Design of a Small-Scale Mixing Section for a Supercritical Water Reactor through the Finite Volume Method

Abstract

Supercritical water (sc-H2O) reactors have been used for biomass gasification and the destruction of hazardous waste. Laboratory scale reactors are typically used for development of chemical kinetic rate parameters. These smaller reactors with lower Reynolds numbers often suffer from slow mixing between the reagent and sc-H2O; this slow mixing increases uncertainty in the data required for calculation of chemical kinetic rate parameters. In this study, we present a multiple-jet in crossflow design for a mixing section, which enables rapid mixing of reagents into sc-H2O. A parametric analysis is conducted to establish an optimum jet-to-crossflow velocity ratio (r) for scalar mixing using three-dimensional computational fluid dynamics (CFD) with Detached Eddy Simulations (DES) for resolving turbulence. Kinetic theory models for calculating physical properties of the fluids at the supercritical state are evaluated against data available in published literature. CFD simulations show that mixing can be characterized by three distinct regimes: (i) under-penetrating jets, (ii) weakly penetrating jets, (iii) jets forming counter-rotating vortex pairs (CVPs), and (iv) impinging jets. The best mixing is observed for jets forming CVPs; under-penetrating jets show the worst mixing. The mechanism of mixing in the three configurations is explained. Decomposition of methanol (MeOH) in a continuous-flow sc-H2O reactor is simulated with CFD using global first-order chemical kinetic rate parameters calculated from published experimental data. This numerical modeling sheds insight into the complex physiochemical processes of organic compound decomposition in the supercritical environment. The modeling approach can be used in industrial process optimization and to improve the design of new and existing systems.