U.S. patent application number 13/140947 was filed with the patent office on 2011-12-29 for process for the production of chemicals.
This patent application is currently assigned to THE UNIVERSITY OF QUEENSLAND. Invention is credited to Korneel P.H.L.A. Rabaey, Rene A. Rozendal.
Application Number | 20110315560 13/140947 |
Document ID | / |
Family ID | 42268180 |
Filed Date | 2011-12-29 |
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United States Patent
Application |
20110315560 |
Kind Code |
A1 |
Rabaey; Korneel P.H.L.A. ;
et al. |
December 29, 2011 |
PROCESS FOR THE PRODUCTION OF CHEMICALS
Abstract
A process for producing one or more chemical compounds
comprising the steps of providing a bioelectrochemical system
having an anode and a cathode separated by a membrane, the anode
and the cathode being electrically connected to each other, causing
oxidation to occur at the anode and causing reduction to occur at
the cathode to thereby produce reducing equivalents at the cathode,
providing the reducing equivalents to a culture of microorganisms,
and providing carbon dioxide to the culture of microorganisms,
whereby the microorganisms produce the one or more chemical
compounds, and recovering the one or chemical compounds.
Inventors: |
Rabaey; Korneel P.H.L.A.;
(Wachtebeke, BE) ; Rozendal; Rene A.; (Amsterdam,
NL) |
Assignee: |
THE UNIVERSITY OF
QUEENSLAND
St Lucia
AU
|
Family ID: |
42268180 |
Appl. No.: |
13/140947 |
Filed: |
December 17, 2009 |
PCT Filed: |
December 17, 2009 |
PCT NO: |
PCT/AU2009/001645 |
371 Date: |
September 8, 2011 |
Current U.S.
Class: |
205/344 ;
205/349; 205/351; 205/413; 205/414; 205/440; 205/452 |
Current CPC
Class: |
Y02W 10/40 20150501;
Y02P 70/56 20151101; C02F 2001/4619 20130101; Y02E 60/527 20130101;
C02F 2201/46115 20130101; C02F 3/005 20130101; C12P 7/40 20130101;
C12P 7/625 20130101; H01M 8/16 20130101; C02F 2201/4618 20130101;
H01M 8/20 20130101; Y02W 10/45 20150501; C02F 1/4618 20130101; C12M
25/08 20130101; Y02E 60/528 20130101; C12M 35/02 20130101; C25B
3/00 20130101; Y02P 70/50 20151101; C12P 7/02 20130101; H01M 8/0612
20130101; Y02E 60/50 20130101; Y02W 10/37 20150501; C02F 2201/4619
20130101 |
Class at
Publication: |
205/344 ;
205/413; 205/351; 205/349; 205/452; 205/440; 205/414 |
International
Class: |
C25B 3/00 20060101
C25B003/00; C25B 13/00 20060101 C25B013/00; C25B 15/02 20060101
C25B015/02; C25B 15/08 20060101 C25B015/08; C25B 3/02 20060101
C25B003/02; C25B 15/00 20060101 C25B015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2008 |
AU |
2008906519 |
Claims
1. A process for producing one or more chemical compounds
comprising the steps of providing a bioelectrochemical system
having an anode and a cathode separated by a membrane, the anode
and the cathode being electrically connected to each other, causing
oxidation to occur at the anode and causing reduction to occur at
the cathode to thereby produce reducing equivalents at the cathode,
providing the reducing equivalents to a culture of microorganisms,
and providing carbon dioxide to the culture of microorganisms,
whereby the microorganisms produce the one or more chemical
compounds, and recovering the one or chemical compounds.
2. A process as claimed in claim 1 wherein the microorganisms that
form the one or more chemical compounds are present in the cathode
compartment, and the process comprises causing oxidation to occur
at the anode and causing reduction to occur at the cathode, wherein
carbon dioxide is supplied to the cathode compartment, and the
microorganisms produce the one or more chemical compounds, and
recovering the one more chemicals from the cathode compartment.
3. A process as claimed in claim 1 wherein the bioelectrochemical
system includes a power supply in the electrical circuit.
4. A process as claimed in claim 1 wherein the carbon dioxide acts
as a carbon-containing feed material to the microorganisms that
receive the reducing equivalents from the cathode or are present in
the cathode compartment and the carbon dioxide comprises the only
carbon-containing feed component supplied to the
microorganisms.
5. A process as claimed in claim 1 wherein the carbon dioxide acts
as a carbon-containing feed material to the microorganisms that
receive the reducing equivalents from the cathode or are present in
the cathode compartment and the carbon dioxide is used in
conjunction with other organic materials by the microorganisms to
produce the chemicals.
6. A process as claimed in claim 1 wherein the microorganisms
provided to the cathode compartment or receiving reducing
equivalents from the cathode compartment comprise a defined
microbial culture containing one or more selected microbial
species.
7. A process as claimed in claim 1 wherein the microbial species do
not form methane in notable quantities when grown in the
cathode.
8. A process as claimed in claim 1 wherein the microorganisms
comprise a mixed, non-selected culture and the process further
comprises the steps of producing the one or more chemicals in the
cathode compartment and recovering the one or more chemicals from
the cathode compartment whilst suppressing formation of methane in
the cathode compartment.
9. A process as claimed in claim 8 wherein methane formation is
suppressed by one or more of adding one or more chemicals to the
cathode compartment that suppress the formation of methane or
suppress the activity of the methanogenic organisms, operating the
cathode compartment such that a low residence time is used in the
cathode compartment, operating the cathode compartment at low pH,
such as below 5.5, or periodically exposing the cathode compartment
to air, oxygen or hydrogen peroxide.
10. A process as claimed in claim 1 wherein the bioelectrochemical
system comprises a bioanode and a biocathode.
11. A process as claimed in claim 1 wherein one of the products
formed in the anode compartment is carbon dioxide and this carbon
dioxide is used as a feed to the cathode compartment.
12. A process as claimed in claim 1 wherein an anion exchange
membrane separates the anode compartment from the cathode
compartment.
