Supercritical fluids gases

Z. Liu, I. Osti ovsky, P. B. Farnsworth and M. L. Lee, Insti umentation for comprehensive two-dimensional capillary supercritical fluid-gas chromatography , Chromato-gmphia 35 567-573 (1993). 

All experiments were performed on a Lee Scientific (Dionex Corporation - Lee Scientific Division - Salt Lake City, UT) -602D Supercritical Fluid/Gas Chromatograph equipped with a 0.5 ml extraction cell. SFC grade carbon dioxide (Scott Specialty Gases, Plumsteadville, PA) was used as the extracting solvent and mobile phase in all experiments. All SFE and SFC investigations were performed under isothermal conditions. Flame ionization detection operating at 325°C was used in all studies. The specific column, conditions, and parameters are listed in the Applications section. 

Dillow AK, Dehghani F, Foster N. Production of polymeric support materials using a supercritical fluid gas antisolvent. Proceedings of the 4th International S5mposium on Supercritical Fluids, Sendai, Japan, 1997, 247-250. 

Such a result is consistent with surface modification of the interface by the supercritical fluid gas (the competitive adsorption effect). An interesting comparison is afforded between the measured differential heat of adsorption for 2-heptanone absorbing on Tenax from helium close to atmospheric pressure (IJ ) and the corresponding values on Tenax from supercritical CO2 at elevated pressures. The differential heat of adsorption for 2-heptanone at 250 atmospheres of CO2 is -1.42 kcal/mole and -0.84 kcal/mole at 350... 

Supercritical fluids/gas/anti- solvent method PEG/PLA, PLGA, PVP, Poly(3-hydro- xybutyrate-co-3- hydroxjrvalerate) (PHBV), Ethylcellulose/ methylcellulose. Phospholipids, Liposomes, Itraconazole, Paclitaxel, Amphotericin B, Oxeglitazar, Beclomethasone, Naproxen, Low molecular weight heparins (LMWH) High encapsulation efficiencies, uniform particle size, easy to scale up, no toxicity due to complete solvent removal High cost of equipment, difficulty in finding proper emulsion solvents [97-99]... 

General. - Reviews have appeared on liquid, supercritical fluid, gas and thin layer chromatography and electrophoresis of carbohydrates (360 iefs.) supercritical fluid chromatography for the analysis of natural products, including sugars and diagnostic microscale reactions of carbohydrate derivatives as a complement to ordinary capillary g.c.-m.s. and h.p.l.c.-m.s. analyses. ... 

In this chapter, latest advancements in solvent engineering in bioreductions and greener needs for bioreaction media have been discussed in depth with recent examples. Solvents for bioreductions may be categorized as (i) aqueous (ii) water/water-miscible (monophasic aqueous-organic system) (iii) water/ water-immiscible (biphasic aqueous-organic system) (iv) nonaqueous (mono-phasic organic system, including solvent-free system) and (v) nonconventional media (e.g., ionic liquids, supercritical fluids, gas-phase media, and reverse micelles). 

Membrane contactors provide a novel approach to the solution of many such problems (especially of the second and third kind) of contacting two different phases, one of which must be a fluid. Essentially, a porous membrane, most often in hollow-fiber form, is the basic element in such a device. Any membrane in flat or spiral-wound or hollow-fiber or any other form has two interfaces since it has two sides. However, conventional separation processes involve usually one interface in a two-phase system, for example, gas-liquid, vapor-liquid, liquid-liquid, hquid-supercritical fluid, gas-solid, liquid-solid, and the like. Membrane contactors allow the creation of one immobilized phase interface between two phases participating in separation via the porous membrane. Three types of immobilized phase interfaces in two-phase configurations are relevant ... 

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. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Supercritical Fluid Extraction. Supercritical fluid (SCF) extraction is a process in which elevated pressure and temperature conditions are used to make a substance exceed a critical point. Once above this critical point, the gas (CO2 is commonly used) exhibits unique solvating properties. The advantages of SCF extraction in foods are that there is no solvent residue in the extracted products, the process can be performed at low temperature, oxygen is excluded, and there is minimal protein degradation (49). One area in which SCF extraction of Hpids from meats maybe appHed is in the production of low fat dried meat ingredients for further processed items. Its apphcation in fresh meat is less successful because the fresh meat contains relatively high levels of moisture (50). 

The WAG process has been used extensively in the field, particularly in supercritical CO2 injection, with considerable success (22,157,158). However, a method to further reduce the viscosity of injected gas or supercritical fluid is desired. One means of increasing the viscosity of CO2 is through the use of supercritical C02-soluble polymers and other additives (159). The use of surfactants to form low mobihty foams or supercritical CO2 dispersions within the formation has received more attention (160—162). Foam has also been used to reduce mobihty of hydrocarbon gases and nitrogen. The behavior of foam in porous media has been the subject of extensive study (4). X-ray computerized tomographic analysis of core floods indicate that addition of 500 ppm of an alcohol ethoxyglycerylsulfonate increased volumetric sweep efficiency substantially over that obtained in a WAG process (156). 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

Erequenfly, the term compressed fluid, a more general expression than supercritical fluid, is used. A compressed fluid can be either a supercritical fluid, a near-critical fluid, an expanded Hquid, or a highly compressed gas, depending on temperature, pressure, and composition. 

A paiticularly attiactive and useful feature of supeicritical fluids is that these materials can have properties somewhere between those of a gas and a hquid (Table 2). A supercritical fluid has more hquid-hke densities, and subsequent solvation strengths, while possessiag transport properties, ie, viscosities and diffusivities, that are more like gases. Thus, an SCF may diffuse iato a matrix more quickly than a Hquid solvent, yet still possess a Hquid-like solvent strength for extracting a component from the matrix. 

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. 

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). 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

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). 

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. 

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]). 

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