Video: Describing Features of Fractional Distillation

Which of the following statements is true? [A] Fractional distillation involves a chemical reaction. [B] Fractional distillation involves the separation of compounds using a pressure gradient. [C] Most fractions of crude oil are liquids at room temperature. [D] All fractions of crude oil are used as fuels. [E] Fractional distillation involves the separation of crude oil into pure compounds.

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Video Transcript

Which of the following statements is true? A) Fractional distillation involves a chemical reaction. B) Fractional distillation involves the separation of compounds using a pressure gradient. C) Most fractions of crude oil are liquids at room temperature. D) All fractions of crude oil are used as fuels. Or E) Fractional distillation involves the separation of crude oil into pure compounds.

The question asks, which of the statements is true? We expect from this one true statement and four false statement. Since the question contains many references to crude oil and fractional distillation, let’s have a quick recap of the process involved. This is a schematic of the equipment necessary to achieve fractional distillation of crude oil on an industrial scale. Crude oil extracted from the ground is passed into a furnace. In the furnace, it is warm to about 400 degrees Celsius. From there, it passes into the bottom of the fractional distillation column.

Crude oil is a mixture of many types of hydrocarbon including chains, rings, and aromatics. Any of the longer hydrocarbons with a boiling point above 400 degrees Celsius will remain a liquid. This residue contains compounds with 20 or more carbon atoms. This residue can be passed to another fractionating column which uses even higher temperatures to separate the fractions. These then go on to be used as fuel oil, tar, and in other applications. Meanwhile, at the bottom of the fractionating column, steam is injected in order to maintain the high temperature.

This means that the majority of the crude oil is in the gas phase and rises up the column. The vapours are forced through bubble caps which mix the vapour with condensing liquid. This helps to improve the separation. This leads us to our next fraction. Our next condensing fraction is lubricating oil, containing molecules with boiling points between 300 and 370 degrees Celsius. This fraction contains molecules with 20 to 50 carbon atoms. The molecules in the lubricating oil will likely have different structures to those in the residue. So even though they have the same number of carbon atoms they have different boiling points. As we move up the fractionating column it gets colder. So the lubricating oil condenses, while other fractions continue to rise.

The next fraction is diesel. It contains molecules that condense at between 250 and 350 degrees Celsius. There is some overlap between the boiling point ranges of some of the fractions. This is because perfect separation is impractical. And vapours from one section can rise up and mix with the next. And liquid can overspill and mix with the fraction below. This isn’t a problem unless it affects the final application of the fraction. The diesel fraction largely contains alkanes with 12 or more carbon atoms. The next fraction following diesel is kerosene containing molecules with boiling points between 175 and 325 degrees Celsius. Kerosene typically contains hydrocarbons with 10 to 18 carbon atoms, both alkanes and aromatics.

Next is the fraction that you’re probably most familiar with, gasoline, the most popular automobile fuel. Gasoline contains molecules with boiling points between 40 and 205 degrees Celsius. It contains hydrocarbons with five to 12 carbon atoms, both alkanes and cycloalkanes. The last liquid fraction is naphtha containing molecules with boiling points between [60] and 100 degrees Celsius. This fraction contains hydrocarbons with five to nine carbon atoms which are all alkanes. Molecules in naphtha are generally reprocessed to form molecules more ideal for gasoline.

The last fraction from the fractional distillation of crude oil is petroleum gas, containing molecules with boiling points at or below 40 degrees Celsius. Petroleum gas contains alkanes with between one and four carbon atoms, substances like methane, ethane, propane, and butane. So the fractionating column maintains a temperature of about 400 at the bottom and about 40 at the top. This means there is a gradient in temperature between the bottom and the top. This is what allows the various fractions to be separated by their boiling points.

In industrial settings, these fractionating columns are many storeys tall. Now that we’ve reviewed fractional distillation of crude oil, we should look at the statements one by one and determine which is true and which are false. The first statement is that fractional distillation involves a chemical reaction. This statement isn’t specifically about fractional distillation of crude oil. However, the principles of fractional distillation do not change from application to application.

Statement A is false, since we know that the only thing done to crude oil during fractional distillation is heating. Heating allows the fractions of crude oil to evaporate and then condense at their boiling points. None of the components of crude oil undergo any reaction. So this is an incorrect answer. The next statement is that fractional distillation involves the separation of compounds using a pressure gradient. This statement is false. Instead, fractional distillation involves heating and establishing a temperature gradient. Therefore, this is an incorrect answer.

Statement C is that most fractions of crude oil are liquids at room temperature. One fraction produced by the fractional distillation of crude oil is petroleum gas which is definitely a gas at room temperature. However, naphtha, gasoline, kerosene, diesel, and lubricating oil are all liquids. They all contain compounds with boiling points above room temperature. Even the residue produced by crude oil contains liquids. Therefore, it’s true that most fractions of crude oil are liquids at room temperature.

To be on the safe side, I’m going to look at the last two statements. The fourth statement is that all fractions of crude oil are used as fuels. All we need to do to prove this false is find one fraction that is not used as a fuel. The heavier fractions of the residue are used to make road material, not fuels. Even though not shown on the diagram, this is considered a separate fraction of crude oil. Therefore, this answer is not a correct answer.

Finally, fractional distillation involves the separation of crude oil into pure compounds. True or false? This statement is most definitely false. As we’ve shown, fractional distillation of crude oil produces many fractions, all of which are mixtures of various hydrocarbons with various boiling points. So this statement is not a correct answer. Of the statements given, the only one that is true is that most fractions of crude oil are liquids at room temperature.

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