U.S. patent application number 16/045863 was filed with the patent office on 2019-03-21 for cyclic process using alkaline solutions created from electrolytically decarboxylated water as an atmosphereic co2 collector followed by repeated electrochemical recovery of co2 with simultaneous production of dihydrogen for liquid hydrocarbon synthesis.
The applicant listed for this patent is The Goverment of the United States of America, as represented by the Secretary of the Navy, The Goverment of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Felice DiMascio, Dennis R. Hardy, Heather D. Willauer, Frederick Williams.
Application Number | 20190085472 16/045863 |
Document ID | / |
Family ID | 65039852 |
Filed Date | 2019-03-21 |
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United States Patent
Application |
20190085472 |
Kind Code |
A1 |
Willauer; Heather D. ; et
al. |
March 21, 2019 |
CYCLIC PROCESS USING ALKALINE SOLUTIONS CREATED FROM
ELECTROLYTICALLY DECARBOXYLATED WATER AS AN ATMOSPHEREIC CO2
COLLECTOR FOLLOWED BY REPEATED ELECTROCHEMICAL RECOVERY OF CO2 WITH
SIMULTANEOUS PRODUCTION OF DIHYDROGEN FOR LIQUID HYDROCARBON
SYNTHESIS
Abstract
A method for the controlled removal of bicarbonate from alkaline
water and its replacement with a strong base that is capable of
chemically absorbing CO.sub.2 from the atmosphere as a carbonate
and bicarbonate solution. This bicarbonate and carbonate solution
is reprocessed in the central compartment of an electrolytic cation
exchange module (E-CEM) to take advantage of the removal of
CO.sub.2 from the air, and as an energetic byproduct of E-CEM
dihydrogen production, and to regenerate the original strong base
absorbent solution. Thus, this process is cyclical in nature, and
no chemicals are needed except an initial source of alkaline
water.
Inventors: |
Willauer; Heather D.;
(Fairfax Station, VA) ; Hardy; Dennis R.;
(Fredericksburg, VA) ; DiMascio; Felice;
(Simsbury, CT) ; Williams; Frederick; (Accokeek,
MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Goverment of the United States of America, as represented by
the Secretary of the Navy |
Washington |
DC |
US |
|
|
Family ID: |
65039852 |
Appl. No.: |
16/045863 |
Filed: |
July 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62537139 |
Jul 26, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2252/1035 20130101;
B01D 53/14 20130101; C25B 1/10 20130101; C25B 1/14 20130101; C25B
1/16 20130101; B01D 2251/606 20130101; C25B 15/08 20130101; B01D
53/1493 20130101; B01D 53/1425 20130101; C10G 2/50 20130101; C25B
9/08 20130101; B01D 2258/06 20130101; B01D 53/1475 20130101; B01D
2251/304 20130101 |
International
Class: |
C25B 1/14 20060101
C25B001/14; C25B 15/08 20060101 C25B015/08 |
Claims
1. A cyclical method for producing a strong alkaline solution for
atmospheric CO.sub.2 capture, subsequently followed by recovery of
the CO.sub.2 along with regeneration of the alkaline solution to
complete the cycle, comprising: feeding an alkaline solution
containing bicarbonate and carbonate ions into an electrochemical
module to form a hydroxide solution; allowing the hydroxide
solution to chemically absorb CO.sub.2 from the atmosphere to form
a re-equilibrated bicarbonate and carbonate solution; and feeding
the re-equilibrated bicarbonate and carbonate solution back into
the electrochemical module.
2. The method of claim 1, wherein the pH of the hydroxide solution
decreases as the CO.sub.2 from the atmosphere is absorbed.
3. The method of claim 1, wherein the hydroxide solution is
regenerated in the electrochemical module from the re-equilibrated
bicarbonate and carbonate solution fed into the electrochemical
module.
4. The method of claim 1, additionally comprising adjusting a
surface to volume ratio of the hydroxide solution to maximize the
absorption rate of CO.sub.2 from the atmosphere into the hydroxide
solution.
5. The method of claim 1, wherein as the pH and hydroxide
concentration increase in the hydroxide solution, the rate of
CO.sub.2 absorption from the atmosphere into the hydroxide solution
increases.
6. The method of claim 1, wherein the hydroxide solution comprises
an alkali metal hydroxide.
