New insights into gas/liquid textures in rotating solid foams

For flexible production of fine chemicals and pharmaceuticals still slurry catalysts in batch reactors are used. However, production efficiency suffers from necessary filtration after reaction is finished and abrasion effects make re-usage of catalyst often not possible. Additionally, the mass transfer rates are limited due to low relative velocities between particles and the gas/liquid phases.
Alternatively, a new three-phase reactor design based on rotating solid foams were developed by the Chemical Reactor Engineering group at Eindhoven University of Technology (Tschentscher et al., 2010). The solid foam is applied both as catalyst support and stirrer in order to mix the gas and liquid phases and to create fine bubbles.
Measurements of the gas-liquid mass transfer revealed clearly improved reactor performance compared to Rushton stirrers. The visual observation suggested that the rotation of the foam block leads to a structuring of the reactor volume into sections with considerable different gas holdup, flow behaviour and bubble size distribution. However, access to the patterns in the solid foam was so far not possible.
In our contribution, results of a comprehensive non-invasive tomographic study will be presented. A high-resolution gamma-ray setup was applied to visualize the gas/liquid textures in the reactor, especially in the solid foam block. Therefore, an acrylic reactor system was built that ensure minimal attenuation. Two different reactor heights, stirrer speed from 150 to 300 rpm and two solid foams with different geometric properties were applied. To study the effect of the liquid viscosity, glycerol-water mixtures as well as pure liquids were used. Additionally, flow patterns of non-Newtonian liquids (carboxy methyl cellulose, CMC), foaming liquids (cetyltrimethyl ammonium bromide, STAB) and non-Newtonian foaming liquids (CMC+STAB) were compared with the air-water system.
The observed flow patterns can be related to the mass transfer characteristics and conclusions on an optimized reactor/foam design can be drawn.