GB2026033A - Production or regeneration of hydrogen halide for electrolysis - Google Patents

Abstract

The hydrogen halide is produced by reacting a halogen with water in the presence of graphitised carbon, ruthenised and/or platinised titanium, or (preferably) ungraphitised carbon. The halogen is preferably generated by electrolysis of a hydrogen halide (in gaseous form or, preferably, in aqueous solution) and the resulting hydrogen halide used to replenish hydrogen halide being electrolysed. The electrolytic process is used for the production of hydrogen, with net consumption of water and, in the preferred case, carbon.

Description

SPECIFICATION Production or regeneration of hydrogen halide for electrolysis The present invention is concerned with the electrolysis of hydrogen halides, particularly for the production of hydrogen, and with the production or regeneration of hydrogen halides.

Hydrogen has great potential for use as a heating fuel or for use as a source of electrical energy in a fuel cell. The most common method of producing hydrogen is by the electrolysis of water which, unfortunately, consumes a large amount of electricity (the decomposition voltage of water is over 2.0 volts D.C.). Efforts have been made to reduce the quantity of electricity consumed, for example, by reducing the decomposition voltage. Thus, it has been suggested that the use of highpressure water electrolysis cells will allow the voltage to be reduced to about 1.6 to 1.7 volts. Even so, the production of hydrogen by the electrolysis of water would involve a substantial input of electrical energy.

It is also known to produce hydrogen by the electrolysis of hydrogen halides (see, for example, U.S. Patents 603058, 1746542, 3236760, 3242065 and 3756930), whereby the input of electrical energy can be reduced.

We have now developed a method of electrolysing hydrogen halides whereby the overall efficiency is improved.

According to the invention, there is provided a method of electrolysing a hydrogen halide so as to electrolytically generate hydrogen and a halogen, in which the halogen is reacted with water in the presence of carbon or of ruthenised and/or platinised titanium to produce hydrogen halide, the latter being used to replenish the hydrogen halide being electrolysed.

The production of a hydrogen halide, such as hydrochloric acid, by reacting the appropriate halogen with water is known and is disclosed in, for example, U.S. Patents 1229509, 1420209, 1485816, 1695522, 1843196, 1843354, 1870308, 2238896 and 3995016. None of these patents suggest that the resulting hydrogen halide should be electrolysed to produce hydrogen; in particular, there is no disclosure that the hydrogen halide could be used to replenish hydrogen halide being electrolysed.

U.S. Patent 4069120 discloses a method of reacting a halogen with water to produce the appropriate hydrogen halide, which is then electrolysed. There is no suggestion that the electrolytically produced halogen should be used to replenish hydrogen halide being electrolysed.

U.S. Patent 4021323 discloses the production of hydrogen by the electrolysis of hydrogen iodide, the latter being replenished by reaction with some of the hydrogen produced.

There is no suggestion of replenishment of the hydrogen iodide by reacting iodine with water.

Furthermore, none of the U.S. patents mentioned above has any disclosure of the production of a hydrogen halide by reaction of water with a halogen in the presence of carbon or ruthenised or platinised titanium.

According to another aspect of the invention, therefore, there is provided a method of producing hydrogen halide, which comprises reacting a halogen with water in the presence of carbon or of ruthenised and/or platinised titanium.

In the method according to the invention, the ungraphitised carbon can be, for example, coal. The by-product of the method is carbon dioxide.

Since hydriodic acid has a much lower electrical disassociation voltage than water or hydrochloric acid, this acid can be used in a more efficient manner than hydrochloric acid.

When hydriodic acid is used in the method of electrolysis according to the invention, the electrolyte preferably also contains hydrogen chloride or hydrogen bromide, the electrolysis voltage used being insufficient to cause decomposition of the chloride or bromide.

The method according to the invention is preferably applied to hydriodic acid or, less preferably, hydrochloric acid. The method may also be applied to hydrobromic acid and, in principle, to hydrofluoric acid (although the latter is not preferred in view of the difficulty of handling the highly active fluorine produced). The hydrogen halide is preferably in solution, although it may be in the gas phase in certain embodiments.

