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Macro-characteristics and micro-mechanisms of Na2SO4 precipitation dissolved by KNO3 molten salt in a continuous supercritical water system

Peng Feng,Donghai Xu,Youwei Zhi,Bing He,Shuzhong Wang,Yaling He,Yang Guo

This work aims to explore the ability of molten salt to solve salt deposition in supercritical water (SCW) related technologies including supercritical water oxidation and supercritical water gasification, with KNO 3 and Na 2 SO 4 as examples. In the pure KNO 3 solution, the two-phase layering of high-density KNO 3 molten salt (settling at the reactor bottom) and low-density saturated KNO 3 -SCW salt solution (flowing out at the top outlet of the reactor) was formed in a kettle-reactor with about 6.5 ratio of depth to inner diameter, thereby improving the accuracy of measured solubilities. The precipitation macro-characteristics of mixed KNO 3 and Na 2 SO 4 in SCW were investigated under different feed concentration conditions. The results showed that Na 2 SO 4 deposition on the reactor sidewall could be reduced by more than 90 % when the mass ratio of KNO 3 to Na 2 SO 4 in the feed was only 0.167. No visible salt deposition was observed on the sidewall when the ratio was 0.374. All solid deposited salts were converted into the liquid molten salt as the ratio reached 3.341, and thus could easily flow out of the reactor, without plugging. ‘Molten salt dissolution’ mechanism may provide a more plausible explanation for mixed KNO 3 and Na 2 SO 4 in SCW. In addition, the precipitation micro-mechanisms of mixed KNO 3 and Na 2 SO 4 , and the critical conditions of avoiding sidewall deposition and reactor plugging were proposed. This work is valuable for overcoming the salt deposition problem in SCW-related technologies.