Linear paraffin diffusion

Comparison of diffusivities and diffusional activation energies for linear paraffins as free liquids and adsorbed on NaX zeolite. 

The concept of shape selectivity relies on the control of the adsorption and diffusion of the molecule in the pore of the catalytic material. Linear paraffins have the smallest critical diameter among hydrocarbons and can therefore penetrate smaller pore structures. In the figure, for example, the n-heptane molecule penetrates the tubular... 

Figure 5 schematically illustrates the concepts of reactant shape selectivity. Only linear paraffins that are able to diffuse and are adsorbed inside the pores can undergo a chemical transformation, e.g., acid catalyzed cracking. The property is exploited in some chemical processes, such as the dewaxing of lubes and middle distillates, through the selective cracking or isomerization of the linear paraffin fraction. 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

A new experimental technique (ZLC) has been developed and applied to study the diffusion of a range of hydrocarbons (xylene, benzene, cyclohexane and linear paraffins) in unaggregated crystals of zeolites A and X. The validity of the method was confirmed by varying the crystal size and the nature and flowrate of the purge gas. The method has advantages of speed and simplicity but the major advantage is that the intrusion of extraneous heat and mass transfer resistances is much less significant than in conventional uptake rate measurements. As a result, the new method can be applied to systems in which diffusion is too rapid to follow in a conventional sorption experiment. 

Diffusion of linear paraffins was studied in a wide range of 5A zeolite crystals including both the small commercial (Linde) crystals and larger crystals synthesized in our laboratory by Charnell s method ( 31.). Whereas in the case of NaX we found no significant difference between the diffusivities for large laboratory synthesized and small commercial crystals (15), for 5A we see large differences. In particular, for n-butane, the large... 

Arrhenius plots showing the temperature dependence of diffusivity for n-butane in various 5A samples are shown in figure 4 while figure 5 shows the trend of activation energy with carbon number. For linear paraffins higher than n-butane, diffusion, even in the large 5A crystals, is too slow to measure by the NMR PFG method so it is only for butane that a direct comparison between ZLC and PFG NMR data is possible. 

The shape of the molecule is very important, and branched or cyclic molecules such as isobutane and cyclohexane are excluded from zeolites that permit entry of linear paraffins of the same or higher molecular weight. There is also a large effect of temperature, with activation energies of 3-15 kcal for hydrocarbon diffusion in zeolites. 

FIGURE 5.19. Comparison of self-diffusivities and diffusional activation energies for linear paraffins as free liquids and adsorbed on 13X zeolite crystals. Data of Karger cl (Reprinted with permission from ref. 46, Copyright 1983 American Chemical Society.)... 

Matthews-Akgerman The free-volume approach of Hildebrand was shown to be valid for binary, dilute liquid paraffin mixtures (as well as self-diffusion), consisting of solutes from Cg to Cig and solvents of Cg and C o- The term they referred to as the diffusion volume was simply correlated with the critical volume, as = 0.308 V. We can infer from Table 5-15 that this is approximately related to the volume at the melting point as = 0.945 V, . Their correlation was vahd for diffusion of linear alkanes at temperatures up to 300°C and pressures up to 3.45 MPa. Matthews et al. and Erkey and Akger-man completea similar studies of diffusion of alkanes, restricted to /1-hexadecane and /i-octane, respectively, as the solvents. 

The data shown in Figure 3.11(a) indicates that the reaction in a solution of the lowest viscosity (hexane) is under kinetic control the functions l/r — 1 and ux — 1 are both linear and indistinguishable as in Eq. (3.75). In contrast, the data in Figure 3.11 (7 ) prove that the reaction in viscous paraffin/heptane solvents are diffusion-controlled in full agreement with Eq. (3.76), the concentration dependence of l/r is strongly nonlinear, unlike that of ux. 

In the group with negative spreading coefficients (e.g., kerosene-in-water and paraffin-in-water emulsions), the values of kLa in both stirred tanks and bubble columns decrease linearly with an increasing oil fraction. This effect is most likely due to the formation of lens-like oil droplets over the gas-liquid interface. A subsequent slower oxygen diffusion through the droplets, and/or slower rates of surface renewal at the gas-liquid surface, may result in a decrease in kLa. 

It follows from the above results that small-pore molecular sieves yield principally hydrocarbons and very small amounts of Cg compounds. This may be related to diffusion constraints and cavity dimensions. Gorring has determined that the diffusion coefficients of n-paraffins in T-zeolite at 340 C decrease as the number of chain C-atoms increase from 2 to 8. Also, for most of the cases the length of zeolite cavities is less or equal to the length of the n-heptane molecule. Thus, it may be assumed that the cavity length imposes a restriction on the formation of Cg linear compounds. The combination of cavity dimensions and pore opening permits attaining high selectivities for C2-C4 linear hydrocarbons. 

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