Water, supercritical

These properties allow SCW to provide an environment and an opportunity to conduct chemistry in a single fluid phase that would otherwise occur in a multiphase system under more conventional conditions. The advantages of a single supercritical-phase reaction medium are that (i) higher concentrations of reactants can often be attained and (ii) there are no inter-phase mass transport processes to hinder reaction rates. 

In contrast to SCCO2, the conditions required to obtain SCH2O are harsh. In particular the temperature requirement of 374 °C precludes its synthetic utility for most organic compounds. That said, syntheses in sub-critical but high-temperature water are well studied and will be covered in the next section. Since many natural minerals and precious stones were formed in water at high temperature and pressure in the Earth s crust, the synthesis 

Carulite (Mn02/Cu0 on alumina) has shown exceptional performance for the complete rapid oxidation of phenol and other difficult substrates at temperatures just above T. The first full-scale SCWO plant has been commercialized by Huntsman, and it is expected that the technology will now become more mainstream as the value of different kinds of supercritical fluid technology becomes generally more widely appreciated and cost effective. 

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Highly pure / -hexane is used to extract oils from oilseeds such as soybeans, peanuts, sunflower seed, cottonseed, and rapeseed. There has been some use of hydrocarbons and hydrocarbon-derived solvents such as methylene chloride to extract caffein from coffee beans, though this use is rapidly being supplanted by supercritical water and/or carbon dioxide, which are natural and therefore more acceptable to the pubHc. 

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. 

M. ModeU, "Detoxification and Disposal of Hazardous Organic Chemicals by Processing with Supercritical Water," Pinal Report, U.S. Medical Research and Development Command, 1985. 

S. H. Timbedake, G. T. Hong, M. Simson, and M. ModeU, "Supercritical Water Oxidation for Wastewater Treatment Preliminary Study of Urea... 

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. 

Various equations of state have been developed to treat association ia supercritical fluids. Two of the most often used are the statistical association fluid theory (SAET) (60,61) and the lattice fluid hydrogen bonding model (LEHB) (62). These models iaclude parameters that describe the enthalpy and entropy of association. The most detailed description of association ia supercritical water has been obtained usiag molecular dynamics and Monte Carlo computer simulations (63), but this requires much larger amounts of computer time (64—66). 

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

Using supercritical water is not without its drawbacks, two of which are the high pressures and temperatures involved. Another difficulty is the extreme corrosive nature of water at supercritical conditions. If halogenated organics are treated, special alloy reactors are requited. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Reaction vessels for supercritical water oxidation must be highly corrosion resistant because of the aggressive nature of supercritical water and oxidation reaction products at extreme temperatures and pressures. Supercritical oxidation of PCBs and some chlorinated hydrocarbons can be difficult... 

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. 

Hydrotheimal oxidation (HO) (also called supercritical water oxidation) is a reactive process to separate aqueous wastes into water, CO9, nitrogen, salts, and other byproducts. It is an enclosed and complete water-treatment process m ng it more desirable to the public than incineration (Fig. 22-25) (Tester et al., op. cit. Gloyna and Li,... 

FIG. 22-25 Hydr othermal-oxidation process (also called supercritical water oxidation) for wastewater purification. (Cowtesy Eco-Waste Technologies.)... 

Supercritical Water Oxidation (SCWO) Wet oxidation occurring in supercritical water at temperatures greater than 374°C (705°F) and pressures greater than 221 bar (3204 psig). 

SCWO Supercritical Water Oxidation TKN Total Kjeldahl Nitrogen TOC Total Organic Carbon TSS Total Suspended Solids WAO Wet Air Oxidation... 

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. 

Diels-Alder reactions using supercritical water as an aqueous solvent medium [79]... 

Supercritical fluid extraction — During the past two decades, important progress was registered in the extraction of bioactive phytochemicals from plant or food matrices. Most of the work in this area focused on non-polar compounds (terpenoid flavors, hydrocarbons, carotenes) where a supercritical (SFE) method with CO2 offered high extraction efficiencies. Co-solvent systems combining CO2 with one or more modifiers extended the utility of the SFE-CO2 system to polar and even ionic compounds, e.g., supercritical water to extract polar compounds. This last technique claims the additional advantage of combining extraction and destruction of contaminants via the supercritical water oxidation process."... 

There are two distinct conditions that have been used above the critical temperature and pressure (374°C and 218 atm) water becomes a supercritical fluid in which the distinction between the liquid and gaseous states disappears. Since supercritical water can dissolve nonpolar compounds, it has been examined for the degradation of such contaminants. Subcritical water in which the liquid state is maintained by the pressure of the containing vessel has also achieved attention. 

Weber R, S Yoshida, K Miwa (2002) PCB destruction in subcritical and supercritical water—evaluation of PCDF formation and initial steps of degradation mechanisms. Environ Sci Technol 36 1834-1844. 

Gasafi, E., Meyer, L., Schebek, L. (2004) Using Life-Cycle Assessment in Process Design Supercritical Water Gasification of Organic Feedstocks. Journal of Industrial Ecology, 7(3M), 75-91. 

FIG. 20-18 Physical properties of water versus temperature at 240 bar. [Reprinted from Kritzer and Dinjus, An Assessment of Supercritical Water Oxidation (SCWO) Existing Froblems, Tossibh Solutions and New Reactor Concepts Chem. Eng. ].,vol. 83(3), pp. 207-214, copyright 2001, withpermission form Elsevier ]... 

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

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