Vacuum Distillation Column (VDU) Overhead System

The vacuum distillation unit (VDU) is the heart of deep crude processing in a petroleum refinery. Atmospheric residue from the crude distillation unit (CDU) is fed to the VDU, where reduced pressure (typically 10–30 mmHg absolute at the column top) allows the heavy fractions — vacuum gasoil, vacuum residue and waxy distillate — to be separated at temperatures well below their thermal cracking threshold. Without vacuum, these heavy fractions would crack before distilling, yielding coke rather than valuable products.

The overhead vacuum system typically consists of a 3–4 stage steam jet ejector train with inter-condensers, designed to handle large volumes of non-condensable gases, light hydrocarbons and steam from the column overhead. Primetech designs these systems to the specific column loading, with each stage sized for the composition and quantity of gas at that stage inlet. Inter-condenser duty is calculated to remove condensable vapours between stages, reducing the load on subsequent ejector stages and minimising total motive steam consumption.

Lube Oil Vacuum Distillation

Lube oil base stock production requires vacuum distillation of vacuum gasoil at very deep vacuum — typically 2–15 mmHg absolute — to separate spindle oil, light machine oil, heavy machine oil and bright stock cuts without thermal degradation. The high molecular weight of lube oil fractions means that even small amounts of cracking produce colour bodies and reduce viscosity index, which destroys product value. Primetech 3–4 stage ejector systems maintain the deep vacuum required to keep column temperatures below the cracking threshold of 380°C.