Catalytic supercritical water oxidation for organic laboratory waste treatment

Abstract

Supercritical water oxidation (SCWO) technology has been widely studied and applied to an extensive variety of wastewater. It converts organic compounds to CO₂ and H₂O via oxidation reactions that exceed the critical point of water (Tc=374 ℃, Pc=218 atm) and needs short residence time for complete destruction of organic compound. However, the treatment of chlorinated organics with this technique may cause the corrosion problem of the reactor wall. In order to solve this problem, a new cascade process that consists of consecutive combination of hydrolysis and SCWO is proposed. Dichloromethane, which is widely used for the solvent of such chemical reactions as organic synthesis and extraction in pharmaceutical laboratory, is chosen as a representative model of chlorinated compounds. There have been many previous studies on the hydrolysis of dichloromethane, which results in co-production of formaldehyde and hydrochloric acid. Compared with the abundance of previous research on dichloromethane hydrolysis there has been less investigated on the kinetics of formaldehyde oxidation in SCW. Therefore, in this study focus on the oxidation of formaldehyde in supercritical water with and without catalyst. A new cascade process where two reactors are consecutively combined, aiming at hydrolysis in the first reactor followed by SCWO in the second reactor, for the complete destruction of dichloromethane is proposed. As a result, formaldehyde can be completely decomposed at 400oC and 25 MPa within a very short contact time in catalytic compact sized system with MnO₂ as catalyst. In addition, the different methods for HCl removal at the exit of first reactor are designed and experimentally investigated.