Operational process, chloralkali

Figure 1. Basic operational process for membrane chloralkali cells.

Figure 7-29 Salt electrolysis to produce CI2 gas and NaOH solution in a chloralkali process. This is the process by which most alkali and chlorine are produced The reactor operates continuously with brine solution flowing into the cell and NaOH and Q2 gas flowing out.

Any of the products of brine electrolysis, chlorine, sodium hydroxide, and hydrogen can be hazardous if released. When releases do occur, it is usually from process upsets or breakdowns, which may be minimized by the construction of fail-safe plants, proper maintenance, and by safe transport and storage practices. Probably of greater long-term concern is the mercury loss experienced through the process streams of a mercury cell chloralkali operation. These losses can also carry over to the products of the diaphragm cell, even though this does not use mercury, if a common brine well or common salt dissolver is used for both sets of cells. 

Chlorine and sodium hydroxide production by the electrolysis of brine solutions necessarily locks the ratio of the two products to the theoretical ratio of the process [45, 46]. When the market for sodium hydroxide exceeds the market for chlorine, the causticization of sodium carbonate to sodium hydroxide (Section 7.3) may be used by some suppliers and consumers to supplement the available sodium hydroxide without producing large amounts of excess chlorine. Another expedient for large-scale chloralkali producers faced with this situation is to stimulate the chlorinated solvent or hydrochloric acid markets in an attempt to increase the consumption of chlorine to restore the balance. These measures are not usually rapid enough to respond over the short term unless the solvent plant is also operated by the chloralkali producer. 

If the chloralkali plant is operating in the vicinity of an oil refinery, it can provide a part of the hydrogen gas requirements of hydrocracking or various hydrotreating processes used in refining. Hydrogen may also be burned in chlorine to profitably produce hydrochloric acid, either for captive use or for sale [52] (Eq. 8.63). 

The commercialization of phosgene processes based on hydrogen chloride, rather than dichlorine, would permit, for example, isocyanate plants to be sited at locations not dependant on chloralkali-producing facilities or large chlorine-consuming (e.g. vinyl chloride) plant. However, it is not known for certain whether any of these processes are currently commercially operated. 

In 2000, 45 Mt of CI2 was manufactured by the chloralkali process this represents 95% of the global supply. The main producers are the US, Western Europe and Japan. Whereas the Japanese chloralkali industry operates almost entirely with the membrane cell, the US favours use of the diaphragm cell, and just over half of the Western European industry retains use of the mercury cell. On environmental grounds, the chloralkali industry is being pressured to replace mercury and diaphragm cells by the membrane cell. This is not the only environmental concern facing the industry demand for CI2 has fallen in the pulp and paper industry and in the production of chlorofluorocarbons, the latter being phased out as a result of the Montreal Protocol for the Protection... 

We mentioned in Section 1.3 some important industrial applications of electrolysis—in the chloralkali industry, metal winning and refining, and organic electrosynthesis. As indicated in Section 1.2, we do not intend to describe electrochemical processes in detail, since there are many books on electrochemical technology. We will discuss the design of individual reactors, with emphasis on modularized, general purpose flow electrolyzers. We will classify reactors by their mode of operation. 

CaCl2 is added to reduce the operating temperature to about 870 K, since pure NaCl melts at 1073 K (see Section 9.12). The design of the electrolysis cell (Fig. 11.2) is critical to prevent reformation of NaCl by recombination of Na and CI2. Although the Downs process is the major manufacturing process for Na, the CI2 produced contributes only 5% of the world s supply. The remaining 95% is produced by the chloralkali process which involves the electrolysis of aqueous NaCl (see Box 11.4). 

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