The method of electrolysing a hydrogen halide according to the invention is carried out in apparatus comprising an electrolysis cell and a hydrogen halide regeneration zone in which the reaction between the halogen and water is effected. The regeneration zone may be in the electrolysis cell itself (which is preferred) or in communication therewith via a conduit system, in which the hydrogen halide is circulated in liquid or gaseous form. The reaction zone may be adjacent a halogencollecting area of the electrolysis cell, the halogen-collecting area being in the anode compartment of the cell.

As will be apparent, in the method of electrolysing hydrogen halide according to the present invention, hydrogen is released for use exterior of the cell. Only water or water and, in the preferred embodiment, carbon must be introduced into the cell. The electrical working voltage of the cell can be substantially less than for the electrolysis of water.

Consequently, hydrogen is obtained at an electrical efficiency not obtainable in a cell which produces hydrogen by direct electro lysis of water.

The method of electrolysing hydrogen halide is preferably operated as a continuous process, which involves dissolving the electrolytically generated hydrogen into the hydrogen halide solution at the halogen collecting area of the electrolytic cell and reacting the dissolved halogen in a reaction area adjacent the halogen collecting area with the solution water of the electrolyte. By using the dissolved halogen in the electrolyte the electrolyte and the dissolved halogen can be circulated through the reaction zone to produce a total unit wherein the continuous addition of water or water and, in the preferred embodiment, coke allows continuous production of hydrogen by the electrolysis process.

Hydrogenation of halogens, even at concentrations of about 3%-5%, allows the system to operate effectively. The hydrogenation rate of the halogen is inversely related to the free energy of the hydrohalic acid which is lowest for hydrogen chloride and is highest for hydrogen iodide. Consequently, hydrogenation, or acid formation, is more rapid and easily accomplished with hydrochloric acid. However, the solubility of the halogen in its respective acid is greatest for iodine in hydriodic acid and less for chlorine in hydrochloric acid.

Taking these factors into consideration, certain modifications in the rate of acid formation in the coke-water-halogen system are proposed.

Thus, the rate of hydrogenation of the halogens in the coke process is increased by treating the coke with nitric acid by passing the coke through hot nitric acid. The increase in hydrogenation rate is by a factor of about ten for iodine and substantially less for chlorine. The reason for this action is not known; however, it is theorized that the nitric acid treatment reduces the absorption of the halogen into the coke or ungraphitised carbon, i.e.

carbon which has not been heated to over about 2000"C. The absorption reduction appears to result in a reduced halogen overpotential at the carbon surface and, thus, increases the rate of hydrogenation, particularly for iodine. After washing with nitric acid, the treated carbon is preferably washed or heated to remove the excess thereof.

Since the solubility of the halogen in its acid is important in the method according to the invention, an improvement in solubility is advantageous. In practice this has been accomplished by adding sodium chloride to hydriodic acid, whereby the solubility of iodine in hydriodic acid is increased but the solubility of chlorine in hydrochloric acid is decreased.

Thus, the rate of hydrogenation may be adjusted by addition of a salt, such as sodium chloride.

The rate of reaction is also affected by the free energy of the acid at the carbon surface of the particles. The higher the free energy, the lower the hydrogenation rate. It has been found that this rate can be affected by adding acids of a lower known free energy to acids of higher free energy. For instance, if hydrochloric acid, which has lower free energy than hydrobromic or hydriodic acid, is added to one of the latter acids, the hydrogenation rate is increased. In a like manner, hydrobromic acid or hydrogen bromide can increase the hydrogenation rate of hydrogen iodide. This phenomenon apparently occurs because of the free energy modification at the carbon surface and because these added acids are not decomposed because they require higher voltage than used for the base acid.

Sufficient solubility of the halogen in its acid is needed to capture the released halogen at the halogen electrode for hydrogenation according to the invention. In addition, low over potential of the ungraphitised carbon at higher concentrations retains the hydrogenation process beyond the normal lower concentrations obtained by merely reacting halogen with water.

