Fractional Distillation Of Crude Oil: Surprisingly Simple

fractional distillation of crude oilWhile the process of refining crude oil is nowhere near as complex as the process for harnessing nuclear energy, it is still quite ingenious and offers an inside look at one of our most controversial, essential commodities. The first step in refining crude oil is known as fractional distillation.

We’ll look at the refining process in a broader view and then break down the steps involved in fractional distillation. If you want to see the bigger picture involved in oil, get a new perspective with this course on exploring the complexities of globalization and how it shapes the economic landscape.

The Refining Process

Crude oil, also known as petroleum, is a mixture of hundreds of valuable and worthless compounds. It is the refining refining process that not only separates these compounds, but even distinguishes different types and grades of valuable substances.

  • Hydrocarbons

The most important substances in crude oil are hydrocarbons (as the name suggests, they are molecules made of hydrogen and carbon atoms). Hydrocarbons are particularly valuable because they are rich in energy (obviously) and exist in diverse forms. From methane to tar, there’s a hydrocarbon for every application. Hydrocarbons are differentiated by their chain length, which refers to the number of carbon atoms per molecule, which in the case of crude oil range from 1 to 70+. The smaller the chain length, the lower the boiling temperature, which is of the utmost importance. Gain a truly elemental understanding of these chemical properties with this awesome online class on chemistry of the elements.

  • Chemicals And Blending

The first thing that happens to crude oil is that it is fractionally distilled, although modern technology has caused a shift to chemical distillation, by using high-tech chemical processes and reactions to separate the different grades of hydrocarbons (although in some cases they want to combine hydrocarbons, a process known as unification). But I’ll save the rest of the post for the distillation process. Once distillation is completed, the compounds are treated to eliminate impurities and then blended into a final product, whether it be gasoline, jet fuel or simple lubricants.

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Fractional Distillation: The Basics

As I mentioned above, crude oil is a mixture of different compounds. These compounds all react differently; their chemical and even physical properties are different. These differences are precisely what allow us to distill oil, however there is one difference in particular that we are interested in: boiling temperatures. Boiling temperatures are what allow fractional distillation to be both relatively easy and affordable. Even substances that have extremely similar boiling temperatures can be accurately separated by fractional distillation.

A Step-By-Step Process

1. Heating

Naturally, to boil a substance, you need to heat it. The different compounds in crude oil boil at extremely high temperatures, and because of the high amounts of potential energy stored in these compounds, it is important to heat them consistently, safely and accurately. This is accomplished via high pressure steam, which is heated to over 1100 degrees Fahrenheit (660 C). At first, it would make sense to heat crude oil slowly, to allow the different compounds to boil at different temperatures. But it turns out that boiling temperatures only come into effect after the oil is boiled.

2. Boiling

Once the crude oil boils, the majority of the substances begin to turn to vapor, but some remain liquid. Substances that do not boil at 1100F include tar, asphalt, waxes, coke, etc. Substances will very low boiling points include gas (65F), gasoline (155F) and kerosene (200F), while things like lubricant-level oil and heavy gas boil above 600F.

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3. Fractional Columns

The substances that boil into vapor rise into a large, tall column (such as those seen in the picture above). This is the fractional distillation column, where all the magic takes place. There are a series of collecting plates that are located at various heights within the column. The plates are designed to attract certain compounds, allowing those with lower boiling temperatures (yes, lower, but more on this soon) to ascend higher in the column. This maximizes the purity and volume of substance collection.

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4. Rising . . . Cooling

As the vapor rises, it cools. The distillation column is not designed such that it sits directly above a boiling vat of oil. This would not provide accurate results as the heat would rise and it would continue to heat the oil as it rose up the column. Instead, the crude oil is first fed into an enclosed steam boiler. Vapors are then transported to the distillation column via piping, thereby allowing the vapors to cool predictably.

5. Condensing

Like I said under “Fractional Columns,” substances with lower boiling temperatures rise higher within the column. This is because the vapors cool as they rise and condense, or turn into liquids. The substances that switch between liquid and gas states at high temperatures will make this transition very quickly, turning back into liquids at the bottom of the column. Other substances, such as gasoline, will cool as they rise and therefore reach a much higher height. The relationship between boiling point and column height is inversely related: the lower the boiling point, the higher in the column it will rise.

6. Collection

As planned, the collection trays trap the liquids with incredible accuracy. Often liquids that are still very hot will be transported to tanks for further cooling. Then it’s off to be blended, stored or chemically processed.

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Life After Fractional Distillation

Chemical processing is starting to replace fractional distillation because it yields higher percentages of usable compounds per barrel of oil. As expected, this process is slightly more complex than fractional distillation and I don’t have time to go into it now. The treating and blending processes that follow fractional distillation (or chemical distillation, for that matter), however, can be discussed more briefly.

Before blending can take place, distilled compounds are treated to eliminate impurities. These vary from compound and compound, but the usual suspects are water, oxygen, nitrogen, sulfur and metals. Water is typically removed in a very old-school process, using relatively simple methods of absorption. Sulfur is treated more chemically, usually by way of specially designed sulfide compounds that collect and bind to sulfur. Most of the rest of the undesirables – nitrogen, oxygen, etc. – are passed through a sulfuric acid column, which speaks for itself.

The final step is perhaps the simplest of all: rigorous blending of pure compounds to make the final products we buy, from gasoline to lubricants to jet fuel. If you’re studying for Regents in chemistry are are trying to brush up on some basic knowledge, check out this five-star, topic-by-topic review course on Regents Chemistry.