Simulation of Hydrodynamics in a Gas-Solid Downer Reactor

The downer reactor is a promising technology that could potentially replace the current riser reactor technology used in fluid catalytic cracking (FCC). The main benefit of the downer is the very narrow distribution of contact times between the solid (catalyst) and the gas (reactive phase), which can lead to improved selectivity control. This work aims to compare the hydrodynamic performance of the two modes of operation of the downer reactor (co- and counter-current) in a pilot plant scale unit: 1.75 m height and 27 mm ID. To achieve this goal, we performed a series of experiments in a downer reactor at ambient conditions, using a commercial FCC catalyst and measuring the pressure drop over various conditions. Then we coded a simulation using a computational particle fluid dynamics (CPFD) approach and validated it by the previously performed experiments. Finally, we used the validated simulation to explore the hydrodynamic performance of each mode of operation and compare one against the other. The simulations show that the counter-current downer reactor has a 69% higher solid holdup than the co-current one, 98% longer residence times of the solid, and both modes approach the plug-flow behavior (for the solid) compared to the riser reactor.