7. The method of claim 1, wherein the hydroxide solution comprises
sodium hydroxide.
8. A cyclical method for producing a strong alkaline solution for
atmospheric CO.sub.2 capture, subsequently followed by recovery of
the CO.sub.2 along with simultaneous production of dihydrogen and
regeneration of the alkaline solution to complete the cycle,
comprising: feeding an alkaline solution containing bicarbonate and
carbonate ions into a center compartment of an electrolytic cation
exchange module (E-CEM), wherein the E-CEM comprises an anode, an
anode compartment adjacent to the anode, a first cation membrane
between the anode compartment and the center compartment, the
center compartment, a cathode compartment, a second cation membrane
between the center compartment and the cathode compartment, and a
cathode adjacent to the cathode compartment; feeding water into the
anode compartment and cathode compartment; applying a source of
electricity to the anode, wherein O.sub.2 is formed in the anode
compartment, CO.sub.2 is formed in the center compartment, and
H.sub.2 and hydroxide are formed in the cathode compartment;
removing the CO.sub.2 formed in the center compartment and the
H.sub.2 formed in the cathode compartment; collecting an effluent
from the cathode compartment comprising the hydroxide formed in the
cathode compartment; allowing the effluent from the cathode
compartment to chemically absorb CO.sub.2 from the atmosphere to
form a re-equilibrated bicarbonate and carbonate solution; and
feeding the re-equilibrated solution back into the center
compartment of the E-CEM.
9. The method of claim 8, wherein the pH of the effluent from the
cathode compartment decreases as the CO.sub.2 from the atmosphere
is absorbed.
10. The method of claim 8, wherein an effluent from the anode
compartment, an effluent from the center compartment, or both are
combined with the effluent from the cathode compartment to form a
combined effluent, and wherein the combined effluent chemically
absorbs CO.sub.2 from the atmosphere to form a re-equilibrated
bicarbonate and carbonate solution.
11. The method of claim 10, wherein the pH of the combined effluent
decreases as the CO.sub.2 from the atmosphere is absorbed.
12. The method of claim 8, wherein hydroxide is regenerated in the
cathode compartment from the re-equilibrated bicarbonate and
carbonate solution fed into the center compartment.
13. The method of claim 8, additionally comprising adjusting a
surface to volume ratio of the cathode effluent to maximize the
absorption rate of CO.sub.2 from the atmosphere into the effluent
from the cathode compartment.
14. The method of claim 8, additionally comprising increasing the
applied electricity to increase the pH and hydroxide concentration
of the effluent from the cathode compartment.
15. The method of claim 8, wherein as the pH and hydroxide
concentration increase in the effluent from the cathode
compartment, the rate of CO.sub.2 absorption from the atmosphere
into the effluent from the cathode compartment increases.
16. The method of claim 8, wherein the hydroxide formed in the
cathode compartment comprises an alkali metal hydroxide.
17. The method of claim 8, wherein the hydroxide formed in the
cathode compartment comprises sodium hydroxide.
Description
PRIORITY CLAIM
[0001] The present application is a non-provisional application
claiming the benefit of U.S. Provisional Application No. 62/537,139
filed on Jul. 26, 2017 by Heather D Willauer et al., entitled "A
CYCLIC PROCESS USING ALKALINE SOLUTIONS CREATED FROM
ELECTROLYTICALLY DECARBOXYLATED WATER AS AN ATMOSPHERIC CO2
COLLECTOR FOLLOWED BY REPEATED ELECTROCHEMICAL RECOVERY OF CO2 WITH
SIMULTANEOUS PRODUCTION OF DIHYDROGEN FOR LIQUID HYDORCARBON
SYNTHESIS AS STORED ENERGY," the entire contents of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to processing seawater or
alkaline solutions for repeated recovery of CO.sub.2 to be used as
feedstock to synthesize fuel.
