US3799860A - Apparatus for preparation of cl2 by electrolysis of hci and polyvalent metal chlorides - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A DUAL ELECTROLYTE SYSTEM UTILIZING A DIAPHRAGM ELECTROLYTIC CELL. THE ANOLYTE AND CATHOLYTE, CONTAINING AQUEOUS HCL AND A POLYVALENT REDUCIBLE METAL CHLORIDE, ARE PROCESSED AND RECYCLED SEPARATELY. THE SYSTEM OF THE PRESENT INVENTION PRODUCES HIGH PURITY CL2 AT HIGHER THAN CONVENTIONAL CURRENT EFFICIENCIES.

Description

' March 26, 1974 G. GRITZNER ETAL APPARATUS FOR PREPARATION OF C12 BY ELECTROLYSS OF HC1 AND POLYVALENT METAL CHLORIDE Original Filed July 24, 1969 United States Patent O 3,799,860 APPARATUS FOR PREPARATION F C12 BY ELECTROLYSIS OF HCl AND POLYVALENI METAL CHLORIDES Gerhard Gritzner and James J. Leddy, Midland, Mich., 1:iguors to The Dow Chemical Company, Midland,

c Original application July 24, 1969, Ser. No. 844,402, now Patent No. 3,635,804. Divided and this application Jan. 14, 1972, Ser. No. 218,005

Int. Cl. C22d I 02 U.S. Cl. 204-258 18 Claims ABSTRACT OF THE DISCLOSURE This is a division, of application Ser. No. 844,402, filed July 24, 1969, now U.S. 3,635,804.

BACKGROUND OF THE INVENTION One method of producing C12 by electrolysis of HC1 utilizes a polyvalent metal -chloride in the electrolyte solution. For example, U.S. Pat. No. 2,468,766 describes an electrolysis method which comprises: introducing an electrolyte containing HC1 and a polyvalent metal chloride, e.g., CuClg, into the space between anode and cathode of a non-diaphragm cell, liberating C12 at the anode, reducing the polyvalent metal chloride at the cathode, withdrawing the electrolyte through the porous cathode and reoxidizing the polyvalent metal chloride with air and HC1 for recycle. Such a method does reduce the cell voltage normally necessary for direct electrolysis of HC1. However, current efficiency suiers due to the inherent ow characteristics of the system which permit back reaction of dissolved chlorine and the lower valence state metal chloride.

German Pat. 1,277,216 discloses an electrolysis system which attempts to change the electrolyte ow pattern by use of a diaphragm between the anode and cathode. By maintaining a pressure differential of zero, the diaphragm reduces reaction of the dissolved C12 (anode side) with the lower valence metal chloride (cathode side). Although this does increase the current eiciency, there is still room for improvement. The German system carries out the reoxidation of the polyvalent metal chloride inside the cathode compartment of the cell. Since reoxidation is the slowest reaction taking place in the cell, this limits cell eiilciency and cell size. Furthermore, improved results are only obtained using oxygen as the oxidizing gas.

It is a principal object of the present invention to provide a method and system of preparing C12 by electrolysis of HC1 and a polyvalent metal chloride.

A further object of the present invention is to provide such a method and system which has high current efficiency.

THE INVENTION The above and other objects and advantages are found iu the present invention which utilizes separate, dual stream ilow of electrolyte containing hydrochloric acid and a reducible polyvalent metal chloride. The invention employs an electrolytic cell with a diaphragm which divides the cell into anode and cathode compartments. Anolyte and catholyte yare fed into the anode and cathode ICC compartments. Preferably the cell is operated to achieve essentially zero uid ow through the diaphragm.

The anolyte passes through the anode compartment where the hydrochloric acid is electrolyzed to form gaseous C12. The CL2-containing anolyte is withdrawn from the cell and the gaseous chlorine separated from the remainder of the anolyte. The anolyte can be replenished in hydrochloric acid by absorption of HCl and recycled through the anode compartment.

The catholyte passes through the cathode compartment where polyvalent metal -chloride is reduced to the lower valence state metal chloride, e.g. CuCl2 reduced to CugClz. This reduced catholyte is removed from the cathode compartment. The metal chloride can be reoxidized to the higher valence state and the reoxidized catholyte recycled through the cathode compartment.

The dual stream method of the present invention produces high purity chlorine at extremely high current efciency, on the order of indicating a minimum of back reactionin the cell between the chlorine produced and the lower valence state metal chloride. Further the reoxidation of the catholyte takes place outside of the cell which permits use of more versatile equipment and apparatus. This permits more eicient operation of the system since the reoxidation rate does not limit the cell reaction.

