Membrane cells/processes caustic system

The major anionic impurity in most brine systems is sulfate. Control of its concentration is an issue mostly in membrane cells. In the diaphragm-cell process, sulfate passes with the rest of the anolyte into the cathode side of the cells. It can be separated from caustic soda in the evaporators and purged from the system as Glauber s salt This is covered in Section 9.4.2.1. Mercury cells are least sensitive to sulfate. Its concentration is frequently allowed to build to the point where dissolution of calcium sulfate from the salt is inhibited. The greatest problem then caused by the sulfate is a reduction in the solubility of NaCl or KCl. 

High-pressure steam may also serve as a driver for turbines. Chlor-alkali plants, which are usually placed in areas of low electrical power cost, are less likely than most other types to justify the use of steam-turbine drives but still may use them as a backup source of power. Furthermore, one of the ways to cope with a major electrical failure is to use steam to operate critical drives imtil all systems are shut down or electrical power is restored. An example of a critical service is the caustic circulation pump on the emergency vent scrubber. A spare pump is always necessary, and it should have an independent source of power. One way to accomplish this is with a steam-turbine drive. Other services may also be considered critical for personnel safety or process security. The latter is especially true in a membrane-cell plant, where some systems are vital for the protection of the membranes from damage. 

Water, after the preliminary treatment methods of Section 12.4.1, can be called purified. Here, we use the term to refer to the higher levels of purification in Table 12.1 or to those processes which remove dissolved contaminants. In the chlor-alkali process, the major uses of purified water are dilution of catholyte, processing of membrane-cell caustic liquor, preparation of ion-exchange system regenerants, manufacture of hydrochloric acid, acidification of brine, and, sometimes, dissolving of salt. It also serves as utility and seal water in the membrane preparation area and in certain parts of the process. 

The desalination of brackish water by electrodialysis and the electrolytic production of chlorine and caustic soda are the two most popular processes using ion-exchange membranes. There are, however, many other processes such as diffusion dialysis, Donnan dialysis, electrodialytic water dissociation, etc. which are rapidly gaining commercial and technical relevance. Furthermore ion-exchange membranes are vital elements in many energy storage and conversion systems such as batteries and fuel cells. 

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