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It is known that the fluid dynamics and transport phenomena in bubble columns depend mainly on the bubble column design (i.e., the column diameter, aspect ratio, and gas sparger openings) and the liquid phase properties. In this communication, we contribute to present-day discussion through an experimental study concerning the combined effects of the gas sparger design and liquid phase properties on both the gas holdup and the main flow regime transition. The experimental study concerning gas holdup measurements was conducted in a large-diameter and large-scale bubble column (with a height of 5.3 m and inner diameter of 0.24 m) operated in the batch mode. Air was used as the dispersed phase (using gas superficial velocities in the range 0.004–0.20 m/s), and various water–monoethylene glycol (MEG) solutions were employed as binary liquid phases. The water–MEG solutions tested have viscosities between 0.9 mPa·s and 7.97 mPa·s, densities between 997.086 kg/m 3 and 1094.801 kg/m 3 , and surface tension between 0.0715 N/m and 0.0502 N/m. Two gas spargers were tested: (a) a spider sparger (“ coarse gas sparger ”) and (b) a needle sparger (“ fine gas sparger ”). The former produced a poly-dispersed homogeneous flow regime resulting in a concave gas holdup curve, whereas the latter produced a mono-dispersed homogeneous flow regime resulting in an S-shaped gas holdup curve. It was observed that the mono-dispersed bubble size distribution stabilized the homogeneous flow regime. The addition of MEG produced different effects depending on the gas sparger design. The addition of MEG in the “ coarse gas sparger ” configuration produced what is usually referred to as “ dual effect of viscosity ”: depending on the MEG concentration, the homogeneous flow regime was stabilized/destabilized, and thus, the gas holdup increased/decreased. Conversely, the addition of MEG in the “ fine gas sparger ” changed the shape of the gas holdup curve from an S-shape to concave, thus rendering it similar to the ones produced by “ coarse gas sparger ”. We speculate that viscous solutions reduce the influence of the inlet conditions in large-diameter and large-scale bubble columns; this is a matter of future research.

It is generally admitted that experimental data obtained in “laboratory-scale” bubble columns are representative of “industrial-scale” reactors if the well-known three “Wilkinson et al. scale-up criteria” are satisfied: (a) the diameter of the bubble column is larger than 0.15 m, (b) the sparger openings are larger than 1–2 mm and (c) the aspect ratio is larger than 5. The aim of this communication is to contribute to the existing discussion. To this end, this communication collects relevant experimental investigation and include new experimental data: in particular, we have experimentally studied the combined effect of the aspect ratio (within the range of 1–15) and the sparger design (considering both “coarse” and “fine” spargers) on the gas holdup in a large-diameter and large-scale gas-liquid bubble column. The bubble column has been operated both in the batch mode and in the counter-current mode. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included deionized water and different aqueous solutions of organic (i.e., ethanol) and inorganic (i.e., sodium chloride, NaCl) active agents. It is found that the “Wilkinson et al. scale-up criteria” are valid for the air-water case in the batch mode for “very-coarse” spargers. Conversely, they are no more valid when considering different liquid velocity, and/or aqueous solutions of active agents, and other sparger openings. Keywords: Bubble column, Scale-up, Gas sparger, Gas holdup, Liquid phase properties

It is generally admitted that the gas holdup is independent of the column dimensions and gas sparger design if three criteria are satisfied: the diameter of the bubble column is larger than 0.15 m, gas sparger openings are larger than 1–2 mm and the aspect ratio is larger than 5. This paper contributes to the existing discussion; in particular, the effect of the aspect ratio (within the range 1–15) in a counter-current gas-liquid bubble column has been experimentally studied and a new gas holdup correlation to estimate the influence of aspect ratio, operation mode and working fluid on the gas holdup has been proposed. The bubble column, equipped with a spider gas sparger, is 5.3 m in height, has an inner diameter of 0.24 m; gas superficial velocities in the range of 0.004–0.23 m/s have been considered, and, for the runs with water moving counter-currently to the gas phase, the liquid has been recirculated at a superficial velocity of −0.0846 m/s. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included tap water and different aqueous solutions of sodium chloride as electrolyte. Gas holdup measurements have been used to investigate the flow regime transitions and the global bubble column hydrodynamics. The counter-current mode has turned out to increase the gas holdup and destabilize the homogeneous flow regime; the presence of electrolytes has resulted in increasing the gas holdup and stabilizing the homogeneous flow regime; the aspect ratio, up to a critical value, has turned out to decrease the gas holdup and destabilize the homogeneous flow regime. The critical value of the aspect ratio ranged between 5 and 10, depending on the bubble column operation (i.e., batch or counter-current modes) and liquid phase properties. Since no correlation has been found in the literature that can correctly predict the gas holdup under the investigated conditions, a new scheme of gas holdup correlation has been proposed. Starting from considerations concerning the flow regime transition, corrective parameters are included in the gas holdup correlation to account for the effect of the changes introduced by the aspect ratio, operation mode and working fluid. The proposed correlation has been found to predict fairly well the present experimental data as well as previously published gas holdup data.