Hydrothermal oxidation

Two Other chemical processes that rely on hydrothermal processing chemistry are wet oxidation and supercritical water oxidation (SCWO). The former process was developed in the late 1940s and early 1950s (3). The primary, initial appHcation was spent pulp (qv) mill Hquor. Shordy after its inception, the process was utilized for the treatment of industrial and municipal sludge. Wet oxidation is a term that is used to describe all hydrothermal oxidation processes carried out at temperatures below the critical temperature of water (374°C), whereas SCWO reactions take place above this temperature. 

Fig. 12. Typical flow diagram of a hydrothermal oxidation process (HO), also known as supercritical water oxidation (SCWO) (73,105).

Extraction of sec-butanol from isobutene Hydrothermal oxidation of organic wastes in water... 

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

Hydrothermal Oxidation Process Processes which involves oxidation/reduction in an aqueous matrix at elevated temperatures. Examples of hydrothermal oxidation processes include ... 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Hydrothermal oxidation (HO) [also called supercritical water oxidation (SeWO)] is a reactive process to convert aqueous wastes to water, CO2, O2, nitrogen, salts, and other by-products. It is an enclosed and complete water treatment process, making it more desirable to the public than incineration. Oxidation is rapid and efficient in this one-phase solution, so that wastewater containing 1 to 20 wt % organics may be oxidized rapidly in SOW with the potential for higher energy efficiency and less air pollution than in conventional incineration. Temperatures range from about 375 to 650°C and pressures from 3000 to about 5000 psia. 

The unique properties of supercritical water, when combined with an oxidant such as air, oxygen, or peroxide, create an excellent reaction medium. The process, called supercritical water oxidation (SCWO), has been proven to be capable of destroying organic contaminants as well as some inorganic substances. SCWO is also known as hydrothermal oxidation (HTO). 

If reduced Fe powder was added to the preceding solution at an optimum pH of 6-7, magnetite (Fe304) was deposited onto Si (100) or k-AIiOb at ca. 140°C over several hours [24]. No other phase was found in the XRD spectrum. It was suggested that the Fe304 formed by reaction between Fe(OH)3 (presumably formed by hydrolysis of the ferric nitrate) and Fe(OH)2 formed by hydrothermal oxidation of the Fe powder. Particle sizes of 150 mn (on AI2O3) and 50 mn (on Si) were measured by SEM. 

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O3/UV/H2O2 AOTs, together with other processes treated in different chapters (such as Fenton oxidation), can be named ambient (temperature and pressure), advanced oxidation technologies, in contrast with other AOTs such as hydrothermal oxidation processes that require pressures and temperatures above 1 MPa and 150°C, respectively, and which are more suitable for the treatment of concentrated wastewaters. It is evident that appropriate ranges of concentrations for the different oxidation technologies cannot be exactly established but some recommended values have been reported [29]. Fig. 1 shows some possible recommended ranges of concentrations for these types of AOTs. 

Fig. 1 shows a graphical representation of these operational regions. WAO and SCWO processes are often referred to as hydrothermal oxidation technologies (HTOs). The major difference between the processes is that, in SCWO, organics are completely oxidized in a relatively short time (seconds to minutes), whereas in WAO, the reaction may require a longer time (minutes to hours). Furthermore, in WAO, some refractory organics are not completely oxidized because of the lower temperature of operation... 

Figure 2 A generic hydrothermal oxidation (WAO, SCWO) process flow diagram. 

Figure 9 Arrhenius plot for hydrothermal oxidation of phenol between 100° and 420°C. (From Ref. 81.)...

Hydrothermal oxidation (especially SCWO) occurs in a pressurized vessel, thus requiring stringent safety and maintenance operations. In addition, SCWO is better suited for processing solutions and slurries, whereas solid wastes require sizing and pretreatment. 

Ross DS, Jayaweera IS, Leif R. Improved method for hydrothermal oxidation of halogenated organic compounds with addition of specific reactants. U.S. Patent 5,746,926, 1998. 

Mill T, Jayaweera IS, Yao D, Ross DS. Hydrothermal oxidation of phenol. Physical chemistry of aqueous systems. Meeting the Needs of Industry, Proceedings of the 12th International. In White HJ Jr, ed. New York Begel House, 1995 589-593. 

Ross DS, Jayaweera IS, Bomberger DC, Lief R. Hydrothermal oxidation of organic compounds with heterogeneous neutralizing reagent. U.S. Patent No. 6,010,632, 2000. 

Shananblesh, A. and Shimizu, Y. (2000) Treatment of sewage sludge using hydrothermal oxidation-technology application challenges. Water Sci. Technol. 41, 85-92. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Alt J. C. (1988) Hydrothermal oxide and nontronite deposits on seamounts in the eastern Pacific. Mar. Geol. 81, 227-239. 

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