Advanced pH measurement and control

This comprehensive treatment of applied instrumentation and control covers all the bases and is presented to more fully educate the industry in basic theories and technologies.

By Gregory K. McMillan and Robert A. Cameron


he pH electrode offers by far the greatest sensitivity and rangeability of any industrial process measurement. To realize the full potential of this opportunity requires extraordinary performance of mixing equipment, control valves, reagent delivery systems, flow meters, control system design, and controller tuning. This book, Advanced pH Measurement and Control, is designed to cover all of the bases but not get the user lost in the details. Since each of these topics could be a book in itself, the emphasis here is on cutting straight to the chase of concepts critical for a successful pH system. Key insights and rules of thumb are inserted to congeal principles and practices that address the essential requirements of pH applications. The book should appeal to both the generalist and the specialist. This book is designed to bring together the chemist, control engineer, instrument technician, process engineer, and project manager to literally and figuratively put them on the same page.

Chapter 1 – The Essentials provides a concise overview of what makes pH applications unique. It covers the total system requirements and is designed for the widest possible audience. It uses an adventurous style and analogies that should help make this chapter interesting enough to read from start to finish. The other chapters follow up on the seeds
planted here and are intended to be used more as a reference for the design of each part of a pH measurement and control system.

Chapter 2 – The Chemistry gets into the engineering units and the conversion of the abscissa of the titration curve into ratios that are useful for pH system analysis and design. It tackles the effect of the crowding of ions at higher concentrations on pH. This should help a control engineer or technician communicate with the process engineer and chemist and vice versa, effectively use a lab titration curve, and understand how pH is affected by the concentrations of acids, bases, and salts and process temperature. The chapter concludes with development of charge balance models that coupled with Coriolis flow meters enable the development of
fast and reliable online pH estimators.

Chapter 3 – Titration Curves discusses how the steepness of the slope in the operating region determines the difficulty of the application and how it changes with composition. Accurate titration curves are the key to pH system design, analysis, online pH estimators, online process efficiency indicators, and real time optimization. Yet they are fraught missing or
misleading information and notorious for graphical deception. The goal of this chapter is to deal with these issues and detail the requirements to generate a useful titration curve in the laboratory, by a charge balance model, or online in the field.

Chapter 4 – Electrodes is the heart of the text because ultimately it all begins and ends with performance of the measurement. If you don’t get this right, nothing else matters. This chapter starts with a proper dose of reality to help the user sort out fact from fiction. Reality is not pretty but it can prevent a lot of false expectations and the consequential astonishments and atonements. This chapter proceeds with a survey of the myriad of measurement and reference electrode designs. The hard data to date is primarily in buffer solutions or developed by the manufacturer. There is a serious need for testing by a third party of all major electrode designs for the wide range of conditions and requirements encountered in the food and beverage, biochemical and pharmaceutical, the chemical and petrochemical, and water treatment processes. Armed with as much information that can be gleaned from assumptions made and features described in the literature and personal application experience, the chapter proceeds to provide as much guidance as possible in the selection, installation, calibration, and troubleshooting of electrodes, and the use of smart transmitters.

Chapter 5 – Mixing Equipment makes sure the biggest capital investment in a pH system is wisely spent. The chapter is designed to get the control engineer, mixing specialist, and process engineer in tune with the dynamics and extraordinary mixing requirements demanded by exceptionally steep titration curves and incredibly small reagent flows. This chapter focuses on the implications of the magnitude of a plug flow volume and its size relative to a back mixed volume. Simple equations are presented to estimate the dead time, time constant, and the attenuation capability of a volume. This should help the user pick the correct type and
size of mixing equipment and recognize when an existing pH loop is doomed to failure.

Chapter 6 – Control Valves deals with the many significant issues that arise from trying to precisely throttle a tiny reagent flow. It focuses on the challenges of manufacturing tolerances, transitions between turbulent and laminar flow, plugging, and stick-slip when trying to make extremely small changes in a flow that might already be the size of a leakage flow for normal sized valves. The chapter shows how meeting the resolution spec found from the titration curve can minimize the cost of mixing equipment and generally satisfy the rangeability requirement. It also discusses how to minimize the nonlinearity, stick-slip, and dead time introduced by split ranging. It concludes with a discussion of the problems and solutions in
the application of metering pumps and lime feeders.

Chapter 7 – Reagents delves into reagent delivery system design in terms of adding dead time and a high-level view of the characteristics of common reagents. It provides some general guidelines on the strength, phases, and consistency of reagents. Physical property handbooks and databases are the best source of more detailed information on reagents.

Chapter 8 – Control Systems digs into the first love of the authors. For us, the book saves the best for last. It covers the basics of feedback, feedforward, and cascade control and then launches into the more sophisticated techniques such as linear reagent demand, adaptive, model predictive, and advanced batch control. It introduces the new opportunities afforded
by online titration curves for online pH estimators, and real-time optimization. The chapter realizes that these topics may only be of passing interest in the route to a successful pH application so the reader is directed to the references noted for the details of the technologies. The chapter then dives into equations and methods for estimating controller tuning and loop performance and concludes with a summation of the best practices and the logic for choosing the best control strategy and technology. Appendices G and H were added as a finishing touch to summarize the guidelines and the use of the enclosed computer programs that can help the user achieve the best design.

ISA’s Independent Learning Modules
This book is an Independent Learning Module (ILM) as developed and published by ISA. The ILMs are the principal components of a major educational system designed primarily for independent self-study. This comprehensive learning system has been custom designed and created for ISA to more fully educate people in the basic theories and technologies associated with applied instrumentation and control.

Paul W. Murrill, Consulting Editor, ILM Series

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