13. A process as claimed in claim 12 wherein bicarbonate ions form
in the cathode compartment and subsequently move through the anion
exchange membrane to the anode compartment to thereby avoid
increases in pH and/or salinity in the cathode compartment that
could kill the microorganisms.
14. A process as claimed in claim 1 wherein the membrane separating
the anode and the cathode comprises a porous membrane that allows
liquid and ions to pass therethrough but prevents microorganisms
from passing therethrough.
15. A process as claimed in claim 14 wherein the anode is operated
as a bioanode and a waste stream is used as a feed material to the
anode and during normal operation liquid passes through the porous
membrane from the anode into the cathode chamber, and protons
generated in an anode reaction are transported through the membrane
to the cathode compartment and react with the hydroxyl ions
generated in a cathode reaction in accordance with equation (4) to
thereby avoid an undesirable increase in the pH in the cathode
compartment: H.sup.++OH.sup.-.fwdarw.H.sub.2O (4)
16. A process as claimed in claim 15 wherein pH and salt
concentration in the cathode chamber remain stable and homeostasis
is maintained.
17. A process as claimed in claim 1 wherein the bioelectrochemical
system is operated with a biocathode only.
18. A process as claimed in claim 17 wherein an acid solution is
provided to the anode compartment and the anode reaction comprises
a proton generating reaction, and the membrane comprises a cation
exchange membrane and protons migrate through the cation exchange
membrane and react with the hydroxyl ions generated in the cathode
reaction.
19. A process as claimed in claim 1 wherein the membrane separating
the anode and the cathode comprises a bipolar membrane.
20. A process as claimed in claim 19 wherein the bipolar membrane
is composed of a cation exchange layer on top of an anion exchange
layer and the anion exchange layer is directed towards the anode
chamber and the cation exchange layer is directed towards the
cathode chamber such that when electrical current flows, water
diffuses in between layers of the bipolar membrane and is split
into protons and hydroxyl ions, and the hydroxyl ions migrate
through the anion exchange layer into the anode chamber where they
compensate for the proton production in the anode reaction and the
protons migrate through the cation exchange layer into the cathode
chamber where they compensate for hydroxyl ion production (or
proton consumption) in the cathode reaction.
21. A process as claimed in claim 1 wherein the effluent of the
anode contains carbon dioxide and the effluent from the anode is
sent to a stripping column or membrane unit to recover gaseous
carbon dioxide for supply to the cathode as a gas.
22. A process as claimed in claim 21 wherein effluent from the
anode is passed through a membrane unit to allow separation of
carbon dioxide from the anode effluent, the membrane unit having a
liquid flow on the other side of the membrane such that the
separated carbon dioxide goes into solution in the fluid on the
other side of the membrane and the carbon dioxide is provided to
the cathode in dissolved form.
23. A process as claimed in claim 22 wherein the fluid passing
through the membrane unit on the other side of the anode fluid
comprises cathode fluid.
24. A process as claimed in claim 22 wherein the anode effluent is
sent through a membrane unit to allow carbon dioxide together with
organic constituents of the anode effluent to pass to a second
liquid and the second fluid is sent to the cathode where reduction
of the organics occurs.
25. A process as claimed in claim 1 wherein a mixture of chemicals
is formed in the cathode compartment and the process further
comprises the steps of removing a mixture of chemical compounds
from the cathode compartment and separating the mixture of chemical
compounds into two or more streams.
26. A process as claimed in claim 1 wherein the cathode compartment
is filled with the microbial culture and the microbial culture is
part of an aqueous mixture in the cathode compartment, or the
microbial culture grows on the electrode surface or the cathode
compartment is filled with part of the microbial culture and
another part of the microbial culture grows on the electrode
surface.
27. A process as claimed in claim 1 wherein the cathode compartment
comprises a first compartment housing the cathode, the first
compartment including a redox shuttle, and a second compartment
containing one or more microorganisms, wherein the redox shuttle is
reduced in the first compartment and a reduced redox shuttle is
provided to the second compartment, the second compartment
containing microorganisms that use the reduced redox shuttle as an
electron donor to facilitate formation of the one more
chemicals.
28. A method as claimed in claim 27 wherein the reduced redox
shuttle is converted to an oxidised redox shuttle in the second
compartment and the oxidised redox shuttle is returned to the first
compartment.
29. A process as claimed in claim 1 wherein the chemical compounds
that are formed include: alcohols such as methanol, ethanol,
propanol, butanol, isobutanol carboxylic acids, such as formic
acid, acetic acid, propionic acid, butyric acid, lactic acid, diols
such as 1,3-propanediol and 1,2-propanediol, biopolymers such as
poly-.beta.-hydroxybutyrate (PHB).
30. A process as claimed in claim 1 wherein the chemical compound
being formed comprises butanol and the bioelectrochemical system
includes chemolithoautotrophic bacteria at the cathode that produce
butanol according to equation (2):
4CO.sub.2+24H.sup.++24e.sup.-.fwdarw.C.sub.4H.sub.9OH+7H.sub.2O
(2)
31. A process as claimed in claim 1 wherein the carbon dioxide
stream being fed to be cathode compartment is derived from an
offgas stream or a flue gas stream from a burner or a boiler.
32. A process as claimed in claim 1 wherein a voltage is applied
between the anode and the cathode of between 0 and 10 V, preferably
between 0 and 1.5 V, more preferably between 0 and 1.0 V and a
volumetric current density in the bioelectrochemical cell of
between 0 and 10,000 A/m.sup.3 of bioelectrochemical cell,
preferably between 10 and 5,000 A/m.sup.3 of bioelectrochemical
cell, more preferably between 100 and 2500 A/m' of
bioelectrochemical cell and/or an area specific current density of
between 0 and 1,000 A/m.sup.2 membrane surface area, preferably
between 1 and 100 A/m.sup.2 membrane surface area, more preferably
between 2 and 25 A/m.sup.2 membrane surface area, is obtained.
33. A process as claimed in claim 1 wherein the carbon dioxide
stream being fed to the cathode compartment comprises biogas
containing a mixture of methane and carbon dioxide or the carbon
dioxide being fed to the cathode is be derived from a coal seam or
layer, in which carbonate rich fluid is pumped from the coal seam
through the cathode compartment.
