As chemists, we often need to separate and purify mixtures of molecules. When we synthesise new chemicals, we rarely make one pure product without side products or unreacted starting materials or catalysts that are present in the mixture with the product. We don’t always make new compounds that need separating, sometimes we wish to extract naturally occurring molecules from mixtures.
There are several ways to separate different chemicals from one another, and the purification of molecules is often the hardest part of a chemist’s job!
When we talk about fractional distillation we are normally discussing the fractional distillation of crude oil. Crude oil is the unrefined substance that we find in the earth. In nature, it contains many different molecules of many different shapes and sizes. Using fractional distillation, we can separate out all of these different components to get more useful products, such as petrol, jet fuel and bitumen, which is the thick, tarry substance that is used to make roads.
- What property of liquids do you think we can use to separate the components of crude oil?
- What practical considerations do scientists need to think about during this process?
- Can you think of any other alternative energy sources we could use instead of burning crude oil? Try and think about the advantages and disadvantages of these different methods.
Scientists have found that heating up crude oil can be used to separate out its different components. They use a fractionating column, which is a giant vertical tank that is very hot at the bottom, and very cool at the top.
Crude oil is vaporised and added to the fractionating column. The molecules that condense at the bottom of the column, where it is hottest, are the heaviest molecules as these have very high boiling points. All of the other gases rise up the column, and as it cools, these gases with different boiling points condense and the liquids are separated. At the very top of the column, the lightest molecules with the lowest boiling points are removed as gases.
Building your own fractional distillation column
YOU WILL NEED
- 2 L Plastic bottle
- Plastic straws
- Acrylic paint (red and blue, can also use purple)
- Hot glue gun (alternative glue e.g. PVA should also work)
- Empty and clean the bottle and remove any labels
- Paint the bottom half of the bottle in red to represent the hottest section
- Paint the top half of the bottle in blue to represent the coolest section
- In the middle mix the colours to show a gradual change from red to blue to show how the temperature cools as you go up the column
- Cut 3 plastic drinking straws in half
- Using a hot glue gun (make sure an adult is helping) glue one of the straws to the completely red fraction – this will be where the crude oil is added to the column
- On the other side of the bottle, at regular intervals, glue four more of the straw pieces – these are the different fractions that are collected from the column
- Glue the final straw piece to the lid of the bottle and screw it on – this shows the gases that are released from the column
Match up these molecules to where you think they will be separated in the column – make sure to count the carbon atoms correctly (this is a common mistake many chemists can make!)
Crude oil contains water, salts and sulfur that need to be removed before the crude oil can be refined to its useful components. To remove the salt and the water, crude oil undergoes a step known as desalting. In this step the crude oil undergoes an initial dilution with the addition of clean water. The new diluted mixture is then emulsified so that the salty water initially present in the crude oil comes into contact with the clean water. The emulsion then enters the desalter which separates the emulsion into two separate layers, a layer of water and a layer of crude oil. Salt easily dissolves in water and because of this it is also separated along with the water. The water is then sent to the water treatment system and the crude oil progresses to the next stage. The next stage is to desulfur the crude oil, this is important as this should reduce the emission of sulfur dioxide (SO2) when any fuel created in the fractional distillation is burnt. Desulfurisation requires heating and pressurising the crude oil to specific conditions and then the addition of hydrogen gas. The addition of the hydrogen gas should cause a reaction with any sulfur present and form hydrogen sulfide (H2S). The hydrogen sulfide is then converted into elemental sulfur and hydrogen, the sulfur can be sold on as a by-product whereas the hydrogen can be recycled and used again in the previous step.
Water treatment is very important in an oil refinery as water is used or removed in most parts of the plant. Initially oil skimmers are used, which are pieces of equipment that remove any oil that would be floating on the surface of the water. The next stage incorporates several large tanks that force air through the water. The water is mixed with biological agents that will consume any unwanted waste in the water and the air that is forced into the water helps promote the growth of these agents. To find out more about water treatment click here.
To separate a fizzy drink into its ingredients by fractional distillation.
YOU WILL NEED
100 mL conical flask
- Tripod/stand and clamp
- 5 Test tubes/vials per group
- Test tube holder
- Heat source – hot plate or tealight (not bunsen burner)
- Fizzy drinks
- Universal indicator
- Large beaker/container to hold ice
- Rubber bung with two holes
- Plastic/ Teflon tubing
- Aluminium foil
NOTE: do not use a bunsen burner for this experiment as it is too difficult to control heating.
Part 1: release of CO2
- Measure out 25 mL of the soft drink into a pre-weighed conical flask and weigh. Record the mass off the soft drink.
- Fill a test tube/vial ¾ of the way full with tap water. Add 1 mL of universal indicator and note the colour and pH of the tap water.
- Push the thermometer through one of the holes in the rubber bung and the tubing through the other. If there is an issue with getting the thermometer or tubing through the bung add a bit of washing up liquid to the the end that you want through the bung. The thermometer should rest above the liquid and measure the temperature of the vapour. Carefully fit the bung on top of the conical flask making sure that there is a tight seal, and put the other end of the tube in the universal indicator solution.
- Clamp the conical flask over the heat source. Gently heat until the thermometer reads around thirty degrees, or there is a consistent release of gas. As the gas passes through the universal indicator solution in the test tube/vial note the colour change and the pH.
Part 2: distillation of odorous compounds
- Fill a beaker or container with ice, and place a second test tube/vial in the ice. Swap the tube that is running into the first test tube and place it in the second test tube.
- Gently increase the heat of the soft drink until it begins to boil and produce a distillate. Collect the distillate up to about 5 ml then remove the test tube/vial from the beaker/container. If you want to increase the rate of distillation, you can wrap the flask and tubing with aluminium foil cladding.
Part 3: distillation of the water
- Place a third test tube/vial in the beaker/container of ice, and swap the tubing from the second test tube to the third test tube/vial. Maintain the heat on the soft drink and collect the remaining distillate.
- Observe what remains in the conical flask. Compare the smell of test tube 2 and test tube 3. Keep collecting fractions until all the liquid has boil
This reaction can take a very long time to run until completion. If you do not have time to allow all of the liquid to boil off and leave a solid, collect whatever liquid you may have and weigh it.
- What happened to the pH of the water after the CO2 from part 1 of the distillation was ran through it and why?
- Which fractions had the highest and lowest boiling points? Write a list of the different fractions that came off the mixture in order of their boiling point from lowest to highest.
- What remains in the conical flask after the distillation was over? What difference would you expect to see in a “diet” version of this drink?
- Calculate the initial density of the fizzy drink? What is the final density of the fizzy drink? Can you give an explanation for any changes? Think about what you have collected in your fractions and what is left in the conical flask.
- What did fractions 2 and 3 smell like? Why is it important that these molecules have relatively low boiling points?
- Below is a diagram of a distillation column. Fill in the boxes for where you think that your fractions from your fizzy drinks would be removed.
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