NGL Recovery Optimization Fundamentals

1. NGL Recovery Fundamentals

Natural Gas Liquids (NGLs) are hydrocarbons heavier than methane that can be extracted from natural gas and sold as separate products. The decision to recover NGLs depends on the "frac spread" - the difference between NGL product value and the value of leaving those components in the gas stream.

NGL Components and Properties

Component	Formula	NBP (°F)	HHV (BTU/SCF)	gal/MSCF	Primary Use
Ethane (C₂)	C₂H₆	-127	1,769	26.59	Ethylene feedstock
Propane (C₃)	C₃H₈	-44	2,516	27.46	Fuel, petrochemical
Isobutane (iC₄)	C₄H₁₀	11	3,252	32.64	Alkylation feed
Normal butane (nC₄)	C₄H₁₀	31	3,262	31.44	Gasoline blending
Natural gasoline (C₅+)	C₅H₁₂+	82+	4,008+	35.95	Gasoline blending

Source: GPSA 14th Edition Table 23-2, GPA 2145

GPM - Gallons Per Thousand SCF

GPM (Gallons Per MSCF) is the standard measure of gas "richness" - the potential liquid yield if 100% of each component were recovered. It's calculated from the gas composition and GPSA liquid yield factors.

GPM Calculation: GPM_total = Σ (y_i × gal/MSCF_i) Where: y_i = Mole fraction of component i gal/MSCF_i = Liquid yield factor from GPSA Table 23-2 Example: 7% C₂, 4.5% C₃, 2% C₄, 0.5% C₅+ GPM = (0.07 × 26.59) + (0.045 × 27.46) + (0.02 × 31.92) + (0.005 × 35.95) GPM = 1.86 + 1.24 + 0.64 + 0.18 = 3.92 GPM

Gas Richness Classification

Classification	GPM Range	Typical Process	Economics
Lean Gas	< 1.5 GPM	Bypass or JT only	Marginal - verify frac spread
Moderate	1.5 - 2.5 GPM	Mech refrigeration or JT	Propane recovery usually economic
Rich	2.5 - 4.0 GPM	Turboexpander	Good economics for C2 recovery
Very Rich	> 4.0 GPM	GSP/RSV/SCORE	Excellent NGL economics

Key Principle: Deeper NGL recovery (more ethane) requires colder temperatures, which demands more compression and refrigeration energy. The optimal recovery level depends on the "frac spread" - the value of NGL products minus the value of the shrinkage gas left behind.

2. Recovery Processes

Modern NGL recovery uses cryogenic processes to achieve deep separation of methane from heavier hydrocarbons. The turboexpander is the heart of most cryogenic plants, providing both cooling and work recovery.

Process Comparison

Process	Cold Temp	C₂ Recovery	C₃ Recovery	Key Feature
Straight Refrigeration	-20 to -40°F	10-25%	70-85%	Simple, low CAPEX
JT + Refrigeration	-60 to -80°F	30-60%	85-95%	No rotating equipment
Basic Turboexpander	-100 to -120°F	60-80%	92-97%	Work recovery
GSP (Gas Subcooled)	-130 to -150°F	80-92%	97-99%	Subcooled reflux
RSV (Recycle Split Vapor)	-140 to -160°F	90-96%	99%+	Vapor recycle
SCORE/CRR	-150 to -170°F	95-99%	99%+	Maximum C2 recovery

GSP Process Description

The Gas Subcooled Process (GSP) is the most common high-recovery cryogenic design:

Inlet Separation: Dehydrated gas enters at 800-1000 psig
Cold Box: Gas-to-gas exchangers cool inlet against cold residue (-120°F approach)
Cold Separator: Two-phase separation at -80°F to -100°F
Subcooling: Separator liquid subcooled as demethanizer reflux
Expansion: Remaining vapor through turboexpander (80-85% isentropic efficiency)
Demethanizer: Trayed/packed column at 200-350 psia separates C1 from NGL
Recompression: Residue recompressed to pipeline pressure

Graph showing NGL component recovery percentage versus coldest process temperature, with S-curves for C5+, C4, C3, and C2 components demonstrating that heavier components recover at warmer temperatures while ethane requires the coldest conditions — Component Recovery vs. Temperature - S-curve behavior showing C5+ recovers first, followed by C4, C3, and C2 requiring coldest temps

