Measuring pH Under Adverse Conditions

AT A GLANCE Flow rate control Final processing Retractable sensor Weekly calibration

Polychemie manufactures coagulant and solution flocculants for its parent company, SNF Inc. SNF is one of the world’s largest suppliers of water-soluble polymers, widely used for water and wastewater treatment and a broad range of specialty applications.

The Pearlington, MS, plant produces a monomer that’s a precursor for Poly DADMAC polymer (poly-diallyl, dimethyl ammonium chloride). The process takes place over two periods: one for charging raw materials and the other for ‘stripping.’

The measurement of pH has different roles during the two stages of the process. Over the course of this process, the reactor’s pH sensor is subjected to an extremely harsh and abrasive environment with temperatures up to 180 °F. Steam coils within the vessel’s jacket maintain the high temperatures.

Control functions

During the initial phase, two components flow into the agitated reactor, mixing with a third material already present. The measured pH controls the flow rate of one of the two compounds to maintain the required stoichiometric ratio of the raw materials in the reactor.

The pH measurement also comes into play during the final process phase. Here the process strips away water and residual organic material that’s not part of the monomer product. The plant recycles excess organic materials. During this phase, the reactor operates under vacuum at elevated temperature and with a tight tolerance on alkalinity (9-10 pH).

The pH value is used to control the rate of caustic added to the reactor to maintain the high alkalinity required for stripping. Salts formed during this phase account for the abrasive nature of the pH sensor’s environment. At the beginning of this phase, the sensor must respond quickly to the new alkaline pH level.

Signals from the pH sensor’s transmitter run to a computerized distributed control system (DCS). The DCS employs PID control algorithms to develop signals for the control valves that set the rate of raw material and caustic additions. The endpoint of a monomer batch occurs when an analysis of the stripping extraction sample reaches a predetermined value.

Early in the process, pH controls flow rate of component A, and later, flow rate of caustic.

Rugged sensor sought

Initially, Polychemie tried several types of pH sensors in the reactor, but none could withstand the harsh environment for very long. Shortly after installation, sensor response times slowed to a point that rendered the pH reading useless. Even flowing reference sensors with conventional glass and unique glass/steel measuring electrodes quickly degraded in the harsh conditions.

The TB557 pH sensor from ABB Instrumentation consists of a hot tap retractable sensor that’s inserted into and out of the reactor through a ball valve. This arrangement allows insertion or removal of the sensor without disturbing the process.

ABB supplies the sensor, Model TB557, complete with a ball valve assembly that bolts to a standard flanged port on the reactor. The sensor has a patented reference comprised of a matrix of immobilized KCl salt fronted by a porous Teflon junction. This ‘solid state’ reference is virtually immune to poisoning, plugging, and pumping problems encountered with conventional liquid, slurry, and gel-based references. Without this reference technology, the sensor would be quickly destroyed during the vacuum and extremely high pH process phases.

The sensor’s pH-sensitive hemispherical glass electrode is rated for a 0-14 pH range at temperatures to 140 °C (284 °F). Sensor body is made of Kynar (PVDF) strengthened by a titanium sheath for the harsh process conditions. Cable and temperature element are integrated.

ABB TB557 pH sensors have been used at the Polychemie plant for three years. Polychemie calibrates the pH sensors and transmitters from the reactors on a weekly schedule. To do so, technicians retract the sensor, close the ball valve, and remove the sensor for field calibration. Technicians check sensor and transmitter accuracies as well as response time.

For more information on the pH sensor mentioned in this article, visitwww.abb.com

Author Information

Kyle Becker is maintenance engineer for Polychemie.

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