Chlor-alkali technology electrolysis technologies

Development of chlorine electrode materials has benefited from the experience of chlor-alkali electrolysis cell technology. The main problem is to find the best compromise between cycle life and cost. Porous graphite, subjected to certain proprietary treatments, has been considered a preferable alternative to ruthenium-treated titanium substrates. The graphite electrode may undergo slow oxidative degradation, but this does not seem to be a significant process. 

Abstract Ion-conducting materials are used as cell separators in electrolysis cells for the double purpose of carrying electric charges between electrodes and separating the products formed at each electrode. The purpose of this chapter is to provide an overview of chlor-alkali technology and associated cell separators. After a brief historical review of the chlor-alkali process, the main reaction characteristics (thermodynamics, cell reactions and kinetics) are detailed in Section 9.1. Main chlor-alkali technologies are described in Section 9.2. Main cell separators are described in Section 9.3 (diaphragm materials) and in Section 9.4 (membrane materials). Some improved electrolysis concepts are described in Section 9.5. 

In Europe, Spain is the third country - after Germany (18) and France (10) -as regards the number of chlor-alkali plants, with a total of nine. Nevertheless, Spain is ahead along with Germany, in respect to the number of plants (8) which are still using Hg-electrolysis technology. The membrane technology represents... 

The Hg-electrolysis technology is one of the major point sources of Hg contamination, and its impact on the environment has been studied worldwide [23-26]. Although mercury cell chlor-alkali industry is obsolete in most of the European Union countries [27], in Spain it will be allowed until the end of 2010. 

The GDE for chlor-alkali electrolysis plants is still a relatively new concept compared with other chlorine technologies. It may be assumed that there is still considerable development potential in this newer technology. The above cost and Rol figures are based on optimistic values and should be regarded as provisional. In... 

All the examples quoted show how costs can be lowered, profit for products increased and the turnover enlarged by selecting KU know-how and technology for chlor-alkali electrolysis plants. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

The development of high current density electrolysis technology is a continuing effort. Asahi Chemical s focus is currently on the confirmation of stable long-term performance and reliability, in preparation for the supply of this process equipment and technology to chlor-alkali producers. 

Chlor-alkali production — With a 63% production volume of the total world chlorine capacity of about 43.4 million tons (in 1998), the chlor-alkali (or chlorine-caustic) industry is one of the largest electrochemical technologies in the world. Chlorine, Cl2, with its main co-product sodium hydroxide, NaOH, has been produced on industrial scale for more than a century by -> electrolysis of brine, a saturated solution of sodium chloride (-> alkali chloride electrolysis). Today, they are among the top ten chemicals produced in the world. Sodium chlorate (NaC103) and sodium hypochlorite (NaOCl, bleach ) are important side products of the... 

Asahi Kasei develops membranes mainly for chlor-alkali electrolysis technology with Aciplex F PFSA membranes. The Aciplex F membrane is employed in plants with a total production capacity of over 5 million tons of sodium hydroxide... 

In the past 30 years, a new process has been developed in the chlor-alkali industry that employs a membrane to separate the anode and cathode compartments in brine electrolysis cells. The membrane is superior to a diaphragm because the membrane is impermeable to anions. Only cations can flow through the membrane. Because neither Cr nor OH ions can pass through the membrane separating the anode and cathode compartments, NaCI contamination of the NaOH formed at the cathode does not occur. Although membrane technology is now just becoming prominent in the United States, it is the dominant method for chlor-alkali production in Japan. 

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

Figure 3.14 Comparison of the energy consumption in the three cell technologies for chlor-alkali production. Full lines represent electrolysis only broken lines represent total energy consumption including evaporation and heating the electrolyte.

M. Seko, J. Omura, and M. Yoshida, Recent Developments in Ion Exchange Membranes for Chlor-Alkali Electrolysis. In K. Wall (ed.) Modem Chlor-Alkali Technology, Vol. 3, EOis Hwwood, Chichester, (1986), p. 178. 

J.H.G. Van der Stegen and P. Breuning, The Formation of Precipitates of Iron Ions Inside Perfluorinated Membranes During Chlor-Alkali Electrolysis. In S. Sealy (ed.) Modem Chlor-Alkali Technology, vol. 7, Royal. Soc. Chem. Cambridge (1995), p. 123. 

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