Fractional Distillation and Cracking
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3.3
The Great Marble Race
You may have seen advertisements that recommend different types of motor oils
for different seasons. Why is it important to choose the right oil for the right
temperature? Find out with this simple demonstration.
Materials: several types of motor oil, designed for different seasons; several
large test tubes of equal size, with stoppers; several glass marbles of equal size;
stopwatch; ice bath; warm-water bath
1. Place one marble in each test tube.
2. Fill each test tube with a different motor oil. Stopper each test tube securely,
and label the test tube to identify the type of oil.
3. Invert all the test tubes at the same time, and start timing (Figure 1). Record
the time each marble takes to travel the entire distance down the test tube.
4. Completely submerge the test tubes in the ice bath for about 5 min, and then
repeat the race.
5. Finally, submerge the test tubes in the warm water bath for about 5 min, and
repeat the race.
(a) Identify any pattern you see in the race results. Provide an explanation
(hypothesis) for your observations.
(b) Why is it important to select a motor oil that is suitable for the season?
6. Dispose of the oil as instructed by your teacher.
People who have the good fortune to “strike oil” have probably drilled deep
into the ground and hit upon a complex mixture of hydrocarbon molecules,
formed from prehistoric plants and animals. This mixture, referred to as
petroleum, contains gases, liquids, and dissolved solids composed of many
different hydrocarbon molecules, some of which may be up to 40 carbon
atoms long.
As you learned earlier, small hydrocarbon molecules (such as methane,
ethane, propane, and butane) exist as gases. Most larger hydrocarbon
molecules are liquids, from light to heavy oils. The heaviest oils are asphalts
and tars. The most valuable hydrocarbons in petroleum are the hydrocarbons
with 5 to 12 carbons, because they are the components of gasoline.
How are the various hydrocarbons separated so that they can be sorted, and
sold, by size? It so happens that molecules of different sizes have different
boiling points. The smallest molecules have the lowest boiling points, which is
why methane, ethane, and propane are all gases at room temperature. They
have already boiled and evaporated at room temperature. The largest
molecules have boiling points over 400°C. Therefore, asphalt can be heated to
high temperatures to pave roads, without evaporating.
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Figure 1
Be sure that you invert all your test
tubes at the same time.
petroleum a mixture of gases and
liquids, composed of hydrocarbon
molecules up to 40 carbon atoms
long
Organic Chemistry 193
Why do hydrocarbons show this correlation between size and boiling point?
Recall, from section 3.1, that the answer is found in the forces of attraction
between neighbouring molecules, called intermolecular bonds. As you learned
in section 1.12, nonpolar molecules like hydrocarbons are attracted to each
other by relatively weak London dispersion forces. As the length of
hydrocarbon molecules increases, the number of intermolecular forces
between the molecules increases as well. Therefore, more heat is required to
pull the molecules apart, meaning that higher temperatures are required to
pull the molecules far enough apart to change into a gas.
Hydrocarbons are most useful to us when they are relatively pure. We do
not want asphalt in our natural gas, and barbecues are designed to run on
propane, not ethyne. How can petroleum be efficiently separated into its
useful components?
Fractional Distillation
fractional distillation the
separation of components of
petroleum by distillation, using
differences in boiling points
Figure 2
Fractionation towers look like tall
columns, with exterior stairs and
platforms for maintenance.
LEARNING
TIP
Heating Up and Cooling Down
Substances boil and condense at
the same temperature. For
example, water boils at 100°C, and
water vapour condenses at 100°C.
194
Unit 3
A method called fractional distillation is used to separate the many
components of petroleum. Essentially, molecules of various sizes are separated
into portions called fractions. Each fraction contains similar-sized molecules.
The lighter fractions boil at lower temperatures, and the heavier fractions boil
at higher temperatures.
In fractional distillation, the entire mixture of hydrocarbons is first heated
to very high temperatures, high enough to evaporate nearly all of the
hydrocarbons, small and large. Then the hot gases are allowed to rise in a tall
fractionation tower (Figure 2). The upper parts of the tower are cooler than
the lower parts. Each gas condenses at its own boiling point. As the hot gases
travel up through the lower, warmer sections, the larger molecules condense.
The smaller molecules with their low boiling points are still gases and ascend
higher, to the top of the tower where the temperatures are lowest (Figure 3).
As each fraction condenses, liquid forms on a tray and is collected. Table 1
shows the various types of hydrocarbons, their boiling points, and their end
uses.
