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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Figure 4. (A) A liquid in equilibrium with its vapor. (B) When a liquid is heated in a sealed container, the density of the vapor pressure increases and that of the liquid decreases slightly. Due to vaporization, the quantity of liquid decreases. (C) At critical temperature, interface between the fluids disappear, at which condition the densities of liquid and vapor are equal.
When a liquid is heated in a sealed vessel, boiling does not occur. Instead, the temperature, vapor pressure, and density of the vapor rise continuously. At the same time, density of the liquid also decreases as a result of its expansion. There comes a stage at which density of the vapor is equal to that of remaining liquid and the surface between two phases disappears. The temperature at which the surface disappears is the critical temperature, Tc, and the corresponding vapor pressure is the critical pressure, Pc. At and above this temperature, a single uniform phase fills the container and an interface no longer exists. That is, above the critical temperature the liquid phase of the substance does not exist.
For a reduced temperature (TR = T/Tc) in the range 0.9-1.2, the reduced fluid density (pR =p/pc) can increase from gas-like values of
0.1 to liquid-like values of 2.5 as the reduced pressure (PR = P/Pc) is increased to values greater than 1.0. But as TR 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.
When utilizing additives in conjunction with a single solvent, binary phase diagrams become useful. Actually single component PT phase diagrams are of limited value for operations such as supercritical chromatography or extraction purposes, because any analyte or solvent present generates a binary phase system. When injecting a sample dissolved in a liquid, ternary or even higher phase diagrams must be considered. Fortunately, in many cases one can look at an ideal diluted solution without pondering over state parameters too much, so it is appropriate to look at the properties of binary mixtures which can differ profoundly from single components. For example, it is not possible to interpolate the critical point for carbon dioxide/ methanol mixtures from the critical point of each component. Depending on the mole fraction and temperature, there exists a distinct pressure maximum, while the critical temperatures lie between those of the pure compounds.
Supercritical fluids make ideal solvents because their density is only about 30% that of a normal fluid, a factor sufficient to provide for good solvent capability, but low enough for high diffiisivity and rapid mass transfer. A sample of gas is supercritical whenever its temperature and pressure are above their critical values, but in practice the operating temperatures are not far above Tc. A simple way to predict the solvent characteristics of a low-boiling substance in its supercritical state is to compare the boiling point (Tb) and critical temperature (Tc) of substance. The Guldbergs rule:
Tb = 2/3 Tc or Tb x \.S = TC (K)
can be applied^12! for low-boiling point liquids. As discussed earlier, supercritical fluids posses properties which resemble gases and liquids. The diffusion coefficient of a SCF is somewhere in the middle between those for gases and liquids and, the viscosity is similar to that of gases while the density is close to that of liquids. If only density is the requirement, one could operate near the critical point. However, the density changes with a maximum rate near the critical point and small pressure differences have a marked effect on density. While raising pressure increases the density, raising temperature will always decrease the density, but will only decrease the solubility at pressures below 350 bar. Solvent properties improve with fluid density and hence can be strongly influenced by changes in pressure. The effects of temperature are more complex.
Supercritical fluids have the ability to dissolve nonpolar solids, and this is what makes them useful for various applications, especially cleaning, though the fact that solids can dissolve in gases remains counterintuitive to most scientists. A number of attempts have been
made to quantify the solvent strength of supercritical fluids, usually through solvatochromic shifts in the UV-visible absorption bands of organic dyes.[13J The precise positions of these bands change as the density of the fluid increases; the solvent strength of the fluid at a particular density is determined by finding a conventional solvent of known strength in which the dye displays an absorption band at the same wavelength as in the supercritical fluid.
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