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

Source page: https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p1/

[Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 5: From crude oil and natural gas to the C1-C4 building blocks](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/) Part 1 of 5

## The raw materials: crude oil, natural gas and the refinery

Nearly every organic chemical starts as a hydrocarbon molecule pumped out of the ground. Before any chemistry happens, crude oil and natural gas are sorted by boiling point and cleaned up. This part looks at what those raw materials contain, how a refinery splits and reshapes them, and why the chemical industry gets only the leftovers of a business built to make fuel.

### 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.

Value chain From crude oil, natural gas and coal to the C1-C4 building blocks

- **Fossil raw materials** oil about 74%, gas 25%, coal 1% of cumulative feedstock since 1970 (IEA)

  - **Natural gas** mostly methane, plus ethane to butanes in wet gas

    - **Methane → synthesis gas (CO + H2)** steam reforming

      - **Ammonia** with N2 from air; about 185 Mt/yr
      - **Methanol** about 100 Mt/yr; to formaldehyde, acetic acid, MTBE, olefins

    - **Ethane, propane, butane (NGLs)** separated at gas plants

      - **Steam cracking** ethylene-rich slate
      - **Propane dehydrogenation** on-purpose propylene

  - **Crude oil → refinery** chemicals take about 6-8% of refinery output

    - **Naphtha**

      - **Steam cracking** ethylene, propylene, butadiene, pyrolysis gasoline
      - **Catalytic reforming** benzene, toluene, xylenes plus hydrogen

    - **Gas oil and vacuum gas oil**

      - **Fluid catalytic cracking** propylene and butenes as by-products
      - **Steam cracking** heavier slate, more fuel oil

    - **Refinery gas and LPG** propane and butanes

  - **Coal** dominant for ammonia and methanol in China

    - **Gasification → synthesis gas** ammonia, methanol, then methanol-to-olefins
    - **Coke ovens** coal tar, benzene, naphthalene

 Source: Wittcoff (2004), Ch 2; IEA (2018), Ch 1-2, Wittcoff pdf pp. 83-87, 93; IEA pdf pp. 24-25, 27-31  In the data

#### Feedstock is a material, not just a fuel

The IEA (2018) points out an accounting oddity. Oil and gas used as chemical feedstock are measured in energy units, because before they enter the plant they are identical to fuel. Yet their carbon and hydrogen atoms end up inside the plastic or fertiliser. Feedstock is more than half the chemical sector's energy input. About 12 million barrels a day of oil products, 105 bcm of natural gas and 80 Mt of coal enter the sector this way each year (2017 figures). The IEA's analogy is wood: it can be burned, or it can be timber in a house.

- 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.

Data Distillation yields of four crudes (wt% of crude)

| Cut | Brent (Norway) | Bonny Light (Nigeria) | Green Canyon (USA) | Ratawi (Mid East) |
|---|---|---|---|---|
| API gravity | 38.3 | 35.4 | 30.1 | 24.6 |
| Sulphur, wt% | 0.37 | 0.14 | 2.00 | 3.90 |
| Light ends | 2.3 | 1.5 | 1.5 | 1.1 |
| Total naphtha | 30.1 | 27.7 | 16.9 | 15.8 |
| Kerosene | 9.9 | 12.5 | 8.5 | 7.4 |
| Atmospheric gas oil | 15.1 | 21.6 | 14.1 | 10.6 |
| Vacuum gas oil (light + heavy) | 30.3 | 31.2 | 32.9 | 32.2 |
| Vacuum residue | 12.3 | 5.5 | 26.1 | 32.9 |

Vacuum gas oil is the sum of the light and heavy VGO rows in the source.
 Source: Kent (2012), Ch 18 Petroleum and Its Products, Table 18.7, pdf p. 713  Common misreading

#### "Naphtha" is not one product

Wittcoff notes that the term is loosely defined. For cracking it usually means material boiling between 70 and 200 C, with five to nine carbon atoms. To add to the confusion, "light naphtha" in cracker economics often means condensate, a cheap C5-plus liquid from gas wells, also called natural gasoline. When someone quotes a naphtha price or a naphtha cracker cost, ask which cut they mean.

- 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.

Data Typical product mix from fluid catalytic cracking of gas oil

| Product | Approximate wt% |
|---|---|
| Propylene | 3.2 |
| n-Butenes and isobutene (no butadiene) | 5.8 |
| Isobutane | 3.3 |
| Branched C5+ (cat cracker gasoline) | 36.5 |
| C10+ (gas oil) | 15.0 |
| Residue and coke | 29.0 |
| Ethylene, ethane, propane, n-butane, H2, H2S | 7.2 |

Little ethylene is made; propylene and butenes come out dilute and must be purified, for example in a propane-propylene splitter.
 Source: Wittcoff (2004), Ch 2, Table 2.9, pdf p. 105  In the data

#### Light ends are priced below crude

Wittcoff's ratios of product value to crude cost in the US show naphtha at 1.2-1.3 times crude, LPG at 1.1-1.4, gasoline at 1.4-1.5 and heavy fuel oil at only 0.6-0.7. Fifteen years later the IEA (2018) found that LPG and naphtha usually trade below crude. That gives refineries little reason to make more of them. The sign differs because the dates and markets differ. The lesson holds in both cases: chemical feedstocks are not what refiners chase.

- 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
[H₂O Water](https://www.materialinsights.org/commodities/water/)[Oil Crude oil](https://www.materialinsights.org/energy/crude_oil/)[Gas Natural gas](https://www.materialinsights.org/energy/natural_gas/)[Pt Platinum](https://www.materialinsights.org/commodities/platinum/)[Re Rhenium](https://www.materialinsights.org/commodities/rhenium/)[Tm Timber](https://www.materialinsights.org/commodities/lumber/)[Cl2 Chlorine](https://www.materialinsights.org/chemicals/chlorine/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[NH3 Ammonia](https://www.materialinsights.org/chemicals/ammonia/)[C2H4 Ethylene](https://www.materialinsights.org/chemicals/ethylene/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[BD Butadiene](https://www.materialinsights.org/chemicals/butadiene/)[Bz Benzene](https://www.materialinsights.org/chemicals/benzene/)

### Check your understanding

Which statement best describes the chemical industry's position inside a refinery?
 Wittcoff puts chemicals at about 6% of US refinery output and rarely above 8% elsewhere. The IEA notes that FCC and reformer chemicals are by-products of units run for gasoline and diesel. Refineries make almost no ethane.

A refinery runs its FCC riser hotter than usual. What is the most likely effect?
 Kent's Ch 18 explains that higher riser outlet temperatures favour olefin-rich light gases, especially propylene, at the expense of gasoline.

Why is ethane, about a third of chemical feedstock, not a refinery product?
 The IEA states that refineries do not produce ethane to any meaningful extent. Ethane and most chemical LPG come from fractionating natural gas liquids.

[← Module contents](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/)[Mark read · Part 2: Steam cracking: the engine of the petrochemical industry →](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p2/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/index.md)
- [Steam cracking: the engine of the petrochemical industry](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p2/index.md)
- [Propylene, the C4 stream and the aromatics](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p3/index.md)
- [C1 chemistry: synthesis gas, methanol, ammonia and coal](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p4/index.md)
- [The economics: feedstock, scale, integration and carbon](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/p5/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/oil-gas-to-building-blocks/review/index.md)