Salt precipitation and scale control in supercritical water oxidation—Part A: fundamentals and research

Supercritical water oxidation (SCWO) is an effective technology for treatment of organics and organic components of aqueous wastes. Commercialization of SCWO processes has been hindered by concerns about corrosion and scale buildup/fouling which, when present, must be accommodated by system design and/or operational procedures. Salts are formed during SCWO when acidic solutions are neutralized to reduce corrosion and may also be present in the waste stream itself. Because salts have low solubility in supercritical water (SCW), they precipitate. Precipitated salts often form agglomerates and coat internal surfaces, thereby inhibiting heat transfer from/to exterior surfaces. When scale buildup is left uncontrolled, plugging of transport lines and/or the reactor can occur. The required cleaning can result in substantial and costly downtime in the SCWO process. General principles and research relevant to SCWO have been reviewed elsewhere. A review of the many technologies available to control scale during SCWO is given in the companion paper by Marrone et al. [J. Supercrit. Fluids (in press)]. Presented here is a review of fundamental principles and research pertinent to the precipitation of salts and scale control at the elevated temperatures and pressures found in an SCWO reactor. First, SCWO is introduced and the physics leading to scale buildup during SCWO is discussed. Next, the phase diagrams of model salt–water systems at relevant conditions are presented. Then, the many phenomena which complicate modeling of heat transfer in SCW (buoyancy, rapidly varying thermophysical properties, etc.) are reviewed and a set of correlations to calculate heat transfer coefficients is provided. Finally, the limited number of controlled experimental studies on scale buildup during SCWO are reviewed.