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We know how to make spirits (we discussed that here), and we know that to make spirits (or liquors) you need to distill fermented grains. Distillation isn’t just used in spirits production though. It is one of any chemical engineer’s favorite production processes, great for separating all sorts of components from one another.
The oil & gas industry couldn’t do without, nor could the pharmaceutical industry, and, of course, liquor manufacturers. Distilling is such a fundamental process in so many areas that it warrants its own explanation and deep dive on the topic.
Why distillation? – Separation
Before even diving into how distilling works, you need to know why you would want to distill. Distillation is one of many separation processes, that is, a process to separate different types of molecules from another. Another example of a separation process is filtration: you use a filter to separate particles (e.g. ground coffee) from the liquid (hot water that has seeped through this coffee). In the case of distillation you want to separate two liquids (or gases) from one another, especially those that mix well and wouldn’t just separate out by themselves (like oil and water would). During distillation no chemical reactions occur, all that happens is separation. Nevertheless, it is vital in so many processes.
What happens during distillation
When you distill a mixture of two liquids (we’ll keep it to two liquids) for simplicity sake) you heat the mixture in order to evaporate one off. You then capture that evaporated component in another part of your equipment, the condenser. In other words, you use their differences in boiling point to separate them from one another. However, you will almost never get a complete separation as easily as that. Often, you need several steps and even then you won’t be able to separate them completely (as is the case for water & alcohol).
Some theory – Moisture/gas equilibria
To properly explain how distillation works we need to cover some thermodynamics theory first.
Imagine you have a closed container of water at room temperature. Almost all of your water will be liquid. However, even at room temperature, some of that water will be present as a gas, floating above the liquid. Even though you can’t see this. This is governed by thermodynamic phenomena.
A good way to visualize this is to take that container and place it next to a heater in an otherwise cold room. You might notice that some water droplets form on the bottom of the lid of that container. Some of the gas touches the cold cover which caused it to condense. It is also the reason why over time, if you leave off the lid from your container, the container will become dry, all the water will have evaporated and disappeared.
The warmer the temperature in that container the more of that liquid will be a gas and float above the water. When you heat it even further up to the boiling point all of the liquid will start to evaporate and transition into a gas.
More theory – Raoult’s law
But, in distillation we don’t have one component, we have a mixture of at least two compounds. In an ideal case Raoult’s law describes what the composition of that as floating above the liquid is. Because, in most cases, that case will be a mixture of the two different molecules! Raoult’s law describes a way to calculate the % of liquid vs. gas molecules for each component in the mixture. When you know the fraction of each of the two components the original liquid mixture (e.g. 50:50), you can use this law to calculate the composition of the gas floating above that mixture. An important concept of this law is that the composition of the gas above a mixture will more often than not not be the same as the composition of the liquid. not be pure or exist of just one component. Also, it explains that differences in boiling point will lead to a difference in ease of evaporation.
An alcohol & water mixture
So now let’s look back at our mixture of two liquids. We’ll use a common example in the drinks industry: alcohol (ethanol) and water. Water has a boiling point of 100C (212F) whereas alcohol has a boiling point of only approx. 78C (172F). Because the alcohol has a lower boiling point it evaporates more easily. As a result, Raoult’s law will tell you that the gas above the mixture will contain a higher percentage of alcohol than the liquid mixture does. During distillation we use this phenomenon to separate our two liquids.
Heating alcohol & water
During distillation you heat up your mixture of liquids (water + alcohol). As a result, part of the liquids will evaporate. This gas is a mixture of the two components but will contain more of the component with the lowest boiling point (alcohol in our case).
This gas then moves to another area, the condenser. This zone is colder and as a result the gas returns back into a liquid. This new liquid will now contain considerably more alcohol than the original liquid did. This is the simplest one-step distillation process.
Multi stage distillation
In most cases though a one step separation will not be good enough. In the case of spirits, we might not yet have enough alcohol in there and the water content may still be too high. Therefore, a lot of distillation processes consist of several steps or stages. In each stage you get a bit more of the component with a lower boiling point.
In between these stages you might condense your mixture, but in most cases the stages are separated by different ‘trays’ in a large distillation tower. The trays are stacked above one another and at each tray level there is a slightly different temperature. It is hottest at the bottom and coldest at the top. Gases travel up through the trays whereas liquids travel down. Since at each tray the temperature is slightly different the balance of ethanol:water will change with each tray. At the top of the tower you will have the highest concentration of alcohol whereas at the bottom there is most water. In large industrial complexes this is a non-stop continuous process with constantly renewing of original mixture and tapping of off the rest streams.
Katzen, R., et al., Ethanol distillation: the fundamentals, chapter 18, link; for those who would really like to understand ethanol/water distillation
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Raoult’s law for distillation, link