February 2008

Special section: Flow/Level

Bringing pH measurement systems up to date

Improvements in sensor life and range of applications can move liquid analysis from one of the most "cursed" functions in the plant, to one that requires only modest attention

Fast Forward

  • If the flow and total acidity or basicity of the stream can vary by a factor of 10,000, use two reagent pumps.
  • The pH analyzer regulates flow and level to maintain a constant level of acid neutrality.
  • When both volume and flow of spent acid or base is high, use multi-mode control.
 
By George Pence and Richard Baril

pH measurement plays a vital role in virtually every major industrial processing industry, from  controlling chemicals in industrial scrubbers, to measuring sulfur dioxide in sugar refineries, to optimizing coagulation in water clarification. 

While pH measurement may seem like a very small part of a total industrial process, it can be the cause of frustration and downtime unless one understands, correctly applies, and controls it. 

This article will address the theory of pH control operation and why it is hard to measure, and look at developments in pH technology making the application of pH easier for plant managers and engineers. 

Starting with pH control

By definition, pH is the negative logarithm of the hydrogen-ion activity in aqueous solution. 

This means a solution having a pH value of four has 10 times more hydrogen ions than a solution whose pH is five. For control systems neutralizing spent acids and bases, pH value provides a control point for neutralization.

Acids and bases are either strong or weak depending on the amount of free hydrogen ions in the solution of a given concentration. For example, nitric acid is strong while acetic acid is weak. 

They have a very different pH value but still have the same total acidity, and therefore, each require the same amount of neutralizing base. 

Titration is the popular method for determining total acidity or basicity of a solution. 

It is necessary in the design of a pH control system to determine the size of the final control elements, particularly the element determining the flow of reagent, and titration is the method for determining this. 

An acid/base titration curve is a plot of pH versus reagent addition and graphically shows how pH changes per unit addition of reagent. It is also an indication of the degree of control obtainable. 

In general, neutralization of a strong acid with a weak base will result in better control than a strong acid/base combination.

The ideal hold-up time 

A pH control system measures the pH of a solution and controls the addition of a neutralizing agent (on demand) to maintain the solution at the pH of neutrality, or within certain acceptable limits. It is, in effect, continuous titration. 

At the heart of the pH-control system is a pH analyzer and one or more pH sensors. The structure of the control system around these instruments may vary considerably including: two-position (on/off) systems; dual-flow one-reagent systems; two-reagent systems; and two-stage systems. 

To select the best system for a given application, the engineer must consider a number of factors. 

Capacity is the ability of the overall system to absorb control agent without change in the process variable. In general, high capacity is favorable for effective control since it levels out abrupt changes and gives time for mixing. However, pH neutralizations are seldom high capacity since the basic nature of pH as a logarithmic function of concentration is capacity limiting. 

A second important consideration is hold-up time, which is required to provide time for the neutralization reaction to go to completion. This is particularly important when a dry feed or slurry is the control agent since the solids must dissolve before they react. 

If adequate mixing and agitation do not occur, the sensing pH electrodes will detect an incorrect pH and continue to call for additional reagent after the correct amount has already added in. 

As a rule of thumb, mixing should be less than 20% of hold-up time, so for example, if hold-up time is 10 minutes, turnover (mixing) should occur in less than two minutes. Increasing hold-up time increases capacity. 

Two final important factors in system design are transfer lag and dead time, both detrimental to effective control. 

Transfer lag results from the inability of the system to supply neutralizing agent instantaneously on demand. Dissolving time and poor mixing are transfer lags. Dead time is delay in any part of the system. Measuring element amplifier, signal converter, and controller lag are part of dead time. So are electrodes that do not respond rapidly to pH change because of coating. 

An engineer can choose a two-position system, one in which the element controlling reagent additions is either fully open or fully closed, for control of continuous processes where waste flow rate is relatively small and hold-up time is relatively large-five minutes or more. 

