Comparison of flow development in high density downer and riser reactors is experimentally investigated using fluid catalytic cracking particles with very high solids circulation rate up to 700 kg/m(2) s for the first time. Results show that both axial and radial flow structures are more uniform in downers compared to riser reactors even at very high density conditions, although the solids distribution becomes less uniform in the high density downer. Solids acceleration is much faster in the downer compared to the riser reactor indicating a shorter length of flow development and residence time, which is beneficial to the chemical reactions requiring short contact time and high product selectivity. Slip velocity in risers and downers is also first compared at high density conditions. The slip velocity in the downer is much smaller than in the riser for the same solids holdup indicating less particle aggregation and better gas-solids contacting in the downer reactors. (C) 2015 American Institute of Chemical Engineers