In modern refinery practice the distillation of atmospheric residue is accomplished under high vacuum conditions in a specially designed tower whose internal equipment ensures a very lower pressure drop. Normally the vacuum conditions in the flash zone of the tower allows about the same percentage of distillate based on the tower feed to be cut in this tower as the distillate on whole crude in the atmospheric unit. Again the flash zone temperature in the vacuum unit is kept below 700?F. Usually there are two or three vacuum distillates from this tower. In a pure energy related refinery there will be two. The heavier of the two say to a cut range of 750–930?F will be the feed to a distillate hydro-cracker or to a fluid catalytic cracker . In both these cases however a small heavier cut is taken off and returned to the tower bottom in order to correct the bottom distillate condradson carbon content to meet the specification required for either of the two downstream processes. This distillate product is usually titled HVGO (Heavy Vacuum Gas Oil).
For those refineries which produce lube oils as non energy products this bottom distillate may be split into two side streams in order to provide the flexibility required in the production of the lube oil blending stock specifications .
The light vacuum distillate is taken off as a top side stream and is usually routed to a hydro-desulfurizer to be sent either to the gas oil pool as heating oil stock or routed to the fuel oil pool as blending stock. This side stream is a cut range of 680–750?F and is usually labeled LVGO (Light Vacuum Gas Oil).
