Crude Distillation (CDU/VDU): The First Step in a Refinery
Key takeaways
- The distillation unit is the first unit in almost every refinery; every barrel passes through it before anything else.
- Separation is by boiling point only: light components rise to the top, heavy ones stay at the bottom.
- Crude in the atmospheric column is not heated much above about 370 °C, because it starts to crack thermally.
- Atmospheric residue goes to a low-pressure (vacuum) column where it boils at a lower temperature, giving vacuum gas oil.
- Main hazards: hot oil leaks above autoignition temperature, HCl corrosion in the overhead, and H2S.
Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
Why distil crude first?
Crude oil is not one substance. It is a mixture of thousands of hydrocarbons, from very light methane to heavy, asphalt-like molecules, and each group boils at a different temperature. The idea of distillation is simple: heat the mixture until much of it vaporizes, then let the vapour rise through a tall column that gets cooler towards the top. Each component condenses at the level that matches its boiling point, and we draw it off there.
That is why the unit is called the crude distillation unit (CDU), or atmospheric distillation unit in some refineries. It is the door every barrel of crude enters, and every other refinery unit takes its feed from it.
Crude lightness is measured in degrees API. A crude with a specific gravity of 0.80 gives:
API = 141.5 ÷ SG − 131.5
API = 141.5 ÷ 0.80 − 131.5 = 45.4
That is a light crude, close to Saharan crudes such as Saharan Blend. Light crude yields more naphtha, kerosene and diesel, and less heavy residue.
If the refinery runs 50,000 bbl/d and the diesel yield is 25%, the column produces 12,500 bbl/d of straight-run diesel before any further treatment.
Stage 1: desalting
Crude carries salty water and fine sand from the field. If the salts reach the heater and column, magnesium and calcium chlorides hydrolyse and form HCl in the column overhead, which eats the piping. So the crude is mixed with a little wash water, warmed, and sent to a large vessel with an electric field. The field merges small water droplets into bigger ones that settle to the bottom, taking the salt with them.
Stage 2: preheat train and fired heater
After the desalter, crude flows through a long train of heat exchangers, picking up heat from the hot products leaving the column, which saves a lot of fuel. It then enters the fired heater and typically leaves at about 340 to 370 °C, largely vaporized.
We do not go much higher, because heavy molecules begin to crack thermally near 400 °C, forming coke inside the heater tubes and upsetting the separation.
Stage 3: the atmospheric column
The hot mixture enters the flash zone in the lower part of the column. Vapour rises through trays while liquid flows down. On every tray the rising vapour meets the descending liquid and they exchange heat and components. The top of the column becomes cool and rich in light components, the bottom hot and rich in heavy ones.
Products are drawn at different levels. The ranges below are approximate and vary between refineries:
- Overhead: gas, LPG and light naphtha. The vapour condenses in the overhead condenser and collects in the overhead drum.
- Heavy naphtha (about 90 to 180 °C): goes to the reformer after treating.
- Kerosene and jet fuel (about 150 to 250 °C).
- Diesel or gas oil (about 250 to 350 °C).
- Atmospheric residue: everything boiling above about 350 °C, leaving the column bottom.
Each side product passes a small side stripper, where stripping steam drives off excess light ends so the product meets its flash-point specification.
Stage 4: vacuum distillation (VDU)
Atmospheric residue still contains valuable oils, but they only vaporize at temperatures that cause cracking. The answer is to lower the pressure: water boils at a lower temperature on a mountain top, and so does oil. The residue is heated in a second heater and fed to a very wide column under low absolute pressure created by steam ejectors.
From the vacuum column we draw:
- Light and heavy vacuum gas oil: the main feed for the FCC and the hydrocracker.
- Vacuum residue: goes to asphalt, a coker, or fuel oil.
What does the operator watch?
- Heater outlet temperature: too high raises the coking risk; too low reduces light products.
- Top temperature and column pressure: they set the cut point between naphtha and kerosene.
- Bottom and tray levels: losing level can run pumps dry.
- Overhead drum water: its chloride and pH show directly how well the desalter works and how high the corrosion risk is.
- Vacuum in the VDU: any air leak into the column weakens the vacuum and can be dangerous with hot hydrocarbons.
Hazards and safety
- Hot oil: heavy streams can be above their autoignition temperature. A leak from a flange or valve can ignite on contact with air, with no spark.
- Overhead corrosion: HCl and water in the overhead cause fast corrosion, so ammonia or neutralizers and corrosion inhibitors are injected and wall thickness is monitored.
- Fired heater: flame impingement or low flow inside the tubes can overheat the metal and rupture a tube.
- H2S: sour crudes release H2S in the overhead and in sour water.
- Opening equipment: iron sulfide (FeS) deposits can ignite spontaneously when exposed to air during maintenance.
Takeaway for the technician
The distillation unit does not change molecules; it only sorts them by boiling point. How well it works sets the quality of everything downstream. In the next lesson we see how heavy molecules are cracked into gasoline and diesel in the conversion units.
Test your understanding
Related lessons
- Hydrostatic Pressure, Gradient and ECD for Engineers: Formulas and Worked Examples
- Inside the separator: how are gas, oil, and water separated?
- Why does production tubing get choked by wax? All about paraffin
Sources
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