distillation, process.

distillation, process. Distillation is a method of separating chemical components of a mixture into fractions based on their relative volatility, most commonly achieved by the application and removal of heat. A substance’s volatility is related to its boiling point, but not precisely equivalent. In engineering, the pertinent physical property of a chemical is termed vapor pressure, which is defined as the pressure that it exerts at a given temperature when both liquid and vapor phases are present.

To better understand the practical meaning of this technical term, picture a perfectly sealed and insulated container, partially filled with only water and nothing else. At room temperature most of the water molecules will be in liquid form, sloshing around at the bottom of the container, but some will have enough energy to escape the surface of the liquid and bounce around in the empty space above it. The individual molecules will be constantly exchanging places, some careering into the surface of the liquid to be buried deep while knocking others free from their surrounding fellows in the process, but the total number of molecules in the liquid space and molecules in the vapor space will stay relatively constant.

The relationship between liquid and gas becomes more interesting and complex when the liquid is made up of more than one component. Consider an equal mixture of two different components, such as ethanol and water, where one has a low vapor pressure and the other has a high vapor pressure. See ethanol. Although the total number of molecules is the same, the relative concentrations are different in the vapor space and the liquid space. The higher vapor pressure component (ethanol) dominates the vapor space, while the lower vapor pressure component (water) dominates the liquid.

If the mixture is unequal, that will also impact the relative concentrations in the vapor space. If there is not very much alcohol present in the ethanol/water mixture, then it will not matter how high its vapor pressure is; both the liquid and the vapor space will still be dominated by the component that is present in greatest abundance.

Understanding how vapor pressure, temperature, and concentration allow us to predict the gas and liquid makeup of a mixture is the foundation of distillation competency. The process of distillation generally manipulates temperature (and sometimes pressure) in order to physically separate components of a mixture based on their relative volatility.

Consider a sample that begins as a 50/50 mixture of two substances that have different volatilities (such as ethanol and water). When heat is applied to that mixture, the more volatile substance will vaporize more quickly, and its concentration in the vapor space will be higher than the less volatile substance. A still provides a way for this vapor to escape the pot to where the heat can be removed, so that the vapor cools and condenses back into a liquid and can be collected, or a cool surface within the pot where the vapor can condense and be drawn off.

The liquid that started as a 50/50 mixture has now been separated into two containers, one that contains mostly the less volatile component and one that contains most of the more volatile component. After this single distillation, the substances have been at least partially separated from one another. This is an example of a batch distillation in a pot still, where one single batch of liquid was placed into the pot still, then the distillation was conducted to produce a single batch of distillate. See still, pot.

A column still acts like a series of pot stills placed on top of each other, with the liquids vaporizing and condensing over and over again on each tray inside the column. This setup allows column stills to achieve greater distillation efficiency. See still, continuous.

Distillation is a versatile and simple means of separation, but its applicability is limited. It does not work well for separating components whose vapor pressures are very similar. It is not efficient for separating components whose vapor pressures are so low that it takes an excessive amount of energy to vaporize them (perhaps even so much energy that it damages the component) or whose vapor pressures are so high that it takes a lot of energy to get them to condense into liquid. Some molecules also have a tendency to be attracted to one another, and the more volatile molecule can sometimes “carry” the less volatile one up into the vapor space, making it difficult to separate them by distillation. In these cases, other means of physical separation might be more appropriate, such as filtration, absorption, adsorption, membrane separation, or extraction.

Kiss, Anton Alexandru. Advanced Distillation Technologies: Design, Control and Applications. New York: John Wiley & Sons, 2013.

McCabe, Warren L., Julian C. Smith, and Peter Harriott. Unit Operations of Chemical Engineering. Boston: McGraw-Hill, 2005.

By: Nicole Austin