Mechanical agitation is used commonly in gas-liquid reactors to improve the homogeneity of dispersion and to enhance the transfer of reacting compounds between gas and liquid. The design and scaleup of gas-liquid reactors is problematic due to non-ideal mixing, heat and mass transfer limitations. In this work, phenomenological models were developed and validated against experiments to investigate local gas-liquid mass transfer in agitated tanks. The aim was to develop more generalized and reliable simulation tools for agitated gas-liquid reactors.Gas-liquid hydrodynamics and mass transfer are related complicatedly to bubble size. Local Bubble Size Distributions (BSD) were measured from several systems in agitated laboratory tanks. The measurements revealed a wide range of existing bubble sizes and a significant spatial inhomogeneity of BSDs. The comparison between capillary suction probe, phase Doppler anemometry and photography showed that BSDs are biased due to limitations of experimental techniques.A dynamic multiblock model with a limited number of ideally mixed subregions was developed to investigate the inhomogeneity of dispersion. Mass transfer fluxes were modelled based on the two-film theory and simplified solution of Maxwell-Stefan diffusion. Local BSDs and mass transfer areas were solved from the population balances for bubbles by the method of classes. Unknown parameters in phenomenological bubble breakage, coalescence, turbulent slip and mass transfer models were fitted against experiments. The multiblock model was used to describe macroscopic inhomogeneities of dispersion in the fitting.The results show that multiblock stirred tank model is an excellent tool for the testing and validation of closure models. The adjusted models describe local BSDs, gas holdups and mass transfer rates under varying agitation conditions and physical properties of dispersion in a limited range. Due to complexity of gas-liquid agitation measured local BSDs alone are not however sufficient for the validation of mechanistic closure models. More basic research and isolated experiments are needed for this.A comparison between multiblock and CFD simulations shows that multiblock model is an optimal trade-off between the accuracy and CPU time, when local mass transfer rates are of interest. The simulations with the validated models predict a significant inhomogeneity of mass transfer, which mostly results from the spatially varying gas-liquid interfacial areas. The developed models relate mass transfer to local physical properties and micro-scale turbulence. They are less dependent on vessel size and geometry than traditional kLa-correlations and therefore suitable for detailed reactor scale-up and design studies.The validated population balance and mass transfer closures for aqueous xanthan systems together with the bioreaction kinetics from literature were incorporated to multiblock fermenter model to investigate batch xanthan fermentation. The simulations show the need of population balances for the detailed investigation of reactive, viscous gas-liquid dispersions in which mass transfer and mixing limitations are present. The model describes the effects of mixing on reactor performance successfully. The results highlight the potential of multiblock modelling for the detailed investigation of complex multiphase reactors.
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