Hydrodynamics and scale-up of bubble columns in the heterogeneous regime: Comparison of bubble size, gas holdup and liquid velocity measured in 4 bubble columns from 0.15 m to 3 m in diameter

Abstract

Hydrodynamics and scale-up of bubble columns in the heterogeneous regime: Comparison of bubble size, gas holdup and liquid velocity measured in 4 bubble columns from 0.15 m to 3 m in diameter. Chemical Engineering Science, Elsevier, 2019, 198, pp.Hydrodynamics and scale-up of bubble columns in the heterogeneous regime: 2 comparison of bubble size, gas holdup and liquid velocity measured 3 in 4 bubble columns from 0.15m to 3m in diameter 4 5 Highlights 19• Bubble Sauter diameter, gas holdup and axial liquid velocity measurements are performed 20• 0.15m, 0.4m, 1m and 3m diameter bubble columns are investigated 21• A wide experimental database is furnished to assist further model developments 22• Void fraction and liquid velocities profiles happen to be self-similar in the heterogeneous 23 regime 24• The entrained liquid flow rate, proportional to D 3/2 , is only set by the column diameter 25• The quantity (gD) 1/2 appears as a natural scale for mean and fluctuations velocities. 26• Strong gas holdup gradients are present in the flow due to convective instabilities 27 28 Abstract 29The development of CFD models coupled with Population Balance is a very promising topic 30 concerning multiphase reactors. In the case of bubbly flows and bubble columns, a serious lack of 31 local hydrodynamic characterizations still harms development and validation of relevant models. To 32 fill partially this gap, a new bubble size measurement technique, previously introduced by Maximiano 33 Raimundo et al. (2016), has been applied on a very wide range of bubble column diameters (from 34 0.15m to 3m) and superficial gas velocities (from 0.06m/s to 0.35m/s). Size measurements have been 35 coupled with others concerning gas holdup and axial liquid velocity, in order to provide an 36 experimental database allowing to clarify the scale-up rules and to assist future modelling works. 37Average bubble sizes have been measured as globally similar at every scale. Measured holdup and 38 average liquid velocity confirm already reported behaviours at lower column diameters. Liquid 39 velocity fluctuations also follow self-similar radial profiles and are proportional to the average liquid 40 velocity at the centre of the column leading to a strong turbulence intensity. The fact that the 41 quantity (gD) 1/2 appears as a natural velocity scale and the presence of strong gas-holdup gradients 42 underline the similarity between bubble columns operated heterogeneous regime and free thermal 43 convection in pipes. 44

Effect of rheology on mass transfer and bubble sizes in a bubble column operated in the heterogeneous regime

“…The bubbles are generated by a perforated flat plate with 92 holes of 2 mm distributed in a triangular pitch of 15 mm. [ 9 ] The choice of the gas sparger is critical for the development of the flow in the column. It has been found in a previous work with water [ 10 ] that the chosen perforate plate induces a fast flow development as it generates bubble sizes that are close to their stable size all along the column.…”

“…The CC measurement method has been detailed in previous articles [ 8,9,11 ] and is succinctly reviewed here. The method relies on the recording of instantaneous phase functions at the tip of two neighbouring optical probes (=1 if gas is present at the tip of the probe, =0 if not).…”

“…Measurements are done at the level of 1.5 m above the distributor ( H / D = 3.75). According to different authors, [ 9,15,16 ] this ratio is high enough to neglect residual axial evolution. Radial profiles are measured on five radial positions, r / R = 0, 0.2, 0.4, 0.6, and 0.8.…”

“…The following fluids were used: (a) tap water, as previously used [ 9 ] ; (b) XG solutions at 0.1 wt% and 0.25 wt%; and (c) CMC solutions at 0.25 wt% and 0.5 wt%. The properties of the water used for these experiments are reported in Table 1.…”

“…The uncertainty of the measurement is typically considered below 15%‐20% [ 8 ] and below 10% with no reverse gas‐flow. It has been successfully validated in the heterogeneous regime and applied since to characterize the effect of scale‐up on bubble sizes, [ 9 ] and the effect of additives. [ 10 ] In addition, the method has been used and validated in stirred reactors [ 11 ] filled with water or shear‐thinning solutions of polymers: carboxymethylcellulose (CMC) and xanthan gum (XG).…”