In the following description, reference will be made to the accompanying drawings, in which: Figure 1 is a schematic illustration of an example of apparatus for performing the method according to the present invention; Figure 2 is a graph of acid concentration against time used in the reaction zone or area of the method constituting the present invention; Figure 3 is a graph illustrating the overvoltage characteristics of ungraphitised carbon compared to graphitised carbon, which may be why ungraphitised carbon will allow a substantially higher hydrogenation of the halide dissolved in the electrolyte of the present invention; Figure 4 is a schematic view illustrating, in more detail, a preferred arrangement for performing the method according to the present invention;; Figure 5 is a schematic view illustrating an example of a method according to the invention, in which the hydrogen halide is gaseous; and Figure 6 is a schematic view illustrating an example of the method according to the invention for stripping chlorine.

Referring now to Fig. 1, there is schematically illustrated a device or cell A for carrying out the method according to the present invention in which hydrogen is made from water. The cell A includes a hydrogen electrode 10 and a halogen electrode 1 2 adjacent to which a halogen, such as chlorine or iodine, is released for immediate absorption into the electrolyte E formed from a solution of hydrohalic acid corresponding to the halogen released at electrode 1 2. An appropriate power supply 11 applies a decomposition voltage across electrodes 10, 1 2 to electrolytically decompose the acid to produce hydrogen at the hydrogen collecting area 14 of compartment 1 6 and the halogen at the halogen collection area 20 of compartment 22.Compartments 1 6 and 22 are connected by an appropriate arrangement including a channel 24 separated by a membrane 30 which may be, for example, of Nafion, e.g. Nafion 1 20 produced by DuPont. (Nafion is a Trade Mark). This membrane may have a thickness of approximately 10 mils. As is well known, Nafion is a perflorosulphonic acid based permselective membrane.

Compartment 1 6 includes a lower drain 32 and compartment 22 includes a lower drain 34. Appropriate valves 36, 38 are used to control the level of liquid in respective compartments. These drains can be used to remove sludge or other unwanted accumulated materials at the bottom of the respective compartments 16, 22. Within compartment 22, which not only forms the halogen collecting area but also the reaction zone for producing replenishment hydrohalic acid, there is provided an inlet 40 through which water and carbon particles C are introduced. An appropriate valve or other metering device 42 controls the amount of carbon particles and/or water which is introduced into the reaction zone formed by compartment 22.

When operation commences, it is desirable to introduce halogen, such as chlorine, into the reaction compartment 22, via halogen inlet 44, which is controlled by an appropriate valve 46.

An impeller 50 is supported in compartment 22 by an appropriate journal mount 52 and is driven by an electric motor 54 to maintain a liquid current flow outwardly and upwardly in the electrolyte E present in compartment 22. Consequently, the electrolyte in the reaction zone is agitated and continuously circulated. This maintains the carbon particles C in suspension within the electrolyte of compartment 22.

In practice, the electrolysis produces sufficient heat to maintain the electrolyte in the reaction zone at above about 80"C. This temperature facilitates the chemical reaction between halogen and water in area 20, although it may be necessary at first to heat the electrolyte, for example using a rod-like heating element 60 connected across terminals 62, 64. An electrical heating control 66 senses the electrolyte temperature by means of thermocouple 68 and controls heating element 60.

In operation, the carbon particles C react with the water to form carbon dioxde which accumulates in the upper portion of compartment 22. Since the carbon dioxide is substantially insoluble in the electrolyte E, an appropriate vent 70 is provided to allow escape of the carbon dioxide through a one-way check valve 72 connected to a water trap 74 containing a body of water 76. This body of water absorbs and dissolves any halogen which may escape with the carbon dioxide through vent 70. As the concentration of halogen within the water trap 74 increases, the water can be drained and replaced by fresh water.

