Total chlorine analyzers from Emerson Process Management are being used by Dow Benelux to improve the monitoring and control of chlorine in cooling water prior to its return to the sea. The improved accuracy and speed of the new system eases compliance with environmental regulations. In addition, maintenance costs have been reduced because the amperometric sensor requires less checking and cleaning than the previously installed sensor.

The Dow Terneuzen site in The Netherlands is the second largest Dow production facility in the world, with 23 plants producing plastics and chemicals. The site is located on the southern shore of the Westerschelde estuary from which it extracts sea water for cooling. To control the microbiological activity in the sea water - preventing blockages and fouling - minimal amounts of chlorine are dosed. The concentration of the chlorine must be monitored and controlled precisely before the water is returned to the estuary to ensure compliance with environmental legislation.

Austin, Texas-based Emerson Process Management’s sample-conditioning system replaces an existing DPD analyzer system that relied on the measurement of color change following the injection of a chemical reagent. Because of the impurities in the water, the measurement accuracy was poor, and an operator had to visit the analyzer daily to clean or reset it.

“Measuring chlorine content in water from the Westerschelde is completely different from measuring chlorine in pool water,” explained Eric Engelen of the process analytical department at Dow Terneuzen. “The water is salty and contains algae, mussels, seaweed and sand.”

Emerson’s Rosemount Analytical Model TCL uses a reagent-based amperometric method to measure total chlorine. A potassium iodide solution injected into the sample reacts with total chlorine to produce iodine. The iodine diffuses through a membrane on the end of the sensor. An electrochemical reaction inside the sensor consumes the iodine and generates a current directly proportional to the diffusion rate. The analyzer measures the current and converts it into an equivalent total chlorine concentration.

This method of measuring chlorine is suited for measuring water containing impurities - and better suited to the application at Dow.