U.S. patent application number 13/322262 was filed with the patent office on 2012-03-22 for method for concentrating dilute sulfuric acid and an apparatus for concentrating dilute sulfuric acid.
This patent application is currently assigned to Outotec OYJ. Invention is credited to Michael Gasik, Anu Lokkiluoto, Helja Peltola, Pekka Taskinen.
Application Number | 20120067740 13/322262 |
Document ID | / |
Family ID | 40680753 |
Filed Date | 2012-03-22 |
United States Patent
Application |
20120067740 |
Kind Code |
A1 |
Gasik; Michael ; et
al. |
March 22, 2012 |
METHOD FOR CONCENTRATING DILUTE SULFURIC ACID AND AN APPARATUS FOR
CONCENTRATING DILUTE SULFURIC ACID
Abstract
A method for concentrating dilute sulfuric acid involves feeding
dilute sulfuric acid through at least two electrolytic cells
depolarized by sulfur dioxide, and through a cation conductive
membrane separating the cathode and anode sides in the electrolytic
cell. Also provided is an apparatus for concentrating sulfuric
acid, wherein at least a first electrolytic cell is in fluid
communication with a second electrolytic cell, and wherein a cation
conductive membrane separates the cathode and anode sides in the
electrolytic cells.
Inventors: |
Gasik; Michael; (Helsinki,
FI) ; Lokkiluoto; Anu; (Helsinki, FI) ;
Peltola; Helja; (Pori, FI) ; Taskinen; Pekka;
(Espoo, FI) |
Assignee: |
Outotec OYJ
Espoo
FI
|
Family ID: |
40680753 |
Appl. No.: |
13/322262 |
Filed: |
May 25, 2010 |
PCT Filed: |
May 25, 2010 |
PCT NO: |
PCT/FI2010/050421 |
371 Date: |
November 23, 2011 |
Current U.S.
Class: |
205/770 ;
204/237; 204/257; 204/258 |
Current CPC
Class: |
C01B 17/88 20130101;
Y02P 20/129 20151101; Y02P 20/132 20151101 |
Class at
Publication: |
205/770 ;
204/257; 204/258; 204/237 |
International
Class: |
C25B 1/22 20060101
C25B001/22; C25B 15/00 20060101 C25B015/00; C25B 9/18 20060101
C25B009/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2009 |
FI |
20095573 |
Claims
1. A method for concentrating dilute sulfuric acid, comprises
feeding sulfuric acid into a first of at least two electrolytic
cells which are depolarized by sulfur dioxide, and wherein a cation
conductive membrane separates the cathode and anode sides in the
electrolytic cells.
2. The method according to claim 1, wherein the concentration of
sulfuric acid is carried out by feeding the sulfuric acid
sequentially through 2 to 5 electrolytic cells that are installed
in a cascading fashion and depolarized by sulfur dioxide.
3. The method according to claim 1, wherein dilute sulfuric acid
and sulfur dioxide are fed to the anode side of a first
electrolytic cell comprising a membrane, so that the following
reaction takes place on the anode side:
SO.sub.2(aq)+2(1+x)H.sub.2O.fwdarw.H.sub.2SO.sub.4(aq)+2(H.sub.2O).sub.xH-
.sup.+(aq)+2e.sup.- (9) where x=1 to 4, in which case the formed
complex ions pass through the membrane of the electrolytic cell to
the cathode side, and the following reaction takes place at the
cathode: 2(H.sub.2O).sub.xH.sup.+(aq)+2e.sup.-.fwdarw.H.sub.2(g)+2x
H.sub.2O (10) where x=1 to 4, and the concentrated mixture of
sulfuric acid and water obtained from the anode side of the first
electrolytic cell is fed to the anode side of the second
electrolytic cell together with the non-reacted and/or added sulfur
dioxide, and the above described reactions take place in said
second electrolytic cell, and the further concentrated sulfuric
acid formed as a reaction product from the anode side of the second
electrolytic cell is recovered or fed to the anode side of the a
successive electrolytic cell.
4. The method according to claim 1, wherein the cation conductive
membrane separating the cathode and anode in the electrolytic cell
sides is a functionally fixed electrolyte that serves both as an
insulator and a proton conductor, and prevents the gases from
flowing from one side of the membrane to the other.
