Nitrogen - less than 1% (basic
compounds with amine groups)
Oxygen - less than 1% (found in
organic compounds such as carbon dioxide, phenols, ketones, carboxylic
acids)
Metals - less than 1% (nickel,
iron, vanadium, copper, arsenic)
Salts - less than 1% (sodium
chloride, magnesium chloride, calcium chloride)
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 to rings.
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:
Paraffins
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 the molecule
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
Other hydrocarbons
Alkenes
general formula: CnH2n
(n is a whole number, usually from 1 to 20)
linear or branched chain
molecules containing one carbon-carbon double-bond
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 double-bonds
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.
The oil refining process starts with a fractional distillation column.
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 way.
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, butane
boiling range = less than 104
degrees Fahrenheit / 40 degrees Celsius
often liquified under pressure
to create LPG (liquified petroleum gas)
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
liquid
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
liquid
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
liquid
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
liquid
long chain (20 to 50 carbon
atoms) alkanes, cycloalkanes, aromatics
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
liquid
long chain (20 to 70 carbon
atoms) alkanes, cycloalkanes, aromatics
boiling range = 700 to 1112
degrees Fahrenheit / 370 to 600 degrees Celsius
Residuals - coke, asphalt, tar, waxes;
starting material for making other products
solid
multiple-ringed compounds with
70 or more carbon atoms
boiling range = greater than
1112 degrees Fahrenheit / 600 degrees Celsius
You may have noticed that all of these products have different
sizes and boiling ranges. Chemists take advantage of these properties when
refining oil. Look at the next section to find out the details of this
fascinating process.
The Refining Process 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 ratings.
Photo courtesy Phillips Petroleum Company An oil refinery
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.
In the next
section, we will look at how we separate crude oil into its components
Fractional Distillation
Photo
courtesy Phillips Petroleum Distillation
columns in an oil refinery
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 through.
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 storage tanks
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.
The oil refining process starts with a fractional distillation column. On
the right, you can see several chemical processors that are described in the
next section.
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.
Chemical Processing You
can change one fraction into another by one of three methods:
breaking large hydrocarbons into smaller
pieces (cracking)
combining smaller pieces to make larger ones (unification)
rearranging various pieces to make desired
hydrocarbons (alteration)
Cracking
Cracking takes large hydrocarbons and breaks them into smaller ones.
Cracking breaks large chains into smaller chains.
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
manufacture chemicals.
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 and sold.
Photo courtesy Phillips Petroleum Company Catalysts used in catalytic cracking or reforming
Catalytic - uses a catalyst to speed up
the cracking reaction. Catalysts include zeolite, aluminum hydrosilicate,
bauxite and silica-alumina.
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.
Unification
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.
A reformer combines chains.
Alteration
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 (see "What
does octane mean?" for details).
Rearranging chains.
Now that we
have seen how various fractions are changed, we will discuss the how the
fractions are treated and blended to make commercial products.
An oil refinery is a combination of all of these units.
