With over 300 units in operation, Fluid Catalytic Cracking (FCC) is currently the dominant refinery conversion process. Over the last years, rapid separation of hydrocarbon vapors and catalyst at the riser outlet has been put forward to limit non-selective decomposition of reaction products, thus enhancing product selectivities.A new riser separator system, called RS², was developed to minimize post-riser cracking, while maintaining flexibility of operation. The development included cold flow testing, CFD and pilot plant testing, and aimed at minimizing both thermal degradation and catalytic overcracking. Cold flow testing was conducted at a representative scale (scale 1/10th) on a large cold flow mock-up. Testing various riser separator configurations provided a thorough understanding, both through visual observations, pressure drop measurements and tracer studies. Hot pilot plant testing was conducted to investigate the effect on yields of the riser separation system in the reactor-stripper under adiabatic operation as a function of operating conditions. The separator had a significant impact on the FCC heat balance and a large reduction of dry gas yield was observed.The resulting technology was successfully implemented on an industrial resid catalytic cracking unit. Operation of the separator was proven smooth and flexible with enhanced performance, while the industrial observations confirm the R&D conclusions. This paper therefore summarizes the R&D study and the industrial operation trends. In particular, the impact of stripper bed level on operation is discussed and explained based on the flow phenomena and design aspects.