(446a) Main Fractionator Control - Fundamental Concepts

The important characteristics that separate main fractionators from other towers are:

• All the heat the tower has comes in with the feed. The feed enters the tower from either a fired heater, a pre-heat train, or a reactor.
• The tower has multiple heat removal areas using either pumparounds or pumpdowns.
• Multiple side-draw products come from the tower.

Main fractionators do not have reboilers.

Typical services include refinery atmospheric crude distillation, vacuum crude distillation, fluid catalytic cracking fractionators, visbreaking delayed coking, delayed coking fractionators, and ethylene plant main fractionators. Due to the size of these plants main fractionator performance has significant economic consequences.

The lack of a reboiler removes one method of controlling the heat balance and product purities around the main fractionators. In operation, main fractionators have completely linked heat balances and material balances. The heat balance-material balance linkage in the main fractionators sets the fundamentals of control around the fractionators. Product qualities, product rates, and heat recovery are completely coupled.

This paper reviews the fundamentals of product recovery, separation effectiveness, and heat recovery in main fractionators. A full understanding of these factors is required for operations, troubleshooting, and advanced control of these units.