Electrolyzer Shutdowns

The intermittent operation of electrolyzers powered from a wind turbine is characterized by a dynamic power fluctuation with periods of varying overload, partial load, and shutdown. The operation of the electrolyzer below 10% of its nominal power remarkably reduced the current efficiency and the purity of the product gases, inducing the shutdown of the electrolyzer, which was programmed at a safety limit of 2 vol% hydrogen-in-oxygen [52],... 

In alkaline electrolyzers, Ni is the only elemental cathode that can be used. It is generally considered as a fairly good electrocatalyst, but in facts it exhibits two shortcomings (i) its activity decreases with time [cf. the AVtterm in Equation (7.16)] especially under conditions of intermittent electrolysis and (ii) shutdown of industrial cells (for maintenance) leads to Ni dissolution at the cathode since this electrode is driven to more positive potentials by short-circuit with the anode. These shortcomings can be alleviated if Ni cathodes are activated, that is, if they are coated with a thin layer of more active and more stable materials. Activation has been attempted with a variety of materials from sulfides to oxides, from alloys to intermetallic compounds. 

Tosoh Co. developed a Ni-Fe alloy cathode containing 20-85 at% Fe in the alloy. This coating, formed by electroplating [190], showed a hydrogen overvoltage of about 120 mV at 4kAm in 32.5% NaOH at 90°C. It was claimed to offer resistance to current interruptions, based on the constancy of the voltage of a bipolar electrolyzer (BiTAC) at 3.23-3.25 V over 250 days with 4 shutdowns. 

Chloride concentrations are typically less than 50 ppm (as NaCl on 50% NaOH basis). The chloride content decreases as current density increases, electrolyzer temperature decreases, caustic concentration increases, or anolyte concentration increases. During current interruptions, the rate of chloride diffusion through a membrane is about five times higher than during operation. Cooling the electrolyzer is the most effective way to reduce the diffusion of chloride ions during shutdowns. These measures are also helpful in controlling the chlorate levels in the catholyte. 

Repairs to an unit cell require shutdown of entire electrolyzer, thoriiy reducing production... 

Leakage currents can be short circuited [64] or biased by placing an opposite current via electrodes placed in the piping [40] or reacted at sacrificial electrodes located in the manifolds [65]. These concepts have been used in the manifolds of MBC electrolyzers of Chlorine Engineers and also in the design of the CME and BiTac cells. Corrosion of catalytic cathodes used in bipolar water electrolyzers during shutdowns is prevented by cathodic protection [9]. 

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. 

Between the electrolyzers and the cooling plant, the gas is sometimes boosted in pressure. This improves performance and helps to reduce contamination by atmospheric air but requires special equipment. The gas will be transported in large pipes, which may have special provisions for the removal of condensate. There will also be some means of pressure relief in case of a shutdown in the gas process. This usually is in the form of a water-filled seal that vents to a caustic scrubber. The pipework conventionally is FRP. It was discussed in some detail in Section 8.4.1.1 A. 

General. Normal operating and monitoring routines for the individual electrolyzers, the cell room, and the associated plant are to be established and followed carefully. Process parameters should remain within the specified ranges. When upsets occur or an important parameter moves out of specification, the appropriate actions, which may include shutdown of the plant, must be taken. 

We also consider the shutdown of individual electrolyzers for maintenance. 

Detailed shutdown procedures vary significantly among technologies. The above should be considered general principles, and the procedures recommended by electrolyzer suppliers should be followed carefully. Some of the differences in procedures are worth highlighting here ... 

Emergency Shutdown. Emergency shutdown procedures should consider the possibility that power and instrument air may not be available. Plant design must include provisions to ensure safety and environmental protection and to prevent damage to the electrolyzers or membranes. These may include ... 

An emergency shutdown should automatically initiate all critical actions. These include those listed in the previous section. Typical minimum requirements for electrolyzers equipped with polarizing rectifiers are ... 

Shutdown of Individual Electrolyzers for Maintenance. In larger monopolar membrane cell rooms, individual electrolyzers may be removed by using an on-load shorting switch to bypass the current around the electrolyzer to be removed. In some small monopolar cell rooms, the entire circuit is shut down, an electrolyzer removed, and a replacement electrolyzer fitted. This practice is justified by the small loss that is sustained in production and the floor area and capital expense that are saved. Shutdowns for cell removal are infrequent, and the change can be made in about 4 hr. 

Full plant shutdowns are necessary in bipolar cell rooms that contain only one electrolyzer. Since work must be carried out on the installed electrolyzer to change modules, electrodes, or membranes, downtime can be significant and is a function of the amount of work to be done and the design of the electrolyzer. Bipolar cell rooms with multiple electrolyzers may have one of several different configurations. Several electrolyzers may be fed in parallel from a common rectifier (common in mercurycell conversions), or individual electrolyzers may be equipped with their own rectifiers (technically, the best modem practice). Electrolyzers also may be made up of a number of packs or frames arranged in electrical series. Section 8.3.1.3 discussed some of these combinations. 

Although there may be some latitude in operation, as the membrane surface will remain alkaline because of the hydroxyl ion flux, the situation becomes more critical when hydroxide ion transfer stops at shutdown. Acid addition must then stop immediately. Additional protection may be obtained from monitoring voltage rise on electrolyzers. The rise caused by overacidification will be rapid, but rapid action to stop acid addition may obviate damage. 

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