If you think you can shortcut proper pH design and, indeed you might get lucky, but I rather doubt it I would suggest that before you try to start up that design, you stock up on a six-month supply of both Prozac and Valium. I suspect you'll require both medicines, advises chemical engineering consultant Ben Horwitz of Penciless Process Design, Cleveland Heights, Ohio.
Lots of plant veterans undoubtedly would agree with Horwitzs admonition. While pH control often plays a crucial role in processing, achieving reliable readings remains challenging at many sites. After all, pH sensors frequently serve in aggressive chemical environments. Additionally, no other type of commonly used measurement covers such a wide range as pH does, says Gregory K. McMillan, an Austin, Texas-based consultant (and one of the instrumentation and control gurus for Ask the Experts).
Fortunately, the pH electrode can respond to changes as small as 0.01 pH (unit), state McMillan and co-author Robert A. Cameron in Advanced pH Measurement and Control, 3rd edition, published in 2005 by ISA, Research Triangle Park, N.C.
Thats impressive, especially since this responsiveness doesnt come from some new high-tech approach. The basic technology for measuring pH has been in use for over seven decades and uses conventional glass pH electrodes, notes Craig Schweitzer, product manager with Cole-Parmer Instrument Co., Vernon Hills, Ill.
Sensor and electrode often are used synonymously but shouldnt be, cautions John P. Connelly, sensor product manager for Invensys Process Systems Measurements and Instruments Division, Foxboro, Mass. The pH sensor contains both a reference electrode and a pH electrode, he explains. When technological advances within the sensor are discussed, electrodes also must be discussed, Connelly says. The speed of response and accuracy and reliability are determined by the design of the glass pH electrode. And advances certainly are taking place here.
Limitations of using glass electrodes are prompting other new developments, says Dave Joseph, Irvine, Calif.-based senior industry manager for the Liquid Division of Emerson Process Managements Rosemount Analytical unit. Conventional pH measurement requires electrodes with thin glass bulbs that allow diffusion and subsequent measurement of hydrogen ions, Schweitzer explains.
[However] the glass bulbs are prone to breakage by mishandling and/or particulates in the measurement stream. Solid-state or non-glass pH sensors, which also are called ion-sensitive field-effect transistors (ISFETs), have eliminated the glass, he notes. [These] can be stored and used in process where the sensor may not be constantly exposed to liquid. But, while ISFETs have been commercially available for approximately 10 to 12 years, they still arent used widely because of cost and potential damage by rapid cycling temperature changes, he adds.
Meanwhile, self diagnostics are beginning to play more of a role in pH sensors.
Gilead Alberta ULC needed sensors that could withstand the full pH range at its Edmonton, Alberta, facility, which custom manufactures active pharmaceutical ingredients and advanced intermediates for biopharmaceuticals. More and more, were finding that pH is a critical parameter that must be controlled to ensure the highest project quality in the highest yield, says Rob Pastushak, the plants senior technical supervisor. It was known that pH was critical, but we didnt have the instrument available to us with the reproducibility in accuracy and compatibility with process.
Gilead Alberta found a device Invensys Foxboro 871 pH sensor that meets its stiff criteria. Most, if not all, other sensors the company tried couldnt tolerate the process chemicals. But the 871, with its standard Ryton polyphenylenesulfide sensor body and custom-ordered Kalrez O-rings, did. Three of the 871s have seen service for about three years in organic and aqueous solutions associated with batch reactors. Monitoring can be one-time monitoring, or several times throughout the processing, Pastushak notes.
With these rebuildable sensors, end users like Gilead Alberta can reuse the sensor body, but interchange plug-in measuring electrodes that include flat and spherical glass pH, antimony pH and gold and platinum oxidation reduction potential, or ORP. The reference junction and reference electrolyte also are field replaceable.
With the 871, the plant gets reproducible accuracy of ±0.03 pH units. The accuracy that we require could be a fraction of a pH unit, Pastushak says, noting that the companys minimum accuracy is 0.1 unit or smaller. Before Foxboro, we were getting accuracies of ±1 pH unit on average.
The changeover not only boosted yields but also saved time. Typically, the cycle average was 10 days, including two days allotted for pH adjustment. But now, by getting the accuracy and reproducibility that we do, we can get through this [pH adjustment] in several hours, Pastushak notes. Annually, that means eight to 10 additional batches.
Savings in probe replacements are a big plus, he adds. With Foxboro, we change out a probe tip every year, not due to process failure, but to someone mishandling it Before Foxboro, we averaged one probe per every two batches. We sometimes were changing up to two probes per batch.
Eastman Chemical Company also is benefiting from longer sensor life at its Longview, Texas, complex, which produces 8.8 million pounds of chemicals and plastics daily. The company installed disposable style Foxboro DolpHin sensors (Figure 1) to manage acid-gas scrubbing there.