U.S. patent application number 12/090052 was filed with the patent office on 2008-09-18 for continuous co-current electrochemical reduction of carbon dioxide.
Invention is credited to Hui Li, Colin Oloman.
Application Number | 20080223727 12/090052 |
Document ID | / |
Family ID | 37942282 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080223727 |
Kind Code |
A1 |
Oloman; Colin ; et
al. |
September 18, 2008 |
Continuous Co-Current Electrochemical Reduction of Carbon
Dioxide
Abstract
In various embodiments, the invention provides electro-chemical
processes for reduction of carbon dioxide, for example converting
carbon dioxide to formate salts or formic acid. In selected
embodiments, operation of a continuous reactor with a three
dimensional cathode and a two-phase (gas/liquid) catholyte flow
provides advantageous conditions for electro-reduction of carbon
dioxide. In these embodiments, the continuous two-phase flow of
catholyte solvent and carbon dioxide containing gas, in selected
gas/liquid phase volume flow ratios, provides dynamic conditions
that favour the electro-reduction of COs at relatively high
effective superficial current densities and gas space velocities,
with relatively low reactor (cell) voltages (<10 Volts). In some
embodiments, relatively high internal gas hold-up in the cathode
chamber (evident in an internal gas to liquid phase volume ratio
>0.1) may provide greater than equilibrium CO.sub.2
concentrations in the liquid phase, also facilitating relatively
high effective superficial current densities. In some embodiments,
these characteristics may for example be achieved at catholyte pH
>7 and relatively low CO.sub.2 partial pressures (<10 bar).
In some embodiments, these characteristics may for example be
achieved under near adiabatic conditions, with catholyte outlet
temperature up to about 80.degree. C.
Inventors: |
Oloman; Colin; (Vancouver,
CA) ; Li; Hui; (Vancouver, CA) |
Correspondence
Address: |
GLENN PATENT GROUP
3475 EDISON WAY, SUITE L
MENLO PARK
CA
94025
US
|
Family ID: |
37942282 |
Appl. No.: |
12/090052 |
Filed: |
October 13, 2006 |
PCT Filed: |
October 13, 2006 |
PCT NO: |
PCT/CA2006/001743 |
371 Date: |
April 11, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60725642 |
Oct 13, 2005 |
|
|
|
Current U.S.
Class: |
205/413 |
Current CPC
Class: |
C25B 11/044 20210101;
C25B 11/075 20210101; B01D 53/326 20130101; C25B 11/04 20130101;
C25B 3/25 20210101; C25B 9/19 20210101; C25B 15/08 20130101; C25B
9/40 20210101; C25B 11/031 20210101; C25B 11/091 20210101; C25B
11/051 20210101; Y02P 20/151 20151101; C25B 13/08 20130101; C25B
15/02 20130101; B01D 2257/504 20130101 |
Class at
Publication: |
205/413 |
International
Class: |
C25B 3/04 20060101
C25B003/04 |
Claims
1. An electrochemical process for reducing carbon dioxide
comprising: a) continuously passing a catholyte mixture through a
cathode chamber of an electrochemical reactor, the catholyte
mixture comprising carbon dioxide gas and a liquid catholyte
solvent containing dissolved carbon dioxide; b) maintaining a
catholyte gas to liquid volumetric hold-up ratio, being the ratio
of the volume of gas to the volume of the liquid catholyte solvent,
in the cathode chamber, greater than about 0.1. c) passing an
electric current between a cathode in the cathode chamber and an
anode, to reduce the dissolved carbon dioxide to form a desired
product.
2. The process of claim 1, wherein the gas (corrected to STP) to
liquid volumetric feed ratio to the cathode chamber is greater than
about 1.
3. The process of claim 1 or 2, wherein the effective superficial
current density at the cathode is greater than 1 kA/m.sup.2
4. The process of claim 1, 2 or 3, wherein the carbon dioxide gas
partial pressure in the cathode chamber is less than 10 Bar.
5. The process of any one of claims 1 to 4, wherein the electric
current is a direct current driven by an electrochemical cell
voltage.
6. The process of claim 5, wherein the electrochemical cell voltage
is less than 10 Volts.
7. The process of any one of claims 1 to 6, wherein fluids in the
cathode chamber are maintained at a cathode temperature above
20.degree. C.
8. The process of any one of claims 1 to 7, wherein the cathode
chamber is maintained at a cathode pressure and the cathode
pressure is in the range of 1 Bar (100 kPa(abs)) to 10 Bar (1000
kPa(abs)).
9. The process of any one of claims 1 to 8, wherein the catholyte
solvent is an aqueous solvent.
10. The process of claim 9, wherein the catholyte solvent
comprises: a dissolved alkali metal bicarbonate or formate; or, a
dissolved
11. The process of claim 9, wherein the bulk pH of the catholyte
solvent is in the range of 4 to 10.
12. The process of claim 9, wherein the catholyte solvent comprises
ammonium cations.
13. The process of any one of claims 1 to 12, wherein the cathode
is a three dimensional electrode that has a thickness in the
dimension of current flow of from 0.5 to 10 mm.
14. The process of claim 13, wherein the cathode has a porosity or
voidage of from about 5% to about 95%.
15. The process of any one of claims 1 to 14, wherein the cathode
comprises tin or lead.
16. The process of any one of claims 1 to 15, wherein the anode is
in an anode chamber, and the anode chamber is separated from the
cathode chamber by an electrochemical cell membrane.
17. The process of claim 16, wherein the anode chamber comprises an
anolyte.
18. The process of claim 17, wherein the anolyte is an aqueous
anolyte.
19. The process of claim 18, wherein the anolyte comprises: a) a
dissolved alkali metal hydroxide; b) a dissolved alkali metal or
ammonium salt; c) a dissolved acid, being H2SO4, HCl, or H3PO4; d)
dissolved sulphuric acid and ammonium sulphate; or
20. The process of claim 18, wherein the anolyte comprises an
ammonium ions.
21. The process of claim 16, wherein the electrochemical cell
membrane is a cation permeable membrane.
22. The process of claim 16, wherein the electrochemical cell
membrane permits selected ions to cross the membrane to balance the
process stoichiometry.
23. The process of any one of claims 17 to 22, further comprising
recycling at least a portion of the anolyte, the recycling anolyte,
from an anolyte chamber outlet to an anolyte chamber inlet.
24. The process of claim 23, further comprising the step of
separating an anode co-product from the recycling anolyte.
25. The process of any one of claims 17 to 24, further comprising
Joule heating of the anolyte to provide heated anolyte.
26. The process of claim 25, further comprising Joule heating of
the anolyte to provide heated anolyte, wherein the heated anolyte
is used to heat the recycling catholyte solvent to separate water
or the desired product from the recycling catholyte solvent by
evaporation.
27. The process of any one of claims 1 to 26, wherein the desired
product comprises a formate salt or formic acid.
28. The process of claim 27, wherein the formate salt is ammonium
formate.
29. The process of any one of claims 1 to 28, further comprising
separating the desired product from the catholyte solvent.
30. The process of any one of claims 1 to 28, further comprising
recycling at least a portion of the catholyte solvent, the
recycling catholyte solvent, from a cathode chamber outlet to a
cathode chamber inlet.
31. The process of claim 30, further comprising the step of
separating the desired product from the recycling catholyte
solvent.
32. The process of claim 30, further comprising reacting recycling
catholyte, comprising formate, with the anolyte, to obtain the
desired product by an acidolysis reaction.
33. The process of claim 32, further comprising recycling at least
a portion of the anolyte, the recycling anolyte, from an anolyte
chamber outlet to an anolyte chamber inlet, and wherein the anolyte
used to obtain the desired product is a portion of the recycling
anolyte.