5. The method according to claim 4, wherein the membrane comprises
a sulfonic acid membrane.
6. The method according to claim 1, wherein the concentration of
the sulfuric acid fed to the anode side of the first electrolytic
cell is 78 wt % or less.
7. The method according to claim 1, wherein the concentration of
the sulfuric acid fed to the cathode side of the first electrolytic
cell is 78 wt % or less.
8. The method according to claim 1, wherein the sulfuric acid is
concentrated at a temperature of 10 to 120.degree. C., preferably
at a temperature of 20 to 100.degree. C.
9. The method according to claim 1, wherein the sulfuric acid is
concentrated at a pressure of 0.5 to 7 bar, preferably at a
pressure of 1 to 5 bar.
10. The method according to claim 1, wherein the concentrated
sulfuric acid is recovered and lead to a sulfuric acid plant.
11. An apparatus for concentrating dilute sulfuric acid, comprises
at least a first electrolytic cell depolarized by sulfur dioxide
and a second electrolytic cell depolarized by sulfur dioxide,
wherein the second electrolytic cell is configured for receiving
sulfuric acid concentrated in the first electrolytic cell from the
first electrolytic cell, and which apparatus comprises a cation
conductive membrane for separating the cathode and anode sides in
the electrolytic cells.
12. The concentration apparatus according to claim 11, wherein the
apparatus comprises a total of 2 to 5 electrolytic cells that are
installed in a cascading fashion and depolarized by sulfur
dioxide.
13. The concentration apparatus according to claim 11, wherein the
first electrolytic cell comprises an anode side and a cathode side,
and that the second electrolytic cell comprises an anode side and a
cathode side, and that the apparatus comprises a first set of feed
elements for feeding dilute sulfuric acid and sulfur dioxide to the
anode side of the first electrolytic cell comprising the membrane,
in order to generate the following reaction on the anode side:
SO.sub.2(aq)+2(1+x)H.sub.2O.fwdarw.H.sub.2SO.sub.4(aq)+2(H.sub.2O).sub.xH-
.sup.+(aq)+2e.sup.- (9) where x=1 to 4, in which case the formed
complex ions pass through the membrane of the electrolytic cell to
the cathode side, and the following reaction takes place at the
cathode: 2(H.sub.2O).sub.xH.sup.+(aq)+2e.sup.-.fwdarw.H.sub.2(g)+2x
H.sub.2O (10) where x=1 to 4, and a second set of feed elements for
feeding the concentrated mixture of sulfuric acid and water
obtained from the anode side of the first electrolytic cell,
together with non-reacted and/or added sulfur dioxide, to the anode
side of the second electrolytic cell, in which second electrolytic
cell the above described reactions take place, and a third set of
feed elements for feeding the further concentrated sulfuric acid
formed as a reaction product on the anode side of the second
electrolytic cell to recovery means or further to the anode side of
a third electrolytic cell.
14. The concentration apparatus according to claim 11, wherein the
membrane in the first electrolytic cell and/or the membrane in the
second electrolytic cell is a cation conductive, functionally fixed
electrolyte separating the anode side and the cathode side,
arranged to serve both as an insulator and a proton conductor, and
to prevent the gases from flowing from one side of the membrane to
the other.
15. The concentration apparatus according to claim 14, wherein the
membrane comprises a sulfonic acid membrane.
16. The concentration apparatus according to claim 11, wherein the
apparatus comprises means for adding sulfur dioxide to the
concentrated mixture of sulfuric acid and water obtained from the
anode side of the first electrolytic cell, which mixture is fed
together with non-reacted and/or added sulfur dioxide to the anode
side of the second electrolytic cell.
17. The concentration apparatus according to claim 11, wherein the
apparatus comprises means for removing hydrogen from the cathode
side of the electrolytic cells, and means for the recovery of
hydrogen.
18. The concentration apparatus according to claim 11, wherein the
apparatus comprises means for recycling catholyte from the cathode
side of one electrolytic cell to the cathode side of another
electrolytic cell, and/or means for recycling the catholyte of the
last electrolytic cell to the anode side of the first electrolytic
cell.
