Supercritical fluid liquid-like density

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

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

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant maybe adjusted over the continuum of gas-like to liquid-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 applications of reactions in supercritical fluids is available (96). 

At more "liquid-like" densities, the solvatochromic shifts in supercritical fluids approach those observed in the corresponding liquids. Figures 1 and 2 depict the pressure dependence of the wavelength of the absorption maximum of 2-nitroanisole in supercritical CO2, N2O, CC1F3 (Freon-13), and NH3. These measurements reflect the effect of pressure (fluid density) on the cybotatic region of these solvents. It is clear that the fluid density affects the cybotatic region, as evidenced by the shift in the absorption maximum with pressure and it is also evident that the magnitude of the shift is fluid dependent. 

Fluids are highly compressible along near-critical isotherms (L01-1.2 Tc) and display properties ranging from gas-like to Liquid-Like with relatively small pressure variations around the critical pressure. The liquid-like densities and better-than-liquid transport properties of supercritical fluids (SCFs) have been exploited for the in situ extraction of coke-forming compounds from porous catalysts [1-6], For i-hexene reaction on a low activity, macroporous a catalyst, Tiltschcr el al. [1] demonstrated that reactor operation at supercritical... 

However, if the solvent is a supercritical fluid (SCF), it is possible to examine the effects of temperature and density on VER independently. The role of other solvent properties, such as viscosity, dielectric constant, and correlation length, can also be studied. A supercritical fluid is a substance that has been heated above its critical temperature (Tc) and, therefore, no longer undergoes the liquid/gas phase transformation. A typical phase diagram for an SCF is shown in Fig. 1. In an SCF, it is possible to fix the temperature and vary the density continuously (by varying the pressure) from gas-like densities to liquid-like densities. It is also possible to vary the temperature at fixed density. 

Employing 1-hexene isomerization on a Pt/y-ALOj reforming catalyst as a model reaction system, we showed that isomerization rates are maximized and deactivation rates are minimized when operating with near-critical reaction mixtures [2]. The isomerization was carried out at 281°C, which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are the main reaction products, the critical temperature and pressure of the reaction mixture remain virtually unaffected by conversion. Thus, an optimum combination of gas-like transport properties and liquid-like densities can be achieved with relatively small changes in reactor pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like) reaction media for the in situ extraction of coke-forming compounds. 

Supercritical fluids are unique solvents and reaction media due to liquid like density and gas like viscosity. Diffusion is not limited by any interface. Under ambient conditions hydrocarbons and water are nearly unmiscible. Phase equilibrium changes significantly in the supercritical region of water (Tc = 647 K, pc = 22.1 MPa). Hydrocarbons and supercritical water become miscible at any ratio, whereas supercritical carbon dioxide and hydrocarbons still have a broad miscibility gap [4],... 

In the last years supercritical fluid (SCF) technology has occupied a significant place in the high pressure chemical engineering. Due to their specific properties as liquid-like densities, gas-like viscosities and diffusivities intermediate between gas and liquid values, SCF have large potential in extraction and separation processes, polymer science and technology and in elaboration of new materials [1-4], In these last cases, to control the size of particles, we have to deal with the kinetics of their formation. ... 

Supercritical fluids have physical properties intermediate to those of gases and liquids. Therefore, they are considered useful as reaction media because of the surprisingly high solubility of liquid and solid solutes in them, particularly when compressed to liquid-like densities. 

Supercritical fluids possess characteristics that make them interesting for use as polymerization media. A supercritical fluid exists at temperatures and pressures above its critical values. In the supercritical state, the fluid exhibits physical and transport properties intermediate between the gaseous and liquid state. This is illustrated in Table 2. SCFs have liquid-like densities, but gas-like diffusivities. These intermediate properties can provide advantages over liquid-based processes. In particular, the higher diffusivities of SCFs reduce mass transfer limitations in diffusion-controlled processes. Additionally, lower energy is required for processing the supercritical fluid because its viscosity is lower than that of most liquids, and because the need to vaporize large quantities of liquid is avoided. 

