FI65281C - FOERFARANDE FOER DRIFT AV EN KLOR-ALKALIELEKTROLYSCELL - Google Patents

Description

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Patent raeddelat ^ T ^ (51) K * .ik.3 / im.a.3 c 25 B 1/46 SUOMI — FINLAND (21) -ριμπομΜο ^ ι 792789 pi) Htk «mtap4ly> - Am6knlng * d * f 07.09 .79 ^^^ (23) Alluipaivt — Glklghetadag 07.09.79 (41) Tulta JulMMfctl - Blhrtt effantllg 08.03. 8l

National Board of Patents and Registration (44) NllrtMW (Wlfton „kuUUJulkatain

Patent · och registerstyreleen An »« Jk »n utbgd och utl. * Krtft» n published 3u. ± ez. oj (32) (33) (31) Requested utuoikiu * —B «glrd prlorttet (71) Diamond Shamrock Corporation, 1100 Superior Avenue, Cleveland,

Ohio Ul + Uh, USA (72) Ronald Lowry LaBarre, Huntington Beach, California, USA (7 U) Berggren Oy Ab (5U) Method for using a chlor-alkali electrolysis cell -Förfarande för drift av en chlor-alkalielectrolyscell This The invention relates generally to the use of an oxygen electrode for use in an electrolytic cell, and more particularly to the production of chlorine and a base (sodium hydroxide) by significantly reducing the voltages required to operate these electrolytic cells and substantially increasing the power efficiencies available from these oxygen electrode cells. More specifically, this disclosure relates to improved methods of using oxygen electrodes using a positive pressure drop from air to liquid on the oxygen supply side of the oxygen electrode to improve performance, control the overall flow of gas eliminating gases such as carbon dioxide to extend the life of the oxygen electrodes by eliminating salts that might form on the porous structure of the oxygen electrode during use. These methods of operation can be used alone or, preferably, in combination to provide higher power efficiencies at lower voltages, providing an energy-efficient oxygen electrode for the electrolytic cell, particularly suitable for the production of chlorine and base (sodium hydroxide).

Chlorine and base are essential mass-produced commodities that are basic chemicals required by all industrial communities. They are prepared almost exclusively electrolytically from aqueous solutions of alkali metal halides or, more specifically, sodium chloride, with most of this production coming from diaphragm-type electrolytic cells. In the electrolytic diaphragm cell process, a brine solution (sodium chloride solution) is continuously fed to the anode compartment to flow through a diaphragm, usually made of asbestos particles formed on a cathode structure perforated in nature. In order to minimize the recovery of hydroxide ions, the flow rate is always considered to be higher than the conversion rate so that unused or unreacted sodium chloride is present in the resulting catholyte solution. Hydrogen ions are released from solution at the cathode in the form of hydrogen gas. The catholyte solution containing alkali liquor (sodium hydroxide), unreacted sodium chloride and other impurities must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and sodium chloride to be reused in electrolysis cells for the production of sodium hydroxide and chlorine. The evolution of hydrogen gas takes advantage of the higher voltage, thus reducing the power-to-feed ratio that is possible from such an electrolytic cell, creating an energy-inefficient means of producing sodium hydroxide and chlorine gas.

With the advancement of technological advances such as dimensionally stable anodes and various coating compositions for them, which allow ever-narrowing gaps between the electrodes, the electrolytic cell has become more efficient because the power efficiency is greatly improved by using these dimensionally stable anodes. Likewise, the hydraulically impermeable membrane has increased the use of a number of electrolytic cells with respect to the selective migration of various ions across the membrane to exclude impurities from the resulting product, eliminating some of the costly purification and concentration steps of treatment. Thus, in addition to the great advances that have been attempts to improve electrolytic cells the anode side and the membrane or the spacer efficiency, more attention is now directed to the electrolysis cell cathode 3 65281 side view to improving electrolytic cells cathodes for use in power efficiency and thus provide a significant energy savings as a result of the production of chlorine and alkali. Turning in more detail a conventional chlorine and emäskennon cathode problem can be seen that in a cell employing a conventional anode and a cathode and a diaphragm therebetween, the electrolysis reaction at the cathode may be represented as follows: 2H20 + 2e "to produce H2 + 20H- this reaction, the standard potential of the electrode is ^ -0.83 volts. The desired reaction under ideal conditions, which is to be promoted by the cathode, would be 2H 2 O + 02 + 4e ”to produce 40H ~ The potential of this reaction is +0.40 volts, which would lead to a theoretical voltage saving of 1.23 volts. The electrical energy necessarily consumed to produce hydrogen gas, which is an undesirable reaction of the cathode of conventional electrolytic cells, has not been efficiently balanced in industry by the use of recovered hydrogen, as it is essentially an undesirable reaction product. Although some excess hydrogen has been utilized, these uses have not complemented the difference in electrical energy consumption required to generate hydrogen, so if hydrogen evolution could be eliminated, it would save electrical energy and thus make chlorine and base production more energy efficient.

