Diffusion isobutane

Mass transfer One more difficulty arises from the fact that there are two phases in the reactor (i) hydrocarbon and (ii) acid. The reaction occurs in the acid phase while reactants are feed in hydrocarbon phase. This implies that, in order to reaction occurs, there is mass transfer from hydrocarbon to acid phase. The mass transfer is a very complex phenomenon which can involve the reaction-diffusion equation. However, such a phenomenon is beyond of the goal of this chapter. Both isobutane and propilene are feed in hydrocarbon phase. Solubility of propylene in acid phase is very... 

The mass diffusivity coefficient of isobutane blowing agent from LDPE foam was found using a onedimensional diffusion model of two concentric cylinders with Dirichlet boundary conditions. An average mass diffusivity coefficient was used to calculate the mass of isobutane remaining in the foam for different boundary conditions. The influence of temperature and additives on diffusion was also examined. The use of the mass diffusivity coefficient in assessing the flammability of PE foam in the post-extrusion period is discussed. 2 refs. USA... 

Fig. 6. Reaction rate coefficients for the combination of f-butyl radicals in Aj n-hexa-decane solvent V, n-dodecane solvent , n-decane solvent X, n-octane solvent 8 n-heptane solvent and of allyl radicals in propane ( ) and melhylallyl radicals in isobutane (O) plotted against the Smoluchowski—Stokes—Einstein rate coefficient, eqn. (30). The broken line is of unit slope. The solid line is a comparison of the steady-state (t-> >) Collins and Kimball rate coefficient [eqn. (26)] with the activation rate coefficient, feact = 1011 dm3 mol-1 s 1 and the diffusion-limited rate coefficient 4irRD replaced by eqn. (30). After Schuh and Fischer [40].

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

Finally our result for isobutane, when compared to the other molecules, is consistent with the fact that its shape should make the diffusion process more difficult. In any case, it follows the expected trend if one considers the values of diffusion coefficient obtained for the other alkanes. 

Molecular mechanics (MM), molecular dynamics (MD), and Monte-Carlo (MC) methods were employed to simulate the adsorption of methane, ethane, propane and isobutane on silicalite and HZSM-5. The silicalite was simulated using the same cluster-model adopted in the diffusion calculations. The H-ZMS-5 structure was constructed according to the procedure suggested by Vetrivel et al. [32], which consists in replacing one atom at the channel intersection by and protonating the oxygen atom bridging the Ta and Tg sites in order to preserve the lattice neutrality. 

It was also found that the methylcyclopentane concentration in the acid phase was about 60 ppm. An order of magnitude calculation indicates that the diffusion of methylcyclopentane Into the bulk acid phase occurs much faster than the rate of formation of Isobutane. Thus the acid phase should be considered to be saturated with methylcyclopentane throughout the reaction In all the kinetic experiments. 

Ideal reaction conditions of high Isobutane concentration, and isobutane diffusion into the acid phase at a rate greater than the olefin diffusion rate, result in high quality alkylate product. Ester formation, exclusive of those formed from feed contaminants,... 

Weckhuysen, B.M., Verberckmoes, A.A., Debaere, J., Ooms, K., Langhans, 1. and Schoonheydt, R.A. (2000) In situ UV-Vis diffuse reflectance spectroscopy-on line activity measurements of supported chromium oxide catalysts relating isobutane dehydrogenation activity with Cr-spedation via experimental design. Journal of Molecular Catalysis A Chemical, 151 (1-2), 115-31. 

Pre-admitting of a noble gas influence the adsorption, while admitting a noble gas subsequently to butane adsorption has no influence at all. This suggests that the observed phenomena is a kinetic effect. Likely, the noble gasses are present in the zeolite channels but no adsorbed, seriously hamper the diffusion of both n- and isobutane. 

The dynamics of methane, propane, isobutane, neopentane and acetylene transport was studied in zeolites H-ZSM-5 and Na-X by the batch frequency response (FR) method. In the applied temperature range of 273-473 K no catalytic conversion of the hydrocarbons occurred. Texturally homogeneous zeolite samples of close to uniform particle shape and size were used. The rate of diffusion in the zeolitic micropores determined the transport rate of alkanes. In contrast, acetylene is a suitable sorptive for probing the acid sites. The diffusion coefficients and the activation energy of isobutane diffusion in H-ZSM-5 were determined. 

