VACUUM DISTILLATION

  Vacuum distillation increases the amount of the middle distillates and produces lubricating oil base stocks and asphalt. The feed to the unit is the residue from atmospheric distillation. In vacuum distillation, reduced pressures are applied to avoid cracking long-chain hydrocarbons present in the feed. The feed is first preheated by exchange with the products, charged to the vacuum unit heater, and then passed to the vacuum tower in an atmosphere of superheated steam. Using superheated steam is important: it decreases the partial pressure of the hydrocarbons and reduces coke formation in the furnace tubes. Distillation normally occurs at a temperature range of 400–440°C and an absolute pressure of 25–40 mmHg. The top tower temperature is adjusted by refluxing part of the gas oil product (top product).   

  The size (diameter) of the vacuum distillation tower is much larger than atmospheric towers because the volume of the vapor/unit-volume of the feed is much larger than in atmospheric distillation.  Products obtained as side streams are vacuum gas oil (VGO), lube oil base stocks, and asphalt. Asphalt may be used for paving roads or may be charged to a delayed coking unit.