Chemistry · Organic chemistry · Grade 10

Natural gas, oil and coal

How are petrol, kerosene and fuel oil obtained from the same black crude oil?

The situation

The gas burning on your kitchen stove comes through pipelines from the Mubarek or Shurtan fields — but first it is cleaned of sulfur compounds at a plant. At the Fergana oil refinery a single black liquid — crude oil — is separated in a column tens of metres tall into petrol, kerosene, diesel fuel and fuel oil. In Angren coal is turned into gas right underground. How can so many different products come from one raw material, and why does a car need “AI-92” petrol?

Topic

Natural gas, oil and coal. Rectification column of the Fergana oil refinery, Shurtan and Mubarek gas, Angren coal

Key ideas

Fractionating column
Cracking, octane number
Coking

Formulas

CH_{4} + 2 O_{2} \overset{}{\to} CO_{2} + 2 H_{2} O

Combustion of natural gas (mostly methane): 1 mol of $CH_{4}$ releases ≈ 890 kJ of heat.

2 H_{2} S + O_{2} \overset{}{\to} 2 S + 2 H_{2} O

The Claus process: sulfur is obtained from the $H_{2} S$ separated from natural gas.

\begin{matrix} gas & < 40^{\circ} C \\ petrol & 40 – 200^{\circ} C \\ kerosene & 200 – 270^{\circ} C \\ diesel (gas oil) & 270 – 350^{\circ} C \\ fuel oil & > 350^{\circ} C \end{matrix}

Oil fractions by boiling point (approximate, conventional limits).

C_{16} H_{34} \overset{\overset{}{t}}{\to} C_{8} H_{18} + C_{8} H_{16}

Cracking: long molecules break down to form petrol (an alkane) and an alkene.

octane number = φ (isooctane), %

The volume fraction of isooctane in an isooctane + n-heptane mixture that knocks in the same way as the petrol.

Theory

Natural gas (2.18)

Natural gas consists mainly of methane (≈ 85–98 % depending on the field), with ethane, propane, butane, nitrogen, carbon dioxide and, in some fields, hydrogen sulfide. In Uzbekistan gas is produced from the Gazli, Shurtan and other fields. Before it is sent into the pipeline the gas is dried and cleaned of “sour” impurities: in the absorber an amine solution takes up $H_{2} S$ and $CO_{2}$ and releases them again when heated in the regenerator; the separated $H_{2} S$ is converted into sulfur by the Claus process: $2 H_{2} S + 3 O_{2} \overset{}{\to} 2 SO_{2} + 2 H_{2} O$ , $2 H_{2} S + SO_{2} \overset{}{\to} 3 S + 2 H_{2} O$ . In this way the Mubarek gas processing plant produces hundreds of thousands of tonnes of technical sulfur every year. Gas is both a fuel and a chemical raw material: it gives hydrogen and ammonia (Navoiyazot), methanol, acetylene, soot, and polyethylene at Shurtan. In GTL technology gas is converted into synthesis gas ( $CO + H_{2}$ ) and then, by the Fischer–Tropsch process, into liquid fuel: the Oltin Yo'l GTL plant in Kashkadarya (2021) is designed to produce ≈ 1.5 million t of fuel a year from 3.6 billion m³ of gas.

Oil and fractional distillation (2.19)

Crude oil is a mixture of hundreds of hydrocarbons (alkanes, cycloalkanes, arenes) with smaller amounts of sulfur, nitrogen and oxygen compounds. It is heated in a tube furnace to ≈ 350 °C and fed into a fractionating column. Inside the column there are dozens of trays: vapour rises, liquid flows down, and on each tray they meet and exchange heat and matter. So the column is hot at the bottom and cold at the top: light substances collect at the top and heavy ones at the bottom. The fractions (approximately) are: gas < 40 °C (C₁–C₄), petrol 40–200 °C (C₅–C₁₁), kerosene 200–270 °C, diesel fuel (gas oil) 270–350 °C, fuel oil > 350 °C; textbooks also give ligroin (150–250 °C) and wider ranges — the limits are conventional. The fractions are then purified: sulfur compounds are converted into $H_{2} S$ with hydrogen over a catalyst and removed (hydrotreating). Fuel oil cannot be distilled at high temperature (it decomposes), so it is distilled under vacuum: lubricating oils (vacuum gas oil) and a residue — tar (bitumen). In Uzbekistan the Fergana refinery (since 1959) and the Bukhara refinery (since 1997, oil and gas condensate) are in operation.

