EP0611835A2

EP0611835A2 - Electrolytic cell with gas electrodes and method for electrolysis by the same

Info

Publication number: EP0611835A2
Application number: EP94830037A
Authority: EP
Inventors: Kazuhiro Hirao; Yasuo Nakajima; Yoshiyuki Kawaguchi; Koji Mochizuki
Original assignee: Permelec SpA; De Nora Permelec SpA; Permelec Electrode Ltd
Current assignee: De Nora SpA; De Nora Permelec Ltd
Priority date: 1993-02-18
Filing date: 1994-02-02
Publication date: 1994-08-24
Also published as: EP0611835A3; JPH06240482A; NO940535L; JP3236693B2; FI940702A; CA2114909A1; NO940535D0; FI940702A0

Abstract

An electrolytic cell of horizontal type constructed such that the electrolytic cell proper is divided horizontally by one or two ion exchange membranes. If one ion exchange membrane is used, the cell is divided into a cathode and an anode compartment holding a gas electrode. If two ion exchange membranes are used, the cell is divided into a cathode compartment (14), an intermediate compartment (15), and an anode compartment (16). This arrangement permits the gas electrode (23) and the current collector (22) to be in contact with each other under a uniform pressure, and also allows for smooth gas discharge. This leads to a reduction in the electrolytic potential of the cell.

Description

FIELD OF THE INVENTION

The present invention relates to an electrolytic cell and a method of electrolysis by the same for the production of an alkali and an inorganic acid (or organic acid) from a salt of an inorganic acid (or organic acid). More particularly, the present invention relates to an electrolytic cell and a method of electrolysis by the same for the production of caustic soda and hydrochloric acid from sodium chloride, the production of an alkali and sulfuric acid from sodium sulfate, and the production of an alkali and amino acid from a salt of an amino acid.

BACKGROUND OF THE INVENTION

A branch of the old basic chemical industry is electrolysis for the production of sodium hydroxide and chlorine from an aqueous solution of alkali chloride, especially brine. Electrolysis in the early stage, which employed mercury as the cathode, yielded alkali hydroxide and chlorine of extremely high purity. However, the use of this conventional method is now restricted because of large energy consumption (approximately 3000 kWh/ton-alkali hydroxide) and environmental pollution due to mercury. The conventional method has been replaced by a new one which employs an asbestos diaphragm. However, it has the disadvantage of yielding low-purity alkali hydroxide which needs subsequent separation from alkali chloride and also yielding chlorine containing a large amount of oxygen. In addition, its energy consumption including that for product purification is equal to or larger than that of the mercury method, although it requires a small amount of energy for electrolysis itself. Moreover, it poses a problem associated with the production of cancer by asbestos. At present, the method for electrolysis of alkali chloride has been shifted to the ion exchange membrane method.
The ion exchange membrane consists of feeding a purified aqueous solution of alkali chloride (especially brine) to the anode compartment of the electrolytic cell (which is divided into the anode and cathode compartments by a cation exchange membrane) and pure water (if necessary) to the cathode compartment, thereby producing chlorine in the anode compartment and alkali hydroxide (30-50%) in the cathode compartment. This method consumes 20-30% less energy than the conventional method (2200-2500 kWh/ton-alkali hydroxide). This method is used for the production of more than 80% of alkali hydroxide in Japan. In addition, this method is also used for electrolysis of other alkali compounds such as sodium sulfate.
The present inventors have proposed a new method for performing electrolysis using a gas electrode as the anode, without evolution of gases (such as chlorine and oxygen) from the anode, which uses less energy than the conventional ion exchange membrane method as disclosed in EP 0 522 382 A1. This new method employs an electrolytic cell constructed as shown in Fig. 1. The electrolytic cell proper 100 is divided into an anode compartment 110, an intermediate compartment 120, and a cathode compartment 130 by two ion exchange membranes (that is, an anion exchange membrane 115 and a cation exchange membrane 125). The anode compartment 110 is further divided into a gas compartment 111 and a solution compartment 112 by a gas electrode 114 provided with a current collector 113.
This electrolytic cell for the ion exchange membrane method, however, has the following disadvantages because the electrodes, current collector, and ion exchange membranes are arranged vertically. First, the gas electrode 114 and current collector 113 in the anode compartment 110 are in contact with each other under a pressure which differs gradually from one place to another in the vertical direction. This leads to an uneven current distribution. Second, the cathode compartment 130 gives off gas bubbles, which causes an uneven current distribution in the vertical direction. Third, condensed water accumulates on the bottom of gas compartment 111. This prevents complete gas diffusion and results in an uneven gas distribution. A gas distributor is necessary to avoid this problem. Fourth, the apparatus requires an external circulating line 140 and a gas-liquid separating column 150 (as shown in Fig. 1) to separate gas effectively from the cathode. The third and fourth disadvantages may be eliminated by the aid of auxiliary equipment, but there are no means to eliminate the first two disadvantages (uneven current distribution).

