2 The Petrochemical Industry 2.1 Refining of crude oil The crude oil which is pumped out of the world’s oil fields is the result of the slow decomposition of plant material in the absence of oxygen. As a consequence, the products are dominated by hydrocarbons: a complex mixture of mainly alkanes and cycloalkanes, with small amounts of alkenes, aromatic compounds, compounds containing sulfur or nitrogen, as well as various inorganic substances. It contains literally thousands of compounds, ranging from low molecular gases to very large compounds with very high melting points. More than 90% of the petroleum produced is used for fuels for heating and transportation. However, petroleum is also an extremely valuable source of petrochemicals, which serve as raw materials for the chemical industry. 90% of the production of synthetic organic chemicals comes from crude oil and natural gas sources. After washing to remove acids and salts, the crude oil is transferred to a distillation tower where various fractions are separated by fractional distillation, as shown in Figure 2.1. The first three liquid fractions comprise the bulk of the distilled products. FIGURE 2.1 Fractions obtained from the distillation of crude oil (from Ashcroft & Hammill, Chemistry of the car, John Murray) 2. The Petrochemical Industry
Within each fraction (except the gases), there are many – possibly hundreds – individual compounds. For example, a thorough GC analysis of one petrol fraction showed the presence of fifty identifiable substances comprising 70% of the fraction, while the balance were not able to separated well enough to determine precisely. The division between the various classes of hydrocarbons is shown in Figure 2.2. Unidentified
14%
Aromatic
6%
Straight chain
11%
Branched
21%
Cyclic
48%
FIGURE 2.2 Composition of petrol fraction The proportions of these various fractions varies from one source to another, but without exception, the yield of the petrol fraction does not provide anywhere near the necessary volume of fuel required to meet current demands for motor vehicle fuel. Similarly, there is far too much of the so‐
called heavy distillates. As a consequence, oil refineries are mostly about chemical conversion of some of the less valuable substances into those which will be useful in car engines, as shown in Figure 2.3. FIGURE 2.3 Composition of crude oil at various refining stages (from Ashcroft & Hammill, Chemistry of the car, John Murray) IPC 2.2 2. The Petrochemical Industry
The “ideal” petrol You might imagine that any hydrocarbon in the right boiling point range would be suitable as a component of motor vehicle fuel, whether it is a straight, branched or cyclic alkane, an alkene or an aromatic compound. They all release fairly similar amounts of energy when burnt. For example, for C‐7 representatives of each class, the energy released (in MJ) per litre is as follows: heptane 33, 2‐
methylhexane 33, cycloheptane 38, 1‐heptene 32, toluene 36. However, the amount of energy of released is not the only consideration with engine fuel: 
the fuel must burn evenly, rather than in one sudden, explosive burst which causes knocking – straight chain alkanes burn too fast, branched or cyclic molecules burn more controllably, as do aromatic compounds 
it must be safe health‐wise; aromatic compounds such as benzene and toluene are proven or suspected carcinogens 
it should not cause other problems in the engine – alkenes can polymerise, instead of burning, leading to the formation of gums in the engine So the ideal petrol has mostly branched and cyclic hydrocarbons, with a low level of “safe” aromatics to improve performance. The measure of a fuel in terms of its performance in an engine is known as the octane rating. Most cars run on octane ratings between 90 and 100. The higher the octane rating, the higher the performance, the smoother the combustion. The octane rating is measured using a special test engine, and a device to measure the combustion for knocks. The test fuel is measured by comparison with standard fuels, which are a blend of heptane and 2‐methylheptane. These standard blends are assigned different octane ratings based on the mix. For example, 30% heptane and 70% 2‐methylheptane is assigned an octane rating of 89.7, while normal unleaded petrol has an octane rating of 92, which is equal in performance to a 73:27 blend. Pure 2‐methylheptane has a rating of 120 and pure heptane a rating of 0. Figure 2.4 shows the octane ratings of a range of pure hydrocarbons in the petrol fraction. FIGURE 2.4 Octane ratings of various hydrocarbon families (A ‐ alkenes, B – aromatics, C – cycloalkanes, D – methylalkanes, E – dimethylalkanes, F – straight chain alkanes) (from Tedder et al, Basic Organic Chemistry Part 5, John Wiley) For many years, there was a simpler answer to improving the octane rating of petrol: the addition of an organometallic compound, tetraethyl lead, Pb(CH2CH3)4. 0.5 grams/L of this increases the octane rating by almost 10 points. However, the pollution caused by this is well‐known, and it is now banned from use. IPC 2.3 2. The Petrochemical Industry
The common petrols sold today in Australia – unleaded, lead replacement and premium unleaded – have octane ratings typically of 92, 96 and 98, respectively. BP Australia provides some specifications for some of its range, the premium unleaded having a very high (40%) concentration of aromatics, which explains the rating. It should be noted that the benzene content is minimal. Conversion Processes To increase the amount and performance of the petrol fraction to meet current demands, there are a number of refining processes, and their purposes fit into one of three categories, as shown in Figure 2.5. break apart
