CA1066655A - Method and apparatus for electrically increasing sulfuric acid concentration in an electrolytic cell in the presence of persulfate ions - Google Patents

Abstract

ABSTRACT
An improved method is provided for concentrating aqueous sulfuric acid solution of concentration greater than about 75 weight per cent H2S04 by electrolysis in an electro-lytic cell wherein platinum electrodes are utilized without the build-up of sulfur deposits thereon. To achieve this end, persulfate ions are provided in the solution and the electrodes are placed in sufficient proximity to one another and sufficient mixing is provided in the solution so that a substantial quantity of persulfate ions are conveyed into the region surrounding the cathode. In an integrated system, drying of a wet fluid stream on a continuous basis is effected by contacting it with concentrated sulfuric acid and purify-ing and recycling the sulfuric acid by the method of this invention, only a small inventory of the sulfuric acid being required.

Description

The invention relates to a method and apparatus for concentrating an aqueous sulfuric acid solution. More specifically, the invention relates to the regeneration of aqueous sulfuric acid of concentration in excess of 75 weight per cent by means of electrolysis in an electro-lytic cell utilizing platinum electrodes.
The problem of satisfactorily, efficiently and economically removing water and other impurities, e.g., organic compounds, from aqueous sulfuric acid solutions containing more than about 7S per cent by weight H2SO4 has been a long standing enigma. For example, evaporation of the water by distillation requires the use of high temperatures at which the hot concentrated acid is highly corrosive and is difficult to contain. Attempts to utilize electrolysis to concentrate sulfuric acid in these high concentration ranges have been frustrated by the great speed at which electrodes and container materials are degraded by the attack of the acid. Platinum electrodes or platinum--coated electrodes are generally required, being the most resistant to sulfuric acid corrosion of all the electrode materials.
However, previous attempts to utilize platinum electrodes in such an electrolytic cell have demonstrated that in addition to the production of hydrogen gas and o~ygen gas thereby decomposing and removing water from the acid, cathodic reduction of the acid itself occurs forming solid elemental sulfur which deposits on the cathode znd dis-perses into the acid. Such sulfuric acid, contaminated - with solid sulfur, is generally unsuitable for use in industrial applications. This has made the eIectrolytic ' , ~F : .

17,454-F -1-.

10~6655 method appear to be unsatisfactory for providing high con-centrations of sulfuric acid on a commercial basis, in that without being able to use platinum electrodes, electrode replacement becomes necessary so often that the electrolysis process becomes economically objectionable.
For example, U. S. Patents 1,992,308 and 1,992,310 utilize a lead, rather than a platinum, cathode to remove sulfur-containing organic compounds from hydrocarbon fluids ;~ by electrolysis. U. S. Patents 2,793,181 and 2,793,182 disclose electrolytically regenerated spent alkylation sul- -furic acid solutions in a cell containing platinum electrodes.
However, free sulfur was formed in the cathode compartment of the cell. U. S. Patent 3,616,337 teaches that the use of a platinum cathode in an electrolytic sulfuric acid regen-eration process was not satisfactory in that sulfates were reduced to free sulfur thereon.
A method has now been discovered that permits platinum to be used for either or both the anode or the cathode in an electrolytic cell which is used to concentrate aqueous sulfuric acid solutions wherein no objectionable sulfur build-up results.
The invention resides in a method of concentrating an aqueous solution of sulfuric acid of concentration greater than about 75 per cent H2SO4 by electrolysis in an electrolytic cell containing at least one corrosion resis-tant anode and at least one corrosion resistant cathode, comprising the step of providing a quantity of persulfate ions in the sulfuric acid solution in a region immediately surrounding and communicating with said cathode sufficient to prevent a build-up of elemental sulfur thereon during electrolysis.

17,454-F -2-.

