U.S. patent application number 12/946537 was filed with the patent office on 2011-03-10 for laminar scrubber apparatus for capturing carbon dioxide from air and methods of use.
Invention is credited to Klaus S. Lackner, Allen Wright.
Application Number | 20110056382 12/946537 |
Document ID | / |
Family ID | 35968210 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110056382 |
Kind Code |
A1 |
Lackner; Klaus S. ; et
al. |
March 10, 2011 |
LAMINAR SCRUBBER APPARATUS FOR CAPTURING CARBON DIOXIDE FROM AIR
AND METHODS OF USE
Abstract
The present invention is directed to methods for carbon dioxide
from air, which comprises exposing solvent covered surfaces to air
streams where the airflow is kept laminar, or close to the laminar
regime. The invention also provides for an apparatus, which is a
laminar scrubber, comprising solvent covered surfaces situated such
that they can be exposed to air streams such that the airflow is
kept laminar.
Inventors: |
Lackner; Klaus S.; (Dobbs
Ferry, NY) ; Wright; Allen; (Tucson, AZ) |
Family ID: |
35968210 |
Appl. No.: |
12/946537 |
Filed: |
November 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12555874 |
Sep 9, 2009 |
7833328 |
|
|
12946537 |
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Current U.S.
Class: |
96/329 |
Current CPC
Class: |
Y02C 10/06 20130101;
Y02C 20/40 20200801; Y02A 50/2342 20180101; B01D 53/62 20130101;
B01D 53/18 20130101; Y02C 10/04 20130101; Y02A 50/20 20180101; B01D
53/1475 20130101 |
Class at
Publication: |
96/329 |
International
Class: |
B01D 47/02 20060101
B01D047/02 |
Claims
1: A scrubber apparatus for capturing carbon dioxide from air,
comprising: a plurality of plates, each of the plates including
front and back surfaces and top and bottom ends, the plates
positioned parallel to one another so as to form gaps therebetween;
a catch tray positioned adjacent the bottom end of the plates; a
pump in fluid communication with a source of carbon dioxide
solvent; a conduit in fluid communication with the pump and at
least the top ends of the plates; and solvent wherein the plates,
catch tray, pump, and conduit are configured such that the plates
can be exposed directly to the air and the carbon dioxide solvent
can be applied to the top ends of the plates so as to flow across
the front and back surfaces of the plates in a direction towards
the bottom ends, and such that, when air flows through the gaps or
otherwise adjacent the plates and contacts the front and back
surfaces covered with carbon dioxide solvent, carbon dioxide in the
air is absorbed by the carbon dioxide solvent and absorbed carbon
dioxide and carbon dioxide solvent flows down the plates and into
the catch tray.
2: An apparatus according to claim 1, further comprising airflow
straighteners for directing the air flow so that it is
substantially perpendicular to a direction of flow of the
solvent.
3: An apparatus according to claim 2, wherein the straighteners are
configured so as to cause the air to flow substantially
horizontally through the gaps.
4: An apparatus according to claim 1, wherein the solvent is
adapted to remove carbon dioxide from open air under ambient
conditions.
5: An apparatus according to claim 4, wherein the solvent is a
hydroxide solution having a hydroxide concentration of about from
about 0.1 molar to about 20 molar.
6: An apparatus according to claim 1, wherein the conduit and the
pump are configured to recycle solvent from the catch tray to the
top end of the plates.
7: An apparatus according to claim 1, wherein the front and back
surfaces include one or more of grooves, dimples, bumps, and other
surface structures that are smaller than the gaps and are
configured so that air flowing through the gaps remains within the
laminar boundary of air flow.
8-14. (canceled)
15: A scrubber apparatus for capturing carbon dioxide from open
air, comprising: a plurality of plates, each of the plates
including front and back surfaces and outer edges, the plates
positioned parallel to one another so as to form gaps therebetween;
a substantially horizontal axis joined to each of the plates
substantially close to its center; a mechanism for rotating the
axis so as to cause the plates to rotate; and a catch tray
configured to contain a carbon dioxide solvent, the catch tray
being positioned beneath the plates so that portions of each of the
plates are at least temporarily submerged in the solvent as the
plates rotate thereby coating portions of each of the plates with
the solvent.
