Detection of gas-liquid dynamics in monolithic channels using ultra-fast x-ray tomography

Monolith structures are a promising catalyst carrier to replace conventional catalyst particle packings in many gas-liquid-solid applications, such as hydrogenation, oxidation and enzymatic reactions. Enhanced reactor performance with respect to mass transfer, selectivity and conversion is mainly attributed to the more intense gas-liquid-solid contact within the regular structure and short diffusion distance due to thin liquid film layer. It is obvious that different flow regimes appear within the channels depending on superficial gas and liquid velocities and monolith channel geometry, e.g. bubble and Taylor flow regimes that feature especially high mass transfer rates and complete wetting of the monolith bed. Furthermore, bubble and slug lengths as well as their frequency and velocity can be tuned by operating conditions for optimization of reactor performance depending on the chemical reaction.
Therefore, gas-liquid dynamics in the channels of a monolith block need to be monitored. However, it was shown that design of gas-liquid distributor is the most crucial issue for uniform utilization of all monolithic channels and the hydrodynamic behavior in the monolith can drastically differ from single channel flow.
CREL laboratory applied gamma-ray tomography to study cross-sectional gas-liquid distribution but gives only time-averaged and blurred data while any temporal gas-liquid in-channel formation remains hidden (Roy and Al-Dahhan, 2005). On the other hand, magnetic resonance tomography from Cambridge University (Professor Gladden) drastically improved chemical engineering imaging, e.g. allowing access inside monolithic channels but exclusively for small diameters and non-metallic reactors (Mantle et al., 2002).
In the present work, a novel ultra-fast X-ray tomograph (Fischer et al., 2008) was applied to study gas-liquid distribution in monolithic structures operated in co-current up-flow. Different monolith configurations were installed and the system was operated at different gas and liquid superficial velocities. Normalized bubbles and slug length frequency distributions, gas and liquid fractions and cross-sectional distribution quality were calculated from image sequences. Furthermore, effect of the monolith in the cross-section on gas-liquid flow pattern was studied with respect to the gas-liquid flow in the pipe without structured internals.