1. Lean Oil Absorption Overview
Lean oil absorption is one of the oldest NGL recovery processes in the gas processing industry. In this process, a heavy hydrocarbon liquid (lean oil or absorption oil) contacts the natural gas stream in a counter-current absorber column, selectively dissolving the heavier hydrocarbon components (C3+, and to a lesser extent C2) from the gas. The resulting rich oil, now containing dissolved NGL components, must be regenerated to recover the NGL product and return the lean oil for reuse.
The Role of Hot Oil Flash
The hot oil flash system is the regeneration section of the lean oil absorption plant. Rich oil from the absorber is heated and flashed in one or more stages to liberate the absorbed NGL components as a vapor stream. The flash vapor is then compressed and fractionated to produce specification NGL products. The regenerated lean oil is cooled and recirculated to the absorber. The efficiency of the flash system directly determines the NGL product purity, lean oil losses, and overall plant energy consumption.
Process Flow Overview
| Step | Equipment | Function |
|---|---|---|
| 1 | Absorber column | Lean oil contacts gas; absorbs C3+ (and some C2, C1) |
| 2 | Rich oil heat exchanger | Rich oil heated against hot lean oil (heat recovery) |
| 3 | Rich oil heater | Supplemental heating to flash temperature |
| 4 | Flash drum(s) | Pressure reduction liberates absorbed components as vapor |
| 5 | Still column (stripper) | Final stripping of light ends from lean oil |
| 6 | Lean oil cooler | Cools regenerated oil before returning to absorber |
| 7 | Lean oil pump | Circulates oil back to absorber at operating pressure |
Comparison with Other NGL Recovery Methods
| Method | C3+ Recovery | C2 Recovery | Capital Cost |
|---|---|---|---|
| Lean oil absorption | 75–90% | 15–40% | Moderate |
| Refrigerated lean oil | 90–95% | 30–60% | Moderate-high |
| JT expansion | 60–80% | 15–30% | Low |
| Turboexpander (cryogenic) | 98%+ | 80–97% | High |
2. Flash Process Fundamentals
The flash process exploits the fact that the solubility of light hydrocarbons in absorption oil decreases with increasing temperature and decreasing pressure. By heating the rich oil and reducing its pressure, the dissolved NGL components vaporize and can be separated from the oil.
Flash Separation Principles
Ki = yi / xi
Where Ki = equilibrium ratio (K-value) for component i, yi = vapor phase mole fraction, xi = liquid phase mole fraction
Higher T → Higher K → More component in vapor
Lower P → Higher K → More component in vapor
Single-Stage vs. Multi-Stage Flash
| Configuration | Recovery Efficiency | Equipment | Application |
|---|---|---|---|
| Single-stage flash | 70–80% of absorbed NGL | One flash drum | Small plants; simple design; lower recovery acceptable |
| Two-stage flash | 85–92% of absorbed NGL | HP and LP flash drums | Most common; good balance of recovery and complexity |
| Three-stage flash | 90–95% of absorbed NGL | HP, IP, and LP flash drums | Large plants; maximum recovery justified |
Why Multiple Stages?
Each flash stage operates at a progressively lower pressure, liberating additional NGL vapors that remain dissolved at higher pressures. The first (high-pressure) flash releases primarily methane and ethane, while subsequent lower-pressure stages release propane, butanes, and heavier components. Multi-stage flashing also produces flash gas at different pressures, allowing staged recompression which reduces total compression power compared to compressing all flash gas from the lowest pressure.
3. Rich Oil Heating
The rich oil leaving the absorber is typically at 80–120°F and must be heated to 250–400°F before flash separation. Heating is accomplished in two stages: heat recovery from the hot lean oil, followed by supplemental heating.
Rich-Lean Oil Heat Exchanger
| Parameter | Rich Oil (Tube Side) | Lean Oil (Shell Side) |
|---|---|---|
| Inlet temperature | 80–120 °F | 300–400 °F |
| Outlet temperature | 200–300 °F | 150–200 °F |
| Pressure | Absorber pressure (300–1000 psig) | Low pressure (50–150 psig) |
| Phase | Liquid (may begin flashing at outlet) | Liquid |
Supplemental Rich Oil Heater
After heat recovery, the rich oil may require additional heating to reach the target flash temperature. Common heating methods include:
| Heater Type | Heat Source | Notes |
|---|---|---|
| Fired heater (direct) | Natural gas fuel | Highest temperature capability; fire tube or cabin heater |
| Hot oil system | Circulating thermal fluid (Dowtherm, Therminol) | Safer; no direct flame near hydrocarbons; common in modern plants |
| Steam heater | HP or MP steam | If steam available; limited to ~350°F with HP steam |
| Waste heat recovery | Engine or turbine exhaust | Energy efficient; common at compressor stations with engines |
Maximum Heating Temperature
The rich oil heating temperature must be limited to prevent thermal cracking (decomposition) of the absorption oil. Most absorption oils begin to degrade above 400–450°F, depending on the oil type and composition. Thermal cracking produces light ends that increase vapor pressure and reduce absorption capacity, and heavy ends (tar and coke) that foul heat exchangers and deactivate the oil. The heater tube skin temperature should be monitored and limited to 25–50°F above the bulk fluid temperature to prevent localized overheating.