Referring now to compartment 16, this compartment includes a hydrogen collection line 80 including a valve 82 to allow escape of hydrogen gas from compartment 1 6 for appropriate subsequent use, such as direct burning or generation of electricity by means of a fuel cell. In the embodiment of Fig. 1, the hydrogen is used as the fuel in a fuel cell 90 which is also supplied with oxygen from an appropriate oxygen source 92. In this manner, an electrical potential is created across leads 94, 96 of fuel cell 90, which leads are used to power any appropriate load, schematically illustrated as load 1 00.

The cell A is used to electrically decompose the hydrohalic acid in the solution which constitutes electrolyte E. The concentration of the acid in the electrolyte is important in maintaining the general efficiency of cell A.

The voltage across electrodes 10, 1 2 is below the decomposition voltage for water; therefore, the water in which the acid is dissolved is not electrolytically decomposed. Because of the high concentration of hydrochloric acid within the electrolyte solution, there is an insignificant amount of hypochlorous acid in the electrolyte. Thus, the over-voltage or over potential associated with oxygen is not a factor in the decomposition process between electrodes 10, 1 2.

In order to replenish the hydrohalic acid, an agent is used in the reaction zone of compartment 22 to promote the hydrogenation of the halogen. In the embodiment described above (and in the following Example I), the agent is ungraphitised carbon particles having a relatively small size. This carbon is oxidized to form carbon dioxide as previously discussed so that the chemical energy of the carbon is used in the chemical process involved in the production of the hydrohalic acid which is subsequently electrolytically decomposed in cell A. The resulting hydrogen has much better burning characteristics than the carbon.

In the past, when halogen has been absorbed in a water solution, a very low acid concentration was obtainable i.e. below about 3%, which is not suitable for efficient electrolysis of the resulting acid solution. It has been found that, according to the invention chlorine, bromine and iodine can be continuously dissolved into the aqueous acid solution formed by these halogens and that the dissolved halogens can be hydrogenated in the presence of carbon particles to produce makeup acids at concentrations higher than about 3%.

We have found that graphitised carbon promotes hydrogenation of halogen to the hydro halic acid solution at a rate which diminishes sharply as added acid strength increases, as illustrated in curve 1 of Fig. 2. (For the measurements, ATJ graphite, from Union;Carbide Corporation, was used). Thus, in hydrochloric acid, graphitised carbon particles promote hydrogenation of the acid at a relatively rapid rate until a concentration of about 5% is reached. Thus, graphitised carbon can be used if the concentration of the hydrochloric acid or other hydrohalic acid is relatively low.

It is advantageous to use higher concentration of the hydrohalic acids however, as this results in a more efficient electrolysis process. It has been found that if the carbon particles are ungraphitised, they promote the hydrogenation of the dissolved halogen into the acid solution at a rate which does not diminish as the concentration of the acid increases at least to a concentration of 20%-30%. This is shown in curve 2 of Fig. 2. (For the measurements, Grade 37 coke-carbon, from Airco Speer Corporation, was used). For this reason, it is preferred that the carbon is ungraphitised.

Fig. 2 also shows curves 3 and 4 which relate to the hydrogenation according to the invention using ruthenised titanium and platinised titanium, respectively. These two catalysts, which can be used with added water only in the reaction zone 22, produce a concentration of hydrochloric acid or other acid generally comparing to that produced using graphitised carbon (curve 1). There is a rapid increase in the acid concentration in a solution containing dissolved halogen below about 3% concentration, and a very slow increase in concentration beyond about 5% of acid. The ruthenised titanium used was prepared by the well known method of air heating porous titanium dipped in a ruthenium chloride solution, while the platinised titanium used was prepared by the well known method of air heating porous titanium dipped in a chloroplatinic acid solution.

