Thermal Design of Supercritical Water Oxidation Reactors

Abstract

Waste destruction using supercritical water oxidation (SCWO) was demonstrated in laboratories in the early 1980’s and in full-size facilities by the early 1990’s. The process offers thorough destruction of toxins in a compact facility without supplementary energy. Early estimates that SCWO could auto-thermally treat wastewaters with as little as 2% weight organic ignored some practical factors, such as corrosion, fouling, heat transfer limitations. In this paper, a thermal model for a SCWO system based on pure water properties and heat transfer correlations is used to estimate heat exchanger size and frictional pressure losses. Information on real mixtures at SCWO conditions is not established to the point needed for rigorous thermal modeling, but the pure-water model can be interpreted using real-fluid properties and experience from operating SCWO systems. It is shown that the waste composition has a direct influence on the SCWO design. Auto-thermal treatment of dilute streams (2% organic) is economic only if inorganic compounds are absent from the waste stream or treated effluent, and the plant is of moderate size. For more corrosive wastes, or those with fouling agents, it becomes very expensive to preheat the feed beyond the critical temperature. Without preheating to supercritical temperatures, some back-mixing of hot products is needed to stabilize reaction, so that plug-flow reactor designs become inappropriate.