Description
FIELD OF THE INVENTION
[0001] The invention is in the field of electrochemistry,
encompassing processes for the electro-reduction of carbon dioxide
in aqueous systems, and apparatus therefor.
BACKGROUND OF THE INVENTION
[0002] The formate salts MHCO.sub.2 (where M is typically Na, K or
NH.sub.4) and formic acid HCO.sub.2H are commercial chemicals that
may be produced by industrial thermochemical processes
(Kirk-Othmer--Encyclopedia of Chemical Technology, 1991). For
example, sodium formate and subsequently formic acid may be
obtained by reaction of sodium hydroxide with carbon monoxide,
followed by acidolysis with sulphuric acid.
NaOH+CO.fwdarw.NaHCO.sub.2
2NaHCO.sub.2+H.sub.2SO.sub.4+2HCO.sub.2H+Na.sub.2SO.sub.4
[0003] Formic acid may also be produced as a co-product in the
oxidation of hydrocarbons and by the hydrolysis of methyl formate
from the carbonylation of methanol. Processes for the synthesis of
formate salts (e.g. KHCO.sub.2) by the electro-reduction of carbon
dioxide are also known (Chaplin and Wragg, 2003; Sanchez et al.,
2001; Akahori et al., 2004; Hui and Oloman, 2005).
[0004] Carbon dioxide Is considered the main anthropogenic cause of
climate change. Methods to sequester CO.sub.2 and/or convert it to
useful products are therefore needed.
[0005] Oloman and Watkinson in U.S. Pat. Nos. 3,969,201 and
4,118,305 (incorporated herein by reference) describe a trickle bed
reactor for electroreduction of oxygen to alkaline peroxide. In
various aspects, that electrochemical cell comprises a pair of
spaced apart electrodes, at least one of the electrodes being in
the form of a fluid permeable conductive mass separated from the
counter electrode by a barrier wall. The electrode mass may be in
the form of a bed of particles or a fixed porous matrix. It is
composed of an electronically conducting material the surface which
is a good electrocatalyst for the reaction to be carried out.
Inlets are provided for feeding liquid electrolyte and gas into the
electrode mass such that the electrolyte and gas move co-currently
through the electrode mass, for example in a direction generally
perpendicular to the direction of the current between the
electrodes. An outlet is provided for removing solutions containing
reaction products from the fluid permeable conductive mass.
SUMMARY OF THE INVENTION
[0006] In various embodiments, the invention provides
electro-chemical processes for reduction of carbon dioxide, for
example converting carbon dioxide to formate salts or formic acid.
In selected embodiments, operation of a continuous reactor with a
three dimensional cathode and a two-phase (gas/liquid) catholyte
flow provides advantageous conditions for electro-reduction of
carbon dioxide. In these embodiments, the continuous two-phase flow
of catholyte solvent and carbon dioxide gas, in selected gas/liquid
phase volume ratios, provides dynamic conditions that favour the
electro-reduction of CO.sub.2 at relatively high effective
superficial current densities. In some embodiments, relatively high
internal gas hold-up in the cathode chamber (evident in a gas to
liquid phase volume ratio >1 in the feed stream, or >0.1
within the porous electrode) may provide greater than equilibrium
CO.sub.2 concentrations In the liquid phase, facilitating
relatively high effective superficial current densities. In some
embodiments, these characteristics may for example be achieved at
catholyte pH >7 and relatively low CO.sub.2 partial pressures
(<10 bar).
[0007] In alternative aspects, the invention involves continuously
passing a catholyte mixture through a cathode chamber of an
electrochemical reactor. The catholyte mixture may include carbon
dioxide gas and a liquid catholyte solvent containing dissolved
carbon dioxide. The catholyte solvent may for example be an aqueous
solvent, it may include a dissolved alkali metal or ammonium
bicarbonate, and may be maintained at a desired pH, such as in the
range of from about 6 to about 9. A catholyte gas to liquid (G/L)
volumetric ratio may be maintained, being the ratio of the volume
of carbon dioxide gas to the volume of the liquid catholyte
solvent. The G/L ratio may be maintained in the cathode chamber,
for example In the feed stream or in a porous cathode within the
chamber. For example, the process may be operated so that the G/L
ratio is greater than a threshold value, such as greater than 1 in
the feed, or greater than 0.1 within the porous (3D) cathode.
[0008] One aspect of the invention involves passing an electric
current between a cathode in the cathode chamber and an anode, to
reduce dissolved carbon dioxide to form a desired product. In some
embodiments, the process may be operated so that the effective
superficial current density at the cathode is greater than a
threshold value, such as 1 kA/m.sup.2 (or 1.5, 2, 2.5, 3, 3.5, 4,
4.5 or 5 kA/m.sup.2). The electric current in the system may for
example be a direct current, driven by an electrochemical cell
voltage, and in some embodiments the process may be capable of
operating at relatively low electrochemical cell voltages, for
example less than 10 Volts.
[0009] Various aspects of the invention work in concert to
facilitate the adoption, in some embodiments, of process parameters
that may improve the economics of processes of the invention. In
some embodiments, the processes of the Invention may be used with
relatively dilute input gas streams, for example the carbon dioxide
gas concentration in the feed gas may be from 1 to 100%, or any
numeric value within this range (in some embodiments yielding a
carbon dioxide partial pressure in the cathode chamber less than a
threshold value, such as 3, 5 or 10 Bar). Similarly, relatively low
system pressures may be used, for example the cathode chamber may
be maintained at a cathode pressure in the range of a minimum value
such as 1, 2, 3, 4 or 5 Bar (1 Bar=100 kPa(abs)) up to a higher
maximum value such as 6, 7, 8, 9 or 10 Bar. In some embodiments, it
may be effective to run processes of the invention at elevated
temperatures, which may avoid the necessity for cooling, for
example at cathode temperatures above a desired threshold such as
20, 25, 30, 35, 40, 45 or 50.degree. C. In this context, it will be
understood that cathode chamber pressures and temperatures may vary
along the cathode height. For example, the inlet pressure may be
greater than outlet pressure (in some embodiments, the pressure
drop may for example range from a minimum of about 10, 20, 30, 40,
or 50 kPa, up to a maximum of about 500, 600, 700, 800 or 900 kPa).
Similarly, the outlet temperature may be greater than inlet
temperature, with a temperature rise from the inlet to the outlet
of from about 1 to 100.degree. C., or any numeric value within this
range. It will be understood that the gas composition (particularly
CO.sub.2 concentration) and the total pressure, fix the CO.sub.2
partial pressure. i.e. ppCO.sub.2=(CO2 fraction).times.(Total
pressure)
[0010] Cathodes for use in the invention may have an effective
thickness in the dimension of current flow, such as a porous
cathode. These may be referred to as three dimensional (3D)
electrodes. Such electrodes may have a selected thickness, such as
less than 6, 5, 4, 3, 2, 1 or 0.5mm, and they may have a selected
porosity, or range of porosities, such as 5 to 95% or any numeric
value within this range, such as 30, 40, 50, 60 or 70%. Cathodes of
the invention may be made from a wide variety of selectively
electro-active materials, such as tin, lead, pewter, mercury,
indium, zinc, cadmium, or other materials such as electronically
conductive or non-conductive substrates coated with selectively
electro-active materials (e.g. tinned copper, mercury amalgamated
copper, tinned graphite or tinned glass).
[0011] The anode may be in an anode chamber, and the anode chamber
may be separated from the cathode chamber by an electrochemical
cell membrane. The anolyte in the anode chamber may be an aqueous
anolyte, and may for example include a dissolved alkali metal
hydroxide, a salt (including an ammonium salt) or an acid, and may
have a pH range of from about 0 to about 14, or any pH value or
range within this range.
[0012] The electrochemical cell membrane may be a cation permeable
membrane, for example a membrane that permits selected ions to
cross the membrane to balance process stoichiometry.
