1. Fractional Distillation
Wednesday, 21 November 2018
Fractional Distillation
L.O: To know that crude oil is a mixture of lots of different compounds, and to know that most of the compounds in crude oil are hydrocarbons. To understand how fractional distillation is used to separate crude oil into fractions.

2. Starter
What are some ways in which we can separate different elements and compounds from compounds and mixtures?
Think – reactions, chemicals, what we have been looking at in the last few topics
Answer in a spider diagram…
Filtration
Thermal decomposition
Separating Compounds
Distillation
Electrolysis

3. Model of a hydrocarbon
KEYWORDS
Crude oil: A mixture of many different compounds. A mixture consists of two (or more) elements or compounds that aren't chemically bonded to each other. Most of the compounds in crude oil are hydrocarbon molecules. Hydrocarbons are basically fuels such as petrol and diesel. They're made of only carbon and hydrogen.
Hydrogen gas
Carbon

4. Properties of mixtures
There are no chemical bonds between the different parts of a mixture, so the different hydrocarbon molecules in crude oil aren't chemically bonded to one another.
This means that they all keep their original properties, such as their boiling points (the temperature at which they turn from a liquid into a gas).
The properties of a mixture are just a mixture of the properties of the separate parts.

5. How can we separate mixtures?
Because the substances in a mixture all keep their original properties, the parts of a mixture can be separated out by physical methods. FOR EXAMPLE: Crude oil can be split up into its separate fractions by fractional distillation. Each fraction contains molecules with a similar number of carbon atoms to each other. This is what we’re going to look at next. It can be quite difficult to get your head round, but it is important that you understand how it works…

6. Fractional Distillation
Crude oil can be SPLIT into separate groups of hydrocarbons using a TECHNIQUE called fractional distillation.
The crude oil is PUMPED into a piece of EQUIPMENT known as a fractionating COLUMN, which works CONTINUOUSLY (it doesn't get switched off).
This fractionating COLUMN has a temperature gradient running through it — it's HOTTER at the BOTTOM and COLDER at the TOP.
Crude oil can be spalten into separate groups of hydrocarbons using a technik called fractional distillation.
The crude oil is auspumpen into a piece of gerät known as a fractionating säule, which works ständig (it doesn't get switched off).
This fractionating säule has a temperature gradient running through it — it's heißesten at the boden and kältesten at the obere.
SPLIT
BOTTOM
TOP
HOTTER
EQUIPMENT
COLDER
CONTINUOUSLY
PUMPED
TECHNIQUE
COLUMN (X2)
SOME OF THE WORDS IN THESE SENTENCES ARE IN GERMAN... CAN YOU WORK OUT WHAT THEY MEAN?
TOO DIFFICULT? CLICK HERE TO GET A WORDBANK…

7. Fractional Distillation
The crude oil is first heated so that it vaporises (turns into a gas) and is then piped in at the bottom of the column. The gas rises up the column and gradually cools. Different compounds in the mixture have different boiling points, so they condense (turn back into a liquid) at different temperatures. This means they condense at different levels in the fractionating column.
Hydrocarbons that have a similar number of carbon atoms in THEM have similar boiling points, so they condense at similar levels in the column.
For example:
Hydrocarbons with lots of carbon atoms in THEM have high boiling points, so they condense near the bottom of the column.
With this in mind, what about… hydrocarbons with a small number of carbon atoms in THEM?
They have low boiling points, so they condense near the top of the column.

8. REMEMBER THIS! The smaller the number of carbon atoms….
The lower the boiling point…
And the lower it condenses!
The higher the number of carbon atoms…
The higher the boiling point…
And the higher it condenses!

9. Fractional Distillation
The groups of hydrocarbons that condense together are called fractions. The various fractions are constantly tapped off from the column at the different levels where they condense.

10. Fractional Distillation Column
Here is a real life fractional distillation column.
Can you see where the different fractions are?
What is the top one?
What is the bottom one?

11. Fact recall What is a mixture? What is a hydrocarbon?
Name the process that is used to separate crude oil into fractions.
Where do compounds with a small number of carbon atoms leave the fractionating column — near the bottom or near the top?

12. Answers – Fact Recall
A substance made from two or more elements or compounds that aren't chemically bonded to each other.
A hydrocarbon is a molecule that only contains hydrogen and carbon.
Fractional distillation
Near the top.
Compounds with a small number of carbon atoms have low boiling points so they condense near the top of the fractionating column where it is cooler.

13. Application
This table shows the number of carbon atoms in some of the hydrocarbons found in crude oil.
Which of the hydrocarbons in the table will condense at the lowest temperature?
Which of the hydrocarbons in the table will condense nearest the bottom of a fractionating column?

14. Application
This table contains data on some of the fractions of crude oil that are separated out during fractional distillation.
Which of the fractions in the table will be removed closest to the top of the fractionating column?
Octane has a boiling point of 125 °C.
At what temperature will octane condense?
In which of the fractions in the table will octane be found?

15. Answers – Application
a) Butane - Butane condenses at the lowest temperature because it has the smallest number of carbon atoms b) Tetracontane - Tetracontane has the largest number of carbon atoms so it will condense at a high temperature near the bottom of the fractionating column.
a) Petrol - The fractionating column is hottest at the bottom and coolest at the top, so the fraction with the lowest boiling temperature range will be removed at the top. This is petrol b) i. 125 °C ii. Naphtha - The boiling point of octane is within the ….boiling temperature range of naphtha, so octane will be ….found in naphtha.