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Coordination of Process Areas

To achieve best overall plant operation, it is often necessary to coordinate the operation of equipment in multiple process areas. In Chapter 16 of Control Loop Foundation – Batch and Continuous Processes, several process examples are shown to demonstrate how multi-loop techniques may be effectively used for such coordination. One of these examples is ammonia plant H/N control. The efficient operation of an ammonia plant (described in Chapter 10 of Control Loop Foundation – Batch and Continuous Processes) depends on the hydrogen-to-nitrogen ratio in the synthesis loop at the back end of the process being tightly maintained. However, this ratio is determined by the gas and air feed to the primary and secondary reformers at the front end of the plant. Thus, the controls needed to maintain the H/N ratio require coordination between these plant areas, as illustrated below.

The throughput of the plant is set by the rate of natural gas flow—primarily methane, CH4—to the primary reformer. Steam must also be added in a fixed proportion to the gas flow: A steam/carbon ratio controller is used to regulate the steam flow based on the gas flow to the primary reformer. Air added to the secondary reformer is the source of nitrogen used in ammonia production, with the air flow requirement based on the gas flow to the primary reformer. The ratio between air flow and gas flow is maintained using an air/gas ratio control. Any changes in the operation of the secondary reformer, and equipment before the synthesis converter will impact the H/N ratio in the feed stream to the synthesis loop. Thus, a measurement of the H/N ratio before the synthesis loop may be used to automatically adjust the air/gas ratio. The H/N ratio in the synthesis loop is maintained at setpoint by regulating the setpoint of the H/N ratio controller before the synthesis loop. Through this control, it is possible to maintain a precise H/N ratio in the synthesis loop. The control module provided for the ammonia plant H/N control workshop is shown below.

By accessing the book’s web site, you may complete ammonia plant H/N control workshop using your web browser. The viewer below may be used to see the solution to this exercise.