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Managing pH in high-temperature chemical applications is critical forconsistent product quality, efficient process reaction, and compliancein environmental regulations. But because accurate measurement requires immersing pH sensors in hot, strongly acidic substances for extended time periods, companies have traditionally had to replace them everyfew days.
Food processor Archer Daniels Midland (ADM), says it has found a pH sensor that can be used for months without replacement, and has applied the sensor in its ethanol production plant in CedarRapids, USA.
Ethanol is a high-alcohol, renewable fuel additive that helps gasoline producers meet environmental regulations. Maintaining accurate pH levels during ethanol production is critical to control the sulphur dioxide levels. Excessive sulphur dioxide in the ethanol corrodes engine components involved in gas distribution.
Making ethanol
Ethanol production involves fermenting and distillingstarch and sugar crops. In the early stages, liquefied starch is converted to sugar (dextrose), which is then fermented. Yeast is added tothe mash to convert the sugars to ethanol and carbon dioxide. The fermented mash, called “beer,” contains about 11 percent alcohol, as wellas all the non-fermentable solids from the corn and the yeast cells. The beer is then heated to 112ºC to evaporate the alcohol content, which is collected and condensed, and run through a rectifier to eliminatethe remaining water. The final product is 99.5% alcohol.
“We tried taking pH readings directly from the process, but the 112º temperature ate the sensors up,” says Lloyd Feickert, instrumentation supervisor at the ADM Cedar Rapids facility. “To reduce excessive sensorcosts we set up a slipstream arrangement, whereby we run a sample of the ethanol offline to cool it down to 60ºC, 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 thelower temperatures, pH sensors could last for weeks, but process conditions caused temperature fluctuations within the slipstreams. These reduced sensor life significantly. We had standardised on the Foxboro 871 rebuildable pH sensors, so we turned to them for the solution,” says Feickert.
High temperature pH sensors
To accommodate ADM’s application, the Foxboro Measurements & Instruments division of Invensys adapted its DolpHinTM pH sensor line to develop a field replaceable measuring electrode that could withstand severe temperature cyclingup to 121ºC.
The newly developed measuring electrode featuresa pH glass formulation that provides superior measurement stability, accuracy, and longer service in high-temperature applications. This pHglass also increases response speed up to five times. The electrodes are available in domed, spherical, or ruggedised flat glass. The domedglass electrode is for the harshest applications: temperatures up to 121ºC and extremes of chemical concentrations.
The spherical glass electrodes are for standard process applications up to 100ºC, andthe flat ruggedised glass electrodes are for applications where the process water contains solid materials with pH between 2 and 12 and temperatures up to 85ºC. All electrodes are interchangeable and their plug -in design facilitates quick and easy replacement to address changes in measurement conditions or application.
1000 per cent life increase
“Rebuildable sensors are definitely the most cost effectiveway to measure pH for our applications,” says Feickert. We have standardised on the Foxboro 871 line and now, with the addition of the hightemperature domed electrode, we have all the tools to cost effectivelyhandle 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 considerthat we use the domed sensor in four beer stills, as well as other processes within the plant, that’s an enormous savings.”
In addition to the product cost savings, ADM has significantly reduced labour and maintenance costs. “Every time you send a person out to work ona sensor, it’s at least an hour’s worth of labour. I estimate that wehave reduced time spent on changing electrodes over the course of a year from 36 hours per electrode to three hours per electrode,” says Feickert.
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