Electrolyzer startup

The hydrogen tank pressure sensor switch controls according to preset values, the cutoff and startup points for the operation of the electrolyzer. 

When the pressure reaches the preset cutoff pressure, a signal is sent to turn the electrolyzer off. When it falls to the preset startup pressure, it will turn the electrolyzer on again. 

The switch itself is installed on the top of the storage tank. As the tank fills and pressure builds, the calibrated metallic bourdon tube bends. When the cutoff pressure level is reached, a mechanical lever is thrown and switches the current off. When the tank pressure falls to the startup pressure level, the mechanical arm is activated and the electrolyzer turns on again. 

Cell voltage measurement at low current density 1. Fast 2. Locates damaged membranes 3. Detects large holes 1. Only possible during startup 2. Only possible with bipolar electrolyzers... 

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. 

Before startup of the electrolyzers, the process systems relating to the supply of electrolyzer feed streams (feed brine, caustic, acid, demineralized water) and to the product streams (depleted brine, product caustic, chlorine, hydrogen) must be commissioned and brought up to operational status ... 

While temperature transmitters are checked and calibrated, sensors may not be checked directly. The operation of some, notably those on the outlets of the electrolyzers, cannot be verified by liquor circulation before startup. A common assumption is that thermocouples (T/Cs) and resistance temperature devices (RTDs) work correctly. If desired, they can be checked by a hot box, if one is available, or by immersion in a flask of hot water or other appropriate fluid. A hot box is a dry-heated temperature-controlled block with drilled holes to accept RTD or T/C sensors. No further calibration of the instrument is necessary after installation, and the DCS indication should be checked against a thermometer of known accuracy. 

Unlined carbon steel tanks are subject to atmospheric corrosion. It may be necessary to descale such a tank. After descaling, it should promptly be filled with caustic solution to passivate the surfaces. Caustic from unlined tanks generally should not be used to fill electrolyzers before startup. 

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. 

Caustic, similarly, should be flowing under control around the catholyte system, through the cell room headers and the electrolyzer bypass. Again, the flow should be appropriate to a current density of 2-3 kA m , and the caustic temperature at the electrolyzers should be about 50°C. A concentration of 30-32% is usually satisfactory in the case of NaOH. The caustic should meet all specifications, and its concentration and iron content should be checked just before startup. 

Electrolyzer Energization. During the cell room startup, it is important to have facilities readily available for analysis of key process streams. Necessary measurements include brine and caustic strengths (by specihc gravity), chlorine gas composition (by gas chromatograph or Orsat), brine pH, and fiee chlorine content of brine (by ORP). 

What one might consider the primary stage of acidification to pH 3 can be fully commissioned prior to cell room startup, and brine at pH 3 may be fed to the electrolyzers at startup and during off-load periods. It is important to allow for effective degassing (Section 7.5.6.2), which may occur in the feed brine tank or a separate degassing vessel. 

Restart of a cell room requires the same attention to detail as the initial start, but it can go more quickly. If electrolyzers have been drained, circulation should be restarted as described above. The cell temperatures for startup should be at least 75°C. Polarization can begin at about 50 C, but not until the hydrogen circuit has been purged properly. 

Every startup provides an opportunity to check the membranes for pinholes by monitoring the current/voltage relationships at low loads (Section 13.8.1.2) and also to check the k-factors of electrolyzers, modules, or cells. This information is valuable in planning future maintenance and cell renewal. 

In a monopolar cell room with a large number of electrolyzers, monthly analysis is usually satisfactory. The current efficiency of a newly installed electrolyzer should be determined soon after its startup. 

The anolyte and catholyte concentrations are maintained by adding more highly concentrated acid, typically 28% HCl, to the circulating streams. Both electrolytes are pumped through filters and heat exchangers to their respective head tanks. They then fall back to the electrolyzers. The concentrated acid is added to the return line between the head tanks and the cells. The heat exchangers remove the waste heat from electrolysis. They also serve as heaters when required (e.g., during startups). 

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