ModelingAndControl.com‘s Greg McMillan was giving his pH Control Solutions webcast a dry run yesterday and it was an opportunity I didn’t want miss. I always learn something when I’m around Greg. Greg will be conducting this live ISA web seminar on pH Control tomorrow, March 10, 2010 at 2-3:30pm U.S. Eastern (GMT -5) time. It’s not free, so visit the ISA webinar registration page to sign up.
It’s a chance to listen to Greg and ask questions about challenges and solutions to your toughest pH Measurement and Control solutions. He derived some of his thoughts from his book Advanced pH Measurement and Control, 3rd Edition.
Those that know Greg know he loves his Top Ten, David Letterman-style lists. In this presentation, he’ll share his top ten signs of a rough pH startup. I’ll share one of them. You know you’ve got a rough pH startup when the plant manager leaves the country.
pH poses measurement challenges in sensitivity and rangeability like no other. Normally, an instrument engineer considers a turndown ratio or rangeability of 100 to 1 as quite large. Try 100,000,000,000,000 to 1 for a pH sensor measuring a pH 0 (1.0 hydrogen ion/0.00000000000001 Hydroxyl ion concentration) to pH 14 (0.00000000000001 hydrogen ion/1.0 Hydroxyl ion concentration). It was enough zeros that I was losing count, so I cut and pasted from Greg’s presentation!
Another big challenge is the non-linear, s-shaped titration curves (pH versus reagent/influent ratio). If you follow the link for titration curves to the Wikipedia link, you’ll see a picture of this non-linear curve. Greg noted that the steep vertical part is deceiving. As you zoom in it’s actually another titration curve. As such, it’s critical to get numerical values and a sufficient number of data points around the setpoint. Greg describes various titration curve scenarios including weak acid/strong base, weak acid/weak base, multiple weak acids/weak bases, and strong acids/weak bases.
Greg describes how large savings in reagent is possible for the flat parts of the titration curves. pH sensor drift can have a large impact on the reagent calculations and Greg discusses the advantages of doing Feedforward flow control on the ratio of reagent to influent flow. The Feedforward control requires pH feedback correction unless the setpoint is in the flat part of the titration curve. He recommends using Coriolis mass flow meters and having constant influent and reagent concentrations.
He covers much more from the construction and operation of double-junction combination pH electrodes to the need for three pH probes and a mid-select algorithm to handle the natural drift in pH measurements. He offers many pH control strategy examples such as cascade, full throttle batch, linear reagent demand batch to name a few.
If you are fighting pH measurement and control issues at your plant, it may be worth the fee and time to hear Greg and have the opportunity to ask your questions of him directly.