Molecular Dynamics And WIDOM Simulations On Corrossion In Supercritical Water

Supercritical water （SCW） is an environmentally benign, nontoxic homogeneousfluid. It is becoming more and more popular and important because of its uniquechemical properties and strong oxidize. So far, supercritical water oxidation technologyhas formed the basis of its application as a green technology. However, it has a hightemperature and pressure, and contains more oxidize in SCW, all of these factors willlead to corrosion or blocked pipes. Therefore, its application and spread have beenhindered in industry.Therefore, we studied the factors which affect the corrosion in SCW by using MDand MC simulations. Firstly, MD simulations were done for H₂O, HCl and NaOH in theNVT ensemble. We obtained the relationships between thermodynamic properties,dissociations, molecular structures and the temperature through the simulations. Theresults show that the dissociations of H₂O, HCl and NaOH increase with temperature,and so does the corrosion in the reactors. At the same time, their changing entropies andinternal energies correspond to the change of dissociations as the temperature increases.Secondly, MD simulations were performed for H₂SO₄and H₃PO₄at100MPa inthe NPT ensemble. The relationships between thermodynamic properties, diffusionvelocities,dissociations,molecular structures and the temperatures were obtained.Results show that the diffusion rate rises as the temperature increases for both the soluteand solvent. Similarly, the ΔpK_a1value of H₂SO₄and the ΔpK_a1, ΔpK_a2, ΔpK_a3values ofH₃PO₄increase as the temperature goes up, that is, their acidities are related to thetemperature. Therefore, the corrosion will go down as the temperature rises. Conversely,ΔpK_a2value of H₂SO₄goes down before623K, and then goes up after the temperatureincreases. So, HSO₄-will lead to heavier corrosion in the reactors near the criticaltemperature.Finally, the WIDOM and MC simulations were performed for the solubility ofnoble gas Ar, organic compounds CH4and NaCl under the high and low density. Itshows that the solubility of noble gas Ar, organic compounds CH4are proportional tothe temperature, and inversely proportional to the density. And NaCl is in contrast tothem.