Propane critical point

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. 

Figure 2-27 gives the saturation envelope for mixtures of methane, propane, and n-pentane at the same temperature as Figure 2-26 but at a higher pressure. The bubble-point and dew-point lines join at a critical point. The critical point gives the composition of the mixture, which has a critical pressure of 1500 psia and a critical temperature of 160°F. 

The critical point of a specific mixture of methane and propane occurs at 1040 psia at this temperature, dot 5. The dew-point and bubble-point lines of the ternary intersect the methane-propane side of the diagram at the composition of this critical point. 

Above this pressure, dot 6, all mixtures of methane and propane are single phase. Thus only the methane-n-pentane binaries have two-phase behavior, and only the methane-n-pentane side of the ternary diagram can show a bubble point and a dew point. The bubble-point and dewpoint lines of the saturation envelope do not intercept another side of the diagram, rather the two lines join at a critical point, i.e., the composition of the three-component mixture that has a critical pressure of 1500 psia at 160°F. 

Apart from polymerization processes with gaseous monomers above their critical points-for example, the synthesis of low-density poly(ethylene) - several SCFs have been tested as inert reaction media, such as ethane, propane, butane, and C02. Among these, scC02 is by far the most widely investigated, because it links positive fluid effects on the polymers with environmental advantages this makes scC02 the main candidate as an alternative to traditional solvents used in polymer syntheses. 

The properties and physical chemistry of liquid and supercritical carbon dioxide have been extensively reviewed (Kiran and Brennecke, 1992), as have many fundamentals and applications for separation, chromatography, and extraction (McHugh and Krukonis, 1994). The phase diagram for pure C02 is illustrated in Figure 1.1. Due to its relatively low critical point, C02 is frequently used in the supercritical state. Other common supercritical fluids require higher temperatures and pressures, such as water with Tc = 374.2 °C and Pc = 220.5 bar, while propane (Tc = 96.7 °C and Pc = 42.5 bar) and ethane (Tc = 32.2 °C and Pc = 48.8 bar) have lower critical pressures but are flammable (McHugh and Krukonis, 1994). 

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

Recently, the first observation of reverse micelles in supercritical fluid (dense gas) solvents has been reported (2) for the surfactant sodium bis(2-ethyhexyl) sulfosuccinate (AOT) in fluids such as ethane and propane. The properties of these systems have several attributes which are relevant to secondary oil recovery. In the supercritical fluid region, where the fluid temperature and pressure are above those of the critical point, the properties of the fluid are uniquely different from either the gas... 

The critical loci of mixture systems are of much interest because they define the boundary in pT space for light alkanes. They are approximately parabolic, connecting the critical points of the pure components and rising to a point of maximum pressure in between. This simple behavior is observed for binary mixtures of light n-parafhns, including methane and a C2-C5 parafQn, ethane and C3-C18 n-parafhn, and propane and some other n-parafhns. For mixtures of greater difference in molecular sizes or attractive forces, the critical loci are more complex due to the formation of solids or immiscible liquids that terminate the critical locus. 

LLV behavior near the critical point of methane T = —82.5°C, Pj. = 46.4 bar) occurs when the ratio of carbon atoms between methane and the second component exceeds 5.0. For binary ethane-hydrocarbon mixtures, LLV behavior occurs near the critical point of ethane T = 32.3°C, Pc = 48.8 bar) when the ratio of carbon atoms between ethane and the second component exceeds 9.5. And for binary propane-hydrocarbon mixtures LLV behavior occurs near the critical point of propane T = 96.7 C, Pc = 42.5 bar)... 

If the temperature is now increased to 82.2°C, the oil-propane binary mixture develops an LV region on the oil-propane axis of the ternary phase diagram and an LLV region appears in the interior of the ternary phase diagram. The three-phase behavior in this ternary diagram is similar to type-II ternary phase behavior described in chapter 3. The LLV behavior occurs because we are in close proximity to the critical point of propane. The degree of separation appears to have improved at this temperature for asphalt-oil feed mixtures which are at least about 60% asphalt. 

Figure 8.12 Pressure-volume diagram for equimolar mixtures of methane + propane, computed from the Redlich-Kwong equation of state. Filled square is the critical point filled circle is the mechanical critical point. The two branches of the saturation curve separate stable states from metastable states. The spinodal separates metastable states from unstable states and the line of incipient mechanical instability separates diffusionally unstable states from states that are both diffusionally and mechanically unstable. Since every point on this diagram represents an equimolar mixture, no tie lines can be drawn.

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