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~066~55 The invention further resides in a method of :~
reducing the H2O content of a wet fluid by intimately con- ~
tacting the wet fluid with a sulfuric acid solution having ~.
a concentration greater than about 75 weight per cent H2SO4 . 5 whereby the H2O from the wet fluid is absorbed by the sul-furic acid solution to produce a dry fluid which is sub-stantially free of H2O, said fluid being incapable of form-ing a chemical reaction with the concentrated sulfuric acid solution, being immiscible with and insoluble in the acid solution and being readily separable from the acid solution after contact therewith, separating the wet sulfuric acid solution having the H2O absorbed therein from the fluid, and thereafter reducing the H2O content of the wet sulfuric acid solution, the improvement which comprises reducing the H2O content of said wet sulfuric acid solution by elec$rol- -ysis in an electrolytic cell having at least one corrosion resistant anode and at least one corrosion resistant cathode while providing a quantity of persulfate ions in the sul-furic acid in the region immediately surrounding and communi-cating with said cathode to prevent a build-up of elemental sulfur thereon during electrolysis.
. The invention further resides in an apparatus for reducing the H2O content of a wet fluid comprising a means `. for contacting the wet fluid with a sulfuric acid solution having a concentration of greater than about 75 weight per cent H2SO4 whereby the H2O from the wet fluid is absorbed by the sulfuric acid solution to produce a dry fluid which is substantially free of H2O, said fluid being incapable of ~- forming a chemical reaction with the concentrated sulfuric 30 acid solution, being immiscible with and insoluble in the .. .
- acid solution and being readily separable from the acid after : contact therewith; a means for separating the wet sulfuric ' ' '' 17,454-F
: ~ , acid solution having the H2O absorbed therein from the fluid, an electro-lysis cell for electrolyzing at least part of the H2O content of said separated sulfuric acid solution, said electrolysis cell being equipped with at least one corrosion resistant anode and at least one corrosion resistant cathode, a first conduit means for conveying the separated wet sulfuric acid solution from the contacting means to the electrolysis cell, a second conduit means for conveying the concentrated dry sulfuric acid solution from the electrolysis cell to the contacting means, the improve-ment comprising means for circulating the sulfuric acid solution in a closed . ..
loop cycle from said contacting means through the first contuit means to the electrolysis cell, through the second conduit means, and back to the contacting means, and means for providing a sufficient quantity of per-sulfate ions in the region immediately surrounding and in communication with the cathode to prevent build-up of elemental sulfur on the cathode during electrolysis.
Figure 1 is a view in vertical section of an embodiment of an apparatus for practicing the method of the present invention.
Figure 2 is a view in vertical section of another embodiment of apparatus used in practicing the method of the present invention in which the oxidizing agent is persulfate ion generated in situ and the H2 and 2 gases formed during electrolysis are segregated.
Figure 3 is a top view in horizontal section, mostly broken away, of yet another embodiment of apparatus used according to a method of the present invention permitting the impure sulfuric acid solution to be sub-jected to electrolysis action while being flowed through an electrolysis cell in a continuous manner.

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- Figure 4 is a schematic illustration of a system for drying sulfuric acid by electrolysis according to the ~ :
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present invention as an integral part of a fluid drying process.
The operation of a basic embodiment of the pre~
sent invention will be better understood upon becoming . familiar with the following description, reference being had to the accompanying drawings. ~ -; Referring now to Figure 1, a container 11 is shown partially filled with aqueous sulfuric acid solution ~' 12 which serves as an electrolyte in which a cathode 13 and an anode 14 are partly immersed. A source 15 provides direct current electricity to the anode 14 and the cathode 13. Hydrogen is generated at the cathode 13 and oxygen is , 15 generated at the anode 14. The mixture of gases is removed , ~, :. ' ' ` , ' ;' ' ''''~

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through a port 16 in the upper wall of container 11. The aqueous H2SO4 to be concentrated is conducted into the container 11 by a pipe 17 extending through the sidewall of the container 11 and which is fitted with an inlet valve 18.
When the desired acid concentrati~n is attained, the acid is removed from the container 11 via the pipe 19 which communicates with the floor of the container and which is - fitted with an outlet valve 20. A mixing device 21, such as a motor driven propeller, the shaft 22 of which extends through a wall in the container 11 and into the acid, may be provided to increase electrolyte circulation.
It has been found that the passage of an electro-lytic current through the aqueous sulfuric acid electrolyte for the primary purpose of electrolyzing the water present also generates reduction products of sulfuric acid such as sulfite ions and sulfur at the cathode together with oxidation products of sulfuric acid such as persulfate ions at the anode of the cell or vessel in which the electrolysis is being conducted. When a sufficient amount of persulfate ion is supplied to the cathode, all or substantially all of the reduction products formed on electrolysis of H2SO4 and other sulfur compounds are oxidized, preventing the build-up of elemental sulfur.
Providing a sufficient quantity of persulfate ions 1 25 at the cathode depends on (1) supplying persulfate ions to ; the solution at a sufficient rate and (2) effecting sufficient diffusion or transport of the persulfate ions from their point of supply to the cathode. Persulfate ions may be added from an external source in salt form, e.g., sodium persulfate, or they may be generated in situ by maintaining an anodic 17,454-F -5_ ! ~

