Supercritical ammonia applications

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

There is no doubt that these applications will grow in the future and that the range of supercritical fluids used (carbon dioxide and methanol modified carbon dioxide, nitrogen dioxide, ammonia, fluoro-hydrocarbons) will increase as will the combination of this technique with mass spectrometric identification of separated compounds. 

SCFs will find applications in high cost areas such as fine chemical production. Having said that, marketing can also be an issue. For example, whilst decaffeina-tion of coffee with dichloromethane is possible, the use of scCC>2 can be said to be natural Industrial applications of SCFs have been around for a long time. Decaffeination of coffee is perhaps the use that is best known [16], but of course the Born-Haber process for ammonia synthesis operates under supercritical conditions as does low density polyethylene (LDPE) synthesis which is carried out in supercritical ethene [17]. 

Liquefied or Supercritical Cases as Solvents for Electrolytes For very special applications, where the increased efforts for low temperature and/or pressurized cells are acceptable, liquefied gases, for example, sulfur dioxide or ammonia, can be interesting solvents for electrolytes (see e.g. [3a]). Supercritical fluids show remarkable properties that are very different from other solvents. Detrimental to electrochemistry is that especially the dielectric constant in the supercritical state becomes low. For supercritical carbon dioxide, no supporting electrolyte with sufficient conductivity is known. 

To access the supercritical fluid state, we must have conditions in excess of the critical temperature and pressure. Given the rating of the autoclave, ammonia would not be suitable because one could not access the supercritical state as a result of the pressure limitation. Methylamine would not be suitable for a room temperature extraction because its Tc is too high. Either methane or tetrafluoromethane would be suitable for this application. 

Cyclic voltammetry in supercritical water-0.2 M NaHS04 [88] and ammonia-0.14 M CF3SO3K [88,332] of some organic compounds shows that this electroanalytical technique was applicable under these conditions. The behavior of phenazine in NH3 at —40°C and under supercritical conditions, for example, was analogous two reversible reductions were found in both cases [88]. Dimethyl carbonate has been prepared from CO and MeOH on anodic oxidation in a supercritical mixture of CO2 and MeOH [89]. 

As an extraction fluid, supercritical carbon dioxide is mostly used because its critical parameters can be rather easy obtained (Tc = 31°C, Pc = 74 bar) and is non-polluting. Other supercritical fluids are nitrogen protoxide (Tc = 36°C, Pc = 71 bar), ammonia (Tc = 132°C, Pc = 115 bar) and water (Tc = 374°C, Pc = 217 bar). The supercritical fluids can be removed at low temperatures, without any toxic wastes but the necessary high pressure can be dangerous in industrial applications. The extractors are tubular devices, pressure resistant where the (semi) solid sample is placed. 

In this section, we describe selected examples of inorganic reactions that are carried out in supercritical water (SCH2O) and ammonia (SCNH3), critical temperatures and pressures of which are listed in Table 8.11. An important application of SCH2O is in the hydrothermal generation of metal oxides from metal salts (or supercritical hydrothermal crystallization). Equations 8.84 and 8.85 summarize the proposed steps for conversion of metal nitrates to oxides where, for example, M = Fe(III), Co(II) or Ni(II). 

In the following sections some aspects of (potential) applications of sc-fluids in the fine chemical industry with respect to product separation/purification and catalytic reactions are discussed. Earlier industrial applications of supercritical fluid reactions, for example the Haber-Bosch process for the synthesis of ammonia, synthesis of methanol from hydrogen and carbon monoxide, or the polymerization of ethene will not be discussed. An extensive overview on the use of sc-fluids in the synthesis of bulk chemicals is given in the book edited by fessop and Leitner [12],... 

A new era was inaugurated by the discovery and application of electronic conduction in crystals (Shockley, 1953). The study of charge transport in the many kinds of solid-state materials has grown into a vast field, and numerous books on the electronic properties of solids have appeared. Works on liquid metals and on liquid and amorphous semiconductors complement this field. Compared to these developments, the study of electronic conduction in liquids has received much less attention. Initially, the research was limited almost entirely to liquid ammonia, water, and aqueous solutions. Later, other polar solvents were included. From a fundamental point of view, studies on the conductivity of supercritical metal vapors (high pressures and high temperatures) gave insight into the important phenomenon of the metal/insulator transition. 

The first industrial SCF applications utilised SCCO2 for the extraction of natural compounds (caffeine, hops) and were successfully established in the early 1970 s. In the following decades, research focus also shifted towards reactions in SCCO2 and SCH2O (however, it is noteworthy that ammonia and methanol syntheses were sometimes considered as supercritical processes). From all these processes, fundamental thermodynamic data and practical experience in high-pressure reaction engineering are available and promote the development of supercritical oxidation processes. 

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