Key Equipment

Equipment	Function	Key Parameter	Typical Value
Cold Box (Brazed Aluminum)	Gas-gas heat recovery	Approach temperature	3-10°F
Turboexpander	Work extraction + cooling	Isentropic efficiency	80-88%
Demethanizer	C1/C2+ separation	Operating pressure	200-350 psia
Residue Compressor	Recompress to pipeline	Discharge pressure	800-1200 psig
Reboiler	Column heat input	Bottoms temperature	100-150°F

CO₂ Freezing Hazard: At temperatures below -100°F, CO₂ can freeze and plug equipment. Inlet CO₂ must be limited to <2% for moderate recovery or <0.5% for deep ethane recovery. Amine treating upstream may be required.

3. Recovery Calculations

NGL recovery calculations use material balance to track each component from inlet gas through the plant. Recovery efficiency is measured as the fraction of each inlet component captured in the NGL product.

Component Recovery

Recovery from material balance: R_i = (n_NGL × y_i,NGL) / (n_inlet × z_i) × 100% Or equivalently from residue: R_i = [1 - (n_res × y_i,res)/(n_inlet × z_i)] × 100% Where: R_i = Recovery of component i (%) n = Molar flow rate (lbmol/hr or equivalent) z_i = Inlet mole fraction of component i y_i = Mole fraction in product stream

Liquid Volume from Gas

Converting recovered gas to liquid gallons: GPD_i = Q_inlet × y_i × R_i × (gal/MSCF)_i Where: GPD_i = Gallons per day of component i Q_inlet = Inlet gas flow (MSCFD) y_i = Inlet mole fraction (decimal) R_i = Recovery fraction (decimal) (gal/MSCF)_i = GPSA liquid yield factor Derivation of gal/MSCF factor: gal/MSCF = (1000 SCF/MSCF) / (379.49 SCF/lbmol) × MW / ρ_liq Example for ethane: gal/MSCF = (1000/379.49) × 30.07 / 2.974 = 26.59 gal/MSCF

Worked Example

Given: 100 MMSCFD inlet gas with 7% C₂, 4.5% C₃, 2% C₄, 0.5% C₅+
Operating at 90% C₂ recovery, 98% C₃ recovery, 99% C₄ recovery

                    Step 1: Calculate recovered component flows (MMSCFD)

                    C₂ recovered = 100 × 0.07 × 0.90 = 6.30 MMSCFD

                    C₃ recovered = 100 × 0.045 × 0.98 = 4.41 MMSCFD

                    C₄ recovered = 100 × 0.02 × 0.99 = 1.98 MMSCFD

                    C₅+ recovered = 100 × 0.005 × 0.999 = 0.50 MMSCFD

                    Step 2: Convert to liquid gallons per day

                    C₂ GPD = 6.30 × 1000 × 26.59 = 167,517 GPD

                    C₃ GPD = 4.41 × 1000 × 27.46 = 121,100 GPD

                    C₄ GPD = 1.98 × 1000 × 31.92 = 63,202 GPD

                    C₅+ GPD = 0.50 × 1000 × 35.95 = 17,975 GPD

                    Total NGL = 369,794 GPD = 8,805 BPD

                    Step 3: Calculate residue gas

                    Shrinkage = 6.30 + 4.41 + 1.98 + 0.50 = 13.19 MMSCFD

                    Residue = 100 - 13.19 = 86.81 MMSCFD

                    Shrinkage % = 13.19%

Residue Gas Heating Value

Mixture heating value (Kay's Rule): HV_mix = Σ (y_i × HV_i) Where: y_i = Residue gas mole fraction of component i HV_i = Gross heating value of pure component i (BTU/SCF) Wobbe Index (burner interchangeability): W = HV / √SG Where: SG = Gas specific gravity = MW_gas / 28.966 Typical pipeline spec: W = 1,310 - 1,390

Shrinkage Calculation

Volume shrinkage: Shrinkage (%) = (Q_inlet - Q_residue) / Q_inlet × 100 Typical shrinkage by recovery mode: Ethane rejection: 3-6% Moderate C2 recovery (70%): 8-12% High C2 recovery (90%): 10-15% Deep C2 recovery (>95%): 12-18%

4. Optimization Parameters

Plant operators optimize NGL recovery by adjusting operating parameters to match current feed conditions and market prices. Key variables include demethanizer pressure, expander operation, and column reflux.

