Comparison of Hydrodynamics and Mass Transfer in Airlift and Bubble Column Reactors Using CFD By J. M. van Baten and R. Krishna* ComputationalFluidDynamics(CFD)isusedtocomparethehydrodynamicsandmasstransferofaninternalairliftreactorwith that of a bubble column reactor, operating with an air/water system in the homogeneous bubble flow regime. The liquid circulationvelocitiesaresignificantlyhigherintheairliftconfigurationthaninbubblecolumns,leadingtosignificantlylowergas holdups.Withintheriseroftheairlift,thegasandliquidphasesarevirtuallyinplugflow,whereasinbubblecolumnsthegasand liquid phases follow parabolic velocity distributions. When compared at the same superficial gas velocity, the volumetric mass transfer coefficient, k L a, for an airlift is significantly lower than that for a bubble column. However, when the results are compared at the same values of gas holdup, the values of k L a are practically identical. 1 Introduction Bubblecolumnsarewidelyusedinindustryforcarryingout a variety of chemical reactions such as hydrogenations, chlorinations, oxidations, and the Fischer Tropsch synthesis [1]. In bubble column slurry reactors, catalyst particle sizes smallerthanabout100 lmcanbeused,thuseliminatingintra- particle diffusion resistances. These catalyst particles are held in suspension due to liquid circulation caused by the rising gas bubbles.Whentheskeletaldensityofthecatalystishighthere is a danger of catalyst settling in the bubble column reactor; this possibility arises, for example, in the hydrogenation of nitriles using Raney catalyst [2,3]. The introduction of a draft tube into the bubble column reactor causes a substantial increase in the liquid circulation, for the same gas flow, and helps to keep the particles in suspension. A bubble column with an internal draft tube is also termed an internal airlift reactor, Fig. 1. The gas sparger is located in the riser, and the other part is thedowncomer.Thedrivingforce,basedonthestaticpressure difference,orthemixturedensitydifference,betweentheriser and the downcomer generates the liquid circulation in a loop. Airlift reactors are finding increasing applications in the chemical industry, biochemical fermentation, and biological wastewater treatment processes [4±6]. Compared with con- ventional reactors, such as stirred tank reactors or bubble columns, shear stress is relatively constant and mild through- out the reactor. Fordesignofanairliftreactor,itisnecessarytohaveaccurate estimates of the phase holdups and velocities in the riser and downcomer. Several literature studies have focused on the estimation of these hydrodynamic parameters [7±9]. In particular, the velocities of the liquid in the downcomer and riser are crucially dependent on the frictional losses, which in turn are determined by the geometry of the reactor and the operatingconditions.Severalempiricalcorrelationshavebeen proposedfortheestimationofthesehydrodynamicparameters; however,thesecorrelationsarerestrictedintheirapplicability tothegeometryforwhichtheyweredetermined.Extrapolation toothergeometries,scales,andoperatingconditionsisfraught with uncertainty. While there are a few experimental studies, which have focused on the interphase mass transfer in airlift reactors [10±12], there appear to be no generally applicable modelsorcorrelationsfortheestimationofthevolumetricmass transfercoefficient, k L a,inairliftreactors. Severalrecentpublicationshaveestablishedthepotentialof Computational Fluid Dynamics (CFD) for describing the hydrodynamics of bubble columns [13±18]. An important advantage of the CFD approach is that column geometry and scale effects are automatically accounted for. The first major objective of the present communication is to develop a CFD model for internal airlift reactors to describe not only the 1074 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ceat.200301796 Chem. Eng. Technol. 2003, 26, 10 Pressure boundary 0.19 m / 38 cells axis of symmetry 3.06 m / 306 cells 0.98 m / 98 cells 2.08 m / 208 cells 0.075 m / 15 cells gas inlet 0.09 m riser wall 2.02 m / 202 cells 0.10 m 0.15 m 0.38 m Gas inlet Figure 1. Schematic of airlift reactor, showing the computational domains and grid details. ± [*] J. M. van Baten, R. Krishna (author to whom correspondence should be addressed, e-mail: krishna@science.uva.nl), Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WVAmsterdam, The Netherlands. Full Paper