Diffusivities of propane

Example 39 Estimate the Infinite Dilution Diffusivity of Propane... 

The self-diffusivity of propane in the coked polycrystalline grains unveils details of coke formation in polycrystalline particles. Figure 44 shows that coking reduces the translational mobility inside the grains. The effect of intracrystalline coke deposition on the translational mobility of propane is indicated in Fig. 43. After a coking time of 1 h, only a slight increase of Di ,ra with increasing observation times occurs. After 12 h, a decrease is ob-... 

Fig. 44. Self-diffusivity of propane (10 CjHg per u.c., 293 K) as a function of the observation time t of self-difTiision for the polycrystalline grains shown in Fig. 43 after different coking times by n-hexane cracking (732) O, starting ZSM-5 Cl h, 3.6 wt% C , 12 h, 4.3 wt% C.

Calculate the relative rate of diffusion of propane (C3H8) molecules to methane (CH4) molecules. 

More detailed uptake rate studies carried out with larger crystals and with a range of different crystal sizes, however, showed no evidence of any surface barrier.(7-9) For diffusion of propane and n-butane in laboratory synthesized 5A crystals the uptake results show reasonably close agreement with the NMR.(10) However, for some other systems such as benzene-NaX the NMR diffusivities were still much higher. The uptake rates were, however, close to the limit of the experimental technique so the possible intrusion of extraneous heat or mass transfer resistances could not be unequivocally excluded. 

Example 39 Estimate the infinite dilution diffusivity of propane (1) in chlorobenzene (2) at25°C. Use Eq. (2-113) ... 

Representative data for surface diffusion of propane in a silica gel adsor-... 

In the figure, D, from single-component runs are also included. All the results are correlated by a single straight line of slope = I, suggesting that the surface pressure driving force with the specific resistance coefficient for each component is acceptable in the case of co-diffusion of propane and butane on activated carbons. 

Ishizuka, S., Miyasaka, K., and Law, C.K., Effects of heat loss, preferential diffusion, and flame stretch on flame-front instability and extinction of propane/air mixtures. Combust. Flame, 45,293,1982. 

A number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

Surface diffusion has been extensively studied in literature. An overview of experimental data is given in Table 6.1. Okazaki, Tamon and Toei (1981), for example, measured the transport of propane through Vycor glass with a pore radius of 3.5 nm at 303 K and variable pressure (see Table 6.1). The corrected gas phase permeability was 0.69 m -m/m -h-bar, while the surface permeability was 0.55 m -m/m -h-bar, and so almost as large as the gas phase permeability (Table 6.1). It is clear from Table 6.1, that the effects of surface diffusion, especially at moderate temperatures, can be pronounced. At higher temperatures, adsorption decreases and it can be expected that surface diffusion will become less pronounced. 

The reaction selectivity is better under these conditions at steady state, because an equihbrated ratio is observed between the resulting higher and lower homolog alkanes. In addition, dynamic conditions allow us to vary the contact time to obtain information about primary products and then about the mechanism. It appears that, in the case of propane metathesis in a stationary regime, conversion increases Hnearly with contact time, which shows that the reaction is under dynamic control with no diffusion Hmitation. Under these conditions, decreasing contact time results in an increase of the selectivity for hydrogen and olefin whereas that of alkanes decreases. Similarly, the alkanes/olefins ratio tends to zero as the contact... 

Nowak et al. (63) presented a comparative study of the diffusivities of rigid models of methane, ethane, and propane in silicalite. (The details of the calculation are reported in the preceding section.) The calculated diffusion coefficients decreased as the length of the carbon chain increased, and the effect was found to be far more pronounced for ethane than propane. The calculated diffusivities, in units of 108 m2/s, were 0.62, 0.47, and 0.41 for methane, ethane, and propane, respectively. The ethane value is in satisfactory agreement with PFG-NMR measurements [0.38 (77), 0.3 (80), 0.4 (42) for silicalite. The value for propane, however, was calculated to be almost an order of magnitude larger than the NMR results of Briscoe et al. (80). [The agreement with the value of Caro et al. (71) is better, but still an overestimation.]... 

Dumont and Bougeard (68, 69) reported MD calculations of the diffusion of n-alkanes up to propane as well as ethene and ethyne in silicalite. Thirteen independent sets of 4 molecules per unit cell were considered, to bolster the statistics of the results. The framework was held rigid, but the hydrocarbon molecules were flexible. The internal coordinates that were allowed to vary were as follows bond stretching, planar angular deformation, linear bending (ethyne), out-of-plane bending (ethene), and bond torsion. The potential parameters governing intermolecular interactions were optimized to reproduce infrared spectra (68). 

Nicholas et al. (67) have performed MD calculations of propane in sili-calite in which the propane molecule is given complete flexibility. The calculations, which have been detailed previously for methane diffusion, employed a large simulation box with multiple sets of adsorbates to ensure good statistics. The framework was kept fixed and data were collected over a 40-ps run. The results predict diffusion coefficients in very good agreement with the values of Caro et al. (71). The calculated values for a concentration of 4 and 12 propane molecules per silicalite unit cell are 0.12 and 0.005 X 10 8 m2/s, respectively. These values for propane are far lower than those of Nowak et al. (63), the reason for this is that Nicholas et al. used flexible adsorbate molecules, whereas Nowak et al. used rigid ones. 

Figure 4 Temperature dependence of the experimental self-diffusion coefficient of propane molecules in NaX.

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