U.S. patent application number 12/745632 was filed with the patent office on 2011-04-14 for scrubber for removing heavy metals from gases.
This patent application is currently assigned to YISSUM RESEARCH DEVELOPMENT COMPANY OF THE HEBREW UNIVERSITY OF JERUSALEM. Invention is credited to Zach Barnea, Mandan Chidambaram, Yoel Sasson.
Application Number | 20110085952 12/745632 |
Document ID | / |
Family ID | 40521980 |
Filed Date | 2011-04-14 |
United States Patent
Application |
20110085952 |
Kind Code |
A1 |
Sasson; Yoel ; et
al. |
April 14, 2011 |
SCRUBBER FOR REMOVING HEAVY METALS FROM GASES
Abstract
A wet scrubber for absorbing a heavy metal from a gas stream, a
liquor for said wet scrubber, and a power-plant comprising said wet
scrubber are described. The wet scrubber has a liquor containing an
oxidizer in ionic liquid. Optionally, the oxidizer has, in pure
state, high vapor pressure, and in the ionic liquid, the oxidizer
forms a complex that has a much lower vapor pressure. In a
preferred embodiment, the liquor is substantially free of any
ligand that binds to the metal to be absorbed by the liquor.
Inventors: |
Sasson; Yoel; (Jerusalem,
IL) ; Chidambaram; Mandan; (Jerusalem, IL) ;
Barnea; Zach; (Jerusalem, IL) |
Assignee: |
YISSUM RESEARCH DEVELOPMENT COMPANY
OF THE HEBREW UNIVERSITY OF JERUSALEM
Jeresalem
IL
|
Family ID: |
40521980 |
Appl. No.: |
12/745632 |
Filed: |
December 2, 2008 |
PCT Filed: |
December 2, 2008 |
PCT NO: |
PCT/IL08/01563 |
371 Date: |
December 17, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60996731 |
Dec 3, 2007 |
|
|
|
Current U.S.
Class: |
423/210 ;
422/168; 548/335.1 |
Current CPC
Class: |
B01D 2257/602 20130101;
B01D 2251/104 20130101; B01D 53/1493 20130101; B01D 2251/108
20130101; B01D 2252/30 20130101; B01D 53/64 20130101 |
Class at
Publication: |
423/210 ;
422/168; 548/335.1 |
International
Class: |
B01D 53/64 20060101
B01D053/64; C07D 233/56 20060101 C07D233/56 |
Claims
1. A wet scrubber for absorbing elemental heavy metal from a gas
stream, the scrubber comprising: a vessel having a gas inlet and a
gas outlet, and in the vessel, a liquor comprising: an oxidizer
capable of oxidizing said elemental heavy metal and an ionic
liquid, the liquor being substantially free of any ligand capable
of binding an ion of the heavy metal.
2. A wet scrubber according to claim 1, wherein said liquor
consists essentially of an ionic liquid and an oxidizer.
3. A wet scrubber according to claim 1, comprising a liquid inlet
and a liquid outlet, the scrubber being configured to contact
liquid with the gas between said liquid inlet and outlet.
4. A wet scrubber according to claim 1, wherein said oxidizer is
selected from the group consisting of chlorine dioxide and
iodine.
5. (canceled)
6. A wet scrubber according to claim 1, wherein said ionic liquid
comprises a cation and an anion, and said anion is a halide.
7. A wet scrubber for absorbing, at a working temperature, a heavy
metal from a gas stream, the scrubber comprising: a vessel having a
gas inlet and a gas outlet, and in the vessel, a liquor comprising:
an ionic liquid comprising an anion and a cation, and an oxidizing
complex capable of oxidizing, at the working temperature, said
heavy metal, the oxidizing complex comprising an oxidizing moiety
and a complexing agent, said oxidizing complex having, in the
liquor, a partial vapor pressure that is at least 10 times smaller
than a vapor pressure of the oxidizing moiety in pure state.
8. A wet scrubber according to claim 7, wherein said complexing
agent is one of said anion and cation.
9. A wet scrubber according to claim 7, wherein said oxidizing
moiety is selected from the group consisting of chlorine dioxide
and a halogen.
10.-11. (canceled)
12. A wet scrubber according to claim 7, wherein said oxidizing
complex is a compound of the general formula X.sub.2Y.sup.-,
wherein each of X and Y is independently selected from Br, Cl, and
I.
13. (canceled)
14. A wet scrubber for absorbing a heavy metal from a gas stream,
the scrubber comprising: a vessel having a gas inlet and a gas
outlet, a liquid inlet and a liquid outlet, and in the vessel, a
liquor moving from the liquid inlet to the liquid outlet, the
liquor comprising an ionic liquid and an oxidizer capable of
oxidizing the heavy metal.
15. A wet scrubber according to claim 14, comprising a filter
configured for separating solids from the liquor exiting from the
vessel through the liquid outlet.
16. A wet scrubber according to claim 14, comprising a liquid path
configured for feeding said liquid inlet with liquor that has
exited from the vessel through the liquid outlet.
17. A wet scrubber according to claim 14, wherein said oxidizer
comprises chlorine dioxide or iodine of a halogen-halide
complex.
18.-19. (canceled)
20. A wet scrubber according to claim 1, wherein the liquor is in
form of a mist.
21. A wet scrubber according to claim 1, wherein the heavy metal is
elemental and the ionic liquid has an oxidation potential that is
insufficient for oxidizing the elemental heavy metal at the working
temperature of the scrubber.
