Methanol and Ethanol Decomposition in Supercritical Water

Abstract

Abstract The decomposition of CH3OH and C2H5OH in supercritical water was studied in a flow reactor tube (Ni/Mo/Cr/Fe alloy) in the temperature range 597 ≤ T/K ≤ 797 at a pressure of p = 315 bar for technical application of scH2O for hazardous chemical waste destruction. The CH3OH and C2H5OH concentrations in the liquid as a function of the residence times were determined by a Raman spectrometer. The [CH3OH] and [C2H5OH] resp. decay followed first order kinetics and a rate constant k 1(653 K) = 1.3 × 10−2 s−1 for CH3OH and k 2(653 K) = 5.5 × 10−2 s−1 for C2H5OH was determined. The rate constant k 1 was found to be independent of the initial CH3OH concentration in the mass fraction range 0.002 ≤ x m ≤ 0.04. The rate depended on the history of the reactor. Treatment with NH4OH, C2H5OH or with H2O2 at T = 653 K, did not change the rate. Treatment with HNO3/H2O2, however, at T = 838 K reduced the rate by about a factor of 1000. The Arrhenius-activation energy over the above temperature range was determined to be E A = 164 kJ/mol for methanol and E A = 145 kJ/mol for ethanol. The major products from methanol decomposition were CH4, H2, and CO2 as observed by gas chromatography and CH4 and CO2 by FTIR-spectrometry. No other products were found. The products were not effected by the pretreatment of the reactor wall. A non-radical mechanism, which explains the formation of only these products, will be discussed.