Distillation is the process of purifying a substance, whereby pure substances are extracted from a mixture. 

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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: 

1. The conversion of the desired liquid from a mixture into vapour

2. The condensation of the purified liquid

3. 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.

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 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:

2. Simple Distillation

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

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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.

5. Azeotropic Distillation

An 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 Distillation

Extractive 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 Distillation

There 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 Distillation

Large-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 Distillation

In 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 Benefits

Distillation 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.

Conclusion

Distillation is an ancient process that was first developed thousands of years ago. The earliest archaeological evidence for distillation dates back to 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.

Distillation

Distillation is the process of heating a liquid until it boils, then condensing and collecting the resultant hot vapors. Mankind has applied the principles of distillation for thousands of years. Distillation was probably first used by ancient Arab chemists to isolate perfumes. Vessels with a trough on the rim to collect distillate, called diqarus, date back to BC.

In the modern organic chemistry laboratory, distillation is a powerful tool, both for the identification and the purification of organic compounds. The boiling point of a compound is one of the physical properties used to identify it. Distillation is used to purify a compound by separating it from a non-volatile or less-volatile material. When different compounds in a mixture have different boiling points, they separate into individual components when the mixture is carefully distilled.

Distillation for Boiling Point Determination

The organic teaching labs employ distillation routinely, both for the identification and the purification of organic compounds. The boiling point of a compound, determined by distillation, is well-defined and thus is one of the physical properties of a compound by which it can be identified. Distillation is used to purify a compound by separating it from a non-volatile or less-volatile material. Because different compounds often have different boiling points, the components often separate from a mixture when the mixture is distilled.

The boiling point is the temperature at which the vapor pressure of the liquid phase of a compound equals the external pressure acting on the surface of the liquid. The external pressure is usually the atmospheric pressure. For instance, consider a liquid heated in an open flask. The vapor pressure of the liquid will increase as the temperature of the liquid increases, and when the vapor pressure equals the atmospheric pressure, the liquid will boil. Different compounds boil at different temperatures because each has a different, characteristic vapor pressure: compounds with higher vapor pressures will boil at lower temperatures.

Boiling points are usually measured by recording the boiling point (or boiling range) on a thermometer while performing a distillation. This method is used whenever there is enough of the compound to perform a distillation. The distillation method of boiling point determination measures the temperature of the vapors above the liquid. Since these vapors are in equilibrium with the boiling liquid, they are the same temperature as the boiling liquid. The vapor temperature rather than the pot temperature is measured because if you put a thermometer actually in the boiling liquid mixture, the temperature reading would likely be higher than that of the vapors. This is because the liquid can be superheated or contaminated with other substances, and therefore its temperature is not an accurate measurement of the boiling temperature.

If you are using the boiling point to identify a solid compound which you have isolated in the lab, you will need to compare its boiling point with that of the true compound. Boiling points are listed in various sources of scientific data, as referenced on the Chemical Information page on this website.

If you look up the boiling point of a compound in more than one source, you may find that the values reported differ slightly. The literature boiling point depends on the method and ability of the technician taking the boiling point, and also on the purity of the compound. While theoretically all boiling points should be constant from source to source, in reality the reported boiling points sometimes vary. Therefore, always reference the source of the physical data which you write in your lab report.

Distillation for Compound Purification

Simple distillations are used frequently in the organic chemistry teaching labs. They are useful in the following circumstances:

• the liquid is relatively pure to begin with (e.g., no more than 10% liquid contaminants)
• the liquid has a non-volatile component, for example, a solid contaminant
• the liquid is contaminated by a liquid with a boiling point that differs by at least 70°C

"Simple" distillation may be a misleading term to the beginning organic chemistry student, since it takes a lot of practice in simple distillation to become proficient in this technique. It is especially important to do a perfect simple distillation when determining a boiling point for identification purposes. You can see detailed photos of a simple distillation set-up here. Be sure to have correct placement of the thermometer, fill the flask to the correct level, and use a boiling chip.

Fractional Distillations

Mixtures of liquids whose boiling points are similar (separated by less than 70°C) cannot be separated by a single simple distillation. In these situations, a fractional distillation is used. You can see photos of a fractional distillation set-up here.

Vacuum Distillations

Vacuum distillation is distillation at a reduced pressure. Since the boiling point of a compound is lower at a lower external pressure, the compound will not have to be heated to as high a temperature in order for it to boil. Vacuum distillation is used to distill compounds that have a high boiling point or any compound which might undergo decomposition on heating at atmospheric pressure. The vacuum is provided either by a water aspirator or by a mechanical pump. You can see photos of a fractional distillation set-up here. Always check for star cracks in the flasks before beginning a vacuum distillation.

Distillation Technique Quiz

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