Supercritical fluid systems

A number of theoretical models have been proposed to describe the phase behavior of polymer—supercritical fluid systems, eg, the SAET and LEHB equations of state, and mean-field lattice gas models (67—69). Many examples of polymer—supercritical fluid systems are discussed ia the Hterature (1,3). 

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). 

Polymer-supercritical fluid systems, phase behavior of, 24 11... 

Shmulovich K. I., Shmonov V. M., and Zharikov V. A. (1982). The thermodynamics of supercritical fluid systems. In Advances in Physical Geochemistry, vol. 2, S. K. Saxena (series ed.). New York Springer-Verlag. 

The phase behaviour of binary polymer - supercritical fluid systems can be modelled with an equation of state model. In general, non-cubic equations of state are used, mainly from the PHCT and SAFT families. Lattice-fluid equations of state are also commonly used for the... 

Vibrational spectroscopy, too, has been used to study supercritical fluid systems. Buback reviewed (59) this area however, much of his discussions are on fluid systems that are well removed from ambient conditions or difficult to handle easily (e.g., H20, HC1). In an early report, Hyatt (21) used IR absorbance spectroscopy to determine the influence of several solvent systems, including COz, on the vibrational frequencies ( ) of solute molecules. Specifically, he studied the vc=o of acetone and cyclohexanone and vs.H of pyrrole. The goal of this work was to determine the suitability of supercritical fluids as reaction solvent. Hyatt concluded that the ketones experienced an environment similar to nonpolar hydrocarbons in COz and that there were no differences between liquid and supercritical CO2. In contrast, the pyrrole studies indicated that the solvent strength of CO2 was between ether and ethyl acetate. This apparent anomalous result was a manifestation of the, albeit weak, degree of pyrrole hydrogen bonding to CO2. 

Yonker and co-workers (60) used near-and mid-IR spectroscopy to study supercritical C02 and binary supercritical fluid systems composed of C02/H20, Kr/H20, and Xe/H20. The C02 results are consistent with increased intermolecular interaction between C02 molecules with increasing density. This parallels previous results using UV-Vis solvatochromism (21-28,32). For an ideal gas/water system an Onsanger electrostatic model (dipole-induced-dipole) sufficed to describe the spectral shifts. In contrast, the C(VH20 system exhibited density-dependent changes in specific intermolecular interactions. 

The experimental solubility data for solid naphthalene in supercritical carbon dioxide, given as moles naphthalene dissolved per liter, are shown in Figure 6. Qualitatively the three pressure-composition isotherms show characteristic behavior for a solid-supercritical fluid system. Each isotherm initially shows a large increase in solubility with increasing pressure, and then a limiting value is reached at higher pressures. 

Mixtures of C02 and methanol were selected for the initial investigation of the solvatochromic behavior in supercritical fluid systems. This combination is of interest as it combines the low critical temperature and pressure of carbon dioxide with a polar, less volatile modifier. This system exhibits relatively simple Type I phase behavior and several groups have published measurements of mixture critical points (19-21). At intermediate compositions the critical pressure for this fluid is much higher than that of either pure C02 or pure methanol, reaching a maximum of approximately 2400 psi (20). 

The main conclusion to be drawn from the experimental data presented here is that fractionation of residuum through the use of a supercritical fluid system incorporating internal reflux produced by retrograde condensation results in sharper fractions than those obtained by ordinary supercritical extraction. The capability of the FDU to process coal-derived residuum in the... 

In the present study, we tested the validity of the EOS-Ge models by applying different treatments for 21 binary solid/supercritical fluid systems listed in table 2. In those systems, the supercritical component is one of the following fluids carbon dioxide, ethane, fluoroform and chlorotrifluoromethane and the solid component is either a nonpolar compound... 

Clean a silicon wafer by rinsing with distilled water, and dry with KimWipes. Place the clean silicon substrate in a tall beaker, and place in the supercritical fluid reactor using long tweezers. With assistance from the instructor, fasten the reactor to the supercritical fluid system. 

Figure C.2. Photograph of the supercritical fluid system used for nanoparticle synthesis. Shown is the 300-mL high-pressure reactor (A), with pressure/temperature controllers (B). The system is rated for safe operation at temperatures and pressures below 200°C and 10,000psi, respectively. The vessel may be slowly vented, or exposed to a dynamic CO2 flow, using a multiturn restrictor valve (C), which provides a sensitive control over system depressurization, allowing for the collection of C02-solvated species in the stainless steel collector (D). For deposition using the rapid expansion of tlie supercritical solution (RESS), nanoparticles were blown onto a TEM grid that was placed under the stopcock below D. Also shown is the cosolvent addition pump (E) used for the synthesis of aluminum oxide nanoparticles, capable of delivering liquids into the chamber against a back-pressure of <5,000 psi.

