Supercritical Water Oxidation (SCWO) Using a Transpiring Wall Reactor: CFD Simulations and Experimental Results of Ethanol Oxidation

Abstract

Supercritical Water Oxidation (SCWO) was studied at the Institute of Technical Chemistry, ITC-CPV. SCWO is a high-pressure-high-temperature process with high space-time yield to destroy organic hazardous compounds present in industrial waste effluents to form water and carbon dioxide. Heteroatoms were mineralized to the corresponding acids or salts; nitrous oxides formation was suppressed due to low oxidation temperatures. Results obtained in a tube reactor system showed destruction efficiency (D.E.) values close to 100% of the organic content, but indicate plugging and corrosion of the tube when treating salt and/or acid containing solutions. Hence, the application of SCWO process is limited. To enlarge the potential of SCWO process for industrial applications, new reactor concepts were developed. Among these, the transpiring wall reactor (TWR) concept is considered to have very good prospects to overcome these limitations. The TWR installed at ITC-CPV is designed for T = 630°C, P = 32 MPa, wastewater flow rate = 20 kg/h, air feed rate = 40 kg/h, transpiring water flow rate = 20 kg/h, and quench water flow rate = 40 kg/h. The TWR can be fed with suspensions of up to 10 wt. %. SCWO experiments with model compounds and industrial waste effluents or waste suspensions of up to 6 wt. % solid material content resulted in D.E. values of up to 99.99%. However, feeding of effluent suspensions may become less reliable at low feed flow rates. Computer simulations of the experiments using the CFD code CFX4 complement the SCWO studies. The Institute for Reactor Safety, IRS, performed 2D and 3D steady-state calculations to get an insight into local flow conditions and species concentrations inside the reactor and around the transpiring wall—information that are hardly accessible to measurements. For validation of the computational results, local temperatures, and the destruction efficiency can be compared with the experimental data. This is illustrated for an experiment in the ITC-TWR with the model compound ethanol.