Azeotropic Distillation of water and ethanol by Harold van Schevensteen

The process of simple distillation is a method of separating mixtures based on their differences in volatility in a boiling liquid mixture. We find out, however, that using this process, a mixture of water and ethanol cannot be separated fully. The latter can only be purified to approximately 96%.

To get to this point, we go through a process whereby through distillation, we reduce the proportion of water in the mixture and increase that of ethanol. For example, if we were to distill a 50/50 mixture of ethanol and water, the distillate would be 80% ethanol and 20% water. Distilling this new mixture produces a mixture of 87% ethanol and 13% water. This goes on until we reach a ratio of 95.63% ethanol and 4.37% water, at which we reach a point called the azeotropic point of the mixture.

At this stage, the mixture is called a positive azeotrope, which is defined when the ratio of the constituents cannot be further changed by simple distillation. This occurs because any further boiling of the mixture will result in a vapour who’s ratio of constituents is the same as the original mixture.

The closer we get to this point, the lower the boiling point of the mixture as a whole. Ethanol has a boiling temperature of 78.4°C and water one of 100°C. When we have reached the azeotropic point, the mixture as a whole boils at a temperature of 78.2°C. Any further distillation will see the azeotrope boil at a temperature which is lower than any of it’s constituents. This introduces a problem for any further attempt to purify the ethanol (>96%), as we will see the azeotrope evaporates before the ethanol does.

This difficulty led some early scientists to believe that azeotropes were actually compounds of their constituents. This however proves not to be true as a compounds’ molecular composition cannot be affected by variations in pressure. Also a more obvious point is that the molar ratio of azeotropic constituents is not generally a ratio of small integers. A perfect example of an actual compound is carbon dioxide (2 moles of oxygen per mole of carbon) which stays in this molecular ratio whatever the pressure under which it is looked at. The fact that azeotropic composition can be affected by pressure suggests a mean by which such a mixture can be separated. (More on that later).

If we now want to get a purer solution of ethanol, we need to find a way to get around the azeotropic point. This is where a process called Azeotropic distillation comes into the equation. In chemical engineering this process refers to the specific technique of adding another component to the azeotrope, generating a new, lower-boiling azeotrope that is heterogeneous. The addition of a separation agent, often referred to as an entrainer, is what helps us get an ethanol solution which is purer than 96%.

We use the word heterogeneous distillation to describe the fact that the entrainer will form an azeotrope with one or more of the components of the initial azeotropic mixture, enabling separation of the remaining constituents by a distillation process. It compares to homogenous distillation whereby the entrainer is completely miscible with the liquid. Depending on whether a single component or all the components of an azeotrope need to be separated, continuous distillation is carried out, until the desired constituent is achieved.

For the case of a water/ethanol azeotrope, we use benzene or cyclohexane as an entrainer. The addition of this third party component changes the molecular interactions between water and ethanol and eliminates the binary azeotrope to form a ternary azeotrope. With cyclohexane as an entrainer the ternary azeotrope is 7% water, 17% ethanol, and 76% cyclohexane. It boils at a temperature of 62.1°C. The addition of the cyclohexane engages all of the water in the tertiary azeotrope so that when the mixture is boiled, the azeotrope vaporizes, leaving a residue composed almost entirely of the excess ethanol.

The downside of using a third party component such as benzene is that the pure ethanol extract is no longer suitable for consumption, as small amounts of benzene used remain in the solution and the latter is known to be carcinogenic. At this stage it is usually used as a gasoline additive.

There are alternatives to azeotropic distillation, such as pressure swing distillation which relies on the pressure dependence of an azeotrope. Composition of azeotropes differs substantially between high and low pressures. The ethanol content of the mixture is greater at higher pressures and thus alternating between low pressure distillation and high pressure distillation can purify our mixture. Another way of separating our mixture is through the addition of calcium oxide. This compound reacts with water to form a non volatile compound, calcium hydroxide, which can be separated by filtration. The filtrate can then be redistilled to obtain pure ethanol.

Finally a method closely related to azeotropic distillation called extractive distillation can be used to separate our mixture. This time the entrainer is less volatile than any of the azeotrope’s constituents and thus does not form an azeotropic mixture with the other constituents. It is used for mixtures which have a low value of relative volatility. The solvent interacts differently with the components of the mixture thereby causing their relative volatilities to change. This enables the new three-part mixture to be separated by normal distillation. Ethanol is now obtained as a top product.