ISA Interchange

Welcome to the official blog of the International Society of Automation (ISA).

This blog covers numerous topics on industrial automation such as operations & management, continuous & batch processing, connectivity, manufacturing & machine control, and Industry 4.0.

The material and information contained on this website is for general information purposes only. ISA blog posts may be authored by ISA staff and guest authors from the automation community. Views and opinions expressed by a guest author are solely their own, and do not necessarily represent those of ISA. Posts made by guest authors have been subject to peer review.

All Posts

How to Obtain Concentration Measurements from Common Inline Sensors

This guest post was written by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). 

 

Ultimately what we want to know and control is the concentration of key components. Relatively inexpensive inline measurements can be used under the right conditions to provide concentration measurements. Understanding the limitations as well as the capabilities is essential to success. Here we look at how we can elevate the use of capacitance, conductivity, density, pH and turbidity to provide process knowledge to control biological and chemical reactor concentration.

The inferential measurement of the concentration of a component (e.g. reactant or reagent) in a feed, exit, or recycle stream or vessel is possible if the component of interest has a large definitive effect on capacitance, conductivity, density, pH, and turbidity. To infer composition in vessels, sensors are installed in nozzles on the vessel, in recirculation lines, or sample lines in close proximity to vessel.

If the component of interest has a higher conductivity than other components or solvent, conductivity can be used if the conductivity is restricted to be always on one side of the peak in the conductivity curve. The process gain changes sign as an operating point crosses the peak, which is disastrous to a control loop because the PID control action sign must be the opposite of the process action sign (assuming valve action is correctly configured).

 

If the PID has the wrong sign, the PID output will ramp to its output limit. Plots of conductivity versus concentration have a peak if the entire concentration range is covered. A few ionic species such as sulfuric have two peaks. The second peak for sulfuric acid occurs at 93 percent concentration.

The measurement and control of biological and chemical reactors is the key to product quality and the yield and production rate of most processes in the process industry. See Greg McMillan's ISA book Advances in Reactor Measurement and Control for an extensive view of practical opportunities for building and effectively using online estimators to improve process knowledge and control.

For product conductivity that traverses across the peak, concentration control cannot start until the conductivity is well to the right of the peak. For a very low concentration of a single acid and base (e.g. < 0.01 percent) pH provides a more sensitive inferential measurement of concentration.

Insight: One or more signal characterizer blocks are used to give a piecewise linear fit that enables the online computation of the X axis ( percent concentration) from the Y axis of a conductivity plot or pH titration curve generated from samples at various operating conditions.

For conductivity and pH, a signal characterizer is commonly used to compute the X axis (e.g. salt, acid, or base ion concentration) from the Y axis (conductivity or pH). The 20 or more data points of signal characterizer is usually enough to provide a piecewise linear fit. The points are more closely packed in the operating regions of greatest interest or nonlinearity. A cascade of characterizers can be used to provide greater resolution where a secondary signal characterizer is added to the output of the primary characterizer. A simple temperature correction should be generated based on the results of samples that cover the entire possible range of temperatures including abnormal operation.

Insight: The effect of process temperature on the actual conductivity and pH of the sample must be measured and used in customized solution pH or conductivity temperature compensation.

For a two component liquid mixture where the density the components differ by more than 1 percent an extremely accurate concentration measurement is attainable by the use of a Coriolis meter. The accuracy of the density measurement in Coriolis meter is 0.0002 gm/cm3. There is no drift and installation effects are essentially negligible. The meter never needs recalibration. There are no upstream and downstream straight run or field calibration requirements. High performance meter designs are now able to measure the concentration of bubbles and solids as well. The use of Coriolis meters on raw material feeds provides not only an incredibly accurate true mass flow measurement independent of composition but also an inferential concentration measurement from density.

Insight: An extremely accurate and drift free measurement of liquid density by Coriolis meter can be used as an inferential measurement of concentration in a two component mixture.