The oxygen electrode offers one possibility to eliminate this reaction, as it uses electrochemically activated oxygen to combine with water and electrons available at the cathode according to the following equation: 2Η2 <3 + 02 + 4e produces 40H "It is easily found that this reaction is more energy efficient due to all hydrogen production this is accomplished by supplying an oxygen-rich fluid, such as air or oxygen, to the oxygen side of the oxygen electrode, where oxygen has easy access to the electrolytic surface where it wears according to the above equation.

65281 4 However, this really requires a slightly different structure for the electrolytic cell itself in order to provide an oxygen compartment on the cathodic side of the cathode so that an oxygen-rich substance can be fed there.

The oxygen electrode itself is well known in the art, as the many NASA projects used to promote space travel in the 1960s also provided funds for fuel cell development, "which utilizes an oxygen electrode and a hydrogen anode to produce hydrogen and oxygen gas to generate electricity for use in the spacecraft. The government-funded main research effort produced many fuel cell components, such as the oxygen electrode, the conditions, and the environment in which the oxygen electrode was to operate were quite different from those encountered in a chlor-alkali cell, so that much of the technology gained during NASA projects has chlorine value. in the alkali industry, as far as the development of the oxygen electrode is concerned, much further development is needed to apply the oxygen electrode to the chlor-alkali cell environment.

Some attention has been paid to the use of an oxygen electrode in a chlor-alkali cell to increase efficiency in a manner described as theoretically possible, but so far the oxygen electrode has not achieved significant interest in producing a commercially efficient and economically viable electrode for chlorine and base production. Although it is recognized that a suitable oxygen electrode is necessary to achieve theoretical efficiencies derived from a chlor-alkali cell, a chlor-alkali cell requires an operating methodology that differs significantly from that of a fuel cell in that an electric potential is applied to the chlor-alkali cell to produce chlorine and base. the strength of the electrochemical reaction to be promoted. Therefore, it would be advantageous to develop an oxygen electrode usage methodology that is specifically focused on maximizing the theoretical electrical efficiencies possible with such an oxygen electrode in an electrolytic chlor-alkali cell for chlorine and base production.

It is therefore an object of the present invention to provide a method of using an oxygen electrode which improves and maximizes the energy efficiencies to be obtained from an oxygen electrode in the vicinity of an electrolytic chlor-alkali cell.

Another object of the present invention is to provide a controlled pressure of the gas supply to the oxygen electrode to promote this maximization.

Another object of the present invention is to control the total flow of gas to the oxygen electrode to maximize its efficiencies.

Yet another object of the present invention is to provide a humidified gas supply to an oxygen electrode to maximize its efficiencies and lifetimes.

It is a further object of the present invention to eliminate contaminants such as CO2 from the gas supply to maximize the life and efficiency of the oxygen electrodes.

These and other objects of the present invention, together with its advantages over the present and prior art, which will become apparent to those skilled in the art from the detailed description of the present invention set forth herein and below, are accomplished with the improvements described, described, and claimed.

It has been found that a chlor-alkali electrolysis cell having an anode compartment, a cathode compartment separated from the anode compartment by a separator, and an oxygen compartment separated from the cathode compartment by an oxygen electrode can be used by a method comprising: feeding an alkali metal halide solution inside the anode compartment; feeding an aqueous solution inside the cathode compartment; feeding the molecular oxygen-containing fluid inside the oxygen compartment at a positive monometer pressure; for the purpose of achieving a total flow rate greater than the theoretical amount of stoichiometric oxygen required for the reaction; applying an electrical potential between the cathode and the anode of the electrolytic cell; removing halogen gas from the anode compartment; removing alkali metal hydroxide from the cathode compartment; and removing the oxygen-depleted fluid from the oxygen compartment while maintaining a positive gauge pressure within the oxygen compartment.

65281 6

It has also been found that a chlor-alkali electrolysis cell having an anode compartment, a cathode compartment separated from the anode compartment by a separator, and an oxygen compartment separated from the cathode compartment by an oxygen electrode can be used by a method comprising: feeding an alkali metal halide solution inside the anode compartment; feeding an aqueous solution inside the cathode compartment; feeding a molecular oxygen-containing, high-carbon dioxide-evacuated gas inside the oxygen compartment; applying an electrical potential between the cathode and the anode of the electrolytic cell; removing halogen gas from the anode compartment; removing alkali metal hydroxide from the cathode compartment; and removing the oxygen-purged humidified gas from the oxygen compartment.

