Deposition of sodium sulfate in a heated flow of supercritical water

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Abstract
Wastes that can be treated by supercritical water oxidation often contain salts. Salts are almost insoluble under supercritical conditions and can result in severe fouling. A simple heat‐ and mass‐transfer model was developed and tested experimentally for sodium sulfate in a fully turbulent flow of water at 25 MPa. This model uses empirical heat‐transfer correlations to estimate mass‐transfer rates. The diffusion coefficient of the salt is calculated from the Stokes‐Einstein relation using a hydrodynamic diameter of 2 to 6 Å. New measurements of solubility showed that the solubility of sodium sulfate decreases by a factor of about 1,000 as the temperature increases from 380°C to 400°C. Salt deposition rates, inferred from the outside temperature of a heated test section, were reasonably close to the model predictions.