34. A process as claimed in claim 1 wherein carbon dioxide is
provided to the cathode compartment via diffusion or transport from
the anode of the bioelectrochemical system.
35. A process as claimed in claim 1 wherein the cathode is also
provided with organic molecules to assist in the production of the
biochemicals.
36. A process as claimed in claim 35 wherein the organic molecules
are selected from glycerol, glucose, lactate, propionate and
butyrate.
37. A process as claimed in claim 36 wherein glycerol is added and
product formation includes 1,3-propanediol or butanol, and glycerol
is added to the cathode compartment, to the anode compartment or to
both.
38. A process as claimed in claim 37 wherein the glycerol can also
be partly converted to propionate prior to entry in the
bioelectrochemical system, and subsequently added to the cathode as
a mixture of glycerol and propionate.
39. A process as claimed in claim 1 wherein redox mediators are
added to the cathode fluid, allowing transport of electrons from
the cathode to the microorganism.
40. A process as claimed in claim 39 wherein the redox mediators
are selected from methyl viologen, neutral red, phenazine
carboxamide, amido black or mixtures of two or more thereof.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a process for producing
chemicals. More particularly, the present invention relates to a
process for producing chemicals using bioelectrochemical
systems.
BACKGROUND
[0002] The global depletion of fossil fuel resources and the
increasing awareness of the possible anthropogenic effect on
climate change are leading to an increasing drive to reduce
greenhouse gas emissions and to develop a more sustainable society.
Besides renewable electricity, such a sustainable society also
needs access to renewably produced fuels and chemicals. To be truly
renewable these chemicals need to be produced from renewable raw
materials such as biomass or from waste products such wastewater
and/or carbon dioxide.
[0003] Recently, bioelectrochemical systems, such as microbial fuel
cells and microbial electrolysis cells, have emerged as potentially
interesting technology for the production of energy and products.
Bioelectrochemical systems are based on the use of
electrochemically active microorganisms, which can either donate
electrons to an anode or accept electrons from a cathode. If
electrochemically active micro-organisms are electrochemically
interacting with an anode, such an electrode is referred to as a
biological anode, bioanode or microbial bioanode. In contrast, if
electrochemically active micro-organisms are electrochemically
interacting with a cathode, such an electrode is referred to as a
biological cathode, biocathode or microbial biocathode.
Bioelectrochemical systems are generally regarded as a promising
future technology for the production of energy from organic
material present from aqueous waste streams (e.g., wastewater).
(Rozendal et al., Trends Biotechnol. 2008, 26, 450-459).
Industrial, agricultural and domestic waste waters typically
contain dissolved organics that require removal before discharge
into the environment. Typically, these organic pollutants are
removed by aerobic treatment, which can consume large amounts of
electrical energy for aeration. Bioelectrochemical wastewater
treatment can be accomplished by electrically coupling a microbial
bioanode to a counter electrode (cathode) that performs a reduction
reaction. As a result of this electrical connection between the
anode and the cathode, the electrode reactions can occur and
electrons can flow from the anode to the cathode. Moreover, the
electrochemically active microorganisms at the anode transfer
electrons to an electrode (anode) while they are oxidising (and
thus removing) (in)organic pollutants in aqueous waste streams
(e.g., wastewater). A bioelectrochemical system may operate as a
fuel cell (in which case electrical energy is produced--e.g. Rabaey
and Verstraete, Trends Biotechnol. 2005, 23, 291-298) or as an
electrolysis cell (in which case, electrical energy is fed to the
bioelectrochemical system--e.g., Patent WO2005005981A2).
[0004] In 2003, Rozendal and Buisman patented biocatalysed
electrolysis, which is a bioelectrochemical system for the
production of hydrogen gas from bio-oxidisable material (WO
2005005981, the entire contents of which are herein incorporated by
cross-reference). The bio-oxisable material used for biocatalysed
electrolysis can, for example, be dissolved organic material in
wastewater. In their invention Rozendal and Buisman introduced
bio-oxisable material into a reactor provided with an anode and a
cathode and containing anodophilic bacteria in an aqueous medium,
applied a potential between the anode and cathode of between 0.1
and 1.5 volt, while maintaining a pH of between 3 and 9 in the
aqueous medium and collected hydrogen gas from the cathode.
[0005] Although hydrogen is an interesting chemical to produce in a
cathode, it would be even more interesting if chemicals with higher
value could be produced, such as fuels and chemicals. Examples of
such fuels and chemicals include alcohols such as methanol,
ethanol, propanol, butanol, etc, carboxylic acids, such as formic
acid, acetic acid, propionic acid, butyric acid, lactic acid, etc,
biopolymers such as poly-.beta.-hydroxybutyrate (PHB), etc., etc.
However, to be able to catalyze these kind complex production
reactions at a cathode, an advanced catalysis mechanism is
required. It might be possible to develop chemical catalysts for
this purpose, but these chemical catalysts are likely to become
very complex and highly expensive as they likely necessitate the
application of precious metals. Alternatively, cathodophilic
microorganisms can be used for catalyzing cathodic reactions for
the production of valuable chemicals. Cathodophilic microorganisms
are microorganisms that can interact with a cathode by accepting
electrons or cathodic reaction products (such as hydrogen or
reduced electron mediators) from the cathode and utilizing these
for the production of valuable chemicals. Such electrode is
referred to as a biological cathode, biocathode or microbial
biocathode. Electrons and cathodic reaction products (such as
hydrogen or reduced electron mediators) are commonly referred to as
reducing equivalents. Reducing equivalents allow reducing electron
acceptors and can serve as electron donor for a microbial
metabolism. Electron mediators are redox-active organic compounds
and are known to person skilled in the art. They include compounds
such as quinones, neutral red, methyl viologen, etc. Electron
mediators shuttle in between the electrode and the microorganisms.
During this shuttling the electron mediators are continuously
reduced by the electrode and subsequently oxidized again by the
microorganism for the production of the chemical products.
