Gas mixing and solids mixing were studied in a geometrically and dynamically scaled cold model fluid coker stripper. Tracer gas (helium) was first injected into the stripper standpipe to quantify total gas entrainment into the underflow stream. Tracer gas was then injected into the upper reactor and lower stripper separately to investigate gas mixing in the stripper. The stripping efficiency was found to depend strongly on operating conditions (solids circulation rate, stripping gas velocity) as well as on the baffle configuration in the stripper. Unsteady state measurements were also obtained in an effort to understand gas dispersion in the stripper. The results show that gas mixing is most intensive in the stripper core. To study solids mixing and residence time distribution in the stripper, solid tracer particles impregnated with salt were injected into the reactor and detected at the top of the stripper and standpipe. The results indicate that axial dispersion of solids in the presence of the baffles could be represented by axially dispersed plug flow. (c) 2005 Elsevier Ltd. All rights reserved.