Numerisches Berechnen der Strömungs- und Konzentrationsfelder in zwei- und dreiphasig betriebenen Blasensäulen

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Inhaltsverzeichnis, Datei (98 KB)
Vorwort, Datei (60 KB)
Leseprobe, Datei (170 KB)

Bubble column reactors are widely used in chemical and biochemical industries. The multiphase flow inside the column can be a two-phase gas-liquid flow or a three-phase gasliquid- solid flow. The transport of mass and momentum between the phases is mainly influenced by the flow field and the bubble size distribution. For the dimension of bubble columns empirical correlations are usually applied, with range of applicability limited to the experimental conditions. Therefore in this work the three-dimensional, time dependent velocity and concentration fields are numerically calculated for two- and three phase bubble columns and compared to existing experimental results. In particular backmixing and mass transfer in these columns are investigated. Multiphase flow in bubble columns can be described by the so-called Eulerian model. In the proposed model the balance equations for mass and momentum are coupled with a transport equation for the mean bubble diameter. Thus the transport of mass and momentum is calculated with a dependence on the local interfacial bubble area density. In the simulations a tracer is injected into the dispersed gaseous phase and the continuous liquid phase. From the calculated three-dimensional, time-dependent concentration field of the tracer the axial dispersion coefficient is obtained and compared with existing experimental results. The model is extended to include mass transfer between the bubbles and the surrounding liquid. The absorption of a gaseous component reduces the momentum transfer between the phases. In the case of high rates of mass transfer between bubbles and liquid, the flow field is altered thus the amount and size of the large-scale liquid vortices are reduced. Finally three-phase flow in bubble columns is considered. For the calculation of flow fields in three-phase operated bubble columns the solid phase is considered by an Eulerian phase. Coalescence of bubbles in suspensions of particles and water is experimentally investigated. The presence of solid particles is found to hinder the coalescence of bubbles. The calculated three-phase flow fields are characterized by large-scale vortices similar to two-phase gasliquid flow. The solid particles are considered as catalyst for a heterogeneous chemical reaction.