[0006] Microbial biocathodes have already been demonstrated for
oxygen reduction (e.g., Rabaey et al., ISME J. 2008, 2, 1387-1396),
nitrate reduction (e.g., Clauwaert et al., Environ. Sci. Technol.,
2007, 41, 7564-7569), dechlorination (e.g., Aulenta et al.,
Environ. Sci. Technol., 2007, 41, 2554-2559), hydrogen production
(e.g., Rozendal et al., Environ. Sci. Technol., 2008, 42, 629-634),
and methane production (e.g., Clauwaert et al., Water Sci.
Technol., 2008, 57, 575-579), but have not been described for the
production of more complex molecules such as those described
above.
[0007] In general, mixed microbial cultures (i.e., multiple
species) are unlikely to produce complex chemicals in high
quantity, concentration or purity, because the natural end products
in a mixed microbial culture are typically simple molecules such as
methane under anaerobic conditions or carbon dioxide under aerobic
conditions. In practice, complex molecules are therefore typically
produced with a defined microbial culture, such as a pure microbial
culture (i.e., single specie) or at least a well-defined co-culture
(i.e., two or more carefully selected species). Therefore, unless
methanogenic activity can be suppressed, a microbial biocathode
capable of producing complex molecules would also require a defined
microbial culture of cathodophilic microorganisms (Rozendal et al.,
Trends Biotechnol. 2008, 26, 450-459), which are unlikely to be
naturally enriched from complex inocula such as wastewater. A
defined microbial culture of cathodophilic microorganisms should be
a carefully selected or genetically engineered pure culture, but
could also be a carefully selected co-culture of two or more
carefully selected or genetically engineered pure cultures. These
pure cultures or co-cultures should consist of microbial species
that are capable of catalyzing the production reaction of the
desired complex molecule.
[0008] A disadvantage of using a defined culture of cathodophilic
microorganisms is that these cultures are susceptible to
contamination with other microorganisms. So unless the activity of
these other micro-organisms can be suppressed, these other
microorganisms will break down the products produced by the defined
culture of cathodophilic microorganisms and consequently limit the
product output of the bioelectrochemical system. Bioelectrochemical
systems can prevent this contamination with other micro-organisms
by applying an ion exchange membrane between the anode and the
cathode. The application of an ion exchange membrane isolates the
cathode from the rest the bioelectrochemical system and can make
the defined culture of cathodophilic microorganisms less
susceptible to contamination. Even more so, because the reducing
equivalents are essentially delivered to cathodophilic
microorganisms sterile in the form of electrons delivered by the
cathode and originally coming from the anode.
[0009] In the context of microbial fuel cells, Torres et al.
(Torres et al., Environ. Sci. Technol., 2008, 42, 8773-8777)
presented a method to decrease the pH difference between the anode
and the cathode of a microbial fuel cell by having an anion
exchange membrane between the cathode chamber and dosing carbon
dioxide to an air cathode (platinum catalyst for oxygen reduction).
This carbon dioxide reacts with the hydroxyl ions and forms
carbonate species. While this decreases the pH of the cathode, the
carbonate species also migrate across the anion exchange membrane
from cathode to anode and increase pH in the latter. As a result,
the pH difference between both is decreased.
BRIEF DESCRIPTION OF THE INVENTION
[0010] In a first aspect, the present invention provides a process
for producing one or more chemical compounds comprising the steps
of providing a bioelectrochemical system having an anode and a
cathode separated by a membrane, the anode and the cathode being
electrically connected to each other, causing oxidation to occur at
the anode and causing reduction to occur at the cathode to thereby
produce reducing equivalents at the cathode, providing the reducing
equivalents to a culture of microorganisms, and providing carbon
dioxide to the culture of microorganisms, whereby the
microorganisms produce the one or more chemical compounds, and
recovering the one or chemical compounds.
[0011] In another aspect, the present invention provides a process
for producing one or more chemical compounds comprising the steps
of providing a bioelectrochemical system having an anode and a
cathode separated by a membrane, the anode and the cathode being
electrically connected to each other, the system having a cathode
compartment and the cathode compartment being provided with
microorganisms that form the one or more chemical compounds in the
cathode compartment, causing oxidation to occur at the anode and
causing reduction to occur at the cathode, wherein carbon dioxide
is supplied to the cathode compartment, and the microorganisms
produce the one or more chemical compounds, and recovering the one
more chemicals from the cathode compartment.
[0012] In some embodiments, the system further includes a power
supply in the electrical circuit. The power supply may comprise a
DC power supply, such as a battery or a DC to AC converter.
[0013] The power supply can be used to apply a voltage on the
system, which increases chemical production rates. The voltage
applied with a power supply between the anode and the cathode may
be between 0 and 10 V, preferably between 0 and 1.5 V, more
preferably between 0 and 1.0 V. This may result in a volumetric
current density in the bioelectrochemical cell of between 0 and
10,000 A/m.sup.3 of bioelectrochemical cell, preferably between 10
and 5,000 A/m.sup.3 of bioelectrochemical cell, more preferably
between 100 and 2500 A/m.sup.3 of bioelectrochemical cell and/or an
area specific current density of between 0 and 1,000 A/m.sup.2
membrane surface area, preferably between 1 and 100 A/m.sup.2
membrane surface area, more preferably between 2 and 25 A/m.sup.2
membrane surface area.
[0014] In embodiments of the present invention, the microorganisms
present in the cathode compartment or receiving reducing
equivalents from the cathode compartment utilise reducing
equivalents produced at the cathode and carbon dioxide to make
organic molecules. Therefore, the carbon dioxide acts as a
carbon-containing feed material to the microorganisms that receive
reducing equivalents from the cathode or are present in the cathode
compartment. Indeed, the carbon dioxide can be the only
carbon-containing feed component supplied to the microorganisms. In
other embodiments, the carbon dioxide is used in conjunction with
other organic materials by the microorganisms to produce the
desired chemicals. Examples of suitable microorganisms in this
regard include chemolithoautotrophic bacteria. For example, in
butanol formation, chemolithoautotrophic bacteria at the cathode
would proceed according to:
4CO.sub.2+24H.sup.++24e.sup.-.fwdarw.C.sub.4H.sub.9OH+7H.sub.2O
(2)
[0015] It will be appreciated that utilising carbon dioxide as a
carbon-containing material for conversion into the desired chemical
products has the desirable effect of reducing carbon dioxide
emissions (and hence reducing greenhouse gas emissions).
