Alkenes, like ethylene and propylene, are crucial in the petrochemical industry. Steam cracking is the main method for producing these compounds, involving high-temperature reactions that break down larger hydrocarbons into smaller, unsaturated molecules.

The process is influenced by factors such as temperature, pressure, and feedstock composition. Ethylene and propylene serve as building blocks for various plastics, fibers, and chemicals, making them essential in manufacturing everyday products we rely on.

Industrial Preparation of Alkenes

Industrial preparation of ethylene and propylene

• Steam cracking primary industrial method for producing ethylene and propylene
  • Heats saturated hydrocarbons (naphtha or ethane) to high temperatures (750-950°C) in the presence of steam
  • Occurs without oxygen to prevent combustion
  • Reaction takes place in a pyrolysis furnace with short residence time (0.1-0.5 seconds)
• Mechanism of steam cracking:
  1. Initial step: high temperature causes homolytic cleavage of C-C bonds in saturated hydrocarbons
  2. Highly reactive free radicals form
  3. Subsequent reactions: free radicals undergo dehydrogenation, isomerization, and decomposition to form smaller unsaturated hydrocarbons (ethylene and propylene)
• Factors affecting product distribution in steam cracking:
  • Feedstock composition impacts product mix
  • Cracking severity (higher temperature and longer residence time) favors lighter products
  • Higher pressure reduces ethylene and propylene yields
  • Diluents (steam) lower partial pressure of hydrocarbons and improve selectivity

Other cracking methods

• Catalytic cracking: uses catalysts to break down larger hydrocarbons into smaller molecules at lower temperatures
• Thermal cracking: uses heat and pressure to break down hydrocarbons without catalysts or steam

Importance of ethylene and propylene

• Ethylene and propylene key building blocks in petrochemical industry
• Ethylene most widely produced organic compound worldwide
  • Manufactures polyethylene (PE) plastics
    • High-density polyethylene (HDPE) used for bottles, pipes, automotive fuel tanks
    • Low-density polyethylene (LDPE) used for plastic bags, packaging films, squeeze bottles
  • Produces ethylene oxide for antifreeze, polyester fibers, surfactants
  • Produces ethylene dichloride for polyvinyl chloride (PVC) plastics
  • Produces ethylbenzene for styrene monomer used in polystyrene plastics
• Propylene second most important petrochemical feedstock
  • Produces polypropylene (PP) for packaging materials, textiles, automotive parts
  • Produces propylene oxide for polyurethane foams, propylene glycol
  • Produces acrylonitrile for acrylic fibers, ABS plastics
  • Produces cumene for phenol and acetone
• Ethylene and propylene are examples of olefins, which are important in the production of various petrochemicals

Thermodynamics of steam cracking

• Steam cracking endothermic process
  • Requires high temperatures (750-950°C) to overcome activation energy barrier
  • Absorbs heat during breaking of C-C bonds in feedstock
• Entropy crucial in thermodynamics of steam cracking
  • Cracking reactions increase number of molecules (one large hydrocarbon breaks into multiple smaller molecules)
  • More molecules lead to higher entropy ($\Delta S > 0$)
• Gibbs free energy ($\Delta G$) determines spontaneity of steam cracking
  • $\Delta G = \Delta H - T\Delta S$
  • At high temperatures, $T\Delta S$ term becomes more significant
  • Positive entropy change ($\Delta S > 0$) makes $\Delta G$ more negative, favoring spontaneity of cracking reactions
• Temperature effects on equilibrium of steam cracking reactions
  • Le Chatelier's principle: higher temperatures shift equilibrium towards endothermic direction (products)
  • Higher temperatures favor formation of lighter alkenes (ethylene and propylene) over heavier hydrocarbons

Feedstock and processing

• Various hydrocarbon feedstocks are used in alkene production, including naphtha and ethane
• Fractional distillation is used to separate different components of crude oil before cracking processes