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.

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✅ 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)

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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.

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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.

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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)

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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.

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✅ 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).

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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).

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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.

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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.

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✅ 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

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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.

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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).

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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.

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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.

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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.

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🚀 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.

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