Chloralkali electrolysis

In a diaphragm cell, the anode and cathode compartments are separated by a porous diaphragm (Figs. 11.1 and 11.2). Formerly, diaphragms were made of asbestos, but now special polymers created for chloralkali electrolysis have been introduced. 

In 1958, Henry Beer invented the dimensionally stable anode (19. 20), which consisted of a titanium anode covered with a catalytic layer composed mainly of a mixture of Ti02 and Ru02. Due to the joint efforts of Beer and of the De Nora Company, the dimensionally stable chlorine anode became accepted everywhere and its introduction revolutionized the technology (4) of chloralkali electrolysis. 

Figure 4 compares the current voltage curves of anodic chlorine evolution measured at graphite and Ru02 anodes in brine solution of a concentration, which is typical for chloralkali electrolysis (300 g NaCl/dm3) (22). 

At pH 2 (the lowest pH value maintained in chloralkali electrolysis anolytes), the equilibrium potential of the oxygen anode is more cathodic than that of the chlorine electrode (+0.96 V vs NHE compared to +1.26 V). Therefore it is of ultimate importance that Ru02 catalyzes chlo-... 

The change from nondimensionally stable carbon anodes to dimensionally stable titanium anodes permitted dramatic innovations in cell design, operation conditions, and reduction of the energy consumption of chloralkali electrolysis. 

The chloralkali electrolysis process is by far the most important source of electrolytically generated hydrogen because hydrogen from alkaline or membrane water electrolysis usually cannot compete with hydrogen from steam reforming followed by shift reaction and PSA purification and therefore is not performed on a large scale (62, 40). 

In chloralkali electrolysis and conventional water electrolysis, the catholyte is strongly alkaline (cNaon, Ckoh = 30 wt%), whereas in orga-noelectrosynthesis it is neutral or acidic depending on whether a divided or undivided cell is used. 

A more obvious method of avoiding the inevitable production of CaCl2 is the combination of the chloralkali electrolysis with the chlorohydrin process. This is also being pursued intensively 54-55>. A modification proposal by Lummus is shown in the block diagram below 56) ... 

This process has been used to make functionalized perlluorovinyl ethers [e.g., F2C = CF0CF2CF(CF3)0CF2CF2S02F] which are copolymerized to form fluorinated membrane materials (Nafion, Flemion), which arc important in chloralkali electrolysis. ... 

Proposal for a Directive concerning the chloralkali electrolysis industry sent to the Council on 20 June 1979, adopted on 22 March 1982 (OJ No L81, 27.3.1982). Proposal for a Directive on other industries in preparation... 

The same situation is true for the most popular electrochemcial process, namely, chloralkali electrolysis. The heart of the process, the cell room, is an even smaller component of the whole process [15]. 

Hydrogen is also formed in large quantities as a byproduct in petrochemical processes, refineries, coking plants (coke oven gas) and in chemical and electrochemical processes e.g. chloralkali-electrolysis. Other processes such as the photochemical production of hydrogen or thermal dissociation of water are only used in special applications and are currently industrially unimportant. 

This hydrogen is, however, mostly used in house. Hydrogen is also produced in other petrochemical and chemical processes (synthesis of olefins, ethyne, styrene, acetone). Coke oven gas contains over 50 volume % of hydrogen, from which it can be isolated. Finally hydrogen occurs as a valuable byproduct in chloralkali-electrolysis (directly with the diaphragm process or indirectly with the amalgam process and hydrochloric acid hydrolysis) see Section 1.7.3.3. The electrolysis processes account for less than 5% of the worldwide production of hydrogen. 

The sodium chloride byproduct can be utilized in chloralkali electrolysis. 

The quantity j (A cm-3) can be considered as a volumetric current density or a current concentration, also shown in Fig. 5 as a function of the characteristic length. Extremely high current densities, for example, 10 kA cm-2 in the chloralkali electrolysis, can be applied only in a limited number of cases, the number of cells with high specific electrode area is restricted to cases of low current densities [13]. 

P. Kohl and K. Lohrberg, Material Problems in the Three Versions of Chloralkali Electrolysis. /. Appl. Electrochem. 19, 589-595 (1989). 

Perfluorinated polyethers have also gained importance as actively functional materials. Ionic polymer membranes (e.g. DuPont s Nafion ) based on sulfonic acid-derivatized perfluoropolyethers have been used for nearly 30 years as ion-con-ducting membranes in chloralkali electrolysis cells, replacing the large amounts of toxic mercury used until then in the classic Castner-Kellner cells (Scheme 4.8.). One of the earliest applications of Nafion was as a membrane in the hydrogen-oxygen fuel cells which powered the Apollo spacecraft carrying the first men to the moon. 

The use of Nafion as a separator in cells for chloralkali electrolysis is described elsewhere. The exceptional chemical inertness and thermal stability coupled with favourable electrical conductivity being of particular advantage in this application. These properties have been exploited in a number of other electrochemical applications. 

The biological methylation of mercury (e.g., from weathering, volcanism, fossil fuels, chloralkali electrolysis) is effected by microorganisms that utilize methylco-balamin (2c) see Section 5.1.2. 

A couple of hydrogen accidents were associated with the chlorine production by chloralkali electrolysis, which are included in Table 8-4 under Electrolysis malfunction . Fire or explosion occurred when for some reason, for example a reversal of cell polarity, the hydrogen entered the chlorine processing equipment downstream of the electrolysis cells [104]. 

Simmrock KH, Poblotzki J (1988) Oxygen consimi-ing cathodes of the carbon and silver type in chloralkali electrolysis. Proc Electrochem Soc 88 369-382... 

In addition to electrochemical energy conversion in fuel cells, the reaction has applications in energy storage in metal-air batteries, in several industrial processes as the chloralkali electrolysis, and it causes corrosion of metals and alloys in the presence of air. That is why the efforts have been focused on elucidating the mechanism of this reaction and developing proper catalysts. 

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