Supercritical critical conditions

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

Supercriticalfluid solvents are those formed by operating a system above the critical conditions of the solvent. SolubiHties of many solutes ia such fluids often is much greater than those found for the same solutes but with the fluid at sub atmospheric conditions. Recently, there has been considerable iaterest ia usiag supercritical fluids as solvents ia the production of certain crystalline materials because of the special properties of the product crystals. Rapid expansion of a supercritical system rapidly reduces the solubiHty of a solute throughout the entire mixture. The resulting high supersaturation produces fine crystals of relatively uniform size. Moreover, the solvent poses no purification problems because it simply becomes a gas as the system conditions are reduced below critical. 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

Studies on reaction mechanisms and by-product analysis have indicated that short-chain carboxylic acids, ketones, aldehydes, and alcohols are the major oxidation intermediates under near-critical conditions, but at supercritical conditions, with T above 650 °C, no intermediate compounds have been found [7]. 

However, even with carbon dioxide as a drying agent, the supercritical drying conditions can affect the properties of a product. Other important drying variables include the path to the critical point, composition of the drying medium, and depressurization. 

At this time sample preparation procedures using extraction solvents at supercritical or near-critical conditions are becoming more and more... 

Figure 11 illustrates the parameter space defined by the equilateral triangle. The initial pressure and conditions for the 3 vertices of the pressure gradient/ temperature triangle were determined arbitrarily from the critical conditions of the supercritical fluid (carbon dioxide), the retention characteristics of nitroaromatic compounds, and the following criteria (i) the first analyte should not co-elute with the sample solvent and (ii) the retention factor of the last analyte should not exceed 30. 

Fig. 4.5 Temperature profiles in subcritical (i>i), critical ( 2), and supercritical (< 3) conditions of heat exchange...

Fig. 4.7 Temperature profiles and second derivative in subcritical (i), critical (2), and supercritical (5) conditions according to the Thomas and Bowes criterion...

As can be seen from Fig. 13, the first peak of the goH(r) correlation function broadens and shifts to longer distances, while the first minimum rises on approaching the critical conditions. In the supercritical state, represented in plot 3, a sharp decrease in the intensity of the first peak can be observed. Although it does not correspond exactly to a complete collapse of the peak, such behaviour is undoubtedly in much closer qualitative accordance with the experimental findings and with the recent simulations performed by Chialvo and Cummings [60] by means of a properly modified model potential. 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

The Diels-Alder cycloaddition reaction of maleic anhydride with isoprene has been studied in supercritical-fluid CO2 under conditions near the critical point of CO2 [759]. The rate constants obtained for supercritical-fluid CO2 as solvent at 35 °C and high pressures (>200 bar) are similar to those obtained using normal liquid ethyl acetate as the solvent. However, at 35 °C and pressures approaching the critical pressure of CO2 (7.4 MPa), the effect of pressure on the rate constant becomes substantial. Obviously, AV takes on large negative values at temperatures and pressures near the critical point of CO2. Thus, pressure can be used to manipulate reaction rates in supercritical solvents under near-critical conditions. This effect of pressure on reacting systems in sc-fluids appears to be unique. A discussion of fundamental aspects of reaction kinetics under near-critical reaction conditions within the framework of transition-state theory can be found in reference [759],... 

In highly supercritical regimes, the combustion process resembles diffusion of a gas from a point or distributed source near critical conditions the strong dependence of the diffusion coefficient of the fuel on temperature can aid in establishing an approach to quasisteady conditions in the fuel-rich zone for situations under which quasisteadiness otherwise would not be expected. The theoretical predictions of [119]-[124] are in qualitative agreement with experimental observations in [77], [88], and [92] made near the critical point. Conditions of criticality may be attained in diesel and rocket combustion. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

Chiu Y-W and Tan C-S. Regeneration of supercritical carbon dioxide by membrane at near critical conditions. J. Supercrit. Fluids 2001 21(l) 81-89. 

Supercritical CO2 is the fluid with more applications at the present time, due to its readily amenable critical conditions (T = 31.3°C = 72.9 atm) and high volatil-... 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

Experimental gas-solid mass transfer data are presented for the well defined supercritical CO 2-naphthalene system at 10-200 atm and 35 C. These data are compared with low pressure gas-solid and liquid-solid systems. It has been found that both natural and forced convection are important under these conditions and that mass transfer rates at near-critical conditions are higher than at lower or higher pressure. 

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