Flash Separation Process

Sour water from crude units, hydrotreaters, and cokers contains dissolved H₂S and ammonia at pressures up to 30 bar. When pressure drops to atmospheric in the flash tank, roughly 30-50% of dissolved gases vaporize and separate. This reduces the load on downstream stripping equipment and captures concentrated acid gas for sulfur recovery rather than diluting it through the stripper.

Vessel Configuration

Vertical or horizontal drum with high-pressure sour water inlet, vapor space for gas disengagement, vapor outlet to acid gas treatment, liquid outlet to stripper feed, and level control maintaining inventory. Inlet momentum breaks cause turbulence promoting vapor release. Mist eliminators prevent liquid carryover in vapor stream.

Design Pressure

Inlet section handles upstream pressure typically 5-30 bar. Flash section operates near atmospheric or slightly elevated. Two-section design with pressure-reducing control valve manages transition. Relief valves protect both sections. Materials resist H₂S corrosion throughout temperature and pressure range.

Material Selection

Carbon steel construction with corrosion allowances works for most sour water service. Stainless steel 316 for severe conditions with high H₂S, chlorides, or cyanides. NACE materials where required. Internal coating options for extended life in aggressive service.

Vapor Recovery

Flash vapor containing 10-30% H₂S and 20-40% ammonia routes to amine treating for H₂S removal. Treated gas goes to fuel or flare. Some facilities incinerate flash vapor directly in sulfur plant incinerators. Capturing concentrated gas improves sulfur recovery economics versus treating dilute stripper overhead.

Liquid Treatment

Partially degassed liquid flows to sour water stripper with 50-70% less dissolved gases. This cuts stripper reboiler steam consumption and reduces column diameter. Lower H₂S concentration decreases corrosion risks in stripper internals and overhead piping.

Operating Conditions

Flash temperature typically 95-120°C depending on upstream pressure and heat content. Residence time 5-10 minutes allows vapor-liquid equilibrium. Proper vapor space prevents entrainment. Temperature and pressure control optimize flash efficiency.

Applications

Refinery sour water systems treating water from crude distillation, FCC, coker, hydrotreating, and catalytic reforming. Reduces stripper operating costs while improving acid gas recovery. Common in refineries processing sour crudes or operating high-severity conversion units.

Level Control

Level transmitter controls liquid discharge to stripper maintaining inventory. High level prevents vapor line flooding. Low level protects against gas breakthrough to liquid outlet. Simple control scheme integrates with stripper operation.

Performance Benefits

Cuts stripper steam consumption 30-40% through vapor pre-removal. Concentrates acid gas for efficient sulfur recovery. Smaller stripper diameter from reduced vapor load. Less corrosive conditions in downstream equipment. Fast payback from energy savings.

Construction Standards

Design per ASME Section VIII with NACE materials for sour service. Vapor outlet sizing prevents backpressure affecting flash efficiency. Complete documentation with flash calculations and material specifications supporting safe corrosive service.

ERGIL sour water flash tanks reduce operating costs and improve acid gas recovery in refinery wastewater treatment systems.