Future space exploration missions will require advanced systems to efficiently handle and recycle a variety of waste streams. Super Critical Water Oxidation (SCWO) processes waste materials mixed in a water slurry at temperatures and pressures above the critical point of water (374 °C and 22.1 MPa). SCWO is an attractive candidate technology for processing solid and liquid wastes for long duration space and extraterrestrial planetary missions because (i) required pre-processing of waste is minimal, (ii) product streams are benign, microbially inert, and easily reclaimed, (iii) waste conversion is complete and relatively fast, and (iv) with proper design and operation, reactions can be self-sustaining. However, an experimental database for the behavior of water near its critical point (i.e., the conditions of pressure, temperature and density at which the distinction between liquid and gaseous phases disappears) as well as SCWO under the microgravity conditions relevant to space and extra-terrestrial environments is currently lacking. The first building block for this microgravity database is the behavior of the critical transition of water from a two-phase liquid-vapor system to supercritical fluid.
To this end, a collaborative experiment between USRA, NASA and the French space agency, CNES, was recently conducted on the International Space Station. This experiment studied the effects of microgravity on phase distribution, heat addition, evolution of bubbles, and hysteresis effects in a small constant volume system during the critical transition process. These data serve as a baseline and precursor for a proposed follow-on experiment that will study the dispersion and precipitation characteristic of salts in near-critical water. This is an important practical problem as the precipitation of salts during SCWO can have significant impacts on the operation of a SCWO reactor by severely limiting the lifetime of system components.
The SCWO reactor is constructed from Hastelloy C-276 and has an internal volume of 480 cm3. The vessel has been ASME certified for a maximum allowable working pressure of 40.7 MPa at 550 °C. Four thermocouples and a Raman probe are inserted into the reactor to provide near real-time measurements of reactant concentrations. In this test configuration, the oxidizer is injected axially into the reactor vessel once the supercritical conditions are attained.