[0013] The desired products of the process include formate salts,
such as ammonium, potassium and sodium formate, or formic acid. The
desired product may be separated from the catholyte solvent in a
variety of ways. For example, a portion of the catholyte solvent,
the recycling catholyte solvent, may be recycled from a cathode
chamber outlet to a cathode chamber inlet, and the desired product
may be separated from the recycling catholyte solvent. Similarly,
at least a portion of the anolyte may be recycled from an anolyte
chamber outlet to an anolyte chamber inlet, and an anode co-product
may be separated from the recycling anolyte,
[0014] In selected embodiments, Joule heating of the anolyte may be
used to provide heated anolyte, and the heated anolyte may be used
to heat the recycling catholyte solvent to separate the desired
product from the recycling catholyte solvent, for example by
evaporation with fractional crystallization or vacuum distillation.
In some embodiments, recycling catholyte, that includes formate,
may be reacted with the anolyte, to obtain the desired product by
an acidolysis reaction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1: is a process flow sheet illustrating aspects of the
process of Example 1, in which A=ammeter, P=pressure gauge,
T=thermometer, V=voltmeter, W=wet gas flow meter, PC=pressure
control.
[0016] FIG. 2: is a schematic illustration of electrochemical cells
of the invention, as described in Example 1, in which the reference
numerals denote the following components: 1 and 2: cell bodies; 2,
7 and 9: gaskets; 3: anode feeder, 4: anode spacer; 5: membrane; 6:
3-D cathode (tin-coated copper mesh, tin shot/granutes and Pb
shot/granules); 8: cathode feeder.
[0017] FIG. 3 shows a sectioned elevation view of the single-cell
reactor of the invention, Reactor A, as described in more detail in
Example 1.
[0018] FIG. 4: shows a sectioned elevation view of the single-cell
reactor of the invention, Reactor B, as described in more detail in
Example 1.
[0019] FIG. 5: is a process flow sheet illustrating various aspects
of a continuous process for conversion of CO.sub.2 to formate salts
or formic acid, involving recycling of catholyte and anolyte.
[0020] FIG. 6: Is a process flowsheet (Flowsheet "A"), illustrating
an embodiment of the process for converting CO.sub.2 gas to
NaHCO.sub.2 (sodium formate) and NaHCO.sub.3 (sodium bicarbonate)
with a byproduct of H.sub.2 (hydrogen) and co-product of O.sub.2
(oxygen).
[0021] FIG. 7: is a formalized version of process Flowsheet A,
forming the basis for a steady-state material balance stream table
for a process of converting approximately 600 tonnes per day of
carbon dioxide gas to sodium formate.
[0022] FIG. 8: illustrates Process Flowsheet B, for which there is
a corresponding material and energy balance stream table in the
examples.
[0023] FIG. 9: illustrates Process Flowsheet C of the Examples.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In various aspects, the invention provides a continuous
reactor for electroreduction of CO.sub.2, which may for example be
used in a process that converts a feed of carbon dioxide plus water
to formate ion (Reaction 1) and consequently produces formate salts
or formic acid.
CO.sub.2+H.sub.2O+2e.sup.-.fwdarw.HCO.sub.2.sup.-+OH.sup.- Reaction
1
[0025] In some embodiments, the invention may utilize an
electrochemical reactor analogous to the trickle bed reactor
described by Oloman and Watkinson in U.S. Pat. Nos. 3,969,201 and
4,118,305. In such embodiments, the Invention may utilize an
apparatus for carrying out electrochemical reactions involving
gaseous reactants comprising an electrochemical cell having a pair
of spaced apart electrodes, at least one of the electrodes, such as
the cathode, being in the form of a fluid permeable conductive mass
and being separated from the counter electrode by an ionically
conductive but electronically insulating layer (such as a membrane
or porous diaphragm). The reactor may be operated in a
"trickle-bed" mode, with co-current flow of reactant gas and
catholyte liquid through a flow-by 3-D cathode. As illustrated in
the Examples, the process parameters of the invention may be
adjusted so that this reactor achieves advantageous reactant supply
(evident for example as a high gas space velocity, a ratio of the
volumetric gas feed flow rate over reactor volume) and mass
transfer characteristics. The co-current fluid flow in the cathode
may be at any orientation relative to gravity, such as upward or
downward.
[0026] In reactors of the invention, an inlet may be provided for
feeding a liquid electrolyte and a gas into the fluid permeable
conductive mass, and an outlet may be provided for removing
solutions containing reaction products from the conductive mass.
The inlet and outlet may be arranged so that the electrolyte and
gas move co-currently through the conductive mass, for example in a
direction generally normal to the flow of electric current between
the electrodes. The reactor may for example be provided with a
cation membrane separator (as described for example in Hui and
Oloman, 2005). In alternative embodiments, other types of reactor
may be used.
[0027] Depending on the desired products and overall material
balance, the process feed may also include: metal hydroxides and/or
metal salts such as MOH, MCl, M.sub.2CO.sub.3, M.sub.2SO.sub.4 and
M.sub.3PO.sub.4 where M is typically an alkali metal (Na, K, etc.)
or NH.sub.4; acids such as H.sub.2SO.sub.4, H.sub.3PO.sub.4, or
HCl; or ammonia (NH.sub.3).
[0028] Flow sheets in various degrees of detail are provided for
alternative processes in FIGS. 1, 5, 6, 7, 8 and 9, illustrating
the range of embodiments encompassed by the invention. In selected
embodiments, the feed CO.sub.2 stream to the process may be
concentrated, for example to above 80% vol CO.sub.2. Alternatively,
a relatively dilute gas stream may be used, such as the product gas
from combustion of a fossil fuel (typically containing about 10%
vol CO.sub.2). Other potentially reactive components of the feed
CO.sub.2 stream include oxygen, sulphur oxides, nitrogen oxides and
hydrogen sulphide. These may be handled in the process in a variety
of ways, for example they may be removed in one or more initial
scrubbing steps, so that they are absent or at low concentrations
(such as below about 1% vol) in the feed stream entering the
reactor. The total pressure and temperature of the feed CO.sub.2
stream may vary over relatively wide ranges, for example from,
respectively, about 100 to 1000 kPa(abs), and about 250 to 550 K.
The conversion of CO.sub.2 per pass through the electrochemical
reactor may be less than 100%, so that the invention may include
provision for recycling the unconverted CO.sub.2 gas as well as
recycling the catholyte liquid.
[0029] Process steps 1 to 5 in FIG. 5 may be included in some
embodiments of the invention, briefly characterized as follows with
reference to the annotations on the Figure.
[0030] Step 1. MIX: Continuously mixes the feed water (plus any
make-up reagents) with the recycling catholyte, which is then
delivered continuously to the reactor cathode chamber.
[0031] Step 2. REACT: [C] Cathode. Continuously drives reaction 1,
along with the side-reaction, Reaction 2, that gives hydrogen by
the electro-reduction of water.
2H.sub.2O+2e.sup.-.fwdarw.H.sub.2+2OH.sup.- Reaction 2
[0032] Anode. Continuously drives the complimentary anode
reaction(s) whose nature depends on the desired products from the
process. For example, if the desired main product is a formate salt
and the co-product Is oxygen then the anode reaction may be
Reaction 3.
2OH.sup.-.fwdarw.1/2O.sub.2+H.sub.2O+2e.sup.- Reaction 3
[0033] If the desired main product is formic acid and the
co-product is oxygen or chlorine then the anode reaction may be
respectively Reaction 4 or 5. Other anode reactions may include the
generation of peroxy-salts of peroxy-acids, such as
peroxy-disulphate
(2SO.sub.4.sup.-.fwdarw.S.sub.2O.sub.8.sup.2-+2e).
