Prediction of residence time distributions in supercritical hydrothermal reactors working at low Reynolds numbers

Abstract

Supercritical water has been demonstrated as a suitable reaction media for the generation of nanomaterials. Many single and complex metal oxides can be obtained using hydrothermal synthesis techniques. This work presents a methodology to characterise continuous hydrothermal flow systems both experimentally and numerically, working at low Reynolds numbers. Under these conditions traditional computational approaches in the field are of limited use, so new tools are in demand. Residence time distributions (RTD) experiments are carried out using a purpose built continuous flow rig featuring an injection loop on the metal salt feed line, allowing injection of a chromophoric tracer. Computational fluid dynamics (CFD) calculations are also conducted using modelled geometry to represent the experimental apparatus, using a specialised approach for modelling turbulence. The performance of the CFD model is tested against the experimental data provided, verifying that the CFD model is able to predict the main flow features with good accuracy. The results indicate that the RTD profile is affected strongly by the mixing step and turbulent diffusion in the annulus section of the reactor. Different mixing regions are identified based on a mixing scale analysis. The internal RTD and its relation to the synthesis of nanomaterials is also investigated.