CA1324783C - Multilayer electrode and electrolytic cell containing such electrode - Google Patents
Multilayer electrode and electrolytic cell containing such electrodeInfo
- Publication number
- CA1324783C CA1324783C CA000561171A CA561171A CA1324783C CA 1324783 C CA1324783 C CA 1324783C CA 000561171 A CA000561171 A CA 000561171A CA 561171 A CA561171 A CA 561171A CA 1324783 C CA1324783 C CA 1324783C
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- layer
- fibers
- cell
- catholyte
- anolyte
- Prior art date
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Abstract
ABSTRACT OF THE INVENTION
An improved multiple layered porous electrode for use in an electrochemical membrane cell is provided having a first perforated support layer, a second layer of fibers, a third layer of fibers less dense than the second layer and a fourth layer of meshed wire cloth.
The electrode may be used as a cathode in the cell for the electrochemical production of an alkali metal hydrosulfite by the reduction of an alkali metal bisulfite component of a circulated aqueous catholyte solution. Additionally, a flow barrier is provided for interrupting the catholyte flowing between the top and bottom of the cell to cause substantially all the catholyte to change flow direction and pass twice through the cathode plate.
An improved multiple layered porous electrode for use in an electrochemical membrane cell is provided having a first perforated support layer, a second layer of fibers, a third layer of fibers less dense than the second layer and a fourth layer of meshed wire cloth.
The electrode may be used as a cathode in the cell for the electrochemical production of an alkali metal hydrosulfite by the reduction of an alkali metal bisulfite component of a circulated aqueous catholyte solution. Additionally, a flow barrier is provided for interrupting the catholyte flowing between the top and bottom of the cell to cause substantially all the catholyte to change flow direction and pass twice through the cathode plate.
Description
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MULTILAYER ELECTRODE AND ELECTROLYTIC
CELL CONTAINING SUCH ELECTRODE
B~CKGROUN~ OF ~H~ TNVFNTION -This invention relates to the improved structure of an electrode for use in an electrochemical membrane cell. More particularly the present invention relates to a multilayered electrode used for the commercial production of concentrated hydrosulfite solutions in an electrochemical membrane cell.
Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites, such as sodium hydrosulfite or potassium hydrosulfite, electrochemically that can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite involves the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, .: ~ ':
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current densities must be employed in a cell which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.
Further, where the solution3, which are strong reducing agents effective as bleaching agents, are to be used in the paper industry, the undesirable byproduct formation of thiosulfate as an impurity from hydrosulfite must be minimized. At high concentrations of hydrosulfite, however, this byproduct reaction becomes more difficult to control.
Additionally, prior electrochemical routes to hydrosulfite have produced aqueous qolutions which are unstable and decompose at a rapid rate. This high decomposition rate of hydrosulfite appears to increase as the pH decreases or the reaction temperature increases. One approach to control the decompo~ition rate is to decrea~e the residence time of the solution in the cell and to maintain the current den~ity as high as possible up to a critical current density above which secondary reactions will occur due to ;
polarization of the cathode.
Some of the proces~es of the prior art, which claim to make hydrosulfite salts electrochemically, require the use of water-miscible organic solvents, such as methanol, to reduce the solubility of the hydrosulfite and prevent it~ decomposition inside the cell. The costly recovery of the methanol and hydrosulfite makes this route uneconomical.
The use of zinc as a stabilizing agent for hydrosulfites in electrochemical proce~ses has also been reported, but because of environmental ~
consideration~, this i8 no longer commercially --practical or desirable.
More recently, U.S. Patent No. 4,144,146 issued March 13, 1979 to B. Leutner et al describes an ~
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electrochemical proces~ for producing hydro3ulfite solutions in an electrolytic membrane cell. The proceRs employs high circulation rates for the catholyte which is passed through an inlet in the bottom of the cell and removed at the top of the cell to provide for the advantageous removal of ga~es produced during the reaction. Catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm per second and the cathode i~ formed of fibrous mats of compressed ~intered fibers with a mesh spacing of 5 mm or less. The process is de3cribed as producing concentrated solutions of alkali metal hydrosulfites at commercially viable current densities;
however, the cell voltages required are high, being in the range of 5 to 10 volts. This results in exces~ive energy consumption. There i8 no indication of the concentrations of thiosulfate impurity in the product solutions. '~
The availability of electrodes with a high ~ -mass transfer surface area having a high surface area ~ ~-to volume ratio and sufficient poro~ity have limited -~
the development of a commercially practical electrochemical cell de~ign for the production of ~ -aqueous solutions of alkali metal sulfites with low concentrations of alkali metal thiosulfates as impurities. -~
These and other problem~ are solved in the design of the present invention employing an improved multiple layer electrode in an electrolytic membrane cell for the production of alkali metal hydrosulfite.
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1324783 :: :
SUMMARY OF THE INVENTION
----It is an object of the pre~ent invention to provide an improved multiple layer electroae for u~e in an electrochemical cell.
It is another object of the present invention to provide an improved multiple layer electrode that i~
highly porous and which cau~es a low overall pre~sure drop of electrolyte fluid flowing through the electrode.
It i8 a further object of the present invention to provide an improved multiple layer cathode for use in an electrochemical membrane cell producing aqueous alkali metal hydro~ulfite solutions with lower alkali metal thiosulfate impurity concentration3 at high current densities.
It is a feature of the pre~ent invention that the improved multiple layer electrode possesses high oxidation and reduction selectivity at high current -~
densities in a non-ga3 evclving reaction.
It is another feature of the present invention that the improved multiple layered electrode has at least a support layer, a first fiber layer and a second less dense fiber layer that are compressed and bonded together.
It is still another feature of the pre~ent invention that the catholyte flow path forces the ~-~
catholyte to make multiple passes through the multiple layered porous cathode formed of sintered wire fiber3 positioned between the perforated ~upport layer or plate and a me3h screen.
It i~ yet another feature of the present invention that a cathode flow barrier i~ employed to -direct the catholyte flow stream through the cathode.
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1~24783 It is an advantage of the present invention that high oxidation/reduction selectivity at high current den ities is obtained in the electrolytic membrane cell using the multiple layered electrode.
It is another advantage of the present --invention that there is a low overall pressure drop of electrolyte hori~ontally acros~ the multiple layered electrode a~ the electrolyte makes its multiple passes therethrough.
It is yet another advantage of the present ,~
, invention that the low overall pressure drop of ¦ electrolyte and the design of the multiple layered electrode does not require the use of a high pressure pump to circulate the electrolyte.
These and other objects, feature~ and ~ -advantages of the invention are provided in an electrolytic membrane cell for the electrochemical production of an alkali metal hydrosulfite by the ,~
reduction of an alkali metal bisulfite component of a -~ -circulated aqueous catholyte solution in a cell having an improved multiple layer extended surface multipass porous cathode and an improved catholyte flow path, the multiple layer cathode having a support layer, a first ! fiber layer, a second fiber layer and a screen mesh ~ 25 layer.
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,, BRIEF DESCRIPTTON OF T~E DRAWI~GS
The objects, features and advantages of the invention will become apparent upon consideration of the ~.
following detailed disclosure of the invention, especially when it is taken in conjunction with the ~ :
accompanying drawings wherein: ~-FIGURE 1 is a diagrammatic exploded view of an electrolytic cell constructed in accordance with the present invention and showing the electrolyte flow paths :.
and the ion flow paths;
FIGURE 2 is a side elevational view of the anode side of the bipolar cell electrode showing a portion of the anode rods that cover the anode backplate, further having some of these shown rods :, :
broken away;
FIGURE 3 is an enlarged partial sectional .
view taken along the lines 3--3 of FIGURE 2 showing the ~
anode rods as they are fastened to the electrode; - ~:
FIGURE 4 is a side elevational view of the cathode side of the bipolar electrode; :~
FIGURE 5 is a side sectional view of the bipolar electrode element of the electrolytic cell showing the flow path of the catholyte through the -:~
porous cathode in the cathode compartment from the - - ::
catholyte distribution slots to the catholyte collection slots or conduits; :;
FIGURE 6, appearing on the same sheet of ~ -Drawings as Figures 2 and 3, is a side elevational view ~ - :
of the separator screen that is positioned between the anode rods and the membrane; and FIGURE 7 is a partial side sectional view of - :
the cathode plate showing the multiple layers of the : :
electrode structure. i ~
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1:3~ 3 DETAILED DESCRIPTION OF THE PREFERRED EMBODIME~T
As 3een in the exploded and partially diagrammatic illustration in FIGURE 1, a filter pres~
membrane electrolytic cell, indicated generally by the numeral 10, i8 shown consisting of an anode backplate 11, separator means 21, cation selective membrane 25, a porou~ cathode plate 26, and a cathode backplate 28.
The anode backplate 11 and cathode backplate 28 form the opposing side~ of the bipolar electrode, which can be machined from a stainle~ steel plate or can be cast ~tainle~ ~teel. The stainless steel plate can, for example, be formed of 304L or 316 stainless steel as thicX as 1 1/4" which i~ resi~tant to corrosion and i~ simply fabricated by machining the flat plate to create chambers through which the anolyte and catholyte fluids can pass into their respective anolyte and catholyte chambers. The thickness of the stainless steel plate provides stiffness and an extremely precise flatness to the structure. The cathode plate 26 is mounted to the cathode plate 28 by screws (not shown) which are screwed into cathode support pedestals 31, while the anode rods 12 may be welded, ~uch as by TIG welding, in place without warping the stainless steel plate.
The anode structure can be seen in ~reater detail in FIGURES 2-4.- As seen in FIGURE 2, the anode backplate 11 has a plurality of parallel positioned, vertically extending anode rods 12 welded at the top and bottom portions of the rods to the anode backplate 11. These rods 12 extend across the entire width of the anode backplate 11, although for simplicity of illu~tration the continuous side-by-sidelarrangement has not been shown in FIGURE 2 since rods in the central portion of the anode backplate 11 have been ~ -~
13247~3 omitted entirely. These rods are, for example, 1/8"
diameter nickel wire rods ~paced apart from each other to create an anode inter-rod gap 20 of approximately 1/16" between adjacent rod These anode rods 12 can be formed from nickel 200, or any other corrosion re istant composition providing low overvoltage characteristics. The vertical positioning of the anode rod~ 12 with the anode inter-rod gap 20, ~ee briefly FIGURE 3, provides clear flow channels from the bottom of the anode backplate 11, where the anolyte fluid ~ enters via anolyte entry port~ 18 into an anolyte 3 distribution groove 15, to the top. Anolyte fluid flows vertically upwardly in the anode inter-rod gaps 3 20 to the anolyte collection groove 16 before the liquid exits the cell through the anolyte exit port~
~ 19. The vertical positioning of the anode rods 12 j provides even current distribution across the anode and ~ avoids gas blinding that can occur from the buildup of 3 ga~ bubbles, which can consequently reduce the current ~ 20 density in the operating cell. ;--¦ Both the anolyte entry ports 18 and the anolyte exit ports 19 have transition slots 18' and i 19', respectivelyi that are machined into the stainle~s teel plate. The anolyte entry port tran~ition slots , 25 18' are machined into the anolyte di~tribution groove 15 to provide a smooth transition surface that is , tapered and avoids erosion corrosion which can ;l interfere with the gmooth flow of the anolyte into the cell 10 and which will provide metal contamination as the erosion and corrosion occurs. The anolyte exit I port transition slots 19' are both ~imilarly positioned ~nd machined.
