In this blog post, we’ll take a natural look at the principles, process, and limitations of atmospheric distillation—a method of refining crude oil by utilizing differences in boiling points.
What Is Atmospheric Distillation?
Whether we realize it or not, petroleum is an integral part of daily life not only in South Korea but around the world. While we typically encounter petroleum directly at gas stations, it is actually used in countless fields—from textiles and plastics to the asphalt on our roads. Because petroleum is such a valuable resource, consumption remains at an enormous level even amid ongoing concerns about resource depletion, and its economic value is correspondingly immense.
However, even this useful resource is nothing more than crude oil—containing substances harmful to the human body—unless it is refined. This is similar to mixing all the side dishes on a dining table together. Yet, most people would find it hard to believe that simply boiling this mixture could separate the ingredients one by one according to their type. This is because, in everyday life, “boiling” usually refers to nothing more than making soup or boiling laundry.
But petroleum is different. With just a distillation tower, simply boiling crude oil is enough to refine it, separating it into a variety of high-value petroleum products. In this article, we’ll explore atmospheric distillation—the most common refining method used to transform crude oil into valuable resources.
The Principle of Separating Crude Oil Based on Boiling Point Differences
Atmospheric distillation is a method of refining crude oil through fractional distillation at 1 atmosphere, utilizing the differences in boiling points of each component. To understand this, we must first understand the principle of fractional distillation.
You’ve probably heard the story at least once from your childhood: if you were stranded on a deserted island, you could obtain drinking water by boiling seawater, collecting the resulting steam, and letting it cool. Fractional distillation works on a similar principle. For example, when a mixture of ethanol and water is heated, the ethanol—which has a lower boiling point—evaporates first, and then the water begins to boil. At this point, if the ethanol vapor produced first is collected separately and then cooled, the water and ethanol can be separated. This separation method is called fractional distillation.
A distillation column is the device that applies this principle to crude oil. When crude oil is heated, most of it turns into a gas in the heated section, regardless of the boiling points of its individual components. As the gas rises up through the distillation column, the temperature gradually decreases, and the components condense into liquid form in order, starting with those that have the highest boiling points.
Petroleum gas, which boils at the lowest temperature, rises all the way to the top, while components such as gasoline, kerosene, diesel, and heavy oil condense into liquid at their respective temperature ranges during the ascent and collect in their corresponding layers. During this process, fresh crude oil continues to be heated and supplied in gaseous form from below, so a variety of components coexist at the bottom of the distillation column.
Similarly, in the fraction where a specific component is separated, components with lower boiling points continue to rise from below and mix with it. For example, in the fraction where heavy oil is separated, diesel, kerosene, gasoline, and petroleum gas flow in together, and in the fraction where kerosene is separated, gasoline and petroleum gas flow in.
In contrast, the petroleum gas at the very top has the highest purity and is recovered first, as there are no components with a lower boiling point than it. The components in the other layers are a mixture of various substances, so separation continues until sufficient purity is achieved, after which they are recovered. Through this process, components are obtained in order from low-boiling-point petroleum gas to gasoline, kerosene, diesel, and heavy oil, while high-boiling-point components such as asphalt remain at the bottom of the distillation column.
Limitations of Atmospheric Distillation and Vacuum Distillation
Although crude oil can be refined to a considerable degree through atmospheric distillation alone, there is a limitation in that some components may not be sufficiently pure or may be of inferior quality. Typical examples are asphalt and residual oil, which remain at the bottom of the distillation column. Since these components have very high boiling points—over 300°C—heating them directly can cause undesirable chemical reactions, such as thermal cracking, making it difficult to process them using atmospheric distillation alone.
For this reason, vacuum distillation is used instead of atmospheric distillation to further separate these remaining components. Vacuum distillation utilizes the principle that as pressure decreases, the boiling point of a liquid also decreases. When external pressure is reduced, liquid molecules can more easily escape into the gaseous state, allowing distillation to occur at lower temperatures. Therefore, vacuum distillation enables a more precise separation of high-boiling-point components—which could not be separated by atmospheric distillation—while minimizing thermal cracking.
Why Atmospheric Distillation Remains Important
Although atmospheric distillation is a method with clear limitations, it is one of the most fundamental and important technologies in the crude oil refining process. Even in today’s refining processes, atmospheric distillation serves as the first key step, followed by vacuum distillation and various subsequent refining processes to produce a range of petroleum products.
The greatest advantage of atmospheric distillation is that it can efficiently separate a wide range of components—from petroleum gas to gasoline, kerosene, diesel, heavy oil, and residual oil—using the relatively simple principle of boiling point differences. Furthermore, since the process of simply heating crude oil can transform crude oil with low utility value into a variety of products used across industry and daily life, atmospheric distillation can be considered the starting point and a critically important technology for the modern oil refining industry.