22.-24. (canceled)
25. A method of scrubbing elemental heavy metal from a gas stream,
the method comprising: providing a liquor free to move in a vessel,
the liquor comprising an ionic liquid and an oxidizer capable of
oxidizing the elemental metal; and directing the gas stream into
the vessel so as to contact with the liquor.
26. A method according to claim 25, wherein providing a liquor
comprises: reacting said ionic liquid with an oxidizing moiety,
thereby obtaining a liquor comprising: the ionic liquid and an
oxidizing complex comprising the oxidizing moiety complexed with an
ion of said ionic liquid.
27.-29. (canceled)
30. A method according to claim 25, wherein said ionic liquid
comprises an anion and a cation, and said anion is selected from
chloride, bromide, and iodide.
31. A method according to claim 25, wherein contacting the gas with
the liquor comprises bubbling a gas into the liquor or contacting
the gas with liquor in the form of droplets or mist.
32.-34. (canceled)
35. A method according to claim 25, comprising: separating heavy
metal salt from the liquor to obtain a liquor with reduced amount
of metal salt; and using the liquor with reduced amount of metal
salt for scrubbing the heavy metal from the gas stream.
36. A compound of the general formula A.sup.+X.sup.-Y.sub.2,
wherein A.sup.+X.sup.-, at 25.degree. C., is an ionic liquid; and X
and Y are each independently F, Cl, Br, or I.
37. A compound according to claim 36, wherein A.sup.+ is a cyclic
tertiary ammonium cation.
38. A compound according to claim 36, wherein A.sup.+ is an
aromatic cation.
39. A compound according to claim 36, wherein A.sup.+ is according
to any of the general formulae I-VII, ##STR00003## wherein each of
R and R' independently is a C.sub.2-C.sub.10 alkyl.
40. A compound according to claim 36, wherein A.sup.+ is selected
from 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and
1-octyl-3-methylimidazolium.
41. A compound according to claim 36, wherein Y is I.
42. A compound according to claim 36, wherein X is Cl.
43. A wet scrubber according to claim 1, wherein said liquor
comprises a compound according to claim 36.
44. A method according to claim 25, wherein the liquor comprises a
compound according to claim 36.
45.-48. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention, in some embodiments thereof, relates
to removing heavy metals from gases, and in particular, for doing
so by contacting the gases with a liquid.
BACKGROUND OF THE INVENTION
[0002] Scrubbers are air pollution control devices, used to remove
particulates and/or gases from industrial exhaust streams. In a wet
scrubber, the exhaust stream contacts a liquid which absorbs the
pollutant, and leaves the device clean from the pollutants, or at
least, cleaner than it enters.
[0003] U.S. Pat. No. 5,354,363, the disclosure of which is
incorporated herein by reference, describes a wet scrubber, which
employs liquid gallium to absorb vapor phase mercury from a hot gas
exhaust stream in an industrial exhaust system. The temperature of
the liquid gallium is elevated for optimum adsorption efficiency,
and subsequently lowered to separate the absorbed mercury.
[0004] U.S. Pat. No. 6,855,859, the disclosure of which is
incorporated herein by reference, describes a wet scrubber for
removing elemental and oxidized mercury from a gas. The scrubber
utilizes two aqueous solutions, each being contacted with the gas
at a different location in the scrubber. A first aqueous solution
is used to oxidize the elemental mercury to obtain a chloride salt,
and the second--to react the chloride salt with a sulfide to obtain
insoluble mercuric sulfide. The mercuric sulfide precipitates, and
the gas leaves the solution with less elemental mercury.
[0005] US 2007/0123660 to deGouvea-Pinto, the disclosure of which
is incorporated herein by reference, describes the use of an
ionic-liquid/ligand combination, immobilized on a silica substrate,
for separating mercuric chloride from nitrogen.
[0006] deGouvea-Pinto states that "the ionic liquid is a chemically
active environment for solubilization or reaction with the metal
vapor. For example, oxidized metals can be made to absorb with high
affinity for subsequent chelation, and elemental metals can be
oxidized by the layer [of ionic liquid] prior to adsorption on the
ligand. [ . . . ] It has been demonstrated that elemental mercury
is captured in this ionic liquid [=pyrrolidinium
bis(trifluoromethane sulfonyl) imide salt] due to simultaneous
oxidation and solvation. Furthermore, the use of additives to
further enhance the reactive environment in the ionic liquid has
also been demonstrated. For example, KMnO.sub.4 (up to 0.25 mmole)
has been used as an additive to enhance the oxidation of elemental
mercury".
[0007] deGouvea-Pinto also states that "a wide variety of
combinations of ionic liquids, chelating agent, or other ligands,
and solid supports can be used together, as well as the ionic
liquid/ligand combination independent of a solid substrate will
function to adsorb/absorb metal atoms in a gaseous stream".
[0008] Other publications of deGouvea-Pinto relating to capture of
mercury by ionic liquids include:
[0009] Mercury extraction by ionic liquids: temperature and alkyl
chain length effect Tetrahedron Letters 48 (2007)1767-1769;
[0010] Pyrrolidiniiom Imides: Promising Ionic Liquids for Direct
Capture of Elemental Mercury from Flue Gas Water Air Siol Pollut:
Focus 8 (2008) 349-358; and
[0011] Room Temperature Ionic Liquids for Mercury Capture from Flue
Gas Ind. Eng. Chem. Res. Published on the web Oct. 1, 2008.
