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Chemistry of Petroleum 3: Distillation of Hydrocarbons

What You Need

Materials

  • Clear drinking glasses, graduated cylinders, or beakers
  • Vegetable oil
  • Water
  • Shale rock sample (if available; a glazed tile can be used)
  • Thermometers
  • Boiling Point of Hydrocarbons Chart from Boiling Points and Structures of Hydrocarbons
 
Chemistry of Petroleum 3: Distillation of Hydrocarbons Photo Credit: Clipart.com

Purpose

To introduce how hydrocarbons in crude oil are distilled and treated in the refinery process to produce useful materials.


Context

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 third 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 this lesson, the Chemistry of Petroleum 3: Distillation of Hydrocarbons, students will be introduced to the distillation and treatment processes by which petroleum is refined to produce useful fuel oils.

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 provides the opportunity to address a number of misconceptions that students have about the physical world. Sometimes, terminology can inadvertently cause students to develop such misconceptions. The term “fossil fuel” can sometimes be misleading as it implies that crude oil is made of or derived from fossils. During the motivation section of this lesson, it may be helpful to clarify the difference between a true fossil and fossil fuels. Fossils are the preserved remains or traces of life forms from the past, often through the process of permineralization. Fossil fuels are produced from decayed organic matter that has changed chemically over geologic time due to high levels of heat and pressure. The term “fossil fuel” is used for substances such as coal, natural gas, and crude oil or petroleum, because the organic life forms lived millions of years ago.

Another misconception among students is that crude oil is composed of individual molecules of diesel, kerosene, and the other refined products. This misconception may emerge if students think of the refinery process as a sieve through which these products are sorted out from crude oil. The analogy of a sieve is a good one if students understand that the refined products are not individual molecules, but a collection of chemically similar molecules. In other words, there is no single molecule known as diesel, or kerosene, or light gases, with a specific chemical formula. In contrast, water is a molecule defined by a specific chemical formula of H2O. Diesel, on the other hand, is a mixture of different molecules that share the same properties. The chemical property that is largely utilized to sort the various hydrocarbons in crude oil is boiling point. The sieve in the refining process separates hydrocarbon molecules of similar size—and therefore, similar chemical properties. Specific collections of similarly sized hydrocarbon molecules are known as diesel, kerosene, light gases, and the other various refined products.

When watching Oil Refining: A Closer Look, it is helpful to also address that the composition and appearance of crude oil will vary depending upon its source. The interactive shows a thick, black, liquid substance; however, crude oil can be brown, yellow, or even green depending upon its chemical composition. It also can be found in a semi-solid form that does not flow easily until after it is heated or diluted.

Research shows that students have many misconceptions about heat, temperature, and phase changes such as boiling. (Benchmarks for Science Literacy, p. 337.) A simple misunderstanding that students may have is that a substance can change phases (i.e., from liquid to gas), while still remaining intact in its chemical composition. In the steam furnace animation in the distillation process, point out to students that the molecular structure of hydrocarbons does not change when they change phase from liquid to gas. It is also important to point out that this phase change is not a function of temperature alone. The boiling point of a liquid is determined not only by heat, but also by the atmospheric pressure around the liquid. In the Development section of the lesson, an opportunity is provided to discuss why water boils at a lower temperature at high altitudes. This will help to clarify the misunderstanding that boiling point is a function of a temperature reading.

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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Motivation

Begin by reviewing what crude oil is, how it is formed, and its relevance to human lives by having students use their Oil Refining student esheet to watch the introductory animation from Oil Refining: A Closer Look. Provide the Understanding Crude Oil student sheet to students as they watch the animation. They should answer the questions on this sheet.

Question 9 provides an opportunity to explain how petroleum rises to the surface over time. To help solidify this concept with students, have them do a quick activity with vegetable oil and water (you can do this as a demonstration if you prefer). They should first pour the vegetable oil into a clear glass, beaker, or cylinder. Ask students:

  • What happens when oil and water are combined?
      (They separate into two phases.)
  • Which phase will be on the top?
      (Allow students to suggest both.)

Now ask students to pour the water on top of the oil. The two phases will separate quickly. Ask students:

  • Which phase is now on top?
      (The oil phase is on top.)
  • Why did the oil rise to the top when water was poured on it?
      (It is less dense than water.)

Explain to students that vegetable oil is a mixture of hydrocarbons from plants, similar to kerogen. Be sure that students understand that vegetable oil is not a fossil fuel like kerogen because it is not produced from organic matter millions of years old that experienced high pressures and temperatures over time. Tell students that if the top of the glass represented the ground, the oil would be sitting on the surface of the earth and would eventually evaporate or be broken down by bacteria. Place your hand over the top of the glass and tell students that it represents non-porous rock, such as shale. Explain that in petroleum exploration, a rock that holds back the petroleum from coming out towards the surface is called a cap rock or seal rock. Because the cap rock is non-porous, the petroleum fills up below it like a reservoir. The rocks directly below the cap rock are porous, because the petroleum was able to move through those rocks. In petroleum exploration, scientists look for these cap rocks above porous rocks to search for deposits. If available, pass around a sample of shale rock, pointing out that it has no pores or openings. A glazed tile also can represent a non-porous cap rock. Allow students to pour vegetable oil onto the rock or tile to see that it will simply roll off and not be absorbed, just as with a cap rock.


