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The
bioconversion of biomass to mixed alcohol fuels can be accomplished using the MixAlco process. Through bioconversion of biomass to a mixed alcohol fuel, more energy from the biomass will end up as liquid fuels than in converting biomass to
ethanol by yeast fermentation.
The process involves a biological/chemical method for converting any
biodegradation material (e.g., urban wastes, such as municipal solid waste,
biodegradable waste, and
sewage treatment, agricultural residues such as
corn stover, sugarcane
bagasse, cotton gin trash,
manure) into useful chemicals, such as
carboxylic acids (e.g., acetic, propionic,
butyric acid),
ketones (e.g., acetone,
methyl ethyl ketone,
diethyl ketone) and
biofuels, such as a mixture of primary
alcohols (e.g., ethanol, Propan-1-ol, butanol) and/or a mixture of secondary alcohols (e.g.,
isopropanol, 2-butanol, 3-pentanol). Because of the many products that can be economically produced, this process is a true biorefinery Advanced Biomass Refinery - Third-Generation 2007 (video) EPA Presentation on the process Application., Texas)The process uses a mixed culture of naturally occurring microorganisms found in natural habitats such as the
rumen of cattle, termite guts, and marine and terrestrial swamps to anaerobic digestion biomass into a mixture of carboxylic acids produced during the
acidogenesis and
acetogenesis stages of anaerobic digestion, however with the inhibition of the
methanogenesis final stage. The more popular methods for production of ethanol and
cellulosic ethanol use enzymes that must be isolated first to be added to the biomass and thus convert the starch or cellulose into simple sugars, followed then by yeast fermentation into ethanol. This process does not need the addition of such enzymes as these microorganisms make their own .
As the microoganisms anaerobically digest the biomass and convert it into a mixture of carboxylic acids, the
pH must be controlled. This is done by the addition of a
buffering agent (e.g.,
ammonium bicarbonate,
calcium carbonate), thus yielding a mixture of carboxylate salts.
Methanogenesis, which, as mentioned, is the natural final stage of anaerobic digestion, is inhibited by the presence of the ammonium ions or by the addition of an inhibitor (e.g.,
iodoform). The resulting fermentation broth contains the produced carboxylate salts that must be dewatered. This is achieved efficiently by vapor-compression evaporation. Further refining of the dewatered fermentation broth may then take place depending on the final product desired.
The condensed distilled water from the vapor-compression evaporation system is recycled back to the fermentation. On the other hand, if sewage or other waste water with high Biochemical oxygen demand in need of treatment is used as the water for the fermentation, the condensed distilled water from the evaporation can be recycled back to the city or to the original source of the high-BOD waste water. Thus, this process can also serve as a water treatment facility.
Because the system uses a mixed culture of microorganisms, besides not needing any enzyme addition, the fermentation requires no sterility or aseptic conditions, making this front step in the process more economical than in more popular methods for the production of cellulosic ethanol. These savings in the front end of the process, where volumes are large, allows flexibility for further chemical transformations after dewatering, where volumes are small.
Carboxylic acids
Carboxylic acids can be regenerated from the carboxylate salts using a process known as "acid springing". This process makes use of a high-molecular-weight tertiary amine (e.g., trioctylamine), which is switched with the cation (e.g., ammonium or calcium). The resulting amine carboxylate can then be thermally decomposed into the amine itself, which is recycled, and the corresponding carboxylic acid. In this way, theoretically, no chemicals are consumed or wastes produced during this step. Williamson, S.A. 2000. Conversion of carboxylate salts to carboxylic acids via reactive distillation. M.S. Thesis
Ketones
There are two methods for making ketones. The first one consists on thermally converting calcium carboxylate salts into the corresponding ketones. This was a common method for making acetone from
calcium acetate during
World War IYeh, H. 2002. Pyrolytic decomposition of carboxylate salts. M.S. thesis. The other method for making ketones consists on converting the vaporized carboxylic acids on a catalyst of
zirconium oxide Ingram, D. 2002. Ketonization of acetic acid. B.S. student report..
Alcohols
Primary alcohols
The undigested residue from the fermentation may be used in gasification to make
hydrogen (H2). This H2 can then be used to hydrogenolysis the
esters over a catalyst (e.g., copper chromite)Bradley, M.W., Harris, N., Turner, K. 1982. Process for Hydrogenolysis of Carboxylic Acid Esters WO 82/03854, Nov. 11, which are produced by esterifying either the ammonium carboxylate salts (e.g., ammonium acetate, propionate, butyrate) or the carboxylic acids (e.g., acetic, propionic, butyric acid) with a high-molecular-weight alcohol (e.g., hexanol,
heptanol) Preparation of esters by reaction of ammonium salts with alcohols. . From the hydrogenolysis, the final products are the high-molecular-weight alcohol, which is recycled back to the esterification, and the corresponding primary alcohols (e.g., ethanol, propanol, butanol).
