Fractionation Design and Operations Reference

Overhead Systems

Module from the Fractionation Design and Operations Reference curriculum.

OVERHEAD SYSTEM DESIGN · Overhead Condenser Design

Configuration	Description	Advantages	Limitations	Application	Pressure Control	Notes
Total condenser	All overhead vapor condensed to liquid	Simplest, liquid reflux	Must handle all condensation duty	Most atmospheric columns	Pressure floats or hot vapor bypass	Standard design
Partial condenser	Only heavy components condensed, vapor exits	Lighter gas removed as vapor	Two-phase system, more complex	Gas + liquid products	Vapor pressure control	Requires liquid-vapor separation
Air-cooled + trim	Air cooler for bulk duty, CW trim for control	Lower CW usage, good control	Air cooler weather-sensitive	Large overhead duty columns	Trim cooler CW controls outlet T	Most common in refining
CW only	Water-cooled shell & tube	Compact, weather-independent	CW usage, fouling	Small/medium duties	CW flow or bypass	Less common now (CW cost)
Reflux Drum Design
Reflux Drum (Overhead Accumulator) Functions: 1. Provide surge volume for reflux pump 2. Separate water from hydrocarbon (boot or separate drum) 3. Disengage any remaining vapor from liquid 4. Provide residence time for level control stability Sizing Guidelines: • Liquid residence time: 5-10 minutes at normal reflux + distillate rate • Minimum: 5 minutes for good level control • Vapor space: minimum 20% of drum volume above NLL • L/D ratio: 2.5:1 to 5:1 (horizontal drum) Water Boot: • Required when free water is present (aqueous phase from condensation) • Boot diameter: typically 60-100% of drum diameter • Boot length: minimum 3 ft or 5 minutes water accumulation • Water boot level control: separate LIC with drain to sour water • Interface detection: DP transmitter or float switch Non-Condensable Venting: • Vent line from top of reflux drum to flare or vent condenser • Size for maximum non-condensable flow (startup, upset conditions) • Consider vent condenser to recover condensable vapors

Source: Fractionation_Design_Ops_Reference_v1.xlsx · Sheet: Overhead Systems

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