Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 196

condensed. Part of the condensate will be taken as the purified product stream and the other
part is returned back to the column as a flux to enhance the rectifying process. In the
stripping section below the feed stream, ethanol is removed from water as the feed moves
towards the bottom of the column. Vapor flows up the column from tray to tray and the
liquid flows down the column. The purpose of the tray is to provide mixing and separation
of vapor and liquid.
In addition, unfermented sugars can negatively affect the efficiency of the distillation
process, as these sugars adhere to the surfaces of the trays in the distillation column.
There are several design factors that affect the size of distillation columns; these include
the amount of vapor generated, the required purity of the product, type of contacting
device (tray or packing), the amount of undissolved solids in the feed and the energy
requirement.
For acetone-butanol-ethanol (ABE) fermentation, the broth contains about 25-35 g/l of
mixed solvents. Butanol concentration is usually less than 20 g/l, which makes its recovery
by distillation expensive. This low butanol concentration in the fermentation broth is related
to the inability of Clostridium species to produce more butanol due to solvent toxicity (Lee
et al ., 2008). Metabolic engineering and advanced fermentation techniques are ongoing to
enhance the organisms' abilities to produce and tolerate higher concentrations of butanol
and increase productivity. Several integrated fermentation and recovery processes, such as
fed-batch fermentation with pervaporation and continuous fermentation with gas stripping,
have been reviewed elsewhere (Lee et al ., 2008 ).
7.7.3 Separation of diols and triols
The recovery of glycerol (1,2,3-propanetriol) from the fermentation broth is difficult because
glycerol is miscible in water and has a high boiling point. Glycerol is separated via vacuum
distillation after the removal of ethanol and precipitation of sulfite, sulfate and phosphate
salts from the fermentation broth (Rehm, 1988). Even so, only about 50% of the glycerol in
the broth is recovered by vacuum distillation. Other methods for glycerol recovery using ion
exchange resins and extraction with different solvents have been reported by Rehm (1988)
and Wang and co-workers (2001).
The recovery of 1,3 propanediol (1,3-PD) from the fermentation broth is done by vacuum
distillation and/or preparative liquid chromatography after removing the cells and other
impurities (Xiu and Zeng, 2008). Other methods for recovery of 1,3-PD, such as extraction
and membrane separation, have been investigated (Xiu and Zeng, 2008). On the other hand,
Syu (2001) summarized several separation techniques for 2,3-butanediol (2,3-BD), which
include pervaporation, membrane distillation and extraction.
7.8 FUTURE APPLICATION AREAS AND EMERGING
DEVELOPMENTS
Emerging areas in fermentation technologies include metabolic engineering, genetic
engineering and synthetic biology, with focus on advanced biofuels and bio-based products
(Ragauskas et al ., 2006 ; Atsumi and Liao, 2008 ; Sillers et al ., 2008 ; Park et al ., 2008 ;
Peralta-Yahya and Keasling, 2010). These areas are being used to develop new organisms
that can tolerate the inhibitory effects of substrate and product, can utilize various substrates
and produced desired products with high titer and productivity.