Dual-Energy X-Ray Tomographic Measurement of Local Phase Fractions in 3-Phase Bubble Columns

For measuring local phase fractions in 2- and 3-phase bubble columns, an X-ray tomographic measurement system is applied. This measurement technique, also referred to as Computer Tomography or CT, is based on the attenuation of X-rays along their path through the bubble column. An X-ray source and an X-ray detector are mounted on opposite sides of the bubble column. The bubble column is irradiated by an X-ray fan beam perpendicular to the bubble column axis. The X-ray intensity measured by each detector pixel is an integral measure for the penetrated material along the path of the X-rays. By rotating the X-ray source and X-ray detector around the bubble column axis, multiple projection measurements of the measurement plane are collected. In a second step, the phase distribution in the measurement plane (the so-called CT-slice) is calculated from the projection measurements by applying mathematical reconstruction algorithms. The reconstructed phase fractions are time-averaged over the measurement interval of 200 seconds for the measurements presented in this work. In order to distinguish all 3 phases, a special dual-energy technique is used. In this technique, 2 separate CT measurements are conducted successively, applying 2 different X-ray wavelengths. By combining the information gained from these 2 measurements, all 3 phase fractions are determined for every image pixel. The local phase fractions of all 3 phases are measured simultaneously for the whole cross-section. The measurement technique is fully non-intrusive. It is not restricted to limited ranges of phase fractions, solid loadings or flow rates of any of the phases. A 244 mm diameter, 7 m high bubble column is examined. It can be operated either with only 2 phases (liquid and gas) or with additional solid particles. Measurements are conducted with air as gas, water as liquid and PVC particles as solid phase. In this paper, the measurement principle of the tomographic technique and the dual-energy algorithm are explained. The experimental setup is described and the results of the measurements are presented.