The adjacent diagram schematically depicts the major hydrocarbon sources used in producing petrochemicals are.
• Methane, ethane, propane and butanes: Obtained primarily from natural gas processing plants.
• Naphtha obtained from petroleum refineries.
• Benzene, toluene and xylenes, as a whole referred to as BTX and primarily obtained from petroleum refineries by extraction from the reformate produced in catalytic reformers.
• Gas oil obtained from petroleum refineries.
Methane and BTX are used directly as feedstocks for producing petrochemicals. However, the ethane, propane, butanes, naphtha and gas oil serve as optional feedstocks for steam-assisted thermal cracking plants referred to as steam crackers that produce these intermediate petrochemical feedstocks:
• Butenes and butadiene
In 2007, the amounts of ethylene and propylene produced in steam crackers were about 115 Mt (megatonnes) and 70 Mt, respectively. The output ethylene capacity of large steam crackers ranged up to as much as 1.0 – 1.5 Mt per year.
Steam crackers are not to be confused with steam reforming plants used to produce hydrogen and ammonia.