Three-Stage Flash

First stage removes bulk vapor. Second stage reduces light ends to low levels. Third stage at near-atmospheric or refrigerated conditions achieves final specification purity. Staged pressure reduction optimizes separation efficiency versus single-stage operation.

Operating Pressure

Third flash typically 1-3 bar, sometimes atmospheric or sub-atmospheric. Low pressure vaporizes remaining traces of ethane and methane. Refrigeration may enhance separation cooling propane below ambient achieving tighter specifications.

Drum Design

Vertical or horizontal vessel with liquid inlet from second stage, minimal vapor space as flash quantity small, vapor outlet to recovery or flare, high-purity propane outlet to storage, and level control. Compact design as vapor volume low.

Final Purity

Reduces light ends to under 1% achieving commercial propane specification typically 95%+ propane purity. Meets vapor pressure limits for transport and use. Critical for propane sold to residential, commercial, or autogas markets.

Material Selection

Carbon steel for propane service. Low-temperature materials if refrigerated operation. Design handles near-atmospheric pressure with vacuum protection if sub-atmospheric operation possible during upsets.

Vapor Handling

Third-stage flash vapor volume minimal compared to first and second stages. May route to fuel gas, flare, or compression for recovery depending on economics. Small flow rate makes recovery marginal unless facility has existing vapor handling.

Applications

Gas plant high-purity propane production, refinery LPG specification upgrading, NGL fractionation final polishing, petrochemical feedstock purification, and applications requiring propane meeting stringent composition limits.

Refrigeration Integration

Some third-stage systems use propane refrigeration cooling liquid before flash. Lower temperature enhances light ends removal achieving HD-5 propane specifications. Refrigeration adds cost but enables premium product.

Economic Considerations

Third stage adds capital cost and complexity. Justified when propane specifications require ultra-low light ends or vapor pressure limits tight. Market pricing differential between standard and high-purity propane determines economic viability.

Level Control

Simple level management as drum operates near steady-state. High-level protection prevents carryover. Integration with upstream flash stages coordinates propane flow through separation train.

Construction Standards

Design per ASME Section VIII or atmospheric vessel codes. Materials for propane service. Vacuum protection if sub-atmospheric operation possible. Complete documentation with phase equilibrium calculations.

ERGIL propane third flash drums achieve final specification purity through low-pressure separation meeting stringent LPG quality requirements in gas processing.