As indicated in Figures 5 and 6, there is a nearly linear relationship between the log[AOT] solubility and the fluid density over several order of magnitude of AOT concentration. This type of behavior would be expected for the solubility of a non-aggregate forming, solid substance in a supercritical fluid (XL). The solubility and phase behavior of solid-supercritical fluid systems has been described by Schneider (2H) and others, and such behavior can be predicted from a simple Van der Waal s equation of state. Clearly, this approach is not appropriate for predicting surfactant solubilities in fluids, because it does not account for the formation of aggregates or their solubilization in a supercritical fluid phase. 

The initial equipment costs and operating costs for the more conventional processes are higher than for supercritical carbon dioxide. The higher equipment costs are due to the additional parts needed for environmental treatments (i.e., scrubbers, vapor incinerators, etc.) The consumables costs are lower for the supercritical fluid processes due to the closed-loop recycle design of the supercritical fluid system, elimination of water for rinsing and the reduced electricity costs associated with not having to dry the parts. 

Phase Equilibrium in Solid-Liquid-Supercritical Fluid Systems... 

We have measured sound velocities of various supercritical fluid systems. An attempt to carry forward such measurements on higher temperature isotherms of formic acid was frustrated by chemical reaction toward products that may include carbon dioxide, carbon monoxide, water, hydrogen and differentiated solid-like products at even higher temperatures and pressures. Nonetheless, the diamond anvil cell provides a unique opportunity to study the chemistry and kinetics of fluids under extreme conditions. We also find that CH2O2 is present during the detonation of some common explosives. 

For solid-supercritical fluid systems the LCEP occurs at the intersection of the low-temperature branch of the solid-liquid-gas line and the critical mixture curve at the LCEP a liquid and gas phase critically merge to form a single fluid phase in the presence of a noncritical solid phase. 

Figure 3.12 (a) P-T-x, (b) P-T, and (c, d, e) P-x diagrams for heavy solid-supercritical fluid system depicted in figure 3.11. 7c, is the critical temperature of the more volatile component. 

Figure 3.21 Examples of binary solid-supercritical fluid systems with a temperature minimum in the SLV line (a) carbon dioxide-solid systems (McHugh and Yogan, 1984) (b) methane-naphthalene system (van Hest and Diepen, 1963).

Lemert, R. M., and K. P. Johnston. 1989. Solid-liquid-gas equilibria in multicomponent supercritical fluid systems. J. Fluid Phase Equil. 45 265-286. 

To date little or no thermodynamic modeling of the phase behavior of the ligand/C02 or metal chelate/C02 systems has been conducted. However, in order for supercritical fluid extraction to be considered as a possible replacement for organic solvent extraction, accurate models must be developed to predict the phase behavior of these systems to allow for both equipment and process design. Equation of state (EOS) modeling was chosen here to model the vapor-liquid equilibrium of the P-diketone/C02 systems studied. Cubic EOSs are the most widely used in modeling high pressure and supercritical fluid systems. This is... 

Prior studies utilizing adsorbents in the presence of supercritical fluid media have been reviewed by King (3), who has commented on the lack of fundamental knowledge on adsorbate(sorbate)/adsorbent(sorbent)/supercritical fluid systems. Indeed, with the exception of the sorbent regeneration studies performed at Critical Fluid Systems in the last... 

Stability of phase boundaries depends on the surface tension. Surface tension in a supercritical fluid system is of major importance for drying, surfactant eflicacy, and extraction. The surface tension of a gas increases with pressure and approaches zero at the critical point while the surface tension of liquid decreases with pressure resulting in dissolution of supercritical components in the liquid phase. The mefliods useful in correlating surface tension include Macleod-Sugden correlation and corresponding states theory. ... 

Polymer supercritical-fluid systems show complex phase behavior (for a general overview see also [80]). Polymer solubihty in these systems depends on (apart from temperature, pressure, and concentration) the chemical nature, molecular weight, and molecular-weight distribution of the polymer and on the comonomer composition in the case of copolymers. 

The study of supercritical fluid systems by Nuclear Magnetic Resonance (NMR) has been accomplished in two ways the high-pressure probe method and the high-pressure cell method. At this present stage, no paper on the study of polymerization reactions in SCF by NMR could be found. Nevertheless, other reactions have been studied in supercritical media and at extreme conditions [13], and these can be compared with polymerization reactions. 

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