The Crude Distillation Unit (CDU) is the first processing unit in a refinery where crude oil is separated into different fractions based on boiling points. Below are key interview questions with detailed answers.
✅ General Process Questions
1️⃣ What is the purpose of the Crude Distillation Unit (CDU)?
🔹 The CDU separates crude oil into different hydrocarbon fractions based on their boiling points using atmospheric distillation. The major fractions include:
✔ Light Ends (LPG, Naphtha)
✔ Kerosene (Jet Fuel, Diesel)
✔ Gas Oil (Vacuum Gas Oil, Diesel Blendstock)
✔ Residue (Fuel Oil, Vacuum Residue)
2️⃣ What are the main sections of a CDU?
🔹 The CDU consists of:
✔ Preheat Train – Heat exchangers use hot streams to preheat crude oil.
✔ Desalter – Removes salts & water to prevent corrosion.
✔ Furnace (Heater) – Raises crude temperature to 340-370°C before entering the main distillation column.
✔ Atmospheric Column – Separates hydrocarbons into different fractions.
✔ Pump-Arounds & Side Strippers – Control heat balance & remove light ends from fractions.
✔ Overhead Condenser & Reflux Drum – Condenses light hydrocarbons & returns reflux to maintain separation.
3️⃣ What is the role of the Desalter in CDU?
🔹 The Desalter removes:
✔ Salts (NaCl, MgCl₂, CaCl₂) – Prevents corrosion & fouling in downstream units.
✔ Water & Sediments – Avoids emulsions & catalyst poisoning.
✔ Metals & Impurities – Reduces contaminants that could poison catalysts in later processes.
👉 Uses electrostatic separation and wash water injection to remove salts.
4️⃣ What is the typical temperature and pressure in the CDU?
✔ Crude Heater Outlet Temperature: 340–370°C (644–698°F)
✔ Atmospheric Column Pressure: 1.2–1.5 bar (low pressure to prevent cracking)
5️⃣ Why is crude oil preheated before entering the furnace?
🔹 To reduce fuel consumption in the furnace and improve energy efficiency.
✔ Heat is recovered from hot distillation products (pump-arounds & reflux streams) using heat exchangers.
✅ Distillation Column Questions
6️⃣ How does separation occur in the CDU distillation column?
🔹 Based on boiling points:
✔ Lighter fractions (naphtha, LPG) vaporize & rise to the top.
✔ Heavier fractions (diesel, gas oil) condense at different levels.
✔ Residue (bottoms) remains for further processing in the Vacuum Distillation Unit (VDU).
7️⃣ What is the function of the reflux in the CDU?
🔹 Reflux (returning condensed liquid to the column):
✔ Improves separation efficiency by maintaining liquid-vapor equilibrium.
✔ Helps achieve sharp separation of products (prevents overlapping boiling ranges).
8️⃣ What are pump-around circuits, and why are they used?
🔹 Pump-around circuits cool down certain sections of the column by removing heat and returning liquid.
✔ Helps control temperature and reduce furnace fuel consumption.
✔ Provides heat recovery for preheating crude oil.
9️⃣ What happens if the column pressure increases abnormally?
🔹 Causes:
✔ Blocked overhead condenser – Poor condensation of vapors.
✔ Overloading of reflux drum – Excessive vapor traffic.
🔹 Solutions:
✔ Check condenser cooling system for leaks or fouling.
✔ Drain excess liquid from the reflux drum.
✔ Reduce column feed rate if necessary.
✅ Troubleshooting & Operational Questions
🔟 What are the common problems in CDU operation and their solutions?
| Problem | Causes | Solutions |
|---|---|---|
| High salt content in crude after desalter | Low wash water injection, poor mixing | Increase wash water, optimize pH |
| High overhead temperature | Insufficient reflux, condenser fouling | Increase reflux, clean condenser |
| Bottom residue too viscous | High heater temperature, poor stripping | Optimize stripping steam, control furnace temp |
| Pressure drop across column increasing | Tray fouling, coking in the bottom section | Inspect trays, clean internals during turnaround |
| Poor separation between diesel & kerosene | Reflux ratio too low | Adjust reflux flow, optimize column temperatures |
1️⃣1️⃣ What is the effect of over-stripping in side strippers?
✔ Removes too many light components, causing flash point issues in products like kerosene & diesel.
✔ Leads to overloading of upper column trays, affecting separation.
✅ Solution: Optimize stripping steam rates.
1️⃣2️⃣ How does the furnace affect CDU performance?
✔ If furnace temperature is too low, heavier fractions won’t vaporize properly, reducing column efficiency.
✔ If too high, it can cause thermal cracking, leading to coke formation & fouling.
✅ Solution: Maintain optimal furnace outlet temperature (340-370°C).
1️⃣3️⃣ What is the impact of steam injection in the CDU?
✔ Reduces partial pressure of hydrocarbons, allowing separation at lower temperatures (energy saving).
✔ Helps prevent coking & overheating.
✔ Used in stripping sections to remove light ends from heavier fractions.
1️⃣4️⃣ What happens if the crude feed has high metals content?
✔ Metals like nickel, vanadium, and iron cause fouling in heat exchangers & affect catalyst life in downstream units.
✔ Desalter performance decreases due to increased contaminants.
✅ Solution: Use demetallization processes & ensure efficient desalting.
1️⃣5️⃣ What is the significance of end-point control in CDU fractions?
✔ Ensures that each product has the right boiling range.
✔ Prevents overlapping between fractions, which affects quality (e.g., high final boiling point of diesel leads to poor combustion).
✅ Adjust column temperature & reflux rates for proper cut points.
🚀 Final Recommendations for CDU Operators & Engineers
✔ Monitor crude feed quality (salt, water, metals).
✔ Optimize reflux & pump-around rates for efficient separation.
✔ Check stripping steam rates to avoid over- or under-stripping.
✔ Maintain furnace efficiency to prevent overheating or coking.
✔ Ensure condenser efficiency to control overhead pressure.