2H.sub.2O.fwdarw.O.sub.2+2H.sup.++2e.sup.- Reaction 4
2Cr.fwdarw.Cl.sub.2+2e.sup.- Reaction 5
[0034] The electrode chambers in the reactor may be separated by a
membrane that selectively allows the transport of cations from
anode to cathode in amounts that balance the desired process
stoichiometry. If the desired main product is a formate salt then
these cations may be alkali metal ions (e.g. Na.sup.+, K.sup.+ or
NH.sub.4.sup.+) fed to the anolyte as hydroxides, salts or NH.sub.3
gas, whereas if the desired main product is formic acid the
transported cations may include protons (H.sup.+) generated in
Reaction 4 and/or fed to the anolyte as an acid.
[0035] Step 3. SEPARATE: Continuously separates the main product
(formate salt or formic acid) and byproduct (hydrogen) from the
recycling catholyte.
[0036] Step 4. MIX: Continuously mixes required anode reagents and
water with the recycling anolyte.
[0037] Step 5. SEPARATE: Continuously separates the anode
co-product(s) from the recycling anolyte.
[0038] In various steps of the process, carbon dioxide and water
may be consumed and/or generated In other reactions, such as
Reactions 6, 7 and 8 that occur in the reactor or elsewhere in the
process.
CO.sub.2+OH.sup.-.fwdarw.HCO.sub.3.sup.- Reaction 6
HCO.sub.3.sup.-+H.sup.+.fwdarw.H.sub.2O+CO.sub.2 Reaction 7
H.sup.+OH.sup.31 .fwdarw.H.sub.2O Reaction 8
[0039] In some embodiments, the process may involve driving the
reactor at a relatively high superficial current density (e.g.
above 0.5 kA/m.sup.2) and current efficiency, for example for
formate production (e.g. above 50%). Processes of the invention may
also involve balancing the material and energy requirements of the
various process steps to match the required process stoichiometry,
while maintaining a low specific energy consumption. For example,
processes of the invention have demonstrated 75% current efficiency
for formate at 1.3 kA/m.sup.2 with a reactor voltage of 3 V at
CO.sub.2 pressure of 200 kPa(abs) and temperature of 300 K. The
management of water may be important to the material balance and
require that water be fed to the cathode and/or anode circuits to
match its rate of reaction, electro-osmotic transport and
evaporation. The consumption of energy in electrochemical reaction,
heating, cooling and pumping may be a contributor to the process
cost, and may be kept relatively low by appropriate reactor design
and by rationalizing the thermal loads In the process. In some
embodiments, non-metallic catalysts may be used. For example, U.S.
Pat. Nos. 5,284,563 and 5,382,332 disclose nickel alkyl cyclam
catalysts that may be used for carbon dioxide reduction.
[0040] In some embodiments, a relatively high gas/liquid (G/L)
phase feed volumetric flow ratio may be used in the electrochemical
reactor (e.g.. G/L flow=1 to 1000 or 10 to 200), as well as a high
gas space velocity (e.g. >100 h.sup.-1). In selected reactors of
the invention, increasing G/L from about 5to 100 increases the
voltage by less than 10%. The optimum G/L phase volume (denoted as
the "G/L hold-up") ratio depends, in general, on the balance
between the effective catholyte conductivity (usually decreasing
with increasing G/L hold-up), the CO.sub.2 mass transfer capacity
(usually increasing with increasing G/L hold-up) and the intrinsic
temperature and pH dependent kinetics of CO.sub.2 conversion to the
un-reactive bicarbonate/carbonate species in the bulk catholyte
liquid phase.
[0041] In various embodiments, there are two separate gas/liquid
(G/L) ratios that may be of importance: [0042] (i) The volumetric
G/L ratio in the reactor feed stream(s), with the gas volume flow
corrected to STP, this may for example have a range of about 1 to
1000, 1 to 500, 10 to 200, or 10 to 100 or any numeric value within
these ranges. For example, gas flow may be 1000 ml/min (corrected
to STP), with liquid flow of 20 ml/min to give a G/L [flow]
=1000/20=50. [0043] (ii) The volumetric G/L ratio within the porous
cathode, i.e. the ratio of gas hold-up to liquid hold-up in the
cathode, which may for example have a range = about 0.1 to 10, or
about 0.2 to 2, or 0.2 to 4, or any numeric value or range within
these ranges. For example, gas hold up may be =0.6, with liquid
hold-up =(1-0.6), to give G/L [hold-up] =0.6/0.4=1.5. Where,
"Hold-up" = fraction of pore space (in 3D cathode) occupied by the
specified phase at a given moment. Assumed to be constant in
steady-state operation of the reactor. G/L[flow] is not equal to
G/L[hold-up] because the gas has a shorter residence time in the
cathode than the liquid (i.e. gas "slips" past the liquid). The
feed stream and internal hold-up values of (i) and (ii) are of
course related, since the value of (ii) depends on the value of (i)
together with the cathode characteristics, such as porosity (or
voidage), shape factor, and particle size. Similarly, the value of
(i) affects the value of (ii), and is also related to the CO.sub.2
mass transfer capacity in the cathode and the gas space velocity of
the reactor.
[0044] The above conditions may be modulated to allow (where
CD=current density): [0045] Effective CD>1.5 kA/m.sup.2 at
CO.sub.2 pressure<3 Bar. [0046] Effective CD=[superficial
CD].times.[current efficiency for desired product (e.g. formate)]
[0047] Format product concentration >0.5 M in a single pass.
[0048] Total reactor voltage at 3 kA/m.sup.2<Volt
[0049] The "superficial current density" is the current passing
through the cell divided by the projected surface area of the
relevant element, such as the cathode. The "projected surface area"
of an element, such as the cathode, is surface area of a projection
of the element on a plane parallel to the element. For flat plate
elements, the projected surface area is equal to the area of the
side of that element facing the other conductive element, for
example the projected surface area of the cathode facing the anode.
For an element in the form of a planar mesh, the projected surface
area is the area within the outline of the mesh as projected onto a
continuous planar surface.
[0050] The "current efficiency" (CE) is the ratio, generally
expressed as a percentage, of the actual reaction rate to the rate
that would be achieved if all of the current passing through the
cell were consumed by relevant reaction, such as the reduction of
carbon dioxide.
[0051] In some embodiments, the invention may operate at or near
adiabatic conditions (T out, up to about 90.degree. C.). In some
embodiments, while reducing the solubility of CO.sub.2 in the
catholyte, an increasing temperature actually favours the intrinsic
kinetics of the electroreduction of carbon dioxide (ERC), and good
CE can be obtained at higher temperatures by manipulating the
factors that promote CO.sub.2 mass transfer in a continuous
reactor. In some embodiments, the ability to operate at high
temperature may be important, because the effects of Joule heating
at high CD under near adiabatic conditions in the continuous
reactor may automatically increase the reaction temperature by up
to about 80.degree. C.
EXAMPLE 1
[0052] FIG. 1 shows a process flow diagram reflecting this example
of the electro-reduction of carbon dioxide (ERC). Pure CO.sub.2 or
the mixture of CO.sub.2 (gas) and N.sub.2 (gas) was combined with
the catholyte (liquid) at a T junction (mixer), from which the gas
and liquid proceeded in slug flow to enter the cathode chamber from
the bottom. Thus, the electrochemical reactor was operated with
co-current upward multi-phase (G/L) flow on the cathode side. The
anolyte, which was an aqueous KOH solution, also flowed upward
through the anode chamber and was recycled to the anolyte storage
tank. All gases and liquids passed through individual rotameters.
