Problems in Supercritical Water Oxidation Process and Proposed Solutions

Abstract

Supercritical water oxidation (SCWO) is a promising green technology to completely convert hazardous wastewaters to innocuous products, allowing energy recovery. This process has been extensively applied to many model compounds and real wastewaters at laboratory scale. However SCWO treatments at the pilot plant scale of real wastewaters are much less extensive in literature. Furthermore, the application of this technology to industrial wastewaters has the two main drawbacks of corrosion and salt deposition, and some other problems to be solved related to management of biphasic wastes, presence of suspended solids, high costs, etc., so currently the industrial scale-up and commercialization of the process is still subject to difficulties. This work reviews the main technical solutions studied by numerous authors to avoid the drawbacks mentioned. Besides, since the economic feasibility of the process will depend on the energy recovery of the reactor effluent, this aspect is also presented in this review.

Salt deposition problems in supercritical water oxidation

“…Hence, the phenomenon of scaling or plugging easily takes place precisely because of low solubilities of these salts in SCWO. 5 Phase equilibrium relationships have important effect on the morphology and size of salt particles formed in SCWO [21]. To better understand the deposition process of salt-type solids, extensive information on soluble salts is required in SCWO, including dissolution characteristics, phase behavior and other thermal physical properties.…”

“…The primary purpose of the source control is to minimize the amount of salts in the feedstock and in the reaction fluid. Vadillo et al [5] pointed out that wastewater treated by SCWO tubular reactors requires to satisfy certain requirements of salt concentrations (e.g. NaCl<200 ppm, 19 Na 2 SO 4 <1 ppm and Na 2 CO 3 <1 ppm, etc.).…”

“…SCW is a non-polar solvent that can be completely miscible with organic compounds and gases such as hydrogen, nitrogen and oxygen. However, it is a poor solvent for polar materials such as inorganic salts and other ionic compounds [3,4], because the dielectric constant of SCW sharply decreases dozens or even hundreds of times than that of the water under ambient conditions, exhibiting non polarity [2,5]. In relation to viscosity, it is small enough to carry out reactions at very high velocities under supercritical conditions [6].…”

Direct and Indirect Applications of Sub- and Supercritical Water in Food-Related Analysis

“…The boundaries among these individual roles of water are somewhat blurred as they depend on the operating conditions (temperature, pressure, the presence of oxidants) as well as on the character of the organic portion of the system. Examples of applications of water as a reaction medium or reactant may include, e.g., supercritical water oxidation (Bermejo and Cocero 2006;Marrone 2013;Vadillo et al 2013), gasification of carbohydrates in supercritical water (Kruse 2008;Pavlovič et al 2013) or hydrothermal liquefaction of biomass to fuels (Kruse et al 2013) although hightemperature water can also serve as a reaction medium for synthesis processes (Kruse and Dinjus 2007;Hayashi and Hakuta 2010;Adschiri et al 2011). Degradation reactions in high-temperature water can broadly be divided into hydrolytic (including hydrothermal) processes (Brunner 2009a) and oxidative processes (Brunner 2009b).…”

Corrosion Properties of Candidate Materials in Supercritical Water Oxidation Process

“…However, there is serious corrosion for materials under the harsh working conditions of high temperature, high pressure and strong corrosive anions. The corrosion problem is the main obstacle to the commercialization and large-scale application of SCWO (12,13,(15)(16)(17)(18)(19)(20)(21).…”