A methodology based on sto
chasti
c modelling is presented to des
cribe the influen
ce of the biorea
ctor heterogeneity on the mi
croorganisms growth and physiology. The sto
chasti
c model is
composed of two sub-models: a mi
croorganism
cir
culation sub-model and a fluid mixing sub-model used for the
chara
cterization of the
con
centration gradient. The first one is expressed by a
classi
cal sto
chasti
c model (with random number generation), whereas the se
cond one is expressed by a sto
chasti
c Markov
chain. Their superimposition permits to obtain the
con
centration profiles experien
ced by the mi
croorganisms in the biorea
ctor. The simulation results are expressed in the form of frequen
cy distributions. At first, the study has been fo
cused on the design of s
cale-down rea
ctors (SDR). This kind of rea
ctor has been reported to be an effi
cient tool to study at a small-s
cale the hydrodynami
c behaviour en
countered in large-s
cale rea
ctor [P. Neubauer, L. Horvat, S.O. Enfors, Influen
ce of substrate os
cillations on a
cetate formation and growth yield in
Escherichia coli glu
cose limited fed-bat
ch
cultivations, Biote
chnol. Bioeng. 47 (1995) 139–146]. Several parameters affe
cting the shape of the frequen
cy distributions have been tested. Among these, it appears that the perturbation frequen
cy, the exposure time and the design of the non-mixed part of the SDR have a signifi
cant influen
ce on the shape of the distributions. The respe
ctive influen
ce of all these parameters must be taken into a
ccount in order to obtain representative results. As a general trend, the in
crease of the re
cir
culation flow rate between the mixed and the non-mixed part of the SDR indu
ce a shift of the frequen
cy distribution for the lower relative
con
centrations, whi
ch suggests an attenuation of the s
cale-down effe
ct. This has been validated by using the SDR in the
case of the
cultivation of
Saccharomyces cerevisiae. However, the influen
ce of the non-mixed part of the SDR is not quite well understood if only taking a
ccount of the frequen
cy distribution analysis, and supplementary experiments are required to elu
cidate the underlying me
chanism.
The aspect of the frequency distributions suggests that both the design and the operating conditions of a scale-down reactor need to be adjusted in order to match the behaviour of a given large-scale reactor. Examples of frequency distributions obtained in the case of large-scale reactors are given.