Claxton, C., Rodman, D., and Sheehan, M. (2018) Influence of ethanol and sulphite dosing on oxygen scavenging in sugar mill boiler feedwater. In: Proceedings of the 40th Annual Conference of the Australian Society of Sugar Cane Technologists. pp. 333-341. From: ASSCT 2018: 40th Annual Conference of the Australian Society of Sugar Cane Technologists, 17-20 April 2018, Mackay, QLD, Australia.

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Abstract

REDUCING CORROSION IN boiler feed water circuits is important to maintaining efficient thermal performance as well as minimising maintenance. Corrosion is typically controlled in boiler feed water through the addition of oxygen scavengers such as sodium sulphites, which act to reduce the dissolved oxygen (DO) concentration in the water. However, poor dissolved oxygen removal when using sulphite oxygen scavengers has been observed throughout the North Queensland sugar industry (Rodman et al., 2016). The cause of reduced oxygen scavenging capabilities of sodium sulphite is hypothesised to be a result of organic contaminants present in sugar mill boiler feedwater, such as ethanol. Previous literature does not provide sulphite oxidation kinetics at typical sugar mill boiler feedwater conditions. The primary focus of this work was to assess the effect of ethanol on sulphite oxidation in the boiler feedwater environment. In this research, a small-scale boiler feedwater system was designed and used to quantify the effect of ethanol on sulphite oxidation rates. Sulphite oxidation rate was quantified using first order reaction kinetics (R2<0.97), with data obtained using a DO analyser. The experimental design was based on ANOVA procedures by considering ethanol concentration and sulphite to dissolved oxygen dose ratio as independent variables, and the first order oxidation rate constant as the dependent variable. To mimic typical industry conditions ethanol concentration was varied between 0 and 300 ppm while the molar sulphite to dissolved oxygen ratio was adjusted from 1:1 to 2:1. Experiments revealed that sulphite oxidation is significantly inhibited by ethanol contamination at typical boiler feedwater conditions. Increased sulphite dosages reduced the effect of ethanol, but were unable to overcome the decrease in oxidation rate. It was found that at recommended sulphite dosages, sulphite oxidation rates were reduced by 48% and 68% at 150 ppm and 300 ppm ethanol respectively. An empirical model correlating oxidation reaction rate to sulphite dosage and ethanol concentration was determined and correlations were high (R2<0.986).

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