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, 132~7~3 An anode gasket groove 14 is machined into the anode backplate 11 about the entire periphery. The groove, for example, is 3/8" wide by 3/16" deep to receive a rectangular anode gasket (not shown) that is 3/8" wide by 3/8" deep. This gasket can have a strip of material, such as material sold under the tradename of GORE-TEX* or TEFLON*, positioned over the gasket to come into contact with the plastic separator means 21 ~-when the cell is compressed and assembled.
The plastic separator means 21 shown in Figure 6 is formed from any material resistant to anolyte corrosion, and preferably polypropylene has been employed. ,~n 8 mesh polypropylene fabric with an approximately 40% open area has been successfully ~ -employed, as has a titanium dioxide filled polyethylene mesh. The separator means 21 has a separator frame 22 that is solid about the periphery and a separator mesh 24 on the interior of the separator frame 22. The mesh 24 is treated with a hydrophilic coating to prevent gas bubbles from adhering to the mesh and the adjacent membrane by capillary action. A coating of titanium dioxide applied to the mesh 24 has been successfully employed as the hydrophilic coating. Preventing the buildup of gas bubbles on the membxane and in the mesh avoids cell voltage fluctuations during operation.
The use of the separator means 21 also has successfully prevented the buildup of regions of locally high acidity in the adjacent membrane where the membrane touches against the nickel anode rods 12.
Having the membrane 25 touch against the nickel anode rods 12 can create pockets of high acidity because the sulfur species become oxidized to sulfuric acid due to the slow migration of the sulfur species back through the membrane during operation of the cell. The nickel oxide coating on the anode rods 12 breaks down and .''' * Trade-mark ...~
lo 1324783 ~ ~ ~
nickel corrosion occur~. This corrosion is tran~ported through the membrane to the cathode side of the cell 10. There this nickel corrosion is reduced to the metallic ~tate by the hydrosulfite solution. This metallic state nickel adheres tightly to the membrane on the cathode side and will impair the transport of ions and f luia through the membrane.
The anode has been designed ~o that the -;
anolyte which is electrolyzed in the cell 10 i8 any suitable electrolyte which i~ capable of supplying ~ -~
alkali metal ions and water molecules to the cathode compartment. Suitable as anolyte~ are, for example, alkali metal halides, alkali metal hydroxides, or alkali metal persulfates. The selection of anolyte i8 : ~ .:.:
in part dependent on the product desired. Where a halogen gas such as chlorine or bromine is wanted, an aqUeOU8 801ution of an alkali metal chloride or bromide i~ used as the anolyte. Alkali metal hydroxide solutions are chosen where oxygen gas or hydrogen 2~ peroxide is to be produced. If persulfuric acid i8 the desired product, an alkali metal persulfate is employed. However, alternate materials of construction, such as titanium group metals for the ~-anolyte wetted parts with an alkali metal chloride anolyte, would be nece3sary for each particular anolyte utilized.
In any case, concen~rated solu~ions of the electrolyte selected are employed as the anolyte. For example, where sodium chloride i3 selected as the alXali metal chloride, suitable solutions as anolytes contain from about 12 to about 25 percent by weight of NaCl. Solutions of alkali metal hydroxides, such a~
~odium hydroxide, contain from about 5 to about 40 porc-nt by wcight o2 ~Oh.
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The cell 10 preferably ha~ been operated with caustic ~oda. Where caustic soda (NaOH~ is u~ed, water and the caustic ~oda enter through the anolyte entry slots 18 and the solution flows along the high velocity flow path between the adjacent anode rods 12 and the anode inter-rod gaps 20 at the rear of the anolyte compartment toward the top of the cell 10.
Thus, most of the anolyte fluid volume flow occurs between the anode rods 12 and within the hydrophilically treated separator mesh 24. The sodium ion3 migrate acros~ the membrane, being produced as a result of the electrolysi~ reaction forming oxygen, water and soaium ions, 4NaOH > 2 + 4~a 1 2H2o. ~ .
Depleted caustic passes out with oxygen and water throug~ the anolyte exit ports 19.
The cathode backplate 28 i8 best seen in :~
FIGURE 4, while the monolithic nature of the electrode that is machined fro~ the solid stainles3 steel plate - -can be seen in FIGURE 5. Since the cell is bipolar, the cathode is on one ~ide of the stainless steel plate :
on the cathode backplata 28 side, while the anode backplate 11 and the anode is on the opposing side. As : -seen best in FIGURE 4, the cathode backplate 28 has catholyte entry ports 35 on the opposing sides of the bottom portion of cathode backplate 28 that feed in catholyte into the catholyte distribution groove 32. ~ : -Catholyte di~tribution groove 32, catholyte entry ports ::~
35, and the machined catholyte transition slots 35' are positioned just above the corresponding anolyte :
distribution groove 15, anolyte ports 18 and the anolyte transition slots 18', but are on the opposite - :-Ride of the solid stainless steel electrode plate. ~ :
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A lower catholyte chamber 38 is positioned .:
immediately above the catholyte distribution groove ~--32. The lower catholyte chamber 38 is separated from the upper catholyte chamber 39 by a generally horizontally positioned cathode flow barrier 30. Flow barrier 30 extends across the entire width of the -~
catholyte chamber and protrudes outwardly from the plane of the catholyte backplate 28, as can be seen ... .. .
al~o in FIGURES 1 and 5. Cathode flow barrier 30 interrupt~ the vertical flow of catholyte fluid upwardly from the lower catholyte chamber 38 into the upper catholyte chamber 39, thereby causing the catholyte fluid to flow in a path shown by the arrows -in FIGURE 1 that takes it twice through the cathode plate 26 enroute to the upper catholyte chamber 39.
This flow path results in a cathode with a highly effective surface area, but require3 the u~e of a very porous cathode plate that will permit at least 30% by ~:
volume of the catholyte fluid to flow through the porous cathode plate 26 rapidly to hold to a minimum the residence time of the catholyte in the cell. As will be described in greater detail hereafter, once the ~ -catholyte fluid has reached the upper catholyte chamber : :
39 it enters the catholyte collection groove 34 and e~its the cell through the machined catholyte exit transition slots ~6' and catholyte exit port~ 360 Weep hole~ 17, as seen in FIGURES 4 and 5, can be used in the cathode flow barrier 30 to permit hydrogen gas to rise from the lower catholyte chamber 38 to the upper catholyte chamber 39. Alternately or concurrently weep holes 33, seen in FIGURE 5, can be ~ :~
u~ed to permit the hydrogen gas to pa 8 out of the :~
interelectrode gap between the walls of the lower and `,'~:,.
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upper catholyte chamber~ 38 and 39 and the cathode plate 26 ~UCt below the cathode flow barrier 30 and then back through the cathode plate 26 oppo~ite the catholyte collection groove 34.
The cathode plate 26 i8 held in place on the catholyte backplate 28 by a plurality of screws (not shown) that seat within the plurality of cathode ~upport pedestals 31 within the lower and upper catholyte chambers 38 and 39.
The cathode plate 26 i~ a highly porous multilayer ~tructure. It comprises a ~upport layer formed of perforated ~tainle~s steel. This ~upport layer forms the mounting base and protects the I innermetal fiber felt layer that is formed of, for 1 15 example, 15~ dense, very fine 4 to 8 micron fibers and 15~ dense 25 micron fibers laid on top of one another.
A wire screen of, for example, la meah with a .009 inch wire diameter i8 then placed atop the fiber felt to form a cathode that has a porosity of preferably ~
between 80 and 85%. The cathode plate 26, thu3, i~ a ~ -four layered ~intered composite with all of the materials made of Ytainless ~teel, preferably 304 or 316 stainless ~teel,-and in the appropriate sheet size. The highly effective surface area of cathode plate 26 is achieved by the use of low density metal felt formed from very fine element~. -A cathode gasket groove 29 is seen in FIGURE
4 extending about the periphery of the cathode backplate 28. Although not shown, a 3/8" round EPDM, ~ ;
ethylene-propylene-diene monomer, gasket is used to ~ -seat within the cathode gasket groove 29 to effect , fluid-tight sealing. -~ -? ~ -'I "
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132~7~3 Reduction occurs at the cathode in the cell -:
10 by the electrolysis of a buffered aqueous ~olution of an alkali metal bisulfite. A typical reaction is ais follows:
4NaHS03 + 2e + 2Na ~ a2S204 + 2~a2S03 + 2H2 Depleted caustic and sulfur dioxide are mixed to form -:
NaHS03 that i~ fed into the catholyte distribution groove 32 via the catholyte entrance ports 35 and the catholyte transition i~lots 35'. This catholyte liquid then ri~e3 vertically upwardly until it passes out through the cathode plate 26, a best seen in FIGURES 5 or 1. The cathode flow barxier 30 acts as a block to the straight vertical flow of the catholyte fluid . : : -upwardly from the lower catholyte chamber 38 into the upper catholyte chamber 39. There i~i an approximately :
1/8" interelectrode cathode gap between the walls of the :~
lower and upper catholyte chambers 38 and 39 and the ;
cathode plate 26 that i~ seated on the cathode support .
pedestals 31. ~he catholyte fluid then pa~ es through the cathode plate 26 and continuesi flowing upwardly : . :-~.
through the cathode-membrane gap until it passes the -cathode flow barrier 30. At this point the catholyte -~
fluid passes back through the highly porous cathode .'~
plate 26 into the upper catholyte chamber 39 and then into the catholyte collection groove 34. The cell :~
product solution containing Na2S204 (dithionite) . : :
exits the cell 10 through the catholyte exit transition ; ~:~
slots 36' and the catholyte exit ports 36.
A buffer solution containing from about 40 to about 80 gpl of bisulfite i~ utilized with the catholyte because of sodium thiosulfate formation resulting from ~:,', , ' 132~783 the reduction and decompo~ition of hydrosulfite (dithionite) and the pH change of the catholyte as bisufite is consumed and sulfite i8 formed according to the reaction Na2S2O4 + 2e + 2Na + 2NaHSO3 ~ Na2S2O3 + 2Na2S3 + H2O-This hydrosulfite decomposition reaction is electrolytically driven by the presence of electron~.
When the potential is increased, ~o is the current density and to a point, the reaction rate of this undesired thiosulfate producing reaction.
The value of the multiple layered cathode plate 26 is particularly evident in its selectivity.