Liquid flow was controlled at the pumps, while gas flows were
controlled by manual valves to assure the appropriate gas and
liquid loads in the reactor. The reactor inlet and outlet pressures
and temperatures were measured by visual gauges at the points
indicated in the flowsheet. In runs during which the catholyte
product temperatures were controlled, pre-cooling or pre-heating of
both anolyte and catholyte was employed to keep the temperature at
a desired level. Liquid product was withdrawn from the sampling
point and analyzed for formate concentration. Gas product from the
gas/liquid separator (a packed bed of graphite felt) was controlled
by a 3-way valve either to an Orsat gas analyzer for CO.sub.2 and
CO analysis, to a wet gas flow meter for flow rate measurement, or
to a Tedlar sampling bag for subsequent hydrocarbon analysis with
gas chromatograph.
[0053] Galvanostatic electrolysis of CO.sub.2 was carried out with
a DC power supply connecting across the anode and cathode. A
voltmeter was also connected to the unit to measure the reactor
voltage. All voltages included anode potential, cathode potential
and IR drop. The individual electrode potentials were not
measured.
[0054] An automatic pressure control valve was used in the anolyte
product line to balance the pressure in the anode chamber against
that in the cathode chamber. Such a pressure balance is required to
prevent catholyte by-passing the 3-D cathode and/or the bursting of
the membrane that can occur when the cathode pressure exceeds the
anode pressure.
[0055] Most runs were conducted with the cathode outlet at the
atmospheric pressure. For some runs in Reactor B a manual back
pressure control valve and pressure gauge were installed in the
catholyte product line to maintain superatmospheric pressure in the
catholyte outlet.
[0056] Processes of the invention were performed first in Reactor A
(small reactor) and then in a seven-fold big Reactor B (big
reactor) to evidence the effects of scale up. Both reactors have
the configuration shown in FIG. 2. The reactors consist of a
cathode feeder plate and a 3-D cathode, a Nafion cation exchange
membrane separator, anode spacer/membrane support, an anode feeder
plate and gaskets. The cathode mesh, anode mesh and the anode
spacer are sealed on their margins by silicone glue, and then the
cell assembly is sandwiched between insulated mild steel plates and
uniformly compressed with SS bolts to give a balanced fluid
distribution.
[0057] FIG. 3 shows a sectioned elevation view of the single-cell
Reactor A. The "flow-by" cathode of this reactor had dimensions of
30 mm width and 150 mm height (geometric surface). The thickness of
the cathode depended on which 3-D cathode material was used. For
tin-coated copper mesh cathode, single or multiple layers of mesh
were placed between the membrane and cathode feeder so the
thickness of the cathode was the total thickness of these all
layers, which ranged from 0.38 to 1.83 mm; for graphite felts and
metal granules or shot, the cathode materials were embedded in two
layers of Neoprene gasket with the back of the cathode in contact
with the cathode feeder, therefore the thickness of the cathode was
that of the gasket, i.e. 3.2 mm. The geometric (a.k.a. superficial)
cathode area perpendicular to the electric current was 30 mm by 150
mm-4.5.times.10.sup.-3 m.sup.2. In Reactor A the applied current
ranged from 1 to 14 A with corresponding superficial current
density from 0.22 to 3.11 kA m-.sup.2.
[0058] In Reactor B, tin-coated copper mesh cathodes or fin granule
cathodes were used. FIG. 4 presents the dimensioned front view and
corresponding dimensions of Reactor B with a tin granule fixed-bed
cathode. To minimize the by-pass of the catholyte at the edges of
the cathode bed, the gasket was purposely made with five triangles
on each side to direct the flow toward the centre of the cathode.
Subtracting the areas taken by those triangles, the superficial
cathode area was 3.22.times.10.sup.-2 m.sup.2, which was about
seven times that of Reactor A (4.5.times.10.sup.-3 l m.sup.2). The
applied current in Reactor B ranged from 20 to 101 A with
corresponding superficial current density 0.62 to 3.20 kA
m.sup.-2.
[0059] Reactor B was assembled with a tin granule fixed-bed
cathode, according to the following procedures: (1) A sanded tin
plate (99.99 wt % Sn, 3mm thick) cathode feeder was put onto the
neoprene gasket; (2) The pretreated tin granules were spread
uniformly into a Durabla gasket (3.2 mm thick) on the tin plate,
and layers of Netlon screen were inserted Into the entrance and
exit regions of the catholyte flow to distribute the fluid and
support the membrane; (3) The wet Nafion 117 membrane was put on
top of the tin granule bed, and then, the PVC screen spacer, anode
SS mesh, and anode feeder (SS plate) were placed on top of one
another In that sequence; (4) Lastly, a cell body was put into
place, and 24 3/8 inch bolts were employed to compress the
sandwiched cell uniformly.
[0060] A variety of cathode materials are available for use In
alternative aspects of the invention. Carbon dioxide can be
electrochemically reduced on almost all groups of metals in the
periodic table to give a variety of products with different levels
of selectivity. The following cathode materials, among others, may
be adapted to particular embodiments: Nanostuctured Cu deposited on
graphite felt; Cu/Sn alloy deposited on graphite felt;
nano-structured Sn on Sn mesh, Sn coated plastic mash, Cu mesh; Sn
deposited graphite felt; Sn coated copper mesh; Pb plate, shot,
granules, grid and Pb-C reticulate; Sn shot and granules. The last
five of the foregoing materials were used in alternative
embodiments for the present Example. In some embodiments, a high
(specific) surface area micro or nano-structured deposit on a 3D
substrate is desirable. Other potential cathodes are.
nano-structured Cu on Cu mesh, nano-structured Sn on Sn mesh, or Sn
coated plastic mesh, alternatively with Pb, In or Hg as the
electroactive surface.
[0061] Reactor A, using granulated tin cathodes (99.9 wt % Sn) and
a feed gas of 100% CO.sub.2 showed slightly better performance than
that of the tinned-copper mesh cathodes. The seven fold scaled-up
Reactor B used a feed gas of 100% CO.sub.2 with the aqueous
catholyte and anolyte respectively [0.5 M KHCO.sub.3+2 M KC1 ] and
2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet
temperature 295 to 325 K. For a superficial current density of 0.6
to 3.1 kA m.sup.-2 Reactor B achieved corresponding formate current
efficiencies of 91 to 63%, with the same range of reactor voltage
as that in Reactor A (2.7 to 4.3 V). Up to 1 M formate was obtained
in the catholyte product from a single pass in Reactor B.
EXAMPLE 2
Recovery Of Cathode Activity
[0062] An electrochemical reactor as described in Example 1 was
constructed and operated as follows: [0063] Anode feeder=316
stainless steel plate [0064] Anode=304 stainless steel number 10
mesh (10 mesh/inch) [0065] Anode spacer=PVC "fly screen" 10 mesh.
[0066] Separator=Nafion 117 cation membrane. [0067] Cathode=ca. 50
mesh On granules. 150 mm high by 32 mm wide by 3 mm thick [0068]
Cathode superficial area=45E4 m2 [0069] Cathode feeder=tin foil
supported on a copper plate
[0070] Operating conditions: [0071] current=6 A (i.e. 1.3 kA/m2),
[0072] catholyte=0.45 M KHCO3+2 M KCl, anolyte=1 M KOH, anolyte
flow=40 ml/min [0073] CO2 gas flow=364 ml (STP)/min, catholyte
flow=20 ml/min, [0074] temperature=300 K, pressure=140-170
kPa(abs).
[0075] With a cathode of fresh tin granules the formate current
efficiency (CE) dropped from about 60% at 30 minutes to 50% at 250
minutes operating time. Recovery of the current efficiency was
achieved by:
[0076] (i). Chemical treatment and recycle of the cathode: The used
cathode tin granules were treated in 11 wt % nitric acid at room
temperature for 2 minutes, washed in deionized water and re-used in
the reactor. Table 1 shows that this treatment regained the cathode
activity at 30 minutes operating time.
