How to purify alcohol by distillation

Ethyl alcohol is one of the most widely used organic chemical compounds in the laboratory. Furthermore, it is one of the few alcohols that can be ingested relatively safely, since most other alcohols can be highly toxic.

Ethanol is the alcohol of two carbon atoms and its molecular formula is CH ₃ CH ₃ OH. Among its multiple properties we can find its use as an organic solvent that is also miscible with water. It has a relatively low boiling point and is also highly combustible.

On the other hand, like all alcohols, ethanol is an important starting material for the synthesis of a wide variety of organic compounds, due to the large number of chemical reactions in which it can participate. These and other reasons make having ethyl alcohol in the laboratory with a good degree of purity very important.

Possible sources of alcohol

Ethyl alcohol can be produced in several ways. At an industrial level, it is usually produced by hydration of ethylene, which is one of the gaseous hydrocarbons found in oil fields and natural gas deposits. In addition, it is also produced in large quantities through the fermentation of carbohydrates by some microorganisms, including yeasts.

Alcohol of industrial origin is often used for organic synthesis at the industrial level and also serves as a source for the preparation of absolute alcohol for use as a solvent or reagent in the laboratory. On the other hand, ethyl alcohol is one of the main components of alcoholic beverages, where we find it mixed with water and a wide variety of other solutes and solvents, all suitable for human consumption.

Since the sale of alcohol for human consumption is heavily regulated and controlled in most of the world, ethyl alcohol intended for other uses is denatured to prevent consumption. This is achieved by adding extremely bitter and even toxic chemicals, in some cases. These substances, in addition to causing these unpleasant effects when consumed, can also interfere with their use as a solvent or as a chemical reagent.

For these and other reasons, the purification of alcohol is a very important process, and the best way to do it is through distillation.

Purification of ethanol by distillation

Distillation is the process of separating liquid mixtures based on the difference in their boiling points. In most of the presentations of alcohol that we find in commerce, whether as alcoholic beverages, rubbing alcohol or as denatured alcohol, it is mixed with water, which has a higher boiling point, which allows its separation by means of of the distillation.

Simple vs. Fractional Distillation

At 1 atmosphere of pressure, pure or absolute ethanol has a boiling point of 78.37 °C, while water boils at 100 °C. This difference in boiling points, in principle, makes it possible to separate both liquids by means of simple distillation. This can be carried out using a distillation equipment like the one shown in the following figure.

This equipment consists of an electric heating plate, a distillation flask with its respective distillation elbow, a condenser, a thermometer to control the temperature and another flask or, alternatively, a beaker to collect the distillate.

Despite the fact that this process allows the separation of ethanol from water quite successfully, the closeness of both boiling points means that the vapor present when the mixture boils still contains significant amounts of water vapor which condenses together with the ethanol and ends up in the distillate. To remove excess water, a second distillation can be carried out, and then a third, and so on.

However, this can usually be avoided by carrying out not the simple distillation several times, but a fractional distillation, using a fractionating column. In these columns, in effect, many small-scale distillations take place as the vapor rises up the column, condenses, and evaporates again.

The distillation method to choose will depend on how pure the ethanol is required. For example, a simple distillation of an ethanol-water mixture that originally contains about 50% by volume of each component, only enriches the alcohol to 62%. Instead, repeating the simple distillation multiple times or using fractional distillation can bring the alcohol up to 95% by volume.

The ethanol-water azeotrope

At 1 atmosphere of pressure, once the alcohol reaches 95% purity by distillation, it cannot be further enriched or purified, no matter how many more times it is distilled in a simple or fractional way. This is because, at that composition, the mixture forms an azeotrope, which consists of a mixture of two substances whose composition in the gas phase is the same as in the liquid phase and which therefore distil together. In these cases, the boiling of the mixture produces a vapor exactly equal to the liquid, therefore, when it is condensed, the same original mixture is also obtained.

At 1 atmosphere of pressure, the ethanol-water azeotrope boils slightly below the boiling point of pure ethanol, at 78.2 °C to be exact, and has a composition of 95% ethanol. This implies that if ethanol with a higher degree of purity is required (such as when it is used as a gasoline additive), we must break the azeotrope. This is achieved by means of the so-called azeotropic distillation.

Azeotropic distillation can be carried out in a number of different ways. One way is by adding benzene or another special additive that prevents the formation of the azeotrope, but with the consequence that the ethanol produced must then be distilled again to remove the benzene.

Another common way to break the azeotrope is to pass the azeotrope mixture through a molecular sieve (such as a zeolite) so that it absorbs even a small part of the water present in the mixture. After the azeotropic mixture is broken, normal fractional distillation can be continued to finish purifying the alcohol.

Lastly, another way to break the azeotrope is by changing the pressure at which the distillation is carried out, either by applying a vacuum or by increasing the pressure. This modifies the composition of the azeotrope, allowing a greater amount of ethanol to be separated from water. Once a mixture above 95% purity is obtained, it can be returned to normal distillation at 1 atmosphere, since once the azeotrope formation point has passed, it cannot be formed again during distillation.

