Bioprocess scale-up strategy based on integration of microbial physiology and fluid dynamics

• Forschungsstelle: EU FRP
• Projektnummer: 95.0881
• Projekttitel Englisch: Bioprocess scale-up strategy based on integration of microbial physiology and fluid dynamics

Schlüsselwörter (Englisch)

Foam; fermentation; mechanical foam control; gas/foam entrainment; multiple impellers; scale-up

Alternative Projektnummern (Englisch)

EU project number: BIO4CT950028

Forschungsprogramme (Englisch)

EU-programme: 4. Frame Research Programme - 4.1 Biotechnology

Kurzbeschreibung (Englisch)

See abstract

Partner und Internationale Organisationen (Englisch)

KTH, Stockholm (S)

Abstract (Englisch)

In aerated stirred vessels foam formation may pose serious operational difficulties. Various ways of foam reduction are used in industry, each with its particular drawbacks. Stirring as Foam Disruption (SAFD) is a novel method to reduce foam in fermentation processes. The principle of this method is to reduce the foam layer with the liquid flow generated by a stirrer placed just underneath the level of the gas-liquid dispersion. The scale-up of the SAFD technique was studied in bioreactors ranging from 20 to 30'000 L. Fermentation broths as well as artificial foaming media were used. Various stirrer configurations were tested. As upper stirrers radial impellers, e.g. Rushton turbines, and axial pumping impellers, such as hydrofoils, were applied. On 750 L scale the power draw of the impellers was measured using strain gauges. Liquid flow patters near the dispersion surface were determined with a magnetic inductive velocimeter.
A mechanistic model was developed, relating the foam height to the horizontal liquid velocity near the dispersion surface.
For a given stirrer configuration and within a certain range of the broth mass the foam height was correlated with the broth mass, i.e. the distance between upper stirrer and dispersion surface. Furthermore, the foam height was correlated with the superficial gas velocity. Increasing the stirrer speed often resulted in reducing the foam height.
The best impeller for foam disruption is considered to be the one that is able to disrupt the foam with the greatest liquid layer between upper impeller and dispersion level for a given specific power draw.
Upward pumping stirrers were particularly effective for foam disruption. Retrofitting a 30 m3 bioreactor by replacing Rushton turbines by a mixed set of Scaba impellers resulted in almost foamless E.coli fermentations.
The SAFD technique for foam disruption proved to be scalable up to industrial scale. For radial pumping stirrers, particularly the Rushton turbines, the above model provided scale independent correlations between foam height and horizontal liquid velocity. With respect to power draw, axial stirrers were more effective for foam disruption. However, for axial stirrers the SAFD model was adequate only for the hydrofoil Scaba 3 SHP1, both in up- and downward pumping mode.
Measured liquid velocities differed from calculated velocities in value but not in order of magnitude. The higher the measured velocity the lower the foam height. Upward pumping hydrofoils create a stronger flow near the dispersion level than downward pumping hydrofoils and are therefore more effective for foam disruption. Furthermore, the flow pattern of the hydrofoils consists of a single loop and not two as does the characteristic flow pattern of radial pumping impellers. This difference means that for the latter roughly half of the power draw gives into producing the upper loop, the only one effective for foam disruption.
Foam entrainment is thought to be the major mechanism behind the SAFD technique. The SAFD technique allows more broth during fermentations and therefore enables a higher bioreactor output.

Datenbankreferenzen (Englisch)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0881