16: An apparatus according to claim 15, further comprising airflow
straighteners for directing the air flow so as to cause the air to
flow substantially horizontally as it flows through the gaps.
17: An apparatus according to claim 15, wherein the solvent is
adapted to remove carbon dioxide from open air under ambient
conditions.
18: An apparatus according to claim 17, wherein the solvent is a
hydroxide solution having a hydroxide concentration of about from
about 0.1 molar to about 20 molar.
19: An apparatus according to claim 15, wherein the plates are
disk-shaped.
20: An apparatus according to claim 15, wherein the front and back
surfaces include one or more of grooves, dimples, bumps, and other
surface structures that are smaller than the gaps and are
configured so that air flowing through the gaps remains within the
laminar boundary of airflow.
Description
CROSS REFERENCE TO PRIOR APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/603,121, filed Aug. 20, 2004, and U.S.
Nonprovisional application Ser. No. 11/207,236, filed Aug. 19,
2005, both of which are incorporated by reference as if disclosed
herein in their entirety.
[0002] This patent disclosure contains material that is subject to
copyright protection. The copyright owner has no objection to the
facsimile reproduction by anyone of the patent document or the
patent disclosure as it appears in the U.S. Patent and Trademark
Office patent file or records, but otherwise reserves any and all
copyright rights.
FIELD OF THE INVENTION
[0003] The present invention relates generally to the field of
extractors, including those that work to extract carbon dioxide.
The present invention relates to carbon dioxide (CO.sub.2) removal
under ambient conditions from the open air without heating or
cooling the air.
BACKGROUND OF THE INVENTION
[0004] Extracting carbon dioxide from ambient air would make it
possible to use carbon based fuels and deal with the greenhouse gas
emissions after the fact. Since CO.sub.2 is neither poisonous nor
harmful in parts per million quantities but creates environmental
problems simply by accumulating in the atmosphere, it is possible
to remove carbon dioxide from air in order to compensate for an
equally sized emission elsewhere and at different times. The
overall scheme of air capture has been described elsewhere.
[0005] The production of carbon dioxide (CO.sub.2) occurs in a
variety of industrial applications, such as the generation of
electricity by burning coal in power plants. Flue gas from
coal-burning power plants typically contains a high percentage of
nitrogen, about 13% CO.sub.2, about 3% oxygen, about 10% water and
less than 1% of various pollutants. To sequester CO.sub.2 during
the operation of coal burners in power plants, CO.sub.2 must be
separated from the flue gas, which is hot, e.g., temperatures from
about 200.degree. C. to about 1000.degree. C. depending on its
specific locations in the flue gas lines of the coal-burning power
plant. In a carbon constrained world, central sources of CO.sub.2
like power plants are likely to capture their own CO.sub.2 from the
power plant stack.
[0006] Hydrocarbons are typically the main components of fuels that
are combusted in combustion devices, such as engines. Exhaust gas
discharged from such combustion devices contains carbon dioxide
gas, which at present is simply released to the atmosphere.
However, as greenhouse gas concerns mount, carbon dioxide emissions
from all sources will have to be curtailed.
[0007] Scrubber designs for separating CO.sub.2 from air already
exist, but they are limited to packed bed type implementations
whose goal is typically to remove all traces of an impurity from
another gas. The disadvantages in the art are addressed and
overcome by the carbon dioxide separation membranes and methods of
use thereof as embraced by the present invention.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to methods for removing
carbon dioxide from air, which comprises exposing solvent covered
surfaces to air streams where the airflow is kept laminar, or close
to the laminar regime. The invention also provides for an apparatus
which is a laminar scrubber, comprising solvent covered surfaces
situated such that they can be exposed to air streams such that the
airflow is kept laminar. The following descriptions of the
invention include many embodiments and aspects, all of which can be
attributable to either the method or the apparatus claimed, even if
not so explicitly stated.
