Hydromechanical stress in aerated stirred tanks

Abstract

Turbulence intensity, or hydromechanical stress, is a controlling parameter in manyindustrially relevant processes that are operated in aerated stirred tanks. These processes areoften characterized by intense aeration and agitation, particularly in the fermentationindustries. Measurement of hydromechanical stress under conditions of intense aeration andagitation is extremely difficult. Very few data on turbulence characteristics exist due to alack of measurement techniques. Therefore, this thesis focused on three main topics: (1) thedevelopment of a measurement technique for hydromechanical stress in aerated stirredtanks, (2) the investigation of the influence of aeration on hydromechanical stress and (3)the characterization of the influence of geometry and scale on hydromechanical stress inaerated stirred tanks.A new measurement method was developed that is based on the well established correlationof maximum stable drop size of a break-up controlled dispersion with hydromechanicalstress. The continuous and dispersed phase properties were selected to be able for the firsttime to apply this principle to aerated operating conditions. This was achieved mainly byapplying a dilute dispersion, a low ionic strength and by incorporating a dispersed phase,paraffin oil, with a negative spreading coefficient. The negative spreading coefficientprevents coalescence due to drop-bubble interactions for aerated operating conditions.This new method was applied to investigate the influence of aeration on hydromechanicalstress for a broad range of aerated and unaerated operating conditions in a 3 m3 reactor.Results from drop dispersion experiments with two different setups of 6-bladed Rushtonturbine impellers with diameters of d = 0.41 m (d/DR = 0.34, setup B-1) and d = 0.51 m(d/DR = 0.43, setup B-3) in a 3 impeller configuration were presented. The results fromexperiments without aeration were well in agreement with the existing literature on dropdispersion. The results from experiments with aeration indicated a strong attenuation ofturbulence intensity in stirred tank reactors by the presence of air. The ratio betweenmaximum and volume-averaged energy dissipation rate, φ, was reduced by aeration by 64 %for the d/DR = 0.34 impeller setup (B-1) and by 52 % for the d/DR = 0.43 impeller setup (B-3) when compared with unaerated operating conditions on the basis of equal volumetricpower input. The value of the aeration rate had no measurable effect in the range of aerationrates applied, which was between 0.1 vvm and 1 vvm.The method was then applied in a broad range of reactor scales (50 L, 3 m3 and 40 m3volume), impeller geometries (Rushton turbines and an Ekato Phasejet/Combijetcombination) and operating conditions to investigate the influence of geometry and scale onhydromechanical stress. Comparison of data between the different scales showed that thereis a scale effect that results in higher values for φ in larger reactors. This behaviour is notcovered by the classic theory of turbulent drop dispersion but is in good agreement with thetheory of turbulence intermittency. The data for all impeller configurations and all aerationrates for the three scales correlated very well when calculated values for φ based on themeasured values for dmax were used to calculate the maximum local energy dissipation rate.Most of the data was within 20 % around the theoretical prediction from the classic theoryof drop dispersion when these values for φ were used. A correlation of the data for all scalesand all impeller configurations in the form φ = 2.3·(φunaerated)0.34·(DR)0.543 was suggested thatsuccessfully models the influence of scale and impeller geometry on φ for aerated operatingconditions. Results for an Ekato Phasejet/Combijet impeller setup showed that thiscorrelation can also be applied successfully to estimate hydromechanical stress for impellergeometries other than Rushton impellers.The measurement method and the data presented in this thesis represent an important stepforward in the characterization of hydromechanical stress in stirred tanks under aeratedoperating conditions. The newly developed measurement method can be applied in thefuture to characterize further agitator and reactor designs under process relevant operatingconditions which was frequently not possible in the past due to a lack of measurementtechniques. The correlation for φ developed in this work can be beneficially used forapproximate calculations of hydromechanical stress for geometries for which experimentaldata is not available. In combination with further important process parameters like masstransfer characteristics and mixing characteristics of impeller setups, characteristics ofhydromechanical stress can be incorporated in reactor and process optimization in a morerelevant manner than possible before