Hydromechanical stress is a crucial
parameter for a broad range
of multiphase processes in the field of (bio)chemical engineering.
The effect of impeller type and geometry on hydromechanical stress
in stirred tanks is important. The present study aims at characterizing
conventional and new impeller types in terms of particle stress. A
two-phase liquid/liquid noncoalescing dispersion system is employed,
and the drop breakage is monitored in-line in a stirred tank. The
published effects of agitation on drop deformation are confirmed and
expanded significantly for five modified new impeller types. Radial
impellers are advantageous for applications where low shear conditions
are desired. A modified propeller with a peripheral ring and the developed
wave-ribbon impellers present remarkable results by producing significantly
low and high hydromechanical stress, respectively. The results obtained
are correlated in terms of mean and maximum energy dissipation rate,
as well as circulation frequency in the impeller swept volume.
A B S T R AC TSince aeration is the largest cost factor in membrane bioreactor (MBR) operation it is clear that the biggest leap towards energy and operational costs savings can be achieved by improving the use of air. Many basics of the complex two-phase fl ow in membrane modules and in the overall MBR tank as well as their interactions, however, are still poorly understood. This work focuses both on fundamental studies on shear stress exerted by rising bubbles and on optimising the geometries of tank and module accordingly in order to obtain an improved deposition control at minimum energy input. For both, parameter studies were carried out by numerical simulations which were validated with experimental measurements. The optimum bubble size/channel width combination depended on the superimposed liquid velocity. The relationship between the liquid circulation velocity and the aeration intensity was measured for different reactor and module geometries. A modifi cation of the Chisti model for airlift loop reactors was also performed which can be used as a design rule for tank and module geometry or aeration rate. At the same gas fl ow rate, a 30-50% increase in liquid circulation velocity was achieved by a simple modifi cation of the sparger and the entry zone to the riser section.
Mainly with respect to biotechnological cases, current developments in the field of impeller geometries and findings for multistage configurations with a specific view on aerated stirred tanks are reviewed. Although often the first choice, in the given case the 6‐straight blade disc turbine is usually not the best option. Furthermore, quantities usable for scale‐up, specifically applicable in this field are discussed. Only quantities taking local conditions into account appear to be able to actually compare different stirrer types and scales.
The experimental accessibility of disperse systems often is a critical factor when it comes to the development of modeling approaches that intend to converge towards an exact solution. Often, integral or pseudo‐homogeneous values are used to reduce the complexity of the system, but a detailed single particle or interface analysis is crucial to understand relevant effects that also affect the swarm behavior. A high number of experimental techniques with respective limitations and advantages is available to quantify these effects. In this work, an overview on measurement techniques for momentum, heat and mass transfer in particle swarms as well as for the particle size distribution and interface characterization is provided. The industrial applicability is addressed by pointing out the vicinity to the process and the costs of different measurement techniques.
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