Demethanizer pressure-temperature operating envelope diagram showing ethane recovery mode zone at lower pressure and colder temperatures, ethane rejection mode zone at higher pressure and warmer temperatures, with CO2 freezing curve constraint — Demethanizer Operating Envelope - Ethane recovery vs. rejection modes with CO₂ freezing constraint

Operating Variables and Effects

Parameter	Increase Effect	Trade-off	Typical Range
Demethanizer pressure	↓ C₂ recovery	↓ Compression power	200-500 psia
Overhead temperature	↓ C₂ recovery	↓ Refrigeration load	-60 to -150°F
Reflux ratio	↑ C₂ recovery	↑ Reboiler duty	0.3-1.0
Expander inlet temp	↓ C₂ recovery	↓ Heat exchange area	-40 to -100°F
Reboiler temp	↑ C1 in bottoms	NGL spec impact	80-150°F

Ethane Rejection Mode

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

Increase demethanizer pressure to 400-500 psia (raises column temperature)
Reduce reflux ratio to allow more C₂ up the column
Reduce expander throughput or bypass to JT valve
Result: C₂ recovery drops to 5-15%, C₃ recovery to 85-95%

Temperature Targets by Recovery Level

Recovery Target	Coldest Temp	Demet Press	Process
C₂ rejection (C₃+ only)	-40 to -60°F	400-600 psia	JT or warm expander
Partial C₂ (60-75%)	-80 to -110°F	300-400 psia	Turboexpander
High C₂ (85-92%)	-120 to -140°F	250-350 psia	GSP
Deep C₂ (>95%)	-150 to -170°F	200-280 psia	RSV/SCORE

Mode Flexibility: Modern plants are designed to switch between ethane recovery and rejection modes within 4-8 hours. This "optionality" adds significant value by allowing operators to respond to daily price movements.

5. Economic Considerations

NGL recovery economics are driven by the "frac spread" - the value of extracted NGL products minus the value of the gas shrinkage left behind. Positive frac spread means extraction adds value; negative spread suggests rejection.

Frac Spread Calculation

Daily Frac Spread: Frac Spread = NGL Revenue - Shrinkage Cost NGL Revenue = Σ (GPD_i × Price_i) Shrinkage Cost = Shrinkage_MMSCFD × HV_shrink × Gas_Price Where: GPD_i = Gallons per day of component i Price_i = Product price ($/gal) HV_shrink = Heating value of shrinkage gas (~1,500-1,800 BTU/SCF) Gas_Price = Residue gas price ($/MMBtu) Breakeven ethane price: The $/gal ethane price where C2 recovery breaks even vs rejection.

Frac spread economics bar chart showing stacked NGL product revenues for ethane, propane, butanes, and C5+ natural gasoline against shrinkage cost deduction, with net frac spread calculation showing daily profitability — Frac Spread Economics - Component revenue contributions vs. shrinkage cost for net daily value

NGL Product Pricing

Product	Spec	Pricing Hub	Typical Range
Ethane	EP Grade (95%+)	Mont Belvieu	$0.15-0.40/gal
Propane	HD-5	Mont Belvieu, Conway	$0.60-1.20/gal
Normal Butane	Commercial	Mont Belvieu	$0.80-1.40/gal
Isobutane	Commercial	Mont Belvieu	$0.90-1.50/gal
Natural Gasoline	Y-Grade	NYMEX-related	$1.20-2.00/gal

Operating Cost Factors

Compression power: Residue recompression is 40-60% of operating cost
Fuel gas: Reboiler heat, regeneration, building heat
Refrigeration: External propane or mixed refrigerant systems
Treating chemicals: Amine, glycol makeup
Maintenance: Rotating equipment, heat exchangers, instrumentation

Rule of Thumb: Total plant operating cost is typically $0.08-0.20 per gallon of NGL produced, depending on plant size, age, and complexity. This translates to ~$3-8 per barrel of NGL.

References

GPSA, Section 16 (Hydrocarbon Recovery)
GPA Standard 2145 - Table of Physical Constants for Hydrocarbons
Campbell, J.M. "Gas Conditioning and Processing", Volume 2
Arnold & Stewart, "Surface Production Operations", Volume 2

NGL Recovery Optimization

85-98%

>98%

-40 to -160°F

Contents

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