(Editor's Note: There is a figure that accompanies this article that can be downloaded at the bottom of this page using the "Download Now" button)
In today’s competitive environment, chemical makers seeking an edge are examining every aspect of their operations to find better ways to meet regulatory requirements, enhance productivity and lower costs. These top-to-bottom efforts involve analyzing everything from plant floor data-collection devices and components to manufacturing process sub-systems, plant equipment, measurement and instrumentation, automation and controls, as well as manufacturing execution and enterprise IT systems.
In plant operations, one device that can represent significant cost is the sensor, particularly one used in a harsh application. Such sensors are critical probes that gather and deliver key data, and are often exposed to or immersed in corrosive solutions and subjected to high operating temperatures and other severe conditions. Sensors support reliable and safe operation, provide regulatory monitoring and validation, and ensure quality control, process efficiency and optimization. It is crucial that these instruments perform such tasks with accuracy, stability and a fast response time.
Eastman Chemical Co. chose Invensys’ Foxboro DolpHin Series pH-sensor line to address these issues in a gas scrubber at the company’s Longview, Texas, manufacturing complex. The new DolpHin Series sensors contributed to significant cost savings in maintenance and equipment by reducing cleaning, calibration and replacement requirements, as well as manufacturing supply costs.
Get a sense of the problem
Eastman Chemical Co.’s Texas facility manufactures more than 60 chemical and plastic products for sale to customers worldwide. The site employs more than 1,800 people to produce and ship 3.2 billion lbs of product annually — or 8.8 million lbs per day.
A gas scrubber at the plant uses a 20% sodium hydroxide caustic solution to remove hydrochloric acid from a flue gas stream before it is vented. To meet its own stringent environmental quality standards and to comply with Federal Resource and Conservation Recovery Act (RECRA) requirements, Eastman must ensure that the acid gas content in the vented stream is within acceptable levels.
It is critical to reliably measure the pH level of the caustic solution for effective scrubbing and meeting regulatory limits. Beyond environmental quality monitoring and control, minimizing caustic chemical consumption costs associated with the waste treatment process also depends on reliable pH measurement.
To monitor the pH level, the scrubber uses a two-probe redundant system to ensure continuous operation in the event one sensor fails. The probes are part of an automatic shutdown system that aborts the scrubbing operation if the pH is outside acceptable limits.
The harsh operating conditions within the scrubber often exact a toll on process pH sensors. These conditions include exposure to liquid caustic at an operating temperature of 185°F and a pH of up to 10.5.
Due to these severe conditions, the old pH sensors had to be replaced frequently, often two weeks after being installed — and as frequently as three times per week. Because the sensors could not withstand the environment, the readings were not accurate. As a result, the operators often added caustic unnecessarily, which resulted in high pH readings and increased use of caustic. The higher pH environment, in turn, would etch the glass on the sensors, resulting in inaccurate readings and faster rates of failure. The results were high manufacturing and maintenance costs.
“Frequent sensor failure caused unreliable pH measurements, which required additional samples to be analyzed by the control room operators,” says Wyatt Partney, senior control systems technician at Eastman’s Longview facility. “That resulted in increased loads on downstream processes.”
Finding a sensible solution
Eastman process improvement engineers working on the scrubber operation and the company’s plant-wide Process Control Group looked at a variety of options for upgrading the existing sensors. After learning that the current sensor manufacturer could not supply upgraded sensors, and other products were not suited for the job, Eastman Chemical decided to test the Foxboro DolpHin high-temperature pH sensors.
The DolpHin Series sensors were an ideal solution since they are designed for process industry applications that span a broad pH range, have strong chemical concentrations and high solids, operate at high temperature, and cycle from high-to-low temperature.
The sensor’s unique pH glass formulation provides measurement stability and accuracy, and longer service life in high-temperature applications up to 250°F (121°C). The pH glass also increases response speed up to five times and provides longer duty cycles compared to conventional sensors.
The sensor’s reference electrode construction includes an ion barrier to protect and stabilize the reference potential in harsh conditions. Both electrodes are packaged in a rugged mechanical housing that facilitates installation, removal, cleaning and calibration, which reduces maintenance time and costs.
Eastman was impressed with the performance of the DolpHin sensors. The patent-pending pH glass formulation increased the pH sensor life to six months — a dramatic improvement from the previous products that operated for three to four weeks at best. The longer life provided by the DolpHin sensors significantly reduced equipment and related maintenance replacement costs.
The DolpHin sensors also provide more accurate pH readings, ensuring that control operators can use the online pH measurements to optimize the efficiency of the scrubber operations. Since installing the new sensors, the amount of caustic the company uses has decreased 50%.
“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 optimized,” Partney says. “In fact, the DolpHin sensors were so successful in solving the problems of high costs, reliable pH measurement and process optimization for our scrubbers that we implemented them in a number of other demanding applications as well.”
Paul Aloia is instrumentation applications engineer at Summit Controls Inc., Plano, Texas. E-mail him at firstname.lastname@example.org.