Technology

The Refrigerated Lean Oil Absorber combines deep chilling via a C₂ refrigerant loop with counter-current oil scrubbing and a de-methanizer, boosting ethane recovery from ~30% (ambient absorption) to ~70–90%, at the price of significant refrigeration and regeneration energy.

Technology Description

The Low-Temperature Lean Oil Absorption unit is designed specifically for ethane (C₂) recovery from a rich feed gas. Unlike conventional ambient-temperature absorbers, deep chilling dramatically improves ethane solubility in the absorption oil.

Flow Path (per diagram, left to right)

1. Inlet Gas Chilling (C₂ Refrigeration)

• Rich inlet gas is first cooled in a chiller using a closed-loop ethylene/ethane (C₂) refrigerant system

• The refrigerant is vaporized in the chiller; the vapor (“C₂ VAPOR TO COMP”) returns to a refrigerant compressor, is condensed, and recirculated

• Feed gas is chilled to roughly −50 to −100 °C — cold enough to condense much of the C₂⁺ content

2. Absorber Column

• The chilled gas enters the bottom of the Absorber

• Chilled lean oil (fed at the top, pre-cooled by the Lean Oil Cooler) flows down counter-currently, dissolving C₂⁺ hydrocarbons

• Outlet gas (methane-rich residue) leaves the top

3. Rich Oil Flash & Presaturation Section

• Rich oil leaving the absorber bottom passes to a Flash Tank, releasing absorbed light ends (CH₄, H₂)

• Flash vapors are routed to the Presaturator Separator, contacted with lean oil to recapture entrained heavies; off-gas goes “TO RECOMPRESSOR” for recycling or fuel

• Liquid from the presaturator is pumped (Presaturated Lean Oil Pump) back into the system

4. De-methanizer Column

• Rich oil is fed to the De-methanizer — a distillation column that strips residual methane overhead (“TO RECOMPRESSOR”), producing a methane-free C₂⁺ liquid

• A reboiler supplies stripping heat at the column bottom

5. Still Tower (Lean Oil Regeneration)

• The de-methanizer bottoms (still rich in absorbed C₂⁺ product) go to the Still Tower

• Heat (via reboiler, “HEAT SOURCE IN”) distills the absorbed product overhead

• Overhead vapors are condensed (Condenser), collected in the Reflux Accumulator, partly refluxed (Reflux Pump), and the net liquid product sent “TO STORAGE” (C₂/C₃⁺ NGL product)

• Regenerated lean oil leaves the still tower bottom, is pumped (Lean Oil Pump) through the Rich Oil/Lean Oil Heat Exchanger (recovering heat), then through the Lean Oil Cooler, and back to the absorber top — completing the closed oil loop

Ethane Recovery Efficiency

Why Chilling Works

• Ethane’s vapor–liquid equilibrium constant (K-value) falls sharply with decreasing temperature

• Absorption factor A = L/(K·V) increases correspondingly → much higher C₂ recovery at practical lean-oil circulation rates

• Chilling also pre-condenses a portion of the C₂⁺ before the absorber, reducing the load on the oil circulation system

Trade-off vs. Turboexpander (Cryogenic) Plants

• Refrigerated absorption achieves comparable or slightly lower ethane recovery than modern turboexpander plants (~90–95%), but at higher energy cost (large oil circulation + refrigeration + reboiler duty)

• This is why it was largely superseded by turboexpander technology for grassroots high-C₂-recovery plants, while remaining relevant in refinery/petrochemical applications (e.g., ethylene plant front ends) and revamps