Supercritical effects

The subject of chemical reactions under supercritical conditions is well outside the scope of matters of major concern to combustion related considerations. However, a trend to increase the compression ratio of some turbojet engines has raised concerns that the fuel injection line to the combustion chamber could place the fuel in a supercritical state that is the pyrolysis of the fuel in the line could increase the possibility of carbon formations such as soot. The 

What follows is an attempt to give some insight into a problem that could arise in some cases related to combustion kinetics, but not necessarily related to the complete held of supercritical use as described in pure chemistry texts and papers. It is apparent that the high pressure in the supercritical regime not only affects the density (concentration) of the reactions, but also the dififusivity of the species that form during pyrolysis of important intermediates that occur in fuel pyrolysis. Indeed, as well, in considering the supercritical regime one must also be concerned that the normal state equation may not hold. 

Following the general chemical approach [75] to evaluate the extent of a given route it is possible to conclude that under supercritical conditions the extent of fuel fouling (PAH formation) could be determined by the ratio of the 

In hydrocarbon oxidation a negative reaction rate coefficient is possible. 

Explain in a concise manner what the essential differences in the oxidative mechanisms of hydrocarbons are under the following conditions  

---

One should thus not regard LCEs as entirely below-critical or above-critical systems, but rather as systems in which both types of phase-transition behaviours may be found, the extent of which is determined by the values of (G) and Gg- We shall introduce the term smeared criticality for such behaviour. In Fig. 16a, the graphical representation illustrates the extent of each type of phase transition behaviour in two LCEs, one prevalently below critical and the other prevalently above critical. A straightforward manifestation of the smeared criticality is the presence of latent heat in many LCE systems that exhibit a supercritical, effective thermodynamic response (see Sect. 5). This latent heat is released by the below-critical component of the LCE whose extent is given by the surface of the shaded area in Fig. 16. 

Supercritical CO2 is used to extract fat from candy bars. Samples are placed in an extraction vessel and its weight determined before and after extraction. The %w/w fat content is determined by difference. The volume of CO2 needed to effect a complete extraction is determined experimentally. Variations in results for different samples illustrate the importance of sampling. 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubiUties in supercritical fluids approach and sometimes exceed those of Hquid solvents as the SCF density increases. SolubiUties typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubiUties, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubiUty (16). 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Removing an analyte from a matrix using supercritical fluid extraction (SEE) requires knowledge about the solubiUty of the solute, the rate of transfer of the solute from the soHd to the solvent phase, and interaction of the solvent phase with the matrix (36). These factors collectively control the effectiveness of the SEE process, if not of the extraction process in general. The range of samples for which SEE has been appHed continues to broaden. Apphcations have been in the environment, food, and polymers (37). 

Cyclopentadiene oligomers up to octamers can be effectively analy2ed and quantified by supercritical fluid chromatography using a chemically bonded methyl siUcone capillary column. 

Supercritical Mixtures Dehenedetti-Reid showed that conven-tionaf correlations based on the Stokes-Einstein relation (for hquid phase) tend to overpredict diffusivities in the supercritical state. Nevertheless, they observed that the Stokes-Einstein group D g l/T was constant. Thus, although no general correlation ap es, only one data point is necessaiy to examine variations of fluid viscosity and/or temperature effects. They explored certain combinations of aromatic solids in SFg and COg. 

Flows are typically considered compressible when the density varies by more than 5 to 10 percent. In practice compressible flows are normally limited to gases, supercritical fluids, and multiphase flows containing gases. Liquid flows are normally considerea incompressible, except for certain calculations involved in hydraulie transient analysis (see following) where compressibility effects are important even for nearly incompressible hquids with extremely small density variations. Textbooks on compressible gas flow include Shapiro Dynamics and Thermodynamics of Compre.ssible Fluid Flow, vol. 1 and 11, Ronald Press, New York [1953]) and Zucrow and Hofmann (G .s Dynamics, vol. 1 and 11, Wiley, New York [1976]). 

THE EFFECT OF PROTON-DONATING MODEFIER ON THE SOLUBILITY ENHANCEMENT OF TRIS(p-DIKETONATO) CHROMIUM(HI) IN SUPERCRITICAL CARBON DIOXIDE... 

---