Secondary alcohols
The secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol) are obtained by hydrogenation over a catalyst (e.g., Raney nickel) the corresponding ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone)Aldrett-Lee, S. 2000. Catalytic hydrogenation of liquid ketones with emphasis on gas-liquid mass transfer. Ph.D. dissertation.
Acetic acid versus Ethanol
Cellulosic-ethanol -manufacturing plants are bound to be net exporters of electricity because a large portion of the lignocellulosic biomass, namely
lignin, remains undigested and it must be burned, thus producing electricity for the plant and excess electricity for the grid. As the market grows and this technology becomes more widespread, coupling the liquid fuel and the electricity markets will become more and more difficult.
Acetic acid, unlike ethanol, is biologically produced from simple sugars without the production of carbon dioxide:
C6H12O6 → 2 CH3CH2OH + 2 CO2 (Biological production of ethanol)
C6H12O6 → 3 CH3COOH (Biological production of
acetic acid)
Because of this, on a mass basis, the yields will be higher than in ethanol fermentation. If then, the undigested residue (mostly lignin) is used to produce hydrogen by gasification, it is ensured that more energy from the biomass will end up as liquid fuels rather than excess heat/electricity Eggeman, T., Verser, D., and Weber, E. (2005), An Indirect Route for Ethanol Production US Department of Energy.
3 CH3COOH + 6 H2 → 3 CH3CH2OH + 3 H2O (Hydrogenation of acetic acid)
C6H12O6 (from
cellulose) + 6 H2 (from lignin) → 3 CH3CH2OH + 3 H2O (Overall reaction)
A more comprehensive description of the
economics of each of the fuels is given on the pages alcohol fuel and ethanol fuel, more information about the economics of various systems can be found on the central page biofuel.
Stage of development
The system has been in development since 1991, moving from the laboratory scale (10 g/day) to the pilot scale (200 lb/day) in 2001. A small demonstration-scale plant (5 ton/day) is under construction as is expected to be operational early in 2008 and a 100 ton/day demonstration plant is expected in 2009.
See also
References
The
bioconversion of biomass to mixed alcohol fuels can be accomplished using the MixAlco process. Through bioconversion of biomass to a mixed
alcohol fuel, more energy from the biomass will end up as liquid fuels than in converting biomass to ethanol by
yeast fermentation.
The process involves a biological/chemical method for converting any biodegradation material (e.g., urban wastes, such as municipal solid waste,
biodegradable waste, and
sewage treatment, agricultural residues such as
corn stover, sugarcane bagasse, cotton gin trash,
manure) into useful chemicals, such as
carboxylic acids (e.g.,
acetic, propionic, butyric acid), ketones (e.g., acetone,
methyl ethyl ketone, diethyl ketone) and
biofuels, such as a mixture of primary alcohols (e.g., ethanol, Propan-1-ol,
butanol) and/or a mixture of secondary alcohols (e.g.,
isopropanol, 2-butanol, 3-pentanol). Because of the many products that can be economically produced, this process is a true biorefinery Advanced Biomass Refinery - Third-Generation 2007 (video) EPA Presentation on the process Application., Texas)The process uses a mixed culture of naturally occurring microorganisms found in natural habitats such as the
rumen of cattle,
termite guts, and marine and terrestrial swamps to anaerobic digestion biomass into a mixture of carboxylic acids produced during the acidogenesis and acetogenesis stages of anaerobic digestion, however with the inhibition of the methanogenesis final stage. The more popular methods for production of ethanol and cellulosic ethanol use enzymes that must be isolated first to be added to the biomass and thus convert the starch or cellulose into simple sugars, followed then by yeast fermentation into ethanol. This process does not need the addition of such enzymes as these microorganisms make their own .
As the microoganisms anaerobically digest the biomass and convert it into a mixture of carboxylic acids, the pH must be controlled. This is done by the addition of a
buffering agent (e.g., ammonium bicarbonate,
calcium carbonate), thus yielding a mixture of carboxylate salts. Methanogenesis, which, as mentioned, is the natural final stage of anaerobic digestion, is inhibited by the presence of the ammonium ions or by the addition of an inhibitor (e.g.,
iodoform). The resulting fermentation broth contains the produced carboxylate salts that must be dewatered. This is achieved efficiently by vapor-compression evaporation. Further refining of the dewatered fermentation broth may then take place depending on the final product desired.