An example of a distillation equipment that allows ethanol to be distilled to a degree greater than 95% is shown below:

Steps for the purification of alcohol by distillation

The steps that must be carried out for the purification of ethanol by distillation are described below. We will start with some security measures.

Security measures

• Ethanol is highly flammable and also considerably volatile. Therefore, distillation should never be carried out using an open flame as a heat source , as this could lead to an explosion. Only an electric iron or electric heating mantle should be used.
• Standard laboratory safety equipment should be used, including gown, safety glasses, and, if possible, a fume hood to prevent buildup of ethanol vapors in the event of a system leak.
• Glassware must be handled with care, especially considering that it will be hot during distillation.
• If denatured alcohol is being distilled, it is not advisable to use the distillate for human consumption, even if fractional distillation has been carried out. This is because some denaturing agents are highly toxic and may still be present in the distillate.

Necessary materials and equipment

The necessary equipment for the fractional distillation of ethanol is presented below, since it is the process that produces, in the fewest number of steps, the best purity.

• Iron or heating blanket.
• Distillation flask of suitable size for the sample and another round bottom flask to collect the distillate.
• Boiling pearls.
• Fractionation column.
• Distillation elbow.
• Water cooled condenser.
• Thermometer.
• Elbow for vacuum distillation.
• Running water source.
• Vacuum pump or tube.
• 2 universal supports with their respective clamps to hold the distillation flask and the distillate flask.
• Grease for glass ground joints.

distillation procedure

1. The heating plate is placed on the universal support.
2. The distillation flask is fixed to the universal support
3. The boiling beads are introduced and the sample to be distilled is added.
4. The ground joints of the fractionating column are greased and it is connected to the balloon.
5. The entire assembly is lowered until the ball touches the heating plate.
6. The same process is repeated to connect the thermometer to the distillation elbow, ensuring that the bulb of the thermometer is level with the opening of the elbow.
7. The lower part of the elbow is connected to the upper part of the column following the same procedure, and the elbow that protrudes from the side is connected to the condenser, which must be previously fixed to a second universal support by means of a clamp.
8. It must be ensured that the side connector of the condenser corresponding to the water inlet is pointing downwards while the water outlet end must be pointing up.
9. The lower part of the condenser is connected to the distillation elbow for vacuum distillation, which should be previously connected to a round bottom flask which, in turn, must also be attached to the universal support.
10. At this point, the condenser must be connected to the cold water source using a hose, and another hose must be connected to the upper water outlet to discharge excess water to the drain. Once this is done, the stopcock is opened so that the water begins to flow through the jacket of the condenser.
11. The heating plate is turned on and the distillation begins.
12. Temperature must be carefully monitored during distillation. If the atmospheric pressure is 1 atm, the temperature during the distillation should remain relatively constant and around 78.2 °C, however this could vary depending on the components of the mixture.
13. When observing an increase in temperature, the distillation must be stopped since, at this point, all the ethanol-water mixture has already distilled and they are probably distilling other substances.

If you want to obtain ethanol with a higher degree of purity, you can distill the azeotrope again, but this time under vacuum. To do this, we start by removing the distillation flask and cleaning it or we can use a new flask and repeat steps 1 to 10 adding the previous distillate instead of the original sample. Then, the following two steps must be carried out:

1. The distillation elbow should be connected to a vacuum system and the vacuum system turned on to ensure there are no air leaks in the system.
2. Once this has been verified, the distillation is started by turning on the heating plate.
3. As before, the temperature should be constantly monitored. In this case, the distillation temperature should be less than that recorded at atmospheric pressure. For example, at 300 mmHg pressure, a new azeotrope is formed that boils at approximately 56°C and is approximately 97.4% ethanol by volume.

Once this new azeotrope is obtained, if further purification is desired, a third distillation at atmospheric pressure can be carried out. In this case, the azeotrope will no longer form again since the mixture has a higher proportion of ethanol, which will only increase with distillation. After this third distillation, absolute ethanol will be obtained, almost completely free of water.

References

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Chemistry.ES. (nd). Azeotropic distillation . https://www.quimica.es/enciclopedia/Destilaci%C3%B3n_azeotr%C3%B3pica.html

Sanz Tejedor, A. (sf). Industrial Organic Chemistry . Industrial Organic Chemistry. https://www.eii.uva.es/organica/qoi/tema-06.php

Tunqui, C., Pardo, A., Tejada, G., & Cjuro, IR (2018). Evaluation of the characteristics of the alcoholic distillate of green anise (Pimpinella anisum L.) obtained by simple distillation. Rev. Soc. Chem. Peru , 84 (4 Lima oct./di). http://www.scielo.org.pe/scielo.php?script=sci_arttext&pid=S1810-634X2018000400003

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