join together
Light
Medium
Heavy
distillates
distillates
distillates
(< C4)
(C4-C10)
(>C10)
rearrange
FIGURE 2.5 Refining processes Producing smaller molecules The basic name given to this process is cracking. Equation 2.1 provides an example of some of the compounds that might be produced. Typically, a wide range of compounds will be produced, including some alkenes. C30H62  C7H16 + C8H16 + C15H30 Eqn 2.1 This was originally done at temperature approaching 1000C (thermal cracking), but it is now almost obsolete, because of its inefficiency. At lower temperatures (about 500°C), the reaction will only progress sufficiently quickly in the presence of a catalyst (catalytic cracking), made from materials known as zeolites. This latter process can be carried out under a hydrogen atmosphere to encourage the formation of alkanes and to help remove nitrogen and sulfur impurities in the form of ammonia and hydrogen sulfide (hydrocracking). Rearranging molecular structure Because C4‐C10 straight chain molecules are not particularly useful as fuels, and the level of aromatics in petrol fractions is too low, it is necessary to change the molecular shape of the undesirable substances to produce branched and aromatic compounds. This is done by isomerisation and reforming, respectively. Isomerisation takes straight chain alkanes, and changes the carbon skeleton by moving an end carbon and joining it along the chain, (Equation 2.2), which shows the rearrangement of heptane to 2‐methylhexane. The process is usually catalysed by aluminium chloride. Eqn 2.2 IPC 2.4 2. The Petrochemical Industry
Reforming converts alkanes and cycloalkanes to aromatic compounds. For example, toluene can be obtained from methylcyclohexane, as shown in Equation 2.3. Special metal catalysts, such as rhenium, are used to promote the conversion of cyclic alkanes to benzene derivatives. Catalytic reforming is also a major source of H2 gas for use elsewhere in the refining process, or sold commercially for other industrial applications, such as the production of ammonia for fertiliser. CH3
CH3
Eqn 2.3 + 3 H2
Joining smaller molecules together The small gaseous alkanes – methane to butane – have commercial value in natural gas and LPG, but some of the small alkenes in the lightest fraction are more usefully combined with other small alkanes or alkenes to form alkanes in the petrol fraction. Alkylation (alkene + alkane) and polymerisation (alkene + alkene) are processes designed to increase the FW of hydrocarbon materials, as shown in Equations 2.4 and 2.5, respectively. Both involve catalysts, usually acidic, such as sulfuric acid. Eqn 2.4 +
Eqn 2.5 +
Of the various processes employed to produce more petrol fraction hydrocarbons, catalytic cracking and reforming are by far the most important. 2.2 Primary petrochemicals The various refining processes described above not only produce an improved yield of desirable petrol fraction hydrocarbons, but also a range of simple hydrocarbons, which form the basis of modern industrial organic chemical production. These are the so‐called primary petrochemicals, and include the following: 
methane 
ethene (old name ethylene) – the single most important organic chemical for production of industrial chemicals; something approaching 100 million tonnes per year are produced and consumed; the bulk is used in the manufacture of a wide range of plastics 
propene (propylene) 
1‐butene (butylene) 
2‐methylpropene (isobutylene) 
ethyne (acetylene) 
benzene 
toluene 
the three isomeric dimethylbenzenes (xylenes) 
naphthalene IPC 2.5 2. The Petrochemical Industry
They are separated from other hydrocarbons and each other by a range of familiar laboratory purification techniques, adapted up to a very large industrial scale, including: fractional distillation 
solvent extraction 
crystallisation 
absorption 
2.3 Secondary petrochemicals The variety of compounds produced from the relatively small set of primary petrochemicals is extraordinary, as is the variety of ways that individual compounds have been made. For example, ethanoic acid has been manufactured in at least half a dozen ways over the years, as shown in Figure 2.6. Alkane
Ethanol
Methanol
mixture
O2
fermentation
CO
Ethene
O
-H2
2
H2O
O2
ETHANOIC
Ethyne
Ethanal
ACID
FIGURE 2.6 Industrial synthetic routes for ethanoic acid manufacture Petrochemical manufacture uses a range of reaction types, some of which you will be familiar from Chemistry of Natural Substances. They include: 
oxidation hydrogenation 
dehydrogenation 
halogenation 
aromatic substitution 

hydrolysis hydroformylation 
TABLE 2.2 Petrochemical reactions Reaction Type Reactant Product Halogenation ethane and chlorine chloroethane Hydrolysis propene and water 2‐propanol Hydrogenation carbon monoxide and hydrogen methanol Dehydrogenation 2‐propanol propanone Oxidation methylbenzene benzoic acid Hydroformylation propene, CO and hydrogen butanal Aromatic substitution benzene nitrobenzene IPC 2.6 2. The Petrochemical Industry
What You Need To Be Able To Do 
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define important terminology list major crude oil fractions explain the need for refining processes outline the basic composition requirements for motor fuel describe and give examples of important refining processes list reaction types used in petrochemical manufacture name four important primary petrochemicals name a important secondary petrochemical made from each of the four primaries and outline its synthesis and uses IPC 2.7