Distillation is the process of purifying a substance, whereby pure substances are extracted from a mixture.
There are different types of distillation processes, including fractional distillation, simple, steam, and vacuum distillation.
Distillation has several commercial and industrial applications. For example, it can be used to distil wine. In fact, the well-documented use of distillation can be traced as far back as the 13th century, where it was used to distil alcohol from wine. It’s able to do this because the difference in the boiling point of water and ethanol makes it possible to isolate purer amounts of alcohol from wine.
Another major application of distillation is in the petroleum industry, where fractional distillation is used to purify crude oil into different types of commercial fuels, like gasoline and diesel.
In this post:
What is Distillation?
The distillation process generally involves three main steps:
- The conversion of the desired liquid from a mixture into vapour
- The condensation of the purified liquid
- The collection of the condensed liquid
Specific types of distillation processes may have several more stages, such as the fractional distillation of crude oil (more on that below).
In general, heat, corresponding to the boiling point of the desired liquid, is applied to the mixture that will be distilled. In other instances, pressure is lowered to optimise the distillation process.
Below is an illustration of a basic laboratory distillation setup:
In this example, freshwater is the liquid being extracted. As heat from the Bunsen burner flame is applied to the flask, which contains seawater, the water boils and evaporates. The water vapour is cooled and redirected by the condenser to the Erlenmeyer flask, drip by drip.
The flowing water in the outer cooling tube surrounding the inner condensing tube facilitates the condensation of the water vapour. Since salt is solid and has a very high boiling point, it doesn’t evaporate with the water. As a result, the distillate is freshwater.
This process is known as the desalination of water, and it’s used on a large-scale in some countries that have freshwater scarcity, such as those in the Middle East.
How Does the Process of Distillation Work?
The distillation process works by ‘exciting’ the molecules of the liquid to be distilled by heating it, thus encouraging evaporation. It’s crucial that the precise boiling point of the intended distillate is known.
Furthermore, the temperature of the liquid mixture must be meticulously monitored, and the temperature must be maintained at or very close to the precise boiling point of the intended distillate. Otherwise, other impurities may evaporate along with the desired distillate.
In mixtures where there are dissolved solids, like salt solutions, it’s relatively easy to isolate the liquid solvent from the mixture. This is because liquid solvents like water generally have lower boiling points than their solutes. In simple cases, such as in the distillation of alcohol from wine, the temperature must simply be maintained at the boiling temperature of the intended distillate.
For instance, ethanol (the alcohol in wines) has a boiling point of 78.37°C. This means that the wine mixture itself, which is mostly water, should not be allowed to boil at 100°C (the boiling point of water). Otherwise, the water will evaporate along with the alcohol. As much as possible, the temperature of the mixture must be maintained at the precise boiling point of the intended distillate.
Once the vapour has been collected, it must be allowed to cool down at room temperature for it to condense into liquid. It’s then collected in a container either for storage or further processing.
Often, distillates are not 100% concentrated, and may still contain some amounts of impurities, particularly the original solvent. Therefore, if you want to have a distillate with higher purity, further distillation is necessary.
Is Distillation a Chemical or Physical Process?
Distillation is a physical process because it involves a phase change from liquid to gas (vapour), and then back again to liquid. Generally, no chemical change is intended to occur during the process of distillation. That said, some incidental or accidental chemical reactions may occur during the process of distillation. The risk of this happening increases as the scale becomes larger.
For example, the distillation of flammable liquids may result in combustion or even explosion. In some cases, liquids may also react with the components of the distillation equipment. Some liquids may even react with the oxygen or impurities in the air once vaporised. Watch this case study presentation about an accidental explosion at a vinyl chloride monomer (VCM) distillation plant.
What Are the Different Types of Distillation?
There are several different types of distillation, owing to its range of industrial applications. The exact step-by-step process and equipment details of each type are beyond the scope of this article, but we’ll outline the generalised process and concepts of each one.
1. Fractional Distillation
As we mentioned earlier on, fractional distillation is commonly used in the petroleum industry to separate the many components of crude oil. This is necessary because the boiling points of these components are too close to each other that simple distillation would not work.
Fractional distillation is a multistage process that separates various types of fossil fuels, like kerosene and heavy oil. The temperatures at various stages are precisely controlled. The illustration below depicting fractional distillation of fossil fuels:
Simple distillation is used when the boiling points of the liquids in a mixture are very different. Typically, it’s used to distil liquids that have at least 100°C difference in boiling points. As the name suggests, it’s relatively easy to separate these liquids because there is sufficient room for temperature variations that will not affect one of the liquids in terms of evaporation.
3. Steam Distillation
As you can tell by the name, steam distillation uses steam to distil the desired liquids. The steam itself isn’t the distillate, but just a means to extract the distillates. It’s commonly used for extracting the essential oils of flowers. See the illustration below for the basic setup:
4. Vacuum Distillation
Vacuum distillation is the distillation process used for liquids with very high boiling points under normal atmospheric pressure. It’s also used for liquids that decompose under normal atmospheric pressure. As pressure is reduced, the boiling point is also reduced, thereby making it easier to evaporate the liquid. It’s commonly used in refining petroleum products.
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