Scale-up effects on the time-averaged and dynamic behavior in bubble column reactors

Abstract

Scale-up effects on the dynamic and time-averaged behavior are investigated in bubble columns with diameters of 200, 400 and 800 mm , respectively. Using a hot-wire probe, the local instantaneous heat transfer rates and the local time-averaged gas holdups were measured at the different locations of the columns over the gas velocity range from 20 to 90 mm/ s . The dyanamic behavior of local heat transfer was characterized by frequency domain and state-space analysis in terms of power spectrum, correlation dimension and Kolmogorov entropy. The power spectra of heat transfer rate fluctuations were found to be insensitive to the bubble column size. The state-space analysis indicates that heat transfer rate exhibits a chaotic behavior with the correlation dimension varying from 2.0 to 3.0. No significant effect of column size on the correlation dimension was observed, whereas the Kolmogorov entropies are apparently dependent on the column diameter. With the increase in column diameter, Kolmogorov entropy decreases significantly and exhibits rather flat radial distribution in the large-scale column. With respect to the time-averaged behavior of local gas holdup, the effects of column size on the radial distribution of gas holdup was examined. It was found that the 800 mm column exhibited a relatively uniform distribution of gas holdup. The dependence of both dynamic and time-averaged behavior on the column scale is considered to be closely related to the different macroscopic flow structure, which is a function of the scale of the column.