Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
1~ Field of the Invention
This invention relates to an electrolytic
cell for carrying out electroche~ical reactions in which
a gas and a liquid electrolyte flow co-currently through
a fluid permeable conductive mass which acts as an
electrode and is related to Canadian Patent lrQ50~477
2. Description of the Prior Art
The literature contains description of fixed
bed electrodes with single phase flow and of the use of
gas to promote turbulences in the electrolyte between
conventional plate electrodes. Packed bed electrodes
have been considered unsuitable for reactions that
generate gases because the presence of gas is supposed
to raise the cell resistance to unacceptable levels.
However gas and liquid flow is commonplace in conducting
chemical (as opposed to electrochemical) reactions where
a liquid and gas must be contacted simultaneously with
a solid catalyst. Such reactors, with co-current,
downward flow of liquid and gas through a bed of
catalyst particles, are called "TRICKLE BED" reactors.
Apart from the effect of the gas on cell
resistance, the difference between the chemical and
electrochemical processes in this connection is that in
the chemical system the reaction occurs over the whole
of the accessible catalyst surface, no matter how large
the catalyst bed, whereas in the electrochemical system
the reaction only occurs over a narrow section of the
bed (up to about 2 cm.) nearest the counter electrode
and normal to the directior. of current flow.
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Recent designs of electrolytic cells are
described in Grangaard, U. S. patents 3,454,477; 3,507,769;
3,459,652 and 3,592,749. ~rangaard used as an electrode a
porous carbon plate with the electrolyte and oxygen
delivered from opposite sides for reaction on the plate.
His porous gas diffusion electrode requires careful
balancing of oxygen and electrolyte pressure to keep the
reaction zone evenly on the surface of the porous plate.
-Moreover, as stated in U.S. patent 3,507,769, the Grangaard
cell gives a peroxide concentration of only 0.5% with an
NaOH/H2O2 ratio of 4/1. As described in U. S. patent
3,459,652, the Grangaard cathode consists of a specially
prepared active carbon ~hich is expensive to produce and
also deteriorates with time.
Another feature of the Grangaard cell is that
it contains an anode chamber and a cathode chamber separated by a
semi-pervious diaphragm and requires the flow of electro-
lyte from the anode to the cathode chamber under a small
hydrostatic head, to prevent the reaction of peroxide on
the anode and a double pass electrolyte feed arrangement
as described in U. S. patent 3,592,749. This has several
disadvantages:
1) It complicates the construction of the
cell;
2~ It increases the electrical resistance
of the cell by the resistance of the liquid in the anode
chamber;
3~ It complicates the operation of the cell,
insofar as the flow of both gas and electrolyte must be
continuously balanced for the proper condition ~o prevail
in the cathode chamber. This becomes particularly
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difficult witll flow arrangement as illustrated in U.S.
patent 3,592,749;
4) The gas generated at the anode must be
collected and pumped back to the cathode.
It is the ob~ect of the present invention to
provide a simplified and improved electrolytic cell for
carrying out electrochemical reactions involving a gas
and a liquid electrolyte.
Summary of the Invention
The electrochemical cell comprises a pair of
spaced apart electrodes, at least one of said electrodes
being in the form of a fluid permeable conductive mass
separated from the counter electrode by a barrier wall
Inlets are provided for feeding liquid electrolyte and gas
into the permeable electrade mass such that the electrolyte
and gas move co-currently through the permeable mass in a
direction perpendicular to the direction of travel of the
current bet~een the electrodes and outlet means are pro-
vided for removing solutions containing reaction products
from the fluid permeable conductive mass. The conduc-
tive mass usually forms the cathode of the cell and can
convenientLy have a thickness of about 0.1 to 2.0 cm. in
the direction of current flow.
The cathode mass can be in the form of a bed
of particles or a fixed porous matrix. It is composed
of a conducting material which is a good electrocatalyst
for the reaction to be carried out.
Graphite has been found to be particularly
suitable for the cathode because it is cheap and required
no special treatment. However, other forms of carbon may
be used as well as tungsten carbide, and certain metals,
such as gold, platinum, iridium, etc. coated on a
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conducting or a non-conducting substrate. In particu-
late form the particles typically have diameters in the
range of about 0.005 to 2.0 cm. and can form either a
fixed or fluidized bed. This bed of particles is made
to act as the cathode in electrochemical reactions.
