production run. Foam buildup can be counteracted mechanically through

devices attached to the stirrer´s shaft or by the addition of surfactants (an-

tifoam agents). The latter, however, can impact negatively the pO2 control as

well as subsequent clarification and DSP steps. For high cell density (HCD)

cultivations exceeding 4E07 cells/mL it might even be required to use two

spargers to provide appropriate aeration for cell growth. Using pure oxygen

instead of air again can help to reduce foaming as the flow rate can be

lowered. However, aeration with pure oxygen results in a CO2-free atmo-

sphere, thus, favoring pH increase. In contrast, if pH decreases too fast it

might be favorable to aerate with pure oxygen [54].

b. Surface aeration and non-contact systems

Bubble-free aeration systems aim to supply oxygen to shear sensitive cell

cultures. In surface aeration, oxygen transfers from the headspace to the

culture through the gas-liquid interface area in either OSB or STR, where

vessel geometry, agitation speed, and working volume influence its rate

[69]. In addition, O2 supply can be enhanced through immersed silicon

tubing aerators, owing to the relatively high O2 permeability in this ma-

terial. Unlike surface aeration, the transfer rate is independent of the

stirring speed due to the fact that the main mass transfer resistance relies

on the silicon tubing wall. By selection of an adequate length and wall

thickness of the silicon tubing as well as the volumetric flow rate and the

O2 content in the flow stream, these aerators can achieve oxygen transfer

rates similar to those of gas spargers [63].

c. Mixing

Short mixing times for dispersion of either gas or nutrients is especially

challenging at larger size of the bioreactor vessels. As this often cannot

be achieved by faster stirring, increase of turbulence and reduction of

stagnant zones is often obtained through vessel and impeller design [70].

In general, the use of radial flow impellers, such as the Rushton type,

results in a better dispersion of the gaseous phase at the expense of

increased power consumption and lower mixing efficiency [71]. The use

of two or more impellers increases mixing efficiency depending on their

spacing along the shaft [72,73]. When radial and axial flow impellers are

combined, lower power consumption and mixing times are obtained.

However, a configuration of three 6-blade Rushton impellers spaced with

a distance equal to the impeller diameter, can deliver the highest kLa per

delivered power unit [74]. The considerable progress and refinement

in computational fluid dynamics (CFD) offers options for an a priori

evaluation of the mixing system. For example, Gelves and coworkers

compared the performance of a typical Rushton impeller system against

novel pitched blade impellers with rotatory microspargers through CFD

mass transfer and hydrodynamic modeling. Experimental and computa-

tional results confirmed an increase a 34-fold increase in kLa with a 50%

saving in power in comparison to the conventional system used as a

reference [75].

Upstream processing for viral vaccines

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