Supercritical physical properties

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Fig. 4. Comparison of physical properties of silica xerogels and aerogels. Note the similar properties of the aerogels prepared with and without supercritical...

Physical property Gases Supercritical fluids Liquids... 

Physical properties of pure supercritical fluids may be found ia many of the standard reference textbooks and journals (10). There are also... 

A fluid is described as supercritical or subcritical if its temperature is above or below its critical temperature. Above the critical temperature the liquid and vapor phases are indistinguishable, the densities of the two phases become identical and the substance is described as a fluid, the physical properties of which are intermediate between those of a liquid and a gas [75]. 

TABLE 20-12 Physical Properties of a Supercritical Fluid Fall between Those of a Typical Gas and Liquid... 

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 ]... 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

Changes to the physical properties of a compound or material can have a dramatic influence on the susceptibility to microwave radiation. For example, ice has dielectric properties (e, 3.2 tan 8, 0.0009 e", 0.0029) that differ significantly from those of liquid water at 25 °C (s, 78 tan <5, 0.16 e", 12.48) [31], rendering it essentially microwave-transparent. Although liquid water absorbs microwave energy efficiently, the dielectric constant decreases with increasing temperature and supercritical water (Tc 374 °C) is also microwave-transparent. 

Pure substance, phase behavior of, 24 663 Pure supercritical fluids, physical properties of, 24 4... 

Comparison of physical properties of liquids, gases and supercritical fluids... 

Supercritical fluids (SCFs) are best known through their use for the decaffeination of coffee, which employs supercritical carbon dioxide (scCC ). In this chapter, we will demonstrate that SCFs also have many properties that make them interesting and useful reaction media. Firstly, the physical properties of SCFs will be explained, then the specialist equipment needed for carrying out reactions under high temperatures and pressures will be described. Finally, we will discuss issues relevant to the use of SCFs as solvents for reactions. 

Supercritical fluids have many features that render their use attractive in synthetic chemistry and separations. Their tunable physical properties allow reactions to be carried out under a variety of conditions and, in some cases, the selectivities and rates of reactions may be altered. The list of reactions that have been carried out in SCFs and compared with those in conventional solvents is continually growing. 

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

To design a supercritical fluid extraction process for the separation of bioactive substances from natural products, a quantitative knowledge of phase equilibria between target biosolutes and solvent is necessary. The solubility of bioactive coumarin and its various derivatives (i.e., hydroxy-, methyl-, and methoxy-derivatives) in SCCO2 were measured at 308.15-328.15 K and 10-30 MPa. Also, the pure physical properties such as normal boiling point, critical constants, acentric factor, molar volume, and standard vapor pressure for coumarin and its derivatives were estimated. By this estimated information, the measured solubilities were quantitatively correlated by an approximate lattice equation of state (Yoo et al., 1997). 

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