19. The method according to claim 6, wherein the concentration of
sulfuric acid fed to the anode side of the first electrolytic cell
is 1 to 25 wt %.
20. The method according to claim 7, wherein the concentration of
sulfuric acid fed to the cathode side of the first electrolytic
cell is 1 to 35 wt %.
21. The method according to claim 8, wherein the method is carried
out at a temperature of 20 to 100.degree. C.
22. The method according to claim 9, wherein the method is carried
out at a pressure of 1 to 5 bar.
Description
FIELD OF INVENTION
[0001] The invention relates to a method for concentrating dilute
sulfuric acid. In particular, the invention relates to a method
where an electrolytic cell is utilized in the concentration of
sulfuric acid. The invention also relates to an apparatus for
concentrating dilute sulfuric acid.
BACKGROUND OF THE INVENTION
[0002] When sulfuric acid is manufactured industrially, it is
generally made of sulfur dioxide, which is obtained for example
from the combustion process of sulfur according to the following
chemical equation (1):
S(s)+O.sub.2(g).fwdarw.SO.sub.2(g) (1)
[0003] In the metallurgical industry, a large quantity of sulfur
dioxide is formed in roasting and smelting processes, i.e. the
exhaust gases contain essentially large quantities of sulfur
dioxide. Sulfuric acid production plants have been built around
metallurgical plants, primarily for environmental reasons, because
sulfur dioxide must be removed from exhaust gases. The sulfuric
acid produced in these kinds of plants is increasingly used for
replacing the conventional acid made of iron sulfide (pyrite).
[0004] The major part of concentrated sulfuric acid is produced in
the so called contact process. The contact process essentially
comprises two separate steps: [0005] catalytic conversion of
SO.sub.2 gas to SO.sub.3, which is called contacting, and [0006]
absorption of SO.sub.3 to a sulfuric acid solution, which is called
absorption.
[0007] These steps can be repeated several times, and the obtained
end product is commercial concentrated sulfuric acid.
[0008] In the contacting step, sulfur dioxide obtained from burning
sulfur or from some other source is oxidized in the presence of
oxygen and a catalyst (for example vanadine(V)oxide) into sulfur
trioxide according to the following chemical equation (2),
whereafter the sulfur trioxide is absorbed in sulfuric acid.
SO.sub.2(g)+1/2 O.sub.2(g).fwdarw.SO.sub.3(g) (cat. V.sub.2O.sub.5)
(2)
[0009] Sulfur trioxide reacts with water contained in or added into
sulfuric acid according to the following chemical equation (3), so
that more sulfuric acid is formed.
SO.sub.3(g)+H.sub.2O(l).fwdarw.H.sub.2SO.sub.4(l) (3)
[0010] It is not practically reasonable to add sulfur trioxide
(SO.sub.3) directly into water, or into dilute sulfuric acid,
because the reaction is extremely exothermal and corrosive sulfuric
acid mist is easily formed instead of liquid. Therefore sulfur
trioxide is absorbed in 98 wt % (18 M) sulfuric acid. This is the
azeotropic content, where the partial pressures of sulfur dioxide,
sulfuric acid and water vapor are at minimum. Sulfuric acid cannot
be concentrated to over 98 wt % by evaporating water, because after
the azeotropic point, more sulfuric acid than water is removed from
the mixture to the vapor phase.
[0011] In many processes of the chemical industry where sulfuric
acid is used large quantities of dilute spent acid are formed and
they contain varying quantities of different organic and inorganic
impurities. One form of impure sulfuric acid is wash acid, which is
formed when washing sulfur dioxide containing gas obtained from
metallurgical processes prior to conducting said gas to the
manufacturing of sulfuric acid.
[0012] When desired, spent acid can be regenerated. In the
regeneration process, the impurities contained in the spent acid
are removed and water is removed from the dilute sulfuric acid
until the product corresponds to concentrated sulfuric acid. The
regeneration of spent acid can also require that it is decomposed.
In that case the impure acid which is concentrated to a suitable
content is decomposed thermally. The formed sulfur dioxide gas is
washed and it is used for manufacturing sulfuric acid in a contact
process.
