Foxboro Develops Solution for pH Sensing in High Temperature Chemical Application

The Foxboro Measurements & Instruments Division of Invensys Process Systems has worked with Archer Daniels Midland (ADM) to develop a solution for pH sensing in high temperature chemical applications.

The application is high-temperature ethanol production at ADM’s Cedar Rapids, Iowa facility, where managing pH is critical for consistent product quality, efficient process reaction, and compliance with environmental regulations.

The ADM/Foxboro collaborative effort resulted in a pH sensor that extends service life from days to months without replacing, thus reducing equipment and labor costs, while maintaining high process quality Ethanol is a high-alcohol, renewable fuel additive that helps gasoline producers meet environmental regulations.

"We tried taking pH readings directly from the process, but the 235 degree temperature ate sensors up," says Lloyd Feickert, instrumentation supervisor at the ADM Cedar Rapids facility.

"To reduce excessive sensor costs we set up a slipstream arrangement, whereby we run a sample of the ethanol offline to cool it down to 140 degrees F, and measure that," says Feickert.

While the slipstream arrangement significantly extended pH sensor service life, inherent process conditions still caused frequent replacement of components in the rebuildable pH sensors.

"At the lower temperatures, pH sensors could last for weeks, but process conditions caused temperature fluctuations within the slipstreams," says Feickert.

"These reduced sensor life significantly. We had standardized on the Foxboro 871 rebuildable pH sensors, so we turned to them for the solution."

To accommodate ADM’s application, the Foxboro Measurements & Instruments division of Invensys worked with Feickert to develop a field replaceable measuring electrode that could withstand severe temperature cycling up to 250 degrees F.

The replaceable electrode incorporates patented technology from the award-winning Foxboro DolpHin™ pH sensor line.

"I heard about the DolpHin line of high temperature pH sensors and asked Foxboro if they could develop a similar product for rebuildable sensors," says Feickert.

"They studied my application and came back with a replaceable electrode that, after a cooperative testing period, increased our sensor service life from days to months."

The measuring electrode features a pH glass formulation that is designed to provide measurement stability, accuracy, and longer service in high-temperature applications.

This pH glass also increases response speed up to five times. The electrodes are available in domed, spherical, or ruggedized flat glass.

The domed glass electrode is for the harshest applications: temperatures up to 250 degrees F and extremes of chemical concentrations.

The spherical glass electrodes are for standard process applications up to 212 degree F, and the flat ruggedized glass electrodes are for applications where the process water contains solid materials with pH between 2 and 12 and temperatures up to 185 degrees F.

All electrodes are interchangeable and their plug-in design facilitates replacement to address changes in measurement conditions or application.

"Rebuildable sensors are definitely the most cost effective way to measure pH for our applications," says Feickert.

"We have standardized on the Foxboro 871 line and now, with the addition of the high temperature domed electrode, we have all the tools to cost effectively handle pH sensing throughout our plant."

"The domed 871PH electrode has increased pH sensor service life from 10 days to four months," says Feickert.

"That’s a 1000 percent increase. When you consider that we use the domed sensor in four beer stills, as well as other processes within the plant, that’s an enormous savings."

"Every time you send a person out to work on a sensor, it’s at least an hour’s worth of labor," says Feickert.

"I estimate that we have reduced time spent on changing electrodes over the course of a year from 36 hours per electrode to three hours per electrode."