When comparing curves 1 to 4 in Fig. 2, it will be seen that the acid concentration when using ungraphitised carbon rises at substantially constant rate even at the lower levels and this rate continues for an acid concentration of up to and beyond about 20%. Thus, dissolved halogen is continuously hydrogenated when using ungraphitised carbon. When using the other three agents, the hydrogenation of the halogen continues until the concentration reaches about 5%, when hydrogenation of the halogen essentially stops. Even so, the 5% concentration is higher than the hydrogenation level obtainable without the selected agents. Thus, any of the four materials used in obtaining the results shown in Fig.

2 can be used to produce hydrogen halide by reaction of a halogen with water.

Although the reason for the continued hydrogenation of the chlorine or other halogen beyond a low level when using ungraphitised carbon is not known, it is believed that this phenomenon is related to the over-potential characteristics of ungraphitised carbon in a hydrohalic acid solution. This is shown in Fig.

3 wherein the over-potential or over-voltage in millivolts for ungraphitised carbon is compared with that of graphitised carbon. The relevant results were obtained by using electrodes of graphitised and ungraphitised carbon in different concentrations of hydrochloric acid and measuring the over-voltage at each concentration. From this graph, it will be noted that both the ungraphitised and graphitised carbon retain a relatively low over-potential up to approximately 20% hydrochloric acid, this being the percentage of hydrochloric acid at which the hydrogen chloride commences to disassociate from the water. Thereafter, the over-potential for graphitised carbon increases substantially whereas that for ungraphitised carbon continues at approximately the same level. In the method according to the invention the hydrogenation of the halogen takes place at the surface of the carbon particles.The concentration of the acid at the particle surfaces is relatively higher than the total acid solution concentration because it is believed that the produced acid accumulates at the reaction surface. Consequently, the ungraphitised carbon continues to hydrogenate the halogen at the carbon surfaces whereas the graphitised carbon has a substantially higher over-potential or over-voltage at this level and stops hydrogenation at about 18% acid in the area of the carbon surfaces. This produces an acid concentration within the electrolyte of approximately 5% whereas the capability of the ungraphitised carbon to retain a low over-voltage or over-potential results in continued hydrogenation even though the concentration of the acid at the surface of the carbon increases substantially above the 18-20% solution or concentration level.

The results shown in Fig. 3 were obtained by measuring the over-voltage between two electrodes and hydrochloric acid having various concentrations at the electrode. Ten milliamps per square inch of current density was used. As the concentration was increased, the over-potential for ungraphitised carbon electrode was substantially constant at about 7 millivolts at 5% hydrochloric acid concentration and raised in a substantially uniform manner to about 8 millivolts at 37% hydrochloric acid concentration. As for graphitised carbon electrode, the over-potential was about 5 millivolts over a range of concentration of 5-1 8%, while as the acid concentration was thereafter increased, there was a very sharp over-potential or over-voltage rise reaching about 45 millivolts at 24% concentration. As the hydrochloric acid concentration increased further, the graphitised carbon showed an over-potential of about 45-46 millivolts at acid concentrations of 24-37% concentra tion, as shown in Fig. 3. Thus, since the ungraphitised carbon electrode in the test indicated no substantial increase in over-potential at the electrode as the acid concentration increased, it has been theorised that when using ungraphitised carbon in the method according to the present invention, the overpotential at the carbon particle surfaces does not substantially increase even though the concentration of the acid within the reaction zone and adjacent the surface does increase.

Referring now to Fig. 4, this figure is a modification of the preferred embodiment shown in Fig. 1 and is used for a hydrohalic acid as previously described. The electrolyte E in this embodiment is preferably hydriodic acid. Cell B includes hydrogen collecting compartment 110 and iodine collecting compartment 11 2. Within compartment 110 is a hydrogen collecting electrode 114 and a halogen collecting electrode 11 6 is provided within compartment 11 2. A conduit 11 8 connects compartments 110 and 11 2 and includes an appropriate membrane 1 20 similar to membrane 30, as mentioned above with reference to Fig. 1.A D.C. power supply schematically illustrated as power supply 1 22 applies a D.C. voltage across electrodes 114, 11 6 which voltage may be in the range 0.6 to 0.7 volts D.C., this voltage being selected to electrically decompose hydriodic acid. Of course, similar arrangements could be used for hydrochloric or hydrobromic acids. A hydrogen outlet 1 30 directs hydrogen from compartment 110 to an appropriate storage or using device.