Description of the Prior Art
[0003] Carbon dioxide (CO.sub.2) is reported to be a cause of
climate change and responsible for ocean acidification as the
world's oceans uptake CO.sub.2 by hydrolysis. Ocean acidification
and decline in the oceans' carbonate-ion concentration is
deteriorating coral reefs and impacting shell-forming marine
organisms. (Orr et al., "Anthropogenic ocean acidification over the
twenty-first century and its impact ion calcifying organisms,
Nature, 437, 681-686 (2005) and Hoegh-Guldbergo Mumby et al.,
"Coral Reefs Under Rapid Climate Change and Ocean Acidification,"
Science, 318, 1737-1742 (2007)). The world's oceans contain
approximately 100 mg/L of total CO.sub.2 of which 96% to 97% is
bound as bicarbonate (HCO.sub.3.sup.-). This amounts to
approximately one third of the concentration of a fossil fuel stack
gas effluent on a weight per volume basis. This
bicarbonate/carbonate is responsible for buffering and maintaining
the ocean's pH which is relatively constant below the first 100
meters. (Takahasi et al., "The Alkalinity and Total Carbon Dioxide
Concentration in the World Oceans," Carbon Cycle Modell., Vol. 16;
SCOPE: NY, USA, pp 271-286 (1982) and Takahasi et al., "Carbonate
Chemistry of the Surface of the Waters of the World Oceans,"
Isotope Marine Chemistry; Goldberg, Horibe, Katsuko, eds.; Uchida
Rokakuho: Tokyo, Japan, pp 291-326 (1980)). This dissolved
bicarbonate and carbonate is essentially bound CO.sub.2, and the
sum of these species along with gaseous CO.sub.2, shown in equation
1, represents the total carbon dioxide concentration
[CO.sub.2].sub.T, of seawater.
.SIGMA.[CO.sub.2].sub.T=[CO.sub.2(g)]+[HCO.sub.3.sup.-]+[CO.sub.3.sup.2--
] (1)
[0004] An electrochemical process has been developed and patented
by the Naval Research Laboratory (NRL) that uses pH to exploit
seawater as a means to recover CO.sub.2 from the sea. (Willauer et
al., "Development of an Electrochemical Acidification Cell for the
Recovery of CO.sub.2 and H.sub.2 from Seawater II. Evaluation of
the cell by Natural Seawater," I&EC, 51, 11254-11260 (2012);
Willauer et al., "Feasibility of CO.sub.2 extraction from seawater
and simultaneous hydrogen gas generation using a novel and robust
electrolytic cation exchange module based on continuous
electrodeionization technology," I&EC, 53, 12192-12200 (2014);
U.S. Pat. No. 9,303,323 to DiMascio et al. (Apr. 5, 2016); Willauer
et al., "Development of an Electrolytic Cation Exchange Module for
the Simultaneous Extraction of Carbon Dioxide and Hydrogen Gas from
Natural Seawater," Energy Fuels, 31, 1723-1730 (2017); and U.S.
Pat. No. 9,719,178 to DiMascio et al. (Aug. 1, 2017)). Johnson, et
al. demonstrated that when the pH of seawater is decreased to 6 or
less, carbonate and bicarbonate in the seawater are re-equilibrated
to CO.sub.2 gas (equation 2). (Johnson et al., "Coulometric
TCO.sub.2 Analyses for Marine Studies: An Introduction," Marine
Chem., 16, 61 (1985)).
HCO.sub.3.sup.-+H.sup.+.revreaction.H.sub.2CO.sub.3.revreaction.H.sub.2O-
+CO.sub.2(g).uparw. (2)
[0005] This method has been the basis for standard quantitative
ocean total [CO.sub.2].sub.T measurements for over 25 years.
(Johnson et al., "Coulometric TCO.sub.2 Analyses for Marine
Studies: An Introduction," Marine Chem., 16, 61 (1985)). In
addition to recovery of CO.sub.2 from seawater, NRL's electrolytic
cation exchange module (E-CEM) simultaneously produces hydrogen gas
through electrolytic dissociation of water at the cathode. Carbon
and hydrogen serve as the principle building blocks to synthesis
liquid hydrocarbons to be used as fuel. Water is broken down at the
anode to H.sup.+ and O.sub.2 (equation 3).
At the Anode: 2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (3)
[0006] The O.sub.2 gas is flushed from the anode compartment with
the flow of the anolyte water. The H.sup.+ ions are driven from the
surface of the anode, through the cation-permeable membrane, and
into the center compartment where they replace the Na.sup.+ in the
flowing seawater. This causes the effluent seawater to be acidified
without the need for any additional chemicals. At a seawater pH
less than or equal to 6, the bicarbonate and carbonate in the
seawater are re-equilibrated to carbonic acid (equation 4).
