The effect of sectional internals on the solid bed cross section in rotary drums is investigated with a novel, fully optical accessible rotary drum apparatus for different internal configurations. The impact on mixing behavior is analyzed for different section numbers and filling degrees as well as material configurations. The mixing kinetics is affected positively by the sections, particularly for lower rotational speeds and filling degrees. The goodness of mixing in segregating particulate systems can be improved by sectional internals. Additionally to mixing effects, the impact of sectional internals on phase interfaces of the cross section is analyzed. A mathematical model description of the solid bed cross section is created and validated with the experimental data. The gas/solid interface is increasing logarithmically compared to the bare drum. The solid/inner wall interface is increasing linearly with the number of sections and the solid/outer wall interface is not affected significantly by sectional internals. (C) 2020 Elsevier B.V. All rights reserved.