For a reduced temperature (7 j = T/Tc) in the range 0.9-1.2, the reduced fluid density (p/ =p/Pc) can increase fi om gas-like values of 0.1 to liquid-like values of 2.5 as the reduced pressure (P/j = P/P is increased to values greater than 1.0. But as is increased to 1.55, the supercritical fluid becomes more expanded and reduced pressures greater than ten are needed to obtain liquid-like densities. By operating in the critical region, the pressure and temperature can be used to regulate density, which in turn regulates the solvent power of a supercritical fluid. 

Supercritical fluids have long been known for their abilities to dissolve organic contaminants. Their ability to display a wide range of solvent characteristics and the ability to tune solubility with small changes in temperature and pressure were identified early on in our search for alternative cleaning methods. The gas-like diffusivity and low surfece tension combined with liquid-like densities were important... 

Liquid-like densities of supercritical gases result in liquid-like solvent powers this property and faster diffusion characteristics due to low-gas viscosity make supercritical fluids attractive extraction agents. Solubility of substances in supercritical gases derives from van der Waals molecular attractive forces and increases with increasing pressure at a constant temperature. The temperature influences the solution equilibria in a more complicated way than does the pressure. Compounds can be selectively dissolved by changing the density of the gas, i.e., pressure and temperature conditions. 

The supercritical fluid mefhod is a relafively new method, which can minimize the use of organic solvents and harsh manufacturing conditions taking advantage of two distinctive properties of supercritical fluids (i.e., high compressibility and liquid-like density). This method can be broadly divided into two parts rapid expansion of supercritical solutions (RESS), which utilizes the supercritical fluid (e.g., carbon dioxide) as a solvent for the polymer, " and supercritical antisolvent crystallization (SAS), using the fluid as an antisolvent that causes polymer precipitation. Recent reviews of the supercritical technology for particle production are available in the literature. ... 

Under supercritical conditions, a gas, such as carbon dioxide, possesses liquid-like density and solubility, and gas-like diffusivity and viscosity, along with zero surface tension. Thus, supercritical fluids work extremely well as a processing media for a wide variety of chemical, biological, and polymer extractions. This solvent power of supercritical fluid has been known since 1879. Nevertheless, its application was not considered until recently when the sharp increases in energy cost, environmental regulations, and performance demands on materials have caused industry to consider alternative... 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

In addition to its unique solubility characteristics, a supercritical fluid possesses certain other physicochemical properties that add to its attractiveness. For example, even though it possesses a liquid-like density over much of the range of industrial interest, it exhibits gas-like transport properties of diffusivity and viscosity (Schneider, 1978). Additionally, the very low surface tension of supercritical fluids allows facile penetration into microporous materials to occur. 

The use of supercritical fluids as reaction media has received increased attention in recent years. The pressure-tunable physical and fransport properties of a supercritical fluid may be exploited to find an optimal reaction medium characterized by liquid-like density and heat capacity, yet significantly better (more gaslike) transport properties. The interest in SCF-based processes is evidenced by several recent reviews involving homogenous (1,2), immobilized (3), and hetero-... 

Supercritical fluids, even when compressed to achieve liquid-like densities, have gas-like viscosities that are frequently more than one order of magnitude lower than those of typical liquids. At its critical point, CO2 has a viscosity of only 0.025 cP, whereas benzene at 25 °C has a viscosity of 0.61 cP [29]. Figure 3.2-9 illustrates that the lower viscosities have a beneficial line-narrowing effect on quadrupolar nuclei stemming from the increased transverse relaxation times, 72, that accompany decreased viscosity [30]. For Co, with an electric quadrupole moment Q of 0.4 x 10 cm, the lines are sufficiently broad in normal liquids to cause (1) partial overlap, even for peaks having substantially... 

With small relative changes in pressure (or temperature), a supercritical fluid can be brought from gas-like densities, where it can hardly dissolve anything, to liquid-like densities, where the molecules of the fluid can cluster around the molecules of solutes and dissolve them into the gas phase. 

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