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.What is Distillation?The distillation process generally involves three main steps: The conversion of the desired liquid from a mixture into vapourThe condensation of the purified liquidThe collection of the condensed liquidSpecific 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.What is the Process of Distillation?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 DistillationAs 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:2. Simple DistillationSimple 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 DistillationAs 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 DistillationVacuum 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.5. Azeotropic DistillationAn azeotropic mixture is a type of mixture that has two or more liquids that cannot be separated by ordinary distillation process. The vapours that are formed by boiling this type of mixture have almost the same proportions as the original liquid. The azeotropic distillation process utilises other substances to facilitate distillation. For example, water and ethanol can be separated through distillation by first adding an entrainer agent such as toluene.6. Extractive DistillationExtractive distillation is used for separating azeotropic mixtures. It’s an alternative to azeotropic distillation and is usually applied to mixtures with a miscible, relatively non-volatile solvent with a high boiling point.Applications of Fractional DistillationThere are several laboratory and industrial-scale applications of fractional distillation. It is highly useful in separating miscible liquid mixtures such as acetone and water, and chloroform and benzene. The process separates mixtures of liquids with different boiling points.Fractional distillation is used in the manufacture of spirits from wine. The ethanol is evaporated from wine and collected in a separate container. This can be done several times to achieve higher ‘proof’ or concentration of alcohol. This has been the traditional method used by both small-scale and large-scale breweries.In laboratories, as well as in factories, distillation is used to purify organic compounds. These include substances such as esters, amide, alcohols, and ketones. Many school required experiments in organic chemistry involve the distillation of organic compounds.Finally, the largest and most important application of distillation is the fractional distillation of hydrocarbons. Different types of hydrocarbon mixtures such as gasoline, diesel, and kerosene are derived through the fractional distillation process.Industrial DistillationLarge-scale industrial distillation involves the extraction and purification of various types of substances that have commercial and manufacturing value. These primarily include alcohol and hydrocarbons.Laboratory Fractional DistillationIn a laboratory setting, fractional distillation can be performed using a ‘fractionating column’, which is a glass tube with specially designed compartments. It is a small-scale setup that can also be used for analytical purposes.Environmental & Energy BenefitsDistillation can be used to extract alternative fuel, such as biofuel. It can make existing types of fuels purer and more energy-efficient. The process can also be used to recycle liquid chemicals for reuse or repurpose, and can remove pollutants from water and allow it to be recycled into closed-loop systems – think of greenhouses in orbiting space stations.ConclusionDistillation is an ancient process that was first developed thousands of years ago. The earliest archaeological evidence for distillation dates back to 1200 BCE, which we know from early Akkadian tablets. It described the process of making perfumes through distillation. Today, distillation is used for both laboratory analytical processes and in large-scale industrial processes. There are several types of distillation, such as fractional and azeotropic. Specific types of substances can be extracted and purified through this process.
