Process For Producing Peroxides

Abstract

An electrolytic cell for the production of peroxide having a cathode, an electrolyte-permeable anode, and a diaphragm separating the cathode and anode, the cell being characterized by the position of the anode against the diaphragm such that the anolyte passes through the anolyte compartment on the rearward face of the anode. The process of operation of the cell in which anolyte flow is across the anode only on the anode face remote from the diaphragm.

Parent Case Text

This application is a divisional application of my copending application Ser. No. 612,515, filed Jan. 30, 1967, and now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates primarily to the preparation of peroxide bleach solutions by the electrochemical reduction of oxygen and is particularly directed to improvements in the organizational arrangement of the electrolytic cells used for the purpose.

2, The Prior Art with Relation to the Invention

Commonly, electrolytic cells for peroxide production involve an electrolyte in communication with an anode and cathode and which latter are spaced from each other by an appropriate diaphragm, the diaphragm being interposed between the electrodes and forming compartments. In the particular electrolytic cells under consideration, the cathode is of sufficient porosity so as to permit the passage of gases, such as oxygen, therethrough. Further, the nature of the cathode is such that it provides a very large catalytic surface upon which the reduction of oxygen occurs according to the following equation:

O.sub.2 +H.sub.2 O+ 2e HO +HO .sub.2

When carrying out the above reaction under alkaline conditions (pH of about 10 and above) solutions of peroxide having an actual peroxide content of the order of 2- 10 grams per liter liter can be obtained at power costs of the order 1.8 to 2.0 kwh./lb. of peroxide.

Since the production of peroxide, within limits, is directly proportional to the quantity of electricity (Faraday's first law), it follows then that the higher the current possible (i.e. amperage), the greater the production of peroxide per unit of time. Further, it follows that the higher the voltage, the higher the current (i.e. amperage) obtained. However, at voltages in excess of about 1.75 to 2 volts, a secondary reaction takes place which tends to decrease the amount of peroxide formed per unit of time. Since this decomposition reaction appears to proceed at a rate somewhat slower than the formation reaction, peroxide concentrations greater than 2 to 10 g./liter are attainable, but the cost in terms of kwh./lb. is substantially higher (viz. 5 to 10 kwh./lb.).

SUMMARY OF THE INVENTION

I have found, however, that higher electrical currents without the necessity of applying higher voltages and, consequently, improved efficiency of operation in terms of grams (or lbs.) of peroxide per hour, as well as compactness of the electrolytic cell structure, may be obtained if certain conditions are followed. Specifically, such benefits are attained if the position of the anode in the cell is changed from what might be termed a normal position where the electrolyte passes across the face of the anode to an unorthodox position where the electrolyte travels primarily across the back face or surface of a permeable anode. More specifically, I have found then that, if the anode is positioned in supporting contact with the diaphragm, substantially higher yields of peroxide can be obtained without the necessity of increasing the applied voltage and/or increasing the size of the electrolytic cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

FIG. 1 of the drawings diagrammatically illustrates an accepted electrode position;

FIG. 2 diagrammatically illustrates the unorthodox, but improved, electrode arrangement;

FIG. 3 is an exploded view illustrating the relationship of components of an operating cell in accordance with the invention;

FIG. 4 is a face view of one of the components of FIG. 3; and

FIG. 5 is a face view of another of the components of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings, the numeral 1 (FIG. 1) indicates the electrically nonconductive casing of an electrolytic cell having a gas porous catalytically surface active cathode 2 and a cathode compartment 3. A semiporous diaphragm 4 of asbestos bounds the side of the cathode compartment 3 opposite the cathode. An electrolytic solution 5 is directed through the cathode compartment from an inlet 6 to an outlet 7. This solution is a dilute aqueous caustic solution, suitably about 2 percent sodium hydroxide. For the purpose of generating the peroxide, oxygen or air is passed through the port designated at 8 and to the porous cathode 2. An electrical current lead is designated at 9 for a purpose to be noted hereinafter. An inlet port 11 and outlet port 12 provide for the passage of a second electrolyte 14 through the anode compartment 13. Commonly (FIG. 1) the anode 15 would be positioned as indicated with the anolyte or electrolyte solution passing the front face of the anode between the anode and diaphragm. I have found, however, that it is highly beneficial to provide the anode as designated generally at 15' (FIG. 2) in contact with the diaphragm 4. For such purpose I form the anode to permit electrolyte communication from the anode rear surface 16 to the cathode compartment. Thus, suitably, the anode is a wire screen, a mesh, or simply a helix of metal such as nickel.

