COMPARISON STUDY OF CORROSION BEHAVIOR AND SEDIMENTATION OF TWO CONVENTIONAL AND OZONE METHODS IN WET COOLING TOWERS AT SEMIINDUSTRIAL SCALE
|M. Ghazi Mirsaeed1, A.Ataei1, A. Torabian2, A.H. Hassani1 and J.Hashmiyan3
|Received: 15 February 2014 Accepted: 20 March 2014|
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Wet cooling tower water is continuously exposed to airborne organic materials, and the buildup of bacteria, algae, fungi, and viruses presents hazards to the tower system. Corrosion occurrence is also probable due to steel-made equipment. Chlorine, as an oxidizer is generally used to inhibit the growth of bacteria and microorganisms and also used to annihilate them in the cooling towers. Chemical Inhibitors are used in order to decrease corrosion, sedimentation and microorganism activity difficulties. Ozone has a stronger oxidization capability than chlorine and destroys the organic materials and bio-films. A cooling tower in semi-industrial scale was built for investigations on corrosion and sedimentation issues in cooling towers. It was equipped with an industrial heat exchanger, a Corrater and a corrosion test rack. Important chemical and operational parameters in the pilot plant are monitored using HMI. In the first forty-five-day period, chlorine with a concentration of 0.5-1 mg/lit was used with conventional chemical inhibitors and in the second period, due to the stronger disinfecting quality of ozone, it was used with a concentration range of 0.05-0.15 mg/lit, While the concentration of chemical inhibitors was also reduced. Anti-corrosion and anti-sedimentation materials dosage was reduced down to 40 percents and biological distributor materials and non-oxidizing biocides were eliminated. Corrosion and sedimentation issues were investigated by means of coupon testing. Daily corrosion rate was also measured via Corrater device. Both ozone and chlorine are known as powerful oxidizers that may destroy the passive layer and cause cathode corrosion when in high concentration or when remaining in the system. In spite of being more powerful, ozone is more harmless due to the facts that its concentration ratio to chlorine was 0.1 and its residence time was only 10 minutes while chlorine remains in the system for 6 hours (Bach wise method). Hence, uniform and pit corrosion rates had been decreased for ozone disinfection. The daily corrosion results obtained using the instantaneous corrosion monitoring device Corrater was confirm using the results of coupon testing. The average corrosion rate for chlorination and ozone disinfection periods were calculated to be 0.028 mm/y and 0.012 mm/y, respectively. However, sedimentation did not demonstrate any differences.