International Association Adopts Guideline Authored by ChBE Alumna, FacultyThe International Association for the Properties of Water and Steam (IAPWS) has adopted a theory-based calculation for the critical parameters of aqueous solutions of sodium chloride, first-authored by Department of Chemical and Biomolecular Engineering (ChBE) alumna Daphne A. Fuentevilla (Ph.D. '12), as its new international guideline. The formulation, recently published in the International Journal of Thermophysics, is expected to be ratified by the member nations of the IAPWS in January 2013.
IAWPS publishes internationally accepted formulations for water and aqueous solutions that can be used to calculate thermodynamic and transport properties, viscosity, and thermal conductivity over a large range of temperatures and pressures. Fuentevilla's work is the latest in a series of contributions to the IAPWS's efforts from the joint research group led by her advisor, Professor Mikhail Anisimov, and Distinguished University Professor Emeritus Jan Sengers.
Fuentevilla, who outside of her doctoral research works for the Materials Branch of the Naval Surface Warfare Center, studies the critical points of water—temperatures, pressures and densities at and beyond which multiple phases of water (such as fluid and vapor) cease to exist and becomes what is known as a supercritical fluid. Near these critical points, water possesses unusual thermodynamic properties, and even small changes in temperature and pressure can measurably alter them. When impurities are introduced—intentionally or naturally—the critical points shift. Industrial engineers using supercritical fluids take advantage of these properties, and need accurate equations describing these parameters in order to efficiently and successfully manage their processes.
In the current study, Fuentevilla set out to refine what is known about the critical points of high-temperature, high-pressure, aqueous solutions of sodium chloride—salt water. Salt water, she says, should serve as a good model for the behavior of other solutions, because it has been well studied, and researchers can access a large body of experimental data.
The last formulation describing the critical parameters of salt water, published by the IAWPS in 1999, was empirical. "What they did was take all of the available experimental data and fit a curve to it," Fuentevilla explains, “but the form of an empirical equation is chosen based on mathematics, the best fit to the data, and does not account for anything else we might know about the physics of the system."
After the release of the empirical guideline, Anisimov and Sengers' colleague, Department of Physics and Institute for Physical Science and Technology professor Michael E. Fisher, published an equation covering the critical parameters of salt water that took a different approach to the problem.
"The key was that Fisher’s equation was based on theory, based on what we know about how ions interact and aqueous salt solutions behave," says Fuentevilla. Fisher's equation, which focused on dilute solutions, matched the corresponding experimental data, but could not be applied to the entire range of concentrations.
Fuentevilla, Anisimov and Sengers used Fisher's theory-based equation as a new starting point. After their revisions, it accurately fit the dilute range, and could also be applied to solutions with higher salt concentrations. "We decided to do two separate fits, dilute and non-dilute, joined together in the same equation and proposed theoretical reasons as to why you would have these two separate parts to the curve," Fuentevilla explains. "We were able to use the theory to fit the entire range of experimental data."
For More Information
D.A. Fuentevilla, J.V. Sengers, and M.A. Anisimov. "Critical Locus of Aqueous Solutions of Sodium Chloride Revisited." Int J Thermophys 33(6):943–958 (2012) Abstract »
Published January 9, 2013