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The chlor-alkali industry is one of the largest electrochemical operations in the world, the main products being chlorine and sodium hydroxide generated simultaneously by the electrolysis of sodium chloride solutions. The chlor-alkali industry serves the commodity chemical business, chlorine and sodium hydroxide (also called caustic soda) being indispensable intermediates in the chemical industry [–].

During the last half of the 19th century, chlorine, used almost exclusively in the textile and paper industry, was made [] by reacting manganese dioxide with hydrochloric acid

The world production capacity of chlorine reached 53 million tons in 2002 from approximately 22 million tons in 1970 [–] and is expected to increase to 65 million tons by the year 2015 []. In this chapter, the major manufacturing processes and the factors affecting the growth pattern of the chlor-alkali industry are presented.

Thermodynamics is a powerful tool for the study of chemical reactions and is intimately related to the atomic and molecular description of the species participating in these reactions. The transformation of energy involved in the reactions depends on the thermodynamic conditions of the reaction, and can be expressed in terms of various thermodynamic functions. One such function is the Gibbs free energy [–], expressed by Eq.(l):

About 97 % of the chlorine and nearly 100% of the caustic soda in the world are produced electrolytically from sodium chloride, while the rest of the chlorine is manufactured by the electrolysis of KC1, HC1, chlorides of Ti and Mg, and by the chemical oxidation of chlorides []. The electrolytic technologies currently used are mercury, diaphragm, and ion-exchange membrane cells. Figures 5.1 and 3.10 show the distribution of these cell technologies in the world and on a regional basis []. Mercury cells had a world share of 45% in 1984 and declined to 18% in 2001 because of the health and environmental concerns associated with mercury. However, it is still the leading technology in Europe.

This chapter presents a general overview of the processing steps in a typical chlor-alkali plant. As shown in Fig. 6.1, we can divide the process into three major steps. These are the preparation of purified brine, electrolysis, and processing of the crude products of electrolysis. The figure also subdivides brine preparation into its main elements. Later sections present flowsheets in block form for each of the process steps. A discussion of the differences among the various types of electrolyzer technology accompanies each flow diagram. Separate flowsheets for the complete processes for the three electrolyzer technologies are also available in the literature [, ].

This chapter considers the preparation of brine from solid salt and the design and operation of each of the major units in a brine purification system. The discussion begins with the sources of electrolysis salts (both sodium and potassium chlorides; both natural and refined) and the methods of handling and storing them and then dissolving them to prepare brine. There is also a brief discussion of the storage and handling of brine. The rest of the chapter is then devoted to the treatment of brine to reach the purity demanded by the various types of cell. The sequence of the discussion follows the flow diagrams of Figs. 6.1 and 6.2.

In their design and construction, electrochemical plants differ from ordinary chemical plants in several ways:

The first three sections of this chapter discuss the processing and handUng of the products of electrolysis. Section 9.1, related to chlorine, comprises most of the chapter. Sections 9.2 and 9.3 then cover hydrogen and caustic soda or potash. Section 9.4 discusses applications of several byproducts that are sometimes found useful.

The purpose of this chapter is to gather in one place some of the basic considerations that apply to the engineering techniques and unit operations that are important in the chloralkali process. Thus, the chapter begins with a discussion of material and energy balances (Section 10.2). These are basic to all of chemical engineering and are used implicitly throughout this book. Here, we present some of the fundamentals. Section 10.3 then covers current distribution. This is uniquely important in electrochemical processing. The presentation discusses methods of predicting and determining the distribution of current in electrochemical reactors of different kinds.