It has also been found that a chlor-alkali electrolysis cell having: an anode compartment adapted to contain an anolyte containing an alkali metal chloride can be used for the production of chlorine and alkali metal hydroxide; a cathode compartment adapted to contain a catholyte containing an alkali metal hydroxide and separated from said anode compartment by a separator; isolator; an oxygen compartment adapted to receive an oxygen-containing fluid free of carbon dioxide, humidified, at a positive gauge pressure and a positive total flow of 1.5 to 10 times the stoichiometric amount of oxygen; an oxygen electrode separating said cathode compartment from said oxygen compartment; an apparatus for adjusting the moisture content of an oxygen-containing substance; an apparatus for regulating the pressure of an oxygen-containing fluid in said oxygen compartment; means for controlling the total flow rate of the oxygen-containing fluid in said oxygen compartment; an apparatus for removing chlorine from said anode compartment; an apparatus for removing alkali metal hydroxide from said cathode compartment; an apparatus for feeding alkali metal chloride to said anode compartment; and means for supplying electrolytic electrical energy to said anode and said cathode.

Preferred embodiments of the present invention are illustrated and described by way of example in this specification without attempting to illustrate all the various forms and variations in which this invention may be practiced, and the invention is measured by the appended claims and not by the details of this specification.

65281 7

In the accompanying drawings, Figure 1 is a schematic view of an electrolytic cell for the production of halogen gas and alkali metal hydroxide in accordance with the concepts of the present invention.

Figure 2 is a graphical representation of the relationships between total flow, pressure difference, and cathode measured potential.

Referring to Figure 1, the number 12 denotes a single-pole split electrolysis cell suitable for use in accordance with the concepts of the present invention. Applicants will appreciate that various other structures for electrolytic cells could be associated with the methods of the present invention, but that for the purposes described, the applicants chose this schematic to describe in more detail the applicants' invention. An electrolytic cell 12 as shown in Figure 1 would generally have some surrounding support structure or foundation to keep each electrolytic cell 12 in precise alignment to build a series of electrolytic cells for production purposes. Details of this surrounding structure are not provided to easily illustrate the concepts of the present invention. The cell itself could be constructed of a variety of materials, either metallic or plastic in nature, insofar as these materials withstand the alkaline environment and temperature properties of the chlorine environment during operation of the basic chlor-alkali cell, which are well known in the art. Such materials generally include, but are not limited to, metallic materials such as steel, nickel, titanium, and other valve metals in addition to plastics such as polyvinyl chloride, polyethylene, polypropylene, fiberglass, and others, of which there are too many to mention. Valve metals include aluminum, molybdenum, niobium, titanium, tungsten, zirconium and their alloys.

It can be seen from the drawing that the electrolytic cell 12 shown has an anode 14, a separator 16 and a cathode 18 for forming three separate compartments in the electrolytic cell, being an anode compartment 20, a cathode compartment 22 and an oxygen compartment 24.

The anode 14 is generally made of a metallic material, although graphitic carbon could be used as in the old electrodes, which are now widely abandoned by the industry. These anodes, especially if they are to be used in a chlor-alkali cell 12, would generally be a 65281 8 active material resistant to an anolyte such as valve metal. The recommended valve metal based on price, availability, and electrical and chemical properties is titanium. There are numerous forms that the substrate can take in electrode fabrication, including, for example, a solid metal material, a stretched metal mesh material with a high percentage free surface area, and a porous titanium with a density of 30-70% pure titanium that can be made by cold sealing titanium powder . Porous titanium appears to be the preferred material today due to its long life properties along with its relative structural strength. If desired, porous titanium can be reinforced with a titanium mesh in the case of large electrodes.

In general, these substrate materials have a surface coating to protect the material from passivation, which is the same as commonly known in the art as a dimensionally stable anode. Most of these coatings contain precious metal, precious metal oxide either alone or in combination with valve metal oxide or other electrocatalytically active corrosion resistant materials. These so-called dimensionally stable anodes are well known and widely used in industry. One type of coating would be, for example, a Beer-type coating, which is disclosed in U.S. Patent Nos. 3,236,756, 3,623,498, 3,711,385, 3,751,296 and 3,933,616. Another type of coating used is one in which tin, titanium and ruthenium oxides are used for surface coating, as can be seen in U.S. Patent Nos. 3,776,834 and 3,855,092. Two other examples of surface coatings are tin and antimony with titanium and ruthenium oxides, as seen in U.S. Patent No. 3,875,043 and tantalum oxide. iridium oxide coating, as seen in U.S. Patent No. 3,878,083.

There are, of course, other coatings available to those skilled in the art for chlor-alkali cells as well as other types of applications where electrodes would be required for electrolytic reactions.

There are a number of materials that can be used in the separator 16 shown in the drawing. One type of material naturally anticipates the use of some kind of hydraulically impermeable or cation exchange membrane, as it is known in the art.

