Vacuum residue is the substance which is obtained by vacuum distillation (= a process of separating substances by reducing pressure to less than atmospheric pressure).
Distillation of crude oil under reduced pressure produces the vacuum residue which is a solid.
The residue from the atmospheric distillation unit is then fed to the vacuum distillationunit, which produces vacuum gas oil and vacuum residue.
Vacuum residue is the substance which is obtained by vacuum distillation.