Formation of Oxalate (and Phosphate) in the LP Saturated Steam of a Combined Cycle Plant During a Period of High Phosphonate-Based Antiscalant Dosage into a RO Feedwater – Part 2: Assessing the Influencing Factors

ABSTRACT

This work suggests that iron played a critical and dual role in determining the amount of oxalate (C₂O₄^2–) and phosphate (PO₄^3–) that formed and persisted in the low-pressure saturated steam from the thermal breakdown of phosphonate which somehow entered the water/steam cycle. The first is a catalytic role in a thermal oxidative and/or hydrolytic breakdown of the phosphonate, which is shown clearly by the strong and steep linear relationships with C₂O₄^2– and PO₄^3– as dependent variables from bivariate analyses involving subsets of the full data at the early stages of the incident when the iron concentration was low (≤ 47 µg ⋅ L^–1). The phosphonate concentration for this scenario was presumably high as it resulted in a large yield of the two breakdown products. Second, iron may have also played the role of a counter-ion in precipitating sparingly soluble iron(II,III)-C₂O₄^2– and iron(II,III)-PO₄^3– salts and/or as a catalyst in the conversion of soluble C₂O₄^2– into lower molecular or gaseous products like CO₂, which was observable in the more gradual linear relationships with C₂O₄^2– and PO₄^3– from the bivariate analyses, which involved most of the data throughout the incident period where the iron concentration covered a wide range ≈ 5 to 5 835 µg ⋅L^–1. The temperature and pressure dependence of C₂O₄^2– and PO₄^3– formation and transformation in the low-pressure saturated steam due to phosphonates shows shifting correlations (positive/negative) at a threshold temperature of 160 °C and threshold pressure of 504 kPa, and coincides with the drum inlet temperature threshold of 137 °C. However, there is evidence that C₂O₄^2– and PO₄^3– could be formed and persist in the low-pressure saturated steam beyond the threshold temperature (160 °C) without deposition and/or decomposition when in lower concentrations (projected as ≤ 62µg⋅L^–1 and ≤ 134µg ⋅L^–1, respectively, based on a previously proposed C₂O₄^2–/PO₄^3– molar ratio of ≈ 0.5 for their formation from the phosphonate).

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