Numerical study on particle behavior in a Y-junction mixer for supercritical water hydrolysis

In the continuous-type supercritical water hydrolysis process, rapid mixing of supercritical and subcritical streams is important to maximize yield and minimize degradation from over-reaction. This work investigated the particle behavior in a Y-junction mixer using large eddy simulation coupled with a discrete phase model, aiming to optimize the supercritical hydrolysis process for biomass conversion. A series of numerical simulations analyzed the influence of the mixer's orientation, flow directions, and flow rates on effective mixing and backflow prevention. The results demonstrated that the most effective mixing occurred in a vertically oriented Y-junction mixer with an upward-directed supercritical water inlet, aligning the momentum direction of natural and forced convection effectively. Consequently, over 80% of particles reached the temperatures close to the mixing temperature of supercritical and subcritical water within the Y-junction mixing zone, indicating enhanced mixing effectiveness and potential for efficient hydrolysis. This configuration also minimized backflow.

A novel spiral infinity reactor for continuous hydrothermal synthesis of nanoparticles

Hydrothermal synthesis is an attractive route to make nanoparticles utilizing inexpensive precursors under moderate process conditions. Though it provides flexibility and robustness in controlling particle characteristics, process scale-up for continuous production is a major challenge. A novel 'infinity-' shaped spiral continuous flow reactor is proposed here, to exploit the large density difference between the precursor solution and supercritical water to provide rapid mixing, leading to uniform conditions for reaction kinetics and particle growth. Hydrothermal synthesis is simulated by coupling computational fluid dynamics with population balance modeling and appropriate reaction kinetics. Simulations indicate three distinct regimes of declining, recovering, and stable flow fields. These regimes are strongly dependent on the flow ratio between the precursor solution and supercritical water. The infinity reactor provides two distinct reaction environments: initial turns of the spiral which serve as a mixed flow reactor facilitating rapid mixing and uniform reaction, followed by a plug flow reactor stabilizing the particle growth. It produces particles with a relatively small mean diameter and a narrow size distribution in comparison to the conventional batch stirred tank reactor and the T-mixer.

Visualization of supercritical water pseudo-boiling at Widom line crossover

Supercritical water is a green solvent used in many technological applications including materials synthesis, nuclear engineering, bioenergy, or waste treatment and it occurs in nature. Despite its relevance in natural systems and technical applications, the supercritical state of water is still not well understood. Recent theories predict that liquid-like (LL) and gas-like (GL) supercritical water are metastable phases, and that the so-called Widom line zone is marking the crossover between LL and GL behavior of water. With neutron imaging techniques, we succeed to monitor density fluctuations of supercritical water while the system evolves rapidly from LL to GL as the Widom line is crossed during isobaric heating. Our observations show that the Widom line of water can be identified experimentally and they are in agreement with the current theory of supercritical fluid pseudo-boiling. This fundamental understanding allows optimizing and developing new technologies using supercritical water as a solvent.

Supercritical water exists in gas- and liquid-like forms, but these have not been distinguished yet at the macroscale. Here the authors investigate supercritical water interacting with microporous carbon by neutron imaging, and observe the coexistence of gas- and liquid-like states upon crossing the Widom line.