Measurements and modeling of deposition rates from a near supercritical aqueous sodium sulfate solution to a heated cylinder

摘要

In the Supercritical Water Oxidation (SCWO) process, organic compounds containing heteroatoms such as S Cl or P are oxidized to the corresponding acid. In order to avoid corrosion, bases are therefore often injected into the reactor. The salts that are formed upon neutralization (sulfates, chlorides, phosphates etc.) have low solubility in SCW and consequently precipitate as solid phases. These salts can form agglomerates and coat internal surfaces, leading to plugging of transport lines and inhibition of heat transfer. The purpose of this study is to develop an understanding of salt deposition kinetics and nucleation phenomena in SCWO reactors. We provide experimental deposition rate data from a sodium sulfate-containing SCW stream to a heated cylinder and develop a predictive model which is buttressed by these data. We also discuss how the deposition rate is linked to the nucleation mechanism and what type of nucleation is most important in the experiments. For the experiments, the test section is a six-port chamber which is fashioned from a 1.91 cm (3/4 inch) diameter Swagelok cross. One port was used to mount a 5.08 mm diameter internally heated cylinder into the center of the chamber and the remaining ports provided fluid cross flow, visual observation capability and instrumentation access. Aqueous sodium sulfate solutions of 4 wt percent salt concentration were pumped at about 250 bar through preheaters that brought the solution to a temperature close to that at which precipitation occurs. The heated cylinder raised the nearby solution above this temperature, thus limiting deposition almost exclusively to the heated cylinder. The rate of deposition was observed to be of order 0.l gm/minute. Natural convection dominated transport at the conditions investigated and the observed deposition rates indicate that all the salt nucleated heterogeneously at the salt layer-solution interface.