Cracking, reforming and octane number

Straight-run petrol is scarce and of poor quality. Cracking is the breaking up of long molecules: thermal (≈ 450–550 °C) and catalytic (aluminosilicate and zeolite catalysts, ≈ 450–500 °C). For example, $C_{16} H_{34} \overset{}{\to} C_{8} H_{18} + C_{8} H_{16}$ : one piece is an alkane and the other an alkene. Catalytic cracking gives many branched and aromatic molecules. Knocking (detonation) is the spontaneous explosion of the fuel mixture before the flame front from the spark reaches it: the engine “knocks”, overheats and is damaged. The resistance of petrol to knocking is measured by its octane number: isooctane (2,2,4-trimethylpentane) has 100, n-heptane 0; the percentage of isooctane in the isooctane + n-heptane mixture that knocks like the petrol is its octane number. Branched alkanes and arenes have high octane numbers: n-pentane 62, isopentane ≈ 92, toluene ≈ 120. Reforming (Pt catalyst) converts n-alkanes into arenes, and isomerisation into branched alkanes. An engine with a high compression ratio needs high-octane petrol (AI-92, AI-95).

Coal (2.20)

Coals: peat, brown coal (Angren), bituminous coal, anthracite — the carbon content rises in this order. Coking is heating coal to ≈ 1000 °C in the absence of air. The products are coke (fuel and reducing agent in the blast furnace), coal tar (a source of benzene, toluene, naphthalene, phenol and pyridine), ammoniacal liquor (ammonium salts — fertiliser) and coke-oven gas (≈ 55–60 % $H_{2}$ , ≈ 25 % $CH_{4}$ , $CO$ ). From 1 t of coal we get roughly 750 kg of coke, 30–55 kg of tar and ≈ 300 m³ of gas. At 500–600 °C, semi-coking gives more tar and less gas. If we heat coal in a test tube in the laboratory, water and tar collect in the U-tube, damp red litmus paper turns blue from the ammonia, the gas burns at the end of the delivery tube, and coke remains in the test tube. Underground gasification: air is pumped into the seam and the coal is burned in place: $C + O_{2} \overset{}{\to} CO_{2}$ , $CO_{2} + C \overset{}{\to} 2 CO$ , $C + H_{2} O \overset{}{\to} CO + H_{2}$ — combustible gas comes to the surface. In Angren this method has been used since the 1960s, and the gas was burned in a power station. Safety: the $CO$ in coke-oven gas is poisonous — do the experiment in a fume cupboard; the mouth of the test tube is tilted slightly downwards.

Efficient use and ecology (2.22)

Oil and gas are non-renewable resources. Deep processing (cracking, reforming, vacuum distillation, coking) gives more light products from a tonne of oil; collecting associated petroleum gas instead of burning it off in a flare, and making polymers and fertilisers from gas, save raw material. Burning fuel releases $CO_{2}$ (the greenhouse effect), sulfur-containing fuel releases $SO_{2}$ (acid rain), and incomplete combustion releases $CO$ and soot. That is why gas and fuels are desulfurised, lead antiknock additives are not added to petrol, and cars use catalytic converters. At home, lighting a gas stove or heater correctly and ventilating the room prevent carbon monoxide poisoning.

University: density and flash point; limits of the model

Density is measured with a hydrometer: it floats in the liquid, and the liquid level on the scale shows the density (it sinks deeper in a light liquid). Typical values (15–20 °C): petrol ≈ 0.74, kerosene ≈ 0.80, diesel ≈ 0.84, fuel oil ≈ 0.96 g/mL. The flash point is the lowest temperature at which the vapour of a product heated in a closed crucible flashes briefly with air when a flame is brought up (burning does not continue); the fire point is the temperature at which burning continues. Kerosene ≈ 40–45 °C, diesel ≈ 60–70 °C (standard ≥ 55 °C), petrol below −40 °C — it is the most dangerous. Model: the distillation curves of the three feedstocks are typical (not the oil of one particular field); the temperature scheme of the column is simplified (the top of a real column is ≈ 100–120 °C, with steam and reflux); blending by octane number is linear only for the reference mixture, by definition; the engine's critical compression ratio and pressure curve are schematic; the gas composition and the $H_{2} S$ fraction are typical, and the output of the Claus unit is taken as 97 % of the sulfur; time is slowed down.

Did you know?

The Fergana oil refinery started operation in 1959 and produces more than 60 types of oil products; its design capacity is about 5.5 million tonnes of oil a year.
The “Uzbekistan GTL” (Oltin Yo'l) plant in Kashkadarya opened in December 2021: it is designed to produce about 1.5 million tonnes of synthetic fuel a year — aviation kerosene, diesel, naphtha and liquefied gas — from 3.6 billion m³ of natural gas.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10
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