Dimensional similitude for scale-up of hydrodynamics in a gas-liquid-(solid) EL-ALR

This study tests the scaling approach for gas-liquid-(solid) external loop airlift reactor (EL-ALR) hydrodynamics based upon geometric and dynamic similitude with a limited number (6) of dimensionless groups. A sixth dimensionless group for sparger characteristic (Scp) is developed. Scp is a convenient parameter to characterize fluid flow performance of EL-ALR and a large cold-flow 0.86 m diameter column. To our knowledge, there is more ambiguity concerning its definition for EL-ALR and a large cold-flow 0.86 m diameter column especially when different sparger structures are used. In this study, reliability of the modified Reynolds and Scp are proposed for such an EL-ALR and a large cold-flow 0.86 m diameter column used different sparger structures. The Scp is based on the pitch and angle between hole normal vector and horizontal line as the characteristic parameters. Experiments were carried out in two systems in which all six dimensionless groups were matched: a 55 wt% aqueous glycerol solution with ceramic catalyst particles in an industrially operated large cold-flow 0.86 m diameter column (system 1) and silicone oil with cylindrical aluminum particles in Taishan Scholar Lab (TSL) EL-ALR with a riser (0.47 m diameter and 2.5 m height) and two downcomers (0.08 m diameter and 2.5 m height) (system 2) with air as the gas in both cases. The micro-conductivity probe and the 3D Laser Doppler Anemometer (LDA) techniques were, respectively, implemented to measure local gas holdup (alpha(Gr)) and Peclet number (Pe) in the riser over a wide range of operation conditions. Although Peclet number was slightly different for the two systems, trends were similar. Gas holdups were always slightly higher for system 1. The dimensionless transition velocities from dispersed to coalesced flow were similar. Differences between the two systems are significant, but generally less than 15%, so the dimensionless similitude approach gives a reasonable basis for estimating global hydrodynamic parameters under the present operating conditions. The differences between the two systems are attributed to the complex coalescence behavior of liquid mixtures and the different sparger structures chosen, suggesting that additional dimensionless groups are needed to fully characterize the local dynamic bed behavior. This agreement proves that the proposed modified dimensionless numbers can be well adapted for engineering purposes and used to compare the flow performance between the two systems. (C) 2009 Elsevier Ltd. All rights reserved.