Hydrogen header control

A. Operation Near Atmospheric Pressure. The simplest approach to hydrogen header control at very low pressure uses large water-sealed vents designed to maintain a fairly precise back-pressiure with little or no fluctuation. Figure 11.35 shows such a vent. Section 9.1.10.1 gives design details for this type of system as a pressure-relief device with little heed to gas flow distribution. For more precise control, it is important to distribute the gas as small bubbles over a wider area of the water seal (10-20 nun deep) in order to provide minimum pressure drop and fluctuation in flow. 

FIGURE 11.35. Hydrogen header pressure control (atmospheric pressure). 

The other principal cause of low pressure is a malfunction of the hydrogen compressor control, which could allow the header pressure to drop very quickly below atmospheric. Section 11.4.2.4 covers the relevant protective systems. 

Diversion to Compressor System. Section 11.4.2.1 discussed the hydrogen header pressure control arrangement. It did not cover the mechanism for diversion of the gas to the compression system. 

B. Hydrogen Users Priority Control. While the discussion immediately above assumed that there was only one user of hydrogen, there are frequently several. Even with only one user, shutdown or curtailment of the offtake can force some of the hydrogen to be vented or diverted for use as fuel. The total demand by multiple hydrogen users, on the other hand, also can exceed the rate of supply. It is, therefore, the usual practice to establish user priorities by staging the hydrogen header pressure control system (Fig. 11.44). 

FIGURE 11.4S. Diaphragm-cell hydrogen header pressure control. 

Cathodic protection of water power turbines is characterized by wide variations in protection current requirements. This is due to the operating conditions (flow velocity, water level) and in the case of the Werra River, the salt content. For this reason potential-controlled rectifiers must be used. This is also necessary to avoid overprotection and thereby damage to the coating (see Sections 5.2.1.4 and 5.2.1.5 as well as Refs. 4 and 5). Safety measures must be addressed for the reasons stated in Section 20.1.5. Notices were fixed to the turbine and the external access to the box headers which warned of the danger of explosion from hydrogen and included the regulations for the avoidance of accidents (see Ref. 4). 

The basic component is a pure nickel, precision groimd, seamless tube (diameter 2 mm., length 5 m., wall thickness 0.1 mm.) soldered shut at one end. Five such tubes are needed. Each is colled into a helix, the helices are intertwined, and the open end of each tube is soldered to a brass header, as shown. The header is provided with a standard tapered male joint n. For ease of handling, the tubes are heated in a Hg stream at 1000°C for two hours, after which they may easily be bent by hand. The helix assembly is Inserted into a quartz reactor tube 1 m. long and 35 mm. in diameter. The front and back headers are cemented to the quartz tube with picein, as shown. The back header is provided with a needle valve v, which serves for fine control of pressure in the tube and through which the gaseous impurities, which are contained in the hydrogen and which accumulate in the reactor, may be released and subsequently burned. The front header has a connection for a mercury manometer. Only the middle part of the quartz reactor is electrically heated. Thus, the soldered points of the nickel tubes remain in the cooler sections of the apparatus. 

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. 

Problems still arise when the gas goes on to processing. It can be difficult, for example, to transfer hydrogen from a zone at low pressure to a compressor with suction throttling while maintaining good control of cell room header pressures. 

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. 

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... 

Improved control of sources can be as simple as providing covers for end boxes and caustic outlets from decomposers. A vacuum system can then collect vapors from these locations for treatment as described below. A typical vacuum in the collecting header is 100-150 mm H2O. Equalizing orifices at pickup points and the eduction of a small amount of fresh air at each point stabilize the pressures and prevent the accumulation of hydrogen in dangerous concentrations. 

In a refinery or chemical plant, compressors are used to compress gases like nitrogen, hydrogen, carbon dioxide, and chlorine. These gases are sent to headers from which they are distributed to a variety of applications. Compressors also provide clean, dry air for instruments and control... 

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