Crude OilCrude 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 examples: methane, ethane, propane, butane, isobutane, pentane, hexane

Aromatics 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)

On average, crude oils are made of the following elements or compounds:

Carbon - 84% Hydrogen - 14% Sulfur - 1 to 3%

(hydrogen sulfide, sulfides, disulfides, elemental sulfur)

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)

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 OilThe 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 ProcessAs 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:

1. 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.

2. 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.

3. Refineries must treat the fractions to remove impurities. 4. 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.

Fractional DistillationThe 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:

1. 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.

2. The mixture boils, forming vapor (gases); most substances go into the vapor phase. 3. 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). 4. The vapor rises in the column. 5. As the vapor rises through the trays in the column, it cools. 6. 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.).

7. The trays collect the various liquid fractions. 8. 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.

Photo courtesy Phillips PetroleumDistillation columns in an oil refinery

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.

I

Chemical ProcessingYou 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)

CrackingCracking 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.

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.

UnificationSometimes, 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.

Photo courtesy Phillips Petroleum CompanyCatalysts used in catalytic cracking or

reforming

A reformer combines chains.

AlterationSometimes, 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. Next Page >>

Treating and Blending the FractionsDistillated 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 following:

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 water

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, such as:

gasoline of various grades, with or without additives lubricating oils of various weights and grades (e.g.

10W-40, 5W-30) kerosene of various various grades jet fuel diesel fuel heating oil chemicals of various grades for making plastics and other polymers

The "crude oil" pumped out of the ground is a black liquid called petroleum. This liquid contains aliphatic hydrocarbons, or hydrocarbons composed of nothing but hydrogen and carbon. The carbon atoms link together in chains of different lengths.

It turns out that hydrocarbon molecules of different lengths have different properties and behaviors. For example, a chain with just one carbon atom in it (CH4) is the lightest chain,

Photo courtesy Phillips PetroleumPlastics produced from refined oil

fractions

known as methane. Methane is a gas so light that it floats like helium. As the chains get longer, they get heavier.

The first four chains -- CH4 (methane), C2H6 (ethane), C3H8 (propane) and C4H10 (butane) -- are all gases, and they boil at -161, -88, -46 and -1 degrees F, respectively (-107, -67, -43 and -18 degrees C). The chains up through C18H32 or so are all liquids at room temperature, and the chains above C19 are all solids at room temperature.

The different chain lengths have progressively higher boiling points, so they can be separated out by distillation. This is what happens in an oil refinery -- crude oil is heated and the different chains are pulled out by their vaporization temperatures. (See How Oil Refining Works for details.)

The chains in the C5, C6 and C7 range are all very light, easily vaporized, clear liquids called naphthas. They are used as solvents -- dry cleaning fluids can be made from these liquids, as well as paint solvents and other quick-drying products.

The chains from C7H16 through C11H24 are blended together and used for gasoline. All of them vaporize at temperatures below the boiling point of water. That's why if you spill gasoline on the ground it evaporates very quickly.

Next is kerosene, in the C12 to C15 range, followed by diesel fuel and heavier fuel oils (like heating oil for houses).

Next come the lubricating oils. These oils no longer vaporize in any way at normal temperatures. For example, engine oil can run all day at 250 degrees F (121 degrees C) without vaporizing at all. Oils go from very light (like 3-in-1 oil) through various thicknesses of motor oil through very thick gear oils and then semi-solid greases. Vasoline falls in there as well.

Chains above the C20 range form solids, starting with paraffin wax, then tar and finally asphaltic bitumen, which used to make asphalt roads.

All of these different substances come from crude oil. The only difference is the length of the carbon chains!

Fractional Distillation of Crude Oil

BOILING POINTS AND STRUCTURES OF HYDROCARBONS

The boiling points of organic compounds can give important clues to other physical properties. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules. Kinetic energy is related to temperature and the mass and velocity of the molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. Then these molecules break away from the liquid forming the gas state.