4. Flash Drum Design
Flash drums are vertical or horizontal pressure vessels that provide residence time for the heated rich oil to separate into vapor (flash gas containing NGL) and liquid (partially stripped oil). The drums must be sized for both vapor-liquid separation and liquid hold-up time.
Flash Drum Sizing Parameters
Vapor velocity limit (Souders-Brown):
Vmax = KSB × √[(ρL − ρV) / ρV]
Where KSB = 0.1–0.35 ft/s (depending on mist eliminator type)
Liquid residence time: 5–10 minutes (typical for oil service)
Typical Flash Conditions
| Flash Stage | Temperature (°F) | Pressure (psig) | Primary Components Flashed |
|---|---|---|---|
| HP flash | 250–350 | 200–500 | Methane, ethane, some propane |
| IP flash | 250–350 | 50–150 | Propane, butanes, remaining light ends |
| LP flash | 300–400 | 15–50 | Remaining C3+, heavier NGL components |
Flash Drum Internals
| Internal | Purpose |
|---|---|
| Inlet device (diverter plate) | Break momentum of incoming two-phase mixture; promote separation |
| Mist eliminator | Remove entrained oil droplets from flash gas; prevent oil carry-over |
| Vortex breaker | Prevent vortex formation at liquid outlet; avoid gas entrainment in oil |
| Level instrumentation | Control liquid level for proper residence time and separation |
Oil Carry-Over in Flash Gas
Oil droplets entrained in the flash gas represent both an oil loss and a downstream contamination problem. Entrained oil fouls flash gas compressor valves, contaminates NGL products, and reduces lean oil inventory. A properly designed mist eliminator should limit oil carry-over to less than 0.1 gallon per MMscf of flash gas. Wire mesh demisters (4–6" thick, 9 lb/ft³ density) are standard; vane-type demisters may be used for high liquid loading or foaming services.
5. Still Column (Stripper)
The still column (also called the oil stripper or regeneration column) provides the final stage of lean oil regeneration by stripping residual light hydrocarbons from the oil using heat. The still column operates at low pressure (5–25 psig) and produces a lean oil with minimal dissolved light ends.
Still Column Configuration
| Parameter | Typical Value | Notes |
|---|---|---|
| Operating pressure | 5–25 psig | Low pressure maximizes stripping |
| Bottom temperature | 350–450 °F | Limited by oil thermal stability |
| Top temperature | 200–300 °F | Overhead product is NGL vapor |
| Number of trays | 8–20 | More trays improve stripping efficiency |
| Reboiler duty | Major energy consumer | Fired heater or hot oil reboiler |
Stripping Medium
| Method | Description | Application |
|---|---|---|
| Heat only (reboiler) | Reboiler provides vapor by boiling the oil itself | Most common; simple operation |
| Steam stripping | Open steam injected into still column bottom | Enhanced stripping; requires water separation from lean oil |
| Inert gas stripping | Nitrogen or residue gas used as stripping medium | Special applications; avoids water issues |
Still Column Overhead Handling
The still column overhead vapor contains the final stripped NGL components plus any vaporized absorption oil. An overhead condenser and reflux accumulator are used to condense the heavier components and return them as reflux, while the lighter NGL components pass overhead as product. Proper reflux control is essential to prevent excessive oil loss in the overhead NGL product. Oil in the NGL product reduces product quality and depletes the lean oil inventory, increasing operating costs.
6. Lean Oil Properties
The absorption oil properties significantly influence NGL recovery efficiency, circulation rate, and overall plant performance. The oil must have adequate absorptive capacity for the target NGL components while remaining stable under operating conditions.