[0013] Depending on the quantity and quality of the impurities
contained in the spent acid, several alternative regeneration
processes can be applied. In addition, sulfuric acid can be
concentrated and decomposed also for other reasons than the
impurities contained therein. In that case it is possible to make
use of the same methods as with impure acid. Various concentration
methods are used depending on the conditions. Below examples of
different concentration methods are presented.
[0014] A relatively dilute sulfuric acid, 70 to 75%, can be
obtained for example by means of vacuum evaporation, Venturi
evaporation or immersion burner evaporation. In vacuum evaporation,
dilute acid is boiled in a vacuum, so that the acid is concentrated
as water evaporates. The most significant feature of Venturi
evaporation is that water is evaporated from dilute sulfuric acid
at an atmospheric pressure, but at a temperature that is remarkably
lower than the acid boiling point. In the Venturi system, the water
removed from sulfuric acid solution is transferred to the bypassing
gas stream, which carries the water vapor away. Immersion burner
evaporation is sometimes used for concentrating sulfuric acid, when
the acid contains a lot of salts as impurities. In this method
liquid or gaseous fuels as well as a burner are used for developing
hot flue gases. These flue gases with a temperature of 1500 to
1600.degree. C. are conducted to the acid through an immersed pipe.
Water vapor is absorbed in the combustion gas, and adiabatic
evaporation cools it down simultaneously.
[0015] When concentrating sulfuric acid to the content of 93 to
98%, either direct or indirect heating can be used; the latter is
possible both at a normal pressure and at low pressure. In
particular, a drum evaporator is used for concentrating spent acids
in the explosives industry. The method is similar to immersion
burner evaporation used for more dilute acids. Flue gases obtained
from burning fuel oil or gas are conducted through an immersion
pipe to the acid to be concentrated, which proceeds in
countercurrent with respect to the gases, through several
containers. Sulfuric acid can be concentrated to 95 to 98% at a
normal pressure by applying indirect heating of the flue gases. On
top of a Paulig-Plinke type heater a column distiller made of an
alloy of iron and silicon is arranged. Sulfuric acid (about 70%) is
fed from the top to the column, and it flows downwardly in the
pipe, in countercurrent with respect to the fumes. The acid is
concentrated to 82 to 85%. The principle can be compared to the
evaporation process of a 70 to 75% acid. Owing to the required high
temperatures, the energy consumption is high and it is not possible
to apply the multiple-effect principle, which would save thermal
energy and thus reduce energy consumption. When concentrating
sulfuric acid to the content of 93 to 98% fumes that contain
remarkable quantities of gaseous sulfuric acid are formed. These
fumes cannot be released to the atmosphere without purification,
which means that the evaporated sulfuric acid is separated from the
water vapor removed from the system by means of a suitable washing
system.
[0016] The evaporation of sulfuric acid is a troublesome process
for several reasons. A drawback in all above described methods is
brought about by corrosion problems caused by the heating of
sulfuric acid. An extremely corrosive sulfuric acid mist is easily
formed in connection with heating. In addition, the use of a vacuum
and high temperatures in the concentration process require a lot of
energy.
[0017] In some thermochemical cycles aiming at the production of
hydrogen sulfuric acid is formed as an intermediate product. In
these methods, sulfuric acid is decomposed, i.e. the decomposition
is part of the cycles and it makes the hydrogen production
possible. There are several thermochemical cycles, among which for
instance the following four have been widely studied: the
hybrid-sulfur cycle, the sulfur-iodine cycle, the Ispra Mark 13
hybrid cycle and the UT-3 cycle.
[0018] Sulfur is recycled for example in a cycle developed by the
Westinghouse Electric Corporation, which cycle is called the
hybrid-sulfur cycle (HyS-process). The hybrid-sulfur cycle is
described for instance in the U.S. Pat. No. 4,412,895. The major
reactions in the cycle are:
H.sub.2SO.sub.4.fwdarw.H.sub.2O+SO.sub.2+1/2 O.sub.2 (4)
SO.sub.2+2H.sub.2O.fwdarw.H.sub.2SO.sub.4+H.sub.2 (5)
[0019] Sulfur is recycled also in the sulfur-iodine cycle. This
cycle is composed of three reactions, where the net reactant is
water, and the net products are hydrogen and oxygen. Sulfur and
iodine are recovered and reused.