65281 9

One type of hydraulically impermeable cation exchange membrane that can be used in the apparatus of this invention is a thin film of a fluorinated copolymer having pendant sulfonic acid groups. The fluorinated copolymer is derived from monomers having the formulas:

1) CF_ = CF (R) SOF

/ n z where the dependent -SO 2 F groups are converted to -SO 2 H groups, and of the monomers of formula 2) CF 1 = CXX 1 Z R 1 * Λ where. R represents a group -CF-CF2-O- (- CFY-CF2-O-), wherein R-1 is a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms; Y is a fluorine atom or a trifluoromethyl group; m is 1, 2 or 3; n is 0 or 1; X is a fluorine, chlorine or trifluoromethyl group? and X 1 is X or CF 2 - (- CF 2 -) a O-, wherein a is O or an integer from 1 to 5.

This results in copolymers with repeating units 3) -CF_-CF-

^ I

(R), n

S03H

and 4) -CF ^ CXX1-

The copolymer should have sufficient repeating units of formula (3) above to give an -SO 2 H equivalent weight of about 800-1600. Materials with a water absorption of about 25% or greater are preferred, as higher cell voltages at any given current density are required for materials with a lower water absorption. Similarly, materials with a film thickness (uncalcined) of about 0.2 mm or greater require higher cell voltages, resulting in lower power efficiency.

Typically, due to the large areas of membranes in commercial cells, the substrate material is laminated and impregnated with a hydraulically permeable, non-conductive, inert reinforcing member, such as a woven or non-woven fabric, 6 5 2 81 10 'made of asbestos, glass, TEF, glass, TEF. . In film / fabric composite materials, it is recommended that the lamination provide an intact surface of the film resin at least to one side of the fabric to prevent leakage through the backing film material.

Materials of this type are described in more detail in the following patents, which are incorporated herein by reference: U.S. Patent Nos. 3,041,317, 3,282,875, 3,624,053, 3,784,399, and British Patent No. 1,184,321. Materials such as those described above are available from the company NO du Pont de Nemours and Co. under the trade name NAFION.

Polymeric materials of formulas 3 and 4 can also be made in which the ion exchange group, instead of being a sulfonic acid exchange group, could have many other types of structures. One special type of structure is a carboxyl group terminating in either an acid, ester or salt to form an ion exchange group similar to a sulfonic acid. In such a group, instead of the SO-F group, there is COOR, where 2 E R may be selected from the group consisting of a hydrogen atom, an alkali metal ion and an organic radical. These polymeric materials are currently available from E.I. du Pont de Nemours & Co. In addition, it has been found that a support material such as NAFION having an ion exchange group or a functional group that can be converted to an ion exchange group or a functional group to which the ion exchange group can be easily attached would contain groups such as oxyacids, salts or carbon, nitrogen, silicon, phosphorus esters of chlorine, arsenic, selenium or tellurium.

Another type of substrate material has a backbone chain of copolymers of tetrafluoroethylene and hexafluoropropylene, and a 50/50 mixture of styrene and alpha-methylstyrene is grafted to this backbone. These grafts can then be sulfonated or carbonized to achieve ion exchange properties. Although, this type of substrate has different dependent groups, it has a fluorinated backbone chain so that the chemical durations are moderately high.

Another type of backing film material would be polymeric materials with pendant carboxylic or sulfonic acid groups and in which the polymeric backbone is obtained by polymerizing an aromatic polyvinyl component 65281 11 with an aromatic monovinyl component in a generally inert solvent under conditions which prevent solvent solubilization. can also be prepared.

The aromatic polyvinyl compound may be selected from the group consisting of divinylbenzenes, divinyltoluenes, divinylnaphthalenes, divinyldiphenyls, divinylphenyl vinyl ethers, their substituted alkyl derivatives with respect to polyionyl derivatives such as dimethyldivinylbenzenes, and the like polymerizable aromatics.

Aromatic monovinyl components commonly present as an impurity in commercial grades of aromatic polyvinyl compounds include: styrene, isomeric vinyltoluenes, vinylnaphthalenes, vinylethylbenzyl-alpha-alkenoyl-alpha-aliphatics, vinylchlorobenzenes, vinylxylenes, and their In cases where high purity aromatic polyvinyl compounds are used, it may be desirable to add aromatic monovinyl compounds such that the aromatic polyvinyl compound comprises 30 to 80 mole percent of the polymerizable material.

Suitable solvents in which the polymerizable material can be dissolved prior to polymerization should be inert to polymerization (so that they do not chemically react with monomers or polymers), should also have a boiling point above 60 ° C, and be miscible with the sulfonation medium.

The polymerization is carried out by any of the well-known means, for example by means of heat, pressure and catalytic accelerators, and is continued until an insoluble, indigestible gel has formed over substantially the entire volume of the solution. The resulting gel structures are then sulfonated in the solvated state and to the extent that up to four equivalents of sulfonic acid groups are formed for each mole of aromatic polyvinyl compound in the polymer and at least one equivalent of sulfonic acid group is formed for every ten moles of polyethyl aromatic in the polymer. As with NAFION-type 65281 12, these materials may require reinforcement with similar materials.