[0012] It is noted that citation or identification of any reference
in this application in general and in the background section in
particular, shall not be construed as an admission that such
reference is available as prior art to the present invention.
SUMMARY OF THE INVENTION
[0013] An aspect of some embodiments of the invention is a wet
scrubber, suitable for removing heavy metals from a gas stream, and
in particular for removing elemental mercury from flue, combustion
and exhaust gases, generally referred to herein as "flue gas". The
wet scrubber preferably holds a liquor that contains an oxidizer
dissolved in a non-volatile liquid. The oxidizer is selected as to
oxidize the elemental metal into a salt; and the liquid is
optionally selected as to stabilize the oxidizer.
[0014] In an exemplary embodiment of the invention, the scrubber is
configured to provide output of a gas clean of the heavy metal and
free from vapors of the liquid or of that of the oxidizer.
[0015] Examples of heavy metals that can be treated with some
embodiments include mercury, uranium, cadmium, arsenic, lead, and
tin.
[0016] There is thus provided, according to an exemplary embodiment
of the invention, a wet scrubber for absorbing elemental heavy
metal from a gas stream, the scrubber comprising:
[0017] a vessel having a gas inlet and a gas outlet, and in the
vessel,
[0018] a liquor comprising: [0019] an oxidizer capable of oxidizing
said elemental heavy metal and [0020] an ionic liquid, the liquor
being substantially free of any ligand capable of binding an ion of
the heavy metal.
[0021] Optionally, the liquor consists essentially of an ionic
liquid and an oxidizer.
[0022] In some embodiments, the wet scrubber comprises a liquid
inlet and a liquid outlet, and the scrubber is configured to
contact liquid with the gas between the liquid inlet and
outlet.
[0023] In some embodiments the oxidizer is selected from fluorine
(F.sub.2), chlorine (Cl.sub.2) Bromine (Br.sub.2), ozone (O.sub.3),
bromine dioxide (BrO.sub.2); hypochlorite (HOCl); sodium chlorite
(NaClO.sub.2); and potassium chlorite (KClO.sub.2).
[0024] In some embodiments, the oxidizer is chlorine dioxide.
[0025] In some embodiments, the oxidizer is a halogen, for example,
iodine.
[0026] In some embodiments, the ionic liquid comprises a cation and
an anion, and said anion is a halide.
[0027] There is further provided, according to an exemplary
embodiment of the invention, a wet scrubber for absorbing, at a
working temperature, a heavy metal from a gas stream, the scrubber
comprising:
[0028] a vessel having a gas inlet and a gas outlet, and in the
vessel,
[0029] a liquor comprising: [0030] an ionic liquid comprising an
anion and a cation, and [0031] an oxidizing complex capable of
oxidizing, at the working temperature, said heavy metal, the
oxidizing complex comprising an oxidizing moiety and a complexing
agent, said oxidizing complex having, in the liquor, a partial
vapor pressure that is at least 10 times smaller than a vapor
pressure of the oxidizing moiety in pure state.
[0032] In some embodiments, the complexing agent is one of the
anion and cation forming together the ionic liquid.
[0033] In some embodiments, the oxidizing complex comprises
chlorine dioxide.
[0034] In some embodiments, the oxidizing moiety is a halogen,
optionally iodine.
[0035] In some embodiments, the oxidizing complex is a compound of
the general formula X.sub.2Y.sup.-, wherein each of X and Y is
independently selected from Br, Cl, and I. Optionally, the wet
scrubber comprises a liquid inlet and a liquid outlet, and the
scrubber is configured to contact liquid with the gas between said
liquid inlet and outlet.
[0036] There is further provided, according to an exemplary
embodiment of the invention, a wet scrubber for absorbing a heavy
metal from a gas stream, the scrubber comprising: a vessel having a
gas inlet and a gas outlet, a liquid inlet and a liquid outlet, and
in the vessel, a liquor moving from the liquid inlet to the liquid
outlet, the liquor comprising an ionic liquid and an oxidizer
capable of oxidizing the heavy metal.
[0037] Optionally, the wet scrubber comprises a filter configured
for separating solids from the liquor exiting from the vessel.
[0038] In some embodiments, the wet scrubber comprises a liquid
path configured for feeding the liquid inlet with liquor that has
exited from the vessel.
[0039] In some embodiments, the oxidizer comprises chlorine
dioxide. In some embodiments, the oxidizer comprises iodine.
[0040] In some embodiments, the oxidizer comprises a halogen-halide
complex.
[0041] In some embodiments, the liquor is in form of small droplets
or mist.
[0042] In some embodiments, the heavy metal is elemental and the
ionic liquid has an oxidation potential that is insufficient for
oxidizing the elemental heavy metal at the working temperature of
the scrubber.
[0043] In some embodiments, the wet scrubber is configured for
removing metal salt from liquor exiting from the vessel, and
returning the liquor to the vessel.
[0044] There is further provided, according to an exemplary
embodiment of the invention, a method of scrubbing elemental heavy
metal, for example mercury, from a gas stream, the method
comprising:
[0045] providing a liquor free to move in a vessel, the liquor
comprising an ionic liquid and an oxidizer capable of oxidizing the
elemental metal; and
[0046] directing the gas stream into the vessel so as to contact
with the liquor.
[0047] In some embodiments, providing a liquor comprises: reacting
the ionic liquid with an oxidizing moiety, thereby obtaining a
liquor comprising: the ionic liquid and an oxidizing complex
comprising the oxidizing moiety complexed with an ion of said ionic
liquid.