Vapor pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy, they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy (velocities) to escape at lower temperatures. The vapor pressure will be higher and therefore the compound will boil at a lower temperature.

BOILING POINT PRINCIPLE:

Molecules which strongly interact or bond with each other through a variety of intermolecular forces can not move easily or rapidly and therefore, do not achieve the kinetic energy necessary to escape the liquid state. Therefore, molecules with strong intermolecular forces will have higher boiling points. This is a consequence of the increased kinetic energy needed to break the intermolecular bonds so that individual molecules may escape the liquid as gases.

THE BOILING POINT CAN BE A ROUGH MEASURE OF THE AMOUNT OF ENERGY NECESSARY TO SEPARATE A LIQUID MOLECULE FROM ITS NEAREST NEIGHBORS.

MOLECULAR WEIGHT AND CHAIN LENGTH TRENDS IN BOILING POINTS

A series of alkanes demonstrates the general principle that boiling points increase as molecular weight or chain length increases (table 1.).

Table 1. BOILING POINTS OF ALKANES

 Formula  Name  Boiling Point C Normal State at Room Temp. +20 C

 CH4  Methane  -161  gas

 CH3CH3  Ethane  - 89  

 CH3CH2CH3  Propane  - 42  

 CH3CH2CH2CH3  Butane  -0.5  

 CH3CH2CH2CH2CH3  Pentane  + 36  liquid

 CH3(CH2)6CH3  Octane  +125  

QUES. State whether the compounds above will be a gas or liquid state at room temperature (20 C). Hint: If the boiling point is below 20 C, then the liquid has already boiled andthe compound is a gas.

The reason that longer chain molecules have higher boiling points is that longer chain molecules become wrapped around and enmeshed in each other much like the strands of

spaghetti. More energy is needed to separate them than short molecules which have only weak forces of attraction for each other.

FOCUS ON FOSSIL FUELS

Petroleum refining is the process of separating the many compounds present in crude petroleum. The principle which is used is that the longer the carbon chain, the higher the temperature at which the compounds will boil. The crude petroleum is heated and changed into a gas. The gases are passed through a distillation column which becomes cooler as the height increases. When a compound in the gaseous state cools below its boiling point, it condenses into a liquid. The liquids may be drawn off the distilling column at various heights.

Although all fractions of petroleum find uses, the greatest demand is for gasoline. One barrel of crude petroleum contains only 30-40% gasoline. Transportation demands require that over 50% of the crude oil be converted into gasoline. To meet this demand some petroleum fractions must be converted to gasoline. This may be done by "cracking" - breaking down large molecules of heavy heating oil; "reforming" - changing molecular structures of low quality gasoline molecules; or "polymerization" - forming longer molecules from smaller ones.

For example if pentane is heated to about 500 C the covalent carbon-carbon bonds begin to break during the cracking process. Many kinds of compounds including alkenes are made during the cracking process. Alkenes are formed because there are not enough hydrogens to saturate all bonding positions after the carbon-carbon bonds are broken.

 

Simple Distillation

The core refining process is simple distillation, illustrated in a stylized fashion at the right.   Because crude oil is made up of a mixture of hydrocarbons, this first and basic refining process is aimed at separating the crude oil into its "fractions," the broad categories of its component hydrocarbons.  Crude oil is heated and put into a still -- a distillation column -- and different products boil off and can be recovered at different temperatures.  The lighter products -- liquid petroleum gases (LPG),  naphtha, and so-called "straight run" gasoline -- are recovered at the lowest temperatures.  Middle distillates -- jet fuel, kerosene, distillates (such as home heating oil and diesel fuel) -- come next.  Finally, the heaviest products (residuum or residual fuel oil) are recovered, sometimes at temperatures over 1000 degrees F.   The simplest refineries stop at this point.  Most in the United States, however, reprocess the heavier fractions into lighter products to maximize the output of the most desirable products, as shown schematically in the illustration, and as discussed below.

