the heat input across multiple points in the stripping section reduces the internal vapor traffic at the bottom of the column, allowing a smaller column diameter or increased capacity.

Turndown and Operating Flexibility

C₃+ recovery plants must operate efficiently across a range of gas flow rates, inlet pressures, and gas compositions. Key design features for operational flexibility include:

• Turboexpander with variable inlet guide vanes or variable-speed capability for turndown to 40–60% of design flow
• JT valve bypass around the expander for startup, shutdown, and low-flow operation
• Split-flow cold box design to allow partial operation
• Deethanizer with adequate tray hydraulic range (typically 3:1 to 4:1 turndown ratio)
• Control system with feed-forward compensation for inlet composition changes

5. Economics and Process Selection

The economic evaluation of a C₃+ recovery project requires balancing the revenue from NGL product sales against the capital investment, operating costs, and the value of gas shrinkage (lost BTU content in the residue gas). Process selection is driven by the intersection of technical feasibility and economic optimization.

Capital Cost Comparison

Capital costs for C₃+ recovery facilities vary significantly based on process type, plant capacity, site conditions, and regulatory requirements. The following table provides relative installed cost comparisons for a 200 MMSCFD plant processing moderately rich gas:

Operating Cost Drivers

The major operating costs for C₃+ recovery plants include:

• Compression: Residue gas recompression is typically the largest single operating cost, consuming 1,000–5,000 HP per 100 MMSCFD depending on the pressure drop through the plant. In turboexpander plants, the expander recovers 30–60% of the recompression power.
• Refrigeration: For mechanical refrigeration plants, the refrigeration compressor power consumption is 500–2,000 HP per 100 MMSCFD. The actual power depends on the chiller temperature, ambient conditions (for air-cooled condensing), and refrigerant type.
• Fuel gas: Gas-driven compression and process heating consume 1–3% of the inlet gas as fuel, depending on the process configuration and heat integration. Electric-drive compression eliminates fuel gas consumption but introduces electricity cost.
• Chemical and consumables: Molecular sieve desiccant replacement (every 3–5 years), amine or glycol makeup, and lubricants represent relatively minor ongoing costs.
• Maintenance: Rotating equipment maintenance (compressors, expander, pumps) and periodic turnarounds for inspection and repair.

NGL Product Values

The revenue from C₃+ recovery depends on the market prices of the individual NGL components. NGL products are typically priced as a percentage of crude oil or on an absolute $/gallon basis:

Gas Shrinkage Calculation

When NGL liquids are recovered from the gas stream, the residue gas volume and heating value are both reduced. This shrinkage represents a cost that must be offset by the value of the recovered NGL products. The shrinkage value is calculated as:

Where V_inlet and V_residue are the inlet and residue gas volumes (MSCFD), and HHV_shrinkage is the heating value of the extracted components. For C₃+ recovery, typical gas shrinkage is 3–8% of the inlet gas volume, depending on gas richness and recovery efficiency.

Breakeven Analysis

The economic decision to build or operate a C₃+ recovery plant depends on the relationship between NGL product values and the cost of gas shrinkage plus processing. The breakeven analysis considers:

At higher gas prices, the shrinkage cost increases, making NGL recovery less attractive unless NGL prices rise proportionally. Conversely, at low gas prices, even modest gas richness can support profitable C₃+ recovery.

Seasonal Considerations

NGL prices exhibit strong seasonal patterns that influence the operating strategy for dual-mode plants:

• Winter: Propane demand increases due to residential and commercial heating, driving propane prices higher. This is typically the most profitable season for C₃+ recovery.
• Summer: Propane demand decreases, and butane demand increases for gasoline RVP blending (though EPA summer RVP limits constrain butane blending). Ethane demand from petrochemical crackers may increase during turnaround season recovery, potentially favoring a switch to C₂ recovery mode.
• Shoulder seasons: Transitional periods where the optimal operating mode may shift. Plants with dual-mode capability can capture value by switching between C₂ and C₃+ recovery based on weekly or monthly price signals.

Environmental and Regulatory Factors

Environmental considerations increasingly influence the design and operation of C₃+ recovery facilities:

• Methane emissions: Fugitive emissions from valves, compressor seals, and pneumatic controllers are subject to EPA regulations (NSPS Subpart OOOOa and state-level rules). Minimizing venting and flaring during mode transitions is a regulatory requirement.
• Flaring: Startup and upset flaring must comply with state environmental permit conditions. Dual-mode transitions should be designed to minimize flaring duration and volume.
• Air quality permits: Compressor engines, fired heaters, and flare systems require air quality permits. The permitting timeline (6–18 months) can significantly affect project schedule.
• Water management: Produced water and condensate from inlet separators must be handled in accordance with state disposal regulations.
• Carbon intensity: Downstream purchasers are increasingly requesting carbon intensity data for NGL products. Efficient plant design and electric-drive compression can reduce the carbon footprint of NGL recovery operations.

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Standard 2145 — Table of Physical Constants of Paraffin Hydrocarbons
3. GPA Standard 2140 — Liquefied Petroleum Gas Specifications
4. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
5. ASME B31.3 — Process Piping