~066655 current density of between 0.5 and 5 amps/sq. in. (0.078 to 0.78 amps/cm2). The rate of supply of persulfate from an external source depends on the rate at which the persulfate source is added to the solution and its solubility therein.
The rate at which persulfate ions are generated at the anode depends on anodic current density, electrolytic temperature, and on the concentration of H2SO4 in the aqueous acid electrolyte.
The diffusion or transport of persulfate ion from anode to cathode depends on electrode spacing and on the degree of mixing provided in the electrolyte. The persulfate ions are believed to contact the cathodic reduction products, rather than the cathode itself, due to the electrostatic repulsive forces existing between the cathode and the per-sulfate ions. However, for simplicity of explanation in the present application, the persulfate ions will be said ~4~ ~crm ~i s 3 to ~contact~ the cathode, by which ~e~ the interaction just described will be denoted. ;~
The cathodic reduction products are believed to include sulfite ions, sulfur and intermediates such as thionate ions, thiosulfate ions, hyposulfite ions, poly-sulfide ions and the like. In the practice of the present method it is believed that the intermediates along the path to formation of free sulfur are actually oxidized before sulfur is allowed to form, although the sulfur itself, if formed (e.g., by suspension of the method of the present invention) will be oxidized back into solution (e.g. to sulfate ion) by the method of the present invention.
When the method of the present invention is utilized, a slight threshold film of sulfur is sometimes 17,454-F -6-- . : , .. . . . . .

found to form on the cathode. At the lower anodic current densities, e.g., about 0.5 amps/sq. in. (0.078 amps/cm ), this amount may be .0001 gm. sulfur/sq. in. (.000016 gm/cm2) of cathode surface area. At higher current densities, e.g., 4 amps/sq. in. (0.62 amps/cm ), this ~ilm is virtually undetectable, i.e., less than about .0001 gm/sq. in.
After formation of this slight initial film however, any further increase in the amount of sulfur on the cathode is prevented by the practice of the present invention. The term build-up is used in this application to denote the further increase in the amount of sulfur on the cathode, which increase may be controlled accord-ing to the present method to limit it to zero or any other predetermined rate less than that which would occur absent the practice of the present invention.
As an example of the build-up of sulfur occurring absent the practice of the present invention, an electrolysis was ~onducted on an 85.2 per cent by weight H2SO4 solution with an anodic current density of 0.15 amp/sq. in. (0.023 amp/cm ); sulfur built-up on the platinum cathode at the rate of about 0.11 gm. sulfur/gm. H2O removed from the acid solution (Table II, Comparison 1).
i The electrolyte used herein is the aqueous sulfuric acid solution to be purified, from which all or a portion of the water and organic compounds present, if any, are to be removed. Aqueous sulfuric acid solutions ; of any concentration may be further concentrated by the method of the present invention. However, acid solutions of initial concentration ranging upward from about 75 weight per cent H2SO4 are the acid solutions wherein sulfur 17,454-F -7-'': . ; . ~ . . .
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build-up has been found to be a problem and thus is defined as the range of applicability of the present invention.
In the course of being concentrated by the present method, the aqueous acid solution may be purified of virtually any organic compound which is oxidizable by persulfate ions in a medium of aqueous sulfuric acid. The persulfate ions generated herein in the aqueous sulfuric acid solution are such a powerful oxidizing agent that almost any organic material is oxidized. The extensively fluorinated organic compounds are less responsive to oxidation by persulfate ion; perfluorinated compounds such as e~}4~ are found not ~-to be oxidized to a noticeable degree. However, the presence of non-oxidized perfluorinated organic compounds in the aqueous sulfuric acid solution is not generally deleterious due to the relative inertness of such compounds. Exemplary, though not exhaustive of organic compounds which are well , oxidized are butylene glycols, phenolics, sulfonated phenols, polystyrene derivatives, e.g., sulfonated polystyrenes, and halohydrocarbons.
~ Any initial concentration of organics may be treated, the higher concentrations requiring a greater quantity of electricity to be utilized to attain a particuiar - final organic compound concentration. This in turn requires a greater electrolytic current flow or the use of a greater period of time to achieve the particular final concentration desired.
Exemplary of normally encountered industrial waste streams are those containing about 200 ppm or more total organic carbon (TOC). The present method is usable on these streams as well as on streams wherein the ~odf~o~k . , 17,454-F -8-.
.
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10666~5 concentration of organic matter is in excess of 2500 ppm TOC. As the organic compounds are in the process of being - removed, the electrolytic current also breaks water down into hydrogen and oxygen, resulting in an increase of the H2SO4~concentration in the aqueous solution. This concentrated solution may be diluted by adding pure water to obtain a yet lower concentration of organic impurities while simultaneously returning the solution to its original concentration of H2SO4.
The aqueous sulfuric acid may be electrolyzed to 100 weight per cent H2SO4 or higher by practicing the method of the invention. Alternatively, the acid may be withdrawn from the electrolytic cell at any desired concentration between the initial value and 100 per cent.
The electrolytic cell or container may be made of any material that does not adversely react with the electrolyte under conditions encountered in the electrolysis, e.g., the presence of a strong oxidizing ager.t, electric current flow, and elevated temperature. Representative, though not exhaus/ive of the usable materials are glass, ceramics, te~lon, and platinum, or other materials of construction having a protective coating formed of one of these acid resistant materials.
Since the electrodes are likewise in contact with the strongly oxidizing concentrated sulfuric acid, platinum is used as the electrode material of construction. Platinum is the element which is most resistant to corrosive degradation, and therefore, ., ' ' ~ ~Oc~

