Monitoring Anhydride and Acid Conversion in Supercritical/Hydrothermal Water by in Situ Fiber-Optic Raman Spectroscopy

The hydrolysis of acetic anhydride to acetic acid in a hydrothermal/supercritical water reactor is monitored via in situ fiber-optic Raman spectroscopy to demonstrate the use of in situ spectroscopy to gain understanding of reaction kinetics and thermodynamics. We discuss the instrumentation used to make measurements in this harsh, corrosive medium, the multivariate mathematical methods used to interpret the spectroscopy, and the mass balance relationships used to convert the result into kinetic and thermodynamic information. From the intensity relationships and temperature information, Ea for the hydrolysis reaction is estimated and compared to literature values. Also, ΔH and ΔS are estimated for a change in solvation environment of acetic acid at high temperatures.