Center Compartment:
2H.sup.++2Na.sup.++2HCO.sub.3.sup.-.fwdarw.2H.sub.2CO.sub.3+2Na.sup.+
(4)
[0007] The CO.sub.2 from the carbonic acid in the effluent
acidified seawater is vacuum stripped by a gas permeable membrane
contactor equation 5. (U.S. Pat. No. 8,663,365 to Willauer et al.
(Mar. 4, 2014)).
Center Compartment Acidified Seawater Effluent:
2H.sub.2CO.sub.3.fwdarw.2H.sub.2O+2CO.sub.2 (5)
[0008] The Na.sup.+ ions from the seawater in the center
compartment are passed through the cation permeable membrane
closest to the cathode. Water is decomposed at the cathode to
H.sub.2 gas and OH.sup.- equation 6.
At the Cathode: 4H.sub.2O+4e.sup.-.fwdarw.4OH.sup.-+2H.sub.2
(6)
[0009] The NaOH and H.sub.2 gas are continuously flushed from the
cathode compartment with the flow of the catholyte water. The
acidified seawater is recombined with the solutions from the
cathode and anode compartment. The overall reaction in equation 7
shows that chemically each mole of HCO.sub.3.sup.- (a weak base) in
seawater is replaced with a mole of OH.sup.- (strong hydroxide
base). (Willauer et al., "Development of an Electrolytic Cation
Exchange Module for the Simultaneous Extraction of Carbon Dioxide
and Hydrogen Gas from Natural Seawater," Energy Fuels, 31,
1723-1730 (2017)).
Overall chemical reaction:
2H.sub.2O+2HCO.sub.3.sup.-.fwdarw..sub.2OH.sup.-+O.sub.2+2H.sub.2+2CO.sub-
.2 (7)
[0010] The industrial state of the art is to use strong alkaline
solutions for the chemical absorption of CO.sub.2 from air or stack
gas into amine-based solvents such as mono-ethanol-amine (MEA). MEA
is highly volatile, highly corrosive, and degrades over time. In
addition, alkaline solutions are very energy intensive because they
cannot be regenerated from carbonate and bicarbonate solution
created by chemical absorption of CO.sub.2. (Yoo et al., J. Envi.
Mang., 53, 512-219 (2013); Mahmoudkhani et al., Inter. J. Green
Gas. Cont., 3, 376-384 (2009); and Baciocchi et al., Chem. Eng.
Proc., 1047-1058 (2006)).
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention provides controlled removal of
bicarbonate and carbonate (in the form of a weak acid,
H.sub.2CO.sub.3) from either natural or synthetic alkaline water
solutions and its replacement with a strong base containing
hydroxide (e.g. sodium hydroxide) that is capable of rapidly
chemically absorbing CO.sub.2 from the atmosphere as, e.g.,
NaHCO.sub.3 solution. NaHCO.sub.3 solution can then be reprocessed
by the electrolytic cation exchange module (E-CEM) to take
advantage of the removal of CO.sub.2 from the air, as an energetic
by-product of E-CEM dihydrogen production. This process is cyclical
in nature, and no chemicals are needed except the initial alkaline
water solution.
[0012] Carbon serves as one of the principle building blocks needed
to synthesize hydrocarbon fuel. Once inorganic carbon (CO.sub.2)
from alkaline water sources is removed, a much stronger alkaline
solution is formed that is capable of re-equilibrating with
CO.sub.2 from the atmosphere by chemical absorption to the alkaline
water that can be subsequently electrolytically reprocessed for
CO.sub.2 recovery. The controlled removal of bicarbonate from
alkaline water and replacement with hydroxide that promotes the
formation of carbonates upon chemical adsorption of CO.sub.2 is a
potential solution to ocean acidification. The feedstocks will
produce renewable hydrocarbons that are superior in performance to
fossil derived hydrocarbons and are drop in replacements for all
current engines.
[0013] Alkaline solutions can be defined in this disclosure to
include artificial solutions of carbonate, bicarbonate, and
hydroxide produced from, e.g., sodium hydroxide, sodium carbonate,
and sodium bicarbonate to be processed by the E-CEM for CO.sub.2
harvesting.
