---
title: "Refining of crude oil and its products"
subject: "Energy Resources"
chapter: 1
key: "semester_6/energy_resources:crude-oil-refining-process"
source: https://www.bctengineer.com/q/semester_6/energy_resources/crude-oil-refining-process
site: BCT Engineering
---

# Refining of crude oil and its products

Energy Resources · Chapter 1: Fossil fuels and their characteristics

- Asked in: year not given, 6 marks
- Video: [How a refinery works – what happens in a refining tower?](https://www.youtube.com/watch?v=8v4hvfY58cs) — JAES Company 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/crude-oil-refining-process
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-1

## Question

Describe the refining of crude oil. Explain fractional distillation with a neat sketch of the column, name the main fractions with their approximate boiling ranges, and briefly explain the role of cracking and reforming.

## Short answer

Crude oil is heated and separated by fractional distillation in a column. Fractions include gas (<30°C), gasoline (40-200°C), kerosene (150-250°C), diesel (250-350°C) and residue. Cracking breaks heavy molecules into lighter fuels; reforming rearranges naphtha molecules to raise octane number.

## Answer

**Refining** separates crude petroleum, a mixture of many hydrocarbons, into useful products and then upgrades them to meet quality requirements.

#### Steps of refining
1. **Desalting and heating:** water and salts are removed, and the crude is heated to about 350–400 °C in a furnace.
2. **Fractional distillation:** the hot vapour-liquid mixture enters an atmospheric column. Vapours rise through bubble-cap trays; lighter components with low boiling points condense higher up, and heavier components condense lower down.
3. **Vacuum distillation** of the bottom residue separates heavy gas oils and lubricating oil without cracking them.
4. **Conversion and treatment:** cracking, reforming, sweetening and blending.

```text
   Gas (C1-C4)     <-- top, below 30 C
  ---------------
   Gasoline        <-- 40-200 C
  ---------------
   Kerosene        <-- 150-250 C
  ---------------
   Diesel/gas oil  <-- 250-350 C
  ---------------
   Fuel oil        <-- 350-450 C
  ---------------
  Crude --> [furnace] --> enters here
   Residue/bitumen <-- bottom
```

#### Main fractions

| Fraction | Approx. boiling range | Use |
|---|---|---|
| Refinery gas, LPG | below 30 °C | Cooking, heating |
| Gasoline (petrol) | 40–200 °C | Spark-ignition engines |
| Kerosene / ATF | 150–250 °C | Lighting, jet fuel |
| Diesel (gas oil) | 250–350 °C | Compression-ignition engines |
| Fuel oil, lubricants | above 350 °C | Boilers, lubrication |
| Residue | non-volatile | Bitumen, coke |

#### Cracking
Cracking breaks the large molecules of heavy oil into smaller, more valuable ones such as gasoline. **Thermal cracking** uses 450–750 °C and high pressure; **catalytic cracking** uses a zeolite catalyst at about 500 °C and gives gasoline of higher octane number.

#### Reforming
Reforming uses a platinum catalyst to rearrange the molecules of low-octane naphtha into branched and aromatic compounds, raising the octane number of gasoline.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In the atmospheric distillation column, where do the lighter, low-boiling components condense?
   A. Higher up in the column
   B. Lower down in the column
   C. At the furnace inlet
   D. Only in the residue at the bottom
   Answer: A. Vapours rise through the trays; light components condense near the top and heavy ones lower down. (Key point: Fractional distillation principle)

2. Why is the bottom residue further distilled under vacuum?
   A. To remove water and salt from the crude
   B. To raise the octane number of gasoline
   C. To convert naphtha into aromatics
   D. To separate heavy gas oils and lubricating oil without cracking them
   Answer: D. Lower pressure lowers boiling points so heavy fractions vaporise without thermal breakdown. (Key point: Vacuum distillation)

3. What does cracking do?
   A. Joins small molecules into larger heavy ones
   B. Removes sulphur by sweetening
   C. Breaks large heavy-oil molecules into smaller, more valuable ones such as gasoline
   D. Rearranges naphtha into aromatics only
   Answer: C. Cracking, thermal or catalytic, turns heavy oil into lighter products; catalytic cracking gives higher octane gasoline. (Key point: Cracking)

4. What is the purpose of catalytic reforming?
   A. Break heavy residue into gas
   B. Rearrange low-octane naphtha into branched and aromatic compounds to raise octane number
   C. Dehydrate crude oil before heating
   D. Separate diesel from kerosene
   Answer: B. Reforming with a platinum catalyst gives branched and aromatic molecules that resist knock. (Key point: Reforming)

## Related in Energy Resources

- [Hydrogen production processes](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Ahydrogen-production-methods) — Energy Resources, chapter 6

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