In order to further distill the residuum from the atmospheric tower at higher temperatures, reduced pressure is required to prevent thermal cracking. The process takes place in one vacuum distillation tower. 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.
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
They are typical used to separate catalytic cracking feedstock from surful residuum.
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