[0014] These and other features and advantages of the invention, as
well as the invention itself, will become better understood by
reference to the following detailed description, appended claims,
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a diagram of an electrolytic cation exchange
module (E-CEM).
[0016] FIGS. 2A and 2B show CO.sub.2 absorption in 0.05 molar NaOH
as a function of time (open circles stirred). FIG. 2A shows how the
pH decreased as a function of time. FIG. 2B shows the measured
CO.sub.2 absorbed in mg/L by coulometry as a function of time.
[0017] FIG. 3 is a carbonate species equilibrium diagram.
[0018] FIGS. 4A and 4B show CO.sub.2 absorption in an electrolytic
cation exchange module (E-CEM) effluent on a laboratory scale. FIG.
4A shows how the pH decreased as a function of time.
[0019] FIG. 4B shows the measured CO.sub.2 absorbed in mg/L as a
function of time.
[0020] FIGS. 5A and 5B show CO.sub.2 absorption in an E-CEM
effluent on a laboratory scale using a larger scale E-CEM. FIG. 5A
shows how the pH decreased as a function of time. FIG. 5B shows the
measured CO.sub.2 absorbed in mg/L as a function of time.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention provides a cyclic process using
alkaline solutions created from electrolytically decarboxylated
water as an atmospheric CO.sub.2 collector followed by repeated
electrochemical recovery of CO.sub.2 with simultaneous production
of dihydrogen for liquid hydrocarbon synthesis as stored energy.
After the inorganic carbon ([CO.sub.2]T) from alkaline water
sources is removed, a much stronger alkaline solution is formed
that is capable of re-equilibrating with CO.sub.2 from the
atmosphere by chemical absorption so that the alkaline water can be
reprocessed electrochemically. This cyclical direct CO.sub.2
capture and recovery from air and simultaneous production of
dihydrogen is the central feature of this invention.
[0022] Seawater (pH 7.4 to 8.4) or any alkaline solution is
processed for repeated recovery of CO.sub.2 to be used as feedstock
to synthesize fuel. The inorganic CO.sub.2 (in the form of
bicarbonate and carbonate) removed from the alkaline solutions is a
weak base and its replacement by the strong hydroxide base results
in the recombined processed water solution.gtoreq.to the pH of the
original alkaline solution. This resulting stronger alkaline
solution is able to re-establish equilibrium with CO.sub.2 from the
atmosphere by chemical absorption at a rapid rate. These
re-equilibrated alkaline solutions can then be reprocessed for
recovery of CO.sub.2 directly from the atmosphere. Strong alkaline
solutions formed by the cathode reaction are able to chemically
absorb CO.sub.2 from the atmosphere. Re-equilibrated cathodic
alkaline solutions can be repeatedly reprocessed for CO.sub.2
recovery from the atmosphere. The entire process requires only a
source of electricity (no chemicals) and alkaline water to produce
the alkaline hydroxide absorbent solutions for recovery of CO.sub.2
from the atmosphere.
[0023] FIG. 1 shows a diagram of the E-CEM, which comprises an
anode, an anode compartment adjacent to the anode, a cation
membrane separating the anode compartment and a center compartment,
another cation membrane separating the center compartment and a
cathode compartment, and a cathode adjacent to the cathode
compartment. An alkaline solution is fed to the center compartment,
water is fed to both the anode compartment and the cathode
compartment, and a source of electricity is applied to the anode.
O.sub.2 is formed in the anode compartment, CO.sub.2 is formed in
the center compartment, and H.sub.2 and NaOH are formed in the
cathode compartment. The CO.sub.2 formed in center compartment and
the H.sub.2 formed in the cathode compartment are removed to be
used as a feedstock to synthesize fuel. The effluent from the
cathode compartment comprising NaOH is collected. This effluent
absorbs CO.sub.2 from the atmosphere to form a re-equilibrated
solution that can be fed back into the center compartment of the
E-CEM.
