Multiphase Bioreactor Design

Table 15.5 Minimal contact time, τ, for cell

attachment to air bubbles cell movement and liquid

drainage, and half life (t0.5) and height of the foam

for different medium compositions (adapted from

Michaels et al. (1995b) (see text for explanation of

the parameters)

Medium formulation τ
ms

Cell movement Thin film drainage t0.5 min h cm

Basal medium <10 Unstable

Serum-free 50–100 Medium Fast 4 7.4

0.1% PVP 20–500 Slow Slow 28 13.3

0.1% PEG 8000 200–1000 Slow Slow 7 15.0

3%FBS 200–1000 Medium Medium 65 21.4

0.1% PVA 1000–10000 Fast Fast 1 3.8

0.1% Pluronic F68 5000–20000 Fast Fast 0 2.0

0.1%Methocel A15LV no attachment Fast Fast 0 4.1

required contact time. The length of this contact time depends on the type and
concentration of the additive used.

Cell Death at the Sparger

The bubble formation process at the sparger can be separated into two stages. In the first
stage the bubble expands, while staying attached to the base. In the second stage the
buoyancy force causes the bubble to move away from the base. Bubble growth continues
through a narrowing neck connecting the bubble to the base. When this neck closes and
the bubble detaches from the base, fluid rushes into the region of the bubble neck. Cell
death near the sparger may have two causes.
One cause of cell death at the sparger may be the hydrodynamic forces associated with
the flow of fluid around a growing and detaching bubble. The maximum velocity with
which the bubble penetrates into the liquid may be calculated from

(11)

where n is the number of nozzles and di (m) is the diameter of a nozzle. Using this
equation at F=l dm^3 .h−^1 , n=1 and di=l mm the injection velocity is equal to 0.088 m.s−^1.
Assuming the flow around the bubble is laminar, shear stresses associated with these
flows may be estimated from

(12)

Multiphase bioreactor design 466