Substrate materials of this type are described in more detail in the following patents, which are incorporated herein by reference: U.S. Patent Nos. 2,731,408, 3,731,411 and 3,887,499. These materials are available from Ionics, Inc. under the tradename IONICS CR6.

Various ways to improve these substrate materials have been sought, one of the most effective of which is the chemical treatment of the substrate surface itself. In general, these treatments involve allowing the dependent groups to react with agents that result in less polar binding and then absorb less water molecules by hydrogen bonding. This tends to reduce the pore openings through which the cations pass so that less hydrated water travels with the cations through the membrane. An example of this would be to allow ethylenediamine to react with pendant groups to link two pendant groups together with two nitrogen atoms in ethylenediamine. Generally, at a film thickness of 0.18 mm, the surface treatment is performed to a depth of approximately 0.05 mm on the other side of the film by adjusting the reaction time. This results in good electrical conductivity and cation transfer with less amount of hydroxide ions and associated water countercurrent.

The separator 16 could also be a porous diaphragm, which can be made of any material that is compatible with the cell fluid environment and suitable bubble pressure and electrical conductivity properties.

One example of such a material is asbestos, which can be used either in the form of a sheet of paper or as a blank layered fiber. Further modification can be accomplished by adding polymeric materials, usually fluorinated, to the slurry from which the diaphragm is deposited. The polymeric materials themselves can also be made porous to the extent that they exhibit diaphragm performance. Those skilled in the art will readily recognize the wide variety of materials currently available for use as separators in chlor-alkali cells.

The third major component in these said cells to be used in accordance with the methods of this invention is cathode 18, as seen in the drawing. In order for cathode 18 to be used in accordance with the methods of this invention, it must necessarily be an oxygen cathode. An oxygen electrode or oxygen cathode can be defined as an electrode to which a fluid containing molecular oxygen is fed to reduce the voltage below that required for the evolution of hydrogen. The basic support of an oxygen cathode usually comprises a current collector which could be made of a parent metal, although carbon black may also be used. The phrase base metal is used herein to mean cheap metals that are commercially available for general structural purposes. Base metals are characterized by low cost, easy availability, and adequate resistance to chemical corrosion when used as a cathode in electrolytic cells. Examples of base metals would be iron, nickel, lead and tin. Base metals also include alloys such as carbon steels, stainless steel, bronze, monel metal, and cast iron. The parent metal is preferably chemically resistant to the catholyte and has high electrical conductivity. Besides, this material is usually a slightly porous material, such as a mesh, when used in the manufacture of an oxygen cathode. The preferred metal, based on price, catholyte resistance and available voltages, is nickel. Other current collectors would be tantalum, titanium, silver, gold, and galvanized base metals. On the other side of this base support material is a coating of porous material either sealed in such a way that it adheres to the nickel support, or held together with some kind of binder to provide a porous substrate material. The recommended porous material based on price is carbon. A catalyst is anchored to the porous portion of the oxygen cathode to catalyze the reaction in which molecular oxygen combines with water molecules to produce hydroxide groups. These catalysts are generally based on silver or a platinum group metal such as palladium, ruthenium, gold, iridium, rhodium, osmium or rhenium, but can also be based on semi-base or base metal, alloys, metal oxides or organometallic complexes. Generally, these electrodes contain a hydrophobic material to make the electrode structure waterproof. Those skilled in the art will, of course, appreciate that the porosity of the carbon material and the amount and type of catalyst material used will affect the voltages and current efficiencies of the resulting electrolytic cell and their lifetimes. The preferred cathode 18 can be made in accordance with U.S. Patent No. 3,423,247, the disclosures of which are incorporated herein by reference.

65281 14

As can be seen from the drawing, the use of the anode 14, separator 16 and oxygen cathode 18 described above results in an electrolysis cell 12 having three compartments, anode compartment 20, cathode compartment 22 and oxygen compartment 24. Of these three compartments The alkali metal halide solution is preferably one that would generate chlorine gas, such as sodium chloride or potassium chloride. An aqueous solution would be found in the cathode compartment 22, which would be transferred there through the aqueous solution supply port 28. The aqueous solution must contain sufficient water molecules to decompose to form the required hydroxide groups required for the reaction. The oxygen compartment 24 is supplied with a fluid flowing through the oxygen supply port 30 that contains a sufficient amount of molecular oxygen to enable the operation characteristics of the cell. Such a material would generally be gas and preferably air from which carbon dioxide has been removed and which has been moistened, or pure molecular oxygen which has been moistened. Reaction products such as chlorine gas would be removed from the anode compartment 20 through the halogen outlet 32 and aqueous NaOH or KOH would be removed from the cathode compartment 22 through the alkali metal hydroxide outlet 34 and the oxygen depleted fluid, preferably in the form of pure residual oxygen or air.

