Processing C4 olefin streams for the maximum production of propylene

A process for the production of propylene from a C4 olefin stream containing n-butenes, isobutylene and paraffins comprising the steps of:

• a. passing said C4 olefin stream in contact with a metathesis catalyst whereby autometathesis occurs including the reaction of isobutylene with n-butenes and an autometathesis product is produced containing ethylene and propylene and heavier olefins including unreacted isobutylene and paraffins;
• b. fractionating said autometathesis product to remove an ethylene stream leaving a deethyleneized stream of said propylene and said heavier olefins and paraffins;
• c. fractionating said deethyleneized stream to remove propylene product leaving a depropyleneized stream of said heavier olefins and paraffins;
• d. processing said depropyleneized stream to remove the C5+ fraction leaving a mixed C4 stream,
• e. processing said mixed C4 stream to remove the isobutylene leaving a remaining normal C4 olefins stream also containing paraffins;
• f. purging a portion of said remaining normal C4 olefins stream to limit paraffin buildup;
• g. admixing the remaining portion of said remaining C4 olefins stream with a quantity of ethylene sufficient for conventional metathesis wherein said quantity of ethylene comprises said removed ethylene stream and an amount of external fresh ethylene selected such that the molar ratio of said external fresh ethylene to the n-butenes in said C4 olefin stream is from zero to 0.8;
• and
• h. passing said admixture in contact with a metathesis catalyst and reacting at least a portion of the C4 olefins stream with ethylene to produce a metathesis product containing additional propylene.