Diffusion coefficients of hydrocarbons

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

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. 

Permeability and diffusion coefficients of hydrocarbons in polyphenylene oxides are also essentially dependent on pressure (see Figure 9.23). It can be seen that in the case of ethylene, with the increase in pressure, the permeability coefficients first decrease, and then begin to rise. Ref. [18] quotes constants of the dual-mode sorption model for a number of hydrocarbons permeation through polyphenylene oxide. 

E. J. M. Hensen, A. M. de Jong, and R. A. van Santen have written Chapter 7, which introduces the tracer exchange positron emission profiling (TEX-PEP) as an attractive technique for in-situ investigations, for example, in a stainless steel reactor, of the adsorption and diffusive properties of hydrocarbons in zeolites under chemical steady-state conditions. Self-diffusion coefficients of hydrocarbons, labeled by proton-emitting C at finite loadings and even in the presence of another imlabeled alkane, may be extracted. The method is illustrated by adsorption and diffusion measurements of linear (n-hexane) and branched (2-methylpentane) alkanes in Fl-ZSM-5 and silicalite-1. 

H2O [6] showed that water eonstituted the eontinuous phase in both, the micellar solution and the ringing gel phase. In the latter, the diffusion coefficients of hydrocarbon and surfactant were lower by one and two orders of magnitude, respectively, as compared to the micellar solution. Thus, we are dealing with an aggregated network where the residual mobilities are caused by exchange of surfactant and solute molecules between neighbouring micelles in the gel. 

The Dp and Dq are the diffusion coefficients of probe and quencher, respectively, N is the number molecules per millimole, andp is a factor that is related to the probability of each collision causing quenching and to the radius of interaction of probe and quencher. A more detailed treatment of fluorescence quenching including multiexponential intensity decays and static quenching is given elsewhere/635 There are many known collisional quenchers (analytes) which alter the fluorescence intensity and decay time. These include O2/19 2( 29 64 66) halides,(67 69) chlorinated hydrocarbons/705 iodide/715 bromate/725 xenon/735 acrylamide/745 succinimide/755 sulfur dioxide/765 and halothane/775 to name a few. 

There is an abrupt decrease in the lateral diffusion coefficient of DPPC upon the phase transition from the GI phase to the Gi phase. This is because the acyl-chain region is being packed even more efficiently in the Gi phase than in the GI phase, and the hydrocarbon volume in the Gi phase is smaller than in the GI phase. Also, in the Gi phase, the lipid acyl-chains from the opposing bilayer leaflets interdigitate. In order for a phospholipid molecule to diffuse it has to circumvent the nearby interdigitated molecules which hinder diffusion. 

Further information on the dependence of structure of microemulsions formed on the alcohol chain length was obtained from measurement of self diffusion coefficient of all the constitutents using NMR techniques (29-34). For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short chain alcohol and C ) the self diffusion... 

Price, W.S. and Soderman, 0. Self-diffusion coefficients of some hydrocarbons in water Measurements and scaling relations,... 

C2. Carmichael, L. T., Reamer, H. H., and Sage, B. H., Diffusion Coefficients in Hydrocarbon Systems. n-Heptane in the Gas Phase of the Methane-n-Heptane System (submitted to Ind. Eng. Chem. for publication). 

Returning to the survival probability, in Fig. 57, the kinetic theory and diffusion equation [cf. eqn. (132)] predictions are compared. Three values of the activation rate coefficient are used, being 0.5, 1.0 and 2.0 times the Smoluehowski rate coefficient for a purely diffusion-limited homogeneous reaction, 4ttoabD. With a diffusion coefficient of 5x 10 9 m2 s1 and encounter distance of 0.5 nm, significant differences are noted between the kinetic theory and diffusion equation approaches [286]. In all cases, the diffusion equation leads to a faster rate of reaction. In their measurements of the recombination rate of iodine atoms in hydrocarbon solvents, Langhoff et al. [293] have noted that the diffusion equation analysis consistently predicts a faster rate of iodine atom recombination than is actually measured. Thus there is already some experimental support for the value of the kinetic theory approach compared with the diffusion equation analysis. Further developments cannot fail to be exciting. 