Table 1
Uses of Hydrocarbons
Number of C atoms
Boiling point
End use
1−5
under 30°C
fuels for heat and cooking
5−6
30°C−90°C
camping fuel and dry-cleaning solvents
5−12
30°C−200°C
gasoline
12−16
175°C−275°C
kerosene and diesel fuel
15−18
250°C−375°C
furnace oil
16−22
over 400°C
heavy greases for lubricating
over 20
over 450°C
waxes, cosmetics, and polishes
over 26
over 500°C
asphalt and tar for roofs and roads
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Section 3.3
liquefied petroleum
gases
gasoline
fractionation
tower
jet fuel
Figure 3
Crude oil, the liquid component of
petroleum, is heated and fed to the
bottom of a fractionation tower. The
gas mixture cools as it rises,
allowing each component of the
mixture to condense at its boiling
point. The smallest hydrocarbons,
with the lowest boiling points,
condense at the top of the tower
where the temperatures are lowest.
kerosene
diesel oil
cooling
lubricating oils
crude oil
fuel oil
wax
asphalt
asphalt
heat
Cracking
Of the many fractions collected, the most valuable and profitable is gasoline.
Particularly in demand is high-octane gasoline, which contains highly
branched alkanes (Figure 4). Fractionation of petroleum produces the less
useful straight-chain hydrocarbons. A process called cracking is used to
convert these straight-chain hydrocarbons into shorter branched-chain
alkanes. In this process, the hydrocarbons are mixed with a catalyst and heated
to temperatures of 400°C to 500°C. Cracking is also used to break apart larger
hydrocarbon molecules, such as kerosene, into smaller molecules, such as
ethene and propene (used in the production of plastics).
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Figure 4
The higher the octane number of a
gasoline, the more efficiently the
gasoline burns to produce power
and, thus, the less “knocking” in the
engine. Highly branched alkanes
have high octane numbers. For
example, iso-octane is assigned an
octane number of 100, while
straight-chain heptane is assigned
an octane number of 0.
cracking the process in which
large straight-chain hydrocarbon
molecules are converted into
smaller branched-chain
hydrocarbon molecules, usually by
catalytic heating
Organic Chemistry 195
Section 3.3 Questions
Understanding Concepts
1. What is the source of petroleum? What are some of
its components?
2. (a) Describe and explain the relationship between
the sizes of hydrocarbon molecules and their
boiling points.
(b) Describe how this relationship can be used to
separate the components of petroleum.
3. A fraction of hydrocarbons with a boiling point of
10°C is collected in a fractional distillation process. Is
this fraction a liquid or a gas at ordinary
temperatures of about 20°C? Explain.
4. Why is cracking an important technological process?
5. The propane in a gas barbecue tank and the wax in a
candle both consist of hydrocarbons. Give the
approximate boiling point of each of these
substances (refer to Table 1). Then give a theoretical
explanation for the difference between their boiling
points.
6. Name two substances you have used that have been
made from a fraction of crude oil condensed in a
fractionation tower, near
(a) the top
(b) the middle
(c) the bottom
Making Connections
8. Research the production of crude oil in Canada.
Write a short report to explain where crude oil is
found and what its main uses are.
GO
www.science.nelson.com
9. Motor oils are available in different blends, which are
appropriate for different seasons. Visit a gas station
or hardware store to research the different brands
and types of motor oils that are available for the
different seasons in Canada. Present your information
in a table. Include composition, brand, seasonal
usage, and cost.
10. Gasoline and home heating oils are referred to as
fossil fuels. Explain why this term applies to these
fuels and why there is a concern about their use.
11. Petroleum production and processing not only
provides Canadians with useful products, it also
generates many jobs in the manufacture of these
products. Figure 5 shows a petrochemical footprint:
an overview of the types and numbers of different
jobs related to the use of ethene (ethylene). Use a
telephone directory to list and count the number of
businesses in your community that provide, directly
or indirectly, products or services related to the
petrochemical footprint shown. For each business
that you list, describe the types of jobs involved.
Applying Inquiry Skills
7. Design a laboratory set-up that would enable you to
separate a mixture of two hydrocarbons
with different boiling points. Explain the
reasons for the apparatus you would
ethylene
need in your set-up. Draw and label a
oxide
diagram of your set-up.
31
gas treating
glycols and
amines
7
polyester
fibre
1353
antifreeze
32
others
801
37
ethylene
(45 000 t)
11
styrene
polystyrene
and latex
100
322
polyethylene
6568
moulding
and film
1660
pipes and
tiles
6000
73
Figure 5
A petrochemical footprint: the types and
numbers of jobs that are generated by the
ethene (ethylene) industry
196
Unit 3
vinyl
chloride
polyvinyl
chloride
116
600
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