electrical resistance of bipolar membrane by limiting carbon dioxide uptake solution.
This invention relates to an electrodialysis method in which bipolar membranes are used to prepare an acid and an alkaline solution from an aqueous salt solution, in particular to an improvement in this method whereby the useful life of the membranes is effectively prolonged.
BACKGROUND ART Acids and bases are important intermediates for a wide variety of products made by the chemical industry. After processing and use these find their way back to nature as salts. A logical route for completing the cycle would be to regenerate the acids and bases directly from these salts. Electrolysis of brine to generate chlorine and caustic soda, in a certain sense, is such a process. Another process is electrodialysis, using bipolar membranes to directly generate acids and bases from their salts. The process is electrically driven and the splitting of the salt to generate the acid and base occurs in an aqueous medium. The process is conceptually a simple one and can be represented by the equation: MX + H20 —► HX + MOH (salt) (acid) (base)
To effect and maintain separation of the various species, ion exchange membranes are used. The most crucial of these is the bipolar membrane, so called because it is composed of two distinct parts which are selective to ions of opposite charges. Under the influence of an applied direct current, such a sandwich membrane is capable of forcibly dissociating water to form equivalent amounts of hydrogen and hydroxyl ions. Used in conjunction with other cation- and anion- selective (i.e., monopolar), membranes, the assembly constitutes a potentially economical water splitting apparatus that generates acid and base. The standard free energy for a process that converts water to one molar hydrogen and hydroxyl ions at 25°C is 19,100 cal/mole. For a reversible process, i.e., a process approaching zero current density, this translates to an energy requirement of 0.022 kWh/mole at 25°C. For production of caustic soda this is equivalent to an energy requirement of 500 k h/ton. An efficient water splitting system is therefore capable of generating acid and base solutions at a fraction of the costs encountered commercially. (2800-3500 kWh/ton) .
Bipolar membranes can be prepared by many different methods. In U.S. Patents 4,024,043 and 4,057,481 (both Dege et al) single film bipolar membranes are prepared from pre-swollen films containing a relatively high amount of an insoluble cross-linked aromatic polymer on which highly dissociable cationic exchange groups are chemically bonded to the aromatic nuclei to a desired depth of the film from one side only; subsequently, highly dissociable anionic exchange groups are chemically bonded to the unreacted aromatic nuclei on the other side of the film. In Japanese Patent Publication Nos. 78-158638 and 79-7196 (both Tokuyama Soda Co. Ltd.), bipolar membranes are prepared by partially covering a membrane with a cover film, sulfonating the surface of the membrane not in contact with the cover film to introduce cation exchange groups, exfoliating the cover film, and introducing anion exchange groups on the exfoliated surfaces.
Bipolar membranes have also been prepared by bonding together separate anion and cation exchange films or membranes. The two monopolar membranes of opposite selectivity can be fused together with the application of heat and pressure. See, for example, U.S. Patent 3,372,101 by Kollsman wherein separate cation and anion membranes are bonded together in a hydraulic press at 150°C at a pressure of 400 lb/sq. inch to form a two ply membrane structure.
However, bipolar membranes formed in this way suffer the disadvantage of high electrical resistance produced by the fusion. Furthermore these membranes are prone to bubble or blister and they are operable for only short time periods at relatively low current densities. These disadvantages make the bipolar membranes formed in this way unattractive for commercial electrodialysis operations. In Electrochimica Acta, 21( 9 ) 1175-1176 (1986), the present inventor disclosed a method for the preparation of bipolar membranes whereby inorganic electrolyte solutions are brushed onto the faces of suitable anionic and cationic membranes, prior to the faces being pressed together. A variety of electrolyte solutions were found to be effective in facilitating the preparation of potassium hydroxide and hydrochloric acid from a potassium chloride solution. It was, however, found that these membranes only remained effective for a few hours when 1 molar acid and base solutions were separated by a membrane and for a few weeks only when a membrane separated potassium chloride solutions.
In International patent application No. PCT/AU88/00279, the present inventor discloses a method for preparing low electrical resistance bipolar membranes wherein separate anion and cation exchange membranes are treated by immersion in a heated alkaline solution containing at least one monovalent or higher oxidation state cation, excluding sodium and potassium, prior to being conjoined by being pressed together. Bipolar membranes prepared in this way have a lower electrical resistance than those formed using the prior art methods wherein separate anion and cation exchange membranes are pressed together without the prior alkaline solution treatment. However the electrical resistance increases slowly with the time under continuous operating conditions. For example bipolar membranes prepared by the method described in PCT/AU88/00279 using Raipore 1030 and 1010 anion and cation exchange films have an initial operating voltage of about 1.0V for a current of 100 mA/cm . when they separate 2 N HC1 and 2 N NaOH solutions. T^voltage is stable under continuous operating conditions for 2-3 months. It then commences to increase.