The so-called "barrier wall" is a physical
insulating barrier which prevents the cathode particles
from coming into actual contact with the anode. It may
be an ion specific membrane or it may be a simple insul-
ating mechanical separator which permits free flow of
electrolyte and the passage of gas between the cathode
and anode. This can conveniently be a plastic fiber
cloth or the like, for example polypropylene, which is
compressed against the anode plate by the cathode bed.
Of course a variety of materials can be used for making
the porous insulating sheet provided they can withstand
attack by alkali solutions and have high electrical
resistance, e.g. asbestos, etc. Preferably the porous
insulating sheet has an air permeability when dry between
about 10 and 100 SCFM/ft at 1/2" water guage pressure
differential. In this respect,it must be suEficiently
permeable to allow gas generated on the anode to escape
into the cathode bed, but its permeability must be bal-
anced against that of the bed so that the bulk gas flow
does not by-pass the bed via the insulating sheet itsel~.
Thus, if an open filament plastic mesh is used, the cell
resistance is very high and the efficiency is very low
because gas flow is predominantly down along the plastic
mesh. On the other hand, a tightly woven asbestos c]oth
30 gave high resistance and low efficiency because it was
impervious to gas generated on the anode.
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According to other preferred features, the
cathode bed has a thickness oE about 0.1 to 2.0 cm. in
the direction oE current flow and a length in the direction
of travel of electrolyte of ab~ut 0.3 to 3 meters.
The electrolytic cell according to this in-
vention has been found to be particularly useful for
processes involving gaseous reactants with low solubility
in the electrolyte. It is also useful for any electro-
chemical process requiring a low real current density,
in which the co-current flow of gas improves the efficiency
of the electrode reactions.
For instance, it can be used for reduction
processes such as the reduction of oxygen to peroxide,
the reduction of sulphur dioxide to produce sodium di-
thionite, the reduction of nitric oxide to hydroxylamine
and the reduction of carbon dioxide to formic acid.
In other reactions, the gas may be an inert
gas in which the inert gas in the cathode bed modifies
the hydrodynamic characteristics of the system and thus
promotes an increased current efficiency. Here the gas
flow may raise the rate of mass transfer of a reactant
already present in the electrolyte, decreasing the liquid
hold-up in the electrolyte, modify the current distribution
in the electrode or otherwise enhance the selectivity of
the electrochemical reaction. The inert gas reactions
can include: (i) electrowinning metals from di.ute solutions
of their ions, e.g. zinc from zinc sulfate, copper from
copper sulfate, etc; (ii) generation of sodium hypochlorite
from dilute sodium chloride solutions; ~iii) electro-
oxidation of cyanide in waste solutions from metal treatingplants; (iv~ oxidation and reduction of organic compounds
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which are sparingly soluble in aqueous electrolytes, such
as the oxidation of phenol to carbon dioxide or the re-
duction of nitrobenzene to aniline.
It was q~ite unexpectedly found, for instance
in the production of peroxides using a porous barrier
wall that the peroxide formed on the cathode is nat
entirely destroyed on the anode and a reasonable current
efficiency for peroxide production can be maintained
even though the electrolyte is allowed to circulate
freely between the cathode and the anode. This allows
for great simplification in reactor design and a decrease
in operating costs. Moreover, it has been found that
with this system it is possible to obtain a product
peroxide concentration of greater than 3~ from a single
pass of the electrolyte through the reactor.
Thus, another feature of this invention relates
to a process for carrying out electrochemical reactions
involving a gaseous component and a liquid electrolyte in
an electrolytic cell having a pair of spaced apart
electrodes, which comprises passing a liquid electrolyte
and n gas simul~aneously in a direction normal to the flow
of electric current, between the electrodes, through a
fluid permeable conductive mass forming an electrode bed in
said cell, said bed being separated from the other electrode
by a porous insulating layer or an ion permeable membrane,
whereby an electrolysis product is generated in the solution
within the electrode bed by reaction between the liquid
electrolyte and gas on the surface of the fluid permeable
conductive mass and removing solutions containing reaction
products and gas from said conductive mass.