Downstream Processing

Additional processing follows crude distillation, "downstream" (or closer to the refinery gate and the consumer) of the distillation process.   Downstream processing is grouped together in this discussion, but encompasses a variety of highly complex units designed for very different upgrading processes.  Some change the molecular structure of the input with chemical reactions, some in the presence of a catalyst, some with thermal reactions. 

In general, these processes are designed to take heavy, low-valued feedstock -- often itself the output from an earlier process -- and change it into lighter, higher-valued output.  A catalytic cracker, for instance, uses the gasoil (heavy distillate) output from crude distillation as its feedstock and produces additional finished distillates (heating oil and diesel) and gasoline.  Sulfur removal is accomplished in a hydrotreater.  A reforming unit produces higher octane components for gasoline from lower octane feedstock that was recovered in the distillation process.   A coker uses the heaviest output of distillation, the residue or residuum, to produce a lighter feedstock for further processing, as well as petroleum coke. 

As noted above and in the section on demand, U.S. demand is centered on light products, such as gasoline.  As shown in the graph, refiners in the United States more closly match the mix of products demand by using downstream processing to move from the natural yield of products from simple distillation, illustrated earlier, to the U.S. demand slate, illustrated here.  After simple distillation alone, the output from a crude oil like Arab Light would be about 20 percent of lightest, gasoline-like products, and about 50 percent of the heaviest, the residuum.  After further processing in the most sophisticated refinery, however, the finished product output is about 60 percent gasoline, and 5 percent residuum.  

Crude Oil Quality

The physical characteristics of crude oils differ.  Crude oil with a similar mix of physical and chemical characteristics, usually produced from a given reservoir, field or sometimes even a region, constitutes a crude oil "stream."   Most simply, crude oils are classified by their density and sulfur content.  Less dense (or "lighter") crudes generally have a higher share of light hydrocarbons -- higher value products -- that can be recovered with simple distillation.  The denser ("heavier") crude oils produce a greater share of lower-valued products with simple distillation and require additional processing to produce the desired range of products.  Some crude oils also have a higher sulfur content, an undesirable characteristic with respect to both processing and product quality.  For pricing purposes, crude oils of similar quality are often compared to a single representative crude oil, a "benchmark," of the quality class. 

The quality of the crude oil dictates the level of processing and re-processing necessary to achieve the optimal mix of product output.   Hence, price and price differentials between crude oils also reflect the relative ease of refining.   A premium crude oil like West Texas Intermediate, the U.S. benchmark, has a relatively high natural yield of desirable naphtha and straight-run gasoline (see graph).  Another premium crude oil, Nigeria's Bonny Light, has a high natural yield of middle distillates.  By contrast, almost half of the simple distillation yield from Saudi Arabia's Arabian Light, the historical benchmark crude, is a heavy residue ("residuum") that must be reprocessed or sold at a discount to crude oil.  Even West Texas Intermediate and Bonny Light have a yield of about one-third residuum after the simple distillation process. 

In addition to gravity and sulfur content, the type of hydrocarbon molecules and other natural characteristics may affect the cost of processing or restrict a crude oil's suitability for specific uses.  The presence of heavy metals, contaminants for the processing and for the finished product, is one example.  The molecular structure of a crude oil also dictates whether a crude stream can be used for the manufacture of specialty products, such as lubricating oils or of petrochemical feedstocks. 

Refiners therefore strive to run the optimal mix (or "slate") of crudes through their refineries, depending on the refinery's equipment, the desired output mix, and the relative price of available crudes.  In recent years, refiners have confronted two opposite forces -- consumers' and government mandates that increasingly required light products of higher quality (the most difficult to produce) and crude oil supply that was increasingly heavier, with higher sulfur content (the most difficult to refine).

