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Deposition of sodium chloride from supercritical water Filipovic, Danijela

Abstract

Supercritical water oxidation (SCWO) is a new technology that promises successful destruction of organic material from hazardous aqueous wastes without harmful emissions. SCWO is made possible due to the special properties of supercritical water (SCW). The very same properties of SCW are also responsible for the low solubility of inorganic salts (coming from processed waste streams or formed during reaction) and consequent salt precipitation and fouling of the system. There are several proposed or implemented systems that attempt to solve the fouling problem by either removing the salt from the stream prior to the reactor or preventing deposition in the reactor itself. Designing a solution to the fouling problem in a SCWO system is closely related to the understanding of the deposition mechanism. Deposition from sodium chloride-water system was the subject to this study because of the system's phase behavior and because no previous deposition studies had been performed on it. Sodium chloride deposition was investigated in a fully developed turbulent flow of SCW through a horizontal, electrically heated test section of the 1 l/min UBC/NORAM SCWO plant. The solubility measurements performed at 24.1, 24.45 and 25.67 MPa and temperature 461-560 °C agreed with the semi-empirical Martynova- Galobardes-Armellini solubility model that assumed equilibrium between solid and vapor as a solvation type reaction. Various flow conditions and concentrations that were higher than the vapor concentration at the three-phase equilibrium were used in deposition experiments. Sodium chloride-water solution in the bulk passed through the two-phase vapor-liquid region before the vapor-solid region was entered. There were two distinct regions of deposition observed, one in the vapor-liquid region and one in the vapor-solid region. The heat transfer coefficient increased by 1-6 kW/m²-K (20-75 %) when a salt solution was introduced to the system. The salt thickness profiles were inferred from outer surface temperatures. The average porosity of the deposit in the vapor-liquid region was calculated as 0.1 and of the deposit in the vapor-solid region as 0.6. These porosities could be uncertain to ±50%. Heat and mass transfer were modeled in both regions. The buoyancy effects for pure water and salt solution were neglected. Three deposition models were developed for vapor-liquid region. The first model assumed molecular mass transfer from the bulk to the wall from both vapor and liquid phases with the same mass transfer coefficient. The second model assumed mass transfer from the vapor phase only (liquid phase frozen). The third model assumed mass transfer from the vapor phase with the vapor and liquid phases in equilibrium (infinitely fast mass transfer between phases). A model assuming combination of molecular mass transfer and particle deposition as a deposition mechanism was developed for the vapor-solid region. All models assumed no surface resistance to molecules or particle attachment. A comparison between experimental results and model predictions showed that the deposition in vapor-liquid region was governed by the mass transfer from the vapor phase with some mass transfer from the liquid phase involved. Deposition in the vapor-solid region was greatly affected by the amount of liquid phase remaining just before the three-phase temperature was exceeded.

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