Biobutanol - Biotechnology in China: Biofuels and Bioenergy - page 96

separation. In Chinese plants, the continuous fermentation process for butanol has
been well established to improve bioreactor productivity by keeping the culture at a
steady state in fermentation. The steady state is controlled by continuous addition of
fresh medium, which is exactly balanced by the removal of cell suspension from the
bioreactor. Compared with batch fermentation and fed-batch fermentation, this
process could reduce the fermentation period, increase the rate of equipment utili-
zation, and decrease the production cost. The use of continuous fermentation for
ABE production in China dates back about 50 years. However, general Clostridium
strains seem not to be suitable for continuous fermentation, because solventogenic
clostridia could form spores in the process and lose their solvent production activity.
In addition, the strains would exhibit physiological decay, referred to as degenera-
tion, after a serious of inoculations in culture medium [ 11 ]. In the 1950s, Chinese
researchers began to study continuous ABE fermentation and its industrial appli-
cation. Chiao published the first academic article on this subject in China [ 6 ]. In the
review by Ni et al. [ 38 ], there was a detailed description of Chinese ABE continuous
fermentation. In brief, the whole system included six to eight 200-500 m 3 fermen-
ters. The first two tanks were activation fermenters with periodic addition of seed
culture from smaller seed tanks. The hallmark of this system was that the cells could
maintain the active phase using this configuration and inoculation procedure. From
the first fermenter to the last, there is a pressure gradient to keep the whole system
continuously operating in an overflow manner. In this way, the ABE process could be
continuously operated for about 170-480 h without sterilization and loading. This
continuous fermentation has higher productivity (20-50%) than batch fermentation.
3.3 Separation
In the ABE fermentation process, when the solvents accumulate to a specific
concentration (usually less 30 g/L), the cells show significant growth inhibition and
limited production activity. Butanol is recognized as the most toxic product among the
solvents. Therefore, developing a series of butanol recovery methods to remove
the toxicity of butanol to cells is an alternative way to improve productivity. Con-
ventionally, the solvents were separated from the fermentation broth by distillation. For
in-situ recovery, several other processes have been established, including absorption,
liquid-liquid extraction, perstraction, reverse osmosis, pervaporation and gas stripping.
(a) Absorption. Absorption is an efficient technique of removing butanol from
the fermentation broth. The principle is that butanol is absorbed by absorbents and
then desorbed by heat or displacers to obtain pure butanol. This method can be
used for in-situ recovery. Polyvinyl pyridine (PVP) and silicalite are the common
absorbents. Compared to conventional batch fermentation, product concentration
and productivity increased by 54 and 130%, respectively, and sugar utilization
increased to 73.3 g/L in absorption-coupling fermentation [ 51 , 52 ]. The results
proved that adsorption coupled with the fermentation process was an effective
method for butanol recovery in improving total product concentration and sugar