Virtual Control of Real pH

Virtual design reduces the hassles of pH control.

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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.

Adaptation of Process Model’s Waste Concentration
Figure 4
The model predictive control (MPC) did a much better than expected job of chasing the acid concentration.
The criterion ignored practical problems in reagent injection and valve design that could cause failure to meet performance objectives despite a large capital investment. A virtual plant can show how advanced control and multiple high-resolution control valves can enable an inline pH system (e.g. static mixer) and a single vessel to replace the three well-mixed vessels for this case and reduce the project cost by 50% or more.

Using a virtual plant, engineers can try out neat stuff such as:

  1. Elimination of the split-ranging of acid and base valves;
  2. Translation of the controlled variable from pH to reagent demand for excursions to the steep portion of the titration curve;
  3. Identification of titration curves;
  4. Diagnostics of production unit problems;
  5. Inferential measurements of concentrations and pH;
  6. Optimization of totalized flow ratios of reagent to influent streams;
  7. Translation of the controlled variable from batch pH to rate of change of reagent demand for linearization and bi-directional self-regulation;
  8. Full throttle set point response of a reagent demand controller (See item 10 in sidebar located at the end of this story).
  9. Virtual discovery is a reality for pH control systems,  (See item 11 in the sidebar located at the end of this story).

Go to 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.

Extra-Credit Reading

Below is a list of additional reading on advanced control and measurement of pH and the use of simulation and “virtual” plants.

  1. McMillan, Gregory K., “A Funny Thing Happened on the Way to the Control Room”
  2. McMillan, Gregory K., Plant Design Category
  3. McMillan, Gregory and Cameron, Robert, Advanced pH Measurement and Control, 3rd edition, ISA, 2005
  4. McMillan, Gregory K. and Cameron, Robert A., Models Unleashed – Virtual Plant and Model Predictive Control Applications, ISA, 2004
  5. Mansy, Michael, M. McMillan, Gregory, K., and Sowell, Mark, S., “Step into the Virtual Plant,” Chemical Engineering Progress, February 2002
  6. Boyes, Walt, Hebert, Dan, O’Brien, Larry, and McMillan, Gregory, “The Light at the End of the Tunnel is a Train,” Control, August, 2005, p.36
  7. McMillan, Gregory K., “Improving Process Loops,” Chemical Processing, October, 2007
  8. McMillan, Gregory K., “A Fine Time to Break Away from Old Control Valve Problems,” Control, November, 2005, p. 57
  9. McMillan, Gregory K, “One Man’s Story—Part 2,” Control Talk, Control, September, 2007, p. 95
  10. McMillan, Gregory, K. “Full Throttle Batch and Startup Response,” Control, May, 2006, p. 67
  11. McMillan, Gregory K, “One Man’s Story—Back to the Future,” Control Talk, Control, October, 2007, p. 91
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