H.sub.2SO.sub.4.fwdarw.H.sub.2O+SO.sub.2+1/2 O.sub.2 (6)
I.sub.2+SO.sub.2+2H.sub.2O.fwdarw.2HI+H.sub.2SO.sub.4 (7)
2HI.fwdarw.I.sub.2+H.sub.2 (8)
[0020] The above described thermochemical cycles are developed
mainly for the utilization of nuclear energy and the utilization of
SO.sub.2 without recycling and decomposition of sulfuric acid has
not been discussed in this connection. Staser et al. have described
the effect of the passage of water on the production of hydrogen
and sulfuric acid in a cell in their article (Staser & Weidner,
J. Electrochem. Soc. 156(1) B16-B21, 2009). In the method described
in said publication, dry sulfur dioxide gas is fed to the anode
side of the cell and the method is discussed only with respect to
water control. The publication does not contain any mention of
concentrating sulfuric acid in an electrolytic cell.
[0021] In the earlier patent publication WO2008087252 by the
applicant, a method for manufacturing hydrogen and sulfuric acid
has been described. In this overall process the utilization of an
electrochemical cell in the production of hydrogen has been
suggested as one alternative. The process comprises steps where a
sulfur dioxide gas stream is divided into two separate sub-streams,
of which the first sub-stream is routed to water splitting and the
second sub-stream is routed to oxidation where sulfur dioxide is
oxidized into sulfur trioxide. The splitting of water is realized
in a partial thermochemical cycle in the production of hydrogen and
sulfuric acid, one suggested embodiment being the electrolytic
splitting of water in an electrolytic cell. The described method
utilizes only one cell for manufacturing hydrogen and sulfuric
acid, and the formed sulfuric acid is dilute. The method does not
utilize or describe the use of an electrolytic cell for
concentrating sulfuric acid; instead for producing commercial
strong sulfuric acid, the method uses a conventional evaporation
step where water is evaporated and partly concentrated sulfuric
acid is thereafter concentrated further in a sulfur trioxide
absorption process.
[0022] The publication JP 8071365 discloses a method where a redox
system is used for sulfur removal of the sulfur oxide present in
exhaust gases and for producing sulfuric acid and hydrogen as side
products. The exhaust gas is contacted with a sulfur dioxide
absorbing solution, which contains iodine dissolved in water. The
solution forms two layers, where the lighter phase contains
sulfuric acid and the heavy phase contains hydrogen iodide. The
hydrogen iodide is electrolyzed for producing hydrogen and iodine.
The separated iodine is reused for absorbing sulfur dioxide gas.
The method is a modification of the sulfur-iodine process without
the decomposition of sulfuric acid. This method produces hydrogen
and sulfuric acid. However, 5 moles water are present along with
each sulfuric acid mole in the light phase, which means that the
acid is a dilute acid which is not a commercial product. The patent
does not specify how the concentration of acid is carried out.
BRIEF DESCRIPTION OF THE INVENTION
[0023] Thus the object of the invention is to provide a method, by
means of which the above described problems can be solved or at
least essentially reduced. In particular, the object of the
invention is to provide a method and an apparatus for concentrating
dilute sulfuric acid without a concentration step carried out at
high temperature or in vacuum. The objects of the invention are
achieved by a method and an apparatus which are characterized by
what is set forth in the independent claim. Preferred embodiments
of the invention are described in the dependent claims.
[0024] Thus the invention relates to a method for concentrating
dilute sulfuric acid. In a method according to the invention, the
concentration of sulfuric acid is carried out by means of at least
two electrolytic cells. The invention is based on the observation
that the electro-osmotic water-drag effect occurring in the cation
conductive membrane of the cell can be utilized in the
concentration of sulfuric acid. The water-drag effect means that
typically 1-4 water molecules are transferred along with one proton
(H.sup.+) from the anode side of the cell to the hydrogen-producing
cathode side.
[0025] In addition to the method, the present invention also
relates to an apparatus for concentrating dilute sulfuric acid,
said apparatus comprising at least a first electrolytic cell (1)
and a second electrolytic cell (2). Thus the apparatus is provided
with a first electrolytic cell (1) for receiving dilute sulfuric
acid, and for producing concentrated sulfuric acid. Moreover, the
apparatus is provided with at least a second electrolytic cell (2)
for receiving the sulfuric acid concentrated in the first
electrolytic cell (1) from the first electrolytic cell (1), and for
further concentrating the sulfuric acid concentrated in the first
electrolytic cell (1).
