The simulation of bubble columns operating under the heterogeneous regime is an ambitious challenge, due to the difficulty of predicting accurately hydrodynamics and bubble size distributions, that requires experimental data for model validation. Gas fraction distributions, liquid and gas velocity profiles and bubble size distributions across bubble columns are deeply interconnected in these systems and only a comprehensive study allows the links between them to be understood. This work reports experimental data obtained by measuring bubble sizes with an innovative technique based on the cross correlation between two optical probes. Particular attention is given

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

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In fluidised bed processes, the solids are in vigorous motion and thus inevitably subjected to mechanical stresses due to inter-particle and particle-wall impacts. These stresses lead to a gradual degradation of the particles by surface wear, abrasion and body fragmentation commonly termed attrition. One significant contribution of attrition comes from the air jets of the fluidised bed distributor. Particles are entrained into the air jet, where they get accelerated and impacted onto the fluidised bed particles. The jet induced attrition only affects the part of the bed which is limited by the jet length, where the mode of attrition is largely collisional. The overall jet attrition rate is therefore the result of the combination of the single particle damage and the flux of particles entering into that region. The attrition behaviour of particles in the jet region is analysed by evaluating their propensity of breakage experimentally and by simulating an air-jet in a bed of particles by CFD-DEM. The frequency of collisions and impact velocities are estimated from which the attrition due to a single air-jet is predicted.

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