Development

Have students proceed to the next part of the interactive, “Let’s Distill.” It is recommended to go through the five hotspots together as a class. This will ensure that students all understand the basic ideas around distillation and treatment of petroleum.

Point out the yellow barrel labeled “crude oil”. Tell students that once the petroleum has been extracted from the ground, it is sent to refineries. The interactive simulates and describes the various chemical processes that petroleum undergoes in order to make useful products.

Ask students:

  • What is crude oil composed of?
      (It is composed of various hydrocarbons of different lengths.)

Click on the "Dump Crude Oil" button and then roll over the hotspot and read the description. Discuss with students that crude oil is made up of different compounds. Ensure that students do not think that the various compounds are floating around within crude oil, but that crude oil itself is the collection of these compounds. Tell students that crude oil will vary in its composition depending on its geographic location. In general, it is composed of a combination of straight-chained hydrocarbons and ringed hydrocarbons. Point out that there are some sulfur and nitrogen compounds mixed with the hydrocarbons as well. Remind students that kerogen also contained sulfur and nitrogen.

Click on the “Dump Crude Oil” button again. Ask students:

  • Where does the crude oil go?
      (It goes to a furnace.)
  • What do you think will happen in the furnace?
      (The crude oil will be heated.)

Before proceeding, provide each student with a table of hydrocarbon boiling points from Boiling Points and Structures of Hydrocarbons. Ask students:

  • When you look at the “name” and “molecular formula” columns, what pattern do you notice as you go down?
      (The number of carbons increases and the hydrocarbon chains or alkanes get longer.)
  • Looking at boiling point, what pattern do you notice?
      (As the number of carbons increases, the boiling point increases.)
  • Now looking at melting point, what pattern do you notice?
      (As the number of carbons increases, the melting point increases.)
  • What general statement can you make about hydrocarbons and boiling point and melting point?
      (The more carbons in an alkane, the higher the boiling point and melting point.)

Review the concept of boiling point with students by having them do a quick hands-on activity of boiling water with a thermometer placed in the water. Have students bring some water to a boil. Once it reaches the boiling point, ask them:

  • What is happening to a substance when it starts to boil?
      (The liquid phase is going into gas phase.)
  • Is this a physical or chemical change? Why?
      (Physical. Because the substance is not changing in its structure or composition.)
  • At what temperature does water boil?
      (It boils at 100 degrees Celsius.)

Students should place the thermometer into the boiling water and read the temperature. A common misconception among students is that the temperature of a boiling substance increases as it boils. To help correct this misconception, allow students to see that the temperature of the boiling water stays at 100 degrees Celsius as it boils. Another common misconception is that the boiling point is when a liquid turns into gas, and not vice versa. It is important to address that boiling point is a phase change between liquid and gas. It is the point when a liquid turns into gas and a gas turns into liquid. Point this out to students by showing that the boiling water can be viewed as liquid turning into gas, or gas turning into liquid.

Tell students that on Mount Everest, water boils at 69 degrees Celsius. Ask students:

  • What makes the summit of Mount Everest different from sea level?
      (It’s higher and there is less air.)
  • If there is less air at the top of Mount Everest, is there more or less air pressure?
      (There is less air pressure.)

Review with students that the boiling point of a substance is not a function of temperature alone. What determines when a substance will change from liquid to gas is both temperature and the surrounding atmospheric pressure. Students are often aware, through their lived experience, that a change in temperature will alter a substance’s phase. For example, water placed in the freezer changes into ice, and water that is heated to 100 degrees Celsius boils and becomes a gas. However, they often do no make the connection that pressure is equally as important in determining the phase of a substance. To help make this point more clear, ask students to imagine a balloon that contains air molecules. Tell students that temperature is a measure of heat. If the balloon is heated, the kinetic energy of the molecules inside the balloon also increases and they begin to break away from each other and move faster. Ask students:

  • What will happen to the size of the balloon as the heat and corresponding kinetic energy of the molecules increase?
      (The balloon will expand from the pressure inside the balloon.)
  • What is keeping the molecules inside the balloon from coming out?
      (The barrier of the balloon itself.)

The barrier of the balloon represents the external air pressure. It holds the air molecules back from expanding out any further. At some point, as the heat increases, the balloon will burst. That bursting represents boiling because at that point, the pressure inside the balloon becomes equal to the pressure outside of the balloon. Similarly, a liquid boils when its internal pressure is equal to the atmospheric pressure.

Going back to the example of Mount Everest, tell students that water boils at a lower temperature on the mountain (69 degrees Celsius) because at that point, its pressure is equal to the low atmospheric pressure on Mount Everest. As you descend from Mount Everest and come to sea level, the atmospheric pressure is more, so correspondingly more heat is needed to cause boiling (100 degrees Celsius).

