Crude Distillation Unit (CDU): The Heart of the Refinery
The CDU is the first and most critical processing step in any oil refinery.
Every drop of gasoline, diesel, or jet fuel that powers the modern world begins its journey in the Crude Distillation Unit (CDU). Often referred to as the “Atmospheric Crude Distillation Unit,” this colossal thermodynamic system separates raw crude oil into usable fractions based on boiling points. It is the bottleneck of the refinery; if the CDU stops, the entire plant stops.
What is a Crude Distillation Unit?
A CDU is a processing plant that uses heat to separate crude oil feedstock into different cuts (fractions). It operates at slightly above atmospheric pressure and typically includes a desalter train, a preheat exchanger network, a fired furnace, and a main fractionation column.
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Feedstock Economics: Brent Oil vs. Crude Oil
Before entering the **Crude Distillation Unit (CDU)**, engineers must understand the feedstock. A common query from non-engineers is the difference between Brent Oil vs Crude Oil.
“Crude Oil” is the generic term for unrefined petroleum. “Brent” is a specific benchmark classification. Brent Crude is a “light, sweet” oil sourced from the North Sea. It is ideal for refining because it contains low sulfur (“sweet”) and has a low density (“light”), making it easier to process into high-value gasoline and diesel compared to “heavy, sour” crudes.
Engineering Formula: API Gravity
Refineries assess the quality of crude using API Gravity. Higher API means lighter fluid (floats on water); lower API means heavier fluid (sinks).
- SG = Specific Gravity of the fluid at 60°F (15.6°C)
- If API > 10, the oil floats on water.
- If API < 10, the oil sinks in water (Extra Heavy Crude).
| Feature | Light Sweet (e.g., Brent, WTI) | Heavy Sour (e.g., Mayan, Arab Heavy) |
|---|---|---|
| API Gravity | > 31.1° | < 22.3° |
| Sulfur Content | < 0.5% (Low Corrosion Risk) | > 1.5% (High Corrosion Risk) |
| Processing Cost | Low (Easy Distillation) | High (Requires metallurgy upgrades) |
| CDU Yield | High Naphtha / Diesel | High Residue / Asphalt |
The Process: From Feed to Fraction
The Atmospheric Crude Distillation Unit is a complex thermal balancing act. The goal is to vaporize the oil without thermally cracking it (coking).
Step 1: The Preheat Train & Desalting
Cold crude oil enters the unit and passes through a series of Heat Exchangers. This “Preheat Train” recovers waste heat from hot products (like diesel or residue) leaving the unit.
Desalting in Refinery operations is the critical intermediate step. Around 120°C – 140°C, the crude enters the Desalter. Here, water is mixed with the oil to dissolve salts (calcium, sodium, magnesium chlorides), and an electrostatic field separates the brine from the oil.
Step 2: The Fired Heater (Furnace)
The desalted crude is pumped into the Fired Heater. This is the energy heart of the CDU. The oil is heated to approximately 340°C – 370°C.
Engineering Limit: We cannot exceed ~380°C. Above this temperature, the hydrocarbons begin to crack (break apart chemically) rather than just boil. This causes coke formation inside the tubes, leading to hotspots and tube rupture.
Step 3: The Main Fractionation Column
The hot mix of vapor and liquid enters the “Flash Zone” of the column.
- Vapors Rise: Light ends (Gas, Naphtha, Kerosene) rise through the trays. As they move up, they cool and condense on trays at their specific boiling temperatures.
- Liquids Fall: Heavy ends (Atmospheric Residue) fall to the bottom.
- Stripping Steam: Superheated steam is injected at the bottom to lower the partial pressure, helping to strip the remaining light ends out of the heavy residue.
Engineering Insight: ASME B31.4 vs B31.3
A common search regarding piping specs is the ASME B31.4 requirements for centrifugal pumps in crude oil pipelines. It is vital to distinguish the scope:
- ASME B31.4: Governs the transportation pipelines bringing crude TO the refinery. It allows for higher allowable stresses and different wall thickness calculations because the environment is generally uniform.
- ASME B31.3 (Process Piping): Governs the piping INSIDE the CDU. This code is far stricter regarding temperature cyclic fatigue and corrosion allowances, as the fluid changes state (liquid/vapor) and temperature rapidly within the unit.