TABLE-US-00001 TABLE 1 Formate CE Reactor voltage Cathode feed
pressure No. of recycles at 30 min % V kPa(abs) 0 (fresh granules)
63 3.73 156 1 61 3.56 156 2 64 3.36 161 3 66 3.30 166
[0077] Similar results for cathode recovery were obtained by
treating the used tin granules with hydrochloric acid and/or
potassium hydroxide.
[0078] (ii). Polarity reversal: Under similar conditions to those
above, with fresh tin granules the formate current efficiency
dropped from 65% at 30 minutes to 48% at 360 minutes operating
time. Polarity reversal was applied to the reactor for 5 minutes at
1 A. The formate current efficiency subsequently increased and was
back to 65% at 400 minutes operating time.
EXAMPLE 3
Scale-Up
[0079] An electrochemical reactor as described in Example 1 was
constructed and operated as follows: [0080] Anode feeder=316
stainless steel plate [0081] Anode=304 stainless steel, number 10
mesh (10 mesh/inch) [0082] Anode spacer=PVC "fly screen", 10 mesh.
[0083] Separator=Nafion 117 cation membrane. [0084] Cathode=ca. 50
mesh tin granules. 680 mm high by 50 mm wide by 3 mm thick [0085]
Cathode superficial area=340E-4 m2 [0086] Cathode feeder=2 mm thick
tin plate.
[0087] Operating conditions: [0088] catholyte=0.45 M KHCO3+2 M KCl,
anolyte =2 M KOH, anolyte flow=60 ml/min [0089] CO2 gas
flow=1600-2200 ml (STP)/min, catholyte flow=20 ml/min, [0090]
temperature in-out=300-314 K, pressure In-out=600-100 kPa(abs).
[0091] Table 2 shows the performance of this reactor.
TABLE-US-00002 TABLE 2 Reactor performance. Current, A 20 40 94 100
Superficial current density, kA/m.sup.2 0.6 1.2 2.9 3.1 Operating
time, min 60 80 100 17 Formate CE, % 91 86 64 63 Formate product
conc, M 0.28 0.54 0.94 1.03 Reactor voltage, V 2.7 3.4 4.1 3.9
EXAMPLE 4
Acid Anolyte
[0092] A reactor was constructed as in Example 1, operation was as
in Example 2, except the anolyte was replaced by an acid sodium
sulphate solution as follows:
[0093] Operating conditions: [0094] catholyte=0.45 M KHCO3+2 M KCl,
[0095] anolyte=0.5 to 2 M Na2SO4+0.5 to 4 M H2SO4, anolyte flow=40
ml/min [0096] CO2 gas flow=500 ml (STP)/min, catholyte flow=20
ml/min, [0097] temperature=300 K, pressure=140-170 kPa(abs).
[0098] The reactor was operated over a current range from 1 to 14 A
(0.2 to 3.1 kA/m.sup.2) with corresponding formate CE from 80 to
30% and reactor voltage from 3.5 to8.0 V.
[0099] This result shows that the process can be operated with an
acid anolyte. The various ratios of Na.sup.+/H.sup.+ in the anolyte
gave different formate current efficiencies, thus indicating that
the formate CE could be improved by manipulating the anolyte
composition.
EXAMPLE 5
Ammonium Cations
[0100] In some embodiments, the invention may utilize ammonium
cations, to produce ammonium formate. A reactor was constructed as
in Example 1, operation was as in Example 4, except the catholyte
potassium cations were replaced by ammonium and the anolyte was
replaced by an acid ammonium sulphate solution, as follows:
[0101] Operating conditions: [0102] current=4 A (i.e. 0.89
kA/m.sup.2) [0103] catholyte=0.45 M NH4HCO3 +2 M NH4Cl, [0104]
anolyte=0.93 M (NH4)2SO4+0.75 4 M H2SO4, anolyte flow=40 ml/min
[0105] CO2 gas flow=500 ml (STP)/min, catholyte flow=20 ml/min,
[0106] temperature=300 K, pressure=140-170 kPa(abs). [0107] The
reactor was operated over 2 hours with formate CE ranging from 35
to 70% and reactor voltage from 4.6 to 5.2 V.
[0108] This result demonstrates that the process can use
exclusively ammonium cations in the catholyte. The ability to use
ammonium cations is illustrated in Process Flowsheets B and C, for
the production of formic acid/or ammonium formate.
EXAMPLE 6
Lead Cathode
[0109] An electrochemical reactor as described in Example 1 was
constructed and operated as follows: [0110] Anode feeder=316
stainless steel plate [0111] Anode=304 stainless steel number 10
mesh (10 mesh/inch) [0112] Anode spacer=PVC "fly screen" 10 mesh.
[0113] Separator=Nafion 117 cation membrane. [0114] Cathode=0.5 mm
diameter lead shot. 150 mm high by 32 mm wide by 3 mm thick. [0115]
Cathode superficial area=45E-4 m2 [0116] Cathode feeder=lead
plate.
[0117] Operating conditions: [0118] current=6 A (i.e. 1.3 kA/m2),
[0119] catholyte=0.45 M KHCO3+2 M KCl, anolyte=1 M KOH, anolyte
flow=40 ml/min [0120] CO2 gas flow=364 ml (STP)/min, catholyte
flow=20 ml/min, [0121] temperature=300 K, pressure=140-180
kPa(abs).
[0122] Operation of this reactor over a period from 2 to 6 hours
showed a constant formate current efficiency of 31+/-1%.
EXAMPLE 7
Process Flowsheet A
[0123] The process of this Example is illustrated in FIG. 6,
showing electro-synthesis of sodium formate from carbon dioxide,
water and sodium hydroxide.
[0124] Based on the concept of FIG. 5 this process (FIG. 6)
converts CO.sub.2 to NaHCO.sub.2 (sodium formate) and NaHCO.sub.3
(sodium bicarbonate) with a byproduct of H.sub.2 (hydrogen) and
co-product of O.sub.2 (oxygen). The feed plus recycle CO.sub.2 is
compressed to about 300 kpa(abs) and delivered to the cathode of
the electrochemical reactor along with the recycling catholyte, an
aqueous solution of NaHCO.sub.2 and NaHCO.sub.3. The cathode outlet
goes to a gas/liquid separator from which the liquid is divided
into a direct recycle and a stream from which NaHCO.sub.2 and
NaHCO.sub.3 are separated by evaporation and fractional
crystallization to give the main cathode products (NaHCO2 and
NaHCO.sub.3). The cathode outlet gas goes to a gas separation
system (e.g. pressure swing adsorption) that recovers H.sub.2 and
delivers the unconverted CO.sub.2 to recycle. The anode side of
this process involves a feed of NaOH (sodium hydroxide) whose
sodium content (Na.sup.+) is transported across the cation membrane
while the hydroxide is converted to oxygen that is recovered as the
co-product from a gas/liquid separator. The recycle streams in this
process include the necessary compressors and pumps along with heat
exchangers (e.g. C1, C2, C3) to control the reactor temperature in
the range of about 300 to 350 K.
[0125] FIG. 7 illustrates Process Flowsheet A, and the steady-state
material balance stream table is set out below, based on 600
tonne/day CO.sub.2. Formate current efficiency=77%. CO.sub.2
conversion/pass=72%.
