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Deposition of sodium carbonate and sodium sulfate in supercritical water oxidation systems and its mitigation Khan, Mohammad Sultan

Abstract

Supercritical water oxidation (SCWO) is a technique to destroy wet organic waste. The oxidation reaction takes place at high temperature (T > 374°C) and pressure (P > 22 MPa). Organics are miscible with water at these conditions but inorganic salts, such as Na₂SO₄ and Na₂CO₃, are not soluble and crystallize on the reactor surface leading to the problem of heat exchanger fouling. Solubility and deposition of these salts on a tubular heat exchanger (reactor) surface have been studied in this work. Experiments were performed to determine solubility of these salts in binary phase and in ternary phase systems, for a wide range of temperatures. A rapid decrease in the salt solubility was observed just above the pseudo-critical temperature. For supercritical conditions, the solubility of each salt in the form of a mixture was quite close to the solubility of pure salt. In order to reduce the net salt deposition, particulate instead of crystalline deposition was encouraged. In the presence of particulate fouling, the deposit buildup was not steady. The flowing fluid partially removed the deposited layer, once it reached a certain thickness, and then the deposition process continued over a number of cycles. Compared to pure crystalline fouling, combined particulate-crystalline fouling resulted in a three times longer operating period, before the system had to be shut down for removing salt deposits. Salt solution leaving the reactor was four times higher than the saturation limit. The structure of the deposits, both pure crystalline and combined particulate-crystalline, were analyzed using Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX). The crystalline scale structure was found to be dense and tenacious, whereas the combined particulate-crystalline deposit was relatively less dense and easy to remove. A computer program has been developed, in MATLAB, to simulate heat and salt mass transfer in order to determine the salt deposition at various reactor locations. The model predicts the clean tube surface temperature quite accurately. The surface temperature change after the salt deposition is also in good agreement with the actual experimental measurements. The calculated location of the peak surface temperature change, due to fouling resistance, was found to be quite close to the experimental data.

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