BRIEF DESCRIPTION OF DRAWINGS
[0026] The invention is described in more detail below, with
reference to the appended drawings, where
[0027] FIG. 1 is a flowchart illustrating an apparatus suitable for
the method, said apparatus comprising two electrolytic cells,
[0028] FIG. 2 is a flowchart illustrating an apparatus suitable for
the method, said apparatus comprising two electrolytic cells,
[0029] FIG. 3 is a flowchart illustrating an apparatus suitable for
the method, said apparatus comprising four electrolytic cells,
[0030] FIG. 4 is a flowchart illustrating an apparatus suitable for
the method, said apparatus comprising five electrolytic cells,
[0031] FIG. 5 is a diagram illustrating an electrolytic cell
suitable for the method, and
[0032] FIG. 6 is a diagram illustrating a process concept applying
a method according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0033] The present invention relates to a method for concentrating
dilute sulfuric acid, where the concentration of sulfuric acid is
carried out by means of at least two electrolytic cells. In one
embodiment of the invention, the concentration of sulfuric acid is
carried out by means of at least three electrolytic cells. The
invention also comprises embodiments where the concentration of
sulfuric acid is carried out by means of at least four or at least
five electrolytic cells.
[0034] In a preferred embodiment of the invention, the electrolytic
cells are cells depolarized by sulfur dioxide. In a preferred
embodiment of the invention, the concentration of sulfuric acid is
carried out by means of electrolytic cells that are installed in a
cascading fashion and depolarized by sulfur dioxide, in which case
in the cascade, there are installed in succession 2 to 11
electrolytic cells, advantageously 2 to 5 electrolytic cells.
[0035] In the present application, the term `dilute sulfuric acid`
refers to a mixture of water and sulfuric acid, where the sulfuric
acid content is 78 wt % or less than 78 wt %. The notion
`relatively dilute sulfuric acid` is also included in the
definition of dilute sulfuric acid. Concentrated sulfuric acid
refers to a mixture of water and sulfuric acid, where the sulfuric
acid content is more than 78 wt %. The term `commercial strong
sulfuric acid` refers to sulfuric acid with a content of over 98 wt
%.
[0036] Dilute sulfuric acid and sulfur dioxide are fed to the anode
side of the first electrolytic cell and the following reaction
takes place on the anode side:
SO.sub.2(aq)+2(1+x)H.sub.2O.fwdarw.H.sub.2SO.sub.4(aq)+2(H.sub.2O).sub.x-
H.sup.+(aq)+2e.sup.- (9)
where x=1 to 4.
[0037] The hydrogen ion (proton) H.sup.+ does not remain as such in
water surroundings, instead water molecules gather around it, so
that complex ions such as (H.sub.2O).sub.xH.sup.+(aq), where x=1 to
4, are formed. The formed complex ions pass through the membrane of
the electrolytic cell to the cathode side, and the following
reaction takes place at the cathode:
2(H.sub.2O).sub.xH.sup.+(aq)+2e.sup.-.fwdarw.H.sub.2(g)+2x H.sub.2O
(10)
where x=1 to 4.
[0038] For example in one embodiment of the invention, each
hydrogen ion carries along three water molecules (x=3) according to
the following chemical equation:
SO.sub.2(aq)+8H.sub.2O.fwdarw.H.sub.2SO.sub.4(aq)+2(H.sub.2O).sub.3H.sup-
.+(aq)+2e.sup.-
[0039] The complex ions according to the example pass through the
membrane of the electrolytic cell to the cathode side and the
following reaction takes place at the cathode:
2(H.sub.2O).sub.3H.sup.+(aq)+2e.sup.-.fwdarw.H.sub.2(g)+6
H.sub.2O
[0040] The concentrated mixture of sulfuric acid and water obtained
from the anode side of the first electrolytic cell is fed together
with non-reacted and/or added sulfur dioxide to the anode side of
the second electrolytic cell, whereafter the described reactions
(9) and (10) take place in the second electrolytic cell. The
further concentrated sulfuric acid formed as a reaction product
from the anode side of the electrolytic cell is recovered or it is
fed to the anode side of the following electrolytic cell.
