Distillation is a purification technique that uses heat to extract or separate components from a mixture. The main product of the distillation process is known as the condensate, which is typically a purer substance.
The distillation of products from a reaction dates as far back as 4,000 years ago, when ancient civilisations were believed to have used the technique to extract essential oils from plants. These oils were then used to make perfume and early medicines.
Today, modern distillation is used in the production of spirits and various petroleum products. The latter is extracted from crude oil through the process of fractional distillation. Another large industrial application of distillation is the desalination of seawater in places that don’t have an adequate supply of fresh water.
Read on to learn more about distillation, the different stages of the process, and the products that are produced.
In this post:
What is distillation?
Distillation is a physical process that involves heating a mixture of liquids to separate the components based on differences in their volatility or pressure. The design of the apparatus used for distillation can vary depending on the liquid mixture and the quantity being processed.
The simplest distillation apparatus in the laboratory is the retort, which is typically a spherical glass vessel with a long neck pointing downwards. It can be used in the simple distillation of spirits from wine. As the ethanol (the alcohol in wines) evaporates, the vapour collects at the tip of the retort and then drips into a separate vessel as it condenses.
A more sophisticated distillation set-up requires the assembly of various parts including a bunsen burner, a distillation flask with a thermometer, a condenser with a coiled or straight interior tube surrounded by an exterior tube with flowing water, and a receiving flask. The apparatus is supported by metal stands and clamps. Sometimes, a water bath is placed under the distillation flask to control the temperature. This is important when dealing with highly volatile liquids with very close vapourisation points.
The design of the distillation apparatus may vary slightly depending on the quantity and type of liquids being processed. However, the basic principles of distillation remain the same; to separate liquids based on their vapourisation points. This underlying principle also applies when distilling petroleum-based products from crude oil through fractional distillation.
Fractional distillation can be performed on either a small or large industrial scale, depending on the requirements. Each chamber within the apparatus corresponds to the fraction of the liquids being distilled. The liquids (or sometimes gases) are separated based on certain temperature ranges or thresholds. For example, in the distillation column used in the fractional distillation of crude oil, lubrication oil is separated from the mixture at between 300°C and 350°C.
What processes are involved in distillation?
Distillation involves three main processes – vapourisation or evaporation, condensation, and finally collection. Heat or pressure differential can be applied to vaporise a liquid or solid.
1. Evaporation
The first step in the distillation process, evaporation is the conversion of a liquid or solid component of a mixture into vapour. Heat is usually applied at normal atmospheric pressure, although for liquids with high boiling points the pressure may be reduced to lower the boiling point.
2. Condensation
Condensation is the process of converting the vapour back into liquid form by lowering the temperature of the vapour. It becomes a condensate once a critical temperature is reached. In some cases, you may need a condensation nucleus or surface to allow the vapour to condense.
3. Collection
The condensed liquid is then collected in a vessel for storage or further processing. The purity of the condensed liquid varies depending on how precise the distillation process is and the types of substances involved. For example, it’s possible to distil highly purified water from seawater by simply evaporating and condensing the water vapour.
What is the product of distillation?
The main product of distillation is the condensate, which is typically a purer substance. In simple distillation, the liquid being distilled might just be a mixture of two components. For example, wine is simply a mixture of water and ethanol plus some organic impurities.
In some cases, however, the distillate may be composed of various substances. Kerosene, for instance, is made up of ten types of hydrocarbons. In this case, other purification methods must also be performed to further separate kerosene into its component hydrocarbons.
It’s important to remember that the distillation process won’t work if the volatility of the substances in a mixture is the same.
What is the basic principle of distillation?
The basic principle of distillation is the separation of two or more liquids, solids, or gases into their purer forms based on the differences in volatility. For the technique to work, the temperature and pressure must be carefully controlled and manipulated.
Although there are different types of distillation techniques, the basic principle and key processes remain the same. The main types of distillation are:
- Simple distillation – a liquid mixture or solution is evaporated and immediately cooled for condensation and collection. The main aim is to separate the dissolved solids and other impurities, such as in the case of the desalination of seawater.
- Fractional distillation – this may involve various liquids with similar boiling points. The liquids are separated from the mixture based on their volatility and then collected in separate levels in a distillation column.
- Steam distillation – this method is designed to separate heat-sensitive components of a mixture. Steam distillation is typically used to extract essential oils from plants. Steam is passed through the plants to heat the oils and herbal distillates.
- Vacuum distillation – a technique used to separate liquids with high boiling points at normal atmospheric pressures. Decreasing the pressure reduces the boiling points of the liquids.
- Air-sensitive vacuum distillation – compounds that are reactive or sensitive to air or a vacuum are distilled in the presence of inert gases.
- Short path distillation – compounds that are unstable at high temperatures are distilled through this process. The pressure is lowered and the distillate follows a short distance before being collected.
- Zone distillation – this involves the partial melting of a substance. The vapour is collected as distillate.
Summary
Distillation is a way of separating components of a mixture that have different boiling points. However, the process becomes less effective if the boiling points or volatility of the components are too close to one another. There are several distillation methods, although they all generally follow the same basic principles. The three main stages of the distillation process are evaporation, condensation, and collection.
