Tracer diffusion of aromatic hydrocarbons in liquid cyclohexane up to its critical temperature

Abstract

Tracer diffusion coefficients in liquid cyclohexane of benzene, toluene, p-xylene, mesitylene, naphthalene, and phenanthrene have been determined from 298.2 to 523.2 K ( TR = 0.54 -0.95) using the Taylor dispersion method. Positive deviations proached, but a rough-hard-sphere theory is found to be adequate for describing the data across the entire temperature range. On the basis of the computer simulation results for hard-sphere fluids, correlations involving solute and solvent critical volumes and their molecular weights have also been developed for practical applications. Tracer diffusivities in supercritical carbon dioxide are also adequately represented by the proposed correlation, as the fluid density is not far removed from that of liquid carbon dioxide. SCOPEThe design of chemical reactors or separation processes often requires a knowledge of binary diffusion coefficients. In particular, current advances in separation technology using dense fluids as solvents have resulted in a need for diffusion data in the range 0.9 < Tx < 1.2 and PR > 1.0. (Paulaitis et al., 1983).In view of the absence of experimental data on binary diffusion in liquids up to the critical point, we have determined the tracer diffusivities of a series of aromatic hydrocarbons in liquid cyclohexane up to TR = 0.95 using the Taylor dispersion method.The results are used to test the performance of a rough-hardsphere theory as well as the Arrhenius equation and togenerate correlations to improve prediction accuracy. CONCLUSIONS AND SIGNIFICANCEThe Taylor dispersion method has been employed to generate new tracer diffusion data in liquids up to their critical temperatures. Using measured values of tracer diffusivities in liquid cyclohexane of benzene, toluene, p-xylene, mesitylene, naphthalene, and phenanthrene, we have demonstrated the success of a.rough-hard-sphere theory. It has also been shown that there are positive deviations from the Arrhenius equations as the critical temperature is approached, which is consistent with what has been observed for the self-diffusion of liquid normal alkanes up to their critical temperatures (Panchenkov et al., 1969). An extension of Dymond's hard-sphere self-diffusion equation, which was developed using the results of molecular dynamics computation results for an infinite number of hard spheres, predicts a linear relationship between D~~IKT and V / V, up to the critical temperature. A second correlation which Correspondence concerning this paper should be addressed to S. H. Chen. explicitly accounts for the solute and solvent molecular mass and size discrepancies, has also been established using the results of computer simulations for the tracer diffusion in a finite number of hard spheres. When the solute and solvent critical volumes are used as parameters (instead of effective hardsphere diameters), both correlations developed above are capable of representing the present diffusion data to within f4% on the average with a maximum error of 13%. In contrast, the WilkeChang equation predicts tracer dif...

Predictive correlation for binary diffusion coefficients in dense carbon dioxide

“…15, as a simplification of Eq. 11, to be valid for binary diffusion coefficients (Sun and Chen, 1985a,b, 1986 , 1987a,b;Erkey et al, 1990;Funazukuri et al, 1991;Liong et al, 1991). Figure 2 shows plots of D,,/dT vs. C02 free volume [u, -( u~)~] for naphthalene, measured by Feist and Schneider (1982), Lauer et al (1983), Sassiat et al (1987), and Funazukuri et al (1989), together with those of D2,/dT for CO, self-diffusion data ( O'Hern and Martin, 1955;Takahashi and Iwasaki, 1966;Robinson and Stewart, 1968).…”

Effects of solvent and solute configuration on restrictive diffusion in hydrotreating catalysts

“…This requires data from diffusion experiments with a solute having a known value of Db For this reason, anthracene was chosen because it has a relatively large molecule size, is easily analyzed, and has a known value of D b The resulting effective diffusivities for anthracene in cyclohexane solvent with the three catalysts, as shown in Table VI, together with a measured Db of 1.2 X 10" cm2/s (Sun and Chen, 1985) yielded from eq 9 a T of 1.37. By applying the same T to the other solutes, values of Db were obtained (Table IV), which are within 10% of the calculated values from eq 7.…”

Diffusion of aromatic compounds in nonaqueous solvents: A study of solute, solvent, and temperature dependences

“…The above experimental results are in agreement with the MCT study of Bhattacharyya and Bagchi,25(b) which found that the effect of solute mass on diffusion is generally quite weak; in particular, the solute-mass dependence can normally be considered as negligibly small when changes of solute mass are small within a factor of 2 or so. It should be pointed out that it 51,53,54 have reported tracer diffusivities of nonassociated aromatic solutes in cyclohexane, ethanol, and n-hexane at temperatures above 298.2 K. We have carried out linear regressions for their data at temperatures below the normal boiling point of solvent (but excluding those data at higher temperatures which were measured at different pressures above 1 atm). All data, except one of Dymond's 14 In view of the correlation coefficients and the average % deviations for the 13 sets of linear regressions in Table II, it is clear that Eq.…”

“…Free volume theory, which has affinity with the kinetic theory, has received considerable attention in the past few decades, and there are many reports on the successful use of that theory for the study of diffusion in the literature. 51,61,[91][92][93][94][95][96] There are different approaches of using the free volume theory, and the simplest is probably the one given by Hildebrand. 91 Based on Batschinski's equation 97 for viscosity, Hiddebrand suggested for self diffusivity D s the following:…”