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`; 1066655 needs to be replaced the least often and contaminates the solution to the least degree possible. Alternatively, platinum may be utilized as a coating on other substrates, such as steel or iron, for example, so long as adequate corrosion resistance is provided.
In the practice of the present electrolytic method, corrosive degradation of a platinum anode is found to be very slow and to involve the substantial redeposition, i.e., about 90 per cent, of the platinum lost from the anode onto the cathode. Accordingly, the flow of electrolytic current may be reversed periodically to permit the platinum anode to regain approximately 81 per cent of its platinum loss dur- -; ~ ~g ing the cycle. The bipolar design ofrelectrolytic cell (Fig.
3) using platinum electrodes is particularly suitable : 15 for the electrolysis of aqueous sulfuric acid because the platinum lost from the anodic surfaces of the interior electrodes, i.e., electrodes between the ends of the array, is substantially regained on the cathodic surfaces of adjacent interior electrodes.
In controlling the build-up of any noticeable -amount of sulfur deposit on the cathode, the persulfate ion may be supplied continuously or intermittently, so long as there is at all times a sufficient amount of ` persulfate ion in contact with the cathode to reoxidize the reduction products of H2SO4 which are formed at the cathode. In some situations temporary build-up of sulfur deposits on the cathode may be tolerable, in which event ` persulfate ions need be supplied to the cathode in suffi-cient amount to reoxidize the H2SO4 reduction products only at the point when the sulfur deposits must finally be removed.
. .