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. 

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Relaxation times T, and T2 depend on the motion of molecules which contain the nuclei (236) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis is on water and hydrocarbons as guest molecules from the fundamental viewpoint species such as Xe, SF6, H2, CH4, and NH3 are of special interest. 

Examples of diffusion coefficients calculated with Eq. (6-18) are summarized in Table 6-3 where they are compared with corresponding experimental values (Reid et al., 1987). Diffusion coefficients of water and hydrocarbons in air are especially important parameters in mass transfer processes between plastic materials and the atmospheric environment. 

Some measured values, D and K, (Koszinowski 1986, and Koszinowski and Pirin-ger, 1990) are contained in Table 9-7. These data deal mainly with the partition coefficients of hydrocarbons, alcohols, phenols and a series of aromas between polyolefins and a liquid phase (ethanol, methanol) as well as diffusion coefficients of these substances in the polymer. 

Estimate the diffusion coefficient of benzene vapor diffusing into air at 100°F and at atmospheric pressure using the following empirical correlation for binary air-hydrocarbon systems at low pressures (less than 30 atm) ... 

FIGURE 9.6 Dependence of diffusion coefficients of various hydrocarbons in polybutadiene (PB) and polyethylene/vinyl acetate copol3fmer (EVAc) on molar volume of liquid hydrocarbons V. (From analysis of data presented in Kamiya, Y., Terada, K., Naito, Y., and Wang, J.S., J. Polym. Sci., B33, 1663, 1995.)... 

Hydrocarbon molecules with an oblong shape have higher diffusion coefficients than branched or flat diene hydrocarbon molecules, and their diffusion coefficients are higher than what could be expected from the correlation. Diffusion coefficients of oblong molecules like frani-2-butene in both polymers are higher compared to branched molecules with a similar molar volume, such as cfr-2-butene or isobutylene. 

In accordance with the equation for the activation energy of diffusion proposed by Meares [31], cohesion energy density (CED) of the polymer has a significant effect on diffusion coefficients of lower hydrocarbons. This is especially typical of mbbery polymers an increase in CED results in reduction of diffusion coefficients. Similar dependencies also apply to glassy polymers [9,32]. For example, Figure 9.8a shows the dependence of diffusion coefficient of propylene on CED of both glassy and mbbery polymers. 

The effect is especially significant in the case of glassy polymers [8,9,32]. It was shown in several studies that diffusion coefficients of penetrants, including hydrocarbons, decrease with the decrease in molar fraction of free volume of the polymer (see, e.g.. Figure 9.8b). 

Very recently, a novel Fourier transform NMR method was employed by Lindman, et al. (21) to obtain multicomponent self-diffusion data for some single phase microenulsion systems. Because of the large values obtained for the self-diffusion coefficients of water, hydrocarbon, and alcohol, over a wide range of concentrations, the authors concluded that there are no extended, well-defined structures in these systems. In other words, the Interfaces which separate the hydrophobic from the hydrophilic regions appear to open up and reform at a short time scale. 

The ZSM-5 family of zeolites show further interesting shape-selective effects. Both normal and methyl-substituted paraffins have access to interior sites, so both hexane and 3-methylpentane are cracked by ZSM-5, but steric constraints cause hexane to be cracked faster than 3-methylpentane. Further shape selectivity was found between 3-methylpentane and 2,3-dimethylbutane. No window effect with paraffin chain length was found with ZSM-5. In the conversion of methanol to hydrocarbons over ZSM-5 catalysts, the distribution 94,152,195 of aromatic products ends at Cio- The distribution of tetramethylbenzenes is not far from equilibrium, but has excess 1,2,4,5-tetramethylbenzene. Measurements of diffusion coefficients of alkyl benzenes show rapid decrease, by orders of magnitude, as ring substitution increases. 

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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