Return back to the table of hydrocarbon boiling points from Boiling Points and Structures of Hydrocarbons. Ask students:

  • Why do you think that the boiling point of alkanes is related to the size of the molecules?
      (As alkanes get longer, there are more internal molecular forces. It takes more heat for the longer alkanes to have enough kinetic energy to become a gas.)

Point out that pentane through dodecane are liquid at room temperature. Ask students:

  • Of the eight alkanes, which one has the highest boiling point?
      (Dodecane.)

Compare dodecane to a long spaghetti strand. Due to its longer length compared to that of a smaller pentane molecule, dodecane molecules will wrap around each other, making it harder to separate them because of these strong forces.

  • If all these alkanes were mixed together in a pot, and the temperature was increased to 250 degrees Celsius, what would happen to all these alkanes?
      (They would turn into gas.)
  • If we just wanted pentane, and not any of the other alkanes, what temperature should we bring the mixture to?
      (36 degrees Celsius.)
  • What will happen at that temperature?
      (The pentane will be at its boiling point, so it can be collected as a liquid.)
  • How can we capture the pentane gas?
      (Allow students to brainstorm suggestions of different collection measures for gas and liquids.)

Return back to the interactive. Remind students that crude oil or petroleum is a mixture of hydrocarbons. The oil is now in the furnace and will be heated. Click on "Start Furnace" and roll over the hotspot and have students focus on a single hydrocarbon molecule. Ask students:

  • If all the hydrocarbons are superheated, what phase will they go into?
      (Gas.)
  • What is happening to the temperature as the hydrocarbon gases rise?
      (It decreases.)

Point out that the furnace has now superheated the crude oil components which are now in vapor form. Indicate that the vapor has now traveled to the distillation column where the vapor rises. Point out the temperature gradations on the side of the distillation column.

Roll over the hotspot on the left side of the distillation column and point out that the animation shows two different hydrocarbon molecules—methane and cyclopentane. Ask students:

  • Which one will have a lower boiling point?
      (Methane.)
  • Why?
      (Because it has fewer carbons than cyclopentane.)
  • Why does methane turn to liquid higher in the distillation column than cyclopentane?
      (Because the lower temperature corresponding to methane’s lower relative boiling point is higher up in the distillation column than the boiling point temperature for cyclopentane.)

Roll over the hotspot at the top right corner of the distillation clumn. Show students that all the smaller chained hydrocarbons collect higher up in the distillation column. The collections of these similar hydrocarbons are called fractions. Examples of lighter fractions are gasoline and naphtha. Rolling over the hotspot at the lower right corner of the distillation column will show students heavier fractions, such as diesel.

Naphtha is a collection of hydrocarbons in the C5, C6, and C7 range. These hydrocarbons are all very light, easily vaporized, and form a clear liquid. Eventually, after treatment, naphthas are used as solvents in dry cleaning, paint, and other industries that need quick-drying products. Gasoline is a collection of hydrocarbons in the C7 and C11 range. Diesel is composed of hydrocarbons in the C16 range. Because they are larger hydrocarbons, their boiling point is much higher and will condense into liquid lower in the distillation column where the temperature is higher. Make sure that students understand that the collected fractions are not one molecule, but rather a collection of hydrocarbons of similar length, and thus, similar boiling point temperatures.

Tell students that distillation is the first part of the petroleum refining process. The fractions must now undergo treatment for two reasons. First, the hydrocarbon fractions still have substances in them such as sulfur, which must be removed. Second, some of the longer hydrocarbon fractions need to be chemically altered and shortened into smaller chains in order to become more useful products.


Assessment

To assess students' understandings of the main concepts covered in this lesson, ask them to complete the Assessing Our Understanding of Crude Oil student sheet. Tell students that the ten questions address misconceptions that the general public has about crude oil. Have students complete the sheet individually. Once all the students have chosen their answers, ask students to work in small groups and share their responses to each question with their group members. Each group must decide on their collective answer to each question. Provide classroom time for student groups to share with one another. Encourage students to discuss misconceptions and why these misconceptions might exist in the general public. For teachers, this is also an excellent opportunity to assess if students are understanding the main ideas through the lessons. The Assessing Our Understanding of Crude Oil teacher sheet provides answers to the questions.


Extensions

Explore the use of vegetable oil, used in the motivation section of this lesson, as a fuel itself. The Bio-Fuel Project, from the Energy Efficiency and Renewable Energy site, provides an extensive, two-week curriculum in which students learn how to make their own biodiesel fuel from waste vegetable oil.


To further emphasize the connection between the boiling point of a substance and atmospheric pressure, use NASA’s lesson, Testing Your Hypothesis by Boiling Water Below Its Boiling Temperature.


Acid Rain: Effects Felt Through the Food Chain, from National Geographic, provides information about the basic causes and effects of acid rain, including photographs and what humans can do to decrease its prevalence.


Climate Change: Basic Information, from the U.S. Environmental Protection Agency, provides comprehensive information on the issue of global warming and climate change.


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

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Lesson Details

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