Forestall pHrustrations

Eliminate operating problems and maintenance costs in pH measurement and control.

By Dave Joseph, Emerson Process Management, Rosemount Analytical

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Let's say you're trying to neutralize acidic industrial waste before discharging it into the plant waste treatment system. Laboratory results don't match the values of the on-line pH analyzers (Figure 1), and you're spending more than you should on chemical reagent for neutralization. Swings of more than 3 in the pH value, possibly caused by extreme temperature changes from day to night, confound your control system. Maintenance costs are high because you're cleaning and calibrating the pH sensors constantly, and plant waste treatment personnel are complaining about your discharge stream.

What do you do in this type of a situation and in other difficult pH control and measurement applications? Read on to find out.

One reason laboratory results don't agree with on-line analyzers' readings might be temperature. When pH is checked by taking grab samples to a laboratory or measuring them with handheld devices, frequently the temperature differs from that at the sensor sampling point.

The pH of a solution can change due to the effect of temperature on the dissociation of weak acids and bases, and especially the dissociation of water. Any solution with a pH of 7 or above will show some degree of temperature dependence. How much depends on the composition of the solution and the process temperature extremes.

The best option is to use a pH analyzer with compensation routines that account for solution pH changes due to temperature. You simply enter the temperature coefficient of the solution (pH change per °C) into the analyzer. If the solution's composition changes, however, the temperature coefficient might require adjustment.

Higher temperatures also affect the amount of reagent necessary. For example, a soap manufacturer was trying to neutralize a solution that was pH 9 at 60°C, but the pH reading would overshoot and drift compared with grab samples. At 25°C, neutralizing 1,000 liters of the waste stream required 10 ml of caustic solution. At 60°C, it took 100 ml — ten times as much.

Water's dissociation constant is much higher at 60°C. The reagent appears less effective because there's more hydroxide ion to neutralize at the elevated temperature. Temperature and nonlinearity are common causes of overshoot in pH control. A bad sensor location or not enough holdup time also can prompt overshoot problems. Table 1 lists common culprits for pH measurement difficulties.

High temperatures can degrade the pH sensor itself so selection of the installation location demands care. For example, pH control is used to prevent corrosion in boilers by keeping the water slightly alkaline. Unfortunately, no pH sensor on the market today can withstand the temperatures found in typical industrial boilers. Therefore, the pH sensor is placed in a cooled and pressure-reduced side stream. In addition, monitoring highly pure boiler water requires specialized sensors.

In a beer-making process, water pushes green beer (wort) to a whirlpool to prepare for fermentation. Normally, electrical conductivity measurements determine the phase interface between the push water and the beer. However, some beer products have conductivity levels similar to that of water. In those cases, pH measurements can be used to detect the phase interface, because beer has a significantly different pH value than push water or clean-in-place (CIP) chemicals like sodium hydroxide.

The wort typically is 90°C, clean-in-place chemicals are 80–90°C, and the push and rinse water are at ambient temperature. This sharp temperature change produces thermal stress that can cause the glass electrode to crack and the sensor to fail. The sensor also must withstand the hot caustic phases, which decrease its sensitivity to pH changes. Finally, the sensor must tolerate high flow velocities.

In these and other problematic cases, you may need to consult with a pH sensor supplier. Sensors are available for almost any application, but it's often difficult for an end-user to match the best specialty pH sensor with the particular application. For example, a sensor that works fine in boiler water service wouldn't suit beer-making, and likely would fail in a dirty process with suspended solids.

The pH scale is logarithmic, so when a solution approaches neutrality (pH = 7), a small addition of reagent can cause a large change in pH (Figure 2). The further the solution is from 7.0, the more reagent is needed to adjust pH.

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