[0009] Capture of carbon dioxide on board of a vehicle while
possible in principle is not practical because of the large amount
of weight involved. Therefore our invention aims at capturing
carbon dioxide from the air at a later time. The purpose of the
removal of carbon dioxide from the air is to balance out the carbon
dioxide emission resulting from the operation of vehicle. While the
most obvious sources of carbon dioxide emissions that could be
remedied by this invention are those for which it would be
difficult or impossible to capture the CO.sub.2 at the point of
emission, the invention is not restricted to such sources but could
compensate for any source as well. Indeed this approach of carbon
dioxide mitigation could be used to lower the atmospheric
concentration of CO.sub.2.
[0010] Efficient capture of carbon dioxide from air requires a
sorbent that can absorb CO.sub.2 with minimum energy costs.
Processes that heat or cool the air, or that change the pressure of
the air by substantial amounts will be energetically
disadvantaged.
[0011] The apparatus consists of a scrubber design which provides
essentially straight flow paths for the air that is blowing through
the device. Sorbent covered surfaces are within millimeters to
centimeters of the flow path of every air parcel. The simplest
embodiment is a set of flat plates with the air moving through the
gaps between the plates and the sorbent flowing over the surfaces.
In the simplest design these plates stand upright so that wetting
of both surfaces can be performed with equal ease. However a
variety of other designs described below can vary from this simple
design. These include but are not limited to corrugated surfaces,
concentric tubes etc.
[0012] In one aspect of the invention, the surfaces are smooth
parallel plates. In another aspect, the surfaces are not entirely
flat, but follow straight parallel lines in the direction of the
airflow. Examples include but are not limited to corrugations,
pipes or tubes, angular shapes akin to harmonica covers. The
invention provides for methods where the surfaces are roughened
with grooves, dimples, bumps or other small structures that are
smaller than the surface spacing and that remain well within the
laminar boundary of the air flow, i.e., the Reynolds number of the
flow around these dimples is small, in an optimum it is between 0
and 100.
[0013] The present invention is directed to implementations of the
above method where surface roughening has been obtained through
sand blasting or other similar means. In one aspect of the
invention, the surface roughening can be obtained by etching.
[0014] In another aspect of the invention, the apparatus contains
surfaces that are part of plates made from steel or other hydroxide
resistant metals. In one aspect of the invention, the plates are
made from glass. In another aspect, the plates are made from
plastics, including but not limited to polypropylene.
[0015] In yet another aspect of the invention, the surfaces are
foils or other thin films that are held taut by wires and supported
by taut wire or wire netting. The invention provides for an
apparatus where all but a supporting wire in the front and the back
run parallel to the wind flow direction. In one aspect, the films
are supported on a rigid structure. For example, the rigid
structure can be a solid plate, a honeycomb, or latticework that
can lend structural rigidity to the films. The invention is not
limited to these examples.
[0016] The invention also provides for an apparatus and method
where the films are made from plastic foils. The invention provides
for an apparatus and method where the plastic foil has been surface
treated to increase the hydrophilicity of the surface. Such
treatments can be state of the art or represent novel treatments.
In another aspect of the invention, an apparatus or method is
provided where surfaces have been coated or treated to increase
hydrophilicity of the plates.
[0017] The method or apparatus of the invention further provides
that the direction of the airflow is horizontal. The method or
apparatus of the invention provides that the surfaces--or the line
of symmetry of the surfaces--is vertical. The invention provides
for where the liquid solvent flow is at right angle to the airflow.
The invention provides for a method and an apparatus where the
surface spacing is between 0.3 centimeters (cm) and 3 cm. In
another embodiment, the surface length at right angle to the
airflow direction is between 0.30 m to 10 m. In another embodiment,
the airflow speed is between 0.1 meters per second (m/s) and 10
m/s. In another embodiment, the distance of airflow between the
surfaces is between 0.10 m and 2 m.
