Supercritical water oxidation conditions

G.L. D Arcy, Corrosion Behaviour of Ti Alloys Exposed to Supercritical Water Oxidation Conditions, Thesis, University of Texas USA, (1995). 

To date, numerous model compounds simulating the pollutants in common waste streams have been studied under laboratory-scale conditions by many researchers to determine their reactivities and to understand the reaction mechanisms under supercritical water oxidation conditions. Among them, hydrogen, carbon monoxide, methanol, methylene chloride, phenol, and chlorophenol have been extensively studied, including global rate expressions with reaction orders and activation energies [58-70] (SF Rice, personal communication, 1998). 

At temperatures above 500 °C more than 90 % of the organic bonded bromine was converted into bromide. By adding oxygen complete breakdown to carbon dioxide, bromide and water is possible. Under supercritical water oxidation conditions more than 99 % of the organic bonded bromine was found in the aqueous phase. A formation of bromine, hydrogen bromide and dioxines as in thermal decomposition was not observed. 

SC WO [supercritical water oxidation] A generic name for processes which destroy organic wastes in water by oxidation under supercritical conditions. The first such process was MODAR, invented in 1980. Since then, several other companies have introduced competing processes. 

The physico-chemical effect of high pressure, especially in the supercritical state, to enhance the solubility and phase conditions of the components involved. Supercritical hydrogenation, or enzymatic syntheses are offer new steps with high pressure. Supercritical water oxidation at high pressure represents an efficient method for the decontamination of wastes. 

Reviewed previous SCWO research with model pollutants and demonstrated that phenolic compounds are the model pollutants studied most extensively under SCWO conditions Studied supercritical water oxidation of aqueous waste Explored reaction pathways in SCWO of phenol Studied catalytic oxidation in supercritical water Explored metal oxides as catalysts in SCWO Studied decomposition of municipal sludge by SCWO Investigated the SCWO kinetics, products, and pathways for CH3- and CHO-substituted phenols Determined oxidation rates of common organic compounds in SCWO... 

Kriksunov, L.B. and MacDonald, D.D., Understanding chemical conditions in supercritical water oxidation systems, ASME Heat Transfer Div., 317(2), 271-279, 1995a. 

Supercritical Water Oxidation (SCWO) has been proved to be a suitable process for treatment of several toxic and hazardous organic wastes due to its high removal efficiency. SCWO requires of hard reaction conditions (22.1 MPa and over 374°C). Special reactors are needed to support these conditions. An original reactor design is presented here wich has been tested in the treatment of alcohols+ammonia solutions in water. Performance results are presented here for ammonia and alcohols. Destruction efficiency greater than 99.9% are reached for both compounds, probing the correct performance of the reactor. 

Fig. 5.8 Examples of oxidative water treatment technologies used in industry, research and development [adapted from FIGAWA (1997), and supplemented by novel methods]. The numbers 1 to 9 refer to the generalized reaction sequences presented in Figure 5-9. a) Oxidation at elevated temperatures between 220°C < T <300°C or supercritical water oxidation at AT >374°C, Ap >221 bar (221000 kPa) (cf Chapter 1) b) oxidation in the presence of bimetallics Fe°/Ni° or Zn°/Ni° (Cheng and Wu, 2001) or heterogeneous oxidation in supercritical water catalyzed by metals Me = Cu, Ag, Au/Ag-alloy c) Fenton reaction at pH <5 d) photo-assisted Fenton reaction, irradiation in the UV-B/VIS range e) the mixture of oxidants O3/H2O2 is called PEROXONE f) ozonation using solid-bed catalysts with conditioned activated carbon (AC) g) vacuum-UV photolysis of water.

Organic pollutants can be oxidized in an aqueous medium above the critical point of water (Tc = 374°C, Pc = 218 atm), whereby the solubility of the organics and of the oxidizing gas is higher than that under normal conditions. This is called supercritical water oxidation (SCWO). A more thorough discussion of supercritical fluids is given in Section 12.3.8. 

More polymerization reactions carried out at supercritical conditions, select biomass conversion supercritical fluid technologies for hydrogen production, wider use of supercritical water oxidation processes, portfolio of self-assembly applications, a spate of opportunities in process intensification, many supercritical fluid aided materials synthesis applications, and numerous reactions for synthesis of specialty chemicals are expected for years to come. 

Supercritical water oxidation poses a unique corrosion problem that has not been experienced in other chemical processes. Development of new materials is needed to address the problem. The material needs to withstand a chemically harsh environment along with the high temperature and pressure conditions. Even if some current materials are available for... 

Residence time for supercritical water oxidation systems may be as short as several minutes at temperatures of 600 to 650°C. More than 99.9 percent conversion of EPA priority pollutants such as chlorinated solvents has been achieved in a pilot-scale plant with retention time less than 5 minutes. The system is limited to treatment of liquid wastes or solids less than 200 microns in diameter. Char formation during reaction may impact the oxidation time of the organics, while separation of inorganic salts during the process may be a problem. Typical materials for the reactor are Hastelloy C-276 and Iconel 625 (high nickel alloys), which can withstand high temperatures and pressmes and the corrosive conditions. 

The reaction is carried out in a 0.478 cm i.d. Inconel 625 reactor at 960 to 1,100 bar and 380° to 450°C with approximately one minute of residence time. Treatment at these conditions is often called supercritical water oxidation (SCWO) however, since the critical parameters of the PBX-9404 hydrolysis product mixture are unknown, the treatment may or may not be above the critical parameters for the hydrolysate. Therefore, the more general term hydrothermal treatment is used in this case. 

The calculations have been done not only for stoechiometric combustion conditions, but also for negative oxygen balances, which are relevant for partial combustion conditions and for thermal decomposition reactions. By taking into account an excess of water, also the reaction products of a supercritical water oxidation can be calculated. 

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