[0048] In some embodiments, the oxidizer comprises chlorine
dioxide.
[0049] In some embodiments, the oxidizer comprises a halogen, for
example, iodine.
[0050] Optionally, the ionic liquid comprises an anion and a
cation, and said anion is selected from chloride, bromide, and
iodide.
[0051] Optionally, contacting the gas with the liquor comprises
bubbling a gas into the liquor.
[0052] Alternatively or additionally, contacting the gas with the
liquor comprises contacting the gas with liquor in the form of
droplets or mist.
[0053] In some embodiments, the ionic liquid has an oxidation
potential that is insufficient for oxidizing the elemental heavy
metal.
[0054] There is further provided, according to an exemplary
embodiment of the invention; compound of the general formula
A.sup.+X.sup.-Y.sub.2, wherein
[0055] A.sup.+X.sup.-, at 25.degree. C., is an ionic liquid;
and
[0056] X and Y are each independently F, Cl, Br, or I.
[0057] Optionally, A.sup.+ is a cyclic tertiary ammonium
cation.
[0058] Optionally, A.sup.+ is an aromatic cation.
[0059] Optionally, A.sup.+ is according to any of the general
formulae I-VIII below,
##STR00001##
wherein each of R and R' independently is a C.sub.2-C.sub.10
alkyl.
[0060] In some embodiments, A.sup.+ is selected from
1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and
1-octyl-3-methylimidazolium.
[0061] In some embodiments, Y is I.
[0062] In some embodiments, X is Cl.
[0063] Optionally, in a wet scrubber and/or a method as described
above, the liquor comprises a compound as described above.
There is further provided, according to an exemplary embodiment of
the invention, a power plant comprising:
[0064] a furnace exhausting a gas; and
[0065] a scrubber, receiving gas from the furnace,
wherein the scrubber is as described above.
[0066] Optionally, the power plant comprises a flue gas
dissulfurization unit, receiving gas from the furnace, wherein the
scrubber receives gas from said unit.
[0067] Optionally, the furnace is coal-fired.
[0068] In some embodiments, the gas comprises mercury.
BRIEF DESCRIPTION OF THE DRAWINGS
[0069] Some embodiments of the invention are herein described, by
way of example only, with reference to the accompanying drawings.
With specific reference now to the drawings in detail, it is
stressed that the particulars shown are by way of example and for
purposes of illustrative discussion of embodiments of the
invention. In this regard, the description taken with the drawings
makes apparent to those skilled in the art how embodiments of the
invention may be practiced.
[0070] In the drawings:
[0071] FIG. 1 is a schematic illustration of a scrubber according
to an embodiment of the invention;
[0072] FIG. 2 is a schematic illustration of a scrubber according
to an embodiment of the invention;
[0073] FIG. 3 is a schematic illustration of an experimental
system, on which a method according to an embodiment of the
invention was practiced;
[0074] FIG. 4 is a schematic illustration of a power plant
according to an embodiment of the invention; and
[0075] FIG. 5 is a graph showing the viscosity of several ionic
liquids with different iodine content.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0076] The present invention, in some embodiments thereof, relates
to removing heavy metals from gases, and in particular, for doing
so by contacting the gases with a liquid liquor.
[0077] For example, an embodiment of the invention is a wet
scrubber, capable of removing heavy metals, for instance mercury,
from a gas stream. In some embodiments, the heavy metal is removed
whether it exists in ionic form, elemental form, or particulate
bound form.
[0078] The invention may find utility, for instance, in coal-fired
power plants, or nay other industrial appliance exhausting flue gas
that contains mercury or other heavy metals. Mercury may be of
particular interest, since it is hazardous, and cannot be removed
efficiently from a flue gas in state of the art methods.
[0079] Various embodiments of the invention utilize scrubbers that
have the structure of known scrubbers, for instance, one of the
structures described in the following web-site:
http://www.epa.gov/ttn/caaa/t1/reports/sect5-4.pdf, incorporated
herein by reference.
[0080] An embodiment of the invention relates to a wet scrubber
that utilizes a non-volatile liquor as a scrubbing liquid. A
non-volatile liquor may be beneficial in that the liquor does not
substantially evaporate out of the scrubber with the clean gas,
thus does not pollute the environment with liquor vapor and does
not require frequent refill.
[0081] A preferred family of non-volatile liquids is the family of
ionic liquids.
[0082] An ionic liquid is a substance that exists mainly in ionic
form, optionally, 100% of the liquid is in form of ions. Ionic
liquids suitable for use in accordance with the present invention
are those that exist as liquids at the operating temperature of the
scrubber. In some embodiments, this temperature depends on the
scrubber's specific position inside a power plant. Some typical
temperature values are in the range of between about 40.degree. C.
and 80.degree. C., although many embodiments are useful within a
broader temperature range, for instance, between about -20.degree.
C. and about 200.degree. C. Some ionic liquids, for example [BMIM]
[Cl], are solids at room temperature. In some embodiments, a solid
ionic liquid may be liquefied without heating, by dissolving a
solute. For example, [BMIM] [Cl] was found to be solid in room
temperature in pure state, and liquid when dissolving 50% iodine.
FIG. 5 is a graph showing the viscosity of several ionic liquids
with different iodine content, measured in relation to the amount
of [BMIM] [Cl]. In another example, [BMIM] [Cl] was found solid in
pure state and liquid when 1% of bromine was added to the [BMIM]
[Cl].