In the three-compartment cell 12 described above in the oxidizing gas-depolarized chlor-alkali cell, the pressure difference is applied across the porous cathode 18 so that the pressure in the oxygen compartment 24 is higher than the pressure in the cathode compartment 22. Increased pressure, which may be zero in the manometer. may be negative in absolute terms, facilitates the mass transfer of an oxidizing gas, such as oxygen, to the cathode 18, thereby preventing oxygen depletion in the reaction zone within the cathode 18 and leading to a longer life of the cathode 18. It should be remembered that this pressure difference is based on the partial pressure of the oxygen present if less than 100% oxygen is used.

Increasing the total flow of depolarizing gas in the oxygen compartment 24 also improves the mass transfer of oxygen to the reaction sites within the cathode 18. This is especially important when using less than 100% pure O 2. Molecular oxygen is consumed in the reaction.

65281 15 which occurs at catalytic sites in the porous material of the oxygen cathode 18. As oxygen is consumed, new amounts must be available continuously and must therefore be continuously fed to the oxygen compartment 24. The recommended total flows are 0-10 times the theoretical stoichiometric amount of oxygen required for the reaction, n.

2.5 times the flow being best.

Pure oxygen gas can be fed to the oxygen compartment 24; however, air can also be used as it contains approximately 23% by weight of free molecular oxygen. However, in the case of air, carbon dioxide must be removed from the air before it is fed to the oxygen compartment 24. It has been found that carbon dioxide promotes the formation of certain carbonate deposits on the cathode, which sharply shortens its service life and power efficiency while increasing voltage. By removing much of the carbon dioxide, this problem was also largely eliminated.

Applicants have found that the presence of nitrogen in the air causes problems because it acts as a diluent, thereby reducing the oxygen concentration present in the oxygen compartment 24 of the electrolytic cell 12. Nitrogen molecules enter the pores of the cathode 18 and must diffuse back out of the pores. This causes a lack of activity in the porous catalyst regions of the oxygen cathode 18, thereby reducing the potential power efficiency and raising the voltage required to operate such a cell. Applicants have further found that this can be minimized by increasing the total flow to provide an abundant oxygen supply to the oxygen compartment 24, thereby minimizing the voltage required to operate the cell while maximizing the potential power efficiency from such an electrolytic cell 12.

In addition, the applicants have found that evaporation and mass transfer are a problem with the oxygen cathode 18 as shown in the electrolytic cells 12. They found that this problem could be overcome by adding oxygen or air to the oxygen compartment 24 by blowing gas at 40-70 ° C. through to provide approximately 85% relative humidity. This in turn reduces evaporation and reduces drying of the cathodes, which may cause the porous 65281 16 material to delaminate from the closed support material of the oxygen cathode 18, and further improve mass transfer across the porous surfaces. The temperature of the gas, when actually fed to the oxygen compartment 24, was generally between 40 and 90 ° C and was therefore saturated. In addition to this, wetting seems to have another effect. In the main rod, the volatility driving force that causes the mass transfer of water from the cathode compartment 22 to the cathode structure 18 causes the electrolyte to crystallize, forming solids that severely shorten the life of the given cathode 18 as the solid clogs the pores. By wetting the feed gas to the oxygen compartment 24, this is greatly reduced by eliminating the volatility driving force associated with the transfer of the liquid electrolyte from the cathode compartment 22.

However, it has been pointed out that if the Dew Point of the gas supply is higher than the surface temperature of the cathode, condensation will occur on the surface of the cathode. When this occurs, the oxygen mass transfer sites become clogged, severely degrading the performance of the given oxygen cathode 18. Therefore, the Dew Point of the gas stream was adjusted to balance the two adverse effects described above, in particular to keep the dew point a few degrees below the cathode surface temperature while maintaining relative humidity in a range that eliminates the volatility driving force associated with the image. It should be noted that higher operating temperatures lower the cell voltage but can shorten the life of the cathode 18. The temperature between 60-85 ° C is considered optimal.

In order that those skilled in the art may more readily understand the present invention and certain preferred aspects by which it may be practiced, the following typical examples are set forth.

Example 1 An oxygen cathode according to U.S. Patent No. 3,423,245 was installed in an electrolysis cell with the carbon side facing the oxygen compartment and the Nikke side facing the cathode compartment into which the electrolyte was placed. A dimensionally stable anode with a catalyst layer consisting of tantalum and iridium oxides was mounted approximately 7 cm from and parallel to the oxygen cathode. Carbon dioxide free air. The flow was introduced into the oxygen compartment of the cell at a flow rate of approximately 790 cm / min, which is approximately 21 times the theoretical stoichiometric amount required when the cell is operating at a current h of 0.15 cm 2. The pressure in the oxygen compartment was adjusted to approximately 110 g / cm above normal pressure by limiting the flow outlet 36. The pressure was maintained at this level throughout the experiment. The electrolyte containing about 400 g / l NaOH was then added to the cathode compartment 22 and stirred continuously with a magnetic stirrer.