1 ,454-F -10-.. . ' ~ : .
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As indicated above, persulfate ions may be supplied either from an external source, e.g., a persulfate salt, or they can be generated in situ by maintenance of an appropriate anodic current density. Since it is the simpler and more efficient technique, the in situ generation of persulfate ions is the preferred technique.
Overall, the rate at which in situ generated persulfate ions are provided to the cathode is a function of the anodic current density, electrolyte temperature, H2SO4 concentration in the aqueous acid electrolyte, inter-electrode separatio~ distance, and degree of electrolyte mixing.
In the preferred practice of this embodiment of the present invention, the aqueous acid will be encountered at a given I
concentration and temperature. Optimum electrode spacing is chosen, an adequate degree of mixing is provided, and ; then a current density at least as great as the minimum anodic current density sufficient to prevent the build-up of elemental sulfur on the cathode is applied to the ;
electrolyte. For example, with electrolyte temperature , 20 50C, electrode separation of 1/4 inch (0.64 cm), H2SO4 i concentration 80 per cent by weight, and with amounts ;~ of mixing varying from mild to vigorous, anodic current densities of from about 0.5 to about 5 amperes per square inch (0.078 to 0.78 amp/cm2) are found suitable. Higher current densities permit faster electrolysis of the aqueous H2SO4 solution and are effective in preventing sulfur formation, but may cause greater materials corrosion and a greater density of gas bubbles to evolve. These bubbles reduce electrolytic conductivity and are thus detrimental to current efficiency. -17,454-F -11-.. . .
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The electrolyte may be maintained at any temperature above its freezing point. Higher temperatures are advantageous in that the cathode reduction products are oxidized faster and the electrolyte conductivity increases with temperature, thus pef~itting persulfate ~-ions to be generated at a faster rate. However, higher temperatures are disadvantageous in that persulfate ion decomposes more rapidly, and corrosivity and resulting materials of construction problems are increased. Presently operating temperatures from about 10C to about 80C
are preferred, although use of higher temperatures may become feasible if appropriate materials of construction are developed.
It is found that use of current densities in the upper end of the preferred range causes a noticeable temperature increase in the aqueous H2SO4 electrolyte.
If the temperature increase becomes objectionable, e.g., from a corrosivity point of view, e.q., at about ~aoc, a cooling device may be provided in or around the electrolyte to prevent the electrolyte from exceeding a desired operating . temperature limit. Such cooling devices are well known in the art of electrolytic cell construction and may be -adapted to the particular embodiment of the present invention practiced.
Positioning the electrodes close to one another ` not only enhances diffusion of persulfate ions from the anode to the cathode, but also reduces the amount of ohmic resistance encountered by the electrolytic current. This in turn reduces the power required to electrolyze the quantity of water present.

17,454-F -12-However, spacing the electrodes too close to one another is to be avoided because the bubbles impair the conductivity of the electrolyte, increasing the power required to electrolyze the water present.
Use of high current densities causes extensive bubble formation and, to maintain sufficient electrolyte conductivity, the electrodes must be spaced further apart than when low current densities are used. It is found that with current densities in the range of O.S to 5 amperes per square inch (0.078 to 0.78 amp/cm2), separation distances of about 1/4 inch (0.64 cm) are preferred. From the ; foregoing it will be apparent to those skilled in the art how to utilize greater or lesser separation distances as may be appropriate to a given system.
, 15 Although the evolution of bubbles of hydrogen and oxygen gas during the electrolysis effects substantial mixing of the electrolyte, it is generally desirable to provide additional mixing, e.g., by means of a stirrer as indicated in Figures 1 and 2. Almost any other known means of mixing, such as an inert gas sparge, may also be used. Mechanical mixing so vigorous that cavitation ~ and aeration of the electrolyte results is to be avoided A due to the reduced electrical conductivity caused by excessive gas bubble formation and entrainment.
Though not essential to the invention, it may be found desirable to provide a liquid permeable membrane ; between the electrodes to separate hydrogen and oxygen as they are evolved, e.g., where they are to be utilized separately or where there exists a risk of ignition of the mixture of H2 and 2 If a membrane is used, it ,"' .

17,454-F -13-.