[0083] In an exemplary embodiment of the invention, the ionic
liquid has an oxidation potential that is insufficient for
oxidizing the elemental metal at the working conditions of the
scrubber. In this embodiment, removing elemental mercury from flue
gas is optionally accomplished by adding an oxidizer to the ionic
liquid.
[0084] In preferred embodiments of the invention, the liquor
contains an oxidizer capable of oxidizing the heavy metal. An
oxidizer capable of oxidizing the heavy metal is capable, at the
working conditions of the scrubber, of oxidizing elemental heavy
metal into a salt. Examples of suitable oxidizers include iodine
and chlorine dioxide.
[0085] Preferably, the oxidizer is of minimal corrosiveness and
toxicity. Iodine and chlorine dioxide are examples of non-corrosive
oxidizers that have low-toxicity. Iodine is easier to handle than
many other oxidizers. However, iodine has a disadvantage in being
very volatile. Its vapor pressure at room temperature is equal to
the atmospheric pressure, and therefore, solid iodine is fuming. If
iodine were used in a scrubber, it would be expected to escape from
the scrubber together with the flue gas, and cause environmental
damage. Furthermore, it could be expected that large amounts of
iodine will be lost, and frequent iodine refill would be
required.
[0086] It has been surprisingly found that iodine does not escape
when it is dissolved in ionic liquids and especially so, when it is
dissolved in ionic liquids having a halide (for example chloride,
bromide, or iodide) as one of their components. Without being bound
to theory, it is suggested that the halide of the ionic liquid
forms an anionic oxidizing complex with the iodine, and this
anionic complex does not evaporate out of the liquor due to the
anion's attraction to the cations of the ionic liquid.
[0087] It was also surprisingly found that at temperatures lower
than 130.degree. C. chlorine dioxide (ClO.sub.2) does not escape
from ionic liquids, and a non-binding explanation of a stabilizing
complex between the ClO.sub.2 and the anion of the ionic liquid may
be suggested for explaining this finding.
[0088] UV-Vis spectra of ionic liquids mixed with iodine have shown
no absorption of free iodine, which normally appears at 460 nm. The
same samples have shown absorption peaks at 250 nm for
bromide-containing ionic liquids and 232 nm for chloride-containing
ionic liquids. These absorption peaks are similar to absorption
peaks observed for I.sub.2Br.sup.- and I.sub.2Cl.sup.- in some
other solvents.
[0089] Iodine-halide complexes are known to form in aqueous
solutions when a metal halide salt, such as potassium iodide, is
added to an aqueous solution of iodine. However, the inventor is
not aware of any scientific publication that reported the formation
of this complex without a metal halide salt. The inventor is also
not aware of any publication relating to formation of
halogen-halide complex in a halide-containing ionic liquid.
[0090] In an embodiment of the invention, iodine is used as an
oxidizer and a halide salt is added to form halogen-halide
complex.
[0091] Preferably, the molar concentration of the halide is much
larger than that of the halogen, such that all the halogen is in
form of halogen-halide complex. Examples of preferable
halogen-halide ratios include 1:100, 1:500 and 1:1000.
[0092] Before explaining at least one embodiment of the invention
in detail, it is to be understood that the invention is not
necessarily limited in its application to the details of
construction and the arrangement of the components and/or methods
described in the following description, drawings and/or the
Example. The invention may be practiced or carried out in various
ways, and is capable of other embodiments than those described
below.
[0093] FIG. 1 is a schematic illustration of a scrubber 100
according to an embodiment of the present invention. FIG. 1
illustrates, inter alia, that the scrubber comprises a liquor 120
that is free to move inside the vessel, as usual with wet
scrubbers, rather than being static, for instance, coating some
solid support, as may be found in some liquid-containing dry
scrubbers. A liquor free to move inside the vessel may be easier to
refresh than an immobilized liquor.
[0094] Scrubber 100 is configured for separating at least one heavy
metal from a gas stream, such as the flue of a power generator or
other burning solid or fluid. The heavy metal is optionally one or
more of vanadium, cadmium, lead, and mercury. In an exemplary
embodiment of the invention, the heavy metal is mercury.
[0095] Scrubber 100 comprises a vessel 105 having a gas inlet 110
and a gas outlet 115. Shown in the vessel 105 is a liquor 120,
comprising an ionic liquid. In an exemplary embodiment of the
invention, scrubber 100 is used to clean a gas stream from heavy
metals. In the embodiment shown, gas entering vessel 105 through
gas inlet 110 bubbles within ionic liquid 120, and exists through
outlet 115.
[0096] In an embodiment of the invention, liquor 120 includes only
an ionic liquid and an oxidizer.
[0097] Alternatively, liquor 120 includes an ionic liquid, an
oxidizer, and additional components. In an exemplary embodiment,
the additional components do not substantially change basic
characteristics of the liquor, as defined below. In this
embodiment, the liquor is said to substantially consist of an ionic
liquid and an oxidizer.
[0098] One way to test if a certain liquor consists essentially of
an ionic liquid and an oxidizer includes: removing all the
additional components; and measuring changes in the basic
characteristics of the liquor. If these changes are less than 10%,
the liquor consists essentially of the ionic liquid and the
oxidizer. Preferably, the characteristics are compared at the
operation temperature of the scrubber.
[0099] Examples to basic characteristics of the liquor include the
capability of the liquor to absorb the heavy metal, the vapor
pressure of the liquor, and the viscosity of the liquor.
