In today’s ultra competitive environment, chemical industry manufacturers seeking an edge are examining every aspect of their operations in order to find better ways to meet regulatory requirements, enhance productivity and lower costs. These top-to-bottom efforts involve analysing everything from sensor to boardroom: from plant floor data collection devices and components to manufacturing process sub-systems, plant equipment, measurement and instrumentation, automation and controls, and on up through manufacturing execution and enterprise IT systems.

Within plant floor operations, one device that can often represent significant cost is the sensor, particularly in harsh applications such as chemical and petrochemical processing. Often exposed to or immersed in highly corrosive solutions and subjected to extremely high operating temperatures and other severe conditions, sensors are the critical front line probes that gather and deliver key data. Sensors support reliable and safe operation, provide regulatory monitoring and validation and ensure quality control, process efficiency and optimisation. And these devices must do so with accuracy, stability and fast response time.

Eastman Chemical Company turned to Foxboro’s Dolphin Series pH sensor line to address these issues at a gas scrubber at the company’s Longview, Texas, manufacturing complex. These new sensors contributed to significant cost savings in maintenance and equipment by reducing cleaning, calibrating, and replacement requirements, as well as manufacturing supply costs (Fig.1).

Eastman Chemical Company’s Texas operations manufacture more than 60 major chemical and plastic products for sale to customers worldwide. The operation employs over 1800 people to annually produce and ship 3.2billion pounds of product – or 8.8million pounds per day. Eastman’s gas scrubber uses water and caustic in a 20percent sodium hydroxide solution to remove hydrochloric acid from a flue gas stream before it is vented.

To meet Eastman’s own stringent environmental quality standards and to comply with Federal Resource and Conservation Recovery Act (RECRA) requirements, the company must ensure that the acid gas content in the vented emissions is within acceptable levels. Reliable measurement of the pH level in the caustic solution is critical for effective scrubbing, and meeting the regulatory limits. Beyond environmental quality monitoring and control, minimising caustic chemical consumption costs associated with the waste treatment process also depends on reliable pH measurement.

Frequent replacement

To monitor the pH levels, the scrubber uses a two-probe redundant system to ensure continuous operation in the event of a single sensor failure. The probes are tied to an automatic shut down system that aborts the scrubbing operation if the pH is outside acceptable limits.

Operational conditions within the scrubber present significant challenges for process pH sensors. These conditions include exposure to the liquid caustic at a nominal operating temperature of 185ºF and pH of up to 10.5.

Due to severe conditions in the scrubber, the old pH sensors had to be replaced frequently – often within two weeks of installation and even as frequently as three times per week. Because the sensors could not withstand the environment, readings were not accurate, which caused operators to have to add caustic, resulting in higher pH values and driving up the use and cost of this manufacturing supply. The higher pH values, in turn, would etch the glass on the sensors, resulting in inaccurate performance and faster rates of failure. The end result of this snowball effect was unacceptably high manufacturing and maintenance costs.

“Frequent sensor failure caused unreliable pH measurements which required additional samples to be analysed by the control room operators,” noted Wyatt Partney, senior control systems technician, Eastman’s Longview facility. “That resulted in increased loads on downstream processes,” he added.

Eastman process improvement engineers working in the scrubber operation and the company’s plant-wide Process Control Group looked at a range of options to upgrade the existing sensors. After determining that the next generation product from the existing sensor supplier was not available, and other options were not adequate for the job, Eastman Chemical tested the new Foxboro Dolphin high temperature pH sensors.

Designed specifically for process industry applications that span the entire pH scale, have strong chemical concentrations and high solids, run at high temperature, and cycle from high-to-low temperature, these sensors were an ideal solution.

The sensor’s novel pH glass formulation provides superior measurement stability and accuracy, and longer service life in high temperature applications of up to 250ºF (121ºC). The pH glass also increases response speed up to five times and provides longer duty cycles, compared to conventional sensors.

The sensor’s new reference electrode construction includes an ion barrier to protect and stabilise the reference potential in harsh conditions. Both electrodes are packaged in a rugged mechanical housing that facilitates installation, removal, cleaning, and calibration, reducing maintenance time and costs.

The results of Eastman’s Dolphin sensors testing were very impressive. The sensors’ patent-pending pH glass formulation increased pH sensor life to six months – a dramatic improvement from the previous products that were operating properly for three to four weeks at best. The significantly longer life also reduced sensor equipment and related maintenance replacement costs eightfold.

“With the Foxboro Dolphin pH sensors, Eastman’s equipment and maintenance costs were eight times lower than with the previous sensors, and the efficiency of our scrubber operation was optimised,“ Partney noted.

The sensors also provide more accurate pH readings, ensuring control operators can use the on-line pH measurements to optimise the efficiency of the scrubber operations. And the sensors have resulted in a 50percent decrease in the amount of caustic used as a result of inaccurate pH level measurement that caused operators to continuously add the sodium hydroxide scrubbing solution.