Novel Reactor for In Situ Dielectric Constant Measurements of Fluids at High Temperature and Pressure

The dielectric constant measures the influence of an electric field on a material. As it probes solute–solvent interactions within a fluid, it could shed light on reactions such as the hydrothermal conversion of biomass to biofuels. However, such measurements are difficult to obtain due to the low temperature and electrical conductivity operating limits of conventional dielectric constant apparati. An apparatus was designed to measure the dielectric constant of fluids up to 220 °C consisting of a cylindrical resonant cavity that operates in TM010 mode at 1 GHz. The liquid sample is contained in a bespoke quartz tube reactor with an internal thermocouple and a pressure transducer. The sample temperature is manipulated with a dual zone heating unit. The system was validated with pure subcritical water between 20 to 220 °C and was employed to in situ detect changes in the dielectric constant of five organic solvents up to 200 °C, beyond values currently available in the literature. The maximum deviation from reference data is approximately 5% for water. The maximum deviation extends to 20% with less polar organic solvents when compared to values using single-Debye models in high-temperature regions.