Crude oil is a mixture of hydrocarbons

Crude oil, also known as petroleum, is a complex mixture of hydrocarbons extracted from underground oil reservoirs. The main components are alkanes, which are saturated hydrocarbons consisting of carbon and hydrogen atoms in straight chains.

Crude oil contains alkanes ranging from small molecules like pentane (C_5_H_12_) to larger alkanes over 50 carbons long.

The varying lengths of the alkane chains result in a broad range of boiling points in crude oil, from $-$162°C to more than 350°C. This range of boiling points is crucial for the separation of crude oil into different components by fractional distillation.

Fractional distillation of crude oil

Fractional distillation is a technique used to separate hydrocarbon components of crude oil based on their boiling points. The process involves heating crude oil to about 350°C in a furnace, causing it to vaporise. The vaporised components are then separated by their ability to condense at different temperatures in a tall column known as a fractionating column.

This technique is effective because the various hydrocarbon components have different boiling points related to their chain lengths. As the vapour mixture rises and cools in the column, fractions with higher boiling points condense first, while heavier residues remain at the bottom.

How fractional distillation works

The key steps in fractional distillation are:

1. Crude oil is heated to above 350°C in a furnace to vaporise the hydrocarbon mixture.
2. These vapours enter a fractionating column that has higher temperatures at the bottom (_{350°C) and lower temperatures near the top (}40°C).
3. As the hot vapours rise through the column, they cool down. When the vapour temperature drops below the boiling point of a hydrocarbon in the mixture, it condenses from gas to liquid on the tray surface.
4. The condensed hydrocarbon liquids that accumulate on each tray are drawn off at specific intervals as fractions (mixtures of hydrocarbons with similar boiling points).

The smallest hydrocarbons, with the lowest boiling points, do not condense at all and remain as gases at the top. Larger molecules, over 50 carbons, have the highest boiling points and condense near the bottom of the comumn to form a tar-like residue.

Key properties of fractions

The separated fractions exhibit consistent trends in key properties as you move down the fractionating column, which makes them suitable for specific uses.

Boiling point - The boiling point increases progressively down the column as the alkane chain length increases in each successive fraction. The lightest gases boil below 0°C, while the undistillable residue requires temperatures over 500°C.

Viscosity - Viscosity steadily increases as you move down the column. Very light distillates, like gasoline or jet fuel, flow freely. Heavier fractions collected towards the bottom demonstrate progressively higher viscosities.

Flammability - Flammability decreases down the fractionating column. Light gases and short-chain hydrocarbons at the top ignite readily, making them excellent fuels due to their high flammability and volatility. In contrast, heavy fractions and long-chain hydrocarbons at the bottom are less flammable and volatile, making them harder to combust and less suitable as fuels.

Meeting demand through cracking

The process of refining crude oil by fractional distillation yields varying amounts of hydrocarbons. This produces larger quantities of long-chain hydrocarbons like bitumen, and smaller quantities of short-chain hydrocarbons, such as petrol, diesel, and naphtha.

There is a higher demand for short-chain hydrocarbons than for long-chain hydrocarbons. This is because they are more efficient as fuels and serve as crucial chemical feedstocks for producing in-demand consumer products, including plastics, fabrics, and packaging materials.

To balance this demand, the heavier, less useful fractions are converted into smaller, more valuable molecules through a process called cracking.

Cracking breaks long-chain alkanes down

Cracking is a key process that involves breaking the carbon-carbon bonds in larger alkane chains to form smaller alkenes and alkanes. It is a type of thermal decomposition reaction.

For instance:

Decane ➔ octane + ethene

C₁₀H_{22 ➔} C₈H₁₈ + C₂H₄

Key points:

• Cracking transforms less useful long-chain alkanes into more valuable smaller hydrocarbons.
• The point at which the chain breaks is random, leading to a variety of product combinations.
• Cracking is also essential for producing alkenes, which are used in manufacturing plastics and polymers.

Thermal and catalytic cracking

Two principal methods of cracking exist:

1. Thermal cracking
2. Catalytic cracking

1. Thermal cracking

• Thermal cracking operates at very high temperatures (approximately 1,000°C) and pressures (about 70 atm).
• This process generates a high yield of alkenes, which are vital for creating numerous valuable products.
• For example, ethene, a common product of thermal cracking, can be polymerised to manufacture polyethene, a widely-used plastic.

2. Catalytic cracking

• Catalytic cracking employs a zeolite catalyst (a hydrated aluminosilicate mineral) and requires moderate temperatures (around 450°C) and pressures than thermal cracking.
• This process primarily produces aromatic hydrocarbons and fuels for vehicles. Aromatic hydrocarbons contain highly stable benzene rings with delocalised electrons.

Catalysts enhance the efficiency of the cracking process in two ways:

• They allow for lower temperatures and pressures, reducing energy consumption and production costs.
• They increase the rate of reaction, enabling faster production of desired products.

The table below compares the conditions and products of thermal and catalytic cracking:

Reforming alkanes

Reforming is a process used to convert less desirable straight-chain alkanes into more valuable branched, cyclic, and aromatic hydrocarbons. This process is crucial in the petroleum industry for improving fuel quality and producing important chemical feedstocks.

The process typically occurs under specific conditions:

Catalyst - Platinum supported on aluminium oxide.

Reaction conditions - High temperatures (450-520°C) and moderate pressures (10-40 atm).

During reforming, alkanes undergo various transformations. For example:

1. Hexane (C₆H₁₄) can be reformed into cyclohexane (C₆H₁₂) and then benzene (C₆H₆): C₆H₁₄ ➔ C₆H₁₂ + H₂

C₆H₁₂ ➔ C₆H₆ + 3H₂

1. Octane (C₈H₁₈) can be reformed to a branched isomer, 2,5-dimethylhexane: C₈H₁₈ ➔ CH₃CH(CH₃)CH₂CH₂CH(CH₃)CH₂CH₃