Chemistry of Petroleum 4: Treatment of Hydrocarbons

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Chemistry of Petroleum 4: Treatment of Hydrocarbons Photo Credit: Clipart.com


To introduce how petroleum fractions are chemically treated in the refinery process to produce useful materials.


This lesson is part of the Energy in a High-Tech World Project, which examines the science behind energy. Energy in a High-Tech World is developed by AAAS and funded by the American Petroleum Institute. For more lessons, activities, and interactives that take a closer look at the science behind energy, be sure to check out the Energy in a High-Tech World Project page.

This lesson is the fourth in a series of lessons about the chemistry of petroleum that are intended for upper-level chemistry students in the 11th and 12th grades. You should be an experienced chemistry teacher to teach these lessons. The goal of these lessons is to introduce high-school students to the use of oil as an energy source in today's high-tech world. In the Chemistry of Petroleum 1: What are Hydrocarbons? students will explore hydrocarbons, the molecular basis of petroleum, and learn to distinguish between organic and inorganic compounds.

In the Chemistry of Petroleum 2: What Happens to Hydrocarbons When They Burn? students will examine the varying amounts of energy produced by the combustion of different hydrocarbons.

In the Chemistry of Petroleum 3: Distillation of Hydrocarbons, students will be introduced to the distillation processes by which petroleum is refined to produce useful petroleum fractions.

The Chemistry of Petroleum 4: Treatment of Hydrocarbons will help students explore the chemical treatment processes by which distilled petroleum fractions are converted to produce useful fuel oils.

This particular lesson is a continuation of lesson 3, which taken together complete the crude oil refining process. Lesson 3 concludes with the crude oil being separated into petroleum fractions, based on the relative boiling points of the hydrocarbons in the original crude oil. In this lesson, students explore how those petroleum fractions are treated chemically to produce useful fuels that are familiar to them, such as diesel, jet fuel, and gasoline.

As students explore the interactive, they may be overwhelmed with the amount of terminology and reactions associated with the treatment process. Students do not need to know or memorize the details of these reactions; rather, they should achieve a general understanding that a series of chemical reactions are important in producing the fuels that are used on an everyday basis.

As in lesson 3, this activity provides an opportunity to clarify the common misconception that crude oil is composed of individual molecules of diesel, kerosene, and the other refined products. By the end of these lessons, students should understand that there is no single molecule known as diesel, or kerosene, or light gases, with a specific chemical formula, like H2O. Fuel oils, like diesel, are a mixture of different molecules that share similar properties. The two main properties shared in a petroleum fraction are the number of carbons in a molecule and the boiling point.

In order for students to do this lesson, as well as the other lessons in this series, they need to have prerequisite knowledge of the basics of atoms and their structure. Basic information about atoms can be found at The Atom. Students also should know basic organic chemistry, including an understanding of what hydrocarbons are and how they are named (i.e., nomenclature).

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Begin by asking students: "We know that petroleum is used to make gasoline. But what other products are made from petroleum?" Ask students to brainstorm in groups, each group generating a list of at least ten items that they have seen or used in the past 24 hours that are petroleum products. A list of items can be found at: A Few Products Made from Petroleum.

As student groups share their product items, generate a class list. Point out that these items and many more are all made with fuels derived from crude oil or petroleum. Ask students:

  • How do you think our use of petroleum products has changed over time?
      (Encourage students to discuss that our use of petroleum products and the customer demand for petroleum-based products has increased.)
  • Given the high customer demand and the fact that crude oil is a limited resource, how do you think refineries make the most out of the crude oil they have?
      (Allow students to brainstorm ideas about how refineries make efficient use of crude oil. Students may suggest that refineries are efficient or find ways to use all the hydrocarbons of crude oil.)


Review with students the first stages of crude oil refinement and treatment by making a flowchart. At the beginning of the flowchart, write the words "crude oil." At the end of the flowchart, place the list of products generated by the students. Begin to fill in the flowchart by asking students what happens to crude oil once it is located in the earth. Next steps should include extraction and taking the crude oil to a petroleum refinery. Once at the refinery, the crude oil undergoes the distillation process. The results of distillation are petroleum fractions. Ask students, "Describe what has happened to the hydrocarbons in the distillation process."