[0016] In one embodiment, the microorganisms provided to the
cathode compartment or receiving reducing equivalents from the
cathode compartment comprise a defined microbial culture containing
one or more selected microbial species. In one embodiment, the
defined microbial culture comprises a pure microbial culture
containing a single species of microorganisms. In another
embodiment, the defined microbial culture comprises a co-culture of
two or more carefully selected microbial species. The microbial
species are selected such that the one or more chemical compounds
are produced by the microbial species. Suitably, the microbial
species do not form methane in notable quantities when grown in the
cathode.
[0017] The defined microbial culture containing one or more
selected microbial species may be formed or selected by any
technique known to be suitable to persons skilled in the art.
[0018] In this embodiment, an essentially "pure" microbial culture
is provided (either in the cathode compartment or to receive
reducing equivalents from the cathode). The essentially "pure"
microbial culture is selected such that the microbial culture
produces the one or more desired chemical compounds. For example,
Clostridium carboxidivorans sp. nov. could be selected for the
production of acetate, ethanol, butyrate and butanol from carbon
dioxide and cathodically produced hydrogen (Liou et al., Int. J.
Syst. Evol. Microbiol., 2005, 55, 2085-2091). In order to ensure
that the essentially pure microbial culture remains essentially
pure, any feed streams to the culture of microorganisms should be
essentially free of other microorganisms. For example, the carbon
dioxide stream fed to the culture of microorganisms should also
free of contaminating microorganisms.
[0019] The carbon dioxide stream being fed to be cathode
compartment may be derived from an offgas stream or a flue gas
stream from a burner or a boiler. It will be appreciated that such
offgas streams or flue gas streams exit the burner or boiler at a
very high temperature and, as a result, will be essentially sterile
(in that they will not contain any contaminating microorganisms).
These streams may simply be cooled and then used as a carbon
dioxide containing feed stream to the cathode compartment. If the
offgas stream or flue gas stream contains other material that may
be toxic to the microorganisms in the cathode compartment, that
other material should be removed therefrom prior to feeding to the
cathode compartment. It will be appreciated that only part of the
offgas stream or flue gas stream may be fed to the cathode
compartment.
[0020] The carbon dioxide stream being fed to the cathode
compartment may also be biogas, containing a mixture of methane and
carbon dioxide (and potentially other gases)
[0021] The carbon dioxide being fed to the cathode may also be
derived from a coal seam or layer, in which carbonate rich fluid is
pumped from the coal seam through the cathode compartment.
[0022] In another embodiment, the microorganisms provided to the
cathode compartment comprised a mixed, non-selected culture and the
process further comprises the steps of producing the one or more
chemicals in the cathode compartment and recovering the one or more
chemicals from the cathode compartment whilst suppressing formation
of methane in the cathode compartment. Persons skilled in the art
will understand that mixed, non-selected cultures typically contain
methanogenic organisms and, if the cathode compartment is operated
without special precautions, the final product from the cathode
compartment is likely to be methane. Therefore, in this embodiment,
the cathode compartment is operated such that methane formation is
suppressed. Methane formation may be suppressed by one or more of
the following: [0023] Adding one or more chemicals to the cathode
compartment that suppress the formation of methane or suppress the
activity of the methanogenic organisms. For example, 2-bromoethane
sulfonate (BES) is known to suppress the activity of methanogenic
organisms. Other chemicals that suppress the activity of
methanogenic organisms may also be used. [0024] Operate the cathode
compartment such that a low residence time is used in the cathode
compartment. In this regard, most methanogenic organisms are slow
growing and utilising a low residence time in the cathode
compartment will suppress the formation or growth of a significant
number of methanogenic organisms in the cathode compartment because
they simply will not have sufficient time to grow in any great
number. In this embodiment, fresh cathode compartment liquid may be
frequently or continuously provided to the cathode compartment.
[0025] Operate the cathode compartment at low pH, such as below
5.5, preferably below pH 5. [0026] Periodically expose the cathode
compartment to air or oxygen.
[0027] In one embodiment, the CO.sub.2 is provided to the cathode
compartment via diffusion or transport from the anode of the
bioelectrochemical system. This transport can occur either through
the membrane separating anode and cathode, or via an additional
conduit.
[0028] In one embodiment, the present invention may be operated
with a bioanode and a biocathode. In embodiments where the anode is
operated as a bioanode, one of the products likely to be formed in
the bioanode compartment is carbon dioxide. This carbon dioxide may
be used as a feed to the cathode compartment. This carbon dioxide
can for example separated from the anode effluent and subsequently
transported to the cathode.
[0029] In embodiments where the anode is Operated as a bioanode, a
waste stream, such as a wastewater stream, may be used as a feed
material to the anode. The anode reaction is then catalyzed by
microorganisms, such as electrochemically active microorganisms,
and generates electrons (e.sup.-) and protons and/or carbon dioxide
and/or other oxidation products (e.g. sulfur):
(In)organic
materials.fwdarw.xHCO.sub.3.sup.-+yH.sup.++ze.sup.-+other products
(3)
[0030] The anode may be located in an anode compartment, with the
anode compartment being separated from the cathode compartment by a
membrane. In the anode compartment, organic and/or inorganic
components in the waste stream are oxidised to liberate electrons
which, in turn, flow through the electrical connection to the
cathode.
[0031] In one embodiment, an anion exchange membrane separates the
anode compartment from the cathode compartment. Anion exchange
membranes are known to the person skilled in the art and include
membranes such as AMI-7001 (Membranes International), Neosepta AMX
(ASTOM Corporation), and fumasep FAA.RTM. (fumatech).
