Process control cell room

Typical other parts of the plant apart from the cell room are storage facilities for starting materials and end products, equipment for safety and environment protection, purification systems for products and by-products, facilities for recycling of solvents and electrolytes, energy supply, and process control. 

The heart of all electrochemical processes is the cell room this contains the electrochemical cells, the piping connecting the cells to the electrolyte containers and their pumps, the rectifiers and power distribution, the process and safety control, and, last but not least, parts of a building. 

Following development of pneumatic field control systems came the idea that a control house was needed to incorporate all controls for a process into a central room. Pneumatic transmitters, including the development of dP cells for flow measurements, provided the next step in instrumentation and moved the local process control room into a central control house for the entire plant. 

Control of electrolyte flows is just one aspect of proper distribution across the entire electrode surface in the cell room. Achieving the desired total flow and the proper flow to each electrolyzer is part of process design. The steps taken to ensure good distribution internally are part of electrolyzer design, discussed in Chapter 5. 

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. 

Diaphragm process. Control of the cell room pressures is most difficult with this system, because the chlorine and hydrogen pressures are so close to atmospheric and to each other. Both hydrogen and oxygen are present in the chlorine, but the use of modified and synthetic diaphragms has greatly reduced their concentrations. 

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. 

Operator Analyses. The operator analysis schedule (Table 13.5) will depend to some extent upon the cell room configuration, the use of feed brine acidification, and the amount of analytical equipment provided. If the last item is limited, manual analysis will form a primary basis of control. If on-line instrumentation (e.g., density meters) is provided, operator analyses serve as checks on the correct functioning of the instruments. The analyses and frequencies given in the table are typical for a multi-electrolyzer bipolar cell room. The plant operators will also carry out routine analyses for process control purposes in other areas, as given in Table 13.6. 

The aspects of the operation of mercury cells that typically differ from those of the other processes are the brine circulation system, the cell room, treatment of the products, measurement and control, and reduction of mercury emissions. 

Negative Feedback. Some of the neurotransmitter diffuses back to the surface of the nerve cell that released it. There are also receptors that tit the neurotransmitter here. When a neurotransmitter binds a receptor (called an autoreceptor) at the axon terminal of the nerve cell that released it, it tells the nerve cell that there s plenty of neurotransmitter already in the synapse. So don t release anymore This process is called negative feedback and is analogous to the way a thermostat works in your home to control room temperature. 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

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