A separate reaction tank 1 40 is provided through which is continuously circulated the electrolyte E by an appropriate inlet 1 42 having a pump 144, which pumps dissolved halogen and the electrolyte into the reaction tank 140. The heat caused by electrolytic decomposition is sufficient to retain the necessary reaction temperature within tank 140, so that additional heat is not required and the electrolyte within tank 1 40 can be heated to the boiling point if sufficient waste heat is created by cell B. Outlet 1 46 directs electrolyte E from reaction tank 140 to cell B through an appropriate filter 1 50 which removes any unwanted impurities within the electrolyte as it is being circulated back to cell B.A centre intake 1 52 is used to collect the electrolyte from tank 1 40. Impeller 1 54 circulates the electrolyte in tank 1 40 outwardly so that the carbon particles are generally spaced from intake 1 52 and are not directed toward filter 1 50. As illustrated, line 1 60 is used for introducing iodine or hydrogen iodide into the tank at the start-up of the process. Thereafter, make-up iodine is not generally required in the continuous operation process. A line 1 62 allows the introduction of hydrochloric acid into the reaction chamber 1 40.

Hydrochloric acid has a decomposition voltage substantially higher than hydrogen iodide and thus can be used with water within electrolyte E without actually entering into the electrolysis process. The hydrochloric acid has the advantage that it reduces the voltage necessary between electrodes 114, 11 6. Any reduction in voltage is a savings in electrical energy which thus increases the electrical efficiency of the method of electrolysing a hydrogen halide according to the invention.

Line 1 64 is used for introducing water into the reaction chamber 1 40 and nitric acid treated coke particles are introduced in the reaction chamber 140 in the desired amount after introduction through line 166, treatment by nitric acid in tank 170, washing in tank 1 70 and removal of residual acid by line 1 74.

An appropriate vent for the carbon dioxide gas is illustrated as line 1 80 having a check valve 1 82. Of course, a water trap could be used as previously described with respect to the embodiment of the cell A as shown in Fig. 1.

The general operation of cell B and its associated reaction chamber 140 is in the same as the operation of cell A shown in Fig. 1. The advantages of using nitric acid washed carbon particles and hydrochloric acid will be explained hereinafter with reference to Example Ill. As mentioned above, ungraphitised carbon particles are peferred but, of course, graphitised carbon could be used, if desired. The carbon is consumed in carrying out the method according to the invention.

Referring now to Fig. 5, a modification of the preferred embodiment is illustrated. In this embodiment the cell D includes a housing 200 with a Nafion membrane 202 having surface mounted electrodes 204, 206. A voltage source applies a working voltage across leads 21 0, 21 2 so that the membrane produces an electrolysis function. A halogen compartment 220 provides a gaseous acid vapour at the electrode face of the membrane to allow electrolysis to form a halogen gas in compartment 220. At the same time, hydrogen is separated and accumulated in compartment 222, from which it exits by way of conduit 224.

Halogen generated, in gaseous form, is passed from compartment 220 to a reaction tank 230 by a means represented as pump or blower 232 in conduit 234. Vapour from conduit 234 is forced into chamber 240 defined by a body of a water 242 and an upper bed of ungraphitised carbon 244. Burners 250 in line 252 heat the water in body 242 to the boiling temperature to produce water vapour. This vapour together with the vapour from conduit 234 reacts with the carbon to hydrogenate the chlorine into the hydrochloric acid vapours which are carried by conduit 260 to the lower portion of halogen compartment 220 wherein the electrolysis releases chlorine for a continuous cycle. Condenser 270 removes water vapour. The heat of the electrolysis process maintains the vapourized condition of the circulated constituents in compartment 220 and conduit 234.