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~ ccording to an alternative arrangement, the
barrier wall can be in the form of a cation specific
membrane which forms separate cathode and anode chambers.
There are then separate anolyte and catholyte flows
through the two chambers.
The system is preferably operated at a super-
atmospheric gas pressure, e.g. in the range of about
0.2 to 30 atmospheres absolute, and this high pressure,
together with the turbulent action of the gas and the
electrolyte through the cathode bed permits the use of
quite high superficial current densities, e.g~ in the
range of 10 3 to 1.0 Amp. cm
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The operating temperature can conveniently
be in the range of 0 - 80C. Increased temperatures
tend to lower the solubility of the gas in the catholyte,
but increase the electrolyte conductivity.
There are a number of general advantages to
the system of the invention, as follows:
(i) The flow of gas together with liquid
enhances the mass transfer in the
electrode and thus allows the use of
higher current densities than would be
possible with the liquid alone at a
given flow rate.
(ii) The gas can supply a reactant for the
electrode process.
(ili) The presence of gas decreases the
liquid hold up in the electrode and
thus suppresses the loss of current
efficiency due to unwanted side
reactions.
(iv) The flow of gas helps to cool the
reactor by evaporstion.
Moreover, there are specific advantages in
the system of the present invention over the systems
described in the prior art as exemplified by the Grangaard
patents. Thus, the cell of the present invention is much
simpler in design as compared with the previous cells
and it can produce a solution containing up to 3% of
hydrogen peroxide with an NaOH/H202 ratio of 2/1. This
ratio is critical to the commercial use of this solution
in pulp bleaching and compared with a peroxide concentra-
tion from the Grangaard cell of only 0.5% with an
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NaOH/H2O2 ratio of 4/1. Moreover, the high pressures
possible with the system of this invention permits much
higher superficial current densities than are permissible
with the Grangaard cell. The cathode material used in
the present unit is cheaper and more readily available
than those described in the prior art and with a single
pass electrolyte flow, where it is not necessary to
separate the catholyte from the anolyte, no problems
of alkalinity build up in the anolyte or sodium ion build
up in the catholyte occur. This is a prevailing problem
in the prior art systems and, for instance, in U. S.
patent 3,592,749 Grangaard required a complicated double-
pass electrolyte flow arrangement to overcome the
problem.
Description of the Preferred Embodiments
Certain specific embodiments of this inven-
tion will now be illustrated by reference to the
following detailed description and accompanying drawings
wherein:
FIG. 1 is a schematic cross-sectional view
of a cell for electrochemical reactions in accordance
with the invention;
FIG. 2 is a cross-sectional view of one
preferred arrangement of the cell shown in FIG. l;
FIG. 3 is a side elevation of the cell
shown in FIG. 2;
FIG. 4 is a side elevation of a graphite
cathode bed;
FIG. 5 is a side elevation of a barrier wall;
FIG. 6 is a cross-sectional view of another
preferred embodiment of the cell, and
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FIG. 7 is a cross-sectional view of yet
another embodiment of the cell.
FIG. 1 is a general schematic illustration
of the cell according to the invention showing the main
components in simplified form. It includes a pair of
current carriers 10 and 11 which are preferably stain-
less steel and adjacent current carrier 11 is a fluid
permeable conductive mass 12 which can be a fixed
porous mass or a bed of discrete particles. On the
opposite side of the conductive mass 12 is an insulating
barrier 13 which can be a porous plastic fabric or an
ion specific membrane. Between the barrier 13 and the
current carrier 10 is a gap 14 but it is also possible
for the barrier 13 to be in actual contact with the
current carrier 10. With this arrangement the conduc-
tive mass 12 becomes one electrode while the current
carrier 10 then becomes the counter electrode.
Streams of liquid electrolyte 15 and of gas
16 are fed in co-currently from the top of the cell and
the product is removed through the bottom outlet 17.
