Diffusion of ethylbenzene

Example 10.6 A commercial process for the dehydrogenation of ethylbenzene uses 3-mm spherical catalyst particles. The rate constant is 15s , and the diffusivity of ethylbenzene in steam is 4x 10 m /s under reaction conditions. Assume that the pore diameter is large enough that this bulk diffusivity applies. Determine a likely lower bound for the isothermal effectiveness factor. 

Figure 4 demonstrates the counter-diffusion of ethylbenzene vs. benzene. In this experiment, benzene was first taken up at 415 K from a stream of benzene in helium, with a benzene partial pressure of 1.15 mbar, until a steady state was reached (spectrum No. 0). Subsequently, ethylbenzene was admixed (1.15 mbar) with the benzene partial pressure and the total rate of the gas ilow remaining constant. Counter-diffusion is indicated by the decrease of the typical benzene absorbance at 1478 cm and the development of the typical ethylbenzene bands at 1605,1496 and 1453 cm (spectra Nos. 1-5). 

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

All experiments in this study were carried out under conditions where C02 and styrene are miscible. The solubilities of C02 and ethylbenzene (a model for styrene) in HDPE were determined at 80 °C and 243 bar. The HDPE samples were immersed in either pure C02 or a 36 wt % ethylbenzene/C02 solution within pressure vessels under these conditions for various times. Figure 10.1 shows results of a typical desorption experiment to determine the mass uptake of ethylbenzene for a given soak time the equilibrium mass uptake was found to be 4% and this was reached after approximately 5 h. Figure 10.2 illustrates the mass uptakes as a function of soak time the diffusivity of ethylbenzene in C02-swollen HDPE under these conditions was calculated by curve fitting to be 9.23 x 10 7 cm2/s. Attempts to determine the equilibrium mass uptake of neat ethylbenzene in HDPE at 80 °C failed because ethylbenzene dissolves polyethylene under these conditions. 

Freshly activated sample (2)-(7) coked samples, tcoking tinie on stream during coking Do [EB] Fickian diffusivity of ethylbenzene n sorption capacity for ethylbenzene at = 395 K under Ap [EB] = 0 115 Pa An/n percentage of loss of sorption capacity A [1490], A [1610] maximum absorbance of typical coke bands at 1490 and 1610 cm respectively, as a measure of coke deposited. The error of the diffusivity values is about... 

The Arrhenius plot of 1/r for benzophenone in poly(methyl acrylate) (PMA) showed another break at 40 °C (above T of PM A), which corresponds to the crossover of k j given by Eq. (14) from a diffusion-controlled to an activation-controlled reaction. The diffusion coefficient D for reacting carbonyl groups calculated from the values of 1/t and B also showed a break at each transition temperature, as exemplified in Fig. 8 for PMMA, polystyrene, and polycarbonate. It should be iK>ted that D in Fig. 8 refers to the reacting functional groups but not to the molecule. The diffusion proc at temperatures below T would be caused by rotation of the benzojdienone molecule and by the cooperative motion of a few successive monomer units of the matrix polymer. Nevertheless, the values of D in these polymers at 100 °C are comparable to the value of D = 5.6 x 10 an /s for mass diffusion of ethylbenzene in polj tyrene at 30 °C. The reaction radius R was estimated to be 3-5 A. The transition temperatures... 

This may be partly the result of increased steric crowding in the transition state of transalkylation. Another contributory factor to the increased selectivity in ZSM-5 is the higher diffusion rate of ethylbenzene vs m-/o-xylene in ZSM-5 and hence a higher steady state concentration ratio [EB]/[xyl] in the zeolite interior than in the outside phase. Diffusional restriction for xylenes vs ethylbenzene may also be indicated by the better selectivity of synthetic mordenite vs ZSM-4, since the former had a larger crystal size. 

Investigation of Diffusion and Counter-diffusion of Benzene and Ethylbenzene in ZSM-5-type Zeolites by a Novel IR Technique... 

In a zeolite catalyst sample, which was coked via dealkylation of ethylbenzene at reaction temperatures somewhat higher than those of the sorption experiments, the diffusion coefficient of ethylbenzene remained essentially unchanged even though the sorption capacity significantly decreased due to deposition of carbonaceous material. 

Fig. 1. Counter-diffusion of (a) Benzene (B) vs. Pyridine (Py) and (b) ethylbenzene (EB) in Hydrogen Mordenite...

Table 1 Diffusivities of benzene and ethylbenzene in fresh and coked H-ZSM-5...

