Supercritical water oxidation vapor phase

T0130 Bohn Biofilter Corporation, Bohn Off-Gas Treatment T0756 Supercritical Water Oxidation—General T0855 Vapor-Phase Biofiltration—General... 

Foster Wheeler Development Corporation (FWDC) has designed a transportable transpiring wall supercritical water oxidation (SCWO) reactor to treat hazardous wastes. As water is subjected to temperatures and pressures above its critical point (374.2°C, 22.1 MPa), it exhibits properties that differ from both liquid water and steam. At the critical point, the liquid and vapor phases of water have the same density. When the critical point is exceeded, hydrogen bonding between water molecules is essentially stopped. Some organic compounds that are normally insoluble in liquid water become completely soluble (miscible in all proportions) in supercritical water. Some water-soluble inorganic compounds, such as salts, become insoluble in supercritical water. 

This paper deals with the degradation of substances like PVC, Tetrabromobisphenol A, y-HCH and HCB in supercritical water. This process is called "Supercritical Water Oxidation", a process which gained a lot of interest in the past. The difference between subcritical and supercritical processes is easy to recognize in the phase diagram of water. The vapor pressure curve of water terminating at the critical point, i.e. at 374 °C and 221 bar. The relevant critical density is 0.32 g/cm3. This corresponds to approx. 1/3 of the density of normal liquid water. Above the critical point, a compression of water without condensation, i.e. without phase transition is possible. It is within this range that supercritical hydrolysis and oxidation are carried out. The vapor pressure curve is of special importance in subcritical hydrolysis as well as in wet oxidation. 

Figure 12 Solubility and phase behavior in supercritical water oxidation systems, (a) O2-H2O at 250 bar (based on data from Refs. 99 and 100 at 250°C and above, other data from Ref. 101) (b) CO2-H2O at 250 bar (based on data from Ref. 102 at HO C and above, other data from Ref. 103 (c) Benzene-H2O at 250 bar (based on data from Ref. 104 below 100°C, Refs. 105 and 106 from 287-295 C, other data from Ref. 107) (d) Benzene-H2O at 100 bar (based on data from Ref. 104 up to 250°C, other data from Ref. 108). (e) NaCl-HoO at 250 bar (vapor phase compositions from Ref. 110 other data from Ref. Ill (f) NaCl-HoO at 100 bar (vapor phase compositions from Ref. 110, other data from Ref. Ill) (g) NaTStC-lCO at 250 bar (based on data from Ref. 116 at 320°C and above other data from Ref. 117) (h) Na2SO4-H2O at 100 bar (based on data from Ref. 116 at 320°C and above, other data from Ref. 117 and Ref. 118.)...

Another recent patent describes a multistep process for the production of ethylene glycol in near-critical or supercritical CO2 (Bhise, 1983). In this instance CO2 is first used as a solvent and then used as a reactant. Normally, ethylene oxide is produced by the vapor phase oxidation of ethylene with molecular oxygen over a supported silver catalyst. In conventional ethylene glycol processing, an effluent stream containing the ethylene oxide is scrubbed with water to recover the ethylene oxide. The ethylene oxide is then recovered for hydrolysis to ethylene glycol. 

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