[0032] In some embodiments, bicarbonate ions form in the cathode
compartment and subsequently move through the anion exchange
membrane to the anode compartment. This may be advantageous in that
pH control in the cathode compartment is also obtained by adding
carbon dioxide to the cathode compartment. In this embodiment, the
carbon dioxide acts as a feed material as a building block for
producing the chemical compounds and also acts to control pH in the
cathode compartment. It will be understood that hydroxyl ions can
be generated by the reactions occurring at the cathode. The
hydroxyl ions can react with the carbon dioxide to form bicarbonate
ions and the bicarbonate ions can subsequently pass through the
anion exchange membrane. In this fashion, an undesirable increase
in pH in the cathode compartment (which has the potential to kill
the culture of microorganisms) can be avoided and homeostatic
conditions can be maintained in the cathode compartment. Carbon
dioxide dosing does not significantly increase salt concentrations
in the cathode compartment. This is advantageous as it will mean
that a homeostatic situation can be achieved in the
bioelectrochemical system.
[0033] In another embodiment, the membrane separating the anode and
the cathode comprises a porous membrane. The porous membrane may
allow liquid and ions to pass therethrough but prevent
microorganisms from passing therethrough. In one embodiment, the
anode may be operated as a bioanode, and a waste stream, such as a
wastewater stream may be used as a feed material to the anode. As
mentioned above, the pore size in the porous membrane may be small
enough to prevent microorganisms from passing through the membrane.
These membranes are known to the person skilled in the art and
include microfiltration and ultrafiltration membranes. During
normal operation liquid passes through the porous membrane from the
anode into the cathode chamber. The protons that are generated in
the anode reaction equation (3) are transported through the
membrane to the cathode compartment and react with the hydroxyl
ions generated in the cathode reaction in accordance with equation
(4):
H.sup.++OH.sup.-.fwdarw.H.sub.2O (4)
[0034] As a result, an undesirable increase in the pH in the
cathode compartment is avoided and no acid needs to be dosed in the
cathode compartment. The pH and salt concentration in the cathode
chamber remain stable and homeostasis is maintained. Dissolved or
gaseous CO.sub.2 can be transferred from the anode to the cathode
alongside with the fluid.
[0035] In yet another one embodiment, the present invention may be
operated with a biocathode only. In this embodiment, the anode may
comprise an essentially conventional anode. In this embodiment an
acid solution (e.g. sulfuric acid) may be provided to the anode
compartment and the anode reaction may be a proton generating
reaction (e.g. oxygen generation from water). The membrane may
comprise a cation exchange membrane. Cation exchange membranes are
known to the person skilled in the art and include membranes such
as CMI-7000 (Membranes International), Neosepta CMX (ASTOM
Corporation), Fumasep.RTM. FKB (Fumatech), and Nafion (DuPont). In
this embodiment protons migrate through the cation exchange
membrane and react with the hydroxyl ions generated in the cathode
reaction. As a result, no acid needs to be dosed in the cathode
compartment the pH and salt concentration in the cathode chamber
remain stable and homeostasis is maintained.
[0036] In yet another embodiment, the electrical current used to
provide the reduction in the cathode is derived from a photo-anode.
Photo-anodes are known to persons skilled in the art and capture
sunlight and transfers the reducing power to the electrical
circuit.
[0037] In yet another embodiment, the electrical current provided
to drive the biochemicals production is derived from a renewable
power source such as solar power, hydro-power, or others as known
to a person skilled in the art.
[0038] In yet another embodiment, the membrane separating the anode
and the cathode comprises a bipolar membrane. Bipolar membranes are
known to persons skilled in the art and include membranes such as
NEOSEPTA BP-1E (ASTOM Corporation) and Fumasep.RTM. FBM (Fumasep).
Bipolar membranes are composed of a cation exchange layer on top of
an anion exchange layer and rely on the principle of water
splitting into protons and hydroxyl ions in between the ion
exchange layers of the membrane, according to equation (5):
H.sub.2O.fwdarw.H.sup.++OH.sup.- (5)
[0039] In embodiments where a bipolar membrane is used as the
membrane in the bioelectrochemical system, the anion exchange layer
is directed towards the anode chamber and the cation exchange layer
is directed towards the cathode chamber. When electrical current
flows, water diffuses in between the ion exchange layers of the
bipolar membrane and is split into protons and hydroxyl ions. The
hydroxyl ions migrate through the anion exchange layer into the
anode chamber, where they compensate for the proton production in
the anode reaction equation (3) and the protons migrate through the
cation exchange layer into the cathode chamber where they
compensate for the hydroxyl ion production (or proton consumption)
in the cathode reaction. As a result, no acid needs to be dosed in
the cathode compartment and the pH and salt concentration in the
cathode chamber remain stable, maintaining homeostasis.
[0040] In another embodiment of the present invention, the effluent
of the anode may be sent to, for example, a stripping column or
membrane unit to recover gaseous carbon dioxide. This carbon
dioxide can be provided to the cathode as a gas.
[0041] In a variation of the above embodiment effluent from the
anode can be passed through a membrane unit to allow separation of
carbon dioxide from the anode effluent, the membrane unit having a
liquid flow on the other side of the membrane. In this manner, the
separated carbon dioxide can go into solution in the fluid on the
other side of the membrane. The carbon dioxide can be provided to
the cathode in dissolved form. The fluid passing through the
membrane unit on the other side of the anode fluid can be cathode
fluid.
[0042] In another embodiment, the anode effluent can be sent
through a membrane unit to allow carbon dioxide together with
organic constituents of the anode effluent to pass to a second
liquid. For example, effluents from fermentation reactors can be
sent through an anode, the effluent of the anode can be sent to a
membrane unit where aside from the carbon dioxide fatty acids such
as propionate, butyrate and others as known to a person skilled in
the art pass through the membrane and become captured in a second
fluid. This fluid can be sent to the cathode where reduction of the
organics can occur. In this embodiment, both carbon dioxide and the
other organic materials provide feed material for the
microorganisms to convert into the desired chemical products.
[0043] In one embodiment, the cathode is also provided with organic
molecules to assist in the production of the biochemicals. Examples
of such organic molecules are glycerol, glucose, lactate, butyrate,
and others known to a person skilled in the art. These compounds
can be added to provide the microorganisms with a source for
adenosyl triphosphate (ATP) formation, which facilitates microbial
growth and product formation.