An electrical current lead designated at 17 is connected to the anode and cooperates through an external circuit with the cathode lead 9 to provide a suitable voltage and current to the cell. Usually 2 volts direct current is sufficient for the purpose. The current through the cell should preferably be as large as possible relative to the applied voltage; that is, it is desirable to maintain the resistance of the cell small.

Quite surprisingly, I have found that, if the anode is positioned as at 15', the actual amperage may be materially increased in a cell of conventional size having an otherwise conventional mode of operation. Improvements in amperage of more than 50 percent to about 150 percent have been found, as is indicated from the following data. These data are based on a comparison of the cell arrangement of the anode as at 15 (FIG. 1) in contrast with an anode positioned as at 15' (FIG. 2), other pertinent factors being: air (or oxygen) is passed through inlet 8 to chamber 18 which is sealed off from the electrolyte; chamber 18 thus serves as a kind of manifold for the feeding of oxygen to the pores of the activated carbon cathode. Gaseous products of the reactions at the anode and cathode are passed from the apparatus by vents above the liquid line such as are shown at 10.

The activated cathode may, for the purposes of the invention, be formed in a number of ways. Suitably however, for long life the cathode is a planar element of pressed activated carbon and paraffin as described in my copending application Ser. No. 604,944 filed Dec. 27, 1966, now U.S. Pat. No. 3,459,652.

Simultaneously, in cell operation electrolyte is passed through the compartments 3 and 13 and the power is applied to the cell. The very large active surface area of the cathode (preferably activated carbon) catalyzes the reduction of oxygen to form perhydroxyl ions in accordance with the following equation:

O.sub.2 +H.sub.2 O+ 2e HO +HO .sub. 2

On a comparative basis in the testing of two different cell structures the following data were obtained: ##SPC1##

Cell "b" differs from cell "a" in two respects. In cell "b" the caustic flow through the cathode compartment was of a greater depth than in cell "a," about three-eights inch versus three-sixteenths inch. This is the dimension between the cathode and diaphragm. Also, the cathode of cell "a" is a graphite base having a coating of activated carbon plus polystyrene as a binder as disclosed in my copending application Ser. No. 605,014 filed Dec. 27, 1966, now U.S. Pat. No. 3,477,940. In contrast, the cathode of cell "b" is a carbon-polyethylene mix material as disclosed in my copending application Ser. No. 604,791 filed Dec. 27, 1966, now U.S. Pat. No. 3,507,773. The anode and diaphragm, as well as the anode compartment arrangement, however, were the same for cells "a" and "b" and the improvement, though somewhat of greater scope in one instance than another, is quite clear, even though other factors influence the degree of improvement. The amperage increases are directly related, however, to hydrogen peroxide production; for example, in a cell using 2 percent sodium hydroxide in a cell of FIG. 1 arrangement, i.e., cell " a" the peroxide yield per hour is only 0.316 grams whereas for the arrangement of FIG. 2 the yield was 0.507 grams; similarly, the yield improved in a cell " b" application from 0.468 to 0.655 grams per hour.

In actual practice the shift of anode position to provide the electrolyte flow rearwardly of the anode has involved an anode displacement in the specific examples noted of about three-sixteenths inch in each case.

For the purposes of illustrating a practical cell arrangement, reference is made now to FIGS. 3 to 5 inclusive.

The numeral 20 in FIG. 3 designates a planar plastic end plate of an electrolytic cell generally designated at 21. Plate 20 is both cut out and planed off to provide a plurality of flat faced knobs 22 which project beyond surface 23 and provide channels 24 for the flow of liquid from an inlet 25 to an outlet 26. Surface 23 is itself additionally partially cut out to provide channels 27 (FIG. 4) between ribs 28. The plate and ribs 28 extend well above the liquid outlet 26 and provide for the venting of gases to the atmosphere.