Now students should review the Oil Refining: A Closer Look interactive by watching the introductory video and going through the distillation process. The movie has an accompanying script that you can print out. The narrator speaks rapidly, so it is a good idea to read the script after watching the movie the first time. After reading the script, students can watch the movie a second time, which will facilitate understanding.

Ask students:

  • What chemical property was used to separate the hydrocarbons in the distillation process?
      (It was the boiling point.)
  • Based on size, where are the smallest hydrocarbons after distillation?
      (They are at the top of the distillation column.)
  • Where are the largest hydrocarbons?
      (They are at the bottom of the distillation column.)

Go through the seven fractions, emphasizing that the fractions are composed of a mixture of hydrocarbons, and are not composed of a single molecule. For example, kerosene is a mixture of hydrocarbon molecules from the original crude oil that contain between 10-16 carbons. This means that there are hydrocarbons in the kerosene fraction that contain 10 carbons and others that contain 11, and so on through 16.

Point out that through the distillation process, the hydrocarbons have not been altered or modified in any way; they have simply been separated by size using the chemical property of boiling point differences. This is a physical change. On the flowchart after distillation and before the students' list of everyday products and oils, write the word "treatment." Define for students that treatment is the series of chemical reactions and technologies to modify the distilled fractions to make useful and marketable oils.

Allow students to explore the treatment section of the interactive on their own using the Treatment of Hydrocarbons student esheet. Students can answer the questions using the Treatment of Hydrocarbons student sheet. Answers are available on the Treatment of Hydrocarbons teacher sheet. Review the questions together as a class. Through discussion and review of the guiding questions, explain that there are three different treatment processes, in general. These are:

  1. Removing impurities in the hydrocarbons, such as metals and sulfur
  2. Breaking down large, heavy hydrocarbons
  3. Reshaping or building hydrocarbons from smaller ones

The goal of these processes is to make useful products, such as diesel, gasoline, jet fuel, and asphalt. In the end, these final products are not individual molecules, but blends of treated hydrocarbons of various lengths and structures.

Go through examples of each of the three processes. Begin with the first process (Removing impurities in the hydrocarbons) by asking students:

  • What are the two processes that are used to remove sulfur?
      (They are amine sweetening and hydrotreating.)
  • Why might it be important to remove sulfur from the hydrocarbons?
      (Encourage students to think of different ideas. Emphasize that impurities such as sulfur can damage equipment in the refinery and the quality of the products. There are also legal limits set by the Environmental Protection Agency on the amount of sulfur that can stay in the products.)
  • What do you think of when you hear the term "hydro"?
      (Answers will vary, but some students should mention hydrogen.)

Tell students that the general idea behind hydrotreating is to add hydrogen gas at high pressure and temperature to the petroleum fractions. The oil and hydrogen then enter a reactor loaded with a catalyst.

Ask students:

  • What is a catalyst?
      (It is something that changes the rate of reaction and is not permanently changed in the reaction.)
  • Would the reaction still happen without the catalyst?
      (Yes, but it would take much longer.)

In hydrotreating, a metal surface is usually used as a catalyst. These metals include palladium, nickel, platinum, cobalt, and iron. Draw a general reaction formula for students to understand hydrotreating.

S + H2 (g)           →           H2S (g)
(Catalyst, heat, and pressure)

If possible, project the treatment page of the interactive for the class. Click on one of the fuels and then on the hydrotreating box  to show students an example of one of the types of reactions that happen in the hydrotreating unit. Solid sulfur is converted to hydrogen sulfide gas, which is then removed.

S8 (s) + 8H2 (g)           →           8H2S (g)
(Catalyst, heat, and pressure)

Tell students that the hydrotreating process also removes nitrogen compounds, which if not removed, would react with the oxygen in the air and produce nitric acid (HNO3).

Begin describing an example of the second process (Breaking down large, heavy hydrocarbons) by asking students:

  • Do large hydrocarbons have high or low boiling points? Why?
      (They have high boiling points due to factors such as a heavier molecular weight, hydrogen bonding, and the amount of branching.)