[0041] The used cation conductive membrane separating the cathode
and anode sides in the electrolytic cell is a functionally fixed
electrolyte. The membrane serves as an insulator, particularly as
an electric insulator and as a proton conductor, and prevents the
gases from flowing from one side of the membrane to the other. In a
preferred embodiment of the invention, the employed membrane
comprises a sulfonic acid membrane, such as a membrane of the
trademark Nafion.RTM..
[0042] In one embodiment of the invention, the concentration of the
sulfuric acid to be fed to the anode side of the first electrolytic
cell is 78 wt % or under 78 wt %, and preferably 1 to 25 wt %. In
one embodiment of the invention, the concentration of the sulfuric
acid to be fed to the cathode side of the first electrolytic cell
is 78 wt % or under 78 wt %, and preferably 1 to 35 wt %.
[0043] In one embodiment of the invention, sulfuric acid is
concentrated at a temperature of 10 to 120.degree. C., preferably
at a temperature of 20 to 100.degree. C., for example at a
temperature of 30.degree. C., 40.degree. C., 50.degree. C.,
60.degree. C., 70.degree. C., 80.degree. C. or 90.degree. C., in
which case the temperature fluctuation can be .+-.5.degree. C. In
an embodiment, sulfuric acid is concentrated at a absolute pressure
of 0.5 to 7 bar, for example at 1 bar, 2 bar, 3 bar, 4 bar, 5 bar
or at a pressure of 6 bar, in which case the pressure fluctuation
can be .+-.0.5 bar; advantageously sulfuric acid is concentrated at
a pressure of 1 to 5 bar, and preferably at a pressure of 1 to 3
bar.
[0044] In a preferred embodiment of the invention, concentrated
sulfuric acid is recovered and lead to a sulfuric acid plant.
[0045] Apart from the method, the present invention also relates to
an apparatus for concentrating dilute sulfuric acid, said apparatus
comprising at least a first electrolytic cell (1) and a second
electrolytic cell (2). Thus the apparatus is provided with a first
electrolytic cell (1) for receiving dilute sulfuric acid and for
producing concentrated sulfuric acid. Moreover, the apparatus also
comprises at least a second electrolytic cell (2) for receiving
sulfuric acid concentrated in the first electrolytic cell (1) from
the first electrolytic cell (1), and for further concentrating the
sulfuric acid concentrated in the first electrolytic cell (1).
[0046] In a preferred embodiment of the invention, the first
electrolytic cell (1) and/or the second electrolytic cell (2)
comprises an electrolytic cell depolarized by sulfur dioxide. In an
embodiment, the apparatus comprises a total of 2 to 5 electrolytic
cells that are installed in a cascading fashion and depolarized by
sulfur dioxide.
[0047] In a preferred embodiment of the invention, the first
electrolytic cell (1) comprises an anode side (1B) and a cathode
side (1E), and the second electrolytic cell (2) comprises an anode
side (2B) and a cathode side (2E). In addition, the apparatus
comprises a first set of feed elements (1A) for feeding dilute
sulfuric acid and sulfur dioxide to the anode side (1B) of the
first electrolytic cell (1) comprising the membrane (1D), in order
to generate the following reaction on the anode side (1B):
SO.sub.2(aq)+2(1+x)H.sub.2O.fwdarw.H.sub.2SO.sub.4(aq)+2(H.sub.2O).sub.x-
H.sup.+(aq)+2e.sup.- (9)
where x=1 to 4, in which case the formed complex ions pass through
the membrane (1D) of the electrolytic cell to the cathode side
(1E), and the following reaction takes place at the cathode:
2(H.sub.2O).sub.xH.sup.+(aq)+2e.sup.-.fwdarw.H.sub.2(g)+2x H.sub.2O
(10)
where x=1 to 4.