[0018] In one embodiment of the invention, liquid solvent is
applied by means of spraying a flow onto the upper edge of the
surface. In another embodiment, the solvent is applied to both
sides of the plates. In another embodiment, the solvent is applied
in a pulsed manner. In another embodiment, the liquid solvent is
collected at the bottom of the surfaces or plates in a catch
tray.
[0019] In another embodiment of the method and apparatus, the
collected fluid or CO.sub.2 solvent is immediately passed on to a
recovery unit. In another embodiment, the collected fluid is
recycled to the top of the scrubbing unit for additional CO.sub.2
collection.
[0020] In another embodiment of the method or apparatus of the
invention, the apparatus is equipped with airflow straighteners to
minimize losses from misalignment between the surfaces and the
instantaneous wind field.
[0021] In another embodiment of the method or apparatus of the
invention, the apparatus is equipped with mechanisms that either
passively or actively steer the surfaces so that they point into
the wind.
[0022] In another embodiment of the method or apparatus of the
invention, the laminar wind scrubber utilizes pressure drops
created by natural airflows. In one embodiment, the pressure drops
created by natural airflows include, but are not limited to: (a)
wind stagnation in front of scrubber; (b) pressure drops created by
flows parallel to the entrance and/or exit into the scrubbers; (c)
pressure drops created by thermal convection as for example in a
cooling tower or by thermal convection along a hill side.
[0023] In one embodiment of the method or apparatus, the surfaces
are rotating disks where wetting is helped by the rotary motion of
the disks, and the air is moving at right angle to the axis. In one
embodiment of the method or apparatus, the axis is approximately
horizontal and the disks dip into the solvent at their rim and the
circular motion promotes distribution of the fluid on the
disks.
[0024] In another embodiment, the liquid is sprayed onto the disk
as it is moved by a radially aligned injector. In another
embodiment, the liquid is extruded onto the disk near the axis.
[0025] In another embodiment of the invention, the surfaces are
concentric tubes of circular or other cross-section shape with the
air flowing in the direction of the tube axis. In another
embodiment, the tubes rotate around the center axis. In one
embodiment of the invention, the tube axis is oriented
approximately vertically and solvent is applied in a manner that it
flows downward on the surfaces of the tube. In another embodiment,
the axis is at some angle to the vertical and the solvent is
inserted at a single point at the upper opening and flows downward
in a spiral motion covering the entire surface.
[0026] In one embodiment of the invention, the solvent used in the
apparatus and in the method is a hydroxide solution. In one aspect,
the hydroxide concentration is from about 0.1 molar to about 20
molar. In another embodiment, the hydroxide concentration is from
about 1 molar to about 3 molar. In one embodiment, the
concentration of the solution exceeds 3 molar. In another aspect of
the invention, the concentration of the solution has been adjusted
to minimize water losses or water gains. In another embodiment of
the invention, the concentration of the solution is allowed to
adjust itself until its vapor pressure matches that of the ambient
air.
[0027] In one embodiment, the hydroxide is sodium hydroxide. In
another embodiment, the hydroxide is potassium hydroxide. In
another embodiment, the solvent is a hydroxide solution where
additives or surfactants have been added. In a further embodiment,
the additives or surfactants work to increase the reaction kinetics
of CO.sub.2 with the solution. Without limitation, such additives
could be state of the art or improvements on the art. In one
embodiment, the additives are intended to reduce the water vapor
pressure over the solution. Such additives could be state of the
art or improvements on the art. In a further embodiment of the
invention, the additives or surfactants change the viscosity or
other rheological properties of the solvent. In one aspect of the
invention, the additives or surfactants improve the absorption
properties of the solvent to scrub gases other than CO.sub.2 from
the air (e.g. ozone). In another embodiment, the method or
apparatus combines additives that create all or part of the
properties disclosed hereinabove.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Additional aspects, features and advantages afforded by the
present invention will be apparent from the detailed description
and exemplification hereinbelow, taken in conjunction with the
accompanying drawings wherein like numerals depict like parts, and
wherein:
[0029] FIG. 1 is an end view, and FIG. 2 is a side elevational view
of a scrubber apparatus in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention is directed to methods and devices to
capture carbon dioxide by absorption into a strongly alkaline
solution. Although general for sorbent recovery already exist, the
present invention includes an apparatus designed to expose alkaline
fluids to atmospheric air where these fluids absorb CO.sub.2.
