Highly durable pH sensor delivers critical measurement

Production of neopentyl glycol requires tight control of pH in tough process conditions. For Sweden’s Perstorp Group, a robust and intelligent pH measurement system was needed to provide assurance of product quality.

Neopentyl glycol (NPG) is an organic chemical compound that is widely used in base resins in the manufacture of coatings and paints, and as an ingredient in synthetic lubricants.

One of the major producers of NPG is the Swedish Perstorp Group, a world leader in several sectors of the specialty chemicals market. Starting as a small family business over 130 years ago, it has developed many innovative products and now has interests including polyols, biofuels and feed additives. It employs about 1600 people and has manufacturing units in Asia, Europe and North America.

NPG synthesis involves an aldol condensation of isobutyraldehyde and formaldehyde, followed by a hydrogenation step. The pH level during production greatly affects final product quality and, in one particular stage, Perstorp technicians found that pH must be maintained between pH 8 and 9 or else the product becomes unusable. The technicians were finding obtaining reliable pH measurements in this step to be very problematic.

The 120°C temperature and oily process medium meant the lifetime of the various pH sensors they tried was no better than one week, despite regular cleaning. As a result, Perstorp approached Mettler Toledo to see if it could provide a better solution.

Mettler Toledo installed a system based on the InPro 4260 i sensor. This probe was selected for a number of reasons. It has a solid polymer electrolyte that provides good resistance to contaminating substances, and instead of having a diaphragm that would quickly become clogged, it has an open junction. These two features provide the extended sensor service life Perstorp were looking for. In addition, the InPro 4260 i includes Mettler Toledo’s Intelligent Sensor Management (ISM) technology.

One of ISM’s main benefits is predictive diagnostics that provide real-time information on sensor condition. The Dynamic Lifetime Indicator (DLI) tool uses past and present process conditions to predict the remaining reliable lifetime of the sensor. This means that Perstorp technicians can see from the display of the connected transmitter when the sensor will need to be replaced. The Adaptive Calibration Timer (ACT) also informs them when sensor calibration should next be performed.

Due to the DLI and ACT, technicians know well in advance when sensor maintenance or replacement will be required; therefore, they can plan maintenance accordingly.

Another valuable feature of ISM is sensor calibration away from the process. Using iSense software for ISM sensors running on a standard PC or laptop, the InPro 4260 i can be calibrated in any convenient location. Once calibrated, the sensors can be stored for quick exchange at the measurement point when required.

Testing of the system showed that reliable process control could be achieved if the sensor was installed in the medium for only five minutes out of every hour. Therefore, Mettler Toledo fitted the sensor to an InTrac 777 retractable housing, which allows removal of the sensor without process interruption or danger of the escape of medium. Once extracted, the sensor can be rinsed in hot water then reinstalled in the housing, ready for the next measurement.

Perstorp is very pleased with the performance of the system and the technical support and service Mettler Toledo provided during the project. The combination of a process-tolerant sensor, retractable housing and ISM has given Perstorp the dependable, durable, pH measurement system it needs to produce high-quality NPG.