SUMMARY OF THE INVENTION

The present invention was completed to eliminate the above-mentioned disadvantages (such as uneven current distribution) involved in the conventional ion exchange membrane method and the conventional electrolytic cell therefor, without resorting to the use of any auxiliary equipment. Accordingly, it is an object of the present invention to provide an electrolytic cell with a gas electrode and a method for electrolysis by said electrolytic cell. The electrolytic cell and method are intended to efficiently produce an alkali and an inorganic acid (or organic acid) from a salt of an inorganic acid (or organic acid).
The present invention is embodied in an electrolytic cell characterized in that the electrolytic cell proper is divided horizontally into an anode compartment and a cathode compartment having a cathode by an approximately horizontal ion exchange membrane, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector. The electrolytic cell may be modified such that said ion exchange membranes have an intermediate compartment formed between them.
The present invention is also embodied in a method for electrolysis by means of an electrolytic cell which is divided horizontally into an anode compartment and a cathode compartment having a cathode by an approximately horizontal ion exchange membrane, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector, said method comprising supplying said solution compartment with an aqueous solution of alkali salt, thereby electrolytically producing an acid in said solution compartment and an alkali hydroxide in said cathode compartment. The method may be modified such that said ion exchange membranes have an intermediate compartment formed between them which is fed with an aqueous solution of alkali salt.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic vertical sectional view showing a conventional vertical electrolytic cell.
Fig. 2 is a schematic vertical sectional view showing an embodiment of the electrolytic cell provided with a gas electrode pertaining to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more detail in the following.
A feature of the present invention resides in positioning the electrolytic cell proper horizontally, thereby also positioning the gas electrode (as the anode), ion exchange membrane, and cathode horizontally. This structure makes uniform the pressure under which the gas electrode and the current collector are in contact with each other, thereby eliminating the uneven current distribution. Moreover, this arrangement permits the condensed water originating from the gas electrode to escape from the system without coming into contact with the gas electrode, thereby eliminating the uneven current distribution for certain. The removal of the condensed water improves the gas distribution, obviating the necessity of adding auxiliary equipment such as a distributor. With the electrolytic cell positioned horizontally, it is possible to reduce the thickness of the cathode compartment in the vertical direction. This alleviates the adverse effect of bubbling due to gas evolution. The overall effect is the reduction in electrolytic potential.
The electrolytic cell pertaining to the present invention may be constructed of substantially the same constituents as in the conventional vertical electrolytic cell. For example, the gas electrode may be the conventional one having a hydrophobic part and a hydrophilic part. It may be formed by coating one side of the substrate carrying a catalyst metal with polytetrafluoroethylene (to render that side hydrophobic). The cathode may be the same in material and shape as the conventional one. A preferred example of the cathode is a perforated nickel plate coated with a noble metal oxide. The hydrogen for depolarization in the anode compartment may be supplied from a separate source or by recycling the hydrogen evolved in the cathode compartment.
The electrolytic cell of the present invention has its inside divided into the cathode compartment and anode compartment by one ion exchange membrane or into the cathode compartment, intermediate compartment, and anode compartment by two ion exchange membranes. The anode compartment is divided into the solution compartment and the gas compartment by the gas electrode. Alternatively, the gas electrode may be provided with an ion exchange membrane on its side facing the solution compartment so as to ensure the gas-liquid separation.
The electrolytic cell of the present invention permits the same electrolytic reaction as the conventional one used for inorganic salts. Therefore, it may be used for the production of caustic soda and hydrochloric acid from sodium chloride by electrolysis, the production of caustic soda and sulfuric acid from sodium sulfate by electrolysis, and the production of alkali and amino acid from a salt of an amino acid by electrolysis.
The intermediate compartment or solution compartment of the electrolytic cell should be fed with an electrolyte which is properly selected according to the intended electrolytic reaction.
The invention will be described with reference to the accompanying drawings.
Fig. 2 is a schematic vertical sectional view showing an embodiment of the electrolytic cell pertaining to the present invention. It is to be noted that two units of the electrolytic cell are placed one on top of the other, each made up of a cathode compartment, an intermediate compartment, and an anode compartment.
Each unit of the electrolytic cell is a box-type electrolytic cell proper 11. The inside of the electrolytic cell proper 11 is divided into a cathode compartment 14, intermediate compartment 15, and anode compartment 16 by a cation exchange membrane 12 and anion exchange membrane 13 both positioned horizontally. In the cathode compartment 14 is a cathode 17 of perforated plate which is in contact with the upper side of the cation exchange membrane 12. The cathode compartment 14 has on its both sides an inlet 18 for water or a dilute aqueous solution of sodium hydroxide and an outlet 19 for hydrogen and caustic soda. The intermediate compartment 15 has on its both sides an inlet 20 for an aqueous solution of sodium sulfate and an outlet 21 for an aqueous solution of sodium sulfate.
A gas electrode 23, provided with a current collector 22, is placed horizontally in an anode compartment 16. The anode compartment 16 is divided into a solution compartment 24 (upper) and a gas compartment 25 (lower) by the gas electrode 23. The solution compartment 24 has on one side an inlet 26 for water or dilute sulfuric acid and on the other side an outlet 27 for sulfuric acid. The gas compartment 25 has on one side an inlet 28 for hydrogen and on the other side an outlet 29 for condensed water.
The electrolytic cell constructed as mentioned above permits the uniform current supply and hence the smooth electrolytic reaction on the gas electrode, because the gas electrode 23 and current collector 22 in the anode compartment 16 are in uniform contact with each other. The fact that the outlet 19 of the cathode compartment is only slightly above the cathode 17 permits the hydrogen evolved by the cathode 17 to be discharged easily from the system. Moreover, the above-mentioned arrangement protects the gas electrode from deterioration because the condensed water evolved by the gas compartment 25 of the anode compartment does not accumulate on the bottom of the gas compartment. This protects the gas electrode 23 and current collector 22 from coming into contact with the condensed water.
The invention will be described with reference to the following examples which demonstrate the electrolysis of an aqueous solution of alkali chloride. The examples are not intended to restrict the scope of the invention.

