Loading Atmospheric Crude Distillation Tower (CDU)...
Continuous multi-cut separation of preheated crude oil. The atmospheric tower fractionates raw petroleum into overhead naphtha, kerosene/jet fuel, atmospheric gasoil, and bottom residue without a reboiler, using bottom steam stripping.
The lightest petroleum hydrocarbons ($C_5 - C_{10}$ fractions, boiling range ~35 °C to 160 °C) rise as overhead vapor through the top nozzle into overhead fin-fan air coolers and condensers.
Condensed liquid accumulates in the overhead accumulator. The hydrocarbon phase is drawn as Full-Range / Light Naphtha (feedstock for isomerization or catalytic reforming), while a portion returns as cold Top Reflux to control top tray temperature and cut-point.
Below the upper rectifying trays, the intermediate kerosene fraction ($C_{10} - C_{14}$, boiling range ~160 °C to 240 °C) is drawn off as a liquid side-stream from a collection pan.
The liquid enters an external Side Stripper Column where low-pressure stripping steam vaporizes entrained light naphtha molecules back into the main tower, adjusting the flash point to meet strict aviation jet fuel specifications (e.g. Jet A-1 flash point ≥ 38 °C).
Further down the tower, the heavier atmospheric distillate fraction ($C_{14} - C_{22}$, boiling range ~240 °C to 360 °C) is withdrawn as Atmospheric Gasoil (AGO / Light Gasoil).
Like the kerosene cut, AGO is routed to a dedicated side stripper where injected superheated steam strips out residual kerosene-range hydrocarbons, ensuring the diesel meets distillation 95% recovery limits and cold-flow properties (cloud/pour point).
Desalted crude oil, preheated by heat exchanger trains and heated in the Crude Fired Furnace to 350 °C - 370 °C, enters through tangential inlet nozzles or vane feed distributors into the flash zone.
Upon entering the lower pressure of the tower (~1.3 - 1.5 bar(a)), adiabatic flashing occurs: 50% to 60% of the crude vaporizes instantly and travels upward across wash trays, while unvaporized heavy liquid cascades down into the bottom stripping zone.
Unlike standard chemical distillation columns, an atmospheric crude tower does not use a reboiler. Reboiling heavy crude at bottom temperatures would cause excessive thermal cracking and severe tube coking.
Instead, superheated medium-pressure stripping steam is sparged directly into the tower base. Steam lowers the hydrocarbon partial pressure, vaporizing remaining diesel/gasoil fractions from the falling liquid without raising temperature. The unvaporized heavy bottoms exit as Atmospheric Residue (AR).
Core thermodynamics, hydraulic mechanisms, and operational foundations of Atmospheric Crude Distillation Units.
Crude oil is a continuous mixture of hundreds of hydrocarbons. Separation is based on boiling point cut ranges (Naphtha: <160 °C, Kerosene: 160-240 °C, Gasoil: 240-360 °C, Residue: >360 °C) rather than binary vapor-liquid equilibria.
Superheated steam injected at the base reduces the partial pressure of hydrocarbons: $P_i = y_i (P_{total} - P_{steam})$. This enables high boiling fractions to vaporize at safe temperatures ($<370\text{ }^\circ\text{C}$), completely preventing thermal coking.
Internal liquid is withdrawn, cooled in crude preheat exchanger trains, and returned several trays higher. This controls internal column vapor traffic, avoids top flooding, and supplies high-grade heat to the incoming cold crude feed.
Koalcore Process Engineering