Emerson Improves Chlorine Monitoring At DOW

DOW Benelux is using Emerson Process Management's total chlorine analysers to improve the monitoring and control of chlorine in cooling water prior to its return to the sea. The greater accuracy and speed of the new system enables easier compliance with environmental regulations. Maintenance costs have also been reduced as the amperometric sensor requires less checking and cleaning than the previously installed sensor. The DOW Terneuzen site has 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 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Model TCL sample-conditioning system replaced an existing DPD analyser system that was unreliable, had become outdated and was costly to run.

The DPD analyser relied on the measurement of colour change following the injection of a chemical reagent. Because of the impurities in the water, the accuracy of the measurement was poor and also an operator had to visit the analyser on a daily basis to either clean or reset it. Eric Engelen, of DOW Terneuzen's process analytical department, said: 'Measuring chlorine content in water from the Westerschelde is completely different from measuring chlorine in pool water. '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,' he added. The 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 analyser measures the current and converts it into an equivalent total-chlorine concentration. This method of measuring chlorine is ideal for measuring water containing impurities and is better suited to the application at DOW. '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. 'This makes it easier for us to meet our legal requirements, which is better for the environment and prevents fines being incurred,' explained Engelen.

The previous analyser required a daily visit from an operator for it to be either reset or cleaned. The Model TCL doses air to assist in iodine formation; this creates a self-cleaning effect, meaning the sensor requires almost no additional cleaning. This leads to lower operational costs than for other sensor types. As well as the amperometric sensor, the Model TCL system includes Emerson's Rosemount Analytical model 1056 dual-input analyser. The 1056 analyser is easy to install and start up and its extensive diagnostic functions make it possible to monitor the status of the sensor and prevent any unplanned downtime.