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Modelling of ClO2-decomposition in Hot Water Systems
Zotz, M. and Glas, K.

Recently an accumulation of corrosion failures of stainless steel equipment having to do with chlorine dioxide have led to increased interest in the topic of the disinfectant?s corrosive properties. So far, studies have mostly focused on the corrsion risk of temporary disinfection steps as usually applied during CIP-processes. However, analyses of corrosion defects in the brewing industry show that a significant number of chlorine dioxide related failures occur in pipes and tanks of hot water circuits. In these areas, influences like high temperatures and stagnant flow conditions can favour pitting corrosion despite lower concentrations. But while electrochemical analysis can only reveal conditions at the time of sampling, these corrosion problems can only be understood if hydrochemical means are considered. The aim of this study was to investigate the stability of chlorine dioxide in hot water distribution systems of brewery plants. For this purpose decomposition kinetics were analysed in laboratory experiments with brewing water and reverse osmosis water at 40??C, 60??C and 80??C. Experimental data was modelled using a second order kinetic approach. Moreover the hydrochemical calculation program PHREEQC was used to simulate changes in water composition due to dosing and decomposition of chlorine dioxide. Results showed that chlorine dioxide decomposition in reverse osmosis water was considerably slower than in brewing water and exhibited less temperature dependency of the reaction rate. Over a period of 48 h a decrease of only 25?% of initial ClO2-concentration was observed in 80??C hot reverse osmosis water. PHREEQC simulations revealed that dosage of acidic chlorine dioxide solution to the mostly unbuffered water caused pH to sink to 4.3 which inhibited decomposition. Because of the combination of constantly high redox potential and acidic pH-values chlorine dioxide treatment of decarbonized waters presents a potential corrosion risk to stainless steel equipment when heated and stored in water distribution networks.

Descriptors: chlorine dioxide, decomposition, water distribution systems, water treatment, PHREEQC, corrosion

BrewingScience, 70 (March/April 2017), pp. 51-56