Hydrodynamics in cellular grid packed bubble columns disclosed with ultrafast X-ray tomography

Michael Wagner 1,2*, Christian-Ole Möller 3, Hendrik Hessenkemper 2, Martina Bieberle 1,2, Uwe Hampel 1,2 and Michael Schlüter 3

1 Technische Universität Dresden, AREVA Endowed Chair of Imaging Techniques in Energy and Process Engineering, 01062 Dresden, Germany

2 Helmholtz-Zentrum Dresden – Rossendorf, Institute of Fluid Dynamics, Bautzner Landstraße 400,01328 Dresden, Germany

3 Technische Universität Hamburg-Harburg, Institute of Multiphase Flows, 21073 Hamburg, Germany

*Corresponding author: Michael.Wagner@hzdr.de

ABSTRACT

The chemical industry is one of the largest consumers of energy and resources. Therefore, process intensification is a field of high interest. Although the chemical reactor is only part of a highly integrated process, its design can significantly affect the overall process efficiency. A very common reactor type is the bubble column in which a liquid and a gas phase react with each other. The mass transfer across the liquid-gas interface is a crucial parameter regarding the process efficiency and should be as high as possible. It can be increased by higher interfacial area and better mixing of the phases. In the past, various internals like static mixers and structured packings were tested in order to control the bubble hydrodynamics in the column. Recent investigations of the authors showed that periodic open cellular structures (POCS) tend to increase the mass transfer significantly. These internals are three- dimensional regular grids with various geometries.

At the Helmholtz-Zentrum Dresden – Rossendorf the ultrafast X-ray computed tomography (CT) system is used to provide insights into the hydrodynamics of such bubble column internals. This imaging technique produces up to 8,000 cross-sectional images per second and provides a spatial resolution of about 1 mm for gas-water contrast. Two simultaneously scanned measurement planes allow determining vertical velocities and with that the extraction of quasi three-dimensional data sets from the original cross-sectional image data. For varying POCS geometries and gas flow rates, the axial bubble velocities, time-averaged gas hold-ups and the Sauter mean diameter are quantified and compared to measurements of unpacked bubble columns.

Keywords packed bubble column, ultrafast X-ray CT