Effect of heat exchanger internals on the main transition velocities in a bubble column

The application of bubble columns to industrial processes, such as Fischer-Tropsch synthesis and liquid-phase methanol synthesis, often involves a large amount of heat generation. That is why, heat exchangers are installed to remove the generated heat. However, the bubble dynamics in bubble columns with dense internals is still not well understood.
In this work, for the first time a flow regime identification in a bubble column with internals (dense vertical heat exchanger tubes) was performed. The bubble column had an inner diameter of 0.1 m and was equipped with a perforated plate distributor (55 holes, ø 0.5×10-3 m). The gas-liquid system consisted of air and deionized water. The clear liquid height Ho was set at 1.1 m. Superficial gas velocities Ug ranging from 0.01 up to 0.15 m/s were employed. The bubble column was equipped with 37 heat exchanger tubes having a square pitch with a diameter of 8×10-3 m and a clearance of 3×10-3 m. Gas holdup fluctuations were recorded by means of a conductivity wire-mesh sensor installed at an axial position of 0.715 m. The time series (60,000 points, sampling frequency (fs)=2000 Hz) were further treated based on the non-linear chaos theory (Schouten et al., 1994; Nedeltchev et al., 2014).
For comparison, the two main transition velocities Utrans in the empty bubble column (Ho=0.66 m) counterpart were also identified. The information entropies were extracted from X-ray data (29,000 points, fs=1000 Hz) recorded by the ultrafast tomographic facility. It was shown that in a bubble column with internals two main transition velocities Utrans can be identified by means of the Kolmogorov entropy (KE) profile. The flow regime boundaries are identifiable on the basis of the sudden change in the decreasing KE rate. It is interesting to note that well-pronounced local KE minima (Nedeltchev et al., 2014), known from empty bubble columns, are not observed in a bubble column with internals.
At Ug=0.05 m/s the bubbly flow (homogeneous) regime transforms itself into the transition regime. The rate of the KE decrease in both flow regimes is different. At Ug=0.11 m/s the heterogeneous regime (mixture of churn-turbulent and slug flow regimes) is formed. It is characterized with constant KE values. The existence of the slug flow regime in the column core is explainable with the relatively small diameter of the column at which, according to Wilkinson et al. (1992), the effect of the column wall is still present.
In an empty bubble column, two well-pronounced local minima of the information entropy values denote the two main Utrans values. The first transition velocity occurs earlier at Ug=0.04 m/s as compared to the column with internals, which means that the presence of internals stabilizes the bubbly flow regime. The second transition velocity (the onset of the heterogeneous flow regime) is unaffected by the internals.
In the full-length contribution, the time-dependent nature of the main transition velocities as well as the effect of the axial height on the Utrans values in bubble columns with and without internals will also be discussed in a comparable manner. A comparison between the transition velocities in the column core and annulus will also be shown.