[0024] To further explain how the effluent solutions are produced
by the E-CEM and the role current has on the process, the following
equations (8-13) are derived using Faraday's constant. Faraday's
constant is defined as the amount of electricity associated with
one mole of unit charge or electron, having the value 96,487
ampere-second/equivalent. For the anode reaction (equation 3),
96,487 A sec will produce 1/4 mole O.sub.2 gas and 1 mole H.sup.+
and for the cathode reaction (equation 6), 96,487 A sec will
produce 1/2 mole H.sub.2 gas and 1 mole OH.sup.-. This allows the
theoretical amount of H.sup.+, OH.sup.-, H.sub.2, and O.sub.2
produced per amp/second of current passed through the electrodes to
be derived as follows:
Anode Reaction _ ( 1 / 4 mole O 2 96 , 487 A - sec ) ( 60 sec min )
= 0.000155 mole O 2 A - min ( 8 ) ( 1 mole H + 96 , 487 A - sec ) (
60 sec min ) = 0.000622 mole H + A - min ( 9 ) Cathode Reaction _ (
1 / 2 mole H 2 96 , 487 A - sec ) ( 60 sec min ) = 0.000311 mole H
2 A - min ( 10 ) ( 1 mole OH - 96 , 487 A - sec ) ( 60 sec min ) =
0.000622 mole OH - A - min ( 11 ) ##EQU00001##
[0025] Therefore, seawater with a HCO.sub.3.sup.- concentration of
142 ppm (0.0023 M) and a flow rate of 1900 milliliters per minute
will require a theoretical applied minimum current of 7.0 A to
lower the pH to less than 6.0 and convert HCO.sub.3.sup.- to
H.sub.2CO.sub.3 (equation 12).
( 0.0023 mole HCO 3 - Liter ) ( 1.89 Liter min ) ( 0.000622 mole H
+ A - min ) = 7.0 A ( 12 ) ##EQU00002##
[0026] The theoretical amount of CO.sub.2 that can be removed from
the acidified seawater is 0.0023 moles per liter. The theoretical
amount of H.sub.2 gas generated at 7.0 A is
( 1 / 2 mole H 2 96 , 487 A - sec ) ( 60 sec min ) ( 7.0 A ) =
0.0022 mole H 2 min ( 13 ) ##EQU00003##
[0027] Equations 8-13 describe the ability to control the removal
of a weak base (HCO.sub.3.sup.-) in the form of a weak acid
H.sub.2CO.sub.3 (equations 4 and 5) from seawater and its
replacement with a strong base using applied current. The strong
base is produced by the primary ionized species in the cathode
effluent water (equation 6) that consists of OH.sup.- and Na.sup.+.
The Na.sup.+ comes from the seawater or alkaline solution processed
in the center compartment of the E-CEM. The Na.sup.+ ion is 8 to 10
times more concentrated in seawater than the other ions present
Mg.sup.2+, Ca.sup.2+, and K.sup.+ on a mole per liter basis.
(Werner et al., "Aquatic Chemistry: An introduction emphasizing
chemical equilibrium in natural waters," Wiley-Interscience, New
York (1970)). The cathode effluent solution or any combination of
anode, cathode, and center compartment effluents become the
CO.sub.2 absorption solutions for atmosphere CO.sub.2 absorption as
shown in equations 14-17. (Yoo et al., "Carbon dioxide capture
capacity of sodium hydroxide aqueous solution," Journal of
Environmental Management, 114, 512-519 (2013)).
NaOH+CO.sub.2.fwdarw.NaHCO.sub.3 (14)
NaHCO.sub.3+NaOH.fwdarw.Na.sub.2CO.sub.2 (15)
Na.sub.2CO.sub.3+CO.sub.2+H.sub.2O.fwdarw.2NaHCO.sub.3 (16)
Overall chemical reaction: CO.sub.2+NaOH.fwdarw.NaHCO.sub.3
(17)
[0028] One mole of NaOH is capable of absorbing up to one mole of
CO.sub.2. The rate and amount of CO.sub.2 absorption into NaOH is
proportional to the NaOH concentration. As shown in equation 17,
the CO.sub.2 is recovered from the atmosphere as NaHCO.sub.3
solution. (Yoo et al., "Carbon dioxide capture capacity of sodium
hydroxide aqueous solution," Journal of Environmental Management,
114, 512-519 (2013)). This demonstrates the chemical completion of
the total cycle.
[0029] FIGS. 2A and 2B demonstrate the absorption of CO.sub.2 into
0.05 molar NaOH solutions left open to the atmosphere in a five
gallon bucket (11.25'' in diameter by 14.25'' high) for 385 hours.
