Chlor-alkali technology diaphragms

T. Florkiewicz and L.C. Curlin, Polyramix Diaphragm A Commercial Reality. In T.C. Wellington (ed.). Modem Chlor-Alkali Technology, vol. 5, Elsevier Applied Science, London (1992), p. 209. 

J.V. Winings and D.H. Porter, Evolutionary Developments in Hooker Diaphragm Cells. In M.O. Coulter (ed.). Modern Chlor-Alkali Technology, Society of Chemical Industry, London (1980), p. 27. 

P.C. Foller, D.W. DuBois, and J. Hutchins, PPG s Tephram Diaphragm, The Adaptable Non-Asbestos Diaphragm. In S. Sealey (ed.). Modem Chlor-Alkali Technology, Vol. 7, Royal Society of Chemistry, Cambridge, UK (1988), p. 162. 

F. Kuntzburger, D. Horbez, J.G. LeHelloco, and J.M. Perineau, New Developments in Built-in Precathode Diaphragm Technology. In Modem Chlor-Alkali Technology, Vol. 7, Society of Chemical Industry, London (1998), p. 181. 

Chlor-alkali technology fundamentals, processes and materials for diaphragms... 

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. 

The major processes for manufacturing chlorine and caustic soda are based on electrolytic procedures and have been since the 1890s. In the UK the introduction of the Castner Kellner process in 1897 was a major development in chlor-alkali technology. At present over 90% of the world s requirement for chlorine is obtained electrolytically from aqueous sodium chloride and the rest from the molten salt. The proliferation of cells used in the 1930s and 1940s has now been reduced to three cell types, two diaphragm cells and the mercury cell. The latter is being phased out, however, because of the hazardous nature of mercury. 

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. 

The use of polyperfluorosulfonic acid membranes as the cell separator was first demonstrated about three decades ago. Yet it was not until the mid-1980s when the economic advantages of membrane cells over the traditional mercury- and diaphragm-cell technology were fully demonstrated—consequent to better membrane performance, higher caustic product concentrations, and lower power consumption. Retrofitting chlor-alkali facilities with membrane cells accounted for much of the growth and sustenance of this industry over the past two decades. 

The choice of technology, the associated capital, and operating costs for a chlor—alkali plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference in operating costs between diaphragm, mercury, and membrane cell plants results from variations in electricity requirements for the three processes (Table 25) so that local eneigy and steam costs are most important. 

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 unit operations in a commercial chlor-alkali plant can be generally classified as follows (1) brine purification, (2) electrolytic cells, (3) H2 and Cl2 collection, and (4) caustic concentration and salt removal. In this section, the general process flowsheets for diaphragm, membrane, and mercury cell technologies are discussed with emphasis on the need for brine purification and the manner in which it is carried out. 

Historically, the Japanese chlor-alkali industry started in 1881, when the LeBlanc process was used to produce caustic soda. Osaka Soda and Hodogaya Chemical commercialized the mercury- and diaphragm-cell technologies in 1915. Asahi Glass started the Solvay process soon after. By 1973, 95% of the chlorine was produced by the mercury-cell process and 5% by diaphragm cells. In 1973, mercury pollution issues... 

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