A study on the influence of the tube layout on sub-channel hydrodynamics in a bubble column with internals

Abstract

In this work, the hydrodynamics of a bubble column with vertical heat exchanger internals in a narrow bubble column of Di=0.1m inner diameter with a clear liquid height of Lc=1.1m was comprehensively studied. We applied ultrafast X-ray tomography to obtain hydrodynamic parameters, such as gas holdup, bubble size distribution, bubble number flux and flow patterns at hitherto inaccessible positions within the sub-channels of the tube bundles. To investigate the influence of the tube bundle pattern, square and triangular pitches were considered. Tubes of do=8 and 13 mm outer diameter were installed to study the effect of tube size, while maintaining approx. Ac=25% coverage of the cross-sectional area, which is typical for, e.g., Fischer-Tropsch processes operated in bubble column reactors. The superficial gas velocity was varied from ug=2 to 20 cm s−1 to cover homogeneous and heterogeneous flow regimes. Internals’ type and tube diameter were found to crucially influence the gas holdup distribution across the column diameter, which is known to generate liquid circulation, to shape the gas velocity profile and to cause intensive bubble interactions. The higher flow resistance induced by triangular tube configurations and dense tube patterns, which is attributed to the smaller hydraulic diameter of the respective configurations, forces bubbles to preferably rise near the column wall. Within the tube bundle, the radial holdup profiles show a pronounced non-parabolic trend, indicating zones of reverse liquid flow directions between the internal tubes. Furthermore, the gas-liquid flow morphology within various sub-channels was analyzed revealing a slug-like flow formation at superficial gas velocities larger than 10 cm s−1.