The anolyte and catholyte can contain hydrochloric acid and any polyvalent metal chloride which exists in at least two oxidation states, e.g. copper chloride, iron chloride and chromium chloride. Copper chloride is preferred. Although concentration ranges can vary quite widely and the concentrations can differ between catholyte and anolyte, a particular preferred concentration range is from about 0.5 to |about 2 Molar CuClz and from about 3 to about 8 Molar HCl. In fact, if diaphragm and process conditions are appropriate, the catholyte can be essentially polyvalent metal chloride and/or the anolyte can be essentially HC1.

The reactions taking place in the system of the present invention using an HCl-CuCl2 electrolyte are as follows:

These reactions add up to an overall reaction of 2HCl-|-1/2O2- Cl2-{H2O PREFERRED EMBODIMENTS The figure is a schematic flow diagram illustrating one embodiment of the dual flow electrolysis system of the present invention.

Referring to the figure, electrolytic cell 10, composed of cathode 11, anode 12 and diaphragm 13, is connected to power source 14. The materials of construction of the electrodes are those normally employed in electrolytic cells, such as carbon or graphite. Diaphragm materials can include synthetic materials which retain substantial strength and dimensional stability such as polypropylene and copolymers of ivinyl chloride and acrylonitrile, and polyvinyl chloride.

The anolyte cycle comprises feeding anolyte containing hydrochloric acid and a polyvalent metal chloride is fed into the anode compartment A of the cell. The HCl in the anolyte is electrolyzed to produce C12 gas. The anolyte is removed from the cell and the chlorine gas separated in a degassing unit 15. The depleted anolyte passes through heat exchanger 22 into an absorption tower 16 where it is contacted wth HCl in gaseous or liquid form, preferably gaseous. HCl is absorbed in sufcient amount to replenish the HC1 consumed during electrolysis. The replenished anolyte is then recycled through a filter 23 into the cell.

The C12 gas produced by the present method can be further processed by passing the gas through a condenser 17, which removes a substantial portion of the HC1 in the C12 gas, and drying in chlorine dryer 18, e.g. utilizing H2804. The HCl-containing condensate can be added back to the anolyte.

In the catholyte cycle, the catholyte is fed through the cathode compartment C where polyvalent metal chloride is reduced to the lower oxidation state. The reduced catholyte is withdrawn from the cell and carried through heat exchanger 22 to an oxidation tower where it is oxidized, for example by admixing it with an oxidizing atmosphere, e.g. oxygen-containing gas or dilute chlorinecontaining gas, to reoxidize the metal chloride. While the oxidizer 19 is shown in the figure as one utilizing concurrent flow, it is understood that any tower or tank unit or the like which permits contact of the oxidizing atmosphere with the reduced anolyte is within the scope of the present system. The reoxidized anolyte is then recycled through a filter 23 into the cathode compartment.

The exit gases blown out of the oxidizing tower contain some HCl which can be recovered by passing the ygases through a condenser 20. The HCl-containing condensate can be recycled to the oxidizer. A more elaborate recovery system, which can |be used but is not necessary to the present invention, is described in U.S. 2,666,024.

The anolyte and catholyte can be cycled for example by means of pumps 21 or other conventional means. It is preferable to control temperature which can be done by use of heat exchangers 22. It is also desirable in many instances to lter the incoming electrolyte streams to remove any solids. Any type of filter means 23 can be employed.

While the ligure shows a single cell system, it is understood that a series of cells connected in series could be employed in the present invention. For example, the cells could be arranged so that one side acts as the cathode for one cell and the other side acts as an anode for a second cell.

The following examples serve to further illustrate the present invention. The following general procedure and apparatus were used for the experiments. Unless otherwise indicated the term electrolyte refers to anolyte and catholyte.

A dual ow system similar to the ligure was set up. Catholyte and anolyte were circulated separately by means of pumps. The flow rate could be regulated from to 650 ml./min. The ow of anolyte and catholyte was measured by means of two rotameters. Copper (I) chloride formed on the cathode was reoxidized with oxygen or air in the oxidizer. Ihe latter was made out of Pyrex glass, having an active volume of about 66 in.3 (21" high). The exit gas stream passed through a condenser. Oxygen or air was fed into the oxidizer through a sintered glass disc (medium size). 'Ihe absorption of hydrochloric acid took place in a glass tube. The tubing for the connection was either polyethylene or pure gum rubber.

'Ihe electrode used for the cell was machined from a graphite block. The active electrode area was 15 in.2, and the electrodes were spaced IAG from the diaphragm. All other parts except the electrode area itself were painted with Saran type cement in order to prevent electrolysis on those surfaces. Teflon or Saran type nipples were used for the inlets and outlets of the electrolyte. Two of these electrodes were clamped together with a cloth diaphragm fbetween them to form the electrolytic cell. Leaks were minimized by a Silastc lilm along the 1" frame of the cell by painting the same area on the diaphragm with Saran cement and by melting the edges of the diaphragm. A copper plate to which the copper leads were soldered was bolted to the external side of each graphite plate by means of two short brass bolts. The complete setup was placed in a temperature controlled hot-box.

