High-severity fluidized catalytic cracking systems and processes having partial catalyst recycle

A system for producing olefins from crude oil, the system comprising:

• a first FCC unit comprising a first cracking reaction zone, the first cracking reaction zone being a downflow cracking reaction zone;
• a first separation zone downstream of the first FCC unit;
• a second FCC unit in parallel with the first FCC unit and comprising a second cracking reaction zone, the second cracking reaction zone being a downflow cracking reaction zone;
• a second separation zone downstream of the second FCC unit;
• a regeneration zone downstream of the first separation zone and the second separation zone and fluidly coupled to the first separation zone and the second separation zone;
• and
• a catalyst recycle fluidly coupling the first separation zone to the second FCC unit.

A method for operating a system having a first FCC unit and a second FCC unit for producing products from a hydrocarbon feed stream, the method comprising:

• introducing the hydrocarbon feed stream to a feed separator;
• separating the hydrocarbon feed stream into a lesser boiling point fraction and a greater boiling point fraction in the feed separator;
• passing the greater boiling point fraction to the first FCC unit;
• passing the lesser boiling point fraction to the second FCC unit, where the first FCC unit and the second FCC unit are downflow FCC units;
• cracking at least a portion of the greater boiling point fraction in the first FCC unit in the presence of a first catalyst at a first cracking temperature of from 500° C. to 700° C. to produce a first cracking reaction product and a spent first catalyst;
• cracking at least a portion of the lesser boiling point fraction in the second FCC unit in the presence of a second catalyst and at a second cracking temperature of from 500° C. to 700° C. to produce a second cracking reaction product and a spent second catalyst;
• passing the at least a portion of the spent second catalyst to the second FCC unit, where the second catalyst comprises the at least a portion of the spent second catalyst and the regenerated catalyst;
• and
• mixing the at least a portion of the spent second catalyst, the regenerated catalyst, and the lesser boiling point fraction in a mixing zone positioned upstream of a cracking reaction zone of the second FCC unit.