Wire-mesh based hydrodynamic and thermal studies in packed beds

Gas/liquid operated packed bed reactors are the workhorses in the wide field of chemical and petrochemical industry. Intense research effort has been undertaken to explore all sub-processes of hydrodynamics, mass and heat transfer, to predict process behaviour, to define reliable scale-up and design rules etc. However, still many details are unsolved and require deeper insights at multiple scales. Particularly in co-current downflow trickling bed reactors (TBR), the interactions between fluid phases and packings and in turn, hydrodynamics and transfer steps are very complex and not yet fully understood.
Catalyst utilization and reactor efficiency suffers clearly from maldistribution of liquid in the cross-section of packings, channelling effects and bypassing liquids. Accompanied by maldistribution, local hot spots may form in packed bed reactors. If not detected early, they can have a detrimental impact on the yield of the desired products and, in addition, they may pose safety hazards, i.e. reaction runaway due to phase transition and accelerated reaction rate which are main reason for major chemical plant accidents.
To take corrective action, indicators of conditions, such as non-irrigated zones and emerging elevated temperature zones must be detected. In the past two decades, considerable effort has been made to visualize and characterize multiphase flows in packings. Since local sensors inserted at selected locations in the bed fail to provide information about spatial distributed phenomena, powerful tomographic methods well known from medical imaging were successfully transferred into chemical engineering applications. However, tomographic techniques are expensive, low speed and don’t yield any information about fluid velocities. Considering industrially suitable temperature monitoring systems, no spatial-resolved array-type system is available.
Prasser et al. (1998) developed an electrode-mesh tomograph for gas-liquid flows in bubble columns which is advantageous considering costs, simplicity and safety. Based on this principle, our contribution presents results of spatially resolved hydrodynamic studies in a pilot-scale TBR using new capacitance wire-mesh sensors for determination of liquid saturation and axial liquid velocities simultaneously for all sensing points of the whole cross-section depending on gas and liquid flow. Furthermore, a new temperature-monitoring sensor was developed installing Pt2000 elements at the cross-points of a wire-mesh based array system.