The results of the inverse experiment, viz. counter-diffusion of benzene into H-ZSM-5, previously loaded with ethylbenzene from an ethylbenzene/helium stream at 415 K, are displayed in Fig. 6. 

Application of the IR method proved to be also suitable for the measurement of diffusivities in coking porous catalysts. This was deihonstrated by uptake experiments with ethylbenzene where the sorbent catalyst, H-ZSM-5, was intermittently coked in-situ via dealkylation of ethylbenzene at temperatures (465 K) somewhat higher than the sorption temperature (395 K). Coke deposition was monitored in-situ via the IR absorbance... 

We have seen previously shape-selective catalysis by ZSM-5 in the conversion of methanol to gasoline (Chapter 15).-7 Other commercial processes include the formation of ethylbenzene from benzene and ethylene and the synthesis of p-xylene. The efficient performance of ZSM-5 catalyst has been attributed to its high acidity and to the peculiar shape, arrangement, and dimensions of the channels. Most of the active sites are within the channel so a branched chain molecule may not be able to diffuse in, and therefore does not react, while a linear one may do so. Of course, once a reactant is in the channel a cavity large enough to house the activated complex must exist or product cannot form. Finally, the product must be able to diffuse out. and in some instances product size and shape exclude this possibility. For example, in the methylu-uon of toluene to form xylene ... 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

Evaluating the catalytic shape-selectivities of these materials by use of the disproportionation of ethylbenzene (23.24) at 523 K at a conversion of 2% in differential reactor mode, it was observed that larger crystals of sample A gave 85% para-diethylbenzene and 15% meta-diethylbenzene. The smaller crystals of sample B with the smoother aluminium gradient yielded 96% para-diethylbenzene and only 4% meta-isomer. In a second series, samples of crushed large crystals with mean sizes of 1-10 (m were examined. No increase in activity was observed as is expected when the reaction is controlled by the diffusion limitation of molecules in the large crystals. However, this treatment created larger non-selective external surface area and hence a smaller selectivity of 87% para-diethylbenzene for sample B was recorded. 

The production of styrene by dehydrogenation of ethylbenzene is a good example (121). When rates of reaction are high, short diffusion lengths are required, suggesting structured, thin-layered catalytic reactors. When selectivity is an issue, this is even more the case. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

In the present concept of styrene dehydrogenation implementation of inorganic membranes is not feasible. Application of Knudsen diffusion membranes with a low permselectivity to hydrogen leads to a considerable permeation of ethylbenzene and thus, to lower yields. Microporous and palladium membranes give better results, but worse than a conventional case, because the conversion is limited by reaction kinetics. The ratio of permeation rate to reaction rate is very important in selecting membranes in a membrane reactor process in which equilibrium shift is foreseen. 

A limited number of sink effect studies have been conducted in full-sized environments. Tichenor et al. [20] showed the effect of sinks on indoor concentrations of total VOCs in a test house from the use of a wood stain. Sparks et al. [50] reported on test house studies of several indoor VOC sources (i.e., p-dichlorobenzene moth cakes, clothes dry-cleaned with perchloroethylene, and aerosol perchloroethylene spot remover) and they were compared with computer model simulations. These test house studies indicated that small-chamber-derived sink parameters and kj) may not be applicable to full-scale, complex environments. The re-emission rate (kj) appeared to be much slower in the test house. This result was also reported by other investigators in a later study [51]. New estimates of and were provided,including estimates of fca (or deposition velocity) based on the diffusivity of the VOC molecule [50]. In a test house study reported by Guo et al. [52], ethylbenzene vapor was injected at a constant rate for 72 h to load the sinks. Re-emissions from the sinks were determined over a 50-day period using a mass-balance approach. When compared with concentrations that would have occurred by simple dilution without sinks, the indoor concentrations of ethylbenzene were almost 300 times higher after 2 days and 7 times higher after 50 days. Studies of building bake-out have also included sink evaluations. Offermann et al. [53] reported that formaldehyde and VOC levels were reduced only temporarily by bake-out. They hypothesized that the sinks were depleted by the bake-out and then returned to equilibrium after the post-bake-out ventilation period. Finally, a test house study of latex paint emissions and sink effects again showed that... 

The deeper oxidation of ethylbenzene over TS-2 can be explained with the slower diffusion of 1-phenylethanol and aeetophenone formed in the zeolite pores where they could undergo additional oxidation to aeetophenone or other products, respectively. Another possible reason could be some differences in the local geometry of the titanium sites due to the different framework structure of the two titanium silicalites. 

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