Hydrothermal supercritical

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

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. 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

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

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. 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagioclase and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Cowan, J.E. and Cann, J. (1988) Supercritical two-phase separation of hydrothermal fluids in the Troodos ophiolite. Nature, 333, 259-261. 

Barne.s, H.L. and Czamanske, G.K. (1967) Solubilities and transport of ore minerals. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits. New York Holt, Rinehart and Win.ston, pp. 334-381. Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagiocla.se and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

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 occurrence of supercritical liquids in hydrothermal systems cannot be excluded. 

Broil et al. (1999) have provided detailed surveys of the variety of reaction mechanisms which can occur in supercritical water. It is possible that supercritical conditions were present in the vicinity of hydrothermal systems, where as yet unknown... 

It could be expected, that combustion reactions and possibly flames can be produced in such dense supercritical mixtures. Technical aspects of hydrothermal oxydation at moderate pressures have already been tested and discussed [7,8]. The study of combustion and flames in supercritical phases offers several possibilities 1. The variation of pressure over wide ranges should influence reaction mechanisms and flame characteristics because the density can be changed from low, gas-like, to high, liquid-like, values. 2. The variable temperature of the dense, fluid environment can have an influence on reactions and flames. 3. The chemical and physical character of this environment can be varied considerably, for example by using supercritical water as the major component, as in the present experiments. Certainly, the knowledge of transport coefficients of gases involved is desirable. For water the viscosity has been determined to... 

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). 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

Fuels - [ALCOHOL FUELS] (Vol 1) - [COALCONVERSIONPROCESSES - LIQUEFACTION] (Vol 6) - [ALCOHOLS,HIGHERALIPHATIC - SURVEY AND NATURALALCOHOLSMANUFACTURE] (Voll) -H202 as oxidant [HYDPOGEN PEROXIDE] (Vol 13) -hydrazine as [HYDRAZINE AND ITS DERIVATIVES] (Vol 13) -hydrothermal oxidation of [SUPERCRITICAL FLUIDS] (Vol 23) -ignition m hot air streams [ACETALDEHYDE] (Voll) -for ironmaking [IRON] (Vol 14) -m pyrotechnics [PYROTECHNICS] (Vol 20) -thorium, uranium, and plutonium as [ACTINIDES AND TRANSACTINIDES] (Vol 1)... 

Steeper, R.R., Methane and methanol oxidation in supercritical water chemical kinetics and hydrothermal flame studies, Sandia Rep., Sand96-8208.UC-1409, 1-150, 1996. 

When deuterium oxide is used as D source, the reaction temperature should be considered. When water in a closed pot is heated beyond the boiling point, it becomes subcritical and, eventually, supercritical [12]. Water under these conditions should also have potential in organic reactions [13, 14]. The same should happen with deuterium oxide. The value of p K%v for subcritical water should be noted. It has the low value of ca. 11 under typical hydrothermal conditions (250°C/4—5 MPa). This means that hydrothermal deuterium oxide ionizes to a greater extent than under ambient conditions (1000 times more) and several acid-catalyzed reactions can actually be performed conveniently under supercritical or subcritical conditions without adding any acid. It is also interesting to perform transition metal-catalyzed reactions under hydrothermal conditions. Under these conditions, one should consider the redox equilibrium shown in Scheme 4 [15]. 

Rofer CK, Buelow SJ, Dyer RB, Wander JD. Conversion of hazardous materials using supercritical water oxidation. U.S. Patent 5,133,877, 1992 Dell orco PC, Foy BR, Robinson JM, Buelow SJ. Hydrothermal treatment of Hanford waste constituents. Hazard Waste Hazard Mater 1993 10 221. 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

Hydrothermal synthesis is often applied to the preparation of oxides. The synthesis of metal oxides in hydrothermal conditions is believed to occur in a two-step process. In the first step, there is a fast hydrolysis of a metal salt solution to give the metal hydroxides. During the second step, the hydroxide is dehydrated, yielding the metal oxide desired. The overall rate is a function of the temperature, the ion product of water, and the dielectric constant of the solvent. The two steps are in balance during the reaction. The hydroxide of the metal salt is favored by a high dielectric constant, while the dehydration of the metal hydroxide is favored by a low dielectric constant. Since the fast reaction is the first step, it is expected that as one approaches supercritical conditions, the rate of reaction increases. 

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