[0048] The apparatus further comprises a second set of feed
elements (1C) for feeding the concentrated mixture of sulfuric acid
and water obtained from the anode side (1B) of the first
electrolytic cell (1), together with non-reacted and/or added
sulfur dioxide, to the anode side (2B) of the second electrolytic
cell (2). The above described reactions (9) and (10) are arranged
to take place also in the second electrolytic cell (2). The
apparatus further comprises a third set of feed elements (2C) for
feeding the further concentrated sulfuric acid, formed as a
reaction product from the anode side of the second electrolytic
cell (2) to the recovery means (6) or further to the anode side of
the third electrolytic cell (3) belonging to the concentration
apparatus.
[0049] In a preferred embodiment of the invention, the membrane
(1D) in the first electrolytic cell (1) and/or the membrane (2D) in
the second electrolytic cell (2) is a cation conductive
functionally fixed electrolyte separating the anode side (1B/2B)
and the cathode side (1E/2E). It is arranged to serve as an
insulator, particularly as an electric insulator and as a proton
conductor, and to prevent the gases from flowing from one side of
the membrane to the other. In a preferred embodiment of the
invention the membrane comprises a sulfonic acid membrane, such as
a membrane of the trademark Nafion.RTM..
[0050] In a preferred embodiment of the invention, the apparatus
comprises means (1I) for adding sulfur dioxide to the concentrated
mixture of sulfuric acid and water obtained from the anode side
(1B) of the first electrolytic cell (1), which mixture is fed
together with non-reacted and/or added sulfur dioxide to the anode
side (2B) of the second electrolytic cell (2).
[0051] In a preferred embodiment of the invention, the apparatus
comprises means for removing hydrogen from the cathode side of the
electrolytic cells, as well as means for the recovery of hydrogen.
Moreover, the concentration apparatus can include means for
recycling catholyte from the cathode side of one electrolytic cell
to the cathode side of the second electrolytic cell, and/or means
for recycling the catholyte of the last electrolytic cell to the
anode side of the first electrolytic cell. The catholyte can be
recycled both concurrently and countercurrently with respect to the
anode side stream, and advantageously the catholyte is recycled
countercurrently. The apparatus can comprise for instance three
electrolytic cells, which are numbered in the order cell 1, cell 2
and cell 3, in which case the stream on the anode side flows from
the cell 1 to the cell 2 and further to the cell 3. In case the
catholyte is recycled countercurrently, the catholyte circulates on
the cathode side from the cell 3 to the cell 2 and further to the
cell 1. According to one embodiment, no catholyte at all is added;
instead the cathode distributor remains moist by means of the water
flowing through the membranes.
[0052] In one embodiment of the invention, the apparatus is
arranged to function at a temperature of 10 to 120.degree. C.,
preferably at a temperature of 20 to 100.degree. C., for instance
at a temperature of 30.degree. C., 40.degree. C., 50.degree. C.,
60.degree. C., 70.degree. C., 80.degree. C. or 90.degree. C., in
which case the temperature fluctuation can be .+-.5 .degree. C. In
an embodiment of the invention, the apparatus is arranged to
function at a absolute pressure of 0.5 to 7 bar, for instance at a
pressure of 1 bar, 2 bar, 3 bar, 4 bar, 5 bar or 6 bar, in which
case the pressure fluctuation can be .+-.0,5 bar. Preferably, the
apparatus is arranged to function at a pressure of 1 to 5 bar, and
more preferably at a pressure of 1 to 3 bar. In one embodiment, one
or several electrolytic cells function in mutually different
temperature and/or pressure circumstances. Also the anode side and
the cathode side can function in mutually different temperature
and/or pressure circumstances according to an embodiment of the
invention, and preferably the pressure on the anode side is higher
than the pressure on the cathode side of the same cell. The
pressure difference between the anode and cathode sides can be even
2 to 6 bar.
[0053] In one embodiment of the invention, sulfur dioxide gas is
used for simultaneously producing hydrogen and concentrated
sulfuric acid (roughly 98 wt %) by dividing the sulfur dioxide gas
stream into two sub-streams. The first sub-stream of the divided
sulfur dioxide gas stream is conducted to the production of
hydrogen and sulfuric acid, and the second sub-stream is conducted
to the production of sulfur trioxide, which sulfur trioxide is used
for concentrating sulfuric acid to a commercial strong sulfuric
acid.
* * * * *