Abstract
AbstractMainly 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.
Highlights
Asking experts on general rules for the layout of stirred tank reactors, simplified spoken, one will most likely hear about 6-straight blade disc turbines with radial primary flow, known as Rushton turbines or propellers with axial primary flow
Local isotropy of the turbulent flow is assumed. When it comes to the scale-up from laboratory to industrial scale, again experience often leads to rules such as keeping the tip velocity of the stirrer wtip constant due to comparable maximum shear rates apparent in the bulk liquid or using a constant specific power input P/V
computational fluid dynamics (CFD) simulations were performed to get a deeper understanding of the overall effect, more work has to be done in this field with multiphase modeling and a rotating stirrer and a flexible shaft
Summary
Asking experts on general rules for the layout of stirred tank reactors, simplified spoken, one will most likely hear about 6-straight blade disc turbines with radial primary flow, known as Rushton turbines or propellers with axial primary flow. The conditions in the aerated stirred tank for the cultivation of microorganisms can cover: – time-dependent, often non-Newtonian rheological behavior of the cultivation broth in combination with – shear-sensitive micro-organisms/cells and, at the same time, – targeting an oxygen transfer rate larger/equal to the oxygen uptake rate while – having substrate concentration-depending metabolisms with different kinetics for metabolism changes from biomass built-up or production to ‘‘survival’’ mode and back This briefly illustrates the complexity of the design task at hand. Nienow [3] gave an overview concerning the important physical aspects such as volumetric mass transfer coefficient kLa, local flow characteristics (shear stress near the stirrer or specific energy dissipation), local and overall homogeneity of the bulk and air-phase mixing, power input, heat transfer or microcarrier suspension He summarized biological aspects such as growth and productivity, local and overall substrate and CO2 concentration, pH value, temperature and shear sensitivity. Different approaches reaching from rather global quantities up to quantities considering local information in the system will be discussed and assessed for their applicability
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