Abstract
Kinetics of homogeneous hydrolysis and/or oxidation reactions in supercritical water is now considerably documented. However, understanding of the heterogeneous reactions on solid materials are still very much limited. In order to analyze the reaction progress around solid materials in sub and supercritical water, flow-type reaction cells were constructed where the reaction progress was directly observed by means of optical methods, mainly shadowgraph observation. In the case of activated carbon particles as one example, the rate of the supercritical water oxidation reaction was mainly determined by the oxygen transport. The reaction progress was also simulated successfully, using CFD calculation adopting an appropriate flow field and reaction mechanism. Similar investigation was carried out for the case of decomposition reactions of wood blocks in sub and supercritical water. In the subcritical region, the phenomenological first-order rate constant for the size decrease obeyed the Arrhenius law with an activation energy of ca. 80 kJ mol-1. However, the rate constant decreased near the critical point and again increased at higher temperatures. This peculiar behavior may be understood considering that the principal reaction mechanism is hydrolysis in subcritical region and changes at higher temperatures to a mechanism of more radical-type. These results are valuable in designing an appropriate reaction process of supercritical water reactions.