FIG. 2A shows how the pH of both 2 liter solutions decreased as a
function of time and the effect of stirring the solution (open
circles) on absorption was minimal. FIG. 2B provides the measured
CO.sub.2 absorbed in mg/L by coulometry as a function of time in
both 0.05 molar solutions. The simple NaOH solutions provide a
timeframe for CO.sub.2 absorption from the atmosphere that contains
only 0.7 to 0.8 mg of CO.sub.2 per liter of air. These solutions
provide a means of concentrating over 1000 times that of the
atmosphere as shown in FIG. 2B. The speciation diagram shown in
FIG. 3 suggests that Na.sub.2CO.sub.3 is the primary product formed
above pH 10.3. (Fleischer et al., "Detailed Modeling of The
Chemisorption of CO.sub.2 Into NaOH In A Bubble Column," Chemical
Engineering Science, 51, 1715-1724 (1996)). As CO.sub.2 is absorbed
and the pH of the solution begins to fall, bicarbonate is the
primary product.
[0030] FIGS. 4A and 4B demonstrate CO.sub.2 absorption into
combined effluent from the E-CEM process. Key West seawater was
processed through a laboratory E-CEM unit at a flow rate of 140
mL/min and deionized water flowrate to the anode and cathode of 14
mL/min and 12 mL/min. The initial pH of the seawater was 8.03 and
was reduced to 3.6. The effluent solutions were collected at a pH
of 3.6 for approximately 20 minutes. The acidified seawater was
sparged with nitrogen. The solutions were combined and measured at
an initial pH of 10.5 (FIG. 4A) and CO.sub.2 content of 9.66 mg/L
(FIG. 4B). Samples were collected over twenty two hours and it was
found that the pH of the combined effluents reduced to as low as
9.5 (FIG. 4A) and the [CO.sub.2].sub.T measured as high as 33.41
mg/L.
[0031] To determine the effect surface area has on CO.sub.2
re-equilibration in E-CEM effluent, Key West seawater was processed
through a larger scale E-CEM at 0.5 gpm and electrode flowrates up
to 0.042 gpm. The effluent solutions were collected and combined
under three separate experimental conditions. In the first
experiment the initial pH of the seawater before processing was
8.3. The E-CEM effluent streams were collected for 160 minutes.
This equates to five thirty-minute polarity cycles. A polarity
cycle is defined by the length of time an electrode is functioning
as an anode/or cathode before the polarity in the module is
switched and the electrode acting as the anode/or cathode becomes
the cathode/or anode. Polarity switching is a common practice in
the Electrodialysis Reversal (EDR) process to desalinate brackish
ground and surface waters and it is designed into this system to
provide electrode regeneration at regular intervals called polarity
cycles. (Dermentzis, "Continuous electrodeionization through
electrostatic shielding," Electrochim. Acta, 53, 2953-2962
(2008)).
[0032] For over 160 minutes the seawater effluent was sparged with
nitrogen as it exited the E-CEM. The processed streams were
collected in a 120''.times.72''.times.18'' rubber pool and allowed
to re-equilibrate with the atmosphere for sixty-nine hours. Over
the course of sixty-nine hours, 5 mL samples were collected twice a
day to measure CO.sub.2 concentration by coulometry and pH. FIGS.
5A and 5B show that the pH and % CO.sub.2 of the combined processed
water measured 9.3 and 19.0% (Large Pool filled black circles).
After sixty-nine hours the [CO.sub.2].sub.T content increased to
55% and the pH dropped to 9.0.
[0033] In the second experiment effluents processed by the E-CEM
were collected in three separate pools 5 feet in diameter (Pool 1
closed squares, Pool 2 closed triangles, Pool 3 open squares). The
first and third pool collected E-CEM effluents over 160 minutes.
The second pool was filled with effluents collected over 80
minutes. The third pool had a circulating pump that recirculated
the water at 75 gph. The processed seawater effluent in each pool
was sparged with nitrogen as it exited the E-CEM. The initial
seawater pH before processing measured 7.8. FIGS. 5A and 5B show
the combined effluent pH initially measured approximately 9 for
each pool. Samples were collected from each pool once a day over
219 hours and the pH decreased to 8.4 for Pools 1 and 3 and to 7.9
for Pool 2. The % CO.sub.2 for the combined effluents initially
measured 16.7% Pool 1, 11.1% Pool 2, and 7.7% Pool 3. These values
increased over 219 hours to 66.3%, 71.9%, and 69.0%.
