Ethane critical parameters

Table 2 shows critical parameters of the fluids most used for SFE. When it comes to choosing a supercritical fluid, the critical pressure and the critical temperature are two important parameters. The critical pressure determines, from a first approximation, the importance of the solvent power of the fluid. Ethane, for example, which has a lower critical pressure than carbon dioxide, will not dissolve a moderately polar soluble in the same way as carbon dioxide. Similarly, fluids with a higher critical pressure are more able to dissolve polar compounds. The critical temperature has practical implications. Indeed, one should always consider the influence of the extraction temperature on the stability of the component to extract. 

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. 

Since both ethylene and ethane have reduced temperatures nearly equal to unity at the extraction conditions of 20 C, (T =. 98) and ethylene (T = 1.04), their respective solvent capacities for butene should be about the same. This is the case as is reflected in the same values for the selectivity against butene for all pure solvent gases. One can conclude that the primary effect of the non-polar solvent is to increase the capacity of the "vapor" phase for the extracted solute near the critical. The influence of the second solvent provides only the option of modifying the physical parameters namely, pressure and temperature, under which the optimal extraction is to be conducted. The evidence for this is the effect of the ammonia on the selectivity as calculated by the EOS in Table V. The higher values for the selectivities in the ethylene mixtures are pronounced. It can be concluded that the solvent mixture interaction parameters must dominate the solubility of butene in the vapor phase. 

Figure 10 (a) Ti (p, T) vs. density for the solvent ethane at 34°C and the best fit theoretically calculated curve. The theory was scaled to match the data at the critical density, 6.88 mol/L. The best agreement was found for u> = 150 cm-1, (b) Ti (p, T) vs. density for the solvent ethane at 50°C and the theoretically calculated curve. The scaling factor, frequency >, and the hard sphere diameters are the same as those used in the fit of the 34°C data. Given that there are no free parameters, the agreement is very good. 

Conversions and enantiomeric excesses (ee), obtained in ethane under supercritical conditions, are shown in Table 1. Favorable conditions for achieving good ee are high hydrogen pressure (>70 bar) and low temperature [8], The latter parameter is limited by the critical temperature of the ethane/hydrogen mixture [10]. Selectivities are about the same as those obtained in the best conventional apolar solvent, toluene. Initial reaction rates of the fast reaction could not be determined accurately. The reaction times, required for complete conversion of ethyl pyruvate, were 3 - 3.5 times lower in ethane than in toluene under otherwise identical conditions, likely due to the absence of mass transfer limitations in ethane. 

The densities of CO2 and ethane were calculated using multi-parameter EOS [17,18], which are accurate near the critical point. The parameters 22 and 33 were obtained from solubility data in binary mixtures (solid/SC fluid and solid/SC entrainer). The Soave-Redlich-Kwong [20] EOS was employed in combination with the classical van der Waals mixing rules as in our previous paper [5]. 

One example will serve to underscore the reason for the advantage over Chao-Seader at high pressure. Figure 1 shows the convergence of RKJZ K-values to unity as the mixture critical pressure is approached, for a temperature and composition on the mixture critical locus for the methane-ethane-butane ternary (20). This mixture was chosen in order to check RKJZ apparent critical pressure vs. the 1972 corresponding-states correlation of Teja and Rowlinson (21), which presumably has a better theoretical basis than the RKJZ method. In these comparisons, the Teja and Rowlinson correlation uses two interaction parameters per binary pair, based primarily on fits to binary critical loci the RKJZ method uses Cij = 0 for all binaries, based on binary VLE data. 

Y. Sun, D. Spellmeyei D. A. Pearlman, and P. A. Kollman, /. Am. Chem. Soc., 114, 6798 (1992). Simulation oiF the Solvation Free Energies for Methane, Ethane, and Propane and Corresponding Amino Acid Dipeptides A Critical Test of the Bond-PMF Correction, a New Set of Hydrocarbon Parameters, and the Gas Phase-Water Hydrophobicity Scale. 

The last point which can be evoked here is conceptually linked to the hot-spot theory. If the limit liquid layers around a bubble are in direct contact with the heated and pressurized bubble content, far above the critical point of the liquid, these layers should be in a supercritical state. 23 This attractive hypothesis (see p. 61) was used by Hoffmann et al to rationalize the sonolysis of nitrophenyl derivatives. Supercritical fluids are characterized by a very high flexibility of important parameters (density, dielectric constant, solubilizing power) as a function of pressure. Experts in the field distinguish gas-like and liquid-like media, in which the kinetics of a reaction can vary over a broad range. For instance, the conjugate addition of piperidine to methyl propiolate was studied in supercritical ethane or fluoroform (Fig. 5). ... 

Rather, an equation of state and semiemplrical correlations are used e.g. the Chueh-Prausnltz correlation scheme (10). In this approach the critical pressure of the mixture is obtained indirectly from a modified version of the Redlich-Kwong-Chudi equation of state after Tcm and have been obtained directly from quadratic mixing rules which employ the respective Chueh-Prausnitz Interaction parameters, I12 and V3 2 An example based on this method is given in Figure 5 for the ethane-n-heptane binary mixture. Agreement is rather good except in the immediate vicinity of the maximum critical pressure of the mixture. 

The introduction of the third parameter, a>, significantly improved the fit to the saturation properties of hydrocarbons for both equations. The SRK equation gives better predictions for the vapor pressure and saturated vapor volumes for alkanes from Ci up to Cio- For saturated liquid densities, the compression factor in the liquid and densities above the critical temperature, the SRK equation gives better results for Ci and C2 only, while the PR equation is better for hydrocarbons higher than ethane (Yu et al. 1986). Thus the choice of the most suitable equation to use will depend on both the size of the molecule and the part of the surface to be considered. 

The regression constants, R are in domain of 1 < R< 0.997. The next step in calculating the function cp(x) was the fitting of the Toth equation (387) to isotherms of ethane, ethylene, carbon dioxide, propane, and propylene measured on microporous adsorbents (activated carbons) at different temperatures below the critical one. These measured data are collected in Valenzuela and Myers handbook [19]. The adsorbents are activated carbons BPL, Nuxit, Columbia L, BPL-P, and Fiber Carbon KF-1500. AH isotherms are Type I without a multilayer plateau. The temperature domains are those shown in Fig. 44. The specific surface areas determined by the BET method, a (N2, 77 K) and the saturation pressures Pq ai collected in Ref 19. The parameters Kj-, and t are also known from the fitting procedure of the Toth equation (387). 

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