is currently again gaining interest in the western world. In contrast, isobutanol is a
new platform molecule, being a precursor for isobutene and xylenes [29], that can be
commercially produced using a genetically modified yeast [9].
A major challenge in any fermentation process for platform molecules is product
inhibition. When the product concentrations become too high in the fermentation
broth, the metabolism of the microorganism slows down, and the production of the
desired product stops. This limits the final concentrations obtain during an alcohol
fermentation process. For example, of the concentration of ethanol in a batch
fermentation is limited to 11–12 wt% [30] and for n-butanol and isobutanol to
2 wt% [9]. Conventionally, these alcohols are recovered using distillation
[21, 31]. However, due the low product concentrations, the energy demand for
these distillation processes is very high. In the case of n-butanol, the recovery via
distillation requires more than 50 MJ/kg [32], while the energy content of n-butanol
itself is only 33 MJ/kg [33]. Therefore, a large amount of research effort is undertaken to increase the efficiency of the downstream processing of bio-alcohols.
Many different techniques have been studied to increase the efficiency of the
separation and purification of fermentatively produced alcohols. Ideally, these separation processes can be partially integrated with the fermentation itself, also called
in situ product recovery, to relieve product inhibition during the fermentation and
increase the yield of the fermentation process. Examples of techniques which have
been studied for bio-alcohol recovery include liquid-liquid extraction [34–36],
gas-stripping [37–40], the use of membranes (pervaporation, membrane extraction,
membrane distillation) [39, 41–46], and recovery via adsorption [37, 47–
61]. Among these different techniques, adsorption-based recovery has been
highlighted for its energy efficiency [53].
An example of the traditional downstream processing of biobutanol is shown in
Fig. 1 [22, 62–64]. Biobutanol is produced in the so-called acetone-butanol-ethanol
(ABE) fermentation process, typically by using bacteria of the genus Clostridium.
These microorganisms also produce acetone and ethanol as side-products, hence the
name of the process. In a first step, prior to the fermentation process, the biomass
source, which can be lignocellulose or starch-based, is pre-treated to obtain fermentable sugars. The substrate is subsequently sterilized and fed to the fermenter. During
the fermentation, acetone, n-butanol, and ethanol are produced. As consequence of
its metabolism, the used Clostridium sp. also produces CO 2 and H 2 . After batch
fermentation, the broth contains about 0.5 wt% acetone, 1.3 wt% butanol, and
0.15 wt% ethanol [64] and is fed to a steam stripper [44, 64–66]. By adding
steam, the ABE products are vaporized, concentrated, and separated from the solid
components (e.g., cells) of the fermentation broth. The remaining solid mass is
subsequently dried in multiple evaporators [64]. In a next step, the different products
are purified in a sequence of distillation towers. The component with the lowest
boiling point, acetone, is distilled first, followed by ethanol [44, 64–66]. n-Butanol
forms an azeotrope with water at 57.5 wt% butanol; however, this azeotrope is
heterogeneous [67], and phase separates in a water-rich (7 wt% butanol) and
n-butanol-rich phase (80 wt% butanol) [67]. This phase separation is exploited in
the purification of n-butanol: in a first distillation column, n-butanol/water mixture is
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
87
new platform molecule, being a precursor for isobutene and xylenes [29], that can be
commercially produced using a genetically modified yeast [9].
A major challenge in any fermentation process for platform molecules is product
inhibition. When the product concentrations become too high in the fermentation
broth, the metabolism of the microorganism slows down, and the production of the
desired product stops. This limits the final concentrations obtain during an alcohol
fermentation process. For example, of the concentration of ethanol in a batch
fermentation is limited to 11–12 wt% [30] and for n-butanol and isobutanol to
2 wt% [9]. Conventionally, these alcohols are recovered using distillation
[21, 31]. However, due the low product concentrations, the energy demand for
these distillation processes is very high. In the case of n-butanol, the recovery via
distillation requires more than 50 MJ/kg [32], while the energy content of n-butanol
itself is only 33 MJ/kg [33]. Therefore, a large amount of research effort is undertaken to increase the efficiency of the downstream processing of bio-alcohols.
Many different techniques have been studied to increase the efficiency of the
separation and purification of fermentatively produced alcohols. Ideally, these separation processes can be partially integrated with the fermentation itself, also called
in situ product recovery, to relieve product inhibition during the fermentation and
increase the yield of the fermentation process. Examples of techniques which have
been studied for bio-alcohol recovery include liquid-liquid extraction [34–36],
gas-stripping [37–40], the use of membranes (pervaporation, membrane extraction,
membrane distillation) [39, 41–46], and recovery via adsorption [37, 47–
61]. Among these different techniques, adsorption-based recovery has been
highlighted for its energy efficiency [53].
An example of the traditional downstream processing of biobutanol is shown in
Fig. 1 [22, 62–64]. Biobutanol is produced in the so-called acetone-butanol-ethanol
(ABE) fermentation process, typically by using bacteria of the genus Clostridium.
These microorganisms also produce acetone and ethanol as side-products, hence the
name of the process. In a first step, prior to the fermentation process, the biomass
source, which can be lignocellulose or starch-based, is pre-treated to obtain fermentable sugars. The substrate is subsequently sterilized and fed to the fermenter. During
the fermentation, acetone, n-butanol, and ethanol are produced. As consequence of
its metabolism, the used Clostridium sp. also produces CO 2 and H 2 . After batch
fermentation, the broth contains about 0.5 wt% acetone, 1.3 wt% butanol, and
0.15 wt% ethanol [64] and is fed to a steam stripper [44, 64–66]. By adding
steam, the ABE products are vaporized, concentrated, and separated from the solid
components (e.g., cells) of the fermentation broth. The remaining solid mass is
subsequently dried in multiple evaporators [64]. In a next step, the different products
are purified in a sequence of distillation towers. The component with the lowest
boiling point, acetone, is distilled first, followed by ethanol [44, 64–66]. n-Butanol
forms an azeotrope with water at 57.5 wt% butanol; however, this azeotrope is
heterogeneous [67], and phase separates in a water-rich (7 wt% butanol) and
n-butanol-rich phase (80 wt% butanol) [67]. This phase separation is exploited in
the purification of n-butanol: in a first distillation column, n-butanol/water mixture is
Efficient Downstream Processing of Renewable Alcohols Using Zeolite Adsorbents
87