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Why produce diesel from the vacuum unit?

Look ahead five years. The economy is likely to keep tightening and the rush to control pollution will inevitably be accompanied by demands for greater energy conservation. Consequence? A growing market for diesel which yields more energy per unit volume. Yet many continue to believe that producing diesel from the vacuum unit is poor design—that it should be produced only from the atmospheric column. Hence many refiners feed 20-30% diesel boiling range material to their FCCs or hydrocrackers. But where most motor fuel is diesel, refiners have long known that producing vacuum unit diesel can increase production up to 5 volume percent on whole crude.

Maximum diesel production is simply not possible in the atmospheric column because flash zone temperature would need to be 760°F or higher. Moreover, the inherent reflux rate below the diesel draw results in molar L/V ratio less than 0.15. But in vacuum columns it can be 0.4 or higher. And distillation is driven by L/V!

Also: eliminating diesel from vacuum unit feed cuts vacuum gas oil (VGO) yield because it is more difficult to vaporize the feed in the vacuum column flash zone. If atmospheric diesel recovery is very high, VGO yield is low. As crudes get heavier it is essential to leave some diesel in the vacuum column feed to achieve reasonable VGO cutpoints.

Lastly, if you process Canadian oils sands Dilbit and Synbit, you can’t run the atmospheric heater hotter than 680°F resulting in very high diesel boiling range material in the vacuum feed. To obtain reasonable diesel recovery from the whole crude you must produce it in the vacuum unit.

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