TABLE-US-00003 Basis: 600 Ton/day CO2 Stream table Flow rate of
components in kmol/h Stream phase Tempt. Volume Density CO2 NaHCO3
HCOONa H2 O2 NaOH H2O K m3/h kg/m3 44 84 68 2 32 40 18 1 G 293
13868 567 0 0 0 0 0 0 2 G 293 19298 789 0 0 0 0 0 0 3 L 288 319
1150 0 0 0 0 0 638 18975 4 G/L 321 222 508 1814 65 0 0 5 G/L 321 0
0 0 0 142 71 6 L 321 319 1150 0 0 0 0 0 71 20235 7 S 298 23 0 0 0 0
0 567 0 8 G 321 3467 0 0 0 0 142 0 0 9 G 321 7024 222 0 0 65 0 0 0
10 L 321 319 1150 0 508 1814 0 0 0 11166 11 G 321 1595 0 0 0 65 0 0
0 12 G 321 5430 222 0 0 0 0 0 0 13 L 321 301 1150 0 479 1711 0 0 0
10528 14 L 321 18 1150 0 29 104 0 0 0 638 15 L 379 178 1150 0 0 0 0
0 0 549 16 L/S 379 L/S L/S L/S L/S L/S L/S 554 17 L 298 123 1350 0
151 1719 0 0 0 2038 18 S 298 27 0 329 0 0 0 0 0 19 L 288 194 1000 0
0 0 0 0 0 600 21 L 298 107 1350 0 131 1490 0 0 0 31780 20 L 288 319
1150 0 160 1596 0 0 0 14359 22 L 298 16 1350 0 20 229 0 0 0 272
EXAMPLE 8
Process Flowsheet B
[0126] FIG. 8 illustrates the electrosynthesis of formic acid from
carbon dioxide and water. The exemplified process converts CO.sub.2
to HCO.sub.2H (formic acid) with a byproduct of H.sub.2 (hydrogen)
and co-product of O.sub.2 (oxygen). The feed plus recycle CO.sub.2
is compressed to about 300 kPa(abs) and delivered to the cathode of
the electrochemical reactor (U1) along with the recycling
catholyte, an aqueous solution of NH.sub.4HCO.sub.2 and
NH.sub.4HCO.sub.3 plus (if required) a supporting electrolyte such
as NH.sub.4Cl or (NH.sub.4).sub.2SO.sub.4. The cathode outlet
stream goes to a gas/liquid separator (U3) from which the liquid is
divided (U5) into a direct recycle and a stream that is passed to a
thermochemical acidolysis reactor/separator (U6,U7) where formic
acid is obtained by reaction 9 with sulphuric acid (generated in
the anolyte) and distilled under partial vacuum to give an overhead
product of aqueous formic acid and a bottoms solution of
(NH.sub.4).sub.2SO.sub.4 that is recycled to the anode via the
mixer U8. The gas stream from U3 passes to a separator (U4) where
H.sub.2 is recovered and CO.sub.2 is recycled to the reactor feed
via mixer U2, along with CO.sub.2 generated by the side-reaction 7
in the acidolyis reactor.
[0127] An aqueous solution of (NH.sub.4).sub.2SO.sub.4 and
H.sub.2SO.sub.4 recycles through the anode circuit, supplying
NH.sub.4.sup.+ and H.sup.+ cations for transport to the catholyte
via the cation membrane. The co-product O.sub.2 gas is generated
with protons (H.sup.+) at the anode by reaction 4 and recovered
from a gas/liquid separator (U9). The recycling acid anolyte is
then divided (U10) to supply H2SO.sub.4 for the acidolysis reaction
(U6) from which the spent reactant is re-combined with the anolyte
(U8).
2NaHCO.sub.2+H.sub.2SO.sub.4.fwdarw.2HCO.sub.2H+Na.sub.2SO.sub.4
Reaction 9
[0128] A material and energy (M&E) balance for Flowsheet B
operating at steady-state is shown In the stream table below. This
M&E balance is based on the assumption of a formate current
efficiency of 80% and 80% conversion of CO.sub.2 per pass through
the electrochemical reactor.
[0129] The primary and secondary net reactions in Flowsheet B are
respectively reactions 10 and 11.
CO.sub.2+H.sub.2O.fwdarw.HCO.sub.2H+1/2O.sub.2 Reaction 10
H.sub.2O.fwdarw.H.sub.21/2O.sub.2 Reaction 11
[0130] The conditions of this process may be chosen to promote the
main net reaction 10. The characteristics of the process of this
example, to promote reaction 10 may be selected as follows: [0131]
i. Appropriate electrode materials, current density, fluid
compositions, fluid loads, pressure and temperature in the
electrochemical reactor. [0132] ii. Maintaining the anolyte
composition with respect to acid and salt to provide cation
transport across the membrane in the correct ratio (e.g.
H.sup.+/NH.sub.4+) that balances the rates of cathode reactions 1
and 2 and holds the catholyte pH in the desired range. [0133] iii.
A bulk catholyte pH in the range about 4 to 10, preferably 6 to 8.
[0134] iv. Maintaining the anolyte composition and flow to provide
protons for the acidolysis reaction that produces HCO.sub.2H in U6
and allow recovery of the aqueous formic acid by vaporization in
U7. [0135] v. A concentration of acid (e.g. H.sub.2SO.sub.4) in the
anolyte of greater than about 1 M. [0136] vi. Maintaining the
formate concentration of the catholyte sufficiently high to allow
formation and separation of HCO.sub.2H in U6. [0137] vii. A
concentration of formate (HCO.sub.2.sup.-) in the recycle catholyte
of greater than about 1 M, preferably about 5 M. [0138] viii.
Feeding water to the cathode and/or anode circuits at the
appropriate rate(s) to maintain the water balance and the
electrolyte concentrations that facilitate both the electrochemical
and thermochemical processes in U1,U6 and U7. [0139] ix.
Maintaining the flow and temperature of the recycle anolyte
sufficiently high to utilize the Joule heating of the
electrochemical reactor for evaporation of formic acid in U6.
[0140] x. A recycle anolyte temperature of greater than about 320
K, with an anolyte flow determined by the energy balance to reduce
the need for heating utilities in the process.
[0141] Operation of the process will typically depend on
interactions among the conditions i to x listed above. Modeling of
this embodiment provided a steady-state material and energy
balance, on the basis of 105 tonne/day CO.sub.2, giving a current
efficiency of 80% and CO.sub.2 conversion/pass of 80%. The material
and energy balance stream table corresponding to Process flowsheet
B is set out below, with the Table continued across the three
sub-tables.
