Diffusion of alkanes

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. 

Diffusion of alkanes in zeolites is widely studied. The experimentally obtained difliisivities, however, vary by orders of magnitude. In many cases noble gasses are used and it is generally believed that they neither influence the adsorption rate nor the adsorption equilibrium. We studied the influence of argon and helium on the adsorption rate and equilibrium of n- and iso-butane in the zeolites BEA, MFI and FER at 383 K and found a pronounced effect on the adsorption equilibrium as well as on the adsorption rate. 

Adsorption and diffusion of alkanes in zeolites and in well-structured porous materials like MCM-41 materials are studied widely [1,5,8,9,11], The reported difusivities however differ sometimes by orders of magnitude. These differences are sometimes attributed to the use of microscopic techniques in stead of macroscopic techniques [12]. We, however, think that a major part of the found differences must be imputed to the use of a carrier gas. Adsorption is often studied in diluted systems with methods as ZLC [3], gaschromatography [4], inverse gas chromatography [10], gravimetry [12], > ile others are not using carriers gasses at all. 

A monotonic decay with increasing concentration over the whole concentration range (type I behavior) is observed for the self-diffusion of alkanes in the wide and medium pore zeolites NaX (see, for instance. Fig. 9) and ZSM-5, respectively. For zeolite NaX, from analysis of the temperature dependence of the longitudinal proton magnetic relaxation times (.see Sec. III.A.l.), the mean... 

P-lO - Adsorption and diffusion of alkanes and their mixtures in silicalite studied with positron emission profiling technique... 

A comparison between sihcalite-1 and H-ZSM-5 teaches that acid sites have a profound influence on the self-diffusivity of alkanes. The self-diffusivities of both components decrease strongly, and we observe a significant preferential adsorption of the linear over the branched hexane. This is caused by the relatively stronger interaction of the linear hexane with the acid sites. On the contrary, 2-methylpentane loadings in mixtures in sihcahte-1 and H-ZSM-5 are very close. In H-ZSM-5, the diffusivity of the hnear alkane in mixtures with the branched alkane is influenced by two factors... 

The subject of this thesis is the study of adsorption and diffusion of alkanes in zeolites by computer simulation. 

As the Fick diffusion coefficient is directly related to the adsorption isotherm and the Maxwell-Stefan diffusion coefficient using the thermodynamic matrix F, we can describe the diffusion of alkane mixture using the Maxwell-Stefan theory. For this, we have assumed that the Maxwell-Stefan diffusivity is independent of the loading of the zeolite. This suggests a possible industrial application for the separation of linear and branched alkanes using a zeolite membrane. 

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