Instrumentation and Control Systems
Chapters 7 through 9 have already covered the processes involved in a chlor-alkali plant, along with some of the essentials of their control. This chapter goes into the details of control systems and hardware. The discussion, where differences exist, focuses primarily on the membrane-cell process. The chlorine and hydrogen processes are essentially the same regardless of the type of cell used. Control of absolute and differential pressures is especially important in the gas systems, and so the discussion is divided primarily according to operating pressure level. Membrane cells require extremely pure brine, and some of the operations used are not necessary with the other types of cell. Otherwise, mercury-cell brine systems are for the most part very similar to those in membranecell plants, but they require their own special features and precautions to prevent the escape of mercury into the environment. Diaphragm cells require approximately the same treatment of new brine, but, unlike the situation with the other cells, there is no direct recycle of the anolyte. Therefore, the discussion of brine systems follows the membranecell process, which is the most comprehensive of the three. The caustic systems for the three manufacturing processes are very different and are discussed separately.
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