Ethanol oxidation and hydrolysis rates in supercritical water

Abstract

Oxidation and hydrolysis experiments with ethanol were investigated in supercritical water using a lab-scale, plug-flow reactor system at temperatures from 433 to 494 °C and a fixed pressure of 246 bar. The residence times ranged from 2 to 12 s, and the initial concentration of ethanol was set to 1 mmol l −1. In the hydrolysis experiments, ethanol did not react to a significant degree relative to the conversions observed in oxidation experiments. The conversion for the hydrolysis experiments was between 1.9 and 7.4%. In the oxidation experiments, the initial oxygen concentration was set to 3 mmol l −1, according to the stoichiometric ratio for complete mineralization. The major products of the reaction were acetaldehyde and formaldehyde in the liquid phase and carbon monoxide and carbon dioxide in the gas phase. An assumed first-order global rate expression was determined with an activation energy of 163.9±3.3 kJ mol −1 and a pre-exponential factor of 10 11.1±4.5 to a 95% confidence level. An induction time was experimentally observed, ranging from 0.5 to 3.8 s, with longer times corresponding to lower temperatures. A second set of experiments was performed to investigate the dependence of the reaction rate on oxygen concentration. The fuel equivalence ratio was varied from 0.5 to 1.9. Experiments were conducted at residence times of 2.5 and 3.0 s at 470 °C and 246 bar. A global rate expression was regressed for the ethanol reaction rate from the complete set of data. The resulting pre-exponential factor was 10 17.23±1.65; the activation energy was 213.9±18.3 kJ mol −1; and the reaction orders for ethanol and oxygen were 1.34±0.11 and 0.55±0.19, respectively.