Chlorine header pressure

A. Chlorine Header Pressure Control. As pointed out above, chlorine header pressure control can be one of the most difficult control tasks, because the pressure is being controlled within a few millimeters of water coliunn and the latitude for error is small. Each chlorine production method has its own control range and limits, but the control method is the same. The strategy, as shown in Fig. 11.18, is simple. The control valve should be a fail-closed butterfly type with a conventional disc and a positioner. Preferably, it should be located after the dry demister downstream of the drying columns (Section 9.1.5). In this location, the wetted parts of the valve can be Monel. This strategy... 

FIGURE 11.18. Chlorine header pressure control Gow pressure). 

Ensure that hydrogen and chlorine header pressures are under control and that the differential is correct. Small header purge flows will be necessary when the... 

Cell room acid addition rate Electrolyzer acid addition rates Caustic export flow rate Cell room outlet caustic temperature Electrolyzer outlet caustic temperatures Chlorine header pressure Hydrogen header pressure Differential pressure between gas headers... 

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

The water seal is installed somewhere on the low-pressure side of the chlorine process, usually between the cell room and the cooling process of Fig. 9.12. It is in communication with the process by a branch on the main chlorine header, as indicated by Fig. 9.44. The branch line terminates inside the seal vessel, slightly below the surface of a pool of water. When the pressure in the gas line exceeds the difference between the water level and the bottom of the branch line, the seal breaks and gas escapes. A source of brine can be used in place of water consideration of the difference in density then is necessary when setting the height of the seal. 

The seal pot vessel and its components must be chlorine-resistant. Rubber-lined and FRP construction are the most common. The vessel must be able to withstand the small pressure and vacuum which may exist in the process. Since it is connected to the chlorine header, thermal expansion and contraction of the pipe must be considered in placement and support of the seal vessel. 

The handling and processing of chlorine are nearly identical for the three different types of cell. Important differences are in the cell room header pressure control and the amounts of hydrogen and oxygen contained in the gas. Pressure control requires special attention because it is necessary to maintain a constant differential between the hydrogen and chlorine gas headers that is a small fraction of the (absolute) operating pressure. 

IB. Chlorine Header Safety Systems. Low-pressure chlorine headers are protected from over- and under-pressures by the use of water-filled seal pots. A typical seal pot relieves at about 50 mm w.c. Some membrane-cell systems operate at pressures too high for effective use of a water seal and must depend upon weighted discs or an automated relieving system. 

The whole chlorine processing train can be upset if air enters the system through a vacuum break. If the pressure in the chlorine header is close to the point where the vacuum seal opens, the chlorine compressor should be shut down. This will prevent air from being drawn into the whole chlorine handling system. 

C. Local Measurements. Local measurements of wet chlorine gas pressures below 10 or 15 kPa can be made with U-tube manometers filled with water. Process connections should minimize the collection of condensate in the manometers. A simple but not foolproof technique is to make a connection in the upper half of the header and force the tubing to rise for some distance before descending to the manometer. 

The alternative to separate control of the header pressures is to control one pressure directly and the other through a differential pressure controller referred to the first. With this alternative, the more frequent choice is to control the chlorine pressure and the hydrogen/chlorine differential pressure. This takes advantage of better control dynamics. An argument for this approach is that it is the quality of the differential pressure control that may determine the life of the membranes, and there is only one control loop variance to deal with. In a poorly designed system, however, responses to the two control signals can be out of phase and introduce fluctuations in the pressures. In order to have a consistent approach in this presentation, we assume that the two header pressures are controlled separately and directly. Differential pressure is measured directly or computed from the outputs of the two pressure transmitters. Monitoring, alarm, and supervisory systems can be appropriate to individual applications. 

Titanium chlorine headers are used in Europe and are claimed to reduce the amount of chlorinated organics and other nonvolatile residues in gaseous and liquid chlorine. However, it suffers from crevice corrosion near the welds. The use of titanium headers for pressure operation of membrane cells is discussed in Section 8.4.1.1 A. 

The chlorine header should be maintained at positive pressure to permit detection and correction of any chlorine piping leaks. The hydrc en header is also maintained at a positive pressure to avoid pulling air into the hydrogen, creating a potentially explosive mixture. The brine header pressure should be maintained to give the desired caustic concentration in the cell liquor. Normal practice is to adjust individual brine feed valves so that each cell receives the correct brine flow rate. 

Chlorine/sulfur dioxide headers in the chlorination (sulfonation) room What if the pressure relief valve sticks open ... 

Most chlorine cell headers operate very close to atmospheric pressure. The accumulated pressure drop causes compressor suctions to be under vacuum, which raises the possibility of infiltration of atmospheric air. This lowers the purity of the delivered gas and makes the liquefaction process less efficient. The front end of the gas processing train therefore should be desired for low pressure drop. To this end, the drying towers... 

The final vent from the system should be regarded as a point of possible chlorine emission. It should be at a high level and away from personnel traffic and ventilation system intakes. Particularly when the low-pressure scrubbing device is a packed bed, booster fans may be added on the tail pipe. The piping arrangement around the fans then should avoid low points that could collect liquid. As with any vent manifold, the designer of a system serving several different units should take precautions to avoid mixtures in headers that may lead to undesirable side reactions. 

The suction pressure control system shown in Fig. 11.24 uses a fail-open recycle valve to return compressed chlorine from discharge to suction. In the common situation with multiple compressors, there is one control system, and the chlorine is piped from the common discharge header to the suction header. The control valve should have an equal percentage characteristic and should be sized to handle the total capacity of the compressors when 90% open. It should have excellent turndown in order to control small recycle flows and should be aided by a valve positioner. 

Too much air or nitrogen in the chlorine can also cause surge. Lower density gas cannot be compressed to the same discharge pressure as chlorine, and thus the compressor may not produce enough pressure for the gas to enter the discharge header. In the approach described below, surge control is combined with a suction pressure control system. 

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