Because the multiple layered electrode has an increased surface area, it requires les~ voltage or a lower potential to drive the primary reduction reaction that produces the desired hydrosulfite product and, thereby, -reauces the amount of the undesired thiosulfate produced by the hydrosulfite decomposition reaction. The ~-increased ~urface area permits the potential to be maintained at the lower level where the primary or desired hydro~ulfite producing reaction predominates and generally below the level where the hydrosulfite decomposition reaction becomes a factor.
The u~e of a monolithic cell body, that is a bipolar cell body or bacXplate formed from a single plate of stainless steel machined to form an anode backplate on one side and a cathode backplate on the opposing side, provide~ ~everal significant inherent operating advantages. Initially, there is no shifting ~ -or dimen3ional instability because of the joining of two separate pieces of material to form the electrode.
There is a reduction in the number of actual cell components from the use of a ~ingle machined plate.
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-16- 132~783 La~tly, and perhap~ most ~ignificantly, there i8 the elimination of electrical 1088 from the contact between two separate anode and cathode elements that would otherwise have ~ome spacing and sizing differences.
~ 5 This particular configuration contribute~ to lower cell ¦ electrical energy consumption.
The hydraulic pressure in cell 10 is e~tablished 80 that the membrane 25 is kept pressed against the separator means 21 and off of the cathode plate 26. Keeping the membrane 25 80 po~itioned al80 : -permits the flow path through the cathode plate to be accomplished. The cathode flow barrier 30 further contributes to the hydraulics of the cell 10 by achieving a uniform pressure across the entire height of -~-lS the cathode due to the flow inversion characteristic~
achieved by the multiple flow paths through the cathode -~
plate 26. ~1-The electrolytic cell 10 is operated at current den~ities which are sufficient to produce solutions of alkali metal hydrosulfites having the concentrations desired. For example, where sodium hydrosulfite is produced for commercial sale, the ~ -¦ solutions contain from about 120 to about 160 grams per i liter. However, since the alkali metal hydrosulfite solutions sold commercially are usually diluted before ~ -use, these dilute aqueous solutions can also be produced I directly by the process.
¦ Current densities of at least 0.5 kiloamperes I per square meter are employed. Preferably the current j 30 density i~ in the range of from about 1.0 to about 4.5, and more preferably at from about 2.0 to about 3.0 kiloamperes per square meter. At these high current densities, the electrolytic cell 10 operate~ to produce the required volume of high purity alkali metal hydrosulfite solution which can be employed commercially without further concentration or purification.
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The electrolytic membrane cell 10 employs a cation exchange membrane between the anode and the cathode compartments which prevents any sub~tantial migration of ~ulfur-containing ions from the cathode compartment to the anode compartment. A wide variety of cation exchange membranes can be employed containing a variety of polymer resins and functional groups, provided the membranes posse~ the requisite ~ulfur ion selectivity to prevent the deposition of sulfur inside membrane~. Such deposition can blind the membranes, the result of ~ulfur 3pecies diffusing through the membranes and then being oxidized to create acid within the membranes that causes hydrosulfite and thiosulfate to decompose to sulfur in acidic conditions. This 3electivity can be verified by analyzing the anolyte for sulfate ions.
Sultable cation exchange membranes are those which are inert, flexible, and substantially impervious to the hydrodynamic flow of the electrolyte and the passage of gas product~ produced in the cell. Cation exchange membranes are well-known to contain fixed anionic group~ that permit intrusion and exchange of cations, and exclude anions, from an external source.
Generally the resinous membrane has as a matrix or a cross-linked polymer to which are attached charged radicals, such as --S03, --COO , - P03, - HP02, - A~03, and - SeO3 and mixtures thereof. The re~in~
which can be used to produce the membranes include, for example, fluorocarbons, vinyl compounds, polyolefins, and copolymers thereof. Preferred are cation exchange membranes such as those comprised of fluorocarbon polymers having a plurality of pendant sulfonic acid groups or carboxylic acid groups or mixtures of sulfonic acid groups and carboxylic acid groups. The terms "sulfonic acid group" and "carboxylic acid groups" are meant to include salts of sulfonic acid or salt~ of ;;~;
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, , -18- 132 ~7 83 carboxylic acid groupY by proces~es such as hydroly~is.
Suitable cation exchange membranes are sold commercially by E. I. DuPont de Nemours & Co., Inc. under the trademark "Nafion", by the Asahi Glass Company under the trademark "Flemion", by the A~ahi Chemical Company under ;
the trademark "Aciplex". Perfluorinated sulfonic acid membranes are also available from the Dow Chemical Company.
The membrane 25 i8 positionea between the anode and the cathode and is separated from the cathode by a cathode-membrane gap which is wide enough to permit the catholyte to flow between the cathode plate 26 and the membrane 25 from the lower catholyte chamber 38 to the upper catholyte chamber 39 and to prevent ga~
blinding, but not wide enough to ~ubstantially increase electrical resi~tance. Depending on the 40rm of cathode plate 26 used, this cathode-membrane gap is a distance of from about 0.05 to about 10, and preferably from about 1 to about 4 millimeters. The cathode-membrane gap can be maintained by hydraulic pres3ure or mechanical means. Thi~ design and the catholyte flow path permits almost all of the catholyte liquid to contact the active area of the cathode. Further, with this design the majority of the electrolytic reaction occurs in the cathode area nearest the anode.
Suitable porous cathode plates 26 u~ed in the cell 10 have at least one layer with a total ~urface area to volume ratio of greater than 100 cm2 per ,~
cm3, preferably 250 cm2 per cm3, and more preferably greater than 500 cm2 per cm3. These structures have a porosity of at leaqt 60 percent and preferably from about 70 percent to about 90 percent, -where porosity is the percentage of void volume. The ratio of total surface area to the projected surface ~r-a of th- porou~ cathod- plat- 26, wh-r- tb- pro~-ct-d e~
' ,--19- ~32~783 surface area i~ the area of the face of the cathode plate 26, i9 at least about 30:1 and preferably at least from about 50:1: for example, from about 80:1 to about 100:1.
~The cathode plate 26 i~ comprised of four layers, as best ~een in FIGURE 7. The first layer i9 a ~upport layer 41 of perforated stainless ~teel plate approximately 0.036 inches thick with 1/16 inch holes 42 on 1/8 inch 60 degree staggered centers. These hole~ 42 in the first layer 41 give the perforated stainless steel plate an open area of approximately 23 percent.
The second layer 44 is preferably formed of 304 ~ -stainles~ steel fibers 45 that are about 20 to about 100 microns in diameter and, preferably, about 25 micron~ in diameter. The second layer 44 has the density of zpproximately 0.62 pounds per square foot. Alternately, the second layer can be a woven screen mesh with a 30 fiber~ per inch by 30 fibers per inch square weave and about a 23% open area. The third layer 48 i8 much less dense than the second layer 44 and is compri~ed of 304 stainles~ steel fibers 49 of about 4 to about 16 microns --in diameter and, preferably, about 8 microns in diameter. The density of the third layer 48 i~ about 0.12 pounds per square foot. The limiting factor in the ~ - -diameter sizing of the fibers of the second layer 44 and the third layer 48 is that the fibers 45 of the second layer 44 can't extend up through the fibers 49 of the third layer 48. The fourth layer 50 is a mesh wire cloth that is preferably 18 x 18 mesh with about a 0.009 inch diameter of the individual wires in the wir0 --cloth. The four layers 41, 44, 48, and 50 are compressed together and bonded by ~intering in a reducing atmosphere, such as hydrogen, ammonia or carbon monoside, to form a single sheet with a preferred thickness of about 0.155 inches, plus or m$nus about 0.008 inches.
~: ' --20- 1 3 2~7 83 The cathode plate 26 of thi~ design provide~ a high mass transfer capability because the surface area of the electrode i~ the same surface area u~ed for electrolysis. Becau~e the cathode plate 26 is relatively thin, it require~ a high surface area to volume ratio, thereby creating a high total surface area. Thi~ is especially applicable to the third layer 48 which is the most active layer and is critical in determining the characteristics of the cathode. The high surface area of the cathode plate 26 is measured as the ratio of the total surface area of the individual fibers 45, 49 to the superficial or projected ~urface area of the total cathode plate 26. The relationship between these parameters can be expres~ed a~ the thickne3s of an individual layer in the cathode plate 26 times the surface area to volume ratio equal~ the superficial surface area. The selection of the fiber~
45 of the second layer 44 with a greater individual diameter than the fibers 49 of the third layer 48 and an equal porosity provide a structure that has a greater ,~
surface area near the membrane 25. Fibers 45 and fibers 49 can equally well be s21ected from different grades of stainless steel, nickel, steel, copper, carbon graphite or ferrous and non-ferrous alloys. The cathode plate 26 could have an electroactive coating, such as ruthenium oxide or platinum, added to the surface area to obtain a more active electrode surface area.
Current is conducted into the cell 10 through anode and cathode current conductor plates (not shown). ,~
Plate~ of c~pper the ~ize of the electrodes are placed against the end cathode and end anode in each cell 10. -~
~lectrical connections are made directly to these copper plates. An insulator plate made, for example, of polyvinyl chloride or other suitable plastic, and a ~-compres3ion plate (both not ~hown) made ~or example, of -21- 1 3 2 ~7 83 ~tainles~ steel or steel, are placed against ea~h end of the cell 10 before it i~ aqsembled to form a sandwich around the de~ired number of electrode~ that are positioned therebetween.
The cell of the instant invention could also be designed a~ monopolar, requiring that both sides of each stainleqs steel plate be identically machined and that half electrodes be used as the end electrodes in the assembled cell. The current conductors in the monopolar design would then be standard copper electrical terminal~ for each electrode.
Additionally the cell of the present invention could be utilized in electrochemical reactions other than the production of hydrosulfite. Typical i~ the production of organic products by electrochemistry, such I as the electrochemical transformations of pyridines ¦ through oxidation or reduction reaction~ in a ¦ cation-exchange membrane divided cell of the instant design.
Employing the novel de~ign of the cell 10, concentrated alkali metal hydrosulfite solutions are ~ -produced having low concentrations of alkali metal thiosulfate~ as an impurity in electrolytic membrane ;-~ ;
cells operat~ng at high current den~ities, substantially reduced cell voltages, and high current efficiencies.
In order to exemplify the re~ults achieved, the following examples are provided without an intent to ~-limit the scope of the instant invention to the discussion therein.
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Example 1 A cell of the type 3hown in FIGURES 1-5 was assembled from three stainles~ steel plates which were mounted on a rack to form two anode/cathode pairs who~e active electrode area was about 0.172 square meters S each. The plates formed two half electrodes, one a cathode and the other an anode, sandwiched about a bipolar electrode with opposing anode and cathode faces. The outside dimen~ions of the electrode plates were about 17 inche~ wide by about 18.5 inches high and about 1.0 inches thick.
The anodes were comprised of about forty-seven (47) 1/8 inch diameter nickel 200 rod welded onto the anode backplate, as shown generally in FIGURE 2, with approximatsly 1/16 inch separation between the rods.
