Distillation & Fractional Distillation

distillation apparatus set up

Core Concepts

In this tutorial, you will learn about the laboratory technique of distillation, different variations of the procedure, and you will be able to use a visual model to help picture the idea of this process.

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What is Distillation?

Distillation is a process of separation of liquids with different boiling points. This technique involves evaporation or having liquids heated to the point where they turn into gas, and condensation, having the gas cool down enough to turn back into a liquid.

The idea of distillation has been around for a long time and was noted to be used by some important people in history. For example, Aristotle looked at the process of making pure water as a distillation of seawater; based on the boiling point of pure water, it was able to be evaporated from all the other contaminants that make seawater and recondensed to make pure water.

Three different temperatures come into play when using a distillation technique; these temperatures have to do with the mixture you are using and the temperature ranges of each liquid within the mixture. The goal is to separate one liquid from a homogenous mixture by using a chemical property that is different than the rest of the liquids, the boiling point.

For example, if you are working with three liquids, A having a BP of 70ºC, B having a BP of 100ºC, and C having a BP of 150ºC, to first remove liquid A you must choose a temperature that will boil liquid A without interfering with liquid B.

Simple Distillation vs Fractional Distillation

These two processes are highly similar in nature, as they have the same purpose; however, one can be chosen over the other based on boiling point similarities. As used in the example above, sometimes a mixture of liquids will have very different boiling points and it can be simple to control this procedure to yield close to pure results. Here you would use simple distillation. However, sometimes a mixture will contain liquids with boiling points that can differ by only a few degrees; when this chemical property is similar, it can be hard to separate the liquids in one run. Fractional distillation, also known as differential distillation, helps with this problem through the use of extra equipment.

Here, a fractionated column is used, which adds an extra measure of precaution, as it requires the gas to rise, preventing the contamination from the other liquids to pass through rather easily. In addition, with this procedure, there will usually be repeated distillation processes in order to try and get the purest amount of the desired mixture component. So the heating/vaporization followed by the cooling/condensation process, also known as theoretical plates, will take place multiple times in a row to try and hone in on the desired liquid. This process of repeated distillations, known as rectification, ensures that despite similar boiling points, the liquids have a possibility of being separated. Fractional distillation can be used in industries such as petroleum refining, as it works with chemicals with similar boiling points.

Multiple Effect Distillation

This is a procedure used on a large scale, such as in commercial plants. Also known as multistage-flash, is like simple distillation; however, instead of using heating properties, it uses atmospheric properties. The mixture is transported from a place of high atmospheric pressure to one of low atmospheric pressure. The drastic pressure change is the same as the heating process, as it causes the desired liquid to vaporize, then is condensed back to a liquid. This shows how different chemical properties can come into play on the same type of procedure. Both temperature and pressure can be used to separate a mixture of liquids!

Vacuum Distillation

When you are working with a mixture of liquids that have extremely high and inconvenient boiling points and can decompose when working with the atmospheric conditions of the process above, you use this reduced pressure process. Here, a vacuum pump is used to produce a high and strong vacuum; this allows for the ambient pressure to change, which will vary the chemical properties of the liquids. This is usually used to lower the boiling points first, before going through the distillation process. The reduction in boiling point can be found using the Clausius- Clapeyron equation.

Steam Distillation

This is an alternative method of lowering boiling points when working with liquids with high boiling point properties; furthermore, this is the best method for when the desired liquid is immiscible, or incapable of mixing, and does not chemically react with water; examples of these are fatty acids and oils. In this process, water vapor is passed through the liquid to supply heat into the system and resulting in evaporation.

Distillation Apparatus

Now that we have mastered the general idea of how distillation works, let’s look at the apparatus itself to be able to picture all the moving parts of the simple distillation technique. Below is a number labeled picture, along with a descriptive key:

  1. Heat source (usually a hot plate or Bunsen burner)
  2. A round bottom flask that holds the mixture of liquids
  3. Distillation head (where liquid travels as it evaporates)
  4. Thermometer (to measure temperature and monitor boiling points)
  5. Condenser (cools gas back down so it can return to liquid)
  6. Water Cooling Hole (allows interaction with ambient temperature)
  7. Water Cooling Hole (allows interaction with ambient temperature)
  8. Rounded Flask (holds desired distillate)
  9. Vacuum spout
  10. Still Receiver (where distillate funnels into receiving flask)
  11. Hot Plate controls
  12. Heat Bath (ensures even heating without direct heat source contact)
  13. Stir bar
  14. Cooling bath (ensures even cooling)