~ specific preferred embodiment is illus-
trated in FIG. 2 and this shows a single cell sandwiched
between a pair of compression plates 20 and 21. Imme-
diately adjacent these compression plates are insulating
layers 22 and 23, these being followed by a 304 stainless
steel cathode current conductor 24 and a 304 stainless
steel anode plate 25 respectively. Within the gap
between the plates 24 and 25 is a cathode bed composed
of graphite particles (UCAR Type No. 1 available from
Union Carbide Corporation~ in the size range 0.42 to
0.30 mm. Positioned between this cathode bed 26 and anode
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25 is a diaphragm of felted polypropylene (National
Felt Company Type PP15) with a permeability of 25 - 35
NCFMlft min. at 1/2" W.G. An inlet 28 and an outlet
29 are provided for flow through the cathode bed 26.
The compression plate 20 is shown in
greater detail in FIG. 3 and includes a flat base plate
30 with upstanding reinforcing webs 31. The base plate
30 includes a series of bolt holes 32 as well as an
inlet opening 33 and an outlet opening 34.
The cathode bed is shown in greater detail
in FIG. 4 and it will be seen that the cathode bed is
retained at the top, bottom and sides between plates 24
and 25 by means of a surrounding casket 37 made from
"Durabla" impregnated asbestos.
The barrier wall 27 is shown in greater
detail in FIG. 5 and it will be seen that the felted
polypropylene material 38 is surrounded by an edge
gasket 39 which engages the edge gasket 37 of the
cathode bed so that when the entire unit is assembled
as shown in FIG. 2 the internal flow region of the
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D cell is enclosed by these eR~ts. Of course, the
entire unit is held together between the compression
plates by means of the series of bolts 35 which pass
through the holes 32 in the compression plates.
FIG. 6 illustrates a unit with five cells,
using bi-polar electrodes. This cell is generally
constructed as shown in FIG. 2 with the same compression
plates 20 and 21 but in place of the single cathode bed
of FIG. 2, there is positioned between the terminal
electrodes 40 and 41 a series of five cathode beds.
These are formed by means of four intermediate electrode
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plates 42 formed ~rom 1/32" thick 304 stainless steel
with appropri~te holes 45 for gas and liquid distribu-
tion between the cells. Adjacent each intermediate
electrode plate 42 is a barrier wall 43 formed from a
woven polypropylene cloth available from the Wheelabrator
Corp. Type S4140 enclosed within a neoprene peripheral
gasket. The space adjacent each barrier wall is filled
with graphite particles 44 as described in relation to
Fig. 2. When particles of graphite are used, it is evident
that these must be retained by some means adjacent the
holes 45 so that they do not touch the counter-electrode
plates. This is conveniently done by means of screen
associated with the holes 45 which retain the graphite
particles while allowing the liquid and gass to pass.
Again the top and bottom and side edges are enclosed by
neoprene gaskets so as to provide a series of parallel
cells to which the liquid electrolyte and gas flow from
inlet 28 to outlet 29.
The cell has dimensions 76 cm long by 5 cm
wide with an active superficial area of about 350 cm
per cell. Current delivered throu~h the terminal elec-
trodes 40 and 41 passes through each cell in series
with the other plates acting as bi-polar electrodes.
~nother embodiment of the cell is shown in
FIG. 7. This includes a pair of 3/h" thick mild steel
compression plates 50 and 51 with a lead cathode feeder
plate 52 and a stainless steel anode plate 53. These
electrodes are spaced from the compression plates by
means of peripheral spacers 54 and 55 forming water
cooling chambers 56 and 57. The chamber 56 has a water
inlet 58 and a water outlet 60 while the chamber 57
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has a water in}et 59 and ~ water olltlet 61. Between the
electrodes 52 and 53 are positioned a membrane support
screen 67 and a cation specific membrane (AMF, Type
C100) with a gap between screen 67 and electrode 52
being filled by tungsten carbide particles in the size
range 0.42 - 0.33 mm and the gap 71 between membrane 68
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and electrode 53 being empty. The cathode region 66 and
the gap 71 are enclosed by means of peripheral gaskets
70.
~ ith this design reactants are fed in through
inlet 62 and these travel co-currently down through the
cathode beds 66 and out through product outlet 63. An
anolyte liquid is passed in a reverse flow through lower
inlet 64 up through the gap 71 and out through anolyte
outlet 65.
lQ The following examples are given to illustrate
the invention but are not deemed to be limiting thereof.
