estimate Njs, Singh and Curtis (1994a) examine the interactions between aggregate size
and agitation conditions on scale-up; on the basis of constant impeller tip speed, for
example, the maximum aggregate size which can be suspended is reduced by
approximately 70%, for a linear scale-up factor of 10.
Laboratory scale reactors have most frequently employed the near-ubiquitous Rushton
turbine. However, with a turbulent power number of approximately S, the specific power
input (P/V) and energy dissipation rate (ε) (equation 3) will be higher for a Rushton
turbine than for a marine impeller (Np, turb~0.4), a helical ribbon impeller (Np, turb~0.3), a
paddle (Np, turb ~2), a pitched blade turbine (Np, turb~1) or a two-stage Intermig (Np,
turb~0.7), operated under comparable conditions in the same vessel. While data for most
commercial applications are proprietary, data from laboratory and pilot-scale facilities
indicates that axial flow impellers, including pitched blade impellers and marine
propellers, are more appropriate for plant cell suspensions (e.g. Furuya et al., 1984;
Leckie et al., 1991b). Hollow paddle impellers have been used with high density
suspensions of Coptis japonica (Matsubara et al., 1989) and helical ribbon impellers
supported biomass levels in excess of 25 g DW L−^1 in suspensions of C. roseus (Kamen
et al., 1992; Jolicœur et al., 1992) and Vitis vinifera (Cormier et al., 1996). Novel
agitation systems which have been employed or recommended for use with plant cell
suspensions include a wide-bladed turbine impeller (w/D=1.84, compared to a value of
0.2 for a standard Rushton turbine) (Hooker et al., 1990); a 2-blade “grid” paddle,
employed in combination with a spiral sparger in a Maxblend Fermenter® (Yokoi et al.,
1993); tangential (Treat et al, 1989) and centrifugal (Wang and Zhong, 1996) cell-lift
impellers. However, there are no reports of large-scale plant cell suspension applications
for any of these systems.
Because of the perceived low-shear advantage, as well as simplicity of operation,
pneumatically impelled reactors, including bubble columns and ALRs (with both internal
and external loops), have been widely used for plant cell applications. Tables 14.1 and
14.2 include references to 1000 L and 1500 L ALRs, used in development trials. Using
available correlations for time-averaged shear rates and shear stresses, Doran (1993)
estimated significantly lower average shear stresses in ALRs than in STRs, under
representative operating conditions. However, for high density broths (>30 g DW L−^1 ),
mixing efficiency in the ALRs is sufficiently reduced to limit oxygen transfer in the broth
and this factor largely explains the failure of this configuration to find wider application
at industrial scale. It is also reflected in operational difficulties encountered in pilot scale
systems (e.g. Park et al., 1992) and experimental observations of pronounced
inhomogeneity in bean suspensions in an internal loop ALR, at a biomass level of 12.5 g
DW L−^1 (Assa and Bar, 1991). In this context, it is interesting to note that the two ALR
systems referenced in Tables 14.1 and 14.2 were ultimately superseded by STRs.
In the design of large-scale microbial fermentation systems, oxygen transfer is often a
limiting factor. On the basis of the lower characteristic oxygen requirements (Section on
Oxygen Requirements), this is not expected to be the case for plant cell suspensions. The
oxygen transfer rate (OTR) is defined as
(6)Bioreactor design for plant cell suspension cultures 435