FCC riser: the gasoline optimum

A fluid catalytic cracking riser — the unit that makes most of a refinery's gasoline — showing the one result that decides how you run it. Gas oil cracks to gasoline, but gasoline overcracks to gas and coke, and the catalyst deactivates as coke lays down. Those three together mean gasoline goes through a MAXIMUM in contact time: past the optimum, running the riser harder destroys product while conversion keeps climbing. Open the sensitivity and the curve draws itself — that hump is the whole point. A first-order network (all hydrocracker can express) makes gasoline rise monotonically with conversion and would recommend exactly the wrong operation. The cat/oil ratio is computed, not chosen: cracking is endothermic and the only heat source is the sensible heat of hot regenerated catalyst, so the circulation follows from a heat balance and lands in the real 5-10 band. That is the number tying the riser to the regenerator an operator actually turns — raise the regenerator temperature and watch it fall. The rate constants here are illustrative and are NOT any real feed's kinetics. The unit op deliberately ships none: an FCC lump matrix is regressed from one feed on one catalyst, it is licensor-proprietary, and a fabricated one would decide the answer while looking authoritative. A riser without constants fails validation and says why. Bring your own regression and this becomes your riser. Bounded: three lumps (no per-cut gasoline detail), isothermal riser (a real one drops 30-60 K as the endotherm bites), no catalyst/vapour slip, and no regenerator — coke burn and the air rate are a separate unit this does not model.