Why is vacuum distillation needed and what products does it produce?

Vacuum distillation is needed because atmospheric residue cannot be heated above 350-400C without thermal cracking (decomposition). Operating under vacuum (25-100 mmHg) lowers boiling points, allowing further separation without cracking. Products: Light vacuum gas oil (LVGO, 350-450C), heavy vacuum gas oil (HVGO, 450-550C), and vacuum residue (>550C). LVGO and HVGO are feeds for conversion units (FCC, hydrocracker). Vacuum residue goes to coker, visbreaker, or asphalt production. Vacuum is maintained by steam ejectors. Column has large diameter due to high vapor volume at low pressure. Packing often used instead of trays to minimize pressure drop.