Thermodynamics of Thermal Cracking

For the most part we are concerned with the breaking of carbon-carbon and carbon-hydrogen bonds and subsequent rearrangement of intermediate free radicals to produce ethylene. These bonds are broken by the simple application of temperature and because the bonds of interest are strong, high temperatures are required. 

Ethylene is produced in large quantities in many countries by the thermal pyrolysis of ethane with the generalised stoichiometry  

Over most of the temperature range, all of the compounds have positive free energies. This means that they are unstable relative to the elements. Thus the most favoured thermodynamic products are carbon and hydrogen. 

If the elements are the most favoured products, success in the production of ethylene will be achieved by preventing thermodynamic equilibrium occurring. This is accomplished by reducing the temperature of pyrolysis as quickly as possible (quenching) in order to prevent the products converting into the elements. In addition, surfaces promote the formation of carbon and there are several proprietary approaches to passivating the surfaces of the equipment in order to reduce carbon formation.  

The lines for the free energy of formation of ethane and ethylene cross at about lOOOK. Relative to ethane, ethylene becomes favoured at this and higher temperatures. The successful pyrolysis of ethane thus takes place at temperatures over lOOOK. 

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