(77d) Novel Reactor for in Situ Measurement of the Dielectric Constant to Tune Hydrothermal Processes

Hydrothermal processes (HTP) thermochemically transform large biomass molecules into energy-dense solid and liquid fuels in subcritical water. The aqueous media’s thermodynamic properties during the HTP strongly influence the products. Under subcritical conditions, water molecules form activated complexes that reduce the activation energy barrier to attacking the chemical bonds of biomass molecules. The hydrogen bonding network in bulk water is also weakened as the mobility of water molecules increases with temperature. The viscosity of water decreases with increasing temperature, in turn enhancing mass transfer to promote the diffusion and convection of reactants and products. This further reduces the energy barrier for HTP reactions. Water also acts as both a solvent and a reactant in hydrolysis reactions, which are essential for breaking down complex organic molecules into simpler compounds. These unique properties of water are a result of its permanent dipole and hydrogen bonding network, which can be screened through the dielectric constant. The dielectric constant decreases as temperature increases, leading to an increase in the solubility of non-polar organic compounds. Therefore, understanding the relationship between and the influence of the dielectric constant’s properties can assist in optimizing hydrothermal conversion processes.

To date, HTP studies focus on temperature and pressure as process design variables. This has led to an incomplete understanding of the reactions occurring during HTP and an inability to precisely control process endpoints. As the wet biomass enters the hydrothermal region, the dielectric constant of water decreases to approach that of polar organic solvents. Water becomes more miscible with what are usually hydrophobic organic compounds. Yet, there is no measurement data available for the dielectric constant of these processes, despite it being a crucial indicator of hydrothermal reactions. Knowing the dielectric constant would enable process improvements by using it as a tunable process parameter to control reaction pathways and produce more selective fuels. This fundamental gap in knowledge is addressed by the design and construction of a reactor to make in situ dielectric measurements - the first of its kind to clarify the role of the dielectric constant during HTP.

We designed and commissioned an apparatus that uses the cavity perturbation method, a well-established technique for dielectric measurements in high temperature applications, to enable real-time measurements during HTP. Such cavity perturbation-based instruments have been utilized for numerous industrial samples up to 1100°C. Our bespoke quartz reactor expands its capabilities into high temperature and pressure reacting fluid samples. Quartz exhibits low dielectric loss, meaning that the measured losses predominantly reflect the sample's properties, and it maintains relatively stable permittivity over a wide temperature range, minimizing the influence of the sample holder's temperature-dependent permittivity on the measurements. The cavity perturbation method builds upon the perturbation theory where the dielectric properties are deduced from the disturbances from the resonance of a hollow metal structure around the measurement frequency. When a small sample is introduced into the cavity, it disrupts the electromagnetic field distribution, causing a shift in the resonant frequency and a change in the quality factor of the cavity. Perturbation theory provides a mathematical framework to relate these changes to the sample's dielectric and magnetic properties.

The apparatus is calibrated across the dielectric constant of 10 to 79 using pure solvents at standard conditions. It provides new measurement data of 5 different solvents at 1 GHz up to 220°C. The dielectric constant of binary (organic solvent-water) mixtures at different mass fractions is also measured to study the changes in the empirical mixing rules at different process conditions. When the model compound mixtures such as glucose and sucrose solutions are used as feedstock, the mixture displays relatively lower dielectric constant values compared to pure water at the beginning but the values approach those of water as solid and gas formation reactions take place in the hydrothermal region.