Molecular Dynamics Calculation of the Diffusivity of Sodium Chloride in Steam

Molecular dynamics simulation is used to calculate the diffusivity of sodium chloride at infinite dilution in high-temperature steam at seven state points representing conditions of interest for the study of impurity deposition in steam turbines. Water is modeled with an existing four-site, polarizable potential, and the ions are modeled as charged, polarizable Lennard-Jones spheres. At the conditions studied, the sodium and chloride ions diffuse together; the simulations determine the diffusivity of this ion pair to within approximately 20%. The results can be fitted satisfactorily to the form of a simple kinetic-theory expression, allowing for extrapolation to lower densities that are less amenable to simulation. At turbine conditions, no experimental diffusivity data are available, and existing semiempirical estimation techniques are unreliable, so the results from this work provide the best values available for this industrially important property.