The fate of chlorine in recirculating cooling towers Field results |
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Authors: | G. Holzwarth R.G. Balmer L. Soni |
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Affiliation: | Exxon Research and Engineering Co., Clinton Township, Annandale, NJ 08801, U.S.A. |
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Abstract: | Chlorine and chloramines are volatile compounds which are stripped (“flashed off”) from recirculating cooling water systems by the large volumes of air which flow through the water in the cooling tower. The fraction of a volatile gas, such as hypochlorous acid (HOCl), which is removed by stripping is determined by Henry's constant H for that gas: H = XG/XL, where XG is the mole fraction of the gas in the air and XL is the mole fraction of the gas in the water. We have measured H for HOCl, OCl−, NH3, NH2Cl, NHCl2 and NCl3 at 20 and 40°C. We found H = 0.076 for HOCl, compared to 0.71 for NH3, at 20°C. At 40°C, H was about 2.5-fold larger for HOCl. This means that 10–15% of the HOCl is stripped from cooling water on each passage through a typical cooling tower. The measured flashoff of free available chlorine (HOCl + OCl−) was markedly pH-sensitive with a pK of 7.5, exactly as expected if HOCl is volatile but OCl− is not. The data permit a quantitative understanding of the fate of chlorine in cooling systems. The values of H at 40°C for NH2Cl, NHCl2 and NCl3 were 1.28, 3.76 and 1067. This means that all of the chloramines are quickly stripped in a cooling tower. |
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Keywords: | chlorine volatility cooling tower water Henry's constant chloramines flashoff HOCl (hypochlorous acid) NH2Cl NHCl2 NCl3 |
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