Electrolyzer chlor-alkali

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. 

Membranes in chlor-alkali electrolysis require highly pure brine feed the water used in membrane water electrolyzers must also be rather pure. 

You have already seen that chlorine gas can be made by the electrolysis of molten sodium chloride. In industry, some chlorine is produced in this way using the Downs cell described earlier. However, more chlorine is produced in Canada using a different method, called the chlor-alkali process. In this process, brine is electrolyzed in a cell like the one shown in Figure 11.32. Brine is a saturated solution of sodium chloride. 

The chlor-alkali cell in this diagram electrolyzes an aqueous solution of sodium chloride to produce chlorine gas, hydrogen gas, and aqueous sodium hydroxide. The asbestos diaphragm stops the chlorine gas produced at the anode from mixing with the hydrogen gas produced at the cathode. Sodium hydroxide solution is removed from the cell periodically, and fresh brine is added to the cell. 

Also, discussions of a number of applications of Nafion are not included in this document and are, at most, mentioned within the context of a particular study of fundamental properties. A number of these systems are simply proposed rather than in actual commercial applications. Membranes in fuel cells, electrochemical energy storage systems, chlor-alkali cells, water electrolyzers, Donnan dialysis cells, elec-trochromic devices, and sensors, including ion selective electrodes, and the use of these membranes as a strong acid catalyst can be found in the above-mentioned reviews. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

The chlor-alkali process,34 in which seawater is electrolyzed to produce Cl2 and NaOH, is the second most important commercial electrolysis, behind production of aluminum. 

D. Bergner, Membrane Electrolyzers for the Chlor-Alkali Industy, Dechema-Monographs, Vol. 123, VCH Vedagsgesellschaft, New York, 1991, pp. 113-131. 

Chlor-alkali process the process for producing chlorine and sodium hydroxide by electrolyzing brine in a mercury cell. (11.8)... 

Membrane electrolyzers for water decomposition use cation exchange membranes of the type used in the chlor-alkali membrane process, in this case made H+-ion conducting by different pretreatment. So, the ions generated at the anode (H2O -> 2H++ I/2O2 + 2e ) pass through the membrane and form at the cathode hydrogen (2H+ + 2e -> H2). 

Remarkable advances in ion exchange membranes have been made since their inception and application to chlor-alkali cells in the 1970fs, and since that time many patents have issued on their applications. Several companies besides duPont have developed proprietary membranes and electrolyzers for commercial application. 

Development of these new zero gap membrane cell electrolyzers represents a major new approach in the membrane cell technology and promises to provide even more rapid development in this quiet revolution of the membrane cell chlor alkali process. 

Our membrane chlor alkali process using Flemion and the electrolyzer is named as the Flemion process. Two commercial plants are in operation in Japan, and another one in Thailand has also started up. 

In the chlor-alkali process, an aqueous solution of NaCl is electrolyzed to give CI2, H2, and NaOH, according to the following anode and cathode reactions ... 

This means that the electrical energy required for electrolysis is reduced by about 56%. It has been reported that the voltage drop between electrodes attains 1.93-1.95 V at a current density of 30 A dm-2 in a semi-commercial electrolyzer.126 Further energy saving is possible in the chlor-alkali process. 

J.T. Keating, Effect of brine purity on performance of commercial electrolyzer, Soda Enso (Soda Chlorine), 1994, 45, 366-375 J.H. Austin, Operation in chlor-alkali plants, 3rd London International Chlorine Symposium, June, 5-7 1985. 

Chlor-alkali process the process for producing chlorine and sodium hydroxide by electrolyzing brine in a mercury cell. (18.9) Chromatography the general name for a series of methods for separating mixtures by employing a system with a mobile phase and a stationary phase. (1.9)... 

Since then, several membrane-cell technologies were developed in Japan, as a pollution-free chlor-alkali process. Japanese contributions include composite membranes and several electrolyzer designs. Japan was the first major chlorine producing country to convert entirely to membrane cell technology. As of January 2003, 35% of world production of chlorine is by membrane-cell technology, generating 52,000 metric tons caustic/day. 

A. k-Factor. A popular diagnostic parameter that is measured and used in the chlor-alkali industry to monitor the performance of mercury and membrane electrolyzers and to control (manually or automatically) the anode-cathode in the mercury cells is the / -factor. The -factor is the slope of the current density-voltage curve, plotted from the cell voltages measured at dififerent loads. 

K.L. Hardee, A Simple Procedure for Evaluation of Membrane Electrolyzer Performance. In R.W. Curry (ed.). Modem Chlor-Alkali Technology, vol. 6, The Royal Society of Chemistry, London (1995), p. 234. 

The polymer in this sulfonyl fluoride form is thermoplastic and can be melt processed with conventional fluoropolymer processing equipment. For use in chlor-alkali electrolyzers, the polymer is extruded to film form and reinforced, if necessary, with... 

S.2. Effects of Operating Conditions Membranes function over a wide range of conditions, allowing chlor-alkali producers to select electrolyzer designs and process conditions. However, if membranes are operated outside the reconunended ranges, their performance may be adversely affected. The effects of the electrolyzer, anolyte, and catholyte operating conditions on membrane performance are addressed here. 

T. Yamashita, Y. Sajima, and H. Ukihashi, The Design and Operating Experiences of AZEC Electrolyzers and Recent Develr ment of Flemirm Membranes. In N.M. Prout and J.S. Moorhouse (eds) Modem Chlor-Alkali Technology, Vol. 4, Elsevier Applied Science, London (1990), p. 109. 

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