Virtual Plant Provides Real Insights

Achieving good pH control can be tough. If a design objective is missed in any part of the pH control system, the system can fail miserably in meeting performance goals [1, 2]. Today, many pH control systems are operating in manual or severely oscillating in automatic. That’s really not too surprising because the extraordinary rangeability and sensitivity of pH measurement translates into exceptional requirements on equipment, piping, the control valve and measurement. For a true strong acid and strong base system, the slope of the titration curve and, hence, the process gain and sensitivity would change by a factor of 1 million. The mixing uniformity required to reduce pH noise from concentration gradients is way beyond the norm. Final reagent adjustments are so small that almost any dip tube or injection volume can create hours of deadtime in starting or stopping reagent delivery [1, 2]. The reagent control-valve-resolution requirement, which determines its rangeability and sensitivity, is 0.0001% for a single stage of neutralization; normally a valve resolution of 0.5% is considered adequate for process control [3].

Effective automatic pH control often should be possible — this belief led us to launch a project at Monsanto’s Luling, La., plant to improve a pH control system for wastewater.

Figure 1. Existing system -- Manual pH adjustment system required intensive efforts from operators. Click on illustration for larger image.

Getting out of the pit
The wastewater, from regeneration of a water demineralizer, now goes to a system that consists of a 40,000-gal. pit with manual pH control (Figure 1). Because the demineralizer was regenerated at most once a day and the pit could hold all of the water from that unit, the operator usually had a day or more to make the adjustment. Our project objective was to replace the pit with a vessel with secondary containment and automatic pH control, which we thought could perform at least as well as an operator.

Veteran operators could get the pit within the permissible range of 6 to 9 pH by first opening the control valve to fill the line and then squirting in reagent by opening the block valve at the pit — but it was a very intensive activity. The relatively new operators didn’t fare as well, as evidenced by the late night phone calls saying the pit was 90% full, the pH wasn’t in range, and the next regeneration or area cleanup and pumpout was imminent.

At first, it seemed that automating the batch pH adjustment would be simple — however, a more detailed look revealed major uncertainty and sensitivity issues. A sensor error of 0.04 pH at 2 or 12 pH could cause a charge error of 20%. Because of the inevitable inaccuracies in the pH measurement and changes in the pH titration curve, at least five charges would be needed to walk one pH unit at a time from 2 or 12 pH to 7 pH. The batch system would need an accurate temperature-compensated titration curve to compute the charge and then would have to wait the entire mixing time of a charge to see the full effect. The last adjustments of reagent with 50% sodium hydroxide and 93% sulfuric acid, needless to say, would be touchy. Coriolis flow meters with 1/8-in. bodies and integrated batch control could make the final reagent charges accurate enough, but plugging was a concern. The real deal-breaker was that two expensive 40,000-gal. tanks would be needed to provide an online spare system for inspection and repair. With an installed cost exceeding $1 million and real estate an issue, we decided to use a modeling and control study to explore innovations.

The operators were using less than $2,500/yr. of reagent; so our goal was to minimize project cost while still meeting the objectives of safe, responsible (i.e., no downstream effluent excursions outside 6 to 9 pH), and reliable operation (i.e., ability to operate automatically despite a single failure anywhere in the equipment or control system). We wanted a control system that would be operator friendly and not require operator intervention. We also realized that the future is continuous pH control — could it meet project objectives and, if so, what size of tanks would be required? According to the traditional guideline, three stages of neutralization are needed because the influent could be below 2 or above 12 pH. Could better reagent injection methods, high-resolution control valves and advanced control reduce the size and number of tanks?

Virtual plant
We opted for a virtual plant to help develop the control system, investigate design details, test whether a prototype would meet project objectives, demonstrate the results to the mechanical, process and automation engineers, enable operator review and input, check out the control system configuration, and train operators for better understanding and operability [4, 5, 6, 7, 8]. Test runs of the virtual plant were used to study the dynamics of the system to guide the decision process and detail the equipment, piping, valve, measurement and control system design.