TABLE-US-00004 Stream table M Stream kmol/h BASIS 105 Ton/day CO2
Comp. kg/kmol 1 2 3 4 5 6 7 8 CO2 44 100.0 139.2 0.0 27.8 27.8 0.0
27.8 0.0 H2 2 0.0 0.0 0.0 25.0 25.0 25.0 0.0 0.0 N2 28 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 H2O 18 314 332 5873 6693 17 0 17 6676 HCOOH 46
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 NH4HCO2 63 0.0 0.0 731.4 831.4 0.0
0.0 0.0 831.4 NH4HCO3 79 0.0 0.0 83.1 94.5 0.0 0.0 0.0 94.5
(NH4)2SO4 132 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 H2SO4 98 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 O2 32 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Total
kg/h 1.0E+04 1.2E+04 1.6E+05 1.8E+05 1.6E+03 5.0E+01 1.5E+03
1.8E+05 Phase G + L G + L L G + L G G G L Pressure kPa(abs) 100 300
300 200 200 800 100 100 Temp K 300 300 300 354 354 300 300 354
Density kg/m3 1083 1083 Volume m3/h 2494 1157 146 1035 78 1128 166
Enthalpy kJ/h -1.2E+08 -1.4E+08 -2.5E+09 -2.8E+09 -1.5E+07 1.5E+03
-1.5E+07 -2.8E+09 Stream table Comp. M 9 10 11 12 13 14 CO2 44 0.0
11.4 0.0 11.4 0.0 0.0 H2 2 0.0 0.0 0.0 0.0 0.0 0.0 N2 28 0.0 0.0
0.0 0.0 0.0 0.0 H2O 18 803 550 550 0 1101 1365 HCOOH 46 0.0 100.0
100.0 0.0 0.0 0.0 NH4HCO2 63 100.0 0.0 0.0 0.0 0.0 0.0 NH4HCO3 79
11.4 0.0 0.0 0.0 0.0 0.0 (NH4)2SO4 132 0.0 0.0 0.0 0.0 181.7 237.4
H2SO4 98 0.0 0.0 0.0 0.0 55.7 0.0 O2 32 0.0 0.0 0.0 0.0 0.0 0.0
Total kg/h 2.2E+04 1.5E+04 1.4E+04 5.1E+02 4.9E+04 5.6E+04 Phase L
G L G L L Pressure kPa(abs) 100 10.0 100 100 100 100 Temp K 354 319
300 300 354 319 Density kg/m3 1083 1061 1356 1322 Volume m3/h 20
175437 14 294 36 42 Enthalpy kJ/h -3.4E+08 -1.7E+08 -2.0E+08
-4.6E+06 -5.8E+08 -6.8E+08 Stream table Comp. M 15 16 17 18 19 20
CO2 44 0.0 0.0 0.0 0.0 0.0 0.0 H2 2 0.0 0.0 0.0 0.0 0.0 0.0 N2 28
0.0 0.0 0.0 0.0 0.0 0.0 H2O 18 18415 17790 61 17729 16628 422 HCOOH
46 0.0 0.0 0.0 0.0 0.0 0.0 NH4HCO2 63 0.0 0.0 0.0 0.0 0.0 0.0
NH4HCO3 79 0.0 0.0 0.0 0.0 0.0 0.0 (NH4)2SO4 132 2981 2925.6 0.0
2925.6 2743.9 0.0 H2SO4 98 841 896.6 0.0 896.6 840.9 0.0 O2 32 0.0
62.5 62.5 0.0 0.0 0.0 Total kg/h 8.1E+05 8.0E+05 3.1E+03 7.9E+05
7.4E+05 7.6E+03 Phase L G + L G L L L Pressure kPa(abs) 300 200 100
100 100 100 Temp K 300 354 354 354 329 300 Density kg/m3 1349 1356
1356 1000 Volume m3/h 585 1804 585 549 8 Enthalpy kJ/h -9.7E+09
-9.4E+09 -1.5E+07 -9.4E+09 -8.8E+09 -1.2E+08
EXAMPLE 9
Process Flowsheet C
[0142] FIG. 9 illustrates electro-synthesis of ammonium formate
from carbon dioxide, ammonia and water. This process converts
CO.sub.2 and NH.sub.3 to NH.sub.4HCO.sub.2 (ammonium formate) with
a byproduct of H.sub.2 (hydrogen) and co-product of O.sub.2
(oxygen).
[0143] The feed plus recycle CO.sub.2 is compressed and delivered
to the cathode of the electrochemical reactor along with the
recycling catholyte, an aqueous solution of NH.sub.4HCO.sub.2 (e.g.
>1 M) with minor amounts of NH.sub.4HCO.sub.2 (ammonium
bicarbonate--e.g. 0.1 M). The cathode outlet stream goes to
separation system that recovers a solution of NH.sub.4HCO.sub.2
plus the byproduct hydrogen and recycles the spent catholyte.
[0144] Ammonia (NH.sub.3 gas or aqueous solution) is fed to the
anolyte circuit where it combines to form (NH.sub.4).sub.2SO.sub.4
(ammonium sulphate). An aqueous solution of
(NH.sub.4).sub.2SO.sub.4 and H.sub.2SO.sub.4 then recycles through
the anode circuit, supplying NH.sub.4.sup.+ and H.sup.+ cations for
transport to the catholyte via the cation membrane. The co-product
O.sub.2 gas is generated with protons (H.sup.+) at the anode by
reaction 4 and recovered from a gas/liquid separator. The ratio
[NH.sub.4.sup.+]/[H.sup.+] is maintained in the anolyte to supply
these species to the catholyte at rates that balance the
stoichiometry of reactions 1 and 2 and produce a catholyte solution
of predominantly ammonium formate at pH in the range about 4 to
8.
[0145] The primary and secondary net reactions in flowsheet C are
respectively reactions 12 and 13.
CO.sub.2+H.sub.2O+NH.sub.3.fwdarw.NH.sub.4HCO.sub.2+1/2O.sub.2
Reaction 12
H.sub.2O.fwdarw.H.sub.2+1/2O.sub.2 Reaction 13
[0146] Variations of this scheme may include for example
replacement of (NH.sub.4).sub.2SO.sub.4 and H.sub.2SO.sub.4 in the
anolyte by (NH.sub.4).sub.3PO.sub.4 and H.sub.3PO.sub.4or by
NH.sub.4Cl and HCl. In the later case the anode co-product may be
Cl.sub.2 by reaction 5. Anode co-products may also include
peroxy-compounds such as ammonium persulphate
(NH.sub.4).sub.2S.sub.2O.sub.8 or persulphuric acid
H.sub.2S.sub.2O.sub.8, etc. by reaction 14.
2SO.sub.4.sup.-.fwdarw.S.sub.2O.sub.8.sup.-+2e.sup.- Reaction
14
REFERENCES
[0147] Kirk-Othmer--Encyclopedia of Chemical Technology. John
Wiley, New York, 1991.
[0148] R. Chaplin and A. Wragg. "Effects of process conditions and
electrode material on reaction pathways for carbon dioxide
electroreduction with particular reference to formate formation" J.
Appl. Electrochem. 33:1107-1123 (2003).
[0149] C. M. Sanchez et al. "Electrochemical approaches to
alleviation of the problem of carbon dioxide accumulation" Pure
Appl. Chem. 73(12), 1917-1927, 2001.
[0150] Y. Akahori et al. "New electrochemical process for CO.sub.2
reduction to formic acid from combustion flue gases". Denki Kagaku
(Electrochemistry) 72(4) 266-270 (2004).
[0151] Li Hui and C. Oloman. "The electro-reduction of carbon
dioxide in a continuous reactor". J. Appl. Electrochem. 35,
955-965, (2005).
[0152] K. Hara and T. Sakata. "Electrocatlytic formation of
CH.sub.4 from CO.sub.2 on a Pt gas diffusion electrode". J.
Electrochem. Soc. 144(2),539-545 (1997).
[0153] M. N. Mahmood, D. Masheder and C. J. Harty. "Use of
gas-diffusion electrodes for high rate electrochemical reduction of
carbon dioxide" J. Appl. Electrochem. 17:1159-1170 (1987).
[0154] K. S. Udupa, G. S. Subramamian and H. V. K. Udupa.
Electrochim Acta 16, 1593, 1976.
[0155] Although various embodiments of the invention are disclosed
herein, many adaptations and modifications may be made within the
scope of the Invention in accordance with the common general
knowledge of those skilled in this art. Such modifications include
the substitution of known equivalents for any aspect of the
invention in order to achieve the same result in substantially the
same way. Numeric ranges are inclusive of the numbers defining the
range. The word "comprising" is used herein as an open-ended term,
substantially equivalent to the phrase "including, but not limited
to", and the word "comprises" has a corresponding meaning. As used
herein, the singular forms "a", "an" and "the" include plural
referents unless the context clearly dictates otherwise. Thus, for
example, reference to "a thing" includes more than one such thing.
Citation of references herein Is not an admission that such
references are prior art to the present invention. Any priority
document(s) and all publications, including but not limited to
patents and patent applications, cited in this specification are
incorporated herein by reference as if each individual publication
were specifically and individually indicated to be incorporated by
reference herein and as though fully set forth herein. The
invention includes all embodiments and variations substantially as
hereinbefore described and with reference to the examples and
drawings.
* * * * *