Crude oil is the term for "unprocessed" oil, the stuff
that comes out of the ground. It is also known as petroleum.
Crude oil is a fossil fuel, meaning that it was made naturally
from decaying plants and animals living in ancient seas millions
of years ago -- anywhere you find crude oil was once a sea bed.
Crude oils vary in color, from clear to tar-black, and in viscosity,
from water to almost solid.
Crude oils are such a useful starting point for so many different
substances because they contain hydrocarbons. Hydrocarbons are
molecules that contain hydrogen and carbon and come in various
lengths and structures, from straight chains to branching chains
There are two things that make hydrocarbons exciting to chemists:
- Hydrocarbons contain a lot of energy. Many of the things
derived from crude oil like gasoline, diesel fuel, paraffin
wax and so on take advantage of this energy.
- Hydrocarbons can take on many different forms. The smallest
hydrocarbon is methane (CH4), which is a gas that is a lighter
than air. Longer chains with 5 or more carbons are liquids.
Very long chains are solids like wax or tar. By chemically
cross-linking hydrocarbon chains you can get everything from
synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon
chains are very versatile!
The major classes of hydrocarbons in
crude oils include:
- general formula: CnH2n+2 (n is a whole number, usually
from 1 to 20)
- straight- or branched-chain molecules
- can be gasses or liquids at room temperature depending upon
- examples: methane, ethane, propane, butane, isobutane, pentane,
- general formula: C6H5 - Y (Y is a longer, straight molecule
that connects to the benzene ring)
- ringed structures with one or more rings
- rings contain six carbon atoms, with alternating double
and single bonds between the carbons
- typically liquids
- examples: benzene, napthalene
Napthenes or Cycloalkanes
- general formula: CnH2n (n is a whole number usually from
1 to 20)
- ringed structures with one or more rings
- rings contain only single bonds between the carbon atoms
- typically liquids at room temperature
- examples: cyclohexane, methyl cyclopentane
- general formula: CnH2n (n is a whole number, usually from
1 to 20)
- linear or branched chain molecules containing one carbon-carbon
- can be liquid or gas
- examples: ethylene, butene, isobutene
Dienes and Alkynes
- general formula: CnH2n-2 (n is a whole number, usually
from 1 to 20)
- linear or branched chain molecules containing two carbon-carbon
- can be liquid or gas
- examples: acetylene, butadienes
From Crude Oil
The problem with crude oil is that it contains
hundreds of different types of hydrocarbons all mixed together.
You have to separate the different types of hydrocarbons to
have anything useful. Fortunately there is an easy way to separate
things, and this is what oil refining is all about.
Different hydrocarbon chain lengths all have progressively higher
boiling points, so they can all be separated by distillation.
This is what happens in an oil refinery - in one part of the
process, crude oil is heated and the different chains are pulled
out by their vaporization temperatures. Each different chain
length has a different property that makes it useful in a different
To understand the diversity contained in crude
oil, and to understand why refining crude oil is so important
in our society, look through the following list of products
that come from crude oil:
- Petroleum gas - used for heating,
cooking, making plastics
- small alkanes (1 to 4 carbon atoms)
- commonly known by the names methane, ethane, propane,
- boiling range = less than 104 degrees Fahrenheit / 40
- often liquified under pressure to create LPG (liquified
- Naphtha or Ligroin - intermediate
that will be further processed to make gasoline
- mix of 5 to 9 carbon atom alkanes
- boiling range = 140 to 212 degrees Fahrenheit / 60 to
100 degrees Celsius
- Gasoline - motor fuel
- mix of alkanes and cycloalkanes (5 to 12 carbon atoms)
- boiling range = 104 to 401 degrees Fahrenheit / 40 to
205 degrees Celsius
- Kerosene - fuel for jet engines
and tractors; starting material for making other products
- mix of alkanes (10 to 18 carbons) and aromatics
- boiling range = 350 to 617 degrees Fahrenheit / 175
to 325 degrees Celsius
- Gas oil or Diesel distillate
- used for diesel fuel and heating oil; starting material
for making other products
- alkanes containing 12 or more carbon atoms
- boiling range = 482 to 662 degrees Fahrenheit / 250
to 350 degrees Celsius
- Lubricating oil - used for
motor oil, grease, other lubricants
- long chain (20 to 50 carbon atoms) alkanes, cycloalkanes,
- boiling range = 572 to 700 degrees Fahrenheit / 300 to
370 degrees Celsius
- Heavy gas or Fuel oil - used
for industrial fuel; starting material for making other products
- long chain (20 to 70 carbon atoms) alkanes, cycloalkanes,
- boiling range = 700 to 1112 degrees Fahrenheit / 370
to 600 degrees Celsius
- Residuals - coke, asphalt,
tar, waxes; starting material for making other products
- multiple-ringed compounds with 70 or more carbon atoms
- boiling range = greater than 1112 degrees Fahrenheit
/ 600 degrees Celsius
As mentioned previously, a barrel of crude oil has a mixture
of all sorts of hydrocarbons in it. Oil refining separates everything
into useful substances. Chemists use the following steps:
- The oldest and most common way to separate things into
various components (called fractions), is to do it using the
differences in boiling temperature. This process is called
fractional distillation. You basically heat crude oil up,
let it vaporize and then condense the vapor.
- Newer techniques use Chemical processing on some of the
fractions to make others, in a process called conversion.
Chemical processing, for example, can break longer chains
into shorter ones. This allows a refinery to turn diesel fuel
into gasoline depending on the demand for gasoline.
- Refineries must treat the fractions to remove impurities.
