In this blog post, we’ll take a simple look at the principles of fractional distillation columns—which separate gasoline from crude oil—and the basic concepts of distillation.
You’ve probably seen headlines like “Gasoline Prices Rise for Third Consecutive Week” or “Crude Oil at ○○ Dollars per Barrel.” As you can see, “gasoline” and “crude oil” are terms we encounter frequently in our daily lives. But what exactly is the difference between gasoline and crude oil? People often use these terms interchangeably without making a clear distinction. However, they are distinctly different substances. Crude oil is a mixture of various components, and gasoline is one of the components contained within it. In this article, I’d like to explain the principle behind the fractional distillation column, which separates gasoline from crude oil.
Did you know that crude oil is naturally black? Gasoline, on the other hand, is clear. The reason a clear product can be produced from naturally black crude oil is precisely because of the principle of “distillation.”
Now, let’s find out what distillation is. In our daily lives, we can easily find various mixtures, ranging from the beverages we drink to the air we need to survive. But how can we isolate only the desired substance from these mixtures? While we can use our eyes to find a black pen among several pens, how can we separate substances that are mixed so uniformly they are indistinguishable from one another—such as sugar dissolved in water? The process of separating a desired substance from a mixture in this way is called “separation.” And the method of separating a liquid mixture by utilizing the differences in boiling points of its components is called “distillation.”
Now that we’ve learned what distillation is, let’s take a closer look at the principles of a distillation column. A distillation column is, quite literally, a piece of equipment that separates mixtures using the principle of distillation—that is, by exploiting differences in boiling points. There are distillation columns designed to separate products after a reaction, as well as fractional distillation columns that separate individual components from mixtures containing multiple components, such as crude oil. To aid understanding, we’ll use the fractional distillation column—which separates crude oil, a substance we’re all very familiar with—as an example.
Crude oil may appear to be a single liquid, but it is actually a mixture of various compounds. Hydrocarbons are among the primary compounds that make up crude oil. Hydrocarbons are compounds consisting of carbon and hydrogen. The type of hydrocarbon varies depending on the number of carbon atoms—for example, methane has one carbon atom, ethane has two, and propane has three. Since different hydrocarbons have different structures, their boiling points also differ. Generally, as the number of carbon atoms increases, the boiling point rises. This can be thought of in a similar way to how a single ball moves easily even with a gentle breeze, but when several balls are connected to each other, a stronger force is required to move them. In this way, we use the differences in boiling points based on the number of carbon atoms to separate the large mixture known as crude oil into various types of compounds.
Now, let’s return to the distillation column. First, crude oil is heated and fed into the bottom of the distillation column. As a result, the bottom of the column maintains the highest temperature, while the temperature gradually decreases as it rises upward. The heated crude oil rises in a gaseous state and condenses back into a liquid when the temperature at each height drops sufficiently. This phenomenon, in which a gas cools and turns into a liquid, is called “condensation.” For example, the phenomenon of water droplets forming on the surface of a cold cup occurs because water vapor in the air comes into contact with the cold surface and liquefies. Based on this principle, components that liquefy within a specific temperature range are separated at each level. At the very top of the distillation column, liquefied petroleum gas (LPG), which has the lowest boiling point, is separated, and just below that, gasoline, which has a slightly higher boiling point than LPG, is separated.
Thus, in a distillation column, components with higher boiling points and a greater number of carbon atoms are separated as you move down the column. Thanks to this principle, the heavy, black hydrocarbon components are separated from gasoline, which has a relatively smaller number of carbon atoms, allowing the gasoline to retain its characteristic transparent color.
Ultimately, distillation can be defined as the process of separating a mixture by utilizing the differences in boiling points of its individual components. Do you understand what distillation is now? In the future, whenever you come across various liquid mixtures—not just when filling up with gasoline—please take a moment to recall this blog post and reflect on the principles of distillation.