If the flow and total acidity or basicity of the stream can vary by a factor of 10,000, then two-reagent pumps are necessary. When the pH of the stream can vary from acid at one time to alkaline at another, then both acid and alkaline reagents are necessary. 

Further, if the concentrations of acid or alkali are greater than 1%, the system may tend to self-oscillate. In this situation, a two-stage method will minimize oscillations caused by overfeeding of a reagent. 

When both volume and flow of spent acid or base is high, it becomes impractical to provide the relatively long hold-up time required by two-position control. 

This situation calls for multi-mode control in which the neutralizing agent adds in continuously per the final control element. The amount of neutralizing agent depends upon the proportionality set up by the system and the controller. 

The ideal hold-up time with multi-mode control is relatively short-30 seconds to three minutes. Beyond 10 minutes, multi-mode control may not get better results than an on/off control mode.

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Advances in pH technology

The selection of the optimum sensor(s) and analyzer for a control system is as important as its proper configuration. 

pH measurement is not simple and furthering the problem is the fact that many, if not most, applications requiring pH measurement are challenging environments involving high temperatures and/or process elements that can foul, coat, or poison a pH sensor. 

In the past, this has resulted in the need for frequent cleaning and replacement of the pH sensor, sometimes once a day in very harsh circumstances, making the measurement costly in both equipment and time. 

While there is no perfect pH sensor yet today, the technology is advancing to the point that the pH sensor is a relatively trouble-free part of the process, so long as one selects carefully and performs appropriate maintenance too. 

The two major areas of advance have been extending the life of the pH sensors and extending their range of applications. 

The most obvious area of advance in pH technology is in the area of glass durability. New glass formulations and low-stress handling techniques provide exceptional resistance to thermal and caustic degradation. 

This translates to less breakage from thermal stress or shock and improved speed of response at near theoretical levels and minimal hysteresis for fast accurate calibrations even after months of service. 

In addition to the more durable glass, new sensor designs also mount the glass bulbs in protected tips that shield the glass from direct impact while in service or during calibration. 

For applications where glass-tipped sensors cannot serve, ion selective field effect transistor (ISFET) technology may be the best choice. 

The response time of these sensors can be as much as 10 times faster than traditional glass electrodes. This response combined with stability make ISFET sensors appropriate for use in cold processes like brine or water for cooling. 

Aging effects caused by temperature fluctuations are also less with ISFET sensors providing longer intervals between maintenance and calibration. However, the ISFET sensor does not typically survive in harsh alkaline or strongly acidic solutions as well as glass does. 

It is non-linear at the ends of the pH spectrum; hence, the ISFET sensor has a range of two to 12 pH versus zero to 14 pH for the glass electrode sensor. If the application requires fast response in a relatively benign environment, or does not allow the use of glass, then an ISFET sensor is the answer. 

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The reference challenge

Most pH measurements fail due to reference electrode problems. The most common of these are fouled or poisoned electrolytes and clogged reference junctions. 

New technologies in this area include improved double junction reference electrodes, designed to excel in specific harsh applications. 

The specially designed porous liquid junctions have a large surface area to maintain a steady reference signal in dirty fouling applications. 

A combination of large surface area and high porosity also minimize junction potentials leading to accurate measurements without standardization.

Interestingly, some of these new designs have come full circle from the all-disposable philosophy of past years to rechargeable reference electrodes, a design feature that was commonplace over 20 years ago. 

These reference electrodes are for ease of use and optimum performance. The reference electrolyte is an inert viscous gel that is unaffected by thermal or pressure cycling. 

After removing the junction, one can recharge the outer reference electrolyte using gel-filled syringes. These reference electrolytes optimize the sensor for maximum resistance to fouling in specific applications. 

Optimizing for applications

For high temperature environments, the electrolyte is a potassium chloride silica gel suitable for highly acidic, basic, or oxidizing solutions as well as high temperatures. 