[0044] In the case of glycerol addition, the product formation may
include 1,3-propanediol or butanol. Glycerol may be added to the
cathode compartment, to the anode compartment or to both. Glycerol
can also be (partially) converted to propionate prior to entry in
the bioelectrochemical system, and subsequently be added to the
cathode as a mixture of glycerol and propionate.
[0045] In one embodiment, the microorganisms in the cathode
compartment are genetically engineered to receive electrons from
the cathode. Examples of modifications include the addition of
hydrogenases, cytochromes, sortases and other enzyme complexes to
the cell. Alternatively, the cathode can be provided with
conductive structures, such as nanowires, to electrically connect
microorganisms with the cathode.
[0046] In another embodiment, redox mediators can be added to the
cathode fluid, allowing transport of electrons from the cathode to
the microorganism. Examples of redox mediators are methyl viologen,
neutral red, phenazine carboxamide, amido black and others as known
to a person skilled in the art. The redox shuttles allow in certain
embodiments to increase the ratio NADH/NAD.sup.+ inside the
microbial cell, which drives the production of reduced
molecules.
[0047] In some embodiments of the process of the present invention,
a mixture of desirable chemicals may be formed in the cathode
compartment. In such embodiments, the present invention further
comprises the steps of removing a mixture of chemical compounds
from the cathode compartment and separating the mixture of chemical
compounds into two or more streams. The mixture of chemical
compounds may be separated using known separation techniques, such
as ion exchange, liquid-liquid extraction, absorption, absorption,
gas stripping, distillation, reverse osmosis, membrane separation,
cryogenic separation, or indeed any other separation technique
known to be suitable to a person skilled in the art. In some
embodiments, one or more of the chemical compounds may be reacted
to form another chemical compound that is more susceptible to
removal from the remaining chemical compounds. In some embodiments,
one or more of the chemical compounds formed in the cathode
compartment may comprise a solid compound. In such embodiments, any
suitable solid/liquid separation technique may be used, including
centrifugation, filtration, settling, clarification, flotation, or
the like. In other embodiments, one or more of the chemical
compounds formed in the cathode compartment may comprise a gaseous
compound. In such embodiments, product can conveniently be
collected with a gas collection device, such as a gas-liquid
separator, as known to a person skilled in the art.
[0048] In some embodiments of the present invention, the cathode
compartment is filled with the microbial culture. The microbial
culture is typically part of an aqueous mixture in the cathode
compartment. In other embodiments of the present invention, the
microbial culture grows on the electrode surface. In yet other
embodiments of the present invention, the cathode compartment is
filled with part of the microbial culture and another part of the
microbial culture grows on the electrode surface.
[0049] In some embodiments of the present invention, the cathode
compartment may comprise a first compartment housing the cathode,
the first compartment including a redox shuttle, and a second
compartment containing one or more microorganisms, wherein the
redox shuttle is reduced in the first compartment and a reduced
redox shuttle is provided to the second compartment, the second
compartment containing microorganisms that use the reduced redox
shuttle as an electron donor to facilitate formation of the one
more chemicals. The reduced redox shuttle is converted to an
oxidised redox shuttle in the second compartment. The oxidised
redox shuttle may be returned to the first compartment.
[0050] Examples of chemical compounds that can be formed using the
present invention include: [0051] alcohols such as methanol,
ethanol, propanol, butanol, isobutanol etc. [0052] carboxylic
acids, such as formic acid, acetic acid, propionic acid, butyric
acid, lactic acid, etc. [0053] diols such as 1,3-propanediol and
1,2-propanediol [0054] biopolymers such as
poly-.beta.-hydroxybutyrate (PHB), etc, [0055] any other organic
chemical that can be produced by micro-organisms from carbon
dioxide and reducing equivalents, in the presence or absence of
organic chemicals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 shows a schematic view of a bioelectrochemical system
suitable for use in embodiments of the present invention;
[0057] FIG. 2 shows a schematic diagram of apparatus suitable for
use in another embodiment of the present invention;
[0058] FIG. 3 shows a schematic diagram of apparatus suitable for
use in a further embodiment of the present invention; and
[0059] FIG. 4 shows a schematic diagram of apparatus suitable for
use in a further embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0060] It will be appreciated that the drawings have been provided
for the purpose of illustrating preferred embodiments of the
present invention. Therefore, it will be understood that the
present invention should not be considered to be limited to the
features are shown in the drawings.
[0061] The bioelectrochemical system 10 shown in FIG. 1 includes an
anode compartment 12 and a cathode compartment 14. The anode
compartment 12 includes an anode 16. The cathode compartment 14
includes a cathode 18. The anode and the cathode are in electrical
connection with each other via an electrical circuit 20 that
contains a power supply 22. The anode compartment 12 is separated
from the cathode compartment 14 by an anion exchange membrane
24.
[0062] The cathode compartment 14 contains a microbial culture
(shown schematically in FIG. 1 as ovals or circles 17 and 19). In
the embodiment shown in FIG. 1, the microbial culture comprises a
defined culture. A defined microbial culture of cathodophilic
microorganisms should be a carefully selected or genetically
engineered pure culture, but could also be a carefully selected
co-culture of two or more carefully selected or genetically
engineered pure cultures. These pure cultures or co-cultures should
consist of microbial species that are capable of catalyzing the
production reaction of the desired complex molecule. The microbial
culture in the cathode compartment 14 is contained in an aqueous
medium (see reference numeral 17) and/or attached to the cathode
(see reference numeral 19).
[0063] The cathode compartment 14 is provided with a gas inlet 26
and a gas outlet 28. A carbon dioxide containing stream is fed into
the gas in 26 and excess gas is removed through gas outlet 28. The
carbon dioxide containing stream that is fed to the cathode
compartment is a sterile carbon dioxide containing stream in that
it contains no contaminating microorganisms. One possible source of
such a carbon dioxide containing stream is an offgas stream from a
boiler or a furnace. Such an offgas stream leaves the boiler or
furnace that elevated temperatures and therefore contained no
microorganisms (and microorganisms would be killed by the high
temperatures encountered in the offgas stream). The offgas stream
may be cooled (in a manner which does not introduce any
contaminating bacteria into the gas stream, such as by using
indirect heat exchange) and subsequently be fed to the cathode
compartment 14.
