Dual riser FCC reactor process with light and mixed light/heavy feeds

A dual riser FCC process, comprising:

• cracking a first hydrocarbon feed having a carbon number less than 12 in a first riser under first-riser FCC conditions to form a first effluent enriched in ethylene, propylene or a combination thereof;
• cracking a second hydrocarbon feed having a carbon number less than 12 in a second riser under second-riser FCC conditions to form a second effluent enriched in ethylene, propylene or a combination thereof, wherein the first hydrocarbon feed comprises mixed olefinic C4's and the second hydrocarbon feed comprises mixed paraffinic C4's and the first-riser and second-riser FCC conditions are independently selected to favor production of ethylene, propylene or a combination thereof;
• recovering catalyst and separating gas from the first and second FCC effluents;
• regenerating the recovered catalyst by combustion of coke in a regenerator to obtain hot, regenerated catalyst;
• recirculating the hot regenerated catalyst to the first and second risers to sustain a continuous operating mode;
• and
• introducing a coke precursor to the first or second riser with the respective first or second hydrocarbon feed at a ratio of from 1 to 40 parts by weight coke precursor to 100 parts by weight fresh hydrocarbon feed, wherein the coke precursor is selected from the group consisting of acetylene, substituted acetylene, and mixtures thereof with diolefins.