[0031] An apparatus that performs this task is the first in a
series of modules that together provide air capture capabilities.
The system discussed here differs from previous CO.sub.2 scrubber
designs in that it is optimized for capturing carbon dioxide from
air rather than scrubbing air clean of CO.sub.2. As a result
uniform extraction from the air or maximum reduction of the
CO.sub.2 content of the air are not at issue, what matters is
maximizing the rate of CO.sub.2 uptake by the sorbent fluid.
[0032] Such technology would provide the ability of delivering
gasoline, diesel or other carbonaceous fuels that are effectively
carbon neutral because already prior to their combustion an amount
of CO.sub.2 has been removed from the air that matches their
ultimate emission. Similarly it is possible to compensate for the
emissions of a car or any other vehicle including airplanes by
removing the amount of CO.sub.2 that will be emitted over the
lifetime of the vehicle before or shortly after their introduction
to the market.
One Embodiment
Description of an Air Scrubber Unit
[0033] The purpose of an air scrubber unit is to remove CO.sub.2
from an airflow that is maintained by a low-pressure gradient. Air
scrubber units could also capture other gases present in the air.
Typical pressure gradients are such that they could be generated by
natural airflows. Pressure drops across the unit range from nearly
zero pressure to a few hundreds of Pascal, a preferred range is
from 1 to 30 Pa and an optimal range may be from 3 to 20 Pa.
However, we explicitly state that we do not limit our claim to
units that are exclusively wind driven. We also consider the use of
fans either with or without ductwork to guide the air and we
explicitly consider units that are driven by convection.
[0034] Flow velocities through the scrubber unit may range from
virtually stagnant to a few tens of meters per second. A preferred
range would be from 0.5 to 15 m/s an optimal range for wind driven
systems ranges from 1 m/sec to 6 m/sec.
[0035] The apparatus of the invention in one embodiment comprises a
flat, hydroxide coated, surfaces approximately centimeters apart.
These large flat sheets are referred to as lamellae. In one
embodiment, a single lamella is bound by two sheets covered in
hydroxide solution. Air flows between the sheets and parallel to
their surfaces. A set of lamellae form a complete and independent
unit, which is referred to herein as a scrubber cell. The typical
depths of these surfaces or lamellae range from tens of centimeters
to a few meters and the height can vary from tens of centimeter to
many meters.
[0036] The surfaces could be made from solid plates, light-weight
mesh like structures covered with thin membranes or films, or from
thin films that are held in place with wire mesh structure.
[0037] There is quite some flexibility in the overall design, but
the following are important design features that distinguish this
approach from others:
[0038] 1) Plate structures are smooth in the direction of the
airflow on scales of the plate separation. (However, incidental or
engineered structures on a much finer scale may be used to improve
the CO.sub.2 transport coefficient.) Variations in shape that are
at right angles to the air flow are of relatively little concern,
as long as they do not interfere with the efficient wetting of the
plates, sheets or surfaces.
[0039] 2) The surfaces are held in place sufficiently tightly or
rigidly for their flexing or flapping not significantly to reduce
pressure variations between the lamellae.
[0040] 3) Flow through openings in the surfaces is inhibited so
that it cannot significantly reduce pressure variations between the
lamellae.
[0041] 4) The spacing between the lamellae is chosen such that the
system does not transition out of the laminar flow or at least does
not deviate much from that regime.
[0042] 5) The depth of the membrane units is kept short enough to
avoid nearly complete depletion of the air in the front part of the
unit.
[0043] 6) For utilization of both sides of the plates it is
preferable to arrange the surfaces vertically. However, deviations
from such a design could be considered for other flow
optimizations.
[0044] 7) The height of the lamella is chosen to optimize wetting
properties of the surfaces and to minimize the need for
reprocessing the fluid multiple times.