Write the formula for docosane on the board: C22H46, and tell students that this large hydrocarbon is found in the gas oil fraction after distillation. The gas oil fraction is composed of a mixture of hydrocarbons with 20–70 carbons. If possible, project the treatment page of the interactive in front of the classroom and click on the red circle next to "gas oil." Then click on the throbbing yellow circle in the gas oil pipe to display the chemical structure of docosane. Ask students:

  • What is the first treatment process for the gas oil fraction?
      (It is hydrotreating.)
  • What is the goal of hydrotreating?
      (The goal is to remove impurities.)
  • What are the other two treatment processes that gas oil undergoes after hydrotreating?
      (It undergoes hydrocracking and catalytic cracking.)

Tell students that cracking processes break down heavy hydrocarbons (you may want to use the term large hydrocarbon molecules) into lighter products (smaller hydrocarbon molecules). Different types of cracking processes are used. Focus on catalytic cracking, telling students that this process uses high heat and a catalyst to break larger molecules into smaller, more useful ones. Draw the following reaction to show students how catalytic cracking works:

C22H46 + heat + catalyst           →           C12H26 + C6H12 + C4H8

The catalyst used is often silica or alumina. In the above reaction, docosane is converted to dodecane, cyclohexane, and butene. The smaller hexane and butene can be combined with other smaller compounds to make gasoline stock. The larger dodecane is a component of diesel stock, which can be used to fuel ships and factories.

Describe an example of the third process (Reshaping or building hydrocarbons from smaller ones) by clicking the red circle next to "light naphtha" on the treatment page of the interactive. Doing so will show two treatment processes: hydrotreating and isomerization. Ask students:

  • Are the hydrocarbons in light naphtha large or small relatively?
      (They are small.)

Tell students that light naphtha is a mixture of mostly pentane and hexane. Have students draw the chemical structures of pentane and hexane. If possible, provide models of each structure. Ask students:

  • What is an isomer?
      (It is a compound with the same molecular formula as another, but with a different structure.)
  • What do you think happens in the isomerization process in the refinery?
      (The hydrocarbons are made into isomers.)
  • If we start with hexane and pentane, after isomerization, will the chemical formulas of the products change?

Write the following reaction on the board:

C5H12           →           C5H12

Tell students that this is the reaction for pentane to isopentane. Ask students to draw the chemical structure of isopentane. Using the model for pentane, remove the fifth carbon from the chain and move it to the second carbon. This new structure is isopentane (also called methylbutane or 2-methylbutane), which has the same chemical formula as straight-chained pentane. Tell students that this isomerization is important in the petroleum treatment process because isomers improve the quality of gasoline as opposed to the straight-chained molecules. In fact, isomers improve the octane rating for gasoline.


Have students work in groups to choose one of the end products of the crude oil distillation and treatment process: diesel, kerosene, gasoline, jet fuel, or asphalt. Ask each group to develop a presentation that discusses how their final product is formed. As they are doing this exercise, they should be sure to classify each step as a physical or chemical change. Their information should draw upon all aspects of the interactive, including the movie. Their presentations should begin with the tiny sea plants living millions of years ago and move through the process of forming kerogen, and ultimately petroleum. Remind students to include drilling before moving into the distillation and treatment process. Students should include a flow chart in their presentations. Encourage students to use different methods of displaying their information. For example, students may choose to present a storyboard, comic book, PowerPoint, essay, or a short film. Assess students based on their holistic understanding of how useful gas oils are made from crude oil through a series of important chemical processes.


Have students explore what octane ratings for gasoline signify about the product. There are a number of misconceptions that students may have. After a lesson, encourage students to survey their parents, teachers, and other drivers about what they know about octane ratings of gasoline. You and your students can review Discovering the Science of Petrol to learn more about octane.

Students can explore how regulations and technology are helping to decrease the production of acid rain at Acid Rain Students Site: What Is Being Done?

Funder Info
American Petroleum Institute
This content was created with support from the American Petroleum Institute.

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Lesson Details

Grades Themes Type Project 2061 Benchmarks National Science Standards State Standards