Headers brine header

It is of course important to control the total flow of brine and at the same time to control the flow to each electrolyzer. This is why brine header flow control often is in fact line pressure control (Section 11.2.2.4D). Maintaining a constant pressure in the line balances the flow into the header with the total flow to all the cells. It also allows individual electrolyzer feed rates to remain steady even dirough manually set valves. It is important that the cell room headers have very small pressure drops. This will cause the pressures at all control points to be essentially the same. In turn, the rates of flow to the individual electrolyzers will also be equal. Header pressure control also offers a simple way to make nearly instantaneous changes in flow to all the electrolyzers during startup, shutdown, or load changes. 

A. Depleted Brine Collection. A depleted brine receiver (or anolyte tank) collects overflowing brine from the electrolyzers (Rg. 11.13). Addition of HCl releases tbe chlorine dissolved in the brine. By way of the depleted brine header, this vessel also receives the flushing brine when the rectifiers trip. Space constraints may prevent the use of a tank with enough freeboard to accept all the flushing brine. This situation is addressed in Section 11.2.2.5E. 

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. 

Figure 20.7 illustrates this process for exit brine temperature. Here the distribution of brine flow from the main header, as well as the efficiency of each cell, influence individual cell exit temperatures in a complex and non-linear way. 

Typical areas where titanium has found widespread industrial use in membrane technology are cells, anodes, anolyte headers, anolyte containers, filters, heat exchangers, chlorate removal systems and various parts of the brine system. 

Unlike in the other two electrolysis processes, the brine is not recirculated and the temperature in the system can be chosen according to optimum conditions and therefore comparatively little titanium is used in a diaphragm cellroom. However, there are some clear candidates. An example is the cell blanket where Permascand has a newly patented design comprising bellows welded to the anode collar. The chlorine header and also the cell top are other components that could be manufactured from... 

In order to handle hot chlorine gas from the cells a header system has been developed (Fig. 23.16) whereby stray current dumpers take care of the brine condensates that would otherwise result in damage. 

In the membrane-cell process, sodium or potassium chloride brine is fed to the cell and distributed equally among the anode compartments, while water fed into a second header flows into the cathode compartments or into an externally recirculating stream... 

A unique feature of the cathode design is the extension of the cathode fingers from the back plate, which allows easy inspection of the cathode surfaces, and the adaptability to use synthetic separators with minor modifications. The anolyte compartment is connected to an independent brine feed tank by flanged connections and chlorine leaves from the top, through the brine feed tank and then to the chlorine header. Each electrolyzer is fitted with a level alarm, which monitors the level of all the cells in the unit. Figure 5.16 is an isometric cutaway of a Glanor V Type 1144 electrolyzer. 

Internal mixing of electrolytes is driven by gas lift. Feed headers run the full width of the electrodes, giving even distribution of electrolyte flows. Internal baffles complete the process by preventing channeling of the gas flow. One intent of these design features is to permit controlled acid injection into the brine in order to limit the oxygen content of the chlorine gas, without the need for extensive recirculation devices. Internal circulation is promoted by the use of a split baffle. 

Recycled anolyte must first be dechlorinated for recycle to the brine plant. Most commonly, this involves acidification with HCl and removal of the evolved chlorine under vacuum. The chlorine goes to the cell gas header for processing. 

Membrane cells, like mercury cells, can switch back and forth between NaOH and KOH production. This operation is much more complex in the membrane-cell case when different types of membrane are recommended for the two different services. We have seen (Chapter 5) that removal and replacement of cells is more or less complicated and time-consuming. Providing separate brine and caustic piping headers to a number of cell berths is also more difficult, because of the much more compact layout of a membrane-cell room. 

The cooling step removes most of the water vapor. The condensate, saturated with chlorine, requires treatment. Figures 6.4 and 6.S showed that membrane- and mercurycell plants require facilities for removal of dissolved chlorine from depleted brine. With proper design, those facilities can also accept the condensate from the chlorine cooling plant. A conunon practice in diaphragm-cell plants is to strip the condensate, after acidification, with steam. The chlorine, again, can return to the main gas header, and the stripped condensate becomes a process waste. 

A. Brine. The depleted brine leaving mercury and membrane cells is saturated with chlorine. The same sorts of materials used in wet chlorine gas systems are suitable here. Brine lines generally are smaller than chlorine gas headers, and there is more scope for the use of common thermoplastics. The chlorinated types are frequently chosen. Especially at the temperatures of depleted brine, mechanical properties are also very important, and proper support is essential. The reason for the superiority of CPVC over PVC in this application may have more to do with better physical properties than with improved corrosion resistance. For added strength, these materials are often wrapped with FRP. 

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. 

A PTFE-lined magnetic flow meter measures the brine feed to the cell room. In a plant with a ratio-controlled anolyte recycle, this flow rate also can furnish the primary flow signal. The recycle anolyte stream returns to the brine feed header downstream of the pressure control valve. A PTFE-lined magnetic flow meter with platinum electrodes measures its flow. The control valve downstream of the flow meter should be a butterfly valve fully lined with PTFE. This valve should close upon rectifier failure. 

Manually operated valves with local indicators control the flow of brine to individual electrolyzers. All the electrolyzers in the circuit are fed from a common header that is maintained at a specified pressure. This approach reduces the interaction between electrolyzer feed rates when any one is adjusted. Total cell room flow can be changed by changing the header pressure. The pressure is measured by a transmitter with a capillary and diaphragm seal. Wetted parts should be tantalum or PTFE. The range is usually 0-200 kPa. The control valve, located upstream of the pressure measurement, should be a fully lined fail-open butterfly valve. 

It would be desirable to add the acid and measure the pH in the piping between the cell room and the depleted brine receiver. However, the header should be designed to be self-venting, and even a horizontal section before the receiver may not run full of liquid. One alternative is to design the tank with a small baffled section for pH control. 

There are three destinations for the depleted brine the brine feed header, the chlorate destruction reactor, and the brine dechlorinator. The last of these is the main process flow. 

Flows to individual electrolyzers can be adjusted by manual valves accompanied by flow indicators, and the total flow can be regulated by the brine feed header pressure controller. The latter is the first step in making global changes as, for example, when changing brine flow to suit a change in the cell room current. Because the individual flow meter/electrolyzer combinations have different nonlinearities, this change may have to be followed by trim adjustments to some of the branches. 

Other control functions on the feed header include temperature and pH. The temperature control loop shown is quite straightforward. However, the cell temperature (or depleted brine temperature) is more important than the feed temperature. Therefore, the control setting must be manipulated to give the desired end result (Section 13.10.6.5). This can be done manually or by feedback of some average outlet temperature or the cell operating load. 

Figure 11.54 shows that the feed header is controlled at a selected pressure so that adjustment of the feed rate to any one electrolyzer does not affect the other flows. The pressure is measured with a Monel or nickel diaphragm seal connected to a pressure transmitter with a capillary system. The fail-open throttling valve is an all PTFE-lined butterfly, controlled by a reverse-acting, proportional-plus-integral controller. Control of the caustic flow to individual electrolyzers is by way of hand valves and local rotameters. This is similar to the arrangement described for brine feed and shown in Fig. 11.11. Again, bipolar systems are amenable to more complete automation.

Preparation for Cell Room Energization. Electrolyzers are isolated from header systems by blanks or slip plates during commissioning of the brine, caustic, chlorine, and hydrogen systems. Immediately before startup of those systems, all blank plates should be removed, leaving only valves closed where it is appropriate to isolate electrolyzers. There should be a record of all blank plates and slip-plates and their locations. This will help to ensure that all are removed before startup. 

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