[0045] Applying liquid solvent to the surfaces could follow
established state of the art approaches, e.g. spray nozzles, liquid
extrusion. It also could be optimized using less conventional
approaches. One aspect of this invention is directed to one
specific approach where the apparatus includes a laminar flow
design that exposes solvent covered surfaces to air streams.
[0046] The apparatus of this invention can be designed in various
ways so long as it is able to perform the functions described
herein. For example, designs could wet vertical surfaces near the
top and let gravity run the fluid over the surface until the entire
area is covered. Alternatively, the surfaces could be shaped as
flat disks which are wetted as they rotate. The motion would
distribute the liquid along these surfaces.
[0047] Examples of designs that are meant as illustration rather
than an exhaustive description include
[0048] 1) flat rectangular surfaces or plates aligned parallel to
each other
[0049] 2) Corrugated surfaces that are lined up parallel to each
other
[0050] 3) Flat disks rotating around a center axis with the air
flowing at right angle to the axis of rotation. Liquid could be
applied by the wheels dipping into fluid near the bottom of the
motion. The standing liquid may only cover the outer rim of the
disks or reach all the way to the axle. Alternatively liquid may be
injected onto the rim by liquid wetting near the axle and flowing
around the disk due to gravity and rotary motion.
[0051] 4) Concentric tubes or similar shapes where air would be
blowing along the tube axis.
[0052] 5) Such tubes could be arranged vertically for counterflow
designs with wetting initiated at the upper rim or,
[0053] 6) nearly horizontally with liquid entering at one end and
one point and getting distributed through a slow rotating motion of
the tubes.
[0054] Referring now to FIGS. 1 and 2, there is illustrated a
scrubber apparatus 10 comprising a plurality of substantially
horizontal concentric tubes 12 each including first and second ends
14, 16. Ends 14, 16 are open so as to allow a flow of open air to
pass through the tubes and directly contact the tube inner surfaces
18. The tubes are mounted for rotation around their center axis 20,
driven by a drive mechanism 22. A CO.sub.2 solvent is applied to
the surfaces 18 of the tubes, pumped from a supply by a pump 24
though a conduit 26. By arranging the tubes 12 nearly horizontal, a
liquid may be introduced through one open end, at one point, and
distributed over the inner surfaces 18 of the tube through a slow
rotating movement of the tube, and finally drained out of the other
end of the tube into catch tray 28.
[0055] If desired, air flow guides 30 may be provided to direct the
air flow substantially horizontally through the tubes, and
preferably for directing the air flow substantially parallel to the
long axis of the tubes 18.
[0056] Also, if desired, a fluid collected in catch tray 28 may be
recycled via conduit 32 to pump 24 and conduit 26.
[0057] If desired, surface 18 of tubes 12 may be provided with one
or more grooves, dimples, bumps or other surface structures 34 so
that air flowing through the tube is remained within the laminar
boundary of air flow.
[0058] Solvents that absorb CO.sub.2 span a wide variety of
options. Here we focus on aqueous hydroxide solutions. These would
tend to be strong hydroxide solutions above 0.1 molar and up to the
maximum possible level (around 20 molar).
[0059] Solvents must wet the surfaces of the scrubber. To this end
we consider various means known in the art. These include surface
treatments that increase hydrophilicity, surfactants in the solvent
and other means.
[0060] Hydroxides could be of a variety of cations. Sodium
hydroxide and potassium hydroxides are the most obvious, but others
including organic sorbents like MEA, DBA etc. are viable
possibilities.
[0061] Hydroxides need not be pure, they could contain admixtures
of other materials that are added to change or modify various
properties of the solvent. For example, additives may improve on
the reaction kinetics of the hydroxide with the CO.sub.2 from the
air. Such catalysts could be surfactants or molecules dissolved in
the liquid. Additions of organic compounds like MEA are just one
example. Other additives may help in reducing water losses by
making the solution more hygroscopic. Yet other additives may be
used to improve the flow or wettability characteristic of the fluid
or help protect the surfaces from the corrosive effects of the
hydroxide solution.