To remove the carbon dioxide from the closed loop, a vent 272 having a small diameter, for example, about 1 /20th of the area of conduit 234, is provided in the coolest part of the circuit. A condenser 274 condenses the water vapour which absorbs any HCI. A large volume of carbon dioxide with traces of halogen then passes from the condenser. A dry carbon bed 276 then removes the halogen, if necessary.

The method of producing hydrogen halide according to the present invention can be used as shown, for example, in Fig. 6, for stripping a halogen from a gas stream, since the obtainable concentration of halogen in water is increased. Referring to Fig. 6, a tank 280 is partially filled with water from valved conduit 282. Graphitised (or ungraphitised) carbon, ruthenised titanium, platinised titanium or mixtures thereof, in particle form, is introduced into the water through feed line 284. A gas stream, illustrated as chlorine, is directed by valved conduit 286 into a diffuser 290 at the lower portion of tank 280. As the halogen bubbles through the water in the presence of the catalyst, the halogen is converted to hydrohalic acid to higher concentrations than obtained by water itself. A drain 292 ultimately drains tank 280.By this arrangement, more chlorine can be stripped from a gas stream. Any insoluble gases pass from tank 280 by outlet 294.

In order that the invention may be more fully understood, the following Examples are given by way of illustration only.

Example I Apparatus as illustrated in Fig. 1 was used, each of hydrogen-collection area 14 and halogen-collection area 20 having a capacity of about 500 ml, hydrogen electrode 10 being of graphite which had been platinised by dipping in chloroplatinic acid and heating, halogen electrode 12 being of graphite which had been treated for 30 hours with boiling nitric acid and the membrane 30 being of Nafion 1 20 of thickness 10 mils. 300 mls of water were charged to the halogen-collection area.

Approximately 20 grams of powdered coke of particle size such that it passed through a standard 6 mesh screen but not through a 14 mesh screen (the coke being marketed by Airco Speer Corporation as Grade 37 cokecarbons) were charged to the reaction compartment and the water was heated to about 80"C. This powder was then slurried in the water and held in suspension by the impeller 50. Thereafter, chlorine gas was introduced into the slurry of the reaction compartment and the HCI concentration continued to rise to between 20%-25% by weight, whereas when using the same procedure with ATJ graphite from Union Carbide Corporation, ruthenised titanium or platinised titanium instead of the ungraphitised coke, the concentration peaked at about 5% HCI.To reach 25% HCI, about 1 40 grams of chlorine reacted with the 20 grams of coke to produce about 1 40 grams HCI in solution.

A voltage of 1.2 volts D.C. was applied across the electrodes 10, 1 2 to produce about 1.0 amps of current flow through the electrolyte. About 1.1 volt D.C. was used for decomposition of the acid. Hydrogen was produced at the hydrogen electrode 10 and passed along collection line 80. The chlorine produced at the chlorine electrode 1 2 reacted with water and carbon to produce make-up HCI and carbon dioxide, the latter passing via vent 70 to trap 74. In the process 20 grams of water and 6 grams of coke were consumed per hour to produce 2 grams of hydrogen per hour while maintaining the acid concentration at about 20%.

After the process came on line, the ohmic heat generated across the electrodes was sufficient to maintain the electrolyte at an elevated temperature above 80"C, so heater 60 was disconnected during operation. Consequently, 6 grams of carbon produce 2 grams of hydrogen per hour with a theoretical heating capacity of 265 Btu. The electrical energy used per hour is about 66 watt-horus (approximately 226 Btu per hour). The electrical energy of 226 Btu produced hydrogen with a converted heating capacity of 264 Btu.

Example II The same basic process as used in Example I was performed in cell A as illustrated in Fig.

1, except that hydriodic acid was used as the electrolyte acid. In this instance the voltage was reduced to about 0.6 to 0.7 volt D.C.

and about 2 grams of coke were used as make-up ungraphitised carbon per hour. This process produced 2 grams of hydrogen at a heating capacity of 264 Btu with 32 watt hours or 110 Btu input per hour of electrical energy.