The anolyte collection and distribution grooves were ~-about 1.25 inches wide and about 0.61 inches deep. ~^
The cathode plate was formed from a four layered sheet cut to size. The first layer wa~ a ~-~
support layer formed of perforated stainless steel 0.036 inches thick with 1/16 inch holes on 1/8 inch 60 staggered centers having a 23% open area. The second layer was a 0.62 pound~ per square foot layer of 304 t;~,~ ,h ~tainless steel fibers about 25 microns in diameter.
The third layer was a 0.12 pound~ per square foot layer of 304 stainless steel fiber~ about 8 microns in diameter. The fourth layer was an 18" x 18" mesh of about 0.009 inch diameter wire cloth. These layers were compress2d together and bonded by sintering in a hydrogen atmosphere to form a single sheet with a thickness of about 0.155 inches. The cathode sheet was cut to form a cathode plate of about 18.5 inches by about 17 inche~ -. . .
.' ' ' The cathode plate was mountsd onto the ~tainless ~teel cathode backplate using 20 ~crew~ of about 1/8 inch diameter that seated into the cathode ~upport pedestals within the catholyte chambers. A
small coating of appropriate electrical joint compound wa~ used on the thread~ of the screwR and a silicon cement was placed over the head of each screw to prevent the screw from becoming an active part of the cathode assembly.
Six (06) 1/6 inch diameter hole3 were drilled in the cathode plate to permit ga~ bubble~ trapped within the cell to escape. Three of the holes were drilled near the top of the cell opposite the catholyte collection groove and three ju~t below the cathode flow -~
barrier. ~-Separator means were formed from polypropylene mesh treated with a coating of titanium dioxide. The ~eparators were mounted in 1/16 inch thick separator frame~ cut to fit just in~ide the gasXet groove in the cell.
Gasket grooves about 0.375 inches wide and about 0.187 inche~ deep were machined into both the anode and cathode backplates. On the anode siae of the cell about a 0.375 inch square gasXet was used with about a 0.5 inch wide strip of about 0.060 thick GORE-TEX~ gasXet tape placed on top. In the cathode gasket groove a rubber O-ring of about a 0.378 inch diameter was used. The cell was as~embled using a portable hydraulic a~embly system de~cribed in U.S.
. .
Patent No. 4,430,179 that compre~sed the cell together -~ ~
80 that approximately a 1/8 inch gap between the anode ~---and the cathode plate~ remained. The cell was then ¦ secured by retaining nuts. -~
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The cell wa~ operated continuously for 42 days. The cell employed a NAFION~ NX 906 perfluorinated membrane that was soaked in about 2%
sodium hydroxide solution for at least 4 hour~ prior to as~embling.
The cell wa~ operated at a temperature of approximately 25C with a total catholyte flow rate of about 6 gpm and a total anolyte flow rate of about 4 gpm. ~xcess anolyte containing about 19% sodium hydroxide was continuously purged and added to the catholyte circulation while the anolyte wa~ continuously replenished with the addition of about 69 grams per minute of about 35% sodium hydroxide solution. About 230 milliliters per minute of deionized water was continuou~ly added to the catholyte, as was sulfur dioxide to the catholyte to maintain a pH of between ~
about 5.4 and about 5.8 and a sulfite to bisulfite molar ~ -ratio of about 1:3 to about 1:8.
Product catholyte was drawn from the cell 2~ continuously at a rate of about 287 milliliters per minute and was analyzed periodically during each day.
The product catholyte reflected in the following Table I
was analyzed from samples taken at the same time each day. These data are representative of the operation of the cell during 4 days of operation under optimized conditions. The catholyte wa~ analyzed for sodium hyarosulfite, sodium thiosulfate, sodium ~ulfite and sodium bisulfite content.
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MULTILAYER ELECTRODE AND ELECTROLYTIC
CELL CONTAINING SUCH ELECTRODE
B~CKGROUN~ OF ~H~ TNVFNTION -This invention relates to the improved structure of an electrode for use in an electrochemical membrane cell. More particularly the present invention relates to a multilayered electrode used for the commercial production of concentrated hydrosulfite solutions in an electrochemical membrane cell.
Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites, such as sodium hydrosulfite or potassium hydrosulfite, electrochemically that can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite involves the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, .: ~ ':
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current densities must be employed in a cell which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.
Further, where the solution3, which are strong reducing agents effective as bleaching agents, are to be used in the paper industry, the undesirable byproduct formation of thiosulfate as an impurity from hydrosulfite must be minimized. At high concentrations of hydrosulfite, however, this byproduct reaction becomes more difficult to control.
Additionally, prior electrochemical routes to hydrosulfite have produced aqueous qolutions which are unstable and decompose at a rapid rate. This high decomposition rate of hydrosulfite appears to increase as the pH decreases or the reaction temperature increases. One approach to control the decompo~ition rate is to decrea~e the residence time of the solution in the cell and to maintain the current den~ity as high as possible up to a critical current density above which secondary reactions will occur due to ;
polarization of the cathode.
Some of the proces~es of the prior art, which claim to make hydrosulfite salts electrochemically, require the use of water-miscible organic solvents, such as methanol, to reduce the solubility of the hydrosulfite and prevent it~ decomposition inside the cell. The costly recovery of the methanol and hydrosulfite makes this route uneconomical.
The use of zinc as a stabilizing agent for hydrosulfites in electrochemical proce~ses has also been reported, but because of environmental ~
consideration~, this i8 no longer commercially --practical or desirable.
More recently, U.S. Patent No. 4,144,146 issued March 13, 1979 to B. Leutner et al describes an ~
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1324783 :
electrochemical proces~ for producing hydro3ulfite solutions in an electrolytic membrane cell. The proceRs employs high circulation rates for the catholyte which is passed through an inlet in the bottom of the cell and removed at the top of the cell to provide for the advantageous removal of ga~es produced during the reaction. Catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm per second and the cathode i~ formed of fibrous mats of compressed ~intered fibers with a mesh spacing of 5 mm or less. The process is de3cribed as producing concentrated solutions of alkali metal hydrosulfites at commercially viable current densities;
however, the cell voltages required are high, being in the range of 5 to 10 volts. This results in exces~ive energy consumption. There i8 no indication of the concentrations of thiosulfate impurity in the product solutions. '~
The availability of electrodes with a high ~ -mass transfer surface area having a high surface area ~ ~-to volume ratio and sufficient poro~ity have limited -~
the development of a commercially practical electrochemical cell de~ign for the production of ~ -aqueous solutions of alkali metal sulfites with low concentrations of alkali metal thiosulfates as impurities. -~
These and other problem~ are solved in the design of the present invention employing an improved multiple layer electrode in an electrolytic membrane cell for the production of alkali metal hydrosulfite.
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1324783 :: :
SUMMARY OF THE INVENTION
----It is an object of the pre~ent invention to provide an improved multiple layer electroae for u~e in an electrochemical cell.
It is another object of the present invention to provide an improved multiple layer electrode that i~
highly porous and which cau~es a low overall pre~sure drop of electrolyte fluid flowing through the electrode.
It i8 a further object of the present invention to provide an improved multiple layer cathode for use in an electrochemical membrane cell producing aqueous alkali metal hydro~ulfite solutions with lower alkali metal thiosulfate impurity concentration3 at high current densities.
It is a feature of the pre~ent invention that the improved multiple layer electrode possesses high oxidation and reduction selectivity at high current -~
densities in a non-ga3 evclving reaction.
It is another feature of the present invention that the improved multiple layered electrode has at least a support layer, a first fiber layer and a second less dense fiber layer that are compressed and bonded together.
It is still another feature of the pre~ent invention that the catholyte flow path forces the ~-~
catholyte to make multiple passes through the multiple layered porous cathode formed of sintered wire fiber3 positioned between the perforated ~upport layer or plate and a me3h screen.
It i~ yet another feature of the present invention that a cathode flow barrier i~ employed to -direct the catholyte flow stream through the cathode.
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1~24783 It is an advantage of the present invention that high oxidation/reduction selectivity at high current den ities is obtained in the electrolytic membrane cell using the multiple layered electrode.
It is another advantage of the present --invention that there is a low overall pressure drop of electrolyte hori~ontally acros~ the multiple layered electrode a~ the electrolyte makes its multiple passes therethrough.
It is yet another advantage of the present ,~
, invention that the low overall pressure drop of ¦ electrolyte and the design of the multiple layered electrode does not require the use of a high pressure pump to circulate the electrolyte.
These and other objects, feature~ and ~ -advantages of the invention are provided in an electrolytic membrane cell for the electrochemical production of an alkali metal hydrosulfite by the ,~
reduction of an alkali metal bisulfite component of a -~ -circulated aqueous catholyte solution in a cell having an improved multiple layer extended surface multipass porous cathode and an improved catholyte flow path, the multiple layer cathode having a support layer, a first ! fiber layer, a second fiber layer and a screen mesh ~ 25 layer.
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,, BRIEF DESCRIPTTON OF T~E DRAWI~GS
The objects, features and advantages of the invention will become apparent upon consideration of the ~.
following detailed disclosure of the invention, especially when it is taken in conjunction with the ~ :
accompanying drawings wherein: ~-FIGURE 1 is a diagrammatic exploded view of an electrolytic cell constructed in accordance with the present invention and showing the electrolyte flow paths :.
and the ion flow paths;
FIGURE 2 is a side elevational view of the anode side of the bipolar cell electrode showing a portion of the anode rods that cover the anode backplate, further having some of these shown rods :, :
broken away;
FIGURE 3 is an enlarged partial sectional .
view taken along the lines 3--3 of FIGURE 2 showing the ~
anode rods as they are fastened to the electrode; - ~:
FIGURE 4 is a side elevational view of the cathode side of the bipolar electrode; :~
FIGURE 5 is a side sectional view of the bipolar electrode element of the electrolytic cell showing the flow path of the catholyte through the -:~
porous cathode in the cathode compartment from the - - ::
catholyte distribution slots to the catholyte collection slots or conduits; :;
FIGURE 6, appearing on the same sheet of ~ -Drawings as Figures 2 and 3, is a side elevational view ~ - :
of the separator screen that is positioned between the anode rods and the membrane; and FIGURE 7 is a partial side sectional view of - :
the cathode plate showing the multiple layers of the : :
electrode structure. i ~
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1:3~ 3 DETAILED DESCRIPTION OF THE PREFERRED EMBODIME~T
As 3een in the exploded and partially diagrammatic illustration in FIGURE 1, a filter pres~
membrane electrolytic cell, indicated generally by the numeral 10, i8 shown consisting of an anode backplate 11, separator means 21, cation selective membrane 25, a porou~ cathode plate 26, and a cathode backplate 28.
