CA1122562A - Electrochemical cell having particulate electrode separated from counter electrode by porous barrier - Google Patents

Abstract

ELECTROCHEMICAL PROCESSES AND APPARATUS THEREFOR

Abstract of the Disclosure A novel electrolytic cell is described for carrying out electrochemical reactions in which a gas and a liquid electrolyte flow co-currently through a fluid permeable conductive mass which acts as an electrode.
The cell has an anode and cathode in spaced apart rela-tionship, with one electrode being in the form of a fluid permeable conductive mass e.g. a porous matrix or a packed bed of graphite particles, separated from the other electrode by a barrier wall. This barrier wall can be either a cation specific membrane dividing the cell into separate cathode and anode chambers or a porous insulating wall permitting free flow of electrolyte between the cathode and anode.
A liquid electrolyte and a gas are passed co-currently through the electrode bed perpendicular to the current flow and the reaction product is generated in the solution within the electrode bed.

Description

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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 ~2~
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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.

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 ~225~;2 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 - 6a -~L2ZS~iZ

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 `
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 ~225~;~
-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 ~2~

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 9`' 5`,ÇC 7L3 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 il~2;~
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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 ` ~ ' t' .
. _ ~ . ' 2~2 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 - lla -..~,:.,,

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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.

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 ~2Z5~2 efficiency Eor peroxide production of 67% and power con-sumption of 2 Kwhr/lb of H202.

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 _~ .
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 ~22~6Z

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 .. , ... .... ...... ... , . .. . . _ _, , ., ~ _ _ . ....

1~2;~SÇi2 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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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for carrying out electrochemical reactions comprising an electrochemical cell having a pair of spaced apart electrodes, at least one of said electrodes being in the form of a fluid permeable conductive mass and being separated from the counter electrode by a barrier wall selected from an ion permeable membrane and a porous insulating layer, said barrier wall having a permeability sufficiently large to allow gas generated on the counter electrode to escape into the fluid permeable conductive mass but not so large as to allow a major portion of the gas to by-pass the fluid permeable conductive mass by travelling within the barrier wall itself, inlet means at one end of said cell for feeding a liquid electrolyte and a gas into said fluid permeable conductive mass and outlet means at the opposite end of said cell for removing solutions con-taining reaction products from said conductive mass, said inlet and outlet being arranged whereby the electrolyte and gas move co-currently through the conductive mass in a direction gener-ally parallel to the electrodes and normal to the flow of electric current between the electrodes.

2. Apparatus according to claim 1 in which the thickness of the fluid permeable conductive mass in the direction of current flow is about 0.1 cm to 2.0 cm.

3. Apparatus according to claim 1 in which the electrode mass is in the form of a bed of conductive particles.

4. Apparatus according to claim 3 in which the conductive particles are in the size range of about 0.005 cm to 2 cm.

5. Apparatus according to claim 1 in which the length of the electrode mass in the direction of liquid flow is from about 0.3 to 3.0 meters.

6. Apparatus according to claim 1 in which the barrier wall is a porous electrically insulating layer.

7. Apparatus according to claim 6 in which the porous sheet is held between the electrode mass and the counter electrode to prevent their contact and permits the flow of gas and liquid between the electrodes.

8. Apparatus according to claim 7 in which the permeability of the sheet is between about 10 and 100 SCFM/ft2 1/2" water gauge differential pressure.

9. Apparatus according to claim 8 wherein the porous sheet is a fabric selected from a polypropylene fabric and an asbestos fabric.

10. Apparatus according to claim 1 in which said ion permeable membrane is positioned to form separate anode and cathode chambers.

11. Apparatus according to claim 3 wherein the conductive particles form a cathode bed, held between said barrier wall and a metallic current conductor plate.

12. Apparatus according to claim 3 in which the conductive particles are composed of materials selected from the group consisting of graphite, tungsten carbide, and conducting and non-conducting substrates coated with metals selected from gold, platinum and iridium.