In water applications where bio-films or algae grow on the sensor such as treated effluent outfalls, aeration basins, cooling towers, or influent water from lakes or rivers, a quaternary ammonium compound, safe for human consumption, is used. 

Poisoning-resistant electrolytes are typically oxidizing solutions that can work in refineries, pulp manufacturing, mining, and wastewater treatment. These solutions work against the chemicals that generally attack the silver wire inside the electrode such as sulfides, mercaptans, and cyanides. 

In industries where light oil and grease foul sensors such as refineries, food processing and many other industrial applications, the electrolyte manifests like a grease-cutting detergent. 

Since all oil applications will eventually foul the sensor no matter what the electrolyte, it is desirable to have a sensor with an easily replaceable junction rather than having to replace the entire sensor. 

Scaling-resistant electrolytes are acidic solutions that combat calcium magnesium salts like gypsum or hard water that may coat the electrode. Such problems occur in applications like limestone scrubbers in power plants and lime treatment in sugar processing. 

There are also applications in which any chloride in the reference gel would react with the process, forming insoluble precipitants. In these applications, including metals and mining and chemical processing, a completely different potassium nitrate gel is necessary.

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Keep it clean

Despite the best technology developments, sensors still need cleaning in dirty applications. 

Spray cleaners attach to the sensor and provide a periodic burst of cleaner onto the sensor electrode, removing the coating that can degrade performance and cause dead time. 

If cleaning is critical and the application is sufficiently benign to not require frequent sensor replacement then a mechanical or pneumatically activated retraction system may be used. 

These systems easily and automatically remove the sensor from the process for cleaning and then return it smoothly, reducing impact on employee time and reducing downtime. 

The systems are costly, so a cost/benefit analysis is appropriate; a long-life sensor that is reliable and easy to install may prove to be a better choice.

Many of these improvements significantly forward the operation of pH sensors in a wide range of applications. Over the last decade, more and more steps to build in sensor diagnostics that alert the operator before a critical failure occurs have come about. 

With advance notice of sensor degradation, orderly maintenance or shutdowns are now possible. 

Most analyzers today are modular in design, allowing the same analyzer to be used with a simple change of a circuit board for multiple applications (pH, ORP, conductivity, chlorine, turbidity, and others), which can significantly speed training and operation. 

In addition, many analyzers allow multiple inputs, permitting a single analyzer to control more than one sensor. 

Finally, pH analyzers can be equipped with advanced digital communications (HART, Profibus, Foundation fieldbus, and others) to allow integration with central databases eliminating "islands of automation," speeding record keeping and trouble-shooting, and complying with requirements in many industries. 

Such advances in pH technology today are something no plant manager can afford to ignore. Significant improvements in sensor life and range of applications can move this liquid analysis from one of the most "cursed" functions in the plant, to one that requires only modest attention. 

Greatly expanded communications and networking capabilities make the pH data collected an integral factor in the complete industrial process. The time to reevaluate pH functions in industrial applications is now. 

ABOUT THE AUTHORS 

George Pence is the instrument technician supervisor for the Water Quality and Operations Business Unit of the Los Angeles Department of Water and Power, serving drinking water to the 3.8 million residents of L.A. With 30 years experience in the industrial control and instrumentation industry, he supervises the service of instrumentation in 23 drinking water treatment facilities. Richard Baril (richard.baril@emerson process.com) is a manager at Emerson Process Management, Rosemount Analytical. He has been active in the drinking water, wastewater, and process industries for more than 20 years. He is an ISA and American Water Works Association member.

 

Terminology

pH is a measure of the acidity or alkalinity of a solution. Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic (alkaline). pH 7 is neutral, and it is the pH of pure water at 25°C.

ISFET is an ion-sensitive field effect transistor that measures ion concentrations in solution. When the ion concentration (such as pH) changes, the current flowing through the transistor will change accordingly.

Electrolyte is a substance containing free ions and that behaves as an electrically conductive medium.

 

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