Hydrochloric acid concentration was determined by indicator.

Copper (II) concentration was measured in the catholyte by the standard idometric way.

Copper (I) concentration was determined by the following procedure. A sample was quickly added to a ferrie ammonium sulfate solution in sulfuric acid. The amount of iron (II) formed was oxidized to iron (III) with 0.1 N ceric sulfate solution using Ferroin as indicator.

Chlorine was absorbed in 600 m1. of a KI solution (400 g./1.) in sulfuric acid, diluted to 1000 ml., and aliquots were titrated with 0.1 N sodium thiosulfate.

Gas analyses were made with the Orsat Method.

Current efficiency is determined by comparing the actual amount of C12 produced with that theoretically producible.

Diaphrarn: Copolymer of vinyl chloride and acrylonitrile Resistance between anode and cathode connectors: 33

Current density, amps/in.

Current eeiency, percent Tempera- Volts ture, C.

Example No.

Table I shows the current eiiciencies of the present system at various current densities and corresponding voltages at two temperatures. As indicated, extremely high current etliciencies, or above in almost all cases, are achieved even at very low current densities, i.e. 0.33 amps/in?, and correspondingly low cell voltages, i.e. less than 1 volt.

TABLE II Conditions:

Composition of electrolyte: 1.46 M CuClz, 5.03 M HC1 Eleetrolyte ow: See table below Diaphragm: Polypropylene Resistance: 17 milliohms Temp.: 70-70.5 C.

Electro` Current lyte now Current density, rate, eiciency, amps/in 2 Volts mlJmin. percent Table l1 shows two aspects of the present system. First, high current eiciencies can be achieved over a wide range of electrolyte ow rates (Examples 11-15 and 16- 19). These examples showed little if any decrease in current efficiency when the flow rate is decreased. Second, current eiiciency can be increased by increasing the current density (Examples 20-24).

TABLE III Conditions:

Composition of electrolyte: 1.40 M CuClz, 6.0 M HC1 Eleotrolyte 110W: 510 ml./min.

Diaphragm: Polypropylene Resistance: 17 milliohms Temp.: 70-70.5 C.

Amount Cu+ in Oz flow reox- Cuirent in oxiidized Current density, dize catholyte, eciency, amps/1n 2 Volts Inl/min. g./l. percent Table III reects yet another feature of the present system-the ability to produce C12 at relatively high efficiencies even with incomplete reoxidation of the polyvalent metal chloride. Examples 25-28 are similar to those shown before lwith a substantial excess of oxidizing gas over that sucient to convert essentially all of the Cu2Cl2 to CuCl2. However, even where the 110W of the oxidizing gas in the oxidizer is reduced, thereby resulting in a significant amount of Cu+ remaining in the recycled catholyte, the present system and method continue to show high current etiiciencies especially at high current densities (Examples 33-36).

A representative chlorine gas analysis for any of the above examples is Percent C12 99.69 CO2 0.24 O2 0.01 N2 0.06

Thus the system and method of the present invention can be employed to produce high purity C12 from electrolysis of HCl and a polyvalent metal chloride at very high current eiciencies over wide ranges of electrolyte concentrations, current dens-ities and liow rates.

EXAMPLES 37-41 A larger diaphragm cell, having an active electrode area of 64 sq. inches, was incorporated into the system of the present invention. The cell design was similar to that used in the previous examples. Air at a ow rate of about 42.5 liters/min. was used in the oxidation tower as the oxidizing gas.

TABLE IV Conditions:

Composition of electrolyte: 1.46 M CuClz, 5.03 M HC1 Electrolyte flow: See table below Diaphragm Polypropylene Resistance: Not measured yg meh.

Table IV demonstrates the high current efficiencies achieved by the present system and method using air as the oxidizing gas over a wide range of electrolyte iiow rates. Examples 40 and 41 were taken from a continuously operated mini-plant setup.