Oil Refineries

A refinery is a factory. A refinery takes a raw material (crude oil) and transforms it into petrol and hundreds of other useful products. A typical large refinery costs billions of pounds to build and millions more to run and upgrade. It runs around the clock 365 days a year, employs  hundreds of people and occupies as much land as several hundred football pitches.

A REFINERY breaks crude oil down into its various components, which then are selectively changed into new products. This process takes place inside a maze of pipes and vessels. The refinery is operated from a highly automated control room.

All refineries perform three basic steps:

Separation (fractional distillation) Conversion (cracking and rearranging the

molecules)

Treatment

Modern separation involves piping crude oil through hot furnaces. The resulting liquids and vapours are passed into distillation towers:-

FRACTIONB Pt 

oCNumber of

carbonsUses

»Refinery gas

1-4 Bottled gas, fuels

»Petrol

40 ~8 Fuel for cars

»Naptha110 ~10

Raw material for chemicals and plastics.

»Kerosine180 ~15 Fuel for Aeroplanes

»Diesel250 ~20 Fuel for cars and lorries

»Oils340 ~35

Fuel for Power Stations, Lubricants and grease

Hot crude »

»Bitumen400+ 40+ Road surfacing.

It is important to realise that the column is hot at the bottom and cool at the top. The crude oil separates into fractions according to weight and boiling point. The lightest fractions, including petrol and liquid petroleum gas (LPG), vapourise and

rise to the top of the tower. Kerosine (aviation fuel) and diesel oil, stay in the middle of the tower Heavier liquids separate lower down. The heaviest fractions with the highest boiling points settle at the very bottom.

TRENDS AS WE UP AND DOWN THE COLUMN

The following table shows how the behaviour of the hydrocarbon molecules alter:

AT THE TOP OF THE COLUMN AT THE BOTTOM OF THE COLUMN

Short carbon chains Long carbon chains Light molecules Heavy molecules Low boiling points High boiling points Gases & very runny liquids Thick, viscous liquids Very volatile Low volatility Highly flammable Not very flammable Light colour Dark colour

Petrol comes off near the top of the column. Does the list above describe petrol?

Fuel oil comes off near the bottom of the column. Does the list above describe fuel oil?

The fractions are now ready for piping to the next areas within the refinery. Some fractions require very little additional processing. However, most molecules require much more processing to become high-value products.

Conversion: cracking and rearranging molecules

Some fractions from the distillation towers need to be transformed into new components . This is where a refinery makes money, because the low-value fractions that aren't in great demand can be converted to petrol and other useful chemicals.

The most widely used conversion method is called cracking because it uses heat and pressure to "crack" heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, reactors and a network of furnaces, heat exchangers and other vessels. Catalytic cracking, or "cat cracking," is the basic petrol-making process. Using intense heat (about 600°C), low pressure and a powdered catalyst (a substance that speeds up a  chemical reaction), the cat cracker can convert most of the heavy fractions into smaller more useful molecules.

Some refineries also have cokers, which use heat and moderate pressure to turn the really heavy fractions into lighter products and a hard, coal like substance that is used as an industrial fuel.

Cracking and coking are not the only forms of conversion. Other refinery processes, instead of splitting molecules, rearrange them to add value. Alkylation makes petrol components by combining some of the gaseous byproducts of cracking. The process, which essentially is cracking in reverse, takes place in a series of large, horizontal vessels.

Reforming uses heat, moderate pressure and catalysts to turn naphtha into high-octane petrol.

Treatment: the finishing touch

Today, a major portion of refining involves blending, purifying, fine-tuning and improving products to meet specific requirements. To make  petrol, refinery workers carefully blend together a variety of hydrocarbons. Technicians also add performance additives and dyes that distinguish the various grades of fuel. By the time the petrol is pumped into a car it contains more than 200 hydrocarbons and additives.

Example: Petrol companies produce different blends of fuels to suit the weather. In winter, they put in more volatile hydrocarbons (with short carbon chains) and in summer they add less volatile hydrocarbons to compensate for the higher temperatures.