The condensed distilled water from the vapor-compression evaporation system is recycled back to the fermentation. On the other hand, if sewage or other waste water with high
Biochemical oxygen demand in need of treatment is used as the water for the fermentation, the condensed distilled water from the evaporation can be recycled back to the city or to the original source of the high-BOD waste water. Thus, this process can also serve as a
water treatment facility.
Because the system uses a mixed culture of microorganisms, besides not needing any enzyme addition, the fermentation requires no sterility or aseptic conditions, making this front step in the process more economical than in more popular methods for the production of cellulosic ethanol. These savings in the front end of the process, where volumes are large, allows flexibility for further chemical transformations after dewatering, where volumes are small.
Carboxylic acids
Carboxylic acids can be regenerated from the carboxylate salts using a process known as "acid springing". This process makes use of a high-molecular-weight tertiary amine (e.g., trioctylamine), which is switched with the cation (e.g., ammonium or calcium). The resulting amine carboxylate can then be thermally decomposed into the amine itself, which is recycled, and the corresponding
carboxylic acid. In this way, theoretically, no chemicals are consumed or wastes produced during this step. Williamson, S.A. 2000. Conversion of carboxylate salts to carboxylic acids via reactive distillation. M.S. Thesis
Ketones
There are two methods for making ketones. The first one consists on thermally converting calcium carboxylate salts into the corresponding ketones. This was a common method for making acetone from calcium acetate during
World War IYeh, H. 2002. Pyrolytic decomposition of carboxylate salts. M.S. thesis. The other method for making ketones consists on converting the vaporized carboxylic acids on a
catalyst of
zirconium oxide Ingram, D. 2002. Ketonization of acetic acid. B.S. student report..
Alcohols
Primary alcohols
The undigested residue from the fermentation may be used in
gasification to make
hydrogen (H2). This H2 can then be used to
hydrogenolysis the
esters over a catalyst (e.g., copper chromite)Bradley, M.W., Harris, N., Turner, K. 1982. Process for Hydrogenolysis of Carboxylic Acid Esters WO 82/03854, Nov. 11, which are produced by esterifying either the ammonium carboxylate salts (e.g.,
ammonium acetate, propionate, butyrate) or the carboxylic acids (e.g., acetic, propionic, butyric acid) with a high-molecular-weight alcohol (e.g.,
hexanol,
heptanol) Preparation of esters by reaction of ammonium salts with alcohols. . From the hydrogenolysis, the final products are the high-molecular-weight alcohol, which is recycled back to the esterification, and the corresponding primary alcohols (e.g., ethanol, propanol, butanol).
Secondary alcohols
The secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol) are obtained by
hydrogenation over a catalyst (e.g., Raney nickel) the corresponding ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone)Aldrett-Lee, S. 2000. Catalytic hydrogenation of liquid ketones with emphasis on gas-liquid mass transfer. Ph.D. dissertation.
Acetic acid versus Ethanol
Cellulosic-ethanol -manufacturing plants are bound to be net exporters of electricity because a large portion of the lignocellulosic biomass, namely
lignin, remains undigested and it must be burned, thus producing electricity for the plant and excess electricity for the grid. As the market grows and this technology becomes more widespread, coupling the liquid fuel and the electricity markets will become more and more difficult.
Acetic acid, unlike ethanol, is biologically produced from simple sugars without the production of
carbon dioxide:
C6H12O6 → 2 CH3CH2OH + 2 CO2 (Biological production of ethanol)
C6H12O6 → 3 CH3COOH (Biological production of
acetic acid)
Because of this, on a mass basis, the yields will be higher than in ethanol fermentation. If then, the undigested residue (mostly lignin) is used to produce hydrogen by gasification, it is ensured that more energy from the biomass will end up as liquid fuels rather than excess heat/electricity Eggeman, T., Verser, D., and Weber, E. (2005), An Indirect Route for Ethanol Production US Department of Energy.
3 CH3COOH + 6 H2 → 3 CH3CH2OH + 3 H2O (Hydrogenation of acetic acid)
C6H12O6 (from cellulose) + 6 H2 (from lignin) → 3 CH3CH2OH + 3 H2O (Overall reaction)
A more comprehensive description of the economics of each of the fuels is given on the pages alcohol fuel and ethanol fuel, more information about the economics of various systems can be found on the central page
biofuel.
Stage of development
The system has been in development since 1991, moving from the laboratory scale (10 g/day) to the pilot scale (200 lb/day) in 2001. A small demonstration-scale plant (5 ton/day) is under construction as is expected to be operational early in 2008 and a 100 ton/day demonstration plant is expected in 2009.
See also
- Anaerobic digestion
- Mechanical biological treatment
References