Example 111 In this example various means for improving the hydrogenation rate were used in a system similar to Example II using hydrogen iodide in the electrolyte solution. Hydrogen was obtained from the electrolyte formed by hydrogenation of iodine which is the preferred system of the present invention. In this example, cell B as shown in Fig. 4 was used.

The ungraphitised carbon used was Airco Speer Grade 37 coke carbon, boiled for 8 hours in constant boiling nitric acid and washed in water, which produced an increase in the iodine hydrogenation rate. The small amount of nitric acid residue on the carbon particles exhibited no apparent deleterious ef fect on the total system. A small amount (1%) of titanium tetrachloride was added to further increase the hydrogenation rate and reduce the corrosiveness of the acid mixture and increase the solubility of the released iodine.

A substantial amount of hydrochloric acid (about 20% by weight per 1% HI) was used in the electrolyte, which lowered the decomposition voltage of the HI electrolyte; however, it did not enter into the electrolysis since the voltage across the electrodes was about 0.3 volt D.C. (which is sufficient to electrically decompose the HI, but not the HCI, which requires about 1.2-1.3 volts). Without a substantial amount of HCI, the voltage of the HI cell was about 0.6 to 0.7 volts D.C. To obtain a 0.3 volt decomposition voltage for the HI cell without the HCI additive, the HI concentration would need to be about 50% HI by weight in water.

The temperature of the cell was about 108"C and the electrolyte was stirred to prevent settling of the carbon particles. The decomposition voltage was 0.3 to 0.4 volt D.C., as mentioned above and hydrogen was produced at 2 amperes of current. The cell was sealed from air to prevent oxidation of the HI electrolyte.

Claims (14)

1. A method of electrolysing a hydrogen halide so as to electrolytically generate hydrogen and a halogen, in which the halogen is reacted with water in the presence of carbon or of ruthenised and/or platinised titanium to produce hydrogen halide, the latter being used to replenish the hydrogen halide being electrolysed.

2. A method according to claim 1, in which the carbon is in particulate form.

3. A method according to claim 2, in which the particulate carbon passes through a 6 mesh screen.

4. A method according to any of claims 1 to 3, in which the carbon is ungraphitised.

5. A method according to any of claims 1 to 4, in which the carbon is pre-treated by washing with nitric acid and removing entrained nitric acid therefrom.

6. A method according to any of claims 1 to 5, in which the hydrogen halide being electrolysed is in gaseous form.

7. A method according to any of claims 1 to 5, in which the hydrogen halide being electrolysed is in the form of an aqueous solution.

8. A method according to claim 7, in which the hydrogen halide is the iodide.

9. A method according to claim 8, in which the aqueous solution also contains hydrogen chloride or bromide, the electrolysis voltage being insufficient to cause decomposition of said chloride or bromide.

10. A method according to any of claims 7 to 9, in which the reaction of the halogen with water is carried out at a temperature between 80"C and the boiling point of the solution.

11. A method according to claim 10, in which the reaction is carried out in a reaction zone also containing the aqueous solution, the latter being circulated through the reaction zone in order to maintain the reaction temperature.

1 2. A method according to any of claims 7 to 11, in which the electrolysis is carried out in an electrolysis cell having respective anode and cathode compartments, the halogen generated at the anode being dissolved in the solution in the anode compartment.

1 3. A method of electrolysing a hydrogen halide, substantially as herein described with reference to Fig. 1, Fig. 4 or Fig. 5 of the accompanying drawings.

14. A method of electrolysing a hydrogen halide, substantially as herein described in any of Examples I to Ill.

1 5. A method of producing hydrogen halide, which comprises reacting a halogen with water in the presence of carbon or of ruthenised and/or platinised titanium.

1 6. A method according to claim 15, in which the carbon is as defined in any of claims 2 to 5.

1 7. A method according to claim 1 5 or 16, in which the halogen is bubbled through a body of the water.

1 8. A method according to claim 15, substantially as herein described with reference to Fig. 6 of the accompanying drawings.