From crude oil and natural gas to the C1-C4 building blocks: The raw materials: crude oil, natural gas and the refinery

1.1 Seven building blocks carry most of the industry, and they come from two raw materials

Wittcoff and colleagues make a simple claim that organises this whole module. A vast organic chemical industry rests on just seven building blocks: ethylene, propylene, the C4 olefins (butenes and butadiene), benzene, toluene, the xylenes and methane. They estimate that about 95% by weight of the organic chemicals the world uses comes from petroleum and natural gas. The IEA (2018) uses a slightly different list of seven "primary chemicals": ammonia, methanol, ethylene, propylene, benzene, toluene and mixed xylenes. Together these account for about two-thirds of the chemical sector's final energy use.

The naming needs a word of explanation. An olefin (or alkene) is a hydrocarbon with a carbon-carbon double bond. That bond is the handle that lets molecules be joined into polymers or reacted with oxygen, chlorine or water. Aromatics are ring-shaped molecules built on the six-carbon benzene ring. The IEA calls the olefins and aromatics together "high-value chemicals" (HVCs). "C2", "C3" and "C4" simply count carbon atoms.

Scale matters for what follows. The IEA (2018) puts world ammonia output at about 185 Mt a year, methanol at about 100 Mt, ethylene plus propylene at about 255 Mt and BTX aromatics at about 110 Mt. Each tonne is made by one of a small number of routes. These are steam cracking of ethane or naphtha, refinery processes such as catalytic cracking and reforming, and steam reforming or gasification to synthesis gas. The tree below maps these routes from raw material to building block. The rest of the module explains each branch and why its cost differs from region to region.

• Wittcoff (2004), Ch 2 Chemicals from Natural Gas and Petroleum · pdf pp. 83, 86
• IEA (2018), Ch 1 and Ch 2 · pdf pp. 17, 24-25, 27-29

1.2 Crude oil is a mixture sorted by boiling point, and naphtha is the cut that matters most

Crude oil is not one substance. Kent (2012) stresses that there are hundreds of crudes with very different compositions. Robinson's chapter there tabulates ten of them, with sulphur from well under 0.1% to over 5% by weight and specific gravity from about 0.80 to just over 1.0. Most of the molecules are hydrocarbons. They come as straight chains (paraffins), rings (naphthenes) and aromatic rings, with some sulphur, nitrogen and metal compounds mixed in.

The first step in a refinery is distillation, which separates by boiling point. Think of a tall column that is hot at the bottom and cool at the top. Light molecules rise and condense high up, and heavy ones stay low. Wittcoff's table gives the cuts. Gases boil below 20 C and are mostly methane, with ethane, propane and butane. Light naphtha (C5-C6) boils at 70-140 C and heavy naphtha (C7-C9) at 140-200 C. Kerosene comes off at 175-275 C, diesel-range gas oil at 200-370 C, and heavy fractions above 370 C. The heavy residue can be distilled again under vacuum (0.07 bar) to recover vacuum gas oil boiling at 430-530 C.

The yield of each cut depends on the crude. The table shows that Brent gives about 30% naphtha by weight, while the heavy Middle East crude Ratawi gives under 16% and leaves a third of the barrel as vacuum residue. The IEA (2018) gives a world average refinery naphtha yield of about 7% and an LPG yield usually below 5%. Refineries make almost no ethane. So the ethane that feeds a third of chemical feedstock comes from gas processing plants, not refineries.

• Kent (2012), Ch 18 Petroleum and Its Products · pdf pp. 710-713
• Wittcoff (2004), Ch 2, Table 2.2 · pdf pp. 85, 87-89, 92
• IEA (2018), Ch 2 · pdf p. 37

1.3 Refineries are built for fuel; chemicals get by-products of cracking and reforming

After distillation, a refinery reshapes molecules to make more of the fuels people buy. Wittcoff stresses that chemicals are the junior partner. They take about 6% of US refinery output and rarely more than 8% anywhere. They compete for raw materials only because their products are worth much more.

Two refinery units matter most for chemicals. Fluid catalytic cracking (FCC) breaks heavy gas oil into gasoline over a zeolite catalyst. A zeolite is a crystalline aluminosilicate full of molecule-sized pores with acidic sites. The reaction runs at about 500 C and near atmospheric pressure, with a contact time of 1-3 seconds. Coke laid on the catalyst is burned off in a regenerator at about 630 C. Kent (2012) adds that higher riser temperatures favour light olefins, especially propylene, at the expense of gasoline. Many FCC units are run that way to feed nearby olefin plants. A typical FCC mix is shown in the table: only about 3% propylene and 6% butenes, but from a very large flow.

Catalytic reforming converts naphtha into aromatics over platinum or rhenium on alumina, at 400-500 C and 25-35 bar. It does three things at once. It strips hydrogen off rings (dehydrogenation), rearranges chains into rings (isomerisation and dehydrocyclisation) and breaks some molecules (hydrogenolysis). About 15% of the feed leaves as gas and hydrogen. The rest is an aromatics-rich liquid called reformate. The hydrogen matters: it feeds the refinery's desulphurisation units.

The IEA (2018) estimates that more than 40% of the world's propylene and about 80% of BTX aromatics come from refineries. These products are still by-products. The refinery is run to maximise gasoline and diesel, not chemicals.

• Wittcoff (2004), Ch 2 sections 2.2.2-2.2.3 · pdf pp. 93, 102-107
• Kent (2012), Ch 18, FCC and catalytic reforming · pdf pp. 721-722, 736-737, 741
• IEA (2018), Ch 2 · pdf pp. 37-39

See it in the table

Check your understanding

Which statement best describes the chemical industry's position inside a refinery?

A refinery runs its FCC riser hotter than usual. What is the most likely effect?

Why is ethane, about a third of chemical feedstock, not a refinery product?