The anode backplate 11 and cathode backplate 28 form the opposing side~ of the bipolar electrode, which can be machined from a stainle~ steel plate or can be cast ~tainle~ ~teel. The stainless steel plate can, for example, be formed of 304L or 316 stainless steel as thicX as 1 1/4" which i~ resi~tant to corrosion and i~ simply fabricated by machining the flat plate to create chambers through which the anolyte and catholyte fluids can pass into their respective anolyte and catholyte chambers. The thickness of the stainless steel plate provides stiffness and an extremely precise flatness to the structure. The cathode plate 26 is mounted to the cathode plate 28 by screws (not shown) which are screwed into cathode support pedestals 31, while the anode rods 12 may be welded, ~uch as by TIG welding, in place without warping the stainless steel plate.
The anode structure can be seen in ~reater detail in FIGURES 2-4.- As seen in FIGURE 2, the anode backplate 11 has a plurality of parallel positioned, vertically extending anode rods 12 welded at the top and bottom portions of the rods to the anode backplate 11. These rods 12 extend across the entire width of the anode backplate 11, although for simplicity of illu~tration the continuous side-by-sidelarrangement has not been shown in FIGURE 2 since rods in the central portion of the anode backplate 11 have been ~ -~
13247~3 omitted entirely. These rods are, for example, 1/8"
diameter nickel wire rods ~paced apart from each other to create an anode inter-rod gap 20 of approximately 1/16" between adjacent rod These anode rods 12 can be formed from nickel 200, or any other corrosion re istant composition providing low overvoltage characteristics. The vertical positioning of the anode rod~ 12 with the anode inter-rod gap 20, ~ee briefly FIGURE 3, provides clear flow channels from the bottom of the anode backplate 11, where the anolyte fluid ~ enters via anolyte entry port~ 18 into an anolyte 3 distribution groove 15, to the top. Anolyte fluid flows vertically upwardly in the anode inter-rod gaps 3 20 to the anolyte collection groove 16 before the liquid exits the cell through the anolyte exit port~
~ 19. The vertical positioning of the anode rods 12 j provides even current distribution across the anode and ~ avoids gas blinding that can occur from the buildup of 3 ga~ bubbles, which can consequently reduce the current ~ 20 density in the operating cell. ;--¦ Both the anolyte entry ports 18 and the anolyte exit ports 19 have transition slots 18' and i 19', respectivelyi that are machined into the stainle~s teel plate. The anolyte entry port tran~ition slots , 25 18' are machined into the anolyte di~tribution groove 15 to provide a smooth transition surface that is , tapered and avoids erosion corrosion which can ;l interfere with the gmooth flow of the anolyte into the cell 10 and which will provide metal contamination as the erosion and corrosion occurs. The anolyte exit I port transition slots 19' are both ~imilarly positioned ~nd machined.
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, 132~7~3 An anode gasket groove 14 is machined into the anode backplate 11 about the entire periphery. The groove, for example, is 3/8" wide by 3/16" deep to receive a rectangular anode gasket (not shown) that is 3/8" wide by 3/8" deep. This gasket can have a strip of material, such as material sold under the tradename of GORE-TEX* or TEFLON*, positioned over the gasket to come into contact with the plastic separator means 21 ~-when the cell is compressed and assembled.
The plastic separator means 21 shown in Figure 6 is formed from any material resistant to anolyte corrosion, and preferably polypropylene has been employed. ,~n 8 mesh polypropylene fabric with an approximately 40% open area has been successfully ~ -employed, as has a titanium dioxide filled polyethylene mesh. The separator means 21 has a separator frame 22 that is solid about the periphery and a separator mesh 24 on the interior of the separator frame 22. The mesh 24 is treated with a hydrophilic coating to prevent gas bubbles from adhering to the mesh and the adjacent membrane by capillary action. A coating of titanium dioxide applied to the mesh 24 has been successfully employed as the hydrophilic coating. Preventing the buildup of gas bubbles on the membxane and in the mesh avoids cell voltage fluctuations during operation.
The use of the separator means 21 also has successfully prevented the buildup of regions of locally high acidity in the adjacent membrane where the membrane touches against the nickel anode rods 12.
Having the membrane 25 touch against the nickel anode rods 12 can create pockets of high acidity because the sulfur species become oxidized to sulfuric acid due to the slow migration of the sulfur species back through the membrane during operation of the cell. The nickel oxide coating on the anode rods 12 breaks down and .''' * Trade-mark ...~
lo 1324783 ~ ~ ~
nickel corrosion occur~. This corrosion is tran~ported through the membrane to the cathode side of the cell 10. There this nickel corrosion is reduced to the metallic ~tate by the hydrosulfite solution. This metallic state nickel adheres tightly to the membrane on the cathode side and will impair the transport of ions and f luia through the membrane.
The anode has been designed ~o that the -;
anolyte which is electrolyzed in the cell 10 i8 any suitable electrolyte which i~ capable of supplying ~ -~
alkali metal ions and water molecules to the cathode compartment. Suitable as anolyte~ are, for example, alkali metal halides, alkali metal hydroxides, or alkali metal persulfates. The selection of anolyte i8 : ~ .:.:
in part dependent on the product desired. Where a halogen gas such as chlorine or bromine is wanted, an aqUeOU8 801ution of an alkali metal chloride or bromide i~ used as the anolyte. Alkali metal hydroxide solutions are chosen where oxygen gas or hydrogen 2~ peroxide is to be produced. If persulfuric acid i8 the desired product, an alkali metal persulfate is employed. However, alternate materials of construction, such as titanium group metals for the ~-anolyte wetted parts with an alkali metal chloride anolyte, would be nece3sary for each particular anolyte utilized.
In any case, concen~rated solu~ions of the electrolyte selected are employed as the anolyte. For example, where sodium chloride i3 selected as the alXali metal chloride, suitable solutions as anolytes contain from about 12 to about 25 percent by weight of NaCl. Solutions of alkali metal hydroxides, such a~
~odium hydroxide, contain from about 5 to about 40 porc-nt by wcight o2 ~Oh.
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The cell 10 preferably ha~ been operated with caustic ~oda. Where caustic soda (NaOH~ is u~ed, water and the caustic ~oda enter through the anolyte entry slots 18 and the solution flows along the high velocity flow path between the adjacent anode rods 12 and the anode inter-rod gaps 20 at the rear of the anolyte compartment toward the top of the cell 10.
Thus, most of the anolyte fluid volume flow occurs between the anode rods 12 and within the hydrophilically treated separator mesh 24. The sodium ion3 migrate acros~ the membrane, being produced as a result of the electrolysi~ reaction forming oxygen, water and soaium ions, 4NaOH > 2 + 4~a 1 2H2o. ~ .
Depleted caustic passes out with oxygen and water throug~ the anolyte exit ports 19.
The cathode backplate 28 i8 best seen in :~
FIGURE 4, while the monolithic nature of the electrode that is machined fro~ the solid stainles3 steel plate - -can be seen in FIGURE 5. Since the cell is bipolar, the cathode is on one ~ide of the stainless steel plate :
on the cathode backplata 28 side, while the anode backplate 11 and the anode is on the opposing side. As : -seen best in FIGURE 4, the cathode backplate 28 has catholyte entry ports 35 on the opposing sides of the bottom portion of cathode backplate 28 that feed in catholyte into the catholyte distribution groove 32. ~ : -Catholyte di~tribution groove 32, catholyte entry ports ::~
35, and the machined catholyte transition slots 35' are positioned just above the corresponding anolyte :
distribution groove 15, anolyte ports 18 and the anolyte transition slots 18', but are on the opposite - :-Ride of the solid stainless steel electrode plate. ~ :
-12- ~3~7~3 ~
A lower catholyte chamber 38 is positioned .:
immediately above the catholyte distribution groove ~--32. The lower catholyte chamber 38 is separated from the upper catholyte chamber 39 by a generally horizontally positioned cathode flow barrier 30. Flow barrier 30 extends across the entire width of the -~
catholyte chamber and protrudes outwardly from the plane of the catholyte backplate 28, as can be seen ... .. .
al~o in FIGURES 1 and 5. Cathode flow barrier 30 interrupt~ the vertical flow of catholyte fluid upwardly from the lower catholyte chamber 38 into the upper catholyte chamber 39, thereby causing the catholyte fluid to flow in a path shown by the arrows -in FIGURE 1 that takes it twice through the cathode plate 26 enroute to the upper catholyte chamber 39.
This flow path results in a cathode with a highly effective surface area, but require3 the u~e of a very porous cathode plate that will permit at least 30% by ~:
volume of the catholyte fluid to flow through the porous cathode plate 26 rapidly to hold to a minimum the residence time of the catholyte in the cell. As will be described in greater detail hereafter, once the ~ -catholyte fluid has reached the upper catholyte chamber : :
39 it enters the catholyte collection groove 34 and e~its the cell through the machined catholyte exit transition slots ~6' and catholyte exit port~ 360 Weep hole~ 17, as seen in FIGURES 4 and 5, can be used in the cathode flow barrier 30 to permit hydrogen gas to rise from the lower catholyte chamber 38 to the upper catholyte chamber 39. Alternately or concurrently weep holes 33, seen in FIGURE 5, can be ~ :~
u~ed to permit the hydrogen gas to pa 8 out of the :~
interelectrode gap between the walls of the lower and `,'~:,.
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upper catholyte chamber~ 38 and 39 and the cathode plate 26 ~UCt below the cathode flow barrier 30 and then back through the cathode plate 26 oppo~ite the catholyte collection groove 34.
The cathode plate 26 i8 held in place on the catholyte backplate 28 by a plurality of screws (not shown) that seat within the plurality of cathode ~upport pedestals 31 within the lower and upper catholyte chambers 38 and 39.
The cathode plate 26 i~ a highly porous multilayer ~tructure. It comprises a ~upport layer formed of perforated ~tainle~s steel. This ~upport layer forms the mounting base and protects the I innermetal fiber felt layer that is formed of, for 1 15 example, 15~ dense, very fine 4 to 8 micron fibers and 15~ dense 25 micron fibers laid on top of one another.
A wire screen of, for example, la meah with a .009 inch wire diameter i8 then placed atop the fiber felt to form a cathode that has a porosity of preferably ~
between 80 and 85%. The cathode plate 26, thu3, i~ a ~ -four layered ~intered composite with all of the materials made of Ytainless ~teel, preferably 304 or 316 stainless ~teel,-and in the appropriate sheet size. The highly effective surface area of cathode plate 26 is achieved by the use of low density metal felt formed from very fine element~. -A cathode gasket groove 29 is seen in FIGURE
4 extending about the periphery of the cathode backplate 28. Although not shown, a 3/8" round EPDM, ~ ;
ethylene-propylene-diene monomer, gasket is used to ~ -seat within the cathode gasket groove 29 to effect , fluid-tight sealing. -~ -? ~ -'I "
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132~7~3 Reduction occurs at the cathode in the cell -:
10 by the electrolysis of a buffered aqueous ~olution of an alkali metal bisulfite. A typical reaction is ais follows:
4NaHS03 + 2e + 2Na ~ a2S204 + 2~a2S03 + 2H2 Depleted caustic and sulfur dioxide are mixed to form -:
NaHS03 that i~ fed into the catholyte distribution groove 32 via the catholyte entrance ports 35 and the catholyte transition i~lots 35'. This catholyte liquid then ri~e3 vertically upwardly until it passes out through the cathode plate 26, a best seen in FIGURES 5 or 1. The cathode flow barxier 30 acts as a block to the straight vertical flow of the catholyte fluid . : : -upwardly from the lower catholyte chamber 38 into the upper catholyte chamber 39. There i~i an approximately :
1/8" interelectrode cathode gap between the walls of the :~
lower and upper catholyte chambers 38 and 39 and the ;
cathode plate 26 that i~ seated on the cathode support .
pedestals 31. ~he catholyte fluid then pa~ es through the cathode plate 26 and continuesi flowing upwardly : . :-~.
through the cathode-membrane gap until it passes the -cathode flow barrier 30. At this point the catholyte -~
fluid passes back through the highly porous cathode .'~
plate 26 into the upper catholyte chamber 39 and then into the catholyte collection groove 34. The cell :~
product solution containing Na2S204 (dithionite) . : :
exits the cell 10 through the catholyte exit transition ; ~:~
slots 36' and the catholyte exit ports 36.
