Production of propylene from steam cracking of hydrocarbons, particularly ethane

A method of producing propylene from ethane comprising the steps of:

• a. steam cracking an ethane or primarily ethane feedstock thereby producing a cracking product containing ethylene, hydrogen, ethane, methane, acetylene and C3 and heavier hydrocarbons;
• b. treating said cracking product in an ethylene plant recovery section including removing said hydrogen, methane and C3 and heavier hydrocarbons therefrom and converting said acetylene therein primarily to ethylene to thereby produce a treated cracking product containing primarily ethylene and ethane and including fractionating said treated cracking product in a C2 fractionator and obtaining a chemical grade ethylene fraction having an ethylene content of less than 99% by volume, a polymer grade ethylene fraction having an ethylene content of at least 99.5% by volume, and a bottoms ethane fraction;
• c. recycling said bottoms ethane fraction to said steam cracking;
• d. reacting by dimerization in a dimerization section a first portion of said chemical grade ethylene fraction thereby producing a butene-rich stream containing heavier hydrocarbons, ethylene and ethane;
• e. separating said heavier hydrocarbons in a butene separator from said butene-rich stream and feeding the remaining butene-rich stream containing ethylene, ethane and butene to said metathesis section;
• f. reacting by metathesis in said metathesis section the butene in said butene-rich stream with a second portion of said chemical grade ethylene fraction thereby producing a propylene-rich stream containing propylene, ethylene and ethane;
• g. separating propylene from said ethylene and ethane in said propylene-rich stream, and
• h. recycling at least a first portion of said ethylene and ethane from said propylene-rich stream to said C2 fractionator.