, , , . ~ .' : ' .: ':: . ' ' must not impede the diffusion of persulfate ion from the anode to the cathode. A generally suitable membrane may be constructed, e.g., of a woven fiberglass net ; with open spaces approximately 0.5 mm. x 0.5 mm. Such an embodiment is illustrated in Figure 2, in which like elements common to both Figuresl and 2 are identified by like reference numerals. Herein a membrane 25 is placed between the electrodes under the surface of the electrolyte. A partition 26 extends from the upper wall of the container 11 to a depth beneath the surface of the electrolyte to join the membrane 25 so as to form a barrier. Hydrogen evolved at the cathode 13 is now removed through port 27 in the upper wall of the container 11 and oxygen evolved at the anode 14 is removed through -' 15 port 28, also in the upper wall of the container 11, thus segregating the gases and eliminating the explosive combustion hazard.
A preferred embodiment of the present invention is illustrated in Figure 3 as a top view, mostly broken away and sXown in horizontal section. Aqueous sulfuric acid solution may be concentrated therein by removal of water and/or organic compound impurities on a continuous basis. Aqueous acid is fed into a bipolar electrolytic cell through an entry pipe 31. The acid is directed past bipolar electrode baffles 32 to flow along the path indicated by the arrows before leaving the cell through the exit port 33. Against the entry end of the cell is placed a cathode 34. Against the exit end of the cell is placed an anode 35. The end electrodes 34 and 35 are supplied with electricity by a source of direct : ' 17,454-F _14_ current electricity 36. Hydrogen gas evolves at the cathode 34 and at the baffle plates 32 on the surface which face toward the anode. Oxygen gas evolves at the anode 35 and at the baffle plates 32 on the surfaces which are closer to the cathode.
These evolved gases are removed through an exit port 37 placed in the upper wall 38 of the vessel 30.
- The considerations relating to choice of materials for a bipolar electrolytic cell for continuous treatment are very similar to those enumerated for the basic embodiment of the invention as illustrated in Figures 1 and 2. The ` electrodes and baffles are all constructed from an electrically conductive material that withstands the corrosivity of the medium, e.g., platinum. The baffles are uniformly separated from the end electrodes and from one another at a distance ot about 1/4 inch (0.64 cmJ, depending on various other para-m^ters as outlined above. The baffles, as mounted, must -; be elctrically insulated from the side walIs unless the ~ walls are made of nonconductive material. An externally ; 20 driven mixer is ordinarily not required; the flow of the electrolyte as directed by the baffles and by the evolved gas bubbles provides sufficient electrolyte circulation.
Aqueous sulfuric acid is fed continuously through the concentrating vessel, generally at as great a rate as is acceptable in terms of the amount of water removal required. This rate will thus depend on the initial concentration of the acid, the current through the vessel and the final concentration of acid desired. For example, with an initial 56 per cent by weight H2SO4 concentration and using an anode current density of about 3 amps/sq. in.

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~066655 (0.47 amp/cm2), a final concentration of 96 per cent H2SO4 was obtained.
A particularly useful application of the present invention is in a closed system drying process for fluids containing water. The H2O content of the fluid is substan-tially removed by intimately contacting the fluid with con-centrated sulfuric acid in cocurrent or countercurrent fashion as is well understood in the art. The H2O is absorbed by the sulfuric acid which is separated from the fluid and then is reconcentrated by subjecting the aqueous sulfuric acid to electrolysis according to the practice of this in-vention.
; The fluid to be dried may be either a liquid or a gas and should not be subject to deleterious reaction with , lS concentrated aqueous sulfuric acid, and it should not be miscible with or soluble in the acid or else it may not be readily separable from the acid after contacting.
Examples of fluids advantageously dried using the present integrated system include the halogens and hydrocarbon liquids and gases. The extent to which H2O must be removed from the H2SO4 solution by the present method depends on the nature of the fluid and the extent to which the H2O-content therein is to be reduced for subsequent use of the acid.
me entire fluid drying process may be operated on continuously recycling one initial charge of sulfuric acid, with occasional small additions of make up acid.
Figure 4 presents the integrated fluid drying process in schematic form. Referring to the drawing in Figure 4, the wet fluid stream 41 to be dried is contacted - with concentrated sulfuric acid furnished by supply stream '' ., 17,454-F -16-, -10666~;5 42 in contacting device 43, wherein the fluid gives up a substantial part of its H2O content to the concentrated sulfuric acid. The fluid stream leaves the contacting device 43 as exit stream 44 substantially reduced in H2O
; 5 content. The sulfuric acid which has absorbed H2O in the contacting device leaves the contacting device as exit stream 45 which is conducted into dilute acid reservoir 46.
Periodically the dilute acid content in reservoir 46 is ; emptied by a feed stream 47 into an electrolytic cell 48.
This cell is provided with a persulfate oxidizing agent according to any of the practices or modifications of the present invention and an electric current is supplied to effect electrolysis which removes water from the aqueous acid without elemental sulfur formation or build-up.
Periodically, after the electrolysis has proceeded to the point that the acid in a relatively simple electrolysis cell has been concentrated to the desired percentage of H2SO4, electrolysis is suspended and the acid content of the electrolytic cell is removed by a pump 49 and is conducted as stream 50 to a concentrated acid reservoir 51. The content of this reservoir is fed continuously as a supply stream 42 back to the contacting device 43 to be brought into contact with the fluid stream 41 to be dried.
' Alternatively the continuous flow electrolysis apparatus illustrated in Figure 3 utilizing a bipolar cell is employed, generally making unnecessary the reservoirs 46 and 51. This modification of the integrated system permits continuous cycling of the aqueous sulfuric acid solution between the contacting device and the electrolytic cell.