In the assembled cell a spacer element 29 of plastic having a central opening 30 is in abutment with surface 23 and bounds the knobs 22. The knobs extend through the opening 30 in the assembled condition of the cell and engage a wire screen anode 31. The anode is planar and is supported in a substantially planar position by the combination of knobs 22 and the surrounding spacer element 29. Positioned against the screen anode and lying thereon is a diaphragm 32. This diaphragm may be of any of a number of materials known to the art but preferably is basically a sheet of asbestos. Such asbestos is relatively soft, particularly when wetted, and the present structural arrangement (FIG. 3) provides for the lending of support to this diaphragm by the screen anode. Rightwardly (FIG. 3) of the diaphragm is an additional support for the diaphragm in the form of a glass fiber mesh 33. This mesh in the assembled cell lies diaphragm plate 34 of plastic which plate, as its principal purpose, serves to retain the porous cathode element 35.

The plate 34, to the depth indicated in FIG. 3, is completely cut out at 36 to provide a peripheral seat 37 for a glass mesh support 38 in the form of a sheet; the support 38 receives the porous cathode 35, the mesh and cathode being cemented, for example, on the seat 37. The plate 34 is also cut through (FIG. 5) to provide a plurality of slots 39 (FIG. 5) bounded laterally by the vertically extending ribs 40. These slots, in the operation of the cell, fill with electrolyte flowing from the inlet 41 to the outlet 42 and serve to communicate the electrolyte with the cathode. Additionally, plate 34 is cut out upwardly at 43 to provide wide channels bounded by rib extensions 44 which are vertically above the outlet 42 in the assembled cell and provide for venting of gases from the cell.

In the structural arrangement shown (FIG. 3) an electrically conductive wire mesh screen 45 overlies the cathode and the plate 34, and an annular gasket 46 seals between the end plate 47 and the cathode structure including plate 34.

The end plate 47 is itself cut out to provide a manifold 48 which is coextensive with the cathode and communicates through a gas inlet port 49 with the exterior of the cell. The manifold in cell operation remains free of liquid as electrolyte does not pass the gas porous cathode.

The cell is retained in an assembled condition by draw bolts 50 cooperating with nuts 51, one draw bolt-nut set being provided at each corner of the cell and two of which set are shown in FIG. 3.

In cell operation about 2 volts is applied between the electrical lead-in 52 attached to the anode and the lead-in 53 attached to the cathode screen 45. The electrolyte for the cathode is fed through inlet 41 to the outlet 42 as already noted. The electrolyte for the anode is separately fed through the inlet 25 to the outlet 26 on the side of the anode remote from the diaphragm as already noted. The liquid electrolyte, of course, wets the diaphragm by some movement through the rear face of the anode to the face fronting on the diaphragm, and the catholyte wets the diaphragm since the latter fronts on the cathode compartment through glass mesh support 33. The cell operation is, of course, so controlled that electrolyte does not overflow the cells above the outlets and, accordingly, only gases pass out through the open top of the cell.

It is not necessary that separate sources of electrolyte be provided for the anode and cathode compartments. In accordance with the principles of this invention the electrolyte may be circulated from the anode to the cathode compartment, for example, if so desired.

In the illustrations as set out in the drawings, the actual cell proportions are somewhat exaggerated for the purpose of clarity in the drawings. Accordingly, it may be noted that in a cell of the type under consideration the cathode compartment is such that the depth of the channels, and hence of the cathode, is commonly about one-eighth to three-sixteenths inch. The anode compartment is of similar dimensions. The cathode itself may be approximately one-eighth inch thick and the chamber 18 about one-eighth to one-half inch. The anode itself is suitably 16.times. 16 nickel wire mesh having a wire diameter of about 0.012 inch and mesh openings of about 0.0505 inch. The mesh open area is about 65.3 percent. The diaphragm 4 is an asbestos sheet suitably supported and is itself about 0.0175 inch in supported The electrodes may be about 5.times. 8 inches and a typical cell 8 inches high by 10 inches wide. Such dimensions are merely illustrative and changes may be made therein to accomplish specific purposes.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

Claims

I claim:

1. In a process for producing a peroxide in an electrolytic cell having a liquid pervious anode, a cathode and a diaphragm separating the anode and cathode, by a reaction at the cathode of the cell induced by the application of a potential difference between the anode and cathode, the improvement which comprises passing the electrolyte of the cell through the anode compartment of the cell only across that face of the anode which is remote from the diaphragm while permitting diffusion of electrolyte through the anode to the diaphragm.

2. A process according to claim 1 in which the electrolyte flow to the anode compartment of the cell is fed to the anode compartment and withdrawn from the compartment separately from the flow of electrolyte through the cathode compartment.