22.2 Petrochemicals

Petrochemicals are organic compounds derived primarily from petroleum. Key examples include ethylene, propylene, butadiene, methanol, benzene, toluene, and xylene. These compounds are mainly hydrocarbons obtained from various fractions of crude oil, gas oil, and petroleum gas. They can also be sourced from coal, natural gas, and renewable resources like corn and sugarcane.

Petrochemicals serve as fundamental raw materials for the petrochemical industry, which manufactures a vast array of chemical products. These products are integral to modern life and include:

• Plastics and resins
• Rubbers
• Synthetic fibres
• Dyes and paints
• Adhesives and coatings
• Pesticides
• Detergents

Everyday items such as clothing, curtains, carpets, floor tiles, and toys are often petrochemical products, highlighting their significant role in our daily lives. The increasing demand for these products necessitates the expansion of refineries and petrochemical plants.

22.2.1 Building Block Processes in Petrochemical Technology

Petroleum fractions are converted into commercially valuable products through several key processes in petroleum refineries. These foundational processes are known as basic block processes.

The three main processes are:

1. Thermal cracking
2. Catalytic cracking
3. Steam reforming

1. Thermal Cracking

Thermal cracking is a process where high molecular weight hydrocarbon fractions are heated to high temperatures and pressures to break them down into smaller, more valuable hydrocarbons.

• Conditions: High temperature ($450-750^{\circ }C$) and high pressure (70 atm).
• Products: Produces a range of products including light oil (gasoline), medium oil (used in diesel fuel), heavy oil, and coke.
• Example Reaction: The cracking of decane into octane and ethene. ${\mathrm{C}}_{10}{\mathrm{H}}_{22}\to {\mathrm{C}}_8{\mathrm{H}}_{18}+\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$

2. Catalytic Cracking

Catalytic cracking uses a catalyst to break down large hydrocarbon molecules into smaller ones. This method is more efficient and selective than thermal cracking.

• Process: High molecular weight petroleum fractions are passed over a layer of a zeolite catalyst, which facilitates the cracking of heavy fractions into lighter hydrocarbons.
• Primary Use: To produce high-octane fuels, making it an ideal process for manufacturing petrol (gasoline).
• Example Reaction: The cracking of heptane into pentane and ethene. ${\mathrm{C}}_7{\mathrm{H}}_{16}\to {\mathrm{C}}_5{\mathrm{H}}_{12}+\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$

Note: Both thermal and catalytic cracking are crucial for producing commercial gasoline and light oils. The gaseous byproducts like methane, ethane, propane, ethene, and propene are vital raw materials for petrochemical plants.

3. Steam Reforming

In steam reforming, lower gaseous hydrocarbons are reacted with steam at high temperatures in the presence of a catalyst to produce synthesis gas (a mixture of hydrogen and carbon monoxide).

• Feedstock: Natural gas, refinery gas, liquefied petroleum gas (LPG), and low-boiling naphtha.
• Conditions: High temperature ($700-1000^{\circ }C$) and a catalyst.
• Products: A mixture of hydrogen (${\mathrm{H}}_2$) and carbon monoxide ($\mathrm{CO}$), with small amounts of carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$).
• Reaction: $\mathrm{C}{\mathrm{H}}_{4(\mathrm{g})}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{g})}\to \mathrm{C}{\mathrm{O}}_{(\mathrm{g})}+3{\mathrm{H}}_{2(\mathrm{g})}$
• Application: The resulting carbon monoxide ($\mathrm{CO}$) and hydrogen (${\mathrm{H}}_2$) are used to synthesize methanol ($\mathrm{C}{\mathrm{H}}_3\mathrm{OH}$), which is another important petrochemical.

Aromatic Petrochemicals

While cracking produces aliphatic compounds, catalytic reforming of naphtha is used to produce aromatic hydrocarbons like Benzene, Toluene, and Xylene (BTX).