[0034] The third experiment collected effluent from the E-CEM for
two polarity cycles in a 32 gallon bucket that was 1.6 feet in
diameter. This container had the smallest surface to volume ratio
of all three experiments. The initial pH and % CO.sub.2 of the
combined effluents from the E-CEM measured 9.2 and 7.7%. The final
pH and % CO.sub.2 measured 8.5 and 29.1% after 219 hours.
[0035] Comparing all three experiments in FIGS. 5A and 5B shows
that the Large Pool (Large Pool filled black circles) had the
fastest rate of CO.sub.2 re-equilibration based on the slope. Pool
2 recorded the most % CO.sub.2 recovery at 71.9%. Finally the %
CO.sub.2 re-equilibrated for the bucket and the large pool
highlight the role surface to volume ratio has on re-equilibration
of CO.sub.2 from the atmosphere into E-CEM effluent.
[0036] Since the primary ionized species in the cathode effluent
water (equation 6) consist of OH.sup.- and Na.sup.+, the catholyte
effluent solution was collected separately and the pH measured over
two twenty minute polarity cycles. Table 1 summarizes the pH of the
effluent catholyte as 60 gpd of freshwater was fed to each
electrode compartment during each cycle. The average pH of the
catholyte solution measured 12.5 over both polarity cycles. The
concentration of NaOH at these pH levels is calculated to be 0.038
mol/L. A portion of the catholyte solution at this high pH could be
used to capture CO.sub.2 from the atmosphere while the rest is used
to mix with the effluent seawater to bring the total pH to 7 or
greater. To enhance absorption of CO.sub.2 from the atmosphere, the
catholyte was passed through a column and treated with air at a
flow rate of 1 lpm and a space time of 2 minutes. Under these
conditions the pH dropped to an average of 12.32 (Table 1).
TABLE-US-00001 TABLE 1 Catholyte Measured Every Five Minutes at 60
gpd Flowrate to Each Electrode Polarity A Polarity B pH pH Min Amp
pH w/air pH w/air 20 30.0 12.57 12.36 12.60 12.51 15 30.0 12.45
12.34 12.58 12.43 10 30.0 12.49 12.29 12.57 12.17 5 30.0 12.43
12.32 12.55 12.39 0 30.0 12.44 12.24 12.57 12.10
[0037] When the flowrates to the electrodes were lowered to 30 gpd,
Table 2 shows the final pH of the catholyte solution measured an
average of 13.5 which equates to 0.5 mol/L of NaOH available for
absorption of CO.sub.2 from the atmosphere. At these lower
electrode flowrates the catholyte solution is more concentrated
causing an increase in pH by approximately 1 pH unit. At these
flowrates the catholyte was passed through a PVC column and treated
with air at a flow rate of 4 lpm at a space time of 16 minutes. The
pH of the catholyte dropped to an average of 13.2.
[0038] These experiments highlight the significance that electrode
flowrate has on the pH of the effluent catholyte stream and NaOH
concentration of the stream. The air bubbling results suggest that
more detailed kinetic studies of dilute CO.sub.2 absorption into
basic hydroxide solutions are needed.
TABLE-US-00002 TABLE 2 Catholyte Collected in Five Gallon Bucket at
30 gpd Flowrate to Each Electrode pH pH w/air Polarity A 13.51
13.21 Polarity B 13.46 13.23
[0039] As shown by these experiments, the higher the catholyte
flowrate into the E-CEM, the lower the pH of the effluent catholyte
solution. Also, the higher current applied to the E-CEM, the higher
the pH of the effluent catholyte solution.
[0040] The above descriptions are those of the preferred
embodiments of the invention. Various modifications and variations
are possible in light of the above teachings without departing from
the spirit and broader aspects of the invention. It is therefore to
be understood that the claimed invention may be practiced otherwise
than as specifically described. Any references to claim elements in
the singular, for example, using the articles "a," "an," "the," or
"said," is not to be construed as limiting the element to the
singular.
* * * * *