A buffer solution containing from about 40 to about 80 gpl of bisulfite i~ utilized with the catholyte because of sodium thiosulfate formation resulting from ~:,', , ' 132~783 the reduction and decompo~ition of hydrosulfite (dithionite) and the pH change of the catholyte as bisufite is consumed and sulfite i8 formed according to the reaction Na2S2O4 + 2e + 2Na + 2NaHSO3 ~ Na2S2O3 + 2Na2S3 + H2O-This hydrosulfite decomposition reaction is electrolytically driven by the presence of electron~.
When the potential is increased, ~o is the current density and to a point, the reaction rate of this undesired thiosulfate producing reaction.
The value of the multiple layered cathode plate 26 is particularly evident in its selectivity.
Because the multiple layered electrode has an increased surface area, it requires les~ voltage or a lower potential to drive the primary reduction reaction that produces the desired hydrosulfite product and, thereby, -reauces the amount of the undesired thiosulfate produced by the hydrosulfite decomposition reaction. The ~-increased ~urface area permits the potential to be maintained at the lower level where the primary or desired hydro~ulfite producing reaction predominates and generally below the level where the hydrosulfite decomposition reaction becomes a factor.
The u~e of a monolithic cell body, that is a bipolar cell body or bacXplate formed from a single plate of stainless steel machined to form an anode backplate on one side and a cathode backplate on the opposing side, provide~ ~everal significant inherent operating advantages. Initially, there is no shifting ~ -or dimen3ional instability because of the joining of two separate pieces of material to form the electrode.
There is a reduction in the number of actual cell components from the use of a ~ingle machined plate.
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-16- 132~783 La~tly, and perhap~ most ~ignificantly, there i8 the elimination of electrical 1088 from the contact between two separate anode and cathode elements that would otherwise have ~ome spacing and sizing differences.
~ 5 This particular configuration contribute~ to lower cell ¦ electrical energy consumption.
The hydraulic pressure in cell 10 is e~tablished 80 that the membrane 25 is kept pressed against the separator means 21 and off of the cathode plate 26. Keeping the membrane 25 80 po~itioned al80 : -permits the flow path through the cathode plate to be accomplished. The cathode flow barrier 30 further contributes to the hydraulics of the cell 10 by achieving a uniform pressure across the entire height of -~-lS the cathode due to the flow inversion characteristic~
achieved by the multiple flow paths through the cathode -~
plate 26. ~1-The electrolytic cell 10 is operated at current den~ities which are sufficient to produce solutions of alkali metal hydrosulfites having the concentrations desired. For example, where sodium hydrosulfite is produced for commercial sale, the ~ -¦ solutions contain from about 120 to about 160 grams per i liter. However, since the alkali metal hydrosulfite solutions sold commercially are usually diluted before ~ -use, these dilute aqueous solutions can also be produced I directly by the process.
¦ Current densities of at least 0.5 kiloamperes I per square meter are employed. Preferably the current j 30 density i~ in the range of from about 1.0 to about 4.5, and more preferably at from about 2.0 to about 3.0 kiloamperes per square meter. At these high current densities, the electrolytic cell 10 operate~ to produce the required volume of high purity alkali metal hydrosulfite solution which can be employed commercially without further concentration or purification.
-17- 132478~
The electrolytic membrane cell 10 employs a cation exchange membrane between the anode and the cathode compartments which prevents any sub~tantial migration of ~ulfur-containing ions from the cathode compartment to the anode compartment. A wide variety of cation exchange membranes can be employed containing a variety of polymer resins and functional groups, provided the membranes posse~ the requisite ~ulfur ion selectivity to prevent the deposition of sulfur inside membrane~. Such deposition can blind the membranes, the result of ~ulfur 3pecies diffusing through the membranes and then being oxidized to create acid within the membranes that causes hydrosulfite and thiosulfate to decompose to sulfur in acidic conditions. This 3electivity can be verified by analyzing the anolyte for sulfate ions.
Sultable cation exchange membranes are those which are inert, flexible, and substantially impervious to the hydrodynamic flow of the electrolyte and the passage of gas product~ produced in the cell. Cation exchange membranes are well-known to contain fixed anionic group~ that permit intrusion and exchange of cations, and exclude anions, from an external source.
Generally the resinous membrane has as a matrix or a cross-linked polymer to which are attached charged radicals, such as --S03, --COO , - P03, - HP02, - A~03, and - SeO3 and mixtures thereof. The re~in~
which can be used to produce the membranes include, for example, fluorocarbons, vinyl compounds, polyolefins, and copolymers thereof. Preferred are cation exchange membranes such as those comprised of fluorocarbon polymers having a plurality of pendant sulfonic acid groups or carboxylic acid groups or mixtures of sulfonic acid groups and carboxylic acid groups. The terms "sulfonic acid group" and "carboxylic acid groups" are meant to include salts of sulfonic acid or salt~ of ;;~;
. .~ . :
, , -18- 132 ~7 83 carboxylic acid groupY by proces~es such as hydroly~is.
Suitable cation exchange membranes are sold commercially by E. I. DuPont de Nemours & Co., Inc. under the trademark "Nafion", by the Asahi Glass Company under the trademark "Flemion", by the A~ahi Chemical Company under ;
the trademark "Aciplex". Perfluorinated sulfonic acid membranes are also available from the Dow Chemical Company.
The membrane 25 i8 positionea between the anode and the cathode and is separated from the cathode by a cathode-membrane gap which is wide enough to permit the catholyte to flow between the cathode plate 26 and the membrane 25 from the lower catholyte chamber 38 to the upper catholyte chamber 39 and to prevent ga~
blinding, but not wide enough to ~ubstantially increase electrical resi~tance. Depending on the 40rm of cathode plate 26 used, this cathode-membrane gap is a distance of from about 0.05 to about 10, and preferably from about 1 to about 4 millimeters. The cathode-membrane gap can be maintained by hydraulic pres3ure or mechanical means. Thi~ design and the catholyte flow path permits almost all of the catholyte liquid to contact the active area of the cathode. Further, with this design the majority of the electrolytic reaction occurs in the cathode area nearest the anode.
Suitable porous cathode plates 26 u~ed in the cell 10 have at least one layer with a total ~urface area to volume ratio of greater than 100 cm2 per ,~
cm3, preferably 250 cm2 per cm3, and more preferably greater than 500 cm2 per cm3. These structures have a porosity of at leaqt 60 percent and preferably from about 70 percent to about 90 percent, -where porosity is the percentage of void volume. The ratio of total surface area to the projected surface ~r-a of th- porou~ cathod- plat- 26, wh-r- tb- pro~-ct-d e~
' ,--19- ~32~783 surface area i~ the area of the face of the cathode plate 26, i9 at least about 30:1 and preferably at least from about 50:1: for example, from about 80:1 to about 100:1.
~The cathode plate 26 i~ comprised of four layers, as best ~een in FIGURE 7. The first layer i9 a ~upport layer 41 of perforated stainless ~teel plate approximately 0.036 inches thick with 1/16 inch holes 42 on 1/8 inch 60 degree staggered centers. These hole~ 42 in the first layer 41 give the perforated stainless steel plate an open area of approximately 23 percent.
The second layer 44 is preferably formed of 304 ~ -stainles~ steel fibers 45 that are about 20 to about 100 microns in diameter and, preferably, about 25 micron~ in diameter. The second layer 44 has the density of zpproximately 0.62 pounds per square foot. Alternately, the second layer can be a woven screen mesh with a 30 fiber~ per inch by 30 fibers per inch square weave and about a 23% open area. The third layer 48 i8 much less dense than the second layer 44 and is compri~ed of 304 stainles~ steel fibers 49 of about 4 to about 16 microns --in diameter and, preferably, about 8 microns in diameter. The density of the third layer 48 i~ about 0.12 pounds per square foot. The limiting factor in the ~ - -diameter sizing of the fibers of the second layer 44 and the third layer 48 is that the fibers 45 of the second layer 44 can't extend up through the fibers 49 of the third layer 48. The fourth layer 50 is a mesh wire cloth that is preferably 18 x 18 mesh with about a 0.009 inch diameter of the individual wires in the wir0 --cloth. The four layers 41, 44, 48, and 50 are compressed together and bonded by ~intering in a reducing atmosphere, such as hydrogen, ammonia or carbon monoside, to form a single sheet with a preferred thickness of about 0.155 inches, plus or m$nus about 0.008 inches.
~: ' --20- 1 3 2~7 83 The cathode plate 26 of thi~ design provide~ a high mass transfer capability because the surface area of the electrode i~ the same surface area u~ed for electrolysis. Becau~e the cathode plate 26 is relatively thin, it require~ a high surface area to volume ratio, thereby creating a high total surface area. Thi~ is especially applicable to the third layer 48 which is the most active layer and is critical in determining the characteristics of the cathode. The high surface area of the cathode plate 26 is measured as the ratio of the total surface area of the individual fibers 45, 49 to the superficial or projected ~urface area of the total cathode plate 26. The relationship between these parameters can be expres~ed a~ the thickne3s of an individual layer in the cathode plate 26 times the surface area to volume ratio equal~ the superficial surface area. The selection of the fiber~
45 of the second layer 44 with a greater individual diameter than the fibers 49 of the third layer 48 and an equal porosity provide a structure that has a greater ,~
surface area near the membrane 25. Fibers 45 and fibers 49 can equally well be s21ected from different grades of stainless steel, nickel, steel, copper, carbon graphite or ferrous and non-ferrous alloys. The cathode plate 26 could have an electroactive coating, such as ruthenium oxide or platinum, added to the surface area to obtain a more active electrode surface area.