17,454-F -17-Example 1 In tests 1-3 and comparative run 1-3 the effect of electrode spacing and electrolyte mixing were evaluated.
In tests 1-3, electrolysis of an aqueous H2SO4 solution was conducted according to the method of the present invention in a U-shaped borosilicate glass tube, into one arm of which were placed a platinum sheet anode and a platinum sheet cathode, and into the other arm of which was placed a glass stirrer. The platinum electrodes were 4 mils thick - 10 (0.0016 cm), had a surface area of 1.43 sq. in. (9.21 cm2) each, and were separated by 1/4 inch (0.64 cm). Stirring was accomplished by rotation of a stirring rod, provision of a N2 stream to bubble beneath and upward past the electrodes, and also by provision of a bridge between the two arms of the U tube at a point above the electrodes.
In comparative runs 1-3 no mixing was provided -except that due to the rising bubbles of H2 and 2 formed by the electrolysis. A borosilicate glass U tube was employedt however, this time the electrode separation distance was 1 inch, the anode being placed in one arm of the U tube and the cathode in the other arm. The platinum electrodes were 4 mils (.0016 cm) thick and had a surface area of 0.71 sq.
in. (4.58 cm2) each.
By comparison of the tests and their individual comparative runs, it can readily be seen that providing effective mixing of the electrolyte and close electrode spacing effects a definite reduction in the amount of sulfur formed during the electrolysis. The difference in surface area between the electrodes in the tests and comparatives is not considered material to the results obtained. The results of Example 1 are summarized in Table I.

i, , 17,454-F -18-Example 2 In tests 1-3 and in comparative runs 1-3 sulfuric acid was concentrated by electrolysis utilizing the apparatus illustrated in Figure 1. In comparative runs 1-3 the anodlc current density used was insufficient to generate enough persulfate ions to prevent sulfur build-up on the cathode. In tests 1-3 the method of the present invention -was practiced in that sufficiently great anodic current densities were utilized to generate enough persulfate ions ~ ~-to prevent the build-up of sulfur deposits on the cathode.
The cell was constructed of borosilicate glass. The electrodes each were constructed of platinum and had a surface area of 1.4 sq. in. (9.0 cm ). They were placed 1/4 inch (0.64 cm) apart. There was no membrane or wall between the cathode and the anode. The starting concentration of the sulfuric acid in each case was 85.2 per cent by weight.
The effects of various electrolyte temperatures and anodic current densities were tested. The parameters and results are presented in Table II.
By comparing comparison runs 1, 2 and 3 with one another and tests 1, 2 and 3 with one another, it is seen that conducting the electrolysis at lower temperatures is beneficial in reducing the amount of sulfur formed.
By comparing test 1 with comparison 1, test 2 with comparison

2, and test 3 with comparison 3, it is seen that conducting the electrolysis at higher anodic current densities permits attaining of the desired H2SO4 concentration in considerably shorter time periods, during which substantially less sulfur is formed. Furthermore, comparing the set of comparative runs 1, 2 and 3 with the set of tests 1, 2 and 3 shows 17,454-F -19-: ~ ..... .. ., : .

that utilization of the appropriate anodic current density in conjunction with sufficiently close electrode spacing essentially eliminates sulfur build-up (i.e., prevents further deposit of sulfur after the initially deposited trace amount) during concentration by electrolysis between about 85 per cent H2SO4 and 95 per cent H2SO4.

TABLE I
, Concentration Current Sulfur H SO in Density Formed Wt2Pe~ Cent amps/ (~ps/ (mgs/g Run Initial Final Sq. In. cm ) H2O) Test 1 85.0 92.6 0.12 0.019 0.37 Comparison 1 85.0 94.1 0.13 0.020 4.8 Test 2 85.3 94.5 0.25 0.039 0.66 Comparison 2 85.0 94.2 0.27 0.042 0.87 Test 3 85.0 94.7 0.67 0.104 0.039 Comparison 3 85.0 95.9 1.44 0.22 0.39 , :' 17,454-F -20-~ 66655 .. , Ll ~ co 1~ ~ ~
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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of concentrating an aqueous solution of sulfuric acid of concentration greater than about 75 per cent H2S04 by electrolysis in an electrolytic cell con-taining at least one corrosion resistant anode and at least one corrosion resistant cathode, comprising the step of providing a quantity of persulfate ions in the sulfuric acid solution in a region immediately surrounding and com-municating with said cathode sufficient to prevent a build--up of elemental sulfur thereon during electrolysis.