EXAMPLE 1
A cell was prepared according to FIGs. 2 to
5 and was used to produce alkaline peroxide solution by
electro-reduction of oxygen. A single electrolyte
solution of sodium hydroxide in water was passed together
with oxygen gas through the inlet 28, down through the
cathode bed 26 and out through outlet 29. The reaction
was carried out under the following conditions:
Sodium hydroxide feed concentration - 2M
Gas feed composltion ~ 99'5~ 2
Electrolyte flow - 10 cm3/min
Oxygen flow - 1500 cm /min S.T.P.
Inlet pressure - 10 Atm Absolute
Outlet pressure - 9.6 Atm Absolute
Inlet temperature - 20C
Outlet temperature - 30C
Current - 30 Amp (= .13A/cm )
Voltage across ceil - l.g Volt
The electrolyte leaving the cell contained
0.62 Molar hydrogen peroxide, corresponding to a current
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efficiency Eor peroxide production of 67% and power con-
sumption of 2 Kwhr/lb of H202.
EXAMPLE 2
An alkaline peroxide solution was also pre-
pared using the five cell unit shown in FIG. 6. The
electrolyte and oxygen were distributed by the manifold
to flow through all five cells in parallel and the
operating conditions were as follows:
Sodium hydroxide feed concentration - 2M
10 Gas feed composition ~ 99 5% 2
Electrolyte flow (total) - 55 cm3/min
Oxygen flow (total) - 7500 cm /min S.T.P.
Inlet pressure - ll Atm.
Exit pressure - 7 Atm.
Exit temperature - 46C
Current - 30 Amp ~= 0.086 A/cm )
Voltage Cell 1 2 3 4 5
1.61 1.57 1.59 1.52 1.64
Electrolyte leaving the cell conta~ned 0.65 M
peroxide, corresponding to a current efficiency of 78%
and a power consumption of 1.44 K~hr/lb ~l22
EXA~PLE 3
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Reduction of Sulphur Dioxide to Dithionite
A cell ~as constructed in the same way as that
of example 1 (Figure 2), with an anode of 1/16 inch lead
sheet and a cathode bed of tungsten carbide particles in
the size range of 0.4 to 0.2 mm. The external dimensions
of the cathode bed were 40 cm high x 5 cm wide x 0.3 cm
thick and the diaphragm was a nylon felt. Water and
30 a mixture of sulphur dioxide and nitrogen gas were passed
co-currently down through the cell and a current passed
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to convert the sulphur dioxi~e to dithionous acid which
was neutralised at the reactor exit and analysed as sodium
dithionite. The following are the conditions and results
of this run.
l~ater flow cm /min 32.5
Sulphur dioxide flow cm3/min tS.T.P.) 500
Nitrogen flow cm /min (S.T.P.) 500
Current Amp 4
Voltage 4-3
Product dithionite conc M 0.02
Current efficiency % 53%
Power consumption Kwh/kg dithionite 3.0
Example 4 Electrowinning of copper
A cell was constructed in the same way as that
of example 1 ~figure 2) with an anode of 1/16 inch
lead sheet and a cathode bed of graphite particles in
the size range -.99+.30 mm. The external dimensions of
the cathode bed were 50 cm high x S cm wide x 0.3 cm thick
and the diaphragm was felted nylon. A solution of copper
sulphate and sulphuric acid in water was pumped downward
through the cell and current passed to deposit copper on
the cathode bed. Two runs were made, one with the copper
sulphate solution alone flowing through the cell and
another with a co-current flow of the inert gas, nitrogen.
The followin~ are the conditions and results of these
runs.
Run 1 Run 2
Concentration of copper sulphate
in electrolyte feed M. 0.018 0.018
pH of electrolyte feed 2.1 2.1
~lectrolyte flow cm3/min 23 23
Gas flow (nitrogen) cm /min (STP) 0 300
Current Amp. 1.0 1.0
Voltage 1.8 1.8
Temperature C. 25 25
Inlet pressure P.S.I.G. 2 8
30 Concentration of copper sulphate
in electrolyte product M. 0.012 0.007
Current efficiency % 45 83
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These results show how the apparatus can be used
with a flow of an inert gas to achieve a good efficiency
of copper removal fro~ the electrolyte.
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