Example 1

Electrolysis of sodium sulfate was carried out using a single unit of the electrolytic cell (shown in Fig. 2) which is specified as follows:
The electrolytic cell proper measures 1274 mm deep, 366 mm wide, and 90 mm high. It is horizontally divided into a cathode compartment (22 mm thick), an intermediate compartment (3 mm thick), and an anode compartment (25 mm thick) having a solution compartment (3 mm thick), by a cation exchange membrane ("Nafion #324" made by DuPont) in the cathode compartment and an anion exchange membrane ("Celemion #AAV" made by Asahi Glass Co., Ltd.) in the anode compartment, respectively. A gas electrode (made by E-TEK Co., Ltd.) is horizontally positioned in the anode compartment and separated from the anolyte by a cation exchange membrane ("Nafion #117"), so that the anode compartment is divided into the solution compartment and the gas compartment. The gas electrode is provided with a current collector of platinum-plated perforated titanium. The upper side of the "Nafion #324" is in contact with an activated cathode (having an electrode effective area of 35 dm²) which is a perforated nickel plate with nickel plating in which ruthenium oxide powder is dispersed.
The cathode compartment was fed with pure water to produce a 19% aqueous solution of caustic soda. The intermediate compartment was fed with a 20% aqueous solution of sodium sulfate which was recycled at a flow rate of 3 liters/minute from a 5-meter high head tank. The anode compartment was fed with 17% sulfuric acid which was recycled at a flow rate of 3 liters/minute. The hydrogen gas evolved in the cathode compartment was immediately discharged from the cathode compartment, washed with water to remove caustic soda mist, and recycled to the hermetically sealed gas compartment. To the hydrogen stream was added moistened hydrogen (at a flow rate of 0.05 m³/h) from a commercial hydrogen bomb. The pressure in the gas compartment was 200 mmAq.
Using the electrolytic cell constructed as described above, electrolysis was carried out at about 60°C with an electric current of 700 A so as to produce caustic soda in the cathode compartment and sulfuric acid in the anode compartment. It was found that the electrolytic potential was 3.10 V, which remained almost unchanged for one month. The current efficiency for caustic soda and sulfuric acid was 90% and 80%, respectively. After continued electrolysis for one month, the electrolytic cell was disassembled for inspection. Nothing anomalous was found in the constituent elements.