Wind Collection with Hydroxide Solvents
[0062] The rate of uptake of CO.sub.2 into a strong hydroxide
solution has been well studied [REFS] and we are using the result
of these studies to design a device that will pull CO.sub.2
directly out of a natural wind flow or out of a flow subject to a
similar driving force, e.g. a thermally induced convection.
[0063] CO.sub.2 uptake into a strong hydroxide solution involves a
chemical reaction that greatly accelerates the dissolution process.
The net reaction is
CO.sub.2(dissolved)+2OH.sup.-.fwdarw.CO.sub.3.sup.--+H.sub.2O
(1)
[0064] There are several distinct pathways by which this reaction
can occur. The two steps that are relevant at high pH are
CO.sub.2(dissolved)+OH.sup.-.fwdarw.HCO.sub.3.sup.- (2)
followed by
HCO.sub.3.sup.-+OH.sup.-.fwdarw.CO.sub.3.sup.-+H.sub.2O (3)
[0065] The latter reaction is known to be very fast, the first
reaction on the other hand proceeds at a relatively slow rate. The
reaction kinetics for reaction (2) is described by
t [ CO 2 ] = K [ OH - ] [ CO 2 ] ##EQU00001##
[0066] Hence the time constant describing the reaction kinetics
is
.tau. = 1 K [ OH - ] ##EQU00002##
[0067] The rate constant K has been measured. At 20.degree. C. and
infinite dilution,
.kappa.=5000 liter mol.sup.-1s.sup.-1=5 m.sup.3
mol.sup.-1s.sup.-1
[0068] The ionic strength correction is given by
.kappa.=.kappa..sub..infin.10.sup.0.13A
[0069] At high concentration of CO.sub.2 in the gas, the rate of
reaction (2) limits the rate of uptake, even though the time
constant for a one molar solution at 0.14 ms is quite short.
[0070] Following standard chemical, engineering models, e.g.
Dankwert or Astarita, one can describe the transfer process in
which a gas component is dissolved or chemically absorbed into a
solvent with a standard model that combines a gas-side flow
transfer coefficient and a liquid side transfer coefficient to
describe the net flow through the interface, The total flux is
given by
F=.kappa..sub.G(.rho.(.chi.=-.infin.)-.rho.(.chi.=0))=.kappa..sub.L(.rho-
.'(.chi.=0)-.rho.'(.chi.=.infin.)
where .rho. and .rho.' are the molar concentrations of CO.sub.2 in
the gas and in the solution respectively. The parameter x
characterizes the distance from the interface. Distances into the
gas are counted negative. At the boundary Henry's law applies
hence
.sigma.(0)=K.sub.H.rho.(0)
Expressed as a dimensionless factor, K.sub.H=0.7.sup.1. .sup.1 Note
that typically, Henry's constant has dimensions, as concentrations
on the gas side are measured as partial pressure, i.e., in units of
Pascal or units of atmospheres (atm), whereas the liquid side
concentrations are typically measured as moles per liter. Thus a
typical unit would be liter/mol/atm.
[0071] For the gas side the transfer constant can be estimated
as
K G = D G .LAMBDA. ##EQU00003##
where A is the thickness of the laminar sublayer that forms on the
surface of the interface. The thickness of this layer will depend
on the geometry of the flow and on the turbulence in the gas flow.
For purposes of this discussion we consider it as given. Our goal
is to determine the optimal choice for A.
[0072] For a fluid package, the standard approach to estimating the
transfer coefficient assumes a residence time .tau..sub.D for the
parcel on the surface of the fluid. This time results from the flow
characteristic of the solvent and it includes surface creation and
surface destruction as well as turbulent liquid mixing near the
surface.
[0073] Since diffusion in the time .tau..sub.D can mix the
dissolved CO.sub.2 into a layer of thickness .lamda.= {square root
over (D.tau..sub.D)}, the flux from the surface is given by
F = D L .differential. .rho. ' .differential. .chi.
