Chemistry: electrical and wave energy – Processes and products
Patent
1989-07-24
1991-05-14
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
204128, 2041531, C25B 116, C25B 1500
Patent
active
050153455
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION OF THE INVENTION
The industrial technologies presently available for chlorine and caustic soda production by electrolysis of aqueous solutions of alkali metal halide, are based on mercury cathode electrolysis cells, porous diaphragm bipolar and monopolar electrolyzers and ion exchange membranes monopolar and bipolar electrolyzers.
The monopolar or bipolar electrolyzers having diaphragm electrolyte permeable diaphragms or ion exchange membranes substantially impermeable to electrolyte flow comprise a row of elementary cells; each cell of which comprises an anode and a cathode separated by a diaphragm such as an ion exchange diaphragm. In the case of a bipolar electrolyzer, an electrolyzing voltage or potential is imposed across the entire row whereby current flows through successive elementary cells of the row from anode to cathode of each cell and then to the anode of the next adjacent cell in the row.
The monopolar electrolyzer comprises a row of separate elementary cells, each cell having an anode and a cathode with the anodes of the cells individually connected to a common positive potential source and the cathodes individually connected to a common negative potential surface.
Typical monopolar electrolyzers of the type contemplated are disclosed in U.S. Pat. No. 4,341,604 and WO 84/02537.
Typical bipolar electrolyzers contemplated are disclosed in U.S. Pat. No. 4,488,946.
The ion exchange membrane technology, notwithstanding a certain depression of the market, is continuously expanding and most certainly will be the preferred choice for plants of future construction. The reasons for this success are essentially based both on lower power consumption, in the range of 2400-2600 kWh/ton of produced chlorine, and absence of ecological problems, which were the reason for the block of the investments on mercury plants.
The improvements attained so far in regard to the anodes and flexible covers lifetime, cleaning of the cell by rakes operated from outside the cell, and on demercurization treatments of gaseous and liquid effluents allow for the construction of mercury cathode electrolyzers which comply with the most severe environment protection requirements; anyway, the fear of mercury pollution (mercury is in fact one of the most poisoning agents both for the environment and for man) causes an emotional rejection by the authorities and the public, so strong that it will never be overcome.
A similar situation is experienced in regards to porous diaphragm electrolyzers: the main component of the diaphragm is asbestos, which is well-known as a cancerogenic element. The problems here arise before the electrolysis cell; the progressive closing of mines due the unbearable expenses for providing safe conditions for the workers, make really troublesome the availability of asbestos.
The above difficulties brought to a great effort and huge investments in research programs directed to finding alternative materials to asbestos. The new types of diaphragm, although more expensive, are today commercially available, but all the same, the porous diaphragm industry today cannot be competitive versus the ion-exchange membrane technology. As a matter of fact, porous diaphragm electrolyzers produce a mixed solution of halide and alkali hydroxide, which mixture must be evaporated and only upon separation of the halide a concentrated alkali hydroxide is obtained. These steps involve a higher power consumption than that of ion exchange membrane plants.
To fully appreciate the advantages of the present invention, the principles of alkali halide electrolysis utilizing ion-exchange membrane plants will be described and the two types of electrolyzers which may be equipped with ion exchange membranes will be discussed.
For the sake of simplicity the following description will make reference only to electrolysis of aqueous solutions of sodium chloride for producing chlorine and sodium hydroxide; anyway, all the concepts and conclusions reported herein also apply to the electrolysis of any aqueous solutions of alkali halide
REFERENCES:
Patent Abstracts of Japan, vol. 10, No. 355(C-388)(2411), Nov. 29, 1986.
Chem. Abstracts, vol. 105, No. 20, Nov. 17, 1986, Columbus, Ohio), p. 531, Abstract 180515q.
Gusmini Carlo
Mojana Corrado
Traini Carlo
Denora Permelec S.p.A.
Gorgos Kathryn
Niebling John F.
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