Supercritical gases, properties

A cosolvent used as a miscible additive to CO2 changed the properties of the supercritical gas phase. The addition of a cosolvent resulted in increased viscosity and density of the gas mixture and enhanced extraction of the oil compounds into the C02-rich phase. Gas phase properties were measured in an equilibrium cell with a capillary viscometer and a high-pressure densitometer. Cosolvent miscibility with CO2, brine solubility, cosolvent volatility, and relative quantity of the cosolvent partitioning into the oil phase are factors that must be considered for the successful application of cosolvents. The results indicate that lower-molecular-weight additives, such as propane, are the most effective cosolvents to increase oil recovery [1472]. 

Table 1. Typical liquid, supercritical fluid (SCF), and gas properties.

Amorphous fluoropolymers have many applications in the areas of advanced materials where they are used in applications requiring thermal and chemical resistance. Their manufacture is hindered by their low solubility in many solvents. Many fluoropolymerizations cannot be carried out in hydrocarbon solvents because the radical abstraction of hydrogen atoms leads to detrimental side reactions. Chlorofluorocarbons (CFCs) were thus commonly used, but their use is now strictly controlled due to their ozone depleting and greenhouse gas properties. Supercritical carbon dioxide is a very attractive alternative to CFCs and it has been shown that amorphous fluoropolymers can be synthesized by... 

Property Symbol Liquid Units Supercritical Gas fluid phase... 

Figure 15.21 shows a schematic representation of the SCCO2 treatment effect for promoting the internal diffusion of metal ions to prepare Rh and RhPt alloy nanoparticles in mesoporous FS-16 and HMM-1. The supercritical phase displays both liquid and gas properties at the same time. SCFs can also dissolve various metal precursors, which promotes their mobiUty and surface-mediated reaction to form nanoparticles by the hydrogen reduction in the mesoporous cavities of... 

Carbon dioxide has a conveniently low critical point (31 °C, 7.39 MPa), and supercritical CO2 has become the most widely used fluid where supercritical solvent properties are required, as it is also inexpensive and nontoxic. The solvent powers of supercritical fluids generally increase with increasing density, which can be regulated at will by varying the pressure. The absence of a gas-liquid interface and associated surface tension in a supercritical fluid enables the fluid to penetrate porous solids freely, and also to... 

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

One of the problems with using a liquid as the extraction solvent is its removal when the extraction is finished. The most recent way to eliminate this problem is to use a supercritical gas, COj being the gas of choice at the moment. A gas in the supercritical state has solvent properties comparable to a liquid but it is less viscous, so it can penetrate the sample faster. When the extraction is complete, the pressure is released, and the gas evaporates away from the extracted components. CO2 is nonpolar so more polar compounds such as methanol are sometimes added in small amounts. This exceWeni supercritical fluid extraction (SEE) technique is described in Chapter 13. 

In the molecular sciences, elueidation of the strueture and property of a van der Waals molecule is an important current researeh subject. The van der Waals molecule is unstable at room temperature, and it is quite difficult to get a concentrated system of van der Waals molecules [14,15]. There is another weakly interactive molecular complex compound, called a moleeular clathrate. Usually, molecular clathrates are unstable at ambient eonditions. Supercritical gas molecules such as N2 and NO are said to tend to produce dimers in micropores [16-20], In particular, NO molecules are adsorbed in micropores of activated carbon fiber (ACF) at ambient conditions in the form of dimers that are typical van der Waals molecules. Water molecules form an organized structure in the carbon micropore [21], The formation of NO hydrate and CH4 hydrate in micropores at a subatmospheric pressure has also been suggested [22,23]. Hence micropores accelerate the formation of van der Waals molecules or molecular clathrate hydrates. In the case of micropores that have a deep potential well, many molecules tend to be adsorbed in the deep potential well. Molecules confined in micropores should form the best dense structure according to the micropore geometry. Therefore, we can control the intermoleeular structure with inicropores even at supercritical conditions of the bulk gas. The molecular field of micropores can stabilize van der Waals molecules and molecular clathrates. 

The properties of SFs make them ideal for extracting analytes from solid matrices such as soils, agricultural products, foods, and solid sorbents. Supercritical fluids have the ability to maximize the extraction selectivity by controlling the temperature and pressure of the supercritical fluid (Figure 11.27) (85). Initially, the solubility of an analyte in a supercritical gas is dependent on solute vapor pressure thus the solubility of the analyte in the gas first decreases with a rise in pressure reaching a point of minimum solubility. As the gas is compressed into the critical phase, there is a rapid increase in analyte solubihty, which ends at a maximum pressure that is determined by the extraction temperature. Any additional increase in pressure will only slightly increase analyte solubility. Also, in... 

BOY Boyer, S.A.E., Klopffer, M.-H., Martin, J., and Grolier, J.-P.E., Supercritical gas-polymer interactions with applications in the petroleum industry. Determination of thermophysical properties, J. Appl. Polym. Sci., 103, 1706, 2007. 

One of the leading cork producers, Sabate, began the development of a cork cleaning process with supercritical gases in 1997 [32, 33]. The tests were very successful and, consequently, the process was patented. With supercritical gas, it was possible to reduce the TCA content below the detectable limit, which is at the moment around 0.2 ng/1. Furthermore, CO2 has special properties regarding reduction of pesticides and inhibition of fungus growth. 

A supercritical fluid is neither a liquid nor a gas but has intermediate qualities to the two. The critical point designates the temperature and pressure at which the substance exhibits supercritical fluid properties. The triple point is the temperature and pressure at which the solid, liquid, and gas forms of a substance coexist. 

Table 2. Comparison of gas, liquid, and supercritical fluid properties...

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. 

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). 

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. 

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). 

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. 

TABLE 22-12 Density and Transport Properties of a GaS/ Supercritical Fluid/ and a Liquid... 

---