The increase in voltage is accompanied by a localised separation of the previously contacting anion and cation exchange membranes due to gas accumulation at the interface of the anion and cation exchange regions. This phenomenon is hereinafter referred to as "ballooning".
DISCLOSURE QF INVENTION The present inventor in identifying the "ballooning" phenomena has recognised that avoidance of this problem may lead to a means by which the life of the bipolar membrane in use may be prolonged. Surprisingly, it has been found that "ballooning" may be substantially avoided through the use of the membranes in an environment from which carbon dioxide is excluded.
Accordingly, the present invention consists in an improvement to a method in which a bipolar membrane is used to generate an acid solution and an alkaline solution by the electrodialysis of an aqueous salt solution, the improvement comprising limiting the uptake of the carbon dioxide by at least one of said solutions to an extent sufficient to minimise the increase in electrical resistance of said bipolar membranes. One means for limiting the uptake of carbon dioxide is to operate the electrodialysis under an atmosphere of inert gas. As used in this description, the term "inert gas" refers to a gas that either does not permit carbon dioxide to be taken up by the electrodialysis solutions or does not produce a compound when dissolved in the electrodialysis solutions that can diffuse to and react with the acid in the interfacial region of the bipolar membrane.
It is believed that helium, neon, argon, krypton and xenon constitute suitable inert gases. In addition to these, nitrogen has been found to be suitable.
Another means for limiting the uptake of carbon dioxide is by degassing the electrodialysis solutions by for example, vacuum degassing. Although any bipolar membrane may be used in this invention, it is preferred that the bipolar membranes of the invention described in PCT/AU88/00279 are used.
The present inventor believes that the reason for the effectiveness of the inert gas atmosphere is that it reduces carbon dioxide absorption by the alkaline solution in the electrodialysis cell. This has the effect of preventing the formation of carbonate ions which can diffuse to the interfacial region and react with protons to yield carbon dioxide. The carbon dioxide accumulates at the interface to cause the "ballooning" phenomenon.
MODES FOR CARRYING OUT THE INVENTION In order to better understand the nature of the present invention, two examples will now be described with reference to the figure, which is a schematic view of the cell used for determining the current efficiency of a bipolar membrane.
In the figure, there is shown an electrodialysis cell 10 which was used in the examples to be described below. The cell 10 comprises a number of chambers 11, 12, 13, 14, 15. Disposed between chambers 12 and 13 is a bipolar membrane 16. Whilst between chambers 11 and 12, 14 and 15 are cation exchange membranes 17. An anion exchange membrane 20 is disposed between chambers 13 and 14. In the headspace 18 of the chambers 11, 12, 13, 14 and 15 there is nitrogen maintained at 16psi (gauge pressure) . The nitrogen is supplied to headspace 18 via inlet 19.
As used in both examples described below, a sodium sulfate solution is present in chambers 11 and 15, a sodium chloride solution is present in chamber 14, and approximately 2 molar sodium hydroxide and hydrochloric acid is present respectively in chambers 12 and 13. EXAMPLE 1 Raipore 1010 and 1030 cation and anion exchange membranes were immersed for 40 minutes in a 5% (W/V) solution of chromic chloride in IN NaOH at 95°C.
The membranes were then washed and pressed together to form a bipolar membrane. The potential difference across the bipolar membrane was 1.0 V when it separated 2N
HC1 and 2N NaOH solutions and the current was 100 A
__2 cm . The current efficiency for acid base production of the bipolar film was greater than 90%. The operating voltage has increased to only 1.15 volts after it had been
_2 in continuous operation at 100 mA cm over a twelve month period in the electrodialysis cell shown in Figure
1, where the solutions adjacent to the membrane were 2N
HC1 and 2N NaOH and the solutions were under a nitrogen atmosphere at a pressure of 16psi (gauge pressure) . EXAMPLE- 2
Raipore 1010 and 1030 cation and anion exchange membranes were immersed for 40 minutes in a 3% (w/v) solution of chromic chloride in 0.2M NaCH at 95°C. The membranes were then washed and pressed together to form a bipolar membrane. The potential difference across the bipolar membrane was I.IV when it separated 2N HCI and 2N
_2 NaOH solutions and the current was 100mA cm . The membrane voltage increased from 1.1-1.3V over the duration of the experiment which was eight months. The solutions in the electrodialysis cell were under a nitrogen atmosphere at a pressure of 16 psi (gauge pressure), as shown in the figure.
Although the present invention has been described with reference to a number of preferred embodiments, it will be recognised by persons skilled in the art that numerous other variations and modifications may be made to the invention without departing from the spirit or scope thereof.