[0064] Liquid from the cathode compartment 14 circulates through
liquid line 30. Pump 32 is used to maintain this liquid
circulation. A separator 34 is used to separate valuable product
from the liquid and the valuable product is recovered at 36. The
nature of the separator will be determined by the valuable
product(s) to be separated from the liquid. The person skilled in
the art will readily appreciate how to design an appropriate
separator for each product being formed.
[0065] FIG. 2 shows a schematic view of an alternative apparatus
suitable for use in the present invention. The apparatus shown in
FIG. 2 includes an anode compartment 112 that contains an anode
116. The apparatus also includes a cathode compartment 114 that
contains a cathode 118. A membrane 124 separates the cathode
compartment from the anode compartment. An electrical circuit 120
that includes a power supply 122 (in the form of a DC power supply,
such as a battery or an AC to DC converter) electrically connects
the anode 116 to cathode 118.
[0066] The apparatus also includes a separate vessel 130. The
vessel 130 has an inlet 132 in which carbon dioxide and oxygen or
an oxygen containing gas can be supplied. The oxygen and carbon
dioxide can be transferred via line 134 to compartment 114. Line
136 returns fluid and excess gas to the vessel 130. The vessel 130
may also be provided with an aqueous medium and the carbon dioxide
and oxygen may dissolve into the aqueous medium, with the aqueous
medium containing dissolved carbon dioxide and oxygen being
transferred to the cathode compartment 114.
[0067] FIG. 3 shows a schematic view of another apparatus suitable
for use in embodiments of the present invention. The apparatus
shown in FIG. 3 includes an anode compartment 212 that contains an
anode 216. The apparatus also includes a cathode compartment 214
that contains a cathode 218. A membrane 224 separates the cathode
compartment from the anode compartment. An electrical circuit 220
that includes a power supply 222 (in the form of a DC power supply,
such as a battery or an AC to DC converter) electrically connects
the anode 216 to the cathode 218.
[0068] The apparatus shown in FIG. 3 also includes a further vessel
230. The vessel 230 has an inlet 232 for admitting carbon dioxide
to the vessel 230. In the embodiment shown in FIG. 3, a redox
shuttle is reduced in the cathode compartment 214. The reduced
redox shuttle is supplied via line 236 to the external compartment
230. A culture of microorganisms in the vessel 230 uses the reduced
redox shuttle as an electron donor for the reduction of carbon
dioxide. The oxidised redox shuttle is then returned to the cathode
compartment 214 via line 238.
[0069] FIG. 4 shows a schematic view of another apparatus suitable
for use in embodiments of the present invention. The apparatus
shown in FIG. 4 includes an anode compartment 312 that contains an
anode 316. The apparatus also includes a cathode compartment 314
that contains a cathode 318. A membrane 324 separates the cathode
compartment from the anode compartment. An electrical circuit 320
that includes a power supply 322 electrically connects the anode
316 to cathode 318.
[0070] The apparatus also includes a vessel 330 that has an inlet
332 for supplying oxygen (and additional carbon dioxide, if
required) thereto. Line 350 transfers oxygen and carbon dioxide to
the cathode compartment 314 and line 352 returns excess oxygen and
carbon dioxide to vessel 330. The oxygen and carbon dioxide may be
transferred as gaseous streams or dissolved in liquid streams.
[0071] In the embodiment shown in FIG. 4, the reaction is taking
place at the anode produce carbon dioxide in the anode compartment
312. An outlet 340 from the anode compartment removes aqueous
liquid containing carbon dioxide from the anode compartment and
passes it to a stripping column 342. In the stripping column,
carbon dioxide is separated from the aqueous liquid. The aqueous
liquid is returned to the anode compartment 312 via line 344. The
stripped carbon dioxide is transferred via line 346 to inlet 334 of
vessel 330. The carbon dioxide and oxygen in the vessel 330 is
transferred to cathode compartment 314, where a selected culture of
microorganisms converts the carbon dioxide into other chemical
compounds. This embodiment is advantageous in that carbon dioxide
that forms at the anode is captured and used as a feed to the
cathode compartment, thereby reducing carbon dioxide emissions.
EXAMPLES
Example 1
Biopolymer Production
[0072] In this example, which uses the apparatus as shown in FIG.
2, bacteria in the cathode chamber use carbon dioxide and electrons
from the cathode as energy and carbon source, in which case they
can produce biopolymer under the form of
poly-.beta.-hydroxybutyrate (PHB). The CO.sub.2 is provided in a
way that a pure culture or a defined mixture of bacteria can be
maintained. Oxygen is supplied to support the PHB synthesis. The
electrons reach the bacteria either directly or indirectly through
e.g. the production of hydrogen at the cathode. An external power
source can provide the required additional reducing power at the
cathode, if required
[0073] Example organism in the cathode: Cupriavidus necator
(formerly Alcaligenes eutrophus or Ralstonia eutropha)
Example 2
Indirect Provision of Reducing Power to Biochemicals Producing
Organisms
[0074] In this example the apparatus as shown in FIG. 3 is used and
a redox shuttle is reduced in the cathode compartment. The reduced
redox shuttle is brought to the external compartment (possibly
through a permeable membrane) where micro-organisms use the reduced
redox shuttle as electron donor for the reduction of an electron
acceptor, being CO.sub.2, and the production chemicals from this
CO.sub.2. An external power source can provide the required
additional reducing power at the cathode, if required
Example 3
Reuse of CO.sub.2 Produced at the Anode to Drive the Cathodic
Reaction
[0075] This example is conducted in the apparatus as shown in FIG.
4. The anode contains micro-organisms that oxidize a carbon source.
The CO.sub.2 produced is stripped in situ, or in an external
stripping reactor, and hence brought to the cathode compartment in
such way that the cathode compartment can contain a well defined
culture or mixed culture of micro-organisms to form the desired
chemicals.
[0076] The present invention presents a cathode system for
producing complex molecules using microbial biocathodes prevents
the abovementioned problems associated with the contamination of
unwanted micro-organisms and/or cathode chamber pH increase and/or
salinity increase.
* * * * *