Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 190

The main advantages of monolithic reactors in catalytic gas-liquid reactions include very
low pressure drop, high mass transfer, high surface-to-volume ratio, good contact between
gas and liquid and ease of reactor scale up. However, there are some drawbacks for
monolithic reactors, such as the high manufacturing cost of the monolithic catalysts, short
residence time, difficulty in uniformly distributing fluid across the monolithic reactor cross-
section and the possibility of clogging monoliths with biofilm formation (Ebrahimi et al .,
2005 ; Cybulski et al ., 2006 ).
7.5 FERMENTATION SCHEMES
Fermentation processes can be conducted in batch, fed-batch or continuous modes. However,
many fermentation processes are operated in batch or fed-batch modes due to the advantages
of these schemes compared to continuous operation. These advantages include easier set-up
and operation, less complication in process control, less susceptibility to microbial
contamination, and high final product titer and yields. On the other hand, continuous
bioreactors have advantages such as consistent quality of product, high productivity and
flexibility in system investigation and analysis.
7.5.1 Batch fermentation
In batch operation, the medium containing the substrate and nutrients is added at time zero.
Then, the medium is sterilized and cooled before addition of the desired microorganism.
Chemical agents are added to control pH, foam formation and contaminating bacteria,
resulting in a minimal change in the net volume of the medium.
Batch fermentation has the disadvantage of low productivity due to the long turn-around
times associated with cleaning and preparation of fermentors and the initial lag phase during
growth of the desired microorganism. Cell recycle from one batch to the second batch has
been successfully used in ethanol fermentation, which significantly reduced the fermentation
cycle time. In addition, the use of high performing strains of microorganisms can increase
product yield and/or reduce total fermentation cycle time. This substantially reduces the size
of fermentor required and lowers operating and capital costs.
7.5.2 Fed-batch fermentation
The fed-batch mode starts the same as batch operation. However, concentrated medium is at
some time continuously or intermittently added into the fermentor until the desired working
volume is reached. The volume of the medium inside the fermentor changes in the fed-batch
mode due to the frequent additions of substrate and nutrients. This is done to keep the
substrate concentration low to minimize substrate inhibition to microorganisms. Higher
product concentration and yield can be obtained in fed-batch compared to batch operation.
Both batch and fed-batch fermentations are run until completion and no products are taken
out from the fermentor until the end of fermentation.
7.5.3 Continuous fermentation
In the continuous fermentation scheme, fermentation is done at a relatively low substrate
concentration and high biomass concentration. The addition of high substrate concentration
in the feed stream can inhibit cells' growth and fermentation capability. Continuous