Design of a Small-Scale Supercritical Water Oxidation Reactor. Part I: Experimental Characterization

Supercritical water oxidation (SCWO) has been previously studied in the context of the destruction of organic compounds; however, information regarding the practical design and the details about the operation of these systems is not well presented in the literature. SCWO reaction rates have been studied in reactors with high surface-to-volume ratios and may not apply to practical reactors where volumetric reactions are prevalent. A modular small-scale supercritical water oxidation reactor is designed and fabricated to study kinetic rates, establish safe operating conditions, and test process control strategies. The reactor is heated by oxidation of pilot fuel (ethanol/water mixture) in an oxidant (H2O2/water mixture). Both streams are introduced coaxially in a downward direction stabilizing the oxidation by buoyancy. The fuel heating value was varied by adjusting the ethanol concentration in the 2-7 mol % ethanol/water range. The oxidant-to-fuel stoichiometric equivalence ratio (Phi(AF)) was varied from 1.1 to 1.5 by adjusting the oxidant mixture flow rate. Higher ethanol concentrations in the pilot fuel stream and operation near-stoichiometric result in a more stratified vertical temperature profile. The steady operation at a fluid temperature >600 degrees C is achieved with a nominal residence time of 25 s at a 7 mol % fuel dilution and Phi(AF) = 1.1. While the higher fluid temperature, the desired destruction of recalcitrant waste streams, was achieved, the wall temperature did not exceed material thresholds, establishing a safe operational envelope for the system. At the lowest pilot fuel dilution (2 mol %), the temperature profile is nearly uniform, approaching a distributed reaction regime with long residence times (>20 s) due to a buoyancy-driven stabilization scheme. The temperature distribution is used to validate the numerical modeling approach, presented in Part II of the study.