(228a) Crude Distillation Sequences

Crude distillation is the initial refinery processing step. Crude distillation produces the major cuts that determine overall refinery yield and sets the composition to downstream units. Properly operating crude units make refineries profitable. Poorly operating crude units make refineries lose money. For example, some refineries operate effectively with high diesel recoveries from the atmospheric unit while others have diesel recovery from the vacuum tower. In contrast, many refineries downgrade diesel to FCC feed, consuming FCC capacity, dropping FCC yields, and degrading middle distillate pool blending quality.

Some simple crude units have evolved into complex, difficult to understand, and nearly impossible to operate sequences using up to five towers. Material shifts between towers may have costs or benefits far from where the material was shifted. Understanding how material shifts change heat duties, yields, and tower capacities provides the basis for designing, operating, and revamping crude units.

The most common approach to understand yield shifts uses the fundamentals of superheat and partial pressures to illustrate how yields change in different crude sequences. The simpler approach of using overall heat balances, while less common, gives equivalent conclusions.

Rigorous analysis compares the partial pressure and heat balance approaches to analyzing crude sequences. Yield shifts for common sequences under different operating conditions are shown. Comparisons include light crude feed versus heavy crude feed for topping units; conventional atmospheric-vacuum distillation; atmospheric preflash towers; gas oil tower configurations; and vacuum preflash towers. Both conventional operating conditions and high-diesel recovery operations are shown for selected options.