Current is conducted into the cell 10 through anode and cathode current conductor plates (not shown). ,~
Plate~ of c~pper the ~ize of the electrodes are placed against the end cathode and end anode in each cell 10. -~
~lectrical connections are made directly to these copper plates. An insulator plate made, for example, of polyvinyl chloride or other suitable plastic, and a ~-compres3ion plate (both not ~hown) made ~or example, of -21- 1 3 2 ~7 83 ~tainles~ steel or steel, are placed against ea~h end of the cell 10 before it i~ aqsembled to form a sandwich around the de~ired number of electrode~ that are positioned therebetween.
The cell of the instant invention could also be designed a~ monopolar, requiring that both sides of each stainleqs steel plate be identically machined and that half electrodes be used as the end electrodes in the assembled cell. The current conductors in the monopolar design would then be standard copper electrical terminal~ for each electrode.
Additionally the cell of the present invention could be utilized in electrochemical reactions other than the production of hydrosulfite. Typical i~ the production of organic products by electrochemistry, such I as the electrochemical transformations of pyridines ¦ through oxidation or reduction reaction~ in a ¦ cation-exchange membrane divided cell of the instant design.
Employing the novel de~ign of the cell 10, concentrated alkali metal hydrosulfite solutions are ~ -produced having low concentrations of alkali metal thiosulfate~ as an impurity in electrolytic membrane ;-~ ;
cells operat~ng at high current den~ities, substantially reduced cell voltages, and high current efficiencies.
In order to exemplify the re~ults achieved, the following examples are provided without an intent to ~-limit the scope of the instant invention to the discussion therein.
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Example 1 A cell of the type 3hown in FIGURES 1-5 was assembled from three stainles~ steel plates which were mounted on a rack to form two anode/cathode pairs who~e active electrode area was about 0.172 square meters S each. The plates formed two half electrodes, one a cathode and the other an anode, sandwiched about a bipolar electrode with opposing anode and cathode faces. The outside dimen~ions of the electrode plates were about 17 inche~ wide by about 18.5 inches high and about 1.0 inches thick.
The anodes were comprised of about forty-seven (47) 1/8 inch diameter nickel 200 rod welded onto the anode backplate, as shown generally in FIGURE 2, with approximatsly 1/16 inch separation between the rods.
The anolyte collection and distribution grooves were ~-about 1.25 inches wide and about 0.61 inches deep. ~^
The cathode plate was formed from a four layered sheet cut to size. The first layer wa~ a ~-~
support layer formed of perforated stainless steel 0.036 inches thick with 1/16 inch holes on 1/8 inch 60 staggered centers having a 23% open area. The second layer was a 0.62 pound~ per square foot layer of 304 t;~,~ ,h ~tainless steel fibers about 25 microns in diameter.
The third layer was a 0.12 pound~ per square foot layer of 304 stainless steel fiber~ about 8 microns in diameter. The fourth layer was an 18" x 18" mesh of about 0.009 inch diameter wire cloth. These layers were compress2d together and bonded by sintering in a hydrogen atmosphere to form a single sheet with a thickness of about 0.155 inches. The cathode sheet was cut to form a cathode plate of about 18.5 inches by about 17 inche~ -. . .
.' ' ' The cathode plate was mountsd onto the ~tainless ~teel cathode backplate using 20 ~crew~ of about 1/8 inch diameter that seated into the cathode ~upport pedestals within the catholyte chambers. A
small coating of appropriate electrical joint compound wa~ used on the thread~ of the screwR and a silicon cement was placed over the head of each screw to prevent the screw from becoming an active part of the cathode assembly.
Six (06) 1/6 inch diameter hole3 were drilled in the cathode plate to permit ga~ bubble~ trapped within the cell to escape. Three of the holes were drilled near the top of the cell opposite the catholyte collection groove and three ju~t below the cathode flow -~
barrier. ~-Separator means were formed from polypropylene mesh treated with a coating of titanium dioxide. The ~eparators were mounted in 1/16 inch thick separator frame~ cut to fit just in~ide the gasXet groove in the cell.
Gasket grooves about 0.375 inches wide and about 0.187 inche~ deep were machined into both the anode and cathode backplates. On the anode siae of the cell about a 0.375 inch square gasXet was used with about a 0.5 inch wide strip of about 0.060 thick GORE-TEX~ gasXet tape placed on top. In the cathode gasket groove a rubber O-ring of about a 0.378 inch diameter was used. The cell was as~embled using a portable hydraulic a~embly system de~cribed in U.S.
. .
Patent No. 4,430,179 that compre~sed the cell together -~ ~
80 that approximately a 1/8 inch gap between the anode ~---and the cathode plate~ remained. The cell was then ¦ secured by retaining nuts. -~
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The cell wa~ operated continuously for 42 days. The cell employed a NAFION~ NX 906 perfluorinated membrane that was soaked in about 2%
sodium hydroxide solution for at least 4 hour~ prior to as~embling.
The cell wa~ operated at a temperature of approximately 25C with a total catholyte flow rate of about 6 gpm and a total anolyte flow rate of about 4 gpm. ~xcess anolyte containing about 19% sodium hydroxide was continuously purged and added to the catholyte circulation while the anolyte wa~ continuously replenished with the addition of about 69 grams per minute of about 35% sodium hydroxide solution. About 230 milliliters per minute of deionized water was continuou~ly added to the catholyte, as was sulfur dioxide to the catholyte to maintain a pH of between ~
about 5.4 and about 5.8 and a sulfite to bisulfite molar ~ -ratio of about 1:3 to about 1:8.
Product catholyte was drawn from the cell 2~ continuously at a rate of about 287 milliliters per minute and was analyzed periodically during each day.
The product catholyte reflected in the following Table I
was analyzed from samples taken at the same time each day. These data are representative of the operation of the cell during 4 days of operation under optimized conditions. The catholyte wa~ analyzed for sodium hyarosulfite, sodium thiosulfate, sodium ~ulfite and sodium bisulfite content.
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~1 ~o ~ CD ' ' 132~783 ~2a~ple 2 A cell 3imilar to the design of E~ample 1 was a~sembled utilizing nine bipolar electrode plate~ and two half electroda plate~, one an anode and one a cathode, having appro~imately a 0 051 ~quare meter active electrode area for each The same type of cathode plate and anode rods were used as in Example 1, except that th- anode and cathode bacXplate~ were about 13 5 inches by about 13 5 inche~ and about 1 188 inche3 thick A perfluorinated sul~onic acid membrane, with a thickne~a of about 2 mils and an equivalent weight of about 1000 (gram~/gram-mole equival0nt e~change capacity), available from the as~igneo of U S Patent ~o 4,470,888 was used The separator mean~ were a mesh made fro~
titaniu~ diosid- filled polyethylene, th~ m0~h being about 0 07 inch thick with appro~i~ately 0 38 inch --openingo and abcut 60% open arsa The ~eparator wa~
trs~ted with a ~lstur- of chromlc ana ~ulfuric acid~
availabl- fro~ Fi~h~r Scienti~ic unde~ the na~
¦ 20 CHROM~GE to obtain th- nece~ary hydrophil~c ~urface ! Th- s~parator meJh W~J ~ount~d on a 1/8 inch ~eparator ¦ fram- th~t estend-d about 1/4 inch beyond the ~dge of ¦ th- cell ~-I Tho coll WA8 ~ealed using about 0 290 inch dia~eter O-ring~ in both th~ anode and cathode backplate ga~ket groovos A ~trip of about 0 875 inch GORE-TEX~
tap- W~8 u~ed betwoon the separator fra~e and the ~ me~brane -I Th- cell operated with a total catholyte flow ¦ 30 rate of 13 gpm and a total anolyte flow rate of 6 gpm ~ The anolyte had continuou~ly addod to it 93 gra~s per ¦ ninuto of 35% ~odiun hydro%id~ ~olution E~ce~ anolyt-containing about 15% ~o~ium hydro~id- was contlnuously;~ -purqed and and add~d to the catholyte circulation * Trade-mark 1~ .
132~783 system. Additionally, about 320 milliliter~ per minute of deionized water was added to the catholyte, while sulfur dioxide was continuously added to the catholyte to maintain a pH of between about 5.4 to about 5.8 and a sulfite to bi~ulfite molar ratio of between about 1:3 to about 1:8.
The cell wa~ operated at a temperature of about 25C with a total catholyte flow rate of about ~,~
13 gpm and a total anolyte flow rate of about 6 gpm.
The cell was operated continuously for over 30 days without significant change in voltage coefficient or ; ~
product composition. -~-Product catholyte was continuously withdrawn from the cell at a rate of about 350 milliliter~ per ¦ 15 minute and was analyzed periodically during each day. ~ --The product catholyte reflected in the following Table ¦ II was analyzed from ~amples taken at the same time each ~ day. These data are repre~entat$ve of the operation of ¦ the cell during 4 days of operation under optimized conditions. The catholyte was analy~ed for sodium ~ -hydrosulite, sodium thio~ulfate, ~odium sulfite and -i -~
~odium bi~ulfite content.
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1 ~ ,, rfs .''' -,', " ' 13~783 While the preferred structure in which the principles of the present invention have been incorporated a~ shown and de~cribed above, it i~ to be -understood that the invention i8 not to be limited to the particular details thus presented, but, in fact, widely different means may be employed in the practice of the broader aspects of this invention. For example, while the anode backplate is shown and described as employing round wire rods on its surface, flat -~
rectangular bars or other appropriate geometrically ~ -shaped structures, such as triangular, pentagonal, hexagonal, octagonal, etc. could be equally well utilized. Additionally the separator mesh could be exposed to hydrophilic containing additive~ or such additives could be in the electrolyte. The separator mesh could al~o be assembled in the cell between the ;~
membrane and the cathode plate, in conjunction with the hydraulic pressure being changed 80 that the membrane i~ -forced off of the anode rods and against the ~eparator ¦ 20 mesh. The scope of the appended claims i8 intended to ¦ encompass all obvious changes in the detail~, material , and arrangement of parts, which will occur to one of skill in the art upon a reading of the disclo~ure.
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~1 ~o ~ CD ' ' 132~783 ~2a~ple 2 A cell 3imilar to the design of E~ample 1 was a~sembled utilizing nine bipolar electrode plate~ and two half electroda plate~, one an anode and one a cathode, having appro~imately a 0 051 ~quare meter active electrode area for each The same type of cathode plate and anode rods were used as in Example 1, except that th- anode and cathode bacXplate~ were about 13 5 inches by about 13 5 inche~ and about 1 188 inche3 thick A perfluorinated sul~onic acid membrane, with a thickne~a of about 2 mils and an equivalent weight of about 1000 (gram~/gram-mole equival0nt e~change capacity), available from the as~igneo of U S Patent ~o 4,470,888 was used The separator mean~ were a mesh made fro~
titaniu~ diosid- filled polyethylene, th~ m0~h being about 0 07 inch thick with appro~i~ately 0 38 inch --openingo and abcut 60% open arsa The ~eparator wa~
trs~ted with a ~lstur- of chromlc ana ~ulfuric acid~
availabl- fro~ Fi~h~r Scienti~ic unde~ the na~
¦ 20 CHROM~GE to obtain th- nece~ary hydrophil~c ~urface ! Th- s~parator meJh W~J ~ount~d on a 1/8 inch ~eparator ¦ fram- th~t estend-d about 1/4 inch beyond the ~dge of ¦ th- cell ~-I Tho coll WA8 ~ealed using about 0 290 inch dia~eter O-ring~ in both th~ anode and cathode backplate ga~ket groovos A ~trip of about 0 875 inch GORE-TEX~
tap- W~8 u~ed betwoon the separator fra~e and the ~ me~brane -I Th- cell operated with a total catholyte flow ¦ 30 rate of 13 gpm and a total anolyte flow rate of 6 gpm ~ The anolyte had continuou~ly addod to it 93 gra~s per ¦ ninuto of 35% ~odiun hydro%id~ ~olution E~ce~ anolyt-containing about 15% ~o~ium hydro~id- was contlnuously;~ -purqed and and add~d to the catholyte circulation * Trade-mark 1~ .
