Modeling Local Gas−Liquid Mass Transfer in Agitated Viscous Shear-Thinning Dispersions with CFD

Abstract

Local mass transfer in viscous gas−liquid systems is investigated, using phenomenological models. A rigorous model incorporating local mass, momentum, turbulence, and population balances for bubbles is developed. Local hydrodynamics and gas−liquid mass transfer are investigated in a viscosity range of 0.001−47.9 Pa·s, specific mixing power range of 0.8−1.4 W/kg(liquid), and gassing rate of 0.7 vvm. The simulation results are verified against mixing time, gas holdup, and mass transfer rate experiments from a 0.2 m3 Rushton agitated vessel. The experimental system is aqueous xanthan, which exhibits viscous pseudoplastic behavior typical to many fermentation broths. The model predicts successfully the cavern formation, gas-slug creation, poor mixing at peripheral areas, and heterogeneous mass transfer, which the vessel averaged models are unable to do. Still, there is room for improvement in the basics of computational fluid dynamics CFD (turbulence and liquid flow) when modeling viscous gas−liquid reactors. Population balances for bubbles are needed to describe viscous gas−liquid dispersion accurately in agitated vessels, since the majority of the mass transfer area is located in small bubbles, whereas most of the gas volume is in the larger ones. In our simulations, over 50% of the mass transfer took place in less than 10% of the reactor volume. The order of magnitude drop of volumetric mass transfer coefficient (k L a) with increasing viscosity is predicted correctly. However, the simulated k L a decreases too rapidly at low (0.25 wt %) xanthan concentrations. The developed model allows qualitative investigation of local conditions in the vessel, thus giving new possibilities for reactor design, operation, scale-up, and troubleshooting.

Modelling mass transfer in an aerated 0.2m3 vessel agitated by Rushton, Phasejet and Combijet impellers

“…Population balances (PB) like multiple size group (MUSIG) and bubble number density (BND) have been recently used to describe G-L stirred vessels. Our work has been done with MUSIG and a Rushton turbine (RT) [4,5], while others have used BND to describe double RT's [6,7], and compared a RT with the lightnin A315 impeller [8]. With bubble columns there exists a single comparison of BND versus MUSIG simulation [9], but the operating conditions are not comparable with stirred 1385 C app apparent baf baffle BR breakage bub bubble C continuous CO coalescence CR critical imp impeller liq liquid max maximum min minimum spa sparger tot total tanks.…”

Coupling of CFD with DPBM for an RDC extractor

“…The parameter fitting is not feasible with CFD due to high computational burden (Moilanen et al, 2007). Therefore, the used parameters in the present study cannot be regarded as universally applicable parameters and are only one possible parameter set in the present case.…”

“…Nowadays, combined CFD-PBM models are widely investigated in the gas-liquid and liquid-liquid fields (e.g. Andersson et al, 2004;Chen et al, 2005;Jakobsen et al, 2005;Lehr et al, 2002;Moilanen et al, 2007;Olmos et al, 2001;Vikhansky and Kraft, 2004;Wang et al, 2006). All the references are based on the assumption that the particle size distribution is modelled with PBM while only averaged particle sizes as the Sauter mean diameter are coupled to the fluid dynamics.…”

Theoretical modelling of the effect of surface active species on the mass transfer rates in bubble column reactors

“…Some research groups have focused on evaluating the effect of the presence of different salts and surface active agents on the liquid-film resistance experimentally [24][25][26][27][28][29][30][31][32][33][34][35][36] and some theoretical models have been proposed [37,38]. However, when it comes to the contact area, its modelling is empirical [26,31,34,36,39] or a number of parameters must be used for each particular case [39,40].…”

“…It accounts for the effect of contaminants on bubble coalescence and break-up using theoretical closures which assume that the presence of surface active species reduce the mobility of the liquid layers surrounding the bubbles affecting bubble rise velocities, the liquid drainage, bubble stability, mass transfer resistances, etc. The use of theoretical closures attempts to give physical meaning and understanding to the number of adjustable parameters used so far by CFD commercial codes [39,40]. The bubbles are assumed to be non-spherical to improve the determination of the specific contact area [41][42][43].…”