Chlor-alkali membrane cell process

A newer chlor-alkali membrane-cell process, in which the diaphragm is replaced by a polymeric membrane to separate the cell compartments, has been adopted in much of the industrialized world. The membrane allows only cations to move through it and only from anode to cathode compartments. Thus, as CF ions are removed at the anode through oxidation to CL, Na ions in the anode compartment move through the membrane to the cathode compartment and form an NaOH solution. In addition to forming purer NaOH than the older diaphragmcell method, the membrane-cell process uses less electricity. 

N. Funiya, H. Syojaku, H. Aikawa, and O. Ichinose, Ag Based Oxygen Cathodes for Chlor-Alkali Membrane Cells. In J.W. Van Zee, P.C. Foller, T.F. Fuller, and F. Hine (eds.). Advances in Mathematical Modelling and Simulation of Electrochemical Processes and Oxygen Depolarized Cathodes and Activated Cathodes for Chlor-Alkali and Chlorate Processes, Proc. vol. 98-10, The Electrochemical Society, Pennington, NJ (1998), p. 243. 

Fig. 1. Flow diagram for chlor-alkali production by a membrane cell process.

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

Summary. Membrane cell processes have become important to modem technology to a great extent because of the development and utilization of perfluorinated membranes. The combination of metal anodes and the perfluorinated membranes has provided a modem revolution in the area of chlor-alkali production. 

Since the first membrane cell installation at the Nobeoka plant by Asahi Chemical Industry in 1975, several membrane cell plants have been constructed, especially in Japan, as a pollution-free chlor-alkali process. By the end of 1982, the total capacity of the membrane cell process in the world was estimated to be about 600,000 tons of NaOH per year.112... 

Currently, major research and developmental emphasis is toward achieving an energy consumption of less than 2000 kWh/M.TNaOH. The membrane technology is so advanced that a reduction of —100 kW h/M.T NaOH will reach the practical minimum value. Thus, the membrane cell process promises to be the main technology for chlor-alkali production in the near future. 

In 1800, Cruickshank was the first to prepare chlorine electrochemically [38] however, the process was of little significance until the development of a suitable generator by Siemens and of synthetic graphite for anodes by Acheson and Castner in 1892. These two developments made possible the electrolytic production of chlorine, the chlor-alkali process, on an industrial scale. About the same time, both the diaphragm cell process (1885) and the mercury cell process (1892) were introduced. The membrane cell process was developed much mpre recently (1970). Currently, more than 95 % of world chlorine production is obtained by the chlor-alkali process. Since 1970 graphite anodes have been superseded by activated titanium anodes in the diaphragm and mercury cell processes. The newer membrane cell process uses only activated titanium anodes. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

Diaphragm cell A family of electrochemical chlor-alkali processes using cells with semi-permiable membranes which minimize diffusion between the electrodes. The overall reaction is 2NaCl + 2H20 = 2NaOH + H2 +C12... 

Their availability has greatly expanded the potential for electrolytic processes in synthesis and fuel cells as well as in environmental control. Perfluorinated cation exchange membranes such as Nafion outlast the material that preceeded them by up to four and a half years [48], Unfortunately very little has been published on their behaviour outside their use in chlor-alkali electrolysis. 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

Chlorine (from the Greek chloros for yellow-green ) is the most abundant halogen (0.19 w% of the earth s crust) and plays a key role in chemical processes. The chlor-alkali industry has been in operation since the 1890s and improvements in the technology are still important and noticeable, for example, the transition from the mercury-based technology to membrane cells [60]. Most chlorine produced today is used for the manufacture of polyvinyl chloride, chloroprene, chlorinated hydrocarbons, propylene oxide, in the pulp and paper industry, in water treatment, and in disinfection processes [61]. A summary of typical redox states of chlorine, standard potentials for acidic aqueous media, and applications is given in Scheme 2. 

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