- Refineries combine the various fractions (processed, unprocessed)
into mixtures to make desired products. For example, different
mixtures of chains can create gasolines with different octane
The products are stored on-site until they
can be delivered to various markets such as gas stations, airports
and chemical plants. In addition to making the oil-based products,
refineries must also treat the wastes involved in the processes
to minimize air and water pollution.
- The various components of crude oil have different sizes,
weights and boiling temperatures; so, the first step is to
separate these components. Because they have different boiling
temperatures, they can be separated easily by a process called
fractional distillation. The steps of fractional distillation
are as follows:
You heat the mixture of two or more substances (liquids) with
different boiling points to a high temperature. Heating is
usually done with high pressure steam to temperatures of about
1112 degrees Fahrenheit / 600 degrees Celsius.
- The mixture boils, forming vapor (gases); most substances
go into the vapor phase.
- The vapor enters the bottom of a long column (fractional
distillation column) that is filled with trays or plates.
- The trays have many holes or bubble caps (like a loosened
cap on a soda bottle) in them to allow the vapor to pass
- The trays increase the contact time between the vapor
and the liquids in the column.
- The trays help to collect liquids that form at various
heights in the column.
- There is a temperature difference across the column
(hot at the bottom, cool at the top).
The vapor rises in the column.
- As the vapor rises through the trays in the column, it cools.
- When a substance in the vapor reaches a height where the
temperature of the column is equal to that substance's boiling
point, it will condense to form a liquid. (The substance with
the lowest boiling point will condense at the highest point
in the column; substances with higher boiling points will
condense lower in the column.).
- The trays collect the various liquid fractions.
- The collected liquid fractions may:
pass to condensers, which cool them further, and then go to
go to other areas for further chemical processing
Fractional distillation is useful for separating
a mixture of substances with narrow differences in boiling points,
and is the most important step in the refining process.
Very few of the components come out of the
fractional distillation column ready for market. Many of them
must be chemically processed to make other fractions. For example,
only 40% of distilled crude oil is gasoline; however, gasoline
is one of the major products made by oil companies. Rather than
continually distilling large quantities of crude oil, oil companies
chemically process some other fractions from the distillation
column to make gasoline; this processing increases the yield
of gasoline from each barrel of crude oil.
You can change one fraction into another by one of three methods:
- breaking large hydrocarbons into smaller pieces
- combining smaller pieces to make larger ones (unification)
- rearranging various pieces to make desired hydrocarbons
Cracking takes large hydrocarbons and breaks them into smaller
There are several types of cracking:
- Thermal - you heat large hydrocarbons at high temperatures
(sometimes high pressures as well) until they break apart.
- steam - high temperature steam (1500 degrees Fahrenheit
/ 816 degrees Celsius) is used to break ethane, butane
and naptha into ethylene and benzene, which are used to
- visbreaking - residual from the distillation tower is
heated (900 degrees Fahrenheit / 482 degrees Celsius),
cooled with gas oil and rapidly burned (flashed) in a
distillation tower. This process reduces the viscosity
of heavy weight oils and produces tar.
- coking - residual from the distillation tower is heated
to temperatures above 900 degrees Fahrenheit / 482 degrees
Celsius until it cracks into heavy oil, gasoline and naphtha.
When the process is done, a heavy, almost pure carbon
residue is left (coke); the coke is cleaned from the cokers
- Catalytic - uses a catalyst to speed up the cracking reaction.
Catalysts include zeolite, aluminum hydrosilicate, bauxite
- fluid catalytic cracking - a hot, fluid catalyst (1000
degrees Fahrenheit / 538 degrees Celsius) cracks heavy
gas oil into diesel oils and gasoline.
- hydrocracking - similar to fluid catalytic cracking,
but uses a different catalyst, lower temperatures, higher
pressure, and hydrogen gas. It takes heavy oil and cracks
it into gasoline and kerosene (jet fuel).
After various hydrocarbons are cracked into smaller hydrocarbons,
the products go through another fractional distillation column
to separate them.
Sometimes, you need to combine smaller hydrocarbons
to make larger ones -- this process is called unification. The
major unification process is called catalytic reforming and
uses a catalyst (platinum, platinum-rhenium mix) to combine
low weight naphtha into aromatics, which are used in making
chemicals and in blending gasoline. A significant by-product
of this reaction is hydrogen gas, which is then either used
for hydrocracking or sold.
Sometimes, the structures of molecules in one
fraction are rearranged to produce another. Commonly, this is
done using a process called alkylation. In alkylation, low molecular
weight compounds, such as propylene and butylene, are mixed
in the presence of a catalyst such as hydrofluoric acid or sulfuric
acid (a by-product from removing impurities from many oil products).
The products of alkylation are high octane hydrocarbons, which
are used in gasoline blends to reduce knocking.
Blending the Fractions
Distillated and chemically processed fractions are treated to
remove impurities, such as organic compounds containing sulfur,
nitrogen, oxygen, water, dissolved metals and inorganic salts.
Treating is usually done by passing the fractions through the
- a column of sulfuric acid - removes unsaturated hydrocarbons
(those with carbon-carbon double-bonds), nitrogen compounds,
oxygen compounds and residual solids (tars, asphalt)
- an absorption column filled with drying agents to remove
- sulfur treatment and hydrogen-sulfide scrubbers to remove
sulfur and sulfur compounds
After the fractions have been treated, they
are cooled and then blended together to make various products,
- gasoline of various grades, with or without additives
- lubricating oils of various weights and grades (e.g. 10W-40,
- kerosene of various various grades
- jet fuel
- diesel fuel
- heating oil
- chemicals of various grades for making plastics and other