Ethane critical point

Figure 4.3 Effects of pressure on residual volumes and compressibility factors along three supercritical isotherms for pure ethane. Broken horizontal lines represent values for the ideal gas. The ethane critical point occurs at = 305.3 K and = 48.7 bar. Note that Z —> 1 as P —> 0,...

When the two components are mixed together (say in a mixture of 10% ethane, 90% n-heptane) the bubble point curve and the dew point curve no longer coincide, and a two-phase envelope appears. Within this two-phase region, a mixture of liquid and gas exist, with both components being present in each phase in proportions dictated by the exact temperature and pressure, i.e. the composition of the liquid and gas phases within the two-phase envelope are not constant. The mixture has its own critical point C g. 

Moving back to the overall picture, it can be seen that as the fraction of ethane in the mixture changes, so the position of the two-phase region and the critical point change, moving to the left as the fraction of the lighter component (ethane) increases. 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

Detailed measurements of the solubility between the lower and upper critical end points have been made only for the solutions in ethylene of naphthalene,14 hexachlorethane,30 and />-iodochloro-benzene.21 Atack and Schneider2 have used dilute solutions of the last-named substance to study the formation of clusters near the gas-liquid critical point of ethane. 

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. 

The quantity Ail/A3, that is, the ratio between the largest perpendicular contraction at the (3, — 1) critical point and the parallel concentration towards the nuclei, is < 1 for closed-shell interactions. For shared interactions, its value increases with bond strength and decreasing ionicity of a bond. It decreases, for example, in the sequence ethylene (4.31), benzene (2.64), ethane (1.63). 

Figure 2-15 shows phase data for eight mixtures of methane and ethane, along with the vapor-pressure lines for pure methane and pure ethane.3 Again, observe that the saturation envelope of each of the mixtures lies between the vapor pressure lines of the two pure substances and that the critical pressures of the mixtures lie well above the critical pressures of the pure components. The dashed line is the locus of critical points of mixtures of methane and ethane. 

When the temperature exceeds the critical temperature of one component, the saturation envelope does not go all the way across the diagram rather, the dew-point and bubble-point lines join at a critical point. For instance, when the critical temperature of a mixture of methane and ethane is minus 100°F, the critical pressure is 750 psia, and the composition of the critical mixture is 95 mole percent methane and 5 mole percent ethane. 

Notice that the locus of the critical points connects the critical pressure of ethane, 708 psia, to the critical pressure of methane, 668 psia. When the temperature exceeds the critical temperature of both components, it is not possible for any mixture of the two components to have two phases. 

When pressure is less than the critical pressures of both components, the bubble-point and dew-point lines join at the vapor pressures of the pure components at either side of the diagram. When the pressure exceeds the critical pressure of one of the components, the bubble-point line and the dew-point line join at a critical point. For instance, a mixture of 98 mole percent methane and 2 mole percent ethane has a critical temperature of minus 110°F at a critical pressure of 700 psia. 

When the pressure of interest exceeds the critical pressures of both components, the phase envelope exhibits two critical points. For instance, mixtures of methane and ethane exhibit critical points at 900 psia and minus 62°F and at 900 psia and 46°F. 

Figure 6-6 gives the viscosity of ethane.2 Note the similarity between this figure and the graph of the densities of pure hydrocarbons given in Chapter 2. The dotted line is the saturation line, and the point of maximum temperature on the dotted line indicates the critical point. 

Note that the viscosity of the saturated liquid is equal to the viscosity of the saturated vapor at the critical point. The isobars above the saturation line give the viscosity of liquid ethane, and the isobars below the saturation line give the viscosity of ethane gas. 

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. 

Although the proposed mechanism is consistent for photolysis of iodine in helium, nitrogen and methane (24), substantive deviations were present at low densities and especially near the critical point of ethane. As Figure 3 shows, the quantum yields at these low densities are consistently below one, the value expected in this high diffusivity regime where kd k i. 

The unusual increase in regioselectivity with a decrease in pressure is not present for the photodimerization of a similar enone, isophorone, in SCF CO2 (31). The difference for the two enones is a result of differences in their phase diagrams and differences in the phase region in which the experiments were performed. As shown in Figure 8, the critical point for the cyclohexenone-ethane mixture is close to that of pure ethane. Hence the compressibility is large. In contrast, the... 

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). 

Fig. 7.17. Ortho-positronium annihilation rates at various values of ethane gas density D, at a temperature of 305.45 K. The solid line is a weighted average of the annihilation rates between 120 and 180 amagat. The broken line is the prediction for free ortho-positronium. The data are due to Sharma, Kafle and Hart (1984). Reprinted from Physical Review Letters 52, Sharma, Kafle and Hart, New features in the behaviour of ortho-positronium annihilation rates near the vapour-liquid critical point of ethane, 2233-2236, copyright 1984 by the American Physical Society.

Sharma, S.C., Kafle, S.R. and Hart, J.S. (1984). New features in the behaviour of orthopositronium annihilation rates near the vapour-liquid critical point of ethane. Phys. Rev. Lett. 52 2233-2236. 

The effect of solid structure on the solubilities of n-alkanes in supercritical ethane has been investigated at a temperature just above the critical point of ethane. Solubilities of n-alkanes containing 28 to 33 carbon atoms in ethane at 308.15K and pressures up to 20 MPa are reported in this work. The enhancement factor is shown to exhibit a regular trend with the number of carbon atoms in the n-alkane, although different trends are exhibited by the odd and even members of the series. 

A PT diagram for the ethane/heptane system is shown in Fig. 12.6, and a yx diagram for several pressures for the same system appears in Fig. 12.7. According to convention, one plots as y and x the mole fractions of the more volatile species in the mixture. The maximum and minimum concentrations of the more volatile species obtainable by distillation at a given pressure are indicated by the points of intersection of the appropriate yx curve with the diagonal, for at these points the vapor and liquid have the same composition. They are in fact mixture critical points, unless y = x = 0 or y = x = 1. Point A in Fig. 12.7... 

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... 

Before preceding, it is useful to consider the form of the force-force correlation function, which is given in Equation (21), with Equations (22), (24), (25), (26) and (27). The form of the force-force correlation function, derived using density functional formalism, is employed because it permits the use of very accurate equations of state for solvents like ethane and CO2 to describe the density dependence and temperature dependence of the solvent properties. These equations of state hold near the critical point as well as away from it. Using the formalism presented above, we are able to build the known density and temperature-dependent properties of the... 

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 value of the charge density at a bond critical point can be used to define a bond order (Bader et al. 1983 Cremer and Kraka 1984). The molecular graphs for ethane, ethylene, and acetylene are shown in Fig. 2.8. In each case the unique pair of trajectories associated with a single (3, — 1) critical point is found to link the carbon nuclei to one another. Multiple bonds do not appear as such in the topology of the charge density. Instead, one finds that the extent of charge accumulation between the nuclei increases with the assumed number of electron pair bonds and this increase is faithfully monitored by the value of p at the bond critical point, a value labelled p, . For carbon-carbon bonds, one can define a bond order n in terms of the values of Ph using a relationship of the form... 

---