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Besides picking the relatively low-hanging fruit in waste treatment systems, what is learned can be carried over to process pH loops to improve product quality. There is a significant opportunity for all pH systems to maximize the return on capital improvements by a performance-based intelligent design of equipment, piping, sensors and valves.
Process control has proven to be able to improve the process metrics (capacity, quality and efficiency) of an existing installation by operating closer to constraints (equipment or process limits). If you had knowledge of the possible improvements by process control during the design of a new plant, you could select equipment that both improves metrics and reduces project cost. A virtual plant provides the tool for exploration and demonstration, testing of the control system and its configuration, and training of operations and technical support groups for these opportunities.
A design criterion dating back to the 1960s cited one stage of neutralization is needed for every 2 pH units the influent is away from the pH target. Each stage was considered to be a well-mixed vessel with a minimum residence time of 10 minutes and a turnover time of less than 0.5 minutes. The guideline also recommended the tanks have significantly different residence times to prevent resonance. An influent at 1 pH and a target of 7 pH would necessitate three vessels with set points of 3, 5, and 7 pH. For a large waste flow (e.g., 1,000 gpm), the minimum residence time translated to large operating volumes (10,000, 50,000 and 250,000 gallons) and agitation power requirements. The installed cost of such a system can be several million dollars.
The model predictive control (MPC) did a much better than expected job of chasing the acid concentration.
Using a virtual plant, engineers can try out neat stuff such as:
Go to www.controlglobal.com/ Gregory K. McMillan is a regular Control contributor and a consultant with Emerson Process Management. Mark S. Sowell is a Process Control Fellow with Solutia.
Below is a list of additional reading on advanced control and measurement of pH and the use of simulation and “virtual” plants.
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