482
Chapter 11
Fig. 11.5.
Impellers used in stirred tank bioreactors, (a) Six-bladed Rushton turbine, (b) four-bladed
Scaba agitator, (c) marine impeller (or propeller), (d) five-bladed Prochem Maxflo T, (e) Intermig.
Adapted from Nienow 1990 and McDonough (1992)
The ratio between the volumetric air flow rate,
vg,
and the cross sectional area of the reactor is
called the superficial gas velocity,
us.
The superficial gas velocity should not be too large, to
ensure an efficient dispersion and utilization of the gas. At too high superficial gas velocities, the
impeller becomes fully surrounded by gas, and the dispersion capacity falls dramatically. This
phenomenon is called flooding.
11.3. Physical Processes of Importance for Scale-up
A number of factors of a physical nature change with the scale of the reactor. In terms of a
rigorous mathematical treatment, these problems are addressed by solving the governing
equations of motion for the fluid in the reactor. This is the field of computational fluid dynamics
(CFD). The mathematical treatment of flow phenomena is rather complex, and considerable
fundamental problems relating to turbulence theory and multi-phase flow still remain to be
solved. We will therefore in the following mainly resort to simplified models to illustrate the
essential changes that occur with respect to mixing, power consumption, heat transfer, mass
transfer and flow patterns in a stirred tank reactor as the scale changes.
11.3.1. Mixing
With
mixing
is understood
the process o f achieving uniformity.
Mixing processes can be divided
according to the number of phases involved in the mixing process; i.e. single phase liquid
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