[0100] Preferably, the liquor has, at the operating temperature of
the scrubber, a vapor pressure that is as low as possible, for
example, lower than 1 mmHg.
[0101] Liquors of lower viscosity are generally preferred, because
they allow bubbling the gas stream into the liquor (or contacting
the gas with the liquor in any other way) more easily than more
viscous liquors. However, low viscosity and low vapor pressure are
usually two contradicting requirements. In some embodiments of the
invention, it is preferable that the viscosity of the liquor, at
the working temperature of the scrubber, is lower than 250 mPas,
optionally between about 50 mPas. and 200 mPas.
[0102] As used in the present description and claims, an ionic
liquid is a substance, which at the working temperature is liquid,
and at least 90% of the liquid, optionally at least 99% of the
liquid, is in the form of ions. Most ionic liquids are salts having
a melting point below the working temperature. In some embodiments,
salts having a melting point below about 100.degree. C., for
instance, below about 25.degree. C., are preferred.
[0103] In an exemplary embodiment of the invention, the liquor
comprises at least 90% ionic liquid (w/w).
[0104] In exemplary embodiments of the invention, the working
temperature of the wet scrubber is between about -20.degree. C. and
about 200.degree. C., optionally more than about 40.degree. and/or
less than about 80.degree. C.
[0105] Non-limiting examples of ionic liquids that may be suitable
for use in accordance with embodiments of the present invention
include liquids having a halide as an anion, and one of the cations
presented below, wherein each of R and R' independently is a
C.sub.1-12 alkyl. In some embodiments, C.sub.4-8 alkyls are
preferred.
##STR00002##
[0106] Optionally, liquor 120 is substantially free of any ligand
capable of binding the heavy metal or an ion thereof.
[0107] Optionally, a liquor is considered to be substantially free
of any ligand capable of binding the heavy metal, if such a ligand
is present in the liquid only in amounts that are not detectable.
Alternatively, a liquor is considered substantially free of the
ligand if the ligand is detectable; but removing the ligand to
below detectable level changes the scrubber efficiency by less than
10%.
[0108] Optionally, a ligand capable of binding the heavy metal is
an agent that forms with an ion of the heavy metal a complex
through covalent (coordinate) bonds with two or more donor groups,
so that one or more rings are formed.
[0109] In an embodiment of the invention, a liquor substantially
free of any ligand capable of binding the heavy metal is
substantially free of ligands that form stable complexes with ions
of the heavy metal, but not of ligands that form complexes of low
stability. In other embodiments, the liquor is free of any ligands
that form with an ion of the heavy metal complexes of stability
larger than 0.
[0110] Complexes of high stability are complexes having a stability
constant of 30 or more, optionally complexes having a stability
constant of 20 or more, optionally, complexes having stability of
constant of 10 or more. Optionally, the difference in stability
constant between complexes of high and low stability is at least
5.
[0111] A definition of the stability K of a complex is
K=log [ML]/[M][L],
[0112] where [ML] is the concentration of the complex, [M] is the
concentration of the heavy metal ion, and [L] is the concentration
of the ligand. All concentrations are in mol/liter.
[0113] In an exemplary embodiment of the invention, liquor 120
comprises an oxidizer. Preferable oxidizers are non-toxic and
non-corrosive, for example, iodine.
[0114] Iodine, however, has a very high vapor pressure, of about 1
atmosphere at 25.degree. C. Thus, iodine is expected to escape from
the scrubber in large quantities, which might cause environmental
hazards and require frequent refill.
[0115] Nevertheless, it has been surprisingly found that iodine
does not contaminate the gas exiting from a liquor comprising, as a
major component, an ionic liquid that has a halide as an anion. It
was also found that chlorine dioxide does not contaminate the gas
exiting from a liquor comprising, as a major component, an ionic
liquid that has a halide as an anion. Without being bound to
theory, these phenomena may be explained by the formation of a
stabilizing complex between the oxidizing iodine (or chlorine
dioxide) and the halide of the ionic liquid. Applicants are not
aware of any prior publication of such a stabilizing complexes in
an ionic liquid.
[0116] Similar stabilizing complexes may form in halidic ionic
liquids with other halogens, for instance, chlorine and
bromine.
[0117] Halogen-halide oxidizing complexes of the kind described
above have the general formula X.sub.2Y.sup.-, wherein each of X
and Y is independently selected from F, Br, Cl, and I. For
instance, if an ionic liquid comprising chloride as the anion is
used with iodine as the oxidizing moiety, it has been found that a
complex of the formula I.sub.2Cl.sup.- is formed.
[0118] FIG. 2 is a schematic illustration of a wet scrubber 200
according to another exemplary embodiment of the invention.
Scrubber 200 is of the kind known in the art as a Spray Tower
Scrubber. In other embodiments of the invention other kinds of
scrubbers may be used, for instance, a Cyclonic Spray Chamber
Scrubber, a Packed Tower Scrubber, a Plate Tower Scrubber, a
Venturi Scrubber, an Orifice Scrubber, or any other kind of wet
scrubber known in the art. A summary of the characterizations of
several known scrubbers may be found, for instance, in
http://www.epa.gov/ttn/caaa/t1/reports/sect5-4.pdf.
[0119] Scrubber 200 is shown to have a vessel 205, a gas inlet 210
and a gas outlet 215. Additionally, scrubber 200 has a liquid inlet
250 and a liquid outlet 255. Scrubber 200 optionally has a demister
(not shown) to collect liquid droplets that may be carried with the
gas towards the exit.
