Resumen de Studies in the development of supercritical water oxidation vessel reactors with hydrothermal flame as an internal heat source
The supercritical water oxidation (SCWO) is a promising technology for the destruction of wastes, but its commercialization has been delayed by the problems of corrosion and salt deposition associated to this technology and as well for its high energetic consumes. The uses of reactors working with a hydrothermal flame as a heat source contribute to overcome many of the challenges presented by this technology.
Injection of the reagents over an hydrothermal flame can avoid the reagents preheating as the feed can be injected into the reactor at low temperatures, avoiding plugging and corrosion problems in a preheating system. Also the kinetics are much faster allowing complete destructions of the pollutants in residence times lower than 1 s. Next to this the high temperatures associated to the hydrothermal flames contribute to a better energy recovery of the heat released in the reaction for electricity production Through this thesis, different studies about the process of supercritical water oxidation working with hydrothermal flames were carried out in order to develop a new reactor design for supercritical water oxidation able to inject feeds at room temperature, to work with wastes containing some inorganic salt and to optimize the energetic use The first step was to study the initiation of the SCWO reaction with concentrated isopropyl alcohol (IPA) solutions, using different tubular reactors at different operational conditions (temperature, fuel concentration and velocity) It was found that at temperatures lower than 450-500ºC, depending on the tubular reactor design, the reaction proceeded slowly, and when reaching higher temperatures a sharp increase in temperature was produced indicating a much faster reaction mechanism. The description of the phenomenon is similar to the ignition of hydrothermal flames described in literature. In these cases TOC removals higher than 99% were reached in residences times as low as 0.4 s. Ignition was not possible injecting feeds at room temperature using tubular reactors.
Using a similar experimental device, the destruction of high concentrations of recalcitrant compounds such as ammonia and an industrial waste containing acetic acid in the presence of a hydrothermal flame were studied using IPA as a co-fuel. For mixtures with acid acetic total elimination of TOC was achieved at temperatures higher than 750ºC and in the case of mixtures with ammonia TOC removal were over 99.9% while maximum removals of N-NH4+ was never higher than 94%, even for reaction temperatures higher than 710ºC The scaling up of vessel reactors was analyzed analyze using a transpiring wall reactor (TWR) operating with a hydrothermal flame as a heat source. Results of the TWR show that steady operation with a hydrothermal flame inside is possible even when reagents are injected at subcritical conditions. Temperature measurements show that reaction is not initiated in the injector but in the reaction chamber, where fluid velocity is similar to the flame front velocity of a hydrothermal flame that is of the order of 0.1 m/s . In this way a design and scale up method for vessel reactors with a hydrothermal flame inside was developed.
Basing on the results obtained using the TWR, a new reactor design was developed. The new design of cooled wall reactor was designed, constructed, installed and tested successfully in different operating conditions.
This new reactor design makes possible to work with lower injection temperatures than previously tested reactors. Stationary and stable operations were feasible even at room injection temperatures with TOC removal over 99.99% (TOC < 10 ppm) in all operating conditions when working with IPA as model compound. It was possible to work with feeds containing up to 2.5% wt of Na2SO4 without plugging, nevertheless low recovery of salt was obtained. The reactor was tested with recalcitrant compounds such as ammonia being able to destroy high concentrations of NH4+ with efficiencies up to 99.99% at temperatures as low as 600ºC in residence times of 20 seconds, and finally it was possible the destruction of synthetic and real sludge with TOC removals of 99.95% (TOC < 80 ppm) was possible.
Finally the new reactor design was tested working with a second effluent from the top of the reactor obtaining an effluent at temperatures between 500 and 600ºC TOC removal over 99.99% were obtained when the fraction of products leaving the reactor in the upper effluent is lower than 70% of the feed flow. Removals higher than 99% of N-NH4+ are achieved in both effluent. Effluents with 2.5% Na2SO4 were injected in the reactor without plugging problems. Average salt recoveries in the lower effluent were of 40% while salt concentration on the upper effluent was as low as 20 ppm. Thus, this top effluent is appropriate for the energy production both by steam generation and expansion in a Rankine cycle or by direct expansion of the effluent.
This new reactor has been presented as an European patent. The reactor is prepared to be tested with real industrial waste in long continuous operational periods.
