Diffusion coefficient intercrystalline

A method is presented for obtaining the diffusion coefficients of Ci to C4 hydrocarbons on H and Na mordenite by analysis of their chromatographic curves. It is shown that in such a transient device the role of the intercrystalline diffusion may be important for the estimation of the total mass-transfer resistance. The diffusion coefficients decrease with increase in the number of carbon atoms. They are about one order of magnitude smaller on mordenite in the Na form than in the H form. The energies of activation are higher for intracrystalline diffusion than for intercrystalline diffusion. The resistance from intercrystalline diffusion makes an important contribution to the total mass-transfer resistance at low temperature. 

For molecular mean free paths much less than the mean free diameters of the intercrystalline void space in the zeolite bed, Din, is controlled by the same mechanism as in the gas phase, with a self-diffusion coefficient Dg. Due to the steric confinement, Dimer is reduced with respect to Dg by a tortuosity factor, Tb, with values typically of the order of 2-3. The mean free path can be estimated through the relation... 

There are essentially two types of diffusion in the zeolite bed which occur in parallel diffusion in macropores (intercrystalline space) and in micropores (intraciystalline pores). Each type of diffusion has a specific characteristic time, Tjnter or Tintra, which depends on the intercrystalline (Djnter) or intraciystalline (Dintra) diffusion coefficient and the distance to cover, the sample length, I, or the radius, R, of the crystallites assumed to be spherical (20 pm for our samples). Tinter and Tjntra are given by [4] ... 

KaX than in smaii-port zeolites such as NaCaA. For hydrocarbons in HaX it has been found that the coefficients of intracrystalline self-diffusion decrease with increasing concentration (concentration dependences of type I and II (6).i. By contrast, for not toe high gas phase concentrations (i.e., as long as molecular transfer in the intercrystalline space proceeds by Knudsen diffusion)... 

By both model considerations for mass transfer in composite systems [195-1991 and simple random walk arguments, the contribution of intracrystalline mass transfer to long-range diffusion may be shown to be much less than that of mass transfer through the intercrystalline space [193]. The coefficient of long-range diffusion therefore may be represented in the form [52,145]... 

Fig. 6 Temperature dependence of the coefficient of long-range self-diffusion of ethane measured by PFG NMR in a bed of crystallites of zeolite Na-X points) and comparison with the theoretical estimate (line). The theoretical estimate is based on the sketched models of prevailing Knudsen diffusion (low temperatures, molecular trajectories consist of straight lines connecting the points of surface encounters) and gas-phase diffusion (high temperatures, mutual collisions of the molecules lead to Brownian-type trajectories in the intercrystalline space). From [92] with permission...

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