Formation of organic acids during the hydrolysis and oxidation of several wastes in sub- and supercritical water

The objective of this work was to evaluate the transformation characteristics of four organic substances in supercritical water, The purpose was to demonstrate the yield and stability of the acetic acid produced under hydrolytic and oxidative conditions. Other organic acids, such as formic, glycolic, and lactic acids, were monitored. Cellulose and coconut oil solutions, as well as brewery and dairy effluents, were used as feedstocks. Batch tests were performed at fixed conditions of 400 degreesC, 27.6 MPa, and 5 min of reaction time. Hydrogen peroxide was the oxidant. Under hydrolytic conditions, 70% of the initial carbon remained as the liquid product. On the contrary, in the presence of excess oxygen, there was a 95% conversion to the gaseous product. Typically, less than 15% of the initial total organic carbon was converted to the acids. The use of catalysts (i.e., TiO2) and additives (i.e., H2SO4) did not enhanced the organic acid yield. However, catalysts addition facilitated feedstock breakdown at lower oxygen levels. To evaluate the effect of alkali addition and the use of lower temperatures, continuous flow tests were conducted using glucose as the substrate. Under alkaline conditions, organic acid production increased. For example, at 250 degreesC and 27.6 MPa with the addition of NaOH (55.6 wt% glucose) and providing 25% stoichiometric oxygen, about 77% glucose was converted to acetic acid (17%), glycolic acid (22%), and formic acid (38%). These preliminary results indicate that valuable compounds could be obtained during the degradation of organic wastes in sub- and supercritical water instead of complete oxidation to CO2 and water.