The cathode was then conditioned using a cell at 60 ° C and a current density of about 0.05 A / 2 cm for about one day. At the end of the conditioning, the current density 2 was increased to approximately 0.15 A / cm. Airflow, pressure, temperature, and current density were kept constant for the remainder of the experiment. It should be noted that only sodium hydroxide electrolyte was used in these experiments and chlor-alkali cells as such were not used. However, the results obtained from these experiments should correlate closely with those obtained using chlor-alkali cells, as the type of anode or the gap between the anode and the cathode are not critical, although the anode must be stable in sodium hydroxide solution.

The cathodes were conditioned at a reduced current density because it was thought that the catalytic platinum layer would be partially oxidized during storage of the cathodes prior to use. The conditioning process restores the catalyst layer to high activity without causing cathode quality deterioration. However, slow run-in may be detrimental to less noble catalysts.

The electrical connection was made to the nickel side of the cathode because it was easier to make a good electrical connection to nickel than to carbon. The cathode reference voltage measured against the mercury / mercury oxide reference electrode cell changed from -0.31 V on the first day to Ί.03 V 98 on the first day the experiment was considered terminated. The lifetime of this particular cathode under these experimental conditions was 2350 hours.

Example 2

The cathode test was performed as described in Example 1 except that the air flow rate was reduced from 790 cm / min to 220 cm / min (approximately 6 times the theoretical stoichiometric amount required for the reaction). The reference voltage changed from -0.43 V on the first day to -2.27 V on the 52nd day. The cathode lifetime was 1240 hours in this experiment compared to 2350 when a higher airflow was used in Example 1. This shows that as the total 65281 18 flow increases, the potentials are lower and the lifetimes are longer. Example 3

The cathode test was performed as described in Example 1 except that

O

an oxygen flow rate of 150 cm / min was used instead of an air flow rate of 790 cmVmin. This oxygen flow rate was about 19 times the theoretical stoichiometric oxygen flow required at a current density of 2 0.15 A / cm to operate the cell. In this experiment, the electrical connection was made to the carbon side of the cathode. The cathode was conditioned by running the cell at a current density of 0.05 A / cm for about 24 1/2 hours, raising it to 0.1 A / cm for approx.

. 2 for 24 hours and finally raising it to about 0.15 A / cm, the level at which it was kept for the rest of the experiment. The reference voltage changed from -0.38 V on day 2 to -0.42 V on day 293. The experiment was stopped on day 293 due to cathode delamination. The life of the cathode was about 7030 hours. This again shows that when the total flow is increased in relation to the stoichiometric amounts of available oxygen, the service life is extended at lower potentials.

Example 4

An oxygen cathode test was performed according to Example 1 at an operating temperature of approximately 85 ° C, a current density of approximately 0.3 A / cm 2 and a solution containing 300 g / l NaOH. The membrane used in the further experiment was the standard NAFION. This test cell was run using different types of cathodes. The comparable cell voltages for the different cathodes were obtained as follows:

Cathode type Avg. Avg. voltage saving compared to hydrogen cell voltage development, volts (% difference)

Steel mesh 4,335

Oxygen electrode with platinum catalyst using pure oxygen 3.039 1.296 (30%)

Oxygen electrode using a silver catalyst account with pure oxygen 3.306 1.029 (24%)

Oxygen electrode using a ho-catalyst catalyst air supply 3,536 0.799 (18%) 65281 19

As can be seen from the above results, each of these cathodes in question, when compared with a standard hydrogen-generating steel mesh cathode, shows excellent performance using the methods of this invention.

Example 5

An oxygen cathode test was performed according to Example 1, in which the run was performed with air from which carbon dioxide was not washed away. The cathode was run in with oxygen and then the air was turned on and the contamination occurred less than 48 hours after turning on the air. This behavior was typical of cathodes fed with carbon dioxide-containing air, indicating the need for carbon dioxide removal for the life of the oxygen cathode. The basic conditions were the same as those included in the runs of Examples 1-3, and the table shows the cell voltages and the reference voltage with comments.