132~783 system. Additionally, about 320 milliliter~ per minute of deionized water was added to the catholyte, while sulfur dioxide was continuously added to the catholyte to maintain a pH of between about 5.4 to about 5.8 and a sulfite to bi~ulfite molar ratio of between about 1:3 to about 1:8.
The cell wa~ operated at a temperature of about 25C with a total catholyte flow rate of about ~,~
13 gpm and a total anolyte flow rate of about 6 gpm.
The cell was operated continuously for over 30 days without significant change in voltage coefficient or ; ~
product composition. -~-Product catholyte was continuously withdrawn from the cell at a rate of about 350 milliliter~ per ¦ 15 minute and was analyzed periodically during each day. ~ --The product catholyte reflected in the following Table ¦ II was analyzed from ~amples taken at the same time each ~ day. These data are repre~entat$ve of the operation of ¦ the cell during 4 days of operation under optimized conditions. The catholyte was analy~ed for sodium ~ -hydrosulite, sodium thio~ulfate, ~odium sulfite and -i -~
~odium bi~ulfite content.
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1 ~ ,, rfs .''' -,', " ' 13~783 While the preferred structure in which the principles of the present invention have been incorporated a~ shown and de~cribed above, it i~ to be -understood that the invention i8 not to be limited to the particular details thus presented, but, in fact, widely different means may be employed in the practice of the broader aspects of this invention. For example, while the anode backplate is shown and described as employing round wire rods on its surface, flat -~
rectangular bars or other appropriate geometrically ~ -shaped structures, such as triangular, pentagonal, hexagonal, octagonal, etc. could be equally well utilized. Additionally the separator mesh could be exposed to hydrophilic containing additive~ or such additives could be in the electrolyte. The separator mesh could al~o be assembled in the cell between the ;~
membrane and the cathode plate, in conjunction with the hydraulic pressure being changed 80 that the membrane i~ -forced off of the anode rods and against the ~eparator ¦ 20 mesh. The scope of the appended claims i8 intended to ¦ encompass all obvious changes in the detail~, material , and arrangement of parts, which will occur to one of skill in the art upon a reading of the disclo~ure.
, "
Claims (33)
1. An electrolytic cell having a top and a bottom and an anolyte and a catholyte flowing therethrough, comprising in combination (a) an anode;
(b) a cation exchange membrane adjacent the anode;
(c) separator means intermediate the anode and the membrane to prevent the membrane from touching the anode;
(d) a porous multilayered cathode plate having a first surface adjacent the membrane and an opposing second surface, the second surface being a support layer of perforated material having a second layer of fibers atop of it, at least a portion of the first surface being formed by a third layer of fibers less dense than the second layer and positioned atop the second layer, the third layer being adjacent the membrane; and (e) a cathode backplate adjacent the opposing second surface of the cathode plate having a flow barrier extending thereacross defining an upper catholyte chamber and a lower catholyte chamber, the flow barrier interrupting the catholyte flowing between the top and the bottom of the cell causing substantially all of the catholyte to change flow direction and pass twice through the porous cathode plate transverse to the first surface and the opposing second surface of the cathode plate to pass beyond the flow barrier and to exit the cell.
(b) a cation exchange membrane adjacent the anode;
(c) separator means intermediate the anode and the membrane to prevent the membrane from touching the anode;
(d) a porous multilayered cathode plate having a first surface adjacent the membrane and an opposing second surface, the second surface being a support layer of perforated material having a second layer of fibers atop of it, at least a portion of the first surface being formed by a third layer of fibers less dense than the second layer and positioned atop the second layer, the third layer being adjacent the membrane; and (e) a cathode backplate adjacent the opposing second surface of the cathode plate having a flow barrier extending thereacross defining an upper catholyte chamber and a lower catholyte chamber, the flow barrier interrupting the catholyte flowing between the top and the bottom of the cell causing substantially all of the catholyte to change flow direction and pass twice through the porous cathode plate transverse to the first surface and the opposing second surface of the cathode plate to pass beyond the flow barrier and to exit the cell.
2. The cell according to claim 1 wherein the second layer of fibers has a density of about 0.62 pounds per square foot and the fibers have an individual diameter of about 25 microns.
3. The cell according to claim 2 wherein the third layer of fibers has a density of about 0.12 pounds per square inch and the fibers have an individual diameter of about 8 microns.
4. The cell according to claim 3 wherein the porous multilayered cathode plate further comprises a fourth layer of meshed wire cloth atop of the third layer of fibers.
5. The cell according to claim 3 wherein the support layer of perforated material has about 23 percent open area.
6. The cell according to claim 5 wherein the fibers of the second layer and the third layer are selected from the group consisting of stainless steel, nickel, steel, copper and carbon graphite.
7. The cell according to Claim 1 wherein the flow barrier further extends generally horizontally across the cathode backplate.
8. The cell according to claim 7 wherein the catholyte flow is generally vertical from the bottom of`
the cell to the top of the cell.
the cell to the top of the cell.
9. The cell according to claim 8 wherein the catholyte enters the cell through at least one catholyte entry port that feeds into the lower catholyte chamber.
10. The cell according to claim 9 wherein the at least one catholyte entry port further feeds into a catholyte distribution slot via a tapered transition slot.
11. The cell according to claim 10 wherein the catholyte exits the cell through at least one catholyte exit port.
12. The cell according to claim 11 wherein the catholyte flow barrier further has at least one gas weep hole extending generally vertically therethrough directly connecting the lower catholyte chamber to the upper catholyte chamber to permit gas to pass therethrough.
13. The cell according to claim 12 wherein the anode further comprises an anode backplate with at least one anolyte entry port for the entry of anolyte into the cell and at least one anolyte exit port for the exit of anolyte from the cell.
14. The cell according to claim 13 wherein the at least one anolyte entry port further feeds into an anolyte distribution groove via at least one tapered anolyte transition slot.
15. The cell according to claim 14 wherein the anode further comprises an anolyte collection groove communicating with said anolyte exit port, said anolyte distribution groove and said anolyte collection groove being parallel and vertically aligned, and a plurality of anode rods extending between said anolyte distribution groove and said anolyte collection groove.
16. The cell according to claim 15 wherein the catholyte comprises a buffered aqueous solution of an alkali metal bisulfite.
17. The cell according to claim 16 wherein the alkali metal bisulfite is sodium bisulfite.
18. The cell according to claim 17 wherein the anolyte comprises a mixture of sodium hydroxide and deionized water.
19. An electrode compressed together and bonded by sintering in a reducing atmosphere for use in an electrochemical cell comprising in combination at least:
(a) a support layer formed of perforated material;
(b) a second layer of fibers atop of the support layer; and (c) a third layer of fibers, the fibers being smaller in diameter than the fibers of the second layer and being positioned atop of the second layer.
(a) a support layer formed of perforated material;
(b) a second layer of fibers atop of the support layer; and (c) a third layer of fibers, the fibers being smaller in diameter than the fibers of the second layer and being positioned atop of the second layer.
20. The electrode according to claim 19 wherein the individual fibers in the second layer of fibers have a diameter of about 20 to about 100 microns.
21. The electrode according to claim 20 wherein the individual fibers of the second layer of fibers have a diameter of about 25 microns.
22. The electrode according to claim 19 wherein the fibers of the second layer are a woven screen mesh.
23. The electrode according to claim 20 wherein the second layer of fibers has a density of about 0.62 pounds per square foot.
24. The electrode according to claim 20 wherein the individual fibers in the third layer of fibers have a diameter of about 4 to about 16 microns.
25. The electrode according to claim 24 wherein the individual fibers in the third layer of fibers have a diameter of about 8 microns.
26. The electrode according to claim 23 wherein the third layer of fibers has a density of about 0.12 pounds per square foot.
27. The electrode according to claim 19 wherein the fibers of the second layer and the third layer are selected from the group consisting of stainless steel, nickel, steel, copper and carbon, graphite.
28. The electrode according to claim 19 wherein the electrode further comprises a fourth layer of meshed wire cloth atop of the third layer of fibers.
29. The electrode according to claim 28 wherein the electrode is a cathode.
30. The electrode according to claim 29 wherein the ratio of the total surface area to the projected surface area of the electrode is at least about 30:1.
31. The electrode according to claim 30 wherein the total surface area to volume ratio of the third layer of fibers is greater than 100 cm2 per cm3.
32. The electrode according to claim 31 wherein the porosity of the electrode is at least about 60 percent.
33. The electrode according to claim 32 wherein the porosity of the electrode is from about 70 to about 90 percent.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US032,803 | 1987-04-01 | ||
| US07/032,803 US4740287A (en) | 1986-12-19 | 1987-04-01 | Multilayer electrode electrolytic cell |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1324783C true CA1324783C (en) | 1993-11-30 |
Family
ID=21866885
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000561171A Expired - Fee Related CA1324783C (en) | 1987-04-01 | 1988-03-10 | Multilayer electrode and electrolytic cell containing such electrode |
Country Status (3)
| Country | Link |
|---|---|
| BR (1) | BR8801439A (en) |
| CA (1) | CA1324783C (en) |
| FI (1) | FI88410C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2652177A4 (en) * | 2010-12-17 | 2016-11-30 | Waikatolink Ltd | An electrolytic cell |
-
1988
- 1988-03-10 CA CA000561171A patent/CA1324783C/en not_active Expired - Fee Related
- 1988-03-14 FI FI881186A patent/FI88410C/en not_active IP Right Cessation
- 1988-03-29 BR BR8801439A patent/BR8801439A/en not_active IP Right Cessation
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2652177A4 (en) * | 2010-12-17 | 2016-11-30 | Waikatolink Ltd | An electrolytic cell |
Also Published As
| Publication number | Publication date |
|---|---|
| BR8801439A (en) | 1988-11-08 |
| FI88410C (en) | 1993-05-10 |
| FI88410B (en) | 1993-01-29 |
| FI881186A (en) | 1988-10-02 |
| FI881186A0 (en) | 1988-03-14 |
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| MKLA | Lapsed | ||
| MKLA | Lapsed |
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