Refinery Cuts: Products & Boiling Points
The science of the CDU relies on Fractional Distillation. Inside the column, a vertical temperature gradient is established: hot at the bottom, cool at the top. As crude vapor rises, it cools. When a hydrocarbon molecule reaches a tray temperature below its boiling point, it condenses into a liquid.
| Fraction Name | Boiling Point Range (°C) | Primary Application |
|---|---|---|
| Off-Gas (LPG) | < 20°C | Refinery fuel gas, heating, cooking (Propane/Butane). |
| Naphtha (Gasoline) | 40°C – 180°C | Feedstock for gasoline blending and petrochemicals. |
| Kerosene (Jet Fuel) | 180°C – 250°C | Aviation Turbine Fuel (Jet A-1), heating. |
| Light Gas Oil (Diesel) | 250°C – 350°C | High-speed diesel for trucks, trains, and cars. |
| Atmospheric Residue | > 350°C | Feed for Vacuum Unit, Asphalt, or Bunker Fuel. |
Anatomy of a CDU Column
To achieve these cuts, the distillation column is not just a hollow pipe. It contains intricate internals designed to maximize vapor-liquid contact.
1. Fractionating Trays
Most CDUs use 20 to 50 physical trays. Common types include Sieve Trays (simple holes) and Valve Trays (movable caps). They hold a layer of liquid that rising vapor must bubble through, ensuring heat transfer.
2. Downcomers
These are vertical channels that allow liquid to flow from an upper tray to the tray below without bypassing the vapor flow. Blocked downcomers are a common cause of column flooding.
3. Pump-Arounds
To improve efficiency, hot liquid is drawn off, cooled in an exchanger, and pumped back into the column a few trays higher. This generates internal reflux and recovers heat.
4. The Flash Zone
The section where the feed enters. It is the hottest part of the column. It must be designed with “wash trays” above it to prevent asphalt particles from entraining upwards into the diesel sections.
Maximizing Profitability: The “Cut Point”
A refinery’s profitability hinges on the CDU’s flexibility. Operators adjust the “Cut Point” (the temperature overlap between fractions) based on market demand.
- Gasoline Mode (Summer): Operators maximize the Naphtha cut to meet high driving demand.
- Diesel Mode (Winter): The operation shifts to maximize heating oil and diesel production.
*Optimization requires precise temperature control. A deviation of just 2°C can shift tons of valuable diesel into low-value residue.
Case Study: The “Preheat Train Crash”
The efficiency of a Crude Distillation Unit (CDU) is dictated by heat recovery. In this analysis, we examine a “Preheat Train Crash” at a mid-sized refinery where a failure in the Desalter unit led to catastrophic fouling of the downstream heat exchangers.
Figure 2: Severe fouling of the tube bundle caused by salt precipitation and asphaltene deposition.
Project Data
Location
Mid-Continent Refinery, USA
Unit
120,000 BPD Atmospheric CDU
Equipment
E-101 A-D (Crude/Resid Interchangers)
Failure Mode
Inorganic Salt Fouling / Flow Restriction
The Problem: Desalter Upset
The refinery switched feedstock to a cheaper, heavier crude with higher salt content (BS&W). The operators failed to adjust the mix valve pressure drop and wash water rates in the Desalting in Refinery unit.
The Cascade Effect:
1. Salt Carryover: Brine slipped through the Desalter and entered the hot Preheat Train.
2. Water Flash: As temperatures rose above 150°C, the water flashed into steam, leaving solid salt crystals behind.
3. Deposition: These salts acted as a binder for asphaltenes, creating a rock-hard layer on the heat exchanger tubes.
4. Furnace Limit: The Furnace Inlet Temperature (FIT) dropped from 220°C to 180°C. The fired heater had to burn 25% more fuel to reach the required transfer temperature, eventually maxing out its firing duty. The unit had to cut rates by 20,000 BPD.
Engineering Solution & Results
Shutting down the CDU for cleaning costs millions in lost production. The engineering team executed an Online Chemical Cleaning strategy.
- Chemical Injection: A specialized terpene-based solvent was injected into the crude side of the preheat train while the unit ran at reduced rates.
- Soak & Surge: The flow was surged to mechanically shear off the softened foulant layers.