SEP vs. Hg / HgO

Cell voltage Reference voltage Remarks 2 1.168 -0.255 V (current 0.05 A / cm) run-in with oxygen 1.044 -0.124 0.995 -0.112 1.790 -0.222 V (current 0.155 A / cm ^) 1.768 -0.212 1.944 -0.354 reversal for compressed air 1.9 0.461 2.120 -0.490 2.745 -1.066 cathode was ruined

Examples 6-12

Oxygen cathode experiments were performed according to Example 1, in which the total amount of air flow was varied according to Fig. 2 of the drawings and also the pressure was varied. As can be seen from Figure 2, the cathode potential decreased with increasing flow rates and also decreased with increasing pressures. In all cases, the air supplied was free of carbon dioxide and humidified.

i 20 6 5 2 81

The cathode tests described above in the examples were stopped in all cases when the reference voltage reached -1.00 V or when delamination of the different layers of the cathode occurred. The air (or oxygen) flow was plotted on the cathode age cell test results sheets as the height of the steel ball (mm) in a Matheson No. 601 flowmeter (except in Example 3 where a Matheson No. 602 flowmeter was used). These readings were then converted to cubic centimeters per minute using appropriate calibration curves. The examples give the results of cathode tests in which the pressure differences were of the order of 100 g / cm. The term pressure differential means the benefit pressure acting between the two sides of the cathode. In this case 2, the difference between the pressure of the oxidizing gas compartment (100 g / cm above normal pressure) and the average hydrostatic pressure 2 produced by the electrolyte on the other side of the cathode (10 g / cm) is approximately 100 g / cm. The hydrostatic pressure is calculated by multiplying the density of the electrolyte (1.33) by an average height above the cathode of 7.5 cm. It is estimated that the useful range of the pressure difference is likely to be between 0.25 and 500 g / cm 2. It is expected that those cathodes using normal pressure or not allowing the gas compartment pressure to exceed normal pressure would have a lifetime of less than 1240 hours as obtained in Example 2, however. It should be noted that all examples give test results using only NaOH electrolyte; chlor-alkali cells were not used because no porous cathodes are currently in commercial use. All experiments described above were performed at a current density of 0.155 A / cm, which was selected for experimental purposes for standardization purposes only and should not be considered as the highest possible value. It is assumed that current densities of the order of 0.30 A / cm or more could be used. The experiments of Examples 1-3 were performed at 60 ° C, which temperature was simply selected as the effortless temperature on which standardization can be based.

Accordingly, it should be apparent from the foregoing description of the preferred embodiments that the methods for using an oxygen / air cathode in electrolytic cells presented and described herein accomplish the purposes of the invention and solve the problems associated with such a system for use in an oxygen-cathode chlor-alkali electrolytic cell. .

I:

Claims (9)

65281 21 A method of operating a chloro-alkali electrolysis cell having an anode compartment, a cathode compartment separated from the anode compartment by a membrane, and an oxygen compartment separated from the cathode compartment by an oxygen electrode, the method an electric potential is generated between the cathode and the anode, halogen gas is removed from the anode compartment, alkali metal hydroxide is removed from the cathode compartment and a deoxygenated medium is removed from the oxygen compartment, characterized in the oxygen-depleted medium is removed from the oxygen compartment so that a positive gauge pressure is maintained in the oxygen compartment. Method according to Claim 1, characterized in that the positive gauge pressure is between 0.25 and 500 g / cm, 2. preferably between 5 and 250 g / cm and most preferably between 100 and 200 g / cm. Method according to Claim 1 or 2, characterized in that the total flow rate is 1.5 to 5 times the theoretical stoichiometric amount of oxygen. Process according to Claim 1, 2 or 3, characterized in that the oxygen-containing medium is degassed air. Process according to one of Claims 1 to 4, characterized in that the oxygen-containing medium with a high humidity is impregnated at 40 to 70 ° C and fed to the oxygen compartment at a temperature in the range from 40 to 90 ° C. A chlor-alkali electrolysis cell for applying the method of claim 1, comprising an anode compartment (20) for an alkali metal chloride-containing anolyte having means (26) for supplying alkali metal chloride thereto and means (32) for removing chlorine therefrom, an alkali metal chloride-containing cathode compartment (22) for a catholyte separated from the anode compartment (20) by a separator (16) and having means (28) for feeding the catholyte thereto and means (34) for removing alkali metal hydroxide therefrom, an oxygen electrode in the oxygen compartment separating the oxygen compartment (24) from the cathode compartment (22) ) and means (30) for supplying a molecular oxygen-containing, wetted and carbon dioxide-free medium to the oxygen compartment (24) and means (36) for removing an oxygen-depleted medium therefrom, characterized in that means (30, 36) for supplying the molecular oxygen medium to the oxygen compartment (24) and to remove the oxygen-depleted medium i are adapted to supply the medium at a positive gauge pressure and a positive total flow of 1.5 to 10 times the stoichiometric amount of oxygen required for the reaction, and to remove the medium so as to maintain a positive gauge pressure in the oxygen compartment (24). Electrolysis cell according to claim 6, characterized in that said separator (16) is a diaphragm. Electrolysis cell according to Claim 6, characterized in that the separator (16) is a cation exchange membrane. Electrolysis cell according to Claim 8, characterized in that the cation exchange membrane (16) is a hydraulically impermeable membrane consisting of a fluorinated copolymer with pendant sulfonic acid groups.