NGL Recovery Optimization

Engineering fundamentals for cryogenic gas processing

1. NGL Recovery Fundamentals

Natural Gas Liquids (NGLs) are valuable hydrocarbons extracted from natural gas streams. Recovery optimization balances product value against processing costs and energy consumption.

NGL Components

Component	Formula	Normal BP (°F)	Primary Use
Ethane (C₂)	C₂H₆	-127	Ethylene feedstock
Propane (C₃)	C₃H₈	-44	Fuel, petrochemical
Isobutane (iC₄)	C₄H₁₀	11	Alkylation, fuel
Normal butane (nC₄)	C₄H₁₀	31	Gasoline blending
Natural gasoline (C₅+)	C₅H₁₂+	82+	Gasoline blending

Recovery Levels

Ethane recovery: Requires cryogenic temperatures (-100 to -150°F)
Propane recovery: Achievable with refrigeration (-40 to -100°F)
Butane+ recovery: Possible with mechanical refrigeration or J-T
Ethane rejection: Operating mode leaving C₂ in residue gas

Key principle: Deeper recovery (more ethane) requires colder temperatures, which means more compression/refrigeration energy. The optimal recovery level depends on product prices and energy costs.

2. Recovery Processes

Modern NGL recovery uses cryogenic turboexpander processes. Different configurations offer varying recovery levels and efficiencies.

Process Comparison

Process	C₂ Recovery	C₃ Recovery	Relative Energy
Straight Refrigeration	20-40%	70-85%	Low
J-T + Refrigeration	40-60%	85-92%	Medium
Turboexpander (basic)	60-75%	92-96%	Medium
GSP (Gas Subcooled)	80-88%	97-99%	Medium-High
RSV (Recycle Split Vapor)	88-93%	99%+	High
SCORE/CRR	93-98%	99%+	Higher

GSP Process Description

The Gas Subcooled Process is the most common cryogenic NGL recovery configuration:

Inlet cooling: Gas-to-gas exchangers cool inlet against cold residue
Separator: Cold separator removes condensed liquids
Subcooling: Liquid subcooled against residue gas
Expander: Remaining gas expanded through turboexpander
Demethanizer: Column separates methane from NGL product

Key Equipment

Equipment	Function	Key Parameter
Cold box exchangers	Gas-gas heat recovery	Approach temperature
Turboexpander	Work extraction, cooling	Isentropic efficiency
Demethanizer	C₁/C₂ separation	Pressure, reflux ratio
Residue compressor	Recompress sales gas	Discharge pressure
Reboiler	Column heat input	Duty, temperature

3. Recovery Calculations

Recovery efficiency is calculated as the fraction of each component captured in the NGL product stream.

Component Recovery

Recovery percentage: R_i = (F_NGL × y_i,NGL) / (F_inlet × z_i) × 100% Where: R_i = Recovery of component i (%) F_NGL = NGL product flow (mol/hr) y_i,NGL = Mole fraction of i in NGL F_inlet = Inlet gas flow (mol/hr) z_i = Mole fraction of i in inlet Or from residue: R_i = [1 - (F_res × y_i,res)/(F_inlet × z_i)] × 100%

GPM Content

Gallons per thousand standard cubic feet: GPM_i = y_i × 1,000 × MW_i / (ρ_i × 379.5) Where: y_i = Mole fraction of component MW_i = Molecular weight (lb/lbmol) ρ_i = Liquid density (lb/gal) 379.5 = scf/lbmol at standard conditions Typical GPM factors: Ethane: 2.63 gal/lbmol Propane: 4.23 gal/lbmol Butanes: 5.66 gal/lbmol Pentanes+: 7.06 gal/lbmol

Example: Recovery Calculation

Given: 100 MMSCFD inlet with 5% C₂, residue has 1.5% C₂ at 92 MMSCFD

C₂ in inlet = 100 × 0.05 = 5.0 MMSCFD
C₂ in residue = 92 × 0.015 = 1.38 MMSCFD
C₂ recovered = 5.0 - 1.38 = 3.62 MMSCFD

Recovery = 3.62 / 5.0 × 100 = 72.4% ethane recovery

Shrinkage Calculation

Volume shrinkage: Shrinkage (%) = (F_inlet - F_residue) / F_inlet × 100 Typical shrinkage: Ethane rejection: 3-6% Ethane recovery: 8-15% Deep ethane recovery: 12-18% BTU shrinkage: Residue gas has lower heating value due to NGL removal. Must meet pipeline HHV spec (typically 950-1,100 BTU/scf)

4. Optimization Parameters

Several operating parameters can be adjusted to optimize recovery for changing feed conditions or product prices.

Key Variables

Parameter	Effect of Increase	Trade-off
Demethanizer pressure	Lower C₂ recovery	Less compression power
Expander inlet temp	Lower recovery	Less refrigeration
Reflux ratio	Higher C₂ recovery	More reboiler duty
Side draw rate	Better fractionation	Process complexity
Residue recompression	Higher recovery	More HP required

Ethane Rejection Mode

When ethane prices are low relative to natural gas, plants operate in "ethane rejection" mode:

Increase demethanizer pressure: Raises column temperature
Reduce reflux: Allows more C₂ up the column
Adjust side draw: May bypass to residue
Result: C₂ stays in residue gas, reduces NGL volume

Temperature Targets

Recovery Target	Coldest Temp (°F)	Typical Pressure (psia)
C₂ rejection (C₃+ only)	-40 to -60	400-600
Partial C₂ (60-70%)	-80 to -100	350-450
High C₂ (85-90%)	-110 to -130	250-350
Deep C₂ (>95%)	-140 to -160	200-280

⚠ CO₂ freezing: At temperatures below -100°F, CO₂ can freeze and plug equipment. Inlet CO₂ must be limited (typically <2%) or removed upstream for deep ethane recovery.

5. Economic Considerations

Recovery optimization is ultimately an economic decision balancing NGL product value against processing costs.

Economic Balance

Net NGL value: Value = Σ(R_i × GPM_i × Q × P_i) - Processing Cost Where: R_i = Recovery of component i GPM_i = Gallons per MCF of component i Q = Inlet flow (MSCFD) P_i = Price of component i ($/gal) Breakeven ethane price: The gas price equivalent where C₂ recovery breaks even with rejection.

Price Relationships

Product	Pricing Basis	Typical Premium to Gas
Ethane	Mont Belvieu, Conway	-20% to +50%
Propane	Mont Belvieu, Conway	+20% to +100%
Butanes	Mont Belvieu, Conway	+30% to +80%
Natural gasoline	NYMEX gasoline related	+50% to +150%

Operating Cost Factors

Compression power: Residue recompression, refrigeration
Fuel gas: Reboiler heat, regeneration
Treating chemicals: Amine, glycol makeup
Maintenance: Rotating equipment, heat exchangers
Utilities: Cooling water, instrument air

References

GPSA Engineering Data Book, Section 16 (Hydrocarbon Recovery)
Gas Processors Association Technical Standards
Campbell, J.M. "Gas Conditioning and Processing"