Fouling rates from a sodium sulphate : water solution in supercritical water oxidation reactors
Creators: name: Teshima, Paul
Publisher: name: University of British Columbia
Year: 2009

description: Supercritical water oxidation (SCWO) is a process for destruction of hazardous organic compounds in aqueous waste streams, and does not produce the environmental and/or publicity problems produced by the air emissions of waste incinerators. The process dissolves organics and an oxidizer into high pressure (25 MPa) and high temperature (300 - 400 °C) water. In a reactor, the temperature is increased (above 400 °C) and the hazardous compounds oxidize, and are converted to carbon dioxide, ash and water. Supercritical water oxidation has been proposed as a treatment technique for pulp sludges, United States Department of Energy special wastes, metabolic byproducts and specialized chemical waste streams. A number of practical difficulties prevent widespread commercialization of SCWO. An important problem is that salts dissolved in the waste streams become insoluble at supercritical conditions and precipitate out of solution in the reactor. These salts then deposit on the reactor tube wall leading to a flow restriction. The fundamental aspects of salt deposition are not understood. The solubility of common salts in supercritical water is not well documented, and the factors that influence deposition rates are not known. In order to design SCWO systems to handle fouling from salts, both solubility and deposition studies must be investigated. A computer program was written to aid in examining the deposition mechanisms of fouling from sodium sulphate, and their influences on friction and heat transfer. The program assumed fully developed flow in a horizontal tube, in which buoyancy was neglected, with Reynolds numbers in the range of 33000 - 180000. Four salt deposition models were incorporated into the computer program which examined the deposition rates from molecular diffusion compared to particle fouling. In each of the deposition models, it was assumed that there was no surface resistance to the attachment of salt molecules and/or particles. In one of the models, it was assumed that particles would nucleate and grow rapidly to a diameter of 2 urn. Another of the models assumed that nucleation was inhibited. A SCWO facility with a design water flow of 1 kg/min was constructed to study the salt deposition problem. Initial experiments discovered that solubilities of sodium sulphate at 25 MPa, and 380 to 500 °C appeared to match higher and lower temperature measurements by other researchers. Within the temperature error (± 1°C) of the solubility measurements, a rapid decrease in the solubility occurred at the pseudo-critical temperature (385.0 °C at 25 MPa). In the present work, salt deposition studies used outside surface temperatures of a fouled, heated tube as a method of inferring salt thickness profiles. Model predictions of fouling rates were reasonably close to the measurements. Predicted and measured peak salt layer heights were within a factor of two, but the location was not accurately predicted. The model predictions were most sensitive to the temperature error in the solubility equation derived from the experiments and the estimation of the properties of sodium sulphate used in calculation of the mass transfer coefficient. Additionally it was discovered that particle deposition was not important for the experimental conditions of this study, as the molecular diffusion rate kept the salt solution below saturation as it was heated. Comparing measured pressure drops with friction calculations, it was apparent that the salt deposit roughness was much less than the thickness itself.