SCF carbon dioxide

The color of the material changed from translucent white to translucent light yellow after treatment with carbon dioxide. The SCF carbon dioxide has plasticized the material as noted by a decrease in both yield strength and modulus. It is not certain whether carbon dioxide has induced further crystallization in the samples, since the ultimate elongation remains high and no increase in modulus after treatment in either condition is noted. 

The solvent strength of a supercritical fluid (compressed gas) may be adjusted continuously from gas-like to liquid-like values, as described qualitatively by the solubility parameter. The solubility parameter, 8 (square root of the cohesive energy density) (2), is shown for gaseous, liquid, and SCF carbon dioxide as a function of pressure in Figure 1. It is a thermodynamic property which can be calculated rigorously as... 

SCF carbon dioxide is a lipophilic solvent since the solubility parameter and the dielectric constant are small compared with a number of polar hydrocarbon solvents. Co-solvents(also called entrainers, moditiers, moderators) such as ethanol have been added to fluids such as carbon dioxide to raise the solvent strength while maintaining it s adjustability. Most liquid cosolvents have solubility parameters which are larger than that of carbon dioxide, so that they may be used to increase yields, or to decrease pressure and solvent requirements. A summary of the large increases in solubility that may be obtained with a simple cosolvent is given at the top of Table I. Cosolvents, unlike carbon dioxide, can form electron donor-acceptor complexes (for example hydrogen bonds) with certain polar solutes to influence solubilities and selectivities beyond what would be expected based on volatilities alone. Several thermodynamic models have been developed to correlate and in some cases predict effects of cosolvent on solubilities( ,2). They are used extensively in SCF research and development... 

SCF technology has spread quickly from molecules such as naphthalene to more complex substances such as polymers, biomolecules, and surfactants. Supercritical fluids can be used to reduce the lower critical solution temperature of polymer solutions in order to remove polymers from liquid solvents(6.26 The technology has been extended to induce crystallization of other substances besides polymers from liquids, and has been named gas recrystallization(4). In other important applications, SCF carbon dioxide has been used to accomplish challenging fractionations of poly(ethylene glycols) selectively based on molecular weight as discussed in this symposium, and of other polymers(. ... 

Since 2009 important part of scientific and educational activity of the NAFU is the common use centre Arctic which is well equipped with the devices for ScF extraction, chromatography, and chemical reactions performing. It helps in the analysis of biomass of different plants composition and properties, and synthesis of new materials in ScF carbon dioxide. Not only postgraduate or doctoral students but also students working on Bachelor s and Master s programmes have free access to such modern equipment during seminars and training as well as for preparation of their own projects. 

There has been an increasing amount of interest in the use of supercritical fluids (SCF) for leaching or extracting compounds from solids or liquids in the food and pharmaceutical industries because of the nontoxic nature of the primary SCF—carbon dioxide. In this section we will briefly consider the properties of SCFs that make them interesting for extraction. Then a typical process for SCF extraction will be explored and several applications will be discussed. 

These values are as much as one hundred times larger than those typically observed in conventional liquids. The improved transport rates in SCFs versus liquid solvents are important in practical appheations including supercritical extraction. Furthermore, carbon dioxide diffuses through condensed-hquid phases (e.g., adsorbents and polymers) faster than do typical solvents which have larger molecular sizes. 

Gas sales contracts for natural gases will limit the concentration of acid compounds. In the United States, typically, gas sates contracts will permit up to 2 to 3% carbon dioxide and 4 grain per 100 scf (approximately 4 ppm) of hydrogen sulfide. The actual requirement for any sales contract nia> vary, depending upon negotiations between seller and purchaser. 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

Among the SCFs, supercritical carbon dioxide (SCCO2) provides additional benefits [73], since it is environmentally benign, inexpensive, available in large quantities, nonflammable, and exhibits low toxicity. Its critical pressure is relatively low (73.4 bar) and it has an ambient critical temperature (31.3 °C). CO2 can be easily removed from reaction mixtures by depressurization [74]. 

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. 

A supercritical fluid (SCF) is a substance above its critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can exist as a gas. The critical pressure is the pressure needed at the critical temperature to liquify a gas. Above the critical temperature and critical pressure, a substance has a density characteristic of a liquid but the flow properties of a gas, and this combination offers advantages as a reaction solvent. The liquidlike density allows the supercritical fluid to dissolve substances, while the gaslike flow properties offer the potential for fast reaction rates. Supercritical carbon dioxide (scC02) has a critical temperature of 31°C and critical pressure of 73 atm. 

Homogeneous molecular catalysts, which have far greater connol over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of mass-transfer problems can be achieved. Supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry (Jessop et al., 1995). 

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