Hydrodynamics and Axial Dispersion in a Gas−Liquid−(Solid) EL-ALR with Different Sparger Designs

Abstract

The gas-liquid-(solid) three-phase hydrodynamics in an external-loop airlift reactor (EL-ALR) with an upward pipe 0.47 m in diameter and 2.5 m in height, two external loop downward pipes 0.08 m in diameter and 2.5 m in height, were investigated using four different gas sparger designs. The microconductivity probe and the three-dimensional (3-D) laser Doppler anemometry (LDA) techniques were, respectively, implemented to measure the local gas holdup in the riser (R Gr ) and liquid phase velocity in the downcomer (U Ld ) using air as the gas phase, water as the liquid phase, and alginate gel beads as the solid phase, over a wide range of operation conditions. The tracer age distribution was measured using the pulse-pursuit response technology. Axial dispersion model (ADM) was used to estimate the model parameter Peclet number (Pe) values as a fitted parameter with the measured data, using the gold partition method for nonlinear programming strategy inequation restrict conditions. The ADM gave better fits to the experimental data at high axial locations and lower superficial gas velocity (U G ) for an EL-ALR used with a large L/D R ratio. A synergistic effect of U Ld , R Gr , Pe, solids loading (SL), and sparger designs on the performance of an EL-ALR was observed in our experiments. The sparger designs were determined to have a noticeable effect on the R Gr and Pe in the lower gas velocity and lower solid loading ranges (U G < 0.025 m/s and SL < 2%), but only a slight effect in the high gas velocity and high solid loading ranges (U G > 0.030 m/s and SL > 3%). However, the effect of sparger designs on the U Ld is greater in the gas velocity from 0.025 m/s to 0.045 m/s. For the lower solids loading, the increase of orifice diameter leads to a decrease in R Gr . This is in accordance with what was presented in the gas-liquid two-phase system. Moreover, the influence of orifice diameters of the spargers is negligible for solids loading of >3%. Although the Pe values decreased with the operating gas velocity, the gas velocity change from 0.03 m/s to 0.04 m/s yielded lower Pe values, as a result of the reduced bubble size. As the gas velocity further increased to 0.06 m/s, the R Gr and the U Ld values increased, while the Pe values negligibly increased. For a gas-liquid two-phase system, Pe decreases with the orifice diameter and, for 1% of solids, Pe is also lower for sparger P-2 (φ 0.6 mm) than for sparger P-1 (φ 0.3 mm). For higher amounts of solids (3%), Pe does not have a defined trend. In addition to the gas velocity and sparger design effects, the solids loading had the effect of decreasing the U Ld values, while such effect became small and flattened at high solid loadings. The U Ld values, especially with VO ) 100%, are 20% lower in three-phase flow than that in two-phase flow. In addition, the U Ld profiles in three-phase flow are flatter than that in two-phase flow with VO ) 50%-100%, actually showing a parabolic shape rather than the almost linear one encountered in two-phase flow. This is very important for ...

Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with Microbubble Generator

“…Consequently, the size of bubbles formed at the sparger has little influence on that prevailing in the riser in rapidly coalescent media, whereas the size of bubbles prevailing throughout the riser remains close to that formed at the sparger in the case of slowly coalescent media. The relative importance of sparger design was thus found to depend strongly on the interfacial properties of the liquid phase [14,16,[39][40][41][42].…”

Influence of the Sparger Type and Added Alcohol on the Gas Holdup of an External‐Loop Airlift Reactor

“…The gas sparger design has a major effect on the initial bubble size and, hence, on the hydrodynamics of the airlift reactor. Cao et al [15] performed experiments in three-phase ELAR with four sparger types. They reported that the sparger design has a noticeable effect on the gas holdup at low gas velocities (< 0.025 m s -1 ) and low solids loading (< 2 %), but only a slight effect at high gas velocities (> 0.030 m s -1 ) and high solids loading (> 3 %).…”

Hydrodynamics of an external-loop airlift reactor with inserted membrane

“…A membrane module inserted in the downcomer represents hydrodynamic resistance to the liquid flow. Higher resistance in the ELAR leads to a decrease in the liquid velocity and an increase in the gas holdup (Bendjaballah et al, 1999;Cao et al, 2008;Merchuk and Stein, 1981;Pošarac, 1988;Verlaan et al, 1989;Vial et al, 2002).…”

“…Gas distributor design has a major effect on the initial bubble size and, hence, on the hydrodynamics of the airlift reactor (Bendjaballah et al, 1999;Cao et al, 2008;Lin et al, 2004;Snape et al, 1995;Vial et al, 2000). Cao et al (2008) performed the most comprehensive study about the influence of the gas distributor on the gas holdup and liquid velocity in the ELAR.…”

“…Cao et al (2008) performed the most comprehensive study about the influence of the gas distributor on the gas holdup and liquid velocity in the ELAR. They observed that the gas distributor had a noticeable effect on the gas holdup up to the superficial gas velocity of 0.25 m/s.…”