Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water

Abstract

Experimental results of the decomposition of glycerol in near- and supercritical water are presented considering measurements in the temperature range of 622–748 K, at pressures of 25, 35, or 45 MPa, reaction times from 32 to 165 s, and different initial concentrations. The reaction was carried out in a tubular reactor and a conversion between 0.4 and 31% was observed. The main products of the glycerol degradation are methanol, acetaldehyde, propionaldehyde, acrolein, allyl alcohol, ethanol, formaldehyde, carbon monoxide, carbon dioxide, and hydrogen. The results are compared with the studies of other working groups. The non-Arrhenius behavior of the overall degradation, as well as the pressure dependence of the reaction rate, and furthermore, the product distribution indicates the occurrence of two competing reaction pathways. One pathway consists of ionic reaction steps, which are preferred at higher pressures and/or lower temperatures. The second reaction pathway is a free radical degradation and dominates at lower pressures and/or higher temperatures. For reaction modeling, both mechanisms, the ionic and the free radical reaction network are compiled into one reaction model. The computer software package chemkin was used for the model calculations. The reaction model and the kinetic parameters were optimized in order to describe the experimental results for glycerol and the main products at 450 bar and all temperatures. This reaction model, consisting of the ionic and the free radical sub-mechanism satisfactorily describes the complex reaction at 450 bar.