Cooling Water's Chlorine Controlled
June 3, 2010
To improve the monitoring and control of chlorine in cooling water prior to its return to the sea, Dow Benelux has begun using total chlorine analyzers from Emerson Process Management, Austin, Texas.
The new system offers greater accuracy and speed, which eases compliance with environmental regulations. Maintenance costs also have been reduced because the amperometric sensor requires less checking and cleaning than did the previous sensor.
The chlorine analyzers are in use at Dow’s Terneuzen site in The Netherlands, which 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 process 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 precisely monitored and controlled before the water is returned to the estuary to ensure compliance with environmental legislation.
Emerson Process Management’s Rosemount analytical model TCL sample-conditioning system replaced an existing unreliable DPD analyzer system that was outdated and costly to run. The DPD analyzer relied on the measurement of color change following the injection of a chemical reagent. Because of the impurities in the water, the accuracy of the measurement was poor, and it required an operator to either clean or reset the analyzer every day.
“Measuring chlorine content in water from the Westerschelde is completely different from measuring chlorine in pool water,” says Eric Engelen of the Process Analytical Department at Dow Terneuzen. “The water is salty and contains algae, mussels, seaweed and sand. This not only influences the accuracy and reliability of the measurement, but it meant that the old sensor had to be cleaned almost daily leading to high maintenance costs.”The Model TCL uses a reagent-based amperometric method to measure total chlorine, which works well for measuring water containing impurities.
“Water pollution and temperature changes don’t influence the reliability of the sensor and because of the faster and more accurate and reliable measurements, we are now able to spot if there is something wrong with the chlorine content much earlier,” Engelen says. “This makes it easier for us to meet our legal requirements, which is better for the environment and prevents fines being incurred.”
The criteria Dow uses to test new technology is stringent. Only technology that has clearly demonstrated it is easier to maintain or use, improves reproducibility and reliability, resists fouling or cuts costs is adopted within the plant.
”We noticed immediately that the Model TCL’s reagents lasted for more than three months and that’s a long time,” says Jos Baart from Cegelec, which was the installing contractor.