Biomedical Engineering Reference
In-Depth Information
Motor
X, V,
[S], [P]
F, [S] in
F, X, [S], [P]
X = P
Impeller
Figure 7.5 Stirred tank reactor.
High agitation and aeration rates can result in foam formation inside the bioreactor, which
can block the exhaust gas line and cause loss of cells and media and possible contamination
when the foam is washed out from the bioreactor. Mechanical foam breakers and antifoam
chemicals are used to control foam formation inside the bioreactor. The working volume of
the bioreactor is usually 70-80% of the vessels' physical volume.
The advantages of the STR include the high mass transfer coefficients that can be achieved
at high agitation rates and impellers that provide uniform mixing to maintain homogenous
composition in the bioreactor. However, high agitation speeds (i.e., high power-to-volume
ratios) are required to achieve good mixing and high gas-liquid mass transfer rates. This
increases operating costs. In addition, high agitation speeds increase the shear rate, which
could damage cells.
7.4.1.2 Bubble column bioreactors
Bubble column bioreactors are structurally very simple and have no mechanical agitators.
A bubble column bioreactor and its modifications are shown in Figure 7.6. Aeration and
mixing are achieved by gas sparging, which requires less energy input compared to a
mechanically agitated STR. The sparger is located at the bottom of the column and it
introduces gas into the bioreactor. Bubbles rise due to their low density compared to the
liquid. The height-to-diameter ratio of the column is greater than two. In the internal and
external gas-lift bioreactors (Figures 7.6b and 7.6c), the gas is separated from the liquid and
the liquid is recirculated in the bioreactor. Bubble columns and their modifications can be
operated counter-currently or co-currently in batch or continuous mode. There are many
important design parameters for bubble column reactors, which include the flow pattern,
bubble size distribution, gas holdup, gas-liquid interfacial area, mass transfer coefficients,
bubble coalescence characteristics, dispersion coefficients and heat transfer characteristics
(Shah et al ., 1982 ).
 
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