Supercritical water oxidation is a technique to destroy wet organic waste. The oxidation reaction takes place at high temperature and pressure. Organics are miscible at these conditions but inorganic salts are not soluble and present fouling problems. Examples of these salts are sodium sulfate and sodium carbonate.
Research is being done in the University of British Columbia to study the fouling behavior of Na2CO3 in the SCWO reactor. The working fluid is heated by passing alternating current through the reactor wall. Since the tube surface was hotter than the bulk fluid, the salt was expected to nucleate on the tube wall. Due to the fouling thermal resistance, the tube surface temperature increased quite rapidly. As a fouling mitigation measure, the salt was encouraged to nucleate in the bulk fluid. It was noticed that the buildup of scale due to homogeneous nucleation of the salt was not steady. Once the deposited layer reached a certain thickness, it was removed by the flowing fluid. In some experiments there was a steady rise in the reactor surface temperature and the reactor did eventually got plugged. However, when the salt was made to nucleate in the bulk fluid, the salt net deposition was reduced and we were able to run the system for longer period of time. This paper discusses the modified procedure to enable homogeneous nucleation of salt in order to extend the operating time of the SCWO system. The results of these experiments are presented and compared with the heterogeneous nucleation case.
M. S. Khan and S. N. Rogak, "Deposition of Na2CO3 in Supercritical Water Oxidation Systems and its Mitigation" in "Heat Exchanger Fouling and Cleaning: Fundamentals and Applications", Paul Watkinson, Hans Müller-Steinhagen, and M. Reza Malayeri Eds, ECI Symposium Series, Volume RP1 (2003). http://dc.engconfintl.org/heatexchanger/8