6 EXAMPLES 42-43 TABLE V Conditions Composition of electrolyte: 1.50 M CuCla, 5.5 M HC1 Electrolyte flow: mL/mln.

Diaphragm Polypropylene Resistance: Not measured HC1 concentrate Current moles/liter Current Example density, efficiency, o. amps/in.I Volts Anolyte Catholyte percent What is claimed is:

1. A system for the production of chlorine by electrolysis which comprises:

(a) at least one electrolytic cell containing an anode, a cathode, a diaphragm therebetween which separates the cell into anode and cathode compartments and connecting means to a power source;

(b) means connected to said cell for feeding anolyte and catholyte into the anode and cathode compartments respectively of the cell, said anolyte and catholyte containing aqueous HC1 and a polyvalent metal chloride; wherein C12 gas is produced at the anode and polyvalent metal chloride is reduced from a higher oxidation state to a lower oxidation state at the cathode;

(c) means connected to said cell for removing the anolyte and catholyte from the cell;

(d) means connected to said anolyte removal means for removing the C12 gas from the anolyte;

(e) means connected to said C12 removal means for absorbing additional HC1 into the anolyte for recycle to the anode compartment of the cell; and

(f) means connected to said catholyte removal means for oxidizing the reduced catholyte to reoxidize polyvalent metal chloride for recycle to the cathode compartment.

2. The system of claim 1 including a series of electrolytic cells connected in series.

3. The system of claim 1 wherein said means to reoxidize the polyvalent metal chloride is spaced apart from the cathode compartment.

4. A system for the production of chlorine by electrolysis which comprises:

at least one electrolytic cell comprising an anode adapted to produce C12 gas positioned in an anode compartment, a cathode adapted to reduce a polyvalent metal chloride from a higher oxidation state to a lower oxidation state positioned in a cathode compartment, a diaphragm adapted to separate the anode compartment from the cathode compartment, and a connecting means to a power source;

a feed means connected to said cell and adapted to provide an anolyte to the anode compartment and a catholyte to the cathode compartment;

an electrolyte removal means connected to said cell and adapted to remove the anolyte and the catholyte from said cell;

a -gas removal means adapted to remove the C12 gas from the anolyte;

an absorber means adapted to absorb HC1 into the anolyte for recycle to the anode compartment and spaced apart from said gas removal means and the anode compartment;

an oxidizing means adapted to reoxidize the reduced polyvalent metal chloride for recycle to the cathode compartment connected to and spaced apart from the cathode compartment.

5. The system of claim 4 including a heat exchanger adapted to control the temperature of the anolyte.

6. The system of claim 4 including a heat exchanger adapted to control the temperature of the catholyte.

7. The system of claim 4 including lter means adapted to remove solids from the anolyte and the catholyte.

8. The system of claim 7 including two heat exchangers adapted to control the temperature of the anolyte and the catholyte, and a condenser adapted to remove HC1 from exhaust gases from said oxidizing means and to recycle HCl condensate to said oxidizing means.

9. The system of claim 8 including a condenser adapted to remove HC1 from the C12 gas.

10. The system of claim 9 including a series of electrolytc cells connected in series.

11. The system of claim 10 wherein the anode, cathode and diaphragm are spaced apart from each other and the system is adapted to achieve a current eiciency of at least about 90 percent at a current density of up to about one ampere per square inch where the polyvalent metal chloride is completely reoxidized.

12. The system of claim 10 wherein the anode and cathode are spaced up to about %2 inch from the diaphragm.

13. The system of claim 4 including a condenser adapted to remove HC1 from exhaust gases from said oxidizing means and to recycle HC1 condensate to said oxidizing means.

14. The system of claim 4 wherein the anode and cathode are spaced up to about %2 inch from the diaphragm.

15. The system of claim 4 including a series of electrolytic cells connected in series.

16. The system of claim 4 wherein the anode and cathode are spaced apart from the diaphragm.

\1-7. The system of claim 4 wherein the anode, cathode and diaphragm are spaced apart from each other and the system is adapted to achieve a current efliciency of at least about percent at a current density of up to about one ampere per square inch where the polyvalent metal chloride is completely reoxidezed.

18. The system of claim 17 including a series of electrolytic cells connected in series.

References Cited UNITED STATES PATENTS 3,481,847 12/1969 Hine et al. 204-128 2,468,766 5/1949 Low 204-128 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. Cl. X.R. 204--257 '12225250 UNITED STATES PATENT OEETQE CERTIFICATE 0F CORRECTION Patent No. 3,799,860 Dated Merenzs, 1974 lnventcnf) Gerhard Gritzner and James J. Leddy It is certified that error appears in the above-identified patent;`v and that said Letters Patent are hereby corrected as shown below:

j co1umn'4, Table 1, "minieme (7oc-)" should be E inserted i'n line 24 after "33": v Column 6; Table V, line 14, after measured" Temp: 72-.73C should beinserted; a Column 6, linev l72, after "compartmentf f---covnnected ,tomshould bek inserted. v

signed end sealed this 3rd dey of December 1976.y

' (SEAL) l Attest:

MecoY M. GIBSON JR. c:. MARSHALL DANN Commissioner of Patents f A.

Attestng Officer'`

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