Crude Oil Distillation Unit (CDU) Working Process System with diagram.

What is a Crude Oil Distillation Unit (CDU)?

Introduction: 

A Crude Oil Distillation Unit (CDU) is the first and most critical processing step in a petroleum refinery. Its main job is to separate crude oil into different hydrocarbon fractions based on their boiling points.

Since crude oil is a complex mixture of hydrocarbons, you can’t use it directly  it needs to be separated and refined into useful products like LPG, gasoline, kerosene, diesel, and heavier oils

Step-by-Step Working Process of a Crude Oil Distillation Unit

a. Preheating and Desalting

Before crude oil enters the distillation column, it must be cleaned and heated:

• Desalting:
  Crude oil contains salts, water, and sediments that can corrode equipment. It’s mixed with fresh water and passed through an electrostatic desalter, where impurities settle out.
  Goal: Remove salts and water to protect downstream units.

• Preheating:
  The clean crude oil passes through a series of heat exchangers, using heat recovered from other refinery streams. This raises the temperature to around 250–300°C.

b. Furnace Heating

After preheating, the crude enters a fired heater (furnace) where it’s further heated to around 340–370°C.
At this temperature, most hydrocarbons vaporize — but some heavy fractions remain liquid.

c. Atmospheric Distillation Column

The heated crude mixture (part vapor, part liquid) then enters the Atmospheric Distillation Column — the main tower where fractional distillation happens.

• The column is tall, with multiple trays or packing that allow vapor and liquid to contact each other.

• As vapors rise through the column, they gradually cool and condense at different levels according to their boiling points.

d. Products drawn from the column:

Fraction	Approx. Boiling Range (°C)	Common Use
Overhead Gas	< 30	Fuel gas, LPG
Naphtha	30–180	Gasoline production
Kerosene	180–250	Jet fuel, lamp oil
Diesel	250–340	Diesel fuel
Atmospheric Gas Oil	340–370	Feedstock for further cracking
Residue (Bottoms)	> 370	Fuel oil or sent to vacuum distillation

e. Vacuum Distillation (Optional Second Step)

The atmospheric residue (bottom product) still contains heavy valuable hydrocarbons. To recover these without cracking them thermally, it’s sent to a Vacuum Distillation Unit (VDU).

• Operating under low pressure, it allows heavier fractions to boil at lower temperatures.

• This produces:

  • Vacuum Gas Oil (VGO) → feedstock for catalytic cracking.

  • Vacuum Residue (VR) → used for asphalt, lube oil, or as fuel.

f. Supporting Systems

A CDU also includes several auxiliary systems:

• Heat Recovery Systems: Recover energy from hot streams.

• Reflux Systems: Return condensed liquid back to improve separation.

• Pump-Around Circuits: Control internal column temperatures.

• Overhead Condensers & Receivers: Condense vapors to collect products.

Simplified Flow Summary

1. Crude oil → Desalter

2. Preheated via exchangers

3. Heated in furnace

4. Sent to atmospheric column → separated into fractions

5. Residue sent to vacuum unit for further separation

In a Nutshell

The Crude Oil Distillation Unit is the refinery’s “heart” — it doesn’t change the chemical structure of hydrocarbons, it simply sorts them by boiling range. Later processes (like cracking or reforming) will upgrade these fractions into finished fuels and petrochemical feedstocks.

Content information from website and original Images Write attached 

✍️Content Regards 

Fiza Engineering 

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SK NAJMUL (munna)

Hi. I’m a Mechanical Engineer, are I’m Designer of Blog Magic. I’m Creative Website, Web Designer, Auto Cad 3D and Interaction Designer, Industrial Machine Designer, Web Developer, Business Enthusiast, CNC, Writer and Photo editing and some time research an Mechanical subjects, Inspired to make things looks better.

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