Common Absorption Oil Types
| Oil Type | MW Range | Boiling Range (°F) | Application |
|---|---|---|---|
| Kerosene-type | 130–170 | 350–500 | General purpose; moderate absorption capacity |
| Heavy naphtha | 110–140 | 300–450 | Higher absorptivity; higher volatility losses |
| Gas oil | 180–250 | 450–650 | Low volatility; lower absorption rate; higher viscosity |
| Specialty oils | 150–200 | 400–550 | Formulated blends; optimized for specific gas compositions |
Key Oil Properties
| Property | Desired Range | Effect on Performance |
|---|---|---|
| Molecular weight | 130–200 | Lower MW improves absorptivity but increases volatility losses |
| Viscosity | < 10 cP at absorber temperature | Lower viscosity improves mass transfer and tray efficiency |
| Vapor pressure | Low at operating temperature | Minimizes oil carry-over in treated gas |
| Thermal stability | Stable to 400–450 °F | Prevents cracking, coking, and degradation in heaters |
| Chemical stability | Non-reactive with gas components | Prevents formation of gums, acids, or emulsions |
Oil Degradation and Makeup
Over time, absorption oil degrades through thermal cracking, oxidation, and accumulation of heavy ends. Degradation manifests as increased viscosity, darker color, higher vapor pressure (from cracked light ends), and reduced absorption capacity. Regular oil analysis (molecular weight, viscosity, distillation curve, and color) monitors oil condition. Periodic oil makeup is required to replace losses from carry-over in treated gas, still overhead, and degradation. Typical oil loss rates are 0.5–2.0 barrels per million standard cubic feet of gas processed.
7. Flash Gas Recovery
Flash gas from each stage of separation contains valuable NGL components mixed with methane and ethane. The flash gas must be compressed, cooled, and routed to NGL fractionation or blended back into the residue gas sales stream.
Flash Gas Composition (Typical)
| Component | HP Flash Gas (mol%) | LP Flash Gas (mol%) |
|---|---|---|
| Methane | 40–60 | 10–25 |
| Ethane | 15–25 | 10–20 |
| Propane | 10–20 | 20–35 |
| Butanes | 5–10 | 15–25 |
| Pentanes+ | 2–5 | 10–20 |
Flash Gas Handling Options
| Option | Description | Application |
|---|---|---|
| Recompression to sales | Compress and cool flash gas; return to residue gas | When NGL fractionation is not available on-site |
| NGL fractionation | Route to deethanizer/depropanizer for specification products | Large plants with fractionation train |
| Fuel gas | Use HP flash gas as plant fuel | Supplements purchased fuel; limited by BTU content |
| Recycle to absorber | Compress and recycle LP flash gas to absorber inlet | Recovers NGL components that would otherwise be lost |
Flash Gas Compression
Flash gas compression is the largest single power consumer in a lean oil absorption plant, typically accounting for 60–80% of total plant power consumption. Multi-stage flash separation reduces compression power by producing flash gas at intermediate pressures. Each stage of compression requires interstage cooling and liquid knockout to prevent compressor damage and improve efficiency. Reciprocating compressors are standard for flash gas service due to the variable composition and relatively low flow rates.
8. Process Optimization
Lean oil absorption plant performance can be optimized by adjusting key operating parameters to maximize NGL recovery while minimizing energy consumption and oil losses.
Key Operating Variables
| Variable | Effect of Increase | Optimization Target |
|---|---|---|
| Oil circulation rate | Higher NGL recovery; higher pumping and heating cost | Minimum rate for target recovery; typically 2–6 GPM/MMscfd |
| Absorber pressure | Higher absorption at higher pressure (higher K-values) | Maximize within pipeline and equipment rating limits |
| Absorber temperature | Lower temperature improves absorption (lower K-values) | As low as practical; limited by hydrate and viscosity |
| Flash temperature | Higher temperature improves stripping; increases energy cost | Balance stripping efficiency vs. energy and oil degradation |
| Number of absorber trays | More trays improve recovery; higher capital and pressure drop | 8–16 trays typical; diminishing returns above 12 |
Energy Efficiency Improvements
| Measure | Energy Saving | Implementation |
|---|---|---|
| Rich-lean oil heat exchange | Recovers 60–80% of heating duty | Shell-and-tube exchanger; close approach temperature |
| Waste heat recovery | Uses engine exhaust for rich oil heating | Exhaust gas heat recovery unit on compressor engines |
| Multi-stage flash | Reduces total compression power 20–40% | Additional flash drums at intermediate pressures |
| Optimized oil circulation | Reduces pumping and heating energy | Match circulation to current gas flow and composition |
When to Consider Conversion to Cryogenic
Lean oil absorption plants are economically competitive for C3+ recovery when gas volumes are moderate (less than 50–100 MMscfd) and ethane recovery is not required. When gas volumes increase, ethane prices justify recovery, or C3+ recovery above 95% is needed, conversion to a turboexpander-based cryogenic plant should be evaluated. Many lean oil plants in the midstream industry have been converted to cryogenic operation over the past three decades as gas volumes and NGL prices have increased.