- In order to further distill the residuum or topped crude from the atmospheric tower at higher temperatures, reduced pressure is required to prevent thermal cracking. The process takes place in one or more vacuum distillation towers.
How It Works
- The principles of vacuum distillation resemble those of fractional distillation and, except that larger-diameter columns are used to maintain comparable vapor velocities at the reduced pressures, the equipment is also similar. The internal designs of some vacuum towers are different from atmospheric towers in that random packing and demister pads are used instead of trays.
- Vacuum units take the 350+ fraction out of crude oil and distil it into a light vacuum gas oil for diesel purposes; it might make a medium and a heavy vacuum gasoil or they might be combined.
- Vacuum towers are typically used to separate HVGO feedstock for the Fluidised Catalytic Cracker or Hydrocracker from surplus residuum
- The vacuum residue can go off to a Delayed Coker, residue hydroconversion units or Visbreakers
- A typical first-phase vacuum tower may produce gas oils, lubricating-oil base stocks, and heavy residual for propane deasphalting. A second-phase tower operating at lower vacuum may distill surplus residuum from the atmospheric tower, which is not used for lube-stock processing, and surplus residuum from the first vacuum tower not used for deasphalting.
- Basically, a vacuum distillation unit consists of a vacuum system, condensation and separation sections to produce diesel and VGO fractions, a vacuum residue (VR) in the bottom of the column, a VR de-entrainment section to keep the bottom of the column from coking and the furnace.