Comparative Example 1

An electrolytic cell of the conventional vertical type as shown in Fig. 1 was constructed from the same constituent elements as in Example 1. The catholyte was circulated spontaneously from an externally installed gas-liquid separating column. The hydrogen separated by the gas-liquid separating column was introduced into the gas compartment adjacent to the anode compartment in which it was used as depolarizing agent as in Example 1. The electrolytic cell was fed with an aqueous solution of sodium sulfate and sulfuric acid in the same manner as in Example 1. Electrolysis was carried out at 60°C with a current of 700 A. The initial electrolytic potential was 3.25 V, which began to increase in the fourth week and eventually reached 3.5 V after one month. Electrolysis was then discontinued.
The electrolytic cell was disassembled for inspection. Whitish discoloration was found in the cation exchange membrane which separates the gas electrode from the anolyte. SEM observations revealed that the discoloration is due to anomalous wear and local deterioration. Discoloration was also found in the current collector and gas electrode at the bottom of the electrolytic cell. The results of analysis revealed that this discoloration was caused by the peeling (anomalous wear) of platinum from the current collector. It is considered that the local corrosion was accelerated by the condensed water which accumulated on the bottom of the gas compartment, exerting a higher pressure on the bottom than to other parts.
The present invention provides an electrolytic cell characterized in that the electrolytic cell proper is divided horizontally into an anode compartment and a cathode compartment having a cathode by an approximately horizontal ion exchange membrane, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector. The electrolytic cell may have two ion exchange membranes which form an intermediate compartment between them.
The electrolytic cell of the present invention offers the following advantages. The gas electrode and the current collector are in contact with each other under a uniform pressure. This allows for uniform current supply and the smooth electrolytic reaction on the gas electrode. The cathode compartment permits hydrogen evolved therein to be discharged easily from the system. The anode compartment does not permit the accumulation of condensed water. This protects the current collector and gas electrode from deterioration by condensed water. The overall effect is the stable electrolytic potential which is lower by about 0.2 V than that of the electrolytic cell of the conventional vertical type.
The electrolytic cell of the present invention allows for an efficient, economical operation for electrolysis.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

An electrolytic cell, comprising an anode compartment and a cathode compartment, said electrolytic cell being divided horizontally into said anode compartment and said cathode compartment having a cathode, said cell being divided by an approximately horizontal ion exchange membrane, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector.
An electrolytic cell, comprising an anode compartment and a cathode compartment, said electrolytic cell being divided horizontally into said anode compartment and said cathode compartment having a cathode, said cell being divided by two approximately horizontal ion exchange membranes, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector, said ion exchange membranes having an intermediate compartment formed between them.
A method for electrolysis by means of an electrolytic cell which is divided horizontally into an anode compartment and a cathode compartment having a cathode by an approximately horizontal ion exchange membrane, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector, said method comprising the steps of:
supplying said solution compartment with an aqueous solution of alkali salt; and
electrolytically producing an acid in said solution compartment and an alkali hydroxide in said cathode compartment.
A method for electrolysis by means of an electrolytic cell which is divided horizontally into an anode compartment and a cathode compartment having a cathode by two approximately horizontal ion exchange membranes, said anode compartment being divided into a solution compartment and a gas compartment by an approximately horizontal hydrogen gas electrode equipped with a current collector, said ion exchange membranes having an intermediate compartment formed between them, said method comprising the steps of:
supplying said intermediate compartment with an aqueous solution of alkali salt; and
electrolytically producing an acid in said solution compartment and an alkali hydroxide in said cathode compartment.
An electrolytic cell according to claim 1, further comprising a hydrogen escape outlet, disposed in close proximity above said cathode, for allowing hydrogen gas to escape from said cathode compartment.
An electrolytic cell according to claim 1, further comprising a water outlet, disposed in said gas compartment, that allows for the removal of condensed water.
An electrolytic cell according to claim 2, further comprising a hydrogen escape outlet, disposed in close proximity above said cathode, for allowing hydrogen gas to escape from said cathode compartment.
An electrolytic cell according to claim 2, further comprising a water outlet, disposed in said gas compartment, that allows for the removal of condensed water.