THE PETROLEUM INDUSTRY

Contents for this page	Related topics
What is petroleum? Petroleum refining Cracking: Octane number Catalytic reforming: The Fischer-Tropf process Additional questions	The chloralkali and Solvay processes The polymer industry (1) The polymer industry (2) The fertiliser industry Ammonia - Nitric acid - Sulphuric acid Electrochemical cells	Data Glossary
Learning Outcomes
After studying this section, you will be familiar with the principles underlying petroleum refining and the Fischer-Tropf process.

What is "petroleum"?:

PETROLEUM, also known as CRUDE OIL is a liquid mixture of hydrocarbons () found in various localities in the earth, not only on land, but also under the sea-bed. Other oxygen- and/or sulphur- containing compounds may also be present. Crude oil is classified into so-called "light", "intermediate" or "heavy", depending on various factors, notably viscosity (). The presence of sulphur compounds is considered to lower the quality of crude oil, and hence its value. The oil obtained from deposits below the North Sea off the Shetland Islands is considered to be of high quality, and is known as "Brent crude", and is used as a standard to establish the price of other oils.

Crude oil typically consists largely of ALKANES () with carbon chains ranging from 1 carbon atom to 60 or more. The constituents having 1-4 carbon atoms are gases, those with 5-19 carbon atoms are, generally speaking, liquids, while the molecules with more than 20 carbon atoms are waxy solids. In addition to alkanes, ALKENES (), AROMATICS and NAPHTHENES are also present in varying amounts.

Above: A cut through part of the lithosphere, showing gas and oil trapped underground. A "gusher", oil driven out by intense pressure below the drill hole. Old photo from a California oilfield in the early 1900's.

Petroleum was formed millions of years ago when organic matter, mainly plants, was covered by sediments and subjected to high pressures over very long periods of time. These sediments found their way underground, where they were eventually covered by what became layers of rocks. The oil, together with natural gas (mainly methane, CH₄), became trapped in underground pockets, from which it is now exploited by drilling.

In South Africa, natural gas is exploited below the sea bed off the coast south of Mossel Bay, and refined by Mossgas.

Petroleum refining:

The refining process involves converting crude oil into FRACTIONS of different boiling points, each having different applications. The process, called FRACTIONAL DISTILLATION, is discussed elsewhere. Not all refineries produce all the fractions listed below, as this will depend on the type of crude oil as well as commercial demands.

As the length of the hydrocarbon chains increase, their physical properties change. Short chain hydrocarbons are either gases or volatile liquids (). Medium chain hydrocarbons are liquids with boiling points below 230 ºC. The boiling point and viscosity increase as the chains become longer.

Another property that changes as the chains get longer, is their FLAMMABILITY, which is their ability to burn in air. A liquid with a high flammability easily catches fire, and so all hydrocarbons with boiling points below 250 ºC should be considered to be a fire risk.

Cracking:

The cracking process involves the breaking of long-chain hydrocarbons into hydrocarbon molecules that have shorter carbon chains, and are thus better suited for use as motor and jet fuel. This operation is carried out at high temperatures in the presence of various catalysts, and typically yields not only alkanes in the C5-C13 range, but also low molecular mass alkenes, such as ethene (CH₂=CH₂) and propene (CH₃CH=CH₂). The cracking process is very complex, but is illustrated in the highly simplified reaction below.

Alkenes are reactive hydrocarbons, and thus the cracking process yields these useful chemicals as by-products, which are known as PETROCHEMICALS. Petrochemicals serve as FEEDSTOCKS for the manufacture of all sorts of useful products, such as plastics, synthetic rubber, medicinal drugs, and solvents, to mention but a few.

Octane number:

The internal combustion engine is designed in such a way that ignition of the petrol vapour-air mixture in the cylinders is ignited at specific time intervals by sparks emitted from the spark plugs. However, when such gas mixtures are heated and compressed, they may ignite spontaneously and prematurely. This leads to a phenomenon known as "knocking" that reduces the efficiency of the engine and increases wear and tear on the moving parts.

It turns out that hydrocarbons that have straight chains of carbon atoms are particularly prone to cause knocking, while those with branched carbon chains have much reduced knocking effects. The hydrocarbon 2,2,4-trimethylpentane has a very low tendency to cause knocking, while n-heptane has a very high tendency to cause knocking.

If a motor fuel has the same knocking properties as a mixture of 93% 2,2,4-trimethylpentane and 7% n-heptane (v/v), then that fuel is rated as 93-octane. It is possible for a fuel to have an octane number greater than 100 and less than 0. The so-called "straight-run" gasoline, produced from crude oil by fractional distillation has an octane number of about 50, and is therefore not suitable as fuel for modern internal combustion engines.

Various anti-knocking additives find their way into commercial petrol, notably tetraethyllead (Pb(CH₂CH₃)₄), which, due to its toxicity and release of lead into the atmosphere, is being phased out in many countries, including South Africa. The so-called "lead-free" petrol does not contain this anti-knocking agent, relying rather on the properties of other additives such as 2-methyl-2-propanol and 2-methoxy-2-methylpropane () . Such a fuel has an octane rating of 95-96.

For diesel fuels, a high tendency for self-ignition under high compression is desirable, and diesel fuels are graded according to their CETANE NUMBERS, which are related to mixtures of n-cetane, CH₃(CH₂)₁₄CH₃ (cetane number = 100), and 1-methylnaphthalene (cetane number= 0) ().

Catalytic reforming:

In order to increase the proportion of motor fuel that can be obtained from petroleum fractions, processes known as CATALYTIC REFORMING are used. What these processes achieve is the conversion of low-octane hydrocarbons (from the higher boiling naphtha fractions) into high-octane fuels. Various methods exist to achieve this, but for the purpose of this discussion, we will look at the reaction that produces branched alkanes from straight-chain alkanes:

This isomerisation takes place in the presence of platinum as a catalyst, at moderate pressures (about 2 MPa) and high temperatures (around 500 ºC).