[0120] In the embodiment shown in FIG. 2, the liquid inlet
comprises sprinkles 260. Sprinkles 260 provide liquid into vessel
205 in droplets, and therefore increase the surface of the liquid
that contacts the gas. Liquid inlet 250 is connected to a liquid
source 265.
[0121] Liquid outlet 255 allows liquid to leave the scrubber 200
during operation of the scrubber. The liquid optionally leaves
through an ionic liquid path 270, which optionally is connected to
the ionic liquid source 265.
[0122] Optionally, path 270 goes through a filter 275 for filtering
ionic liquid leaving the vessel.
[0123] As a rule, the scrubber is more efficient as the droplets
are smaller.
[0124] In an exemplary embodiment of the invention, the liquid used
with scrubber 200 is a liquor comprising an ionic liquid and an
oxidizer.
[0125] Optionally, the liquor also comprises agents for reducing
surface tension of the liquor, to allow formation of smaller
droplets. Examples of such agents are detergents, for example,
organic detergents and/or borax.
[0126] In operation, a gas is streamed through gas inlet 210 and
impinges on droplets of liquor moving from sprinklers 260.
Elemental heavy metal, for example mercury, comprised in the gas
stream is oxidized into a salt by the oxidizer comprised in the
liquor to form a salt. The salt is dissolved in the ionic liquid,
and the gas leaves through outlet 210 with substantially reduced
amount of mercury, either in ionic, elemental, or particulate bound
form. In preferred embodiments of the invention, the amount of
mercury is reduced in at least 80%, optionally at least 90%, and
many times in 99% or more.
[0127] When the liquor is saturated with mercury salt, the salt
precipitates, and is optionally filtered out by filter 275.
[0128] FIG. 3 illustrates an experimental system 300 similar to the
one at which the invention was first practiced. Experimental system
300 included an air pump 305, a mercury source 310, a vessel 320,
and a mercury monitoring device 330. Vessel 320 had a liquor 325
according to an embodiment of the invention.
[0129] In operation, air was pumped by air pump 305 into mercury
source 310 through tube 312 at about 2 lit/min. The mercury (314)
in source 310 was optionally heated with the aid of heater 316.
Without heating, air with about 75 ppb (=parts per billion) mercury
vapor left mercury source 310 through a tube 318, and entered
vessel 320 through gas inlet 322. The air and the mercury bubbled
into liquor 325 out to the atmosphere above the liquor, from where
the material exiting was collected through a gas outlet 324 and
monitored for the existence of mercury with monitoring device 330.
The efficiency of the system was shown to be constant for about
five months without refilling or regenerating iodine or ionic
liquid. The ionic liquids were prepared according to the procedure
described by R. S. Varma et al. in Chem. Commun., 2001, pages
643-644, the contents of which is incorporated herein by
reference.
[0130] Table I presents physical properties of some ionic liquids
used with the system of FIG. 3. Aliquat 336 listed in the table is
a commercially available ionic liquid of the chemical name
trioctylmethylammonium chloride salt.
TABLE-US-00001 TABLE I Dencity Viscosity Decomposition Entry Ionic
Liquid Yield (%) (g/cm.sup.3).sup.a (mPa s).sup.b temp. (.degree.
C.).sup.c 1 [BMIM][Br--] 93 1.18 70 240 2 [OMIM][Br--] 85 1.08 162
252 3 [BMIM][Cl--] 82 0.99 12 220 4 [HMIM][Cl--] 88 1.03 32 209 5
[OMIM][Cl--] 86 1.10 225 212 6 Aliquat-336.sup.d -- 0.88 197 --
.sup.aDensity measurement was performed at 20.degree. C.;
.sup.bViscosity measurement was performed at 50.degree. C. using
Brookfield DV-II+ viscometer; .sup.cMeasured by TGA and DSC
analytical tools; Purchased from Aldrich.
[0131] The cations of the ionic liquids are abbreviated as follows:
BMIM stands for 1-butyl-3-methylimidazolium, HMIM stand for
1-hexyl-3-methylimidazolium and OMIM stands for
1-octyl-3-methylimidazolium.
[0132] Table II summarizes absorption obtained in the system of
FIG. 3 with liquors consisting of the liquids of table I and iodine
in the amounts indicated in the table. When 7.0 mg iodine was used
per 12 g ionic liquid, all the liquids of table I except for
Aloquat 336 provided more than 99% removal of mercury. Aloquat 336
did not provide mercury removal of over 88.6%, no matter how much
iodine was added to it, up to 40.0 mg/12 g iodine/aloquat.
[0133] Without being bound to theory, it is suggested that the
performance of aliquat was not as good as those of the other ionic
liquids examined because it has a small cationic core hidden
between long alkyl chains. If this suggestion is correct, it may be
preferable to use ionic liquids with aromatic cations and
relatively short alkyl chains, for instance, chains of 2-8 carbons.
Optionally, the aromatic cation comprises a cyclic cation with a
positive charge resonating among the entire cycle. Optionally, the
cyclic cation is a heterocycle. Optionally, the cyclic cation
comprises a 5-membered or 6-membered ring. Optionally, one or more
of the 5 or 6 members is a nitrogen atom, and the rest, carbon
atoms.