##EQU00004##
[0074] Approximating the gradient by
.differential. .rho. .differential. .chi. = .rho. ' ( 0 ) - .rho. '
( .infin. ) .lamda. ##EQU00005##
Shows that for a diffusion driven absorption process
K L = D L .lamda. = D L .tau. D ##EQU00006##
[0075] Here D.sub.L is the diffusion rate of CO.sub.2 in the
solvent.
[0076] In the presence of a fast chemical reaction where the
reaction time .tau..sub.R<<.tau..sub.D, the layer that
absorbs CO.sub.2 is characterized by this shorter time, hence the
transfer coefficient is given by
K L = D L .tau. R ##EQU00007##
[0077] In the presence of a chemical the transfer coefficient is
increased therefore by a factor
.tau. D .tau. R ##EQU00008##
[0078] However, this enhancement can only be maintained if the
supply of reactant in the solvent is not limited. In the case of
carbon dioxide neutralizing a hydroxide solution, it is possible to
deplete the hydroxide in the boundary layer. The layer thickness
.lamda. contains an a real density of hydroxide ions of
.rho..sub.OH.sup.-.lamda. and the rate of depletion is 2
K.sub.L.rho.'.sub.CO.sup.2. Thus for the fast reaction limit (eqn.
x) to apply,
.rho. OH - 2 .rho. ' CO 2 .tau. R .tau. D << 1
##EQU00009##
[0079] In our case,
.rho. OH - .tau. R = 1 K ##EQU00010##
[0080] Hence the condition can be rewritten as
2.rho.'.sub.CO.sub.2' .kappa..tau..sub.D>I
[0081] The critical time for transitioning from fast reaction
kinetics to instantaneous reaction kinetics is approximately 10 sec
for ambient air. The transition does not dependent on the hydroxide
concentration in the solution. However, once past the transition,
the rate of uptake is limited by the rate at which hydroxide ions
can flux to the surface. It is therefore lower than in the fast
limit, and the CO.sub.2 flux is given by
F = 1 2 D OH - .tau. D .rho. OH - 2 .rho. CO 2 ' ##EQU00011##
By forcing F into the form in equation x, we find that
K L = D OH - D L .rho. OH - 2 .rho. CO 2 ' ##EQU00012## K L 0 = D
OH - .tau. D .rho. OH - 2 .rho. CO 2 ' ##EQU00012.2##
[0082] Here K.sub.L.sup.0 is the transfer coefficient in the
absence of chemical reactions. In the instantaneous regime the flux
is independent of the CO.sub.2 concentration in the boundary
layer.
[0083] The flux can be characterized by an effective transfer
coefficient, which can be written as
F=.kappa..sub.eff(.rho..sub.CO.sub.2-.rho.'.sub.CO.sub.2/K.sub.H
[0084] Here the molar concentrations are for the asymptotic values
in the far away gas and far away liquid. In the case of hydroxide
solutions, the latter is zero. Hence,
F = .kappa. eff .rho. CO 2 ##EQU00013## and ##EQU00013.2## .kappa.
eff = ( 1 K G ) + ( 1 K L + K H ) - 1 ##EQU00013.3##
[0085] An optimal design is close to the border between gas side
limitation and liquid side limitation. Therefore, we establish a
design value for the air side boundary thickness A.
.LAMBDA. .apprxeq. D G D L / .tau. R ##EQU00014##
[0086] This is approximately 4 mm for air based extraction of
CO.sub.2.
[0087] These constraints together very much limit a practical
design. For a 1 molar solution, the total flow has been measured as
6.times.10.sup.-5 mol m.sup.-2s.sup.-1, which translates into an
effective value of 0-4 cm/s which is close to the theoretical
value.
[0088] All patent applications, published patent applications,
issued and granted patents, texts, and literature references cited
in this specification are hereby incorporated herein by reference
in their entirety to more fully describe the state of the art to
which the present invention pertains.
[0089] As various changes can be made in the above methods and
compositions without departing from the scope and spirit of the
invention as described, it is intended that all subject matter
contained in the above description, shown in the accompanying
drawings, or defined in the appended claims be interpreted as
illustrative, and not in a limiting sense.
* * * * *