- Monitoring: Operations monitored the “U-Value” (Heat Transfer Coefficient) in real-time.
Key Result
Within 48 hours, the Furnace Inlet Temperature recovered by 35°C. The furnace firing duty dropped back to normal range, and the unit returned to full 120,000 BPD capacity. This case highlights why the Crude Distillation Unit Process is not just about fractionation—it is about strict contamination control starting at the desalter.
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Technical Addendum: Classifications & Specific Codes
Classifications: Types of Crude Oil
Crude oil is not uniform. It is classified into four primary types of crude oil based on API Gravity (density) and Sulfur Content. Refineries are often designed to process a specific “diet” of these types.
1. Light Distillate Rich
API > 31.1°
High yield of gasoline and kerosene. Includes “Brent” and “WTI”. Easiest to process.
2. Medium Crude
API 22.3° – 31.1°
Balanced yield. Common feedstock for complex refineries.
3. Heavy Crude
API 10° – 22.3°
Requires extensive heating and vacuum distillation. High yield of asphalt/residue.
4. Extra Heavy (Bitumen)
API < 10°
Sinks in water. Requires “Upgraders” or dilution (Dilbit) to flow in pipelines.
Engineering Specific: ASME B31.4 Requirements for Centrifugal Pumps
For engineers searching specifically for ASME B31.4 requirements for centrifugal pumps in crude oil pipelines, the code (Pipeline Transportation Systems for Liquids) dictates the following key constraints compared to refinery pumps:
- Casing Design Pressure: The pump casing must be designed to withstand the maximum discharge pressure plus the suction pressure (in series operation) under transient surge conditions.
- Temperature Limits: Unlike API 610 refinery pumps (which handle 370°C+), B31.4 pipeline pumps are typically limited to ambient or slightly heated crude (max 250°F/120°C) to prevent seal vaporization.
- Material Verification: Ductility is prioritized over high-temperature creep resistance. Materials must pass Charpy V-Notch impact testing if operating in cold environments (e.g., Alaska/Canada pipelines).
Note: While ASME B31.4 covers the system, the pump itself is usually specified to API 610 standards to ensure reliability.
Frequently Asked Questions
What is the difference between Atmospheric (CDU) and Vacuum (VDU) distillation?
The Atmospheric Crude Distillation Unit operates slightly above atmospheric pressure to separate lighter fractions (Naphtha, Diesel). However, the “bottoms” (Residue) cannot be heated further without cracking. This residue is sent to the Vacuum Distillation Unit (VDU), which operates at near-vacuum pressure. This lowers the boiling point, allowing the recovery of heavy vacuum gas oils (VGO) without thermal degradation.
What are Fractional Distillation Column Internals?
Fractional Distillation Column Internals refer to the hardware inside the tower that facilitates mass transfer between vapor and liquid. The two main types are:
- Trays (Sieve, Valve, Bubble Cap): Used in CDUs for distinct separation stages. They hold a liquid level that vapor bubbles through.
- Packing (Random, Structured): Used to minimize pressure drop, common in Vacuum units but less common in the fouling environment of a CDU flash zone.
Why is stripping steam injected into the CDU?
Superheated steam is injected into the bottom of the main column and side strippers to lower the Partial Pressure of the hydrocarbons. According to Dalton’s Law, reducing the partial pressure allows the lighter volatile components (like diesel trapped in the residue) to vaporize at a lower temperature, increasing the yield of valuable products without adding more heat.
How does the choice of Brent vs. Heavy Crude affect the CDU?
Processing Brent Oil vs Crude Oil (Heavy) requires different operational settings. Brent is light and sweet, meaning the CDU tower top (Naphtha/Kerosene sections) will be hydraulically loaded. Conversely, Heavy Crude loads the bottom of the tower and the furnace, often requiring higher temperatures and better metallurgy (like 9-Chrome steel) to resist sulfur corrosion (Sulfidation) and naphthenic acid attack.
Final Thoughts for 2026
The Crude Distillation Unit (CDU) remains the heartbeat of the energy sector. Whether you are optimizing the preheat train to save energy or selecting the right metallurgy to handle high-TAN crudes, understanding the physics of the process is non-negotiable. As refineries evolve in 2026, the focus is shifting toward efficiency, digitalization, and handling “opportunity crudes” without compromising safety.
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