Diffusivity linear hydrocarbon

Fig. 6. Simulated self-diffusivity diftusion constant as a function of linear hydrocarbon chain length in mordenite [78].

At higher temperature (>560K), over poisoned or unpoisoned catalyst, increased rates of diffusion of hydrocarbon products allow the shape-selectivity of the catalyst to become apparent. The isomerization reactions within the channels which lead to linear and singly branched hydrocarbons may also occur in reverse on the active sites of the external surface (most active sites at the zeolite surface occur at the mouth of the channels), which may thus mask some of the shape-selectivity of the zeolite. 

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. 

The current trends identified for n-pentane are strikingly similar to those for n-hexane. TEX-PEP provides the unique possibility to study diffusion of two linear hydrocarbon in two separate sets of experiments. In a first set, n-pentane is labeled and its diffusion coefficients are determined in a mixture with non-labeled n-hexane, while the reverse is done in a second set. The experiments have been performed at a temperature of 433 K and the total hydrocarbon pressure was kept constant at 6.6 kPa by varying the ratio between n-hexane and n-pentane in the gas phase. Figure 19 shows the loadings of both components in the mixture as a function of the n-hexane fraction in... 

Diffusivity correlates linearly with the ratio of temperature and viscosity. Therefore the diffusivity can also be expected to correlate with relaxation time because the latter correlates with temperature and viscosity according to Eq. (3.6.1). Figure 3.6.3 illustrates the correlation between relaxation time and diffusivity with the gas/oil ratio as a parameter [13]. The correlation between diffusivity and relaxation time extends to hydrocarbon components in a mixture and there is a mapping between the distributions of diffusivity and relaxation time for crude oils [17]. 

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). 

A useful comparison may be made between bent, flat, and linear molecules by considering the diffusion coefficients for ethane, ethene, and ethyne. In the sinusoidal channel, ethane diffuses the slowest, ethene approximately 30% faster, and ethyne 3 times as fast. In the straight channel and parallel to the z-axis, ethene and ethyne both diffuse approximately 3 times faster than ethane. These ratios are consistent with the relative cross-sectional area of the three C2 hydrocarbons. 

The primary requirement for an economic adsorption separation process is an adsorbent with sufficient selectivity, capacity, and life. Adsorption selectivity may depend either on a difference in adsorption equilibrium or, less commonly, on a difference in kinetics. Kinetic selectivity is generally possible only with microporous adsorbents such as zeolites or carbon molecular sieves. One can consider processes such as the separation of linear from branched hydrocarbons on a 5A zeolite sieve to be an extreme example of a kinetic separation. The critical molecular diameter of a branched or cyclic hydrocarbon is too large to allow penetration of the 5A zeolite crystal, whereas the linear species are just small enough to enter. The ratio of intracrystalline diffusivities is therefore effectively infinite, and a very clean separation is possible. 

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... 

Jama, M.A. E)elmas, M.P.F., Ruthven, D.M., Diffusion of linear and branched C6 hydrocarbons in silicalite studied by the wall-coated capillary diromatographic Method. Zeolites 18 (1997) pp. 200-204. 

Diffusion coefficients of hydrocarbons are less influenced by temperature than those of alcohols and diethyl ether, for which the dependence is close to that observed in a normal gas-in-gas diffusion. Equation 17 was derived for strong sorbable gases thus, this equation could not be used for n-hexane isotherms in the higher temperature range, where the isotherm is almost linear. 

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