Operating pressure, chlor-alkali

Many types of filter are supplied for this sort of operation. In chlor-alkali brine treatment, the two most frequently encountered are the pressure leaf filter and the candle filter. Both types are capable of removing submicron particles and producing a filtrate with less than 1 ppm of suspended solids. A leaf filter, as the name implies, contains a number of thin, flat elements that are active on both sides. In chlor-alkali brine plants, the leaves normally are suspended vertically in a tank. The tank may be horizontal, in which case the leaves are circles or rounded squares, or vertical, in which case the leaves are approximately rectangular and of different widths. 

A wide variety of designs and operating procedures exist in the chlor-alkali industry. These vary from once-through to recycle operation with the caustic solution, from continuous to batch addition of the caustic make-up as well as in the means of provision of a back-up reservoir of caustic for containment of chlorine-laden pressure relief and emergency shutdown streams. 

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. 

Early operation should also be at less than design current density. A typical load during this period is 2-4 kA m . The load should be held stable while monitoring brine and caustic temperatures and concentrations. The differential pressure between hydrogen and chlorine should also remain stable. The rest of the chlor-alkali plant operation can also be checked for deviations and problems during this time. 

The swings in the chlor-alkali industry during the 1980s, coupled with environmental pressures on the use of mercury and asbestos in chlor-alkali cells, led to numerous plant closures. In 1989 there were 26 U.S. companies producing chlorine at 51 sites with an operating capacity of 12,275 short tons. Dow Chemical U.S.A. accounted for almost a third of U.S. capacity. The top four producers shown in Table 12.20 accounted for over 70 percent of the installed capacity. 

---