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Fractional
Distillation
Definition and Examples
What You Need to Know About Fractional Distillation
by Anne Marie Helmenstine, Ph.D.
Fractional Distillation
Definition
Fractional
distillation is a process by which components in a chemical mixture are
separated into different parts (called fractions) according to their different
boiling points.
Fractional distillation is
used to purify chemicals and also to separate mixtures to obtain their
components.
It's
used as a lab technique and in industry, where the process has vast commercial
significance.
The chemical and petroleum industry rely on fractional
distillation.
How
Fractional Distillation Works
Vapors from
a boiling solution are passed along a tall column, called a fractionating
column.
The column is packed with
plastic or glass beads to improve the separation by providing more surface area
for condensation and evaporation.
The temperature of the column
gradually decreases along its length. Components with a higher boiling point
condense on the column and return to the solution; components with a lower
boiling point (more volatile) pass through the column and are collected
near the top.
Theoretically, having more
beads or plates improves the separation, but adding plates also increases the
time and energy required to complete a distillation.
Fractional
Distillation of Crude Oil
Gasoline
and many other chemicals are produced from crude oil using fractional
distillation. Crude oil is heated until it evaporates.
Different fractions condense at certain temperature ranges. The
chemicals in a certain fraction are hydrocarbons with comparable numbers of
carbon atoms.
From
hot to cold (largest hydrocarbons to smallest), the fractions may be residue
(used to make bitumen), fuel oil, diesel, kerosene, naphtha, gasoline, and
refinery gas.
Fractional
Distillation of Ethanol
Fractional
distillation cannot completely separate the components of a mixture of ethanol
and water, despite the different boiling points of the two chemicals.
Water boils
at 100 °C while ethanol boils at 78.4 °C. If an
alcohol-water mixture is boiled, the ethanol will concentrate in the vapor, but
only up to a point because alcohol and water form an azeotrope.
Once the mixture reaches the
point where it consists of 96% ethanol and 4% water, the mixture is more
volatile (boils at 78.2 °C) than the ethanol.
Simple
vs Fractional Distillation
Fractional
distillation differs from simple distillation because the fractionating
column naturally separates compounds based on boiling point.
It's possible to isolate
chemicals using simple distillation, but it requires careful control of the
temperature since only one "fraction" can be isolated at a time.
How
do you know whether to use simple distillation or fractional distillation to
separate a mixture?
Simple distillation is
faster, simpler, and uses less energy, but it's really only useful when there
is a large difference between the boiling points of the desired fractions (more
than 70 degrees Celsius).
If there is only a small
temperature difference between the fraction, fractional distillation is your
best bet.
|
Simple
Distillation
|
Fractional
Distillation
|
Uses
|
Used
for separating relatively pure liquids that have large boiling point
differences. Also useful for separating liquids from solid impurities.
|
Used
to isolate components of complex mixtures with small boiling point
differences.
|
Advantages
|
· faster
· requires less energy input
· simpler, less expensive
equipment
|
· results in better
separation of liquids
· better at purifying
liquids containing many different components
|
Disadvantages
|
· only useful for relatively
pure liquids
· requires a large boiling
point difference between components
· doesn't separate fractions
as cleanly
|
· slower
· requires more energy
· more complicated and
expensive set-up
|
Anne
Marie Helmenstine, Ph.D.
· Ph.D.
in biomedical sciences from the University of Tennessee at Knoxville - Oak
Ridge National Laboratory.
· Science
educator with experience teaching chemistry, biology, astronomy, and
physics at the high school, college, and graduate levels.
· ThoughtCo
and About Education chemistry expert since 2001.
· Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
Anne
Marie Helmenstine, Ph.D.
ThoughtCo
and Dotdash
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