In the temperature range of 373-573 K no transformation of the C1-C5 alkanes occurred on the H-ZSM 5 samples. The adsorption of the small methane molecules was very weak, while the large neopentane molecules could not enter the narrow zeolitic channels. Thus, the response to the applied pressure modulation was too small to record meaningful FR spectra with these molecules. For propane and isobutane the FR results suggest that diffusion in the micropores is the rate limiting process of transport over the entire temperature range. 

Figure 5. The FR spectra of the isobutane/H-ZSM-5 diameter. The apparent activation energy of diffusion (E.) of systems at 373 K and 133 isobutane obtained from the Arrhenius plot was 21 kJ mol Pa (A) Z57, (B) Z34 and (Figure 6, open symbols). This value is about three times higher (C) Z15. The sample than that obtained for the diffusion n-butane [12]. At 373 K and amount was 50 mg.

Neutron Scattering (QENS) [13], and Temporal-Analysis of Products (TAP) [14] (Figure 6, solid symbols). The isobutane diffusivities determined by the macroscopic methods, such as MEMBRANE, TAP, and FR show reasonable agreement. The self-difiusivity coefficient derived from QENS is about one order of magnitude lower. The E, values of 34 kJ mof and 25 kJ mol obtained by the MEMBRANE and the TAP methods, respectively, are higher than those determined by methods where the conditions of the mesurements correspond to sorption equilibrium or quasi-equilibrium, such as QENS (17 kJ mol )orFR(21 kJ mol ). 

It can be shown that the frequency, where the out-of-phase characteristic FR Figure 6. The temperature dependence of the, ction of diffusion is a maximum, isobutane diffusivity (D) in the micropores of H- j e particle geometry [7, 8]. 

T. J.H. Vlugt, C. Dellago, and B. Smit (2000) Diffusion of Isobutane in Silicalite studied by Transition Path Sampling. J. Chem. Phys., 113, p. 8791... 

Another set of experiments tested the effect of temperature while maintaining a constant isobutane density of 0.41 g/cm To obtain a constant density of the SC isobutene, pressure was adjusted at each temperature between 150 °C (111 bar) and 210 °C (220 bar). The results showed an apparent maximum in the TMP regeneration effectiveness between 190-210 °C, at about 78-83% recovered activity. Any increase in temperature increases all reaction rates and hence slow diffusing species may condense more rapidly and produce higher molecular weight hydrocarbons that could not to be extracted from the zeolite pores due to their larger size. 

This section summarizes the chemistry of the SC isobutane regeneration process. To understand the nature of the hydrocarbons that remain adsorbed on the surface of the USY zeolite catalyst both before and after SC isobutane regeneration, a series of ex-situ temperature-programmed oxidation (TPO), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and ultraviolet-visible (UV-vis) analyses was performed on samples submitted to different TOS 10 under isobutane/butene reaction conditions. 

Example 11-4 Rothfeld has measured diffusion rates for isobutane, in the isobutane-helium system, through a -in.-Iong pelleted cylinder of alumina (diameter in.). The measurements were at 750 mm Hg total pressure and 25°C, and the diffusion direction was through the pellet parallel to the central axis. The following data are available for Harshaw alumina, type A1-0104-T ... 

Solution a) According-to the procedure of Example 11-1 (the Chapman-Enskog equation), the bulk diffusivity in the isobutane-helium system at 750 mm Hg pressure and 25°C is... 

Lee and Ma (1977) observed that the intracrystalline diffusion coefficients (A) of n-butane, isobutane and 1-butene in the Na-, Ca- and La-forms of synthetic faujasite, determined in a constant volume, decrease in the following order for all three hydrocarbons (table 4) A(La-X) > A(Ca-X) > A(Na-X). This is caused by the... 

The coke deposited during the isobutane alkylation with C4 olefins on zeolites was extracted with a mixture of methanol/toluene, and with CI2CH2. None of these solvents were effective to remove the coke. Only a very small fraction was removed, which was attributed to the dissolution of external coke. The coke molecules formed inside the channels have a large size and therefore cannot diffuse out of the pores . 

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