TABLE-US-00002 TABLE II Iodine amount (mg/12 g of ionic liquid) 0
2.2 4.6 7.0 9.4 Entry Ionic liquid Absorption of Hg.sup.0(%) 1
[BMIM][Br--] 2.4 79.9 94.6 99.2 99.8 2 [OMIM][Br--] 2.6 77.9 84.3
99.1 99.1 3 [BMIM][Cl--] 2.9 90.5 97.1 99.8 99.8 4 [HMIM][Cl--] 2.5
90.3 96.7 99.2 99.7 5 [OMIM][Cl--] 2.8 86.1 96.3 98.9 99.5 6
Aliquat-336 1.6 58.0 68.7 85.2 88.6
[0134] In the experiments summarized in table II, the initial flow
of gaseous mercury was kept as 75.5 ppb (100%). The amount of all
ionic liquids have been kept as 12.0 g to maintain the uniform
contact time of gaseous mercury in the ionic liquids. This contact
time was in all the experiments 0.2 sec.
[0135] A similar experiment (with [BMIM]Cl.sup.-) was carried out
with motorcycle exhaust gas replacing the air. The gas entering the
liquor contained about 60 ppb mercury, and the gas exiting the
liquor had mercury below the detection threshold of monitoring
device 330, which was an ICP-OES (Vista-MPX by Varian).
[0136] It should be noted that iodine did not escape from the
system, and the only loss of iodine was due to its reaction with
elemental mercury. In contrast, iodine did escape under similar
conditions from water and from organic liquids that are not ionic
liquids, even if they included a metal halide salt that formed an
iodine-halide complex with the iodine. This surprising stability of
the halogen-halide complex in ionic liquids, may be explained,
perhaps, in the high concentration of the halide, and/or
stabilization of the complex by the cation of the ionic liquid.
[0137] Since in the oxidation of elemental mercury by iodine, 1 mol
of iodine reacts with one mole of mercury, about 1.25 g of iodine
are required for absorbing Ig of mercury. Since coal usually has
less than 125 ppb mercury, of which only about half is elemental
mercury, which may be oxidized by the iodine, 1.25 g iodine are
sufficient to react with at least about 16 tons of coal. Thus, the
iodine needs refreshing only at substantial time intervals.
[0138] FIG. 4 is a schematic illustration of a power plant (400)
according to an embodiment of the invention. Power plant 400 has a
furnace 402 that receives coal and air and lets out flue gas. The
flue gas goes first through a filter 404 for removing ash, and
generally large particulate matter. Non-limiting examples of
filters suitable for use as a filter 404 include fabric filters and
electrostatic separators. Optionally, gas exiting filter 404 goes
into a flue gas dissulfurization unit (FGD) (406), which further
separates acidic components of the general formula SO.sub.x from
the flue gas.
[0139] In the embodiment shown, FGD 406 functions as a gas source,
and gas exiting from unit 406 is fed into gas inlet 110 in scrubber
100. Scrubber 100 directly receives gas from unit 406, and
indirectly receives gas from filter 404.
[0140] Optionally, flue gas going from furnace 402 is cooled on its
way to filter 404 with heat exchangers (not shown). Alternatively
or additionally, other units in the power plants, for instance,
filter 404, unit 406, and/or other units, not shown, cool the flue
gas, such that the flue gas enters scrubber 100 at a temperature
lower than the decomposition temperature of the ionic liquid in the
scrubber.
[0141] It is expected that during the life of a patent maturing
from this application many relevant scrubbers, and ionic liquids
will be developed and the scope of the terms scrubber, and ionic
liquid, are intended to include all such new scrubbers, and ionic
liquids a priori.
[0142] For instance, other structures of scrubbers may be developed
or adapted for use in accordance with some embodiments of the
present invention, allowing for contacting the gas with the liquid,
introducing and/or releasing the liquid, and/or introducing and/or
releasing the clean gas in ways other than those described
therein.
[0143] In another instance, other ionic liquids may be developed or
found to be effective in allowing for efficiently stabilizing an
oxidizing agent without hindering the stabilized oxidizing agent so
as to substantially slow down its reaction with elemental heavy
metals.
[0144] It is also expected that during the life of a patent
maturing from this application many relevant liquors will be
developed and the scope of the term liquor is intended to include
all such new liquors a priori.
[0145] For instance, other nonvolatile liquids, including, but not
limited to ionic liquids, and/or other green oxidizers may be
developed.
[0146] It is appreciated that certain features of the invention,
which are, for clarity, described in the context of separate
embodiments, may also be provided in combination in a single
embodiment. Conversely, various features of the invention, which
are, for brevity, described in the context of a single embodiment,
may also be provided separately or in any suitable subcombination
or as suitable in any other described embodiment of the invention.
Certain features described in the context of various embodiments
are not to be considered essential features of those embodiments,
unless the embodiment is inoperative without those elements.
[0147] The terms "comprises", "comprising", "includes",
"including", "having" and their conjugates mean "including but not
limited to".
[0148] When a first element is described or claimed to comprise a
second element, this should be construed as if the first element
comprises, one or more of said second element, and as a disclosure
of two kinds of a first element: one kind in which the first
element comprises only one second element, and another kind, in
which the first element comprises a plurality of second elements.
This definition holds also if the first element is said to include,
have, contain, etc. a second element.
[0149] The words "the" and "said" are used interchangeably, and the
appearance of one of them rather than the other should not be used
for construing the sentence or claim, in which the word
appears.
[0150] The phrases "between X and Y" and "from X to Y", where X and
Y are two indicated numbers, are used herein interchangeably and
are meant to include the first and second indicated numbers and all
the fractional and integral numerals therebetween.
* * * * *
References