Turbidity offers an inferential measurement of biomass concentration for bioreactors. The turbidity measurement does not distinguish the number of cells or how many are alive (viable) or dead (lysed). The addition of a capacitance probe to provide a dielectric spectroscopy can provide inferential measurements of cell size, homogeneity, and membrane integrity. Viable cells have membranes intact whereas lysed cells have holes or fractures in their membranes.

Insight: Dielectric spectroscopy in combination with an inferred measurement of biomass from turbidity can provide inferential measurements of cell size, and the relative concentration of viable (live) versus lysed (dead) cells.

The inferential measurement of concentration must be periodically corrected by taken a fraction of the error between a synchronized computed concentration and an at-line or off-line analysis result. The synchronization is achieved by passing the change in concentration through dead time and integrator blocks with the proper sign of process feedback to match process dynamics and then adding the change to an original corrected value and passing the result through a final dead time block representing the analysis delay.

To get the most out of inline meters and probes, fundamental relationships should be used to compute concentration measurements. The computed concentration measurements must be gradually corrected by comparing the computed concentration synchronized with an analysis result somewhere and sometime whether in the field, plant lab, or offsite lab.

Greg McMillan


About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

Connect with Greg:
LinkedIn

Greg McMillan
Greg McMillan
Greg McMillan has more than 50 years of experience in industrial process automation, with an emphasis on the synergy of dynamic modeling and process control. He retired as a Senior Fellow from Solutia and a senior principal software engineer from Emerson Process Systems and Solutions. He was also an adjunct professor in the Washington University Saint Louis Chemical Engineering department from 2001 to 2004. Greg is the author of numerous ISA books and columns on process control, and he has been the monthly Control Talk columnist for Control magazine since 2002. He is the leader of the monthly ISA “Ask the Automation Pros” Q&A posts that began as a series of Mentor Program Q&A posts in 2014. He started and guided the ISA Standards and Practices committee on ISA-TR5.9-2023, PID Algorithms and Performance Technical Report, and he wrote “Annex A - Valve Response and Control Loop Performance, Sources, Consequences, Fixes, and Specifications” in ISA-TR75.25.02-2000 (R2023), Control Valve Response Measurement from Step Inputs. Greg’s achievements include the ISA Kermit Fischer Environmental Award for pH control in 1991, appointment to ISA Fellow in 1991, the Control magazine Engineer of the Year Award for the Process Industry in 1994, induction into the Control magazine Process Automation Hall of Fame in 2001, selection as one of InTech magazine’s 50 Most Influential Innovators in 2003, several ISA Raymond D. Molloy awards for bestselling books of the year, the ISA Life Achievement Award in 2010, the ISA Mentoring Excellence award in 2020, and the ISA Standards Achievement Award in 2023. He has a BS in engineering physics from Kansas University and an MS in control theory from Missouri University of Science and Technology, both with emphasis on industrial processes.

Books:

Advances in Reactor Measurement and Control
Good Tuning: A Pocket Guide, Fourth Edition
New Directions in Bioprocess Modeling and Control: Maximizing Process Analytical Technology Benefits, Second Edition
Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance
101 Tips for a Successful Automation Career
Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology, Fourth Edition
The Funnier Side of Retirement for Engineers and People of the Technical Persuasion
The Life and Times of an Automation Professional - An Illustrated Guide
Advanced Temperature Measurement and Control, Second Edition
Models Unleashed: Virtual Plant and Model Predictive Control Applications

Related Posts

ISA Landmark Turnaround Fuels Future Growth

In our 2023 annual report and in our recent State of the Society speech at the 2024 ISA Automation Summit...
Prabhu Soundarrajan Oct 8, 2024 7:00:00 AM

High Integrity Pressure Protection Systems (HIPPS)

High integrity pressure protection systems (HIPPS) are used in the process industries to reduce the risk ...
Ankeet Anil Kaji Oct 4, 2024 7:00:00 AM

Highlighting Cybersecurity Awareness Month with ISA's Cybersecurity Library

The start of Cybersecurity Awareness Month in October offers a timely reason to refocus on securing criti...
Kara Phelps Oct 1, 2024 7:00:00 AM