2. The method as in Claim 1, wherein at least part of the persulfate ions are supplied from an external source in the form of a persulfate salt which is soluble in the sulfuric acid solution.

3. The method as in Claim 1, wherein all of the persulfate ions are generated in situ at the anode and the sulfur build-up is prevented upon spacing the anode and cathode at a distance of about 1/4 inch, and mixing the sulfuric acid solution in the vicinity of the electrodes to provide a sufficient quantity of persulfate ions.

4. The method as in Claim 3, wherein the elec-trolysis is carried out by maintaining an anodic current density of 0.5 to 5 amperes per square inch.

5. The method as in Claim 1, wherein the tem-perature of the aqueous sulfuric acid solution being sub-jected to electrolysis is between 10°C and 80°C.

6. In a method of reducing the H20 content of a wet fluid by intimately contacting the wet fluid with a sulfuric acid solution having a concentration greater than about 75 weight per cent H2S04 whereby the H20 from the wet fluid is absorbed by the sulfuric acid solution to produce a dry fluid which is substantially free of H20, said fluid being incapable of forming a chemical reaction with the concentrated sulfuric acid solution, being immiscible with and insoluble in the acid solution and being readily separ-able from the acid solution after contact therewith, separ-ating the wet sulfuric acid solution having the H20 absorbed therein from the fluid, and thereafter reducing the H20 con-tent of the wet sulfuric acid solution, the improvement which comprises reducing the H20 content of said wet sul-furic acid solution by electrolysis in an electrolytic cell having at least one corrosion resistant anode and at least one corrosion resistant cathode while providing a quantity of persulfate ions in the sulfuric acid in the region immed-iately surrounding and communicating with said cathode to prevent a build-up of elemental sulfur thereon during elec-trolysis.

7. The method as in Claim 6, wherein at least part of the persulfate ions are supplied from an external source in the form of a persulfate salt which is soluble in said sulfuric acid solution.

8. The method as in Claim 6, wherein all of the persulfate ions are generated in situ at the anode and the sulfur build-up is prevented by spacing the anode and cathode at a distance of about 1/4 inch from each other while maintaining a current density of 0.5 to 5 amperes per square inch, and mixing the sulfuric acid solution in the vicinity of the electrodes to provide said sufficient quantity of per-sulfate ions.

9. The method as in claim 6, wherein the wet fluid is a wet hydro-carbon, a wet chlorine, or a wet perchloroethylene.

10. In an apparatus for reducing the H20 content of a wet fluid com-prising:
(a) means for contacting the wet fluid with a sulfuric acid solution having a concentration of greater than about 75 weight per cent H2S04 where-by the H20 from the wet fluid is abosrbed by the sulfuric acid solution to produce a dry fluid which is substantially free of H20, said fluid being incapable of forming a chemical reaction with the concentrated sulfuric acid solution, being immiscible with and insoluble in the acid solution and being readily separable from the acid solution after contact therewith;
(b) means for separating the wet sulfuric acid solution having the H20 absorbed therein from the fluid;
(c) an electrolysis cell for electrolyzing at least part of the H20 content of said separated sulfuric acid solution, said electrolysis cell being equipped with at least one corrosion resistant anode and at least one corrosion resistant cathode;
(d) first conduit means for conveying the separated wet sulfuric acid solution from the contacting means to the electrolysis cell;
(e) second conduit means for conveying the concentrated dry sulfuric acid solution from the electrolysis cell to the contacting means;
(f) the improvement comprising means for circulating the sulfuric acid solution in a closed loop cycle from said contacting means through the first conduit means to the electrolysis cell; through the second conduit means, and back to the contacting means, and means for providing a sufficient quantity of persulfate ions in the region immediately surrounding and in communication with the cathode to prevent build-up of elemental sulfur on the cathode during electrolysis.

11. The apparatus as in claim 10, wherein the means for providing sufficient persulfate ions in the region immediately surrounding and in communication with the cathode includes electrodes spaced about 1/4 inch from each other, and means for mixing the sulfuric acid in the vicinity of the electrodes whereby build-up of elemental sulfur on the cathode is pre-vented during electrolysis.

12. The apparatus as in claim 10, wherein the electrolytic cell is a bipolar cell with a plurality of anodes and cathodes.