where kLa is frequently related to the operating conditions in a STR by correlations of the
following form:
(7)
For ALRs, empirical correlations of the following form have been employed:
(8)
Evaluation of μa depends, of course, on the specification of an appropriate shear rate
(Section on Shear Sensitivity)
In practical terms, for a high density (30 g DW L−^1 ) suspension, with a qo of 0.3 mmol
O 2 (g DW)−^1 h−^1 , a ccrit of 25% and assuming an equilibrium dissolved oxygen
concentration (c) of 0.25 mmol L−^1 , equation 6 indicates that the system oxygen
demands can be met by a kLa of approximately 48 h−^1. This value lies comfortably within
the performance range of conventional STRs and is representative of values reported for
large-scale systems (e.g. Hashimoto and Azechi, 1988). If either the biomass level or ccrit
is higher than the assumed values, then a higher kLa will be required. It is obvious, from
equations 7 and 8, that kLa can be increased by increasing N (and thus, P/V) and/or the
aeration rate (and thus, usr). However, neither of these approaches may be appropriate for
a shear-sensitive organism or a high-foaming broth. An alternative is to increase c, by
either oxygen enrichment or increasing system pressure. Bioreactors are typically
designed to withstand sterilisation and are generally rated for pressures of up to 2.5–2.7
bar. Thus, the costs associated with operating at pressures of up to 2 bar are minimal.
However, although few organisms exhibit sensitivity to pressures within this range, the
resulting effects of elevated concentration of other dissolved components, including CO2
and ethylene (Section on Aeration in Plant Bioreactors) are still unclear. Oxygen-
enrichment is commonly employed in laboratory and pilot-scale fermentations, but is a
costly exercise at industrial scale. This approach was adopted by Matsubara et al. (1989)
to achieve kLa values of up to about 80 h−^1 , at a gassing rate of 0.05 vvm, in a high
density C. japonica system. Such higher values notwithstanding, analysis of the available
data suggests that kLa values of between about 10 and 30 h−^1 are typical for plant cell
cultures. In the development of novel bioreactors/ mixing systems for use with plant cell
suspensions, a premium has been placed on the achievement of high kLa values, under
conditions of reduced aeration/agitation intensity (e.g. Tanaka et al., 1983, Kamen et al.,
1992; Yokoi et al., 1993).
In a recent analysis of mixing systems for plant cell suspension in STRs, Doran (1999)
simultaneously considers the phenomena of gas dispersion, solids suspension, oxygen
mass transfer and shear damage for a variety of impeller geometries, in a representative
10 m^3 vessel of standard configuration. Large-scale commercial bioreactors typically
have larger aspect ratios (H/T) and consequently, are often equipped with more than one
impeller. Nonetheless, the flow regime maps for a Rushton turbine (Figure 14.5 (a)) and
an upward-pumping pitched blade turbine (Figure 14.5 (b)) give a clear, qualitative
indication of the relative merits of these impellers and of the pertinent issues for mixing
system design. The shaded regions, indicating approximate limits for shear-related
damage, are evaluated on the basis of a maximum cumulative energy dissipation of 10^7 J
Multiphase bioreactor design 436