FRACTIONAL DISTILLATION - 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 might be residue (used to make bitumen), fuel oil, diesel, kerosene, naphtha, gasoline, and refinery gas. 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.

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Fractional Distillation

Definition and Examples

Fractional distillation is used to purify chemicals and separate mixtures

by Anne Marie Helmenstine, Ph.D. 

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 to separate mixtures to obtain their components.

The technique is used in labs and in industry, where the process has vast commercial significance.

The chemical and petroleum industry rely on fractional distillation.

How It 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.

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 might be residue (used to make bitumen), fuel oil, diesel, kerosene, naphtha, gasoline, and refinery gas.

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 degrees Celcius while ethanol boils at 78.4 degrees Celcius.

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 degrees Celcius) than the ethanol.

Simple vs. Fractional Distillation

Fractional distillation differs from simple distillation because the fractionating column naturally separates compounds based on their boiling points.

It's possible to isolate chemicals using simple distillation, but it requires careful control of the temperature because 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 Celcius).

If there is only a small temperature difference between the fractions, fractional distillation is your best bet.

Here's a breakdown of the differences between simple and fractional distillation:

	Simple Distillation	Fractional Distillation
Uses	Separating relatively pure liquids that have large boiling point differences. Also separating liquids from solid impurities.	Isolating components of complex mixtures with small boiling point differences.
Advantages	· Faster · Requires less energy input · Simpler, less expensive equipment	· 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 setup

Anne Marie Helmenstine, Ph.D.

Chemistry Expert

Education

Ph.D., Biomedical Sciences, University of Tennessee at Knoxville

B.A., Physics and Mathematics, Hastings College

Introduction

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.

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