(442c) Investigating Bubble Size Distribution in Fermentation Reactors with CFD

Authors: 
Eppinger, T., CD-adapco
Aglave, R., Siemens PLM Software
Lo, S., Siemens PLM

Fermentation is a common method for producing
bio-products such as alcohols and pharmaceutical compounds or alternative
feedstocks. In the primary stage, presence of oxygen drives the metabolism and
hence growth of the organic substance. Oxygen is supplied via air in stirred
vessel type fermenters. While a certain configuration tested in a lab may
result in satisfactory distribution of the dissolved oxygen (DO), large scale
fermenters need careful design considerations.

Computational methods (CFD) have been reported
in large amount of literature to investigate the flow patterns and consequently
the gas hold up in such equipment. However, the air bubble injected vary in
size throughout the reactors due to the choice of impellers, rpm, baffles, and
other internals such as coils or dip tubes. The size of the air bubbles affects
directly the gas hold up and the interfacial area. Therefore it is necessary to
account for the varying bubble size distribution due to coalescence and
breakup.

In this work, we present the use of a size
distribution model to account for the bubble size distribution called S-gamma
model in the commercial code STAR-CCM+. This model shows the differences in
assuming constant bubble size but is not as computationally expensive as the
quadrature (DQMOM/QMOM) type of models.

Features of the S-gamma model will be discussed
and a case study presented where the two methods are compared. An outlook on
future work and advances will also be given.

Gas distribution and total gas hold-up for a plant scale bioreactor assuming a constant bubble size (left) and a bubble size distribution including coalescence and break-up (right).

References:

Lo, S. and Zang, D.,
Modelling of Break-up and Coalescence in Bubbly Two-Phase Flows, Journal of
Computational Multiphase Flows, Vol 1(1), p 23, 2009

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