Ethane critical enhancements

It should be pointed out here that the asymptotic description of the thermal conductivity is valid only extremely close to the critical point. Measurements on He + He (Cohen et al. 1982), methane + ethane (Friend Roder 1985 Roder Friend 1985) and CO2 + ethane (Mostert et al. 1992) seem to indicate that the thermal conductivity exhibits a critical enhancement similar to that observed for pure fluids. In Figure 6.7, as an example the experimental thermal-conductivity results for CO2 + ethane for a mole fraction of 25% CO2 in the one-phase region close to the critical isochore are presented, which were obtained by Mostert (1991). To reconcile the experimental data with the asymptotic result of equation (6.54), again a crossover theory is needed. Thermophysical quantities in fluid mixtures near a plait point undergo two types of crossover as the... 

Up to now there has been no rigorous analysis of the critical enhancement as carried out, for instance, for argon (see Section 14.1), ethane (Section 14.3) and R 134a (Section 14.5). Furthermore, the equation of state applied here cannot represent the data accurately within the temperature range 0.99 Tc critical temperature has been excluded from this correlation. 

Figure 14.17 illustrates the size and the extent of the critical enhancement of the viscosity of ethane. 

The excess viscosity has been determined for each datum by the use of equation (14.46) and subtracting the dilute-gas value, and the critical enhancement, A c, from the experimental value, T). For this purpose, reported by the experimentalists, rather than the value obtained from equation (14.47), has been preferred. This choice minimizes the influence of systematic errors in the individual measurements and forces the data of each author to a proper asymptotic behavior for the dilute-gas state. Furthermore, the experimental excess viscosity obtained in this fashion is independent of the choice for a dilute-gas viscosity correlation. Unfortunately, the majority of measurements on viscosity of ethane have been performed at pressures above 0.7 MPa and hence only a few authors reported a >7 value. Therefore, in order to estimate the experimental zero-density viscosity of each isotherm, again an iterative procedure had to be used (Hendl et al. 1994). The correction introduced by the extrapolation to zero density is small and in general does not exceed 0.5%. 

Figure 15.5 contains a plot of the predicted critical enhancement of the viscosity of the same carbon dioxide-ethane mixture, which also includes the background contribution... 

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. 

The results of an experimental Investigation are presented for the separation of mixtures of 1,3-butadiene and 1-butene at near critical conditions with mixed and single solvent gases. Ammonia was used as an entrainer to enhance the separation. Several non-polar solvents were used which included ethylene, ethane and carbon dioxide, as well as mixtures of each of these gases with ammonia in concentrations of 2, 5, 8 and 10% by volume. Each solvent and solvent mixture was studied with respect to its ability to remove 1-butene from an equimolar mixture of 1,3-butadiene/ 1-butene. Maximum selectivities of 1.4 to 1.8 were measured at a pressure of 600 psia and a temperature of 20 C in mixtures containing 5%-8% by volume of ammonia in ethylene. All other solvents showed little or no success in promoting separation of the mixture. The experimental results are reported for ethylene/ ammonia mixtures and are shown to be in fair agreement with VLE flash calculations predicted independently by a modified two parameter R-K type of equation of state. 

One aspect of the last set of experiments on W(CO)6 in supercritical ethane that we have not yet discussed involves the possible role of local density enhancements in VER and other experimental observables for near-critical mixtures. The term local density enhancement refers to the anomalously high solvent coordination number around a solute in attractive (where the solute-solvent attraction is stronger than that for the solvent with itself) near-critical mixtures (24,25). Although Fayer and coworkers can fit their data with a theory that does not contain these local density enhancements (10,11) (since in their theory the solute-solvent interaction has no attraction), based on our theory, which is quite sensitive to short-range solute-solvent structure and which does properly include local density enhancements if present, we conclude that local density enhancements do play an important play in VER and other spectroscopic observables (26) in near-critical attractive mixtures. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Carbon dioxide, water, ethane, ethylene, propane, ammonia, xenon, nitrous oxide, and fluoroform have been considered useful solvents for SEE. Carbon dioxide has so far been the most widely used as a supercritical solvent because of its convenient critical temperature, 304°K, low cost, chemical stability, nonflammability, and nontoxicity. Its polar character as a solvent is intermediate between a truly nonpolar solvent such as hexane and a weakly polar solvent. Moreover, COj also has a large molecular quadrupole. Therefore, it has some limited affinity with polar solutes. To improve its affinity, additional species are often introduced into the solvent as modifiers. For instance, methanol increases C02 s polarity, aliphatic hydrocarbons decrease it, toluene imparts aromaticity, R-2-butanol adds chirality, and tributyl phosphate enhances the solvation of metal complexes. 

Friend, D. G. Roder, H. M. (1985). Thermal-conductivity enhancement near the liquid-vapour critical line of binary methane-ethane mixtures. Phys. Rev. A, 32, 1941-1944. 

Water is known to be essential for the enzyme activity. Small amounts of water enhance enzyme activity, however excess water hinders the rate of some enzyme catalyzed reactions. Also, supercritical water cannot be used as the reaction medium either, because its critical temperature and pressure are too high for the enzymes used in biotransformations. The active site concentration on enzymes, hence the enzyme activity is found to be higher in the presence of hydrophobic supercritical fluids (ethane, ethylene) as compared to hydrophilic supercritical carbon dioxide. 

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