Ethylbenzene desorption

Figure 10.1. An example of an ethylbenzene desorption experiment used to determine the equilibrium mass uptake of ethylbenzene in HDPE. The specimen was soaked in a 36 wt % ethylbenzene/C02 solution for 5 h at 80 °C and 243 bar.

ESTD, ex situ thermal desorption VOCs, volatile organic compounds TPH, total petroleum hydrocarbons BTEX, benzene, toluene, ethylbenzene, xylene PCBs, polychlorinated biphenyls PAHs, polycyclic aromatic hydrocarbons ISTD, in situ thermal desorption. 

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. 

Figure 6. Simulation of ethylbenzene ethylation over SK-600 at 577°K and C8 C2 = 0.2 by product desorption limitation model...

Desorption of similar products from cumene- and propylene-deactivated parent H-mordenite is a result analogous to that of Venuto and Hamilton (3). They found that deactivation of rare earth X (REX) faujasite by alkylation of benzene with ethylene to ethylbenzene resulted in trapped products similar to those for deactivation with ethylene alone. 

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. 

Hydrogen and water desorption measurements indicated that only the a-Fe203 (0001) face can dissociate hydrogen, which then reacts with lattice oxygen to form water, thereby reducing the oxide film. This result highlights the role of water in the catalytic dehydrogenation of ethylbenzene. Water not... 

The uptake process was followed by purging for 7 min, and subsequently the TPD was carried out. The results were obtained by monitoring the decrease, during desorption, of the absorbance A measured in arbitrary units (au), of a typical IR band of the adsorbate molecules, where A is proportional to N (A = K x N), and where N is the amount of adsorbate in the material and K, a proportionality constant [98,99], For benzene, the region between 1450 and 1550 cm-1 was integrated to obtain a measure of the intensity of the band around 1482 cm-1. For toluene and ethylbenzene, the segment between 1477 and 1517 cm-1 was integrated to obtain an intensity measure for the band around 1497 cm-1 [97,98],... 

The FTIR-TPD profiles (A vs. T) for the desorption of benzene, toluene, and ethylbenzene from high-silica H-ZSM-5 is reported in Figure 4.40 [97], These results were fitted with the complementary error function, that is,... 

It has been shown that single ring aromatic alkylation reactions such as benzene to ethylbenzene take place primarily within the 12- ring (12-MR) system, and that the 10-ring (10-MR) system contributes little to the ethylbenzene reaction. A key feature of MCM-22 is its ability to operate stably at low benzene-to-ethylene ratios with minimal production of polyethylbenzenes (PEBs) or ethylene oligomers. The excellent ethylbenzene selectivity of the MCM-22 catalyst is likely due to confinement effects within this 12-MR pore system and to the very facile desorption... 

Figures 1,2, and 3 are provided to illustrate one protocol often used to evaluate sink materials [20,32,42-47] however, other methods are also used. For example, Krebs and Guo [48] reported on a unique method involving two test chambers in series. The first chamber is injected with a known concentration of a pollutant (in this case, ethylbenzene). The outlet from the first chamber provides a simple first-order decay that is injected into the inlet of the second chamber that contains the sink material (gypsum board). Thus, this method exposes the sink test material to a changing concentration typical of many wet VOC sources. The sink adsorption rate and desorption rate results are comparable to one-chamber tests and are achieved in a much shorter experimental time. Kjaer et al. [31] reported on using a CLIMPAC chamber and sensory evaluations coupled with gas chromatography retention times to evaluate desorption rates. Finally, Funaki et al. [49] used AD PAG chambers and exposed sink materials to known concentrations of formaldehyde and toluene and then desorbed the sinks using clean air. They reported adsorption rates as a percentage of concentration differences.

The anhydrous bulk zirconium sulfate preparation did not display any activity in the trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3). At 473 K the silica-supported, gas-phase sulfated zirconia showed a very small activity, which rapidly dropped to a negligible level (Fig. 2). The conclusion is that Lewis acid sites are not active with sulfated zirconia catalysts. The low activity of the silica-supported catalyst is due to adsorption of some water leading to Bronsted acid sites. Desorption of water at 473 K leads to the decrease in activity with time. Pre-hydration of the supported catalyst brings about a slightly higher activity as apparent from Fig. 2 the activity drops again due to the loss of water. 

Temperature-programmed desorption (TPD) and surface reaction (TPSR) were carried out at 450 torr with a temperature increment of 15 K.min. A 598S GC-MS mass spectrometer was used for detection of the desorbed species. The sample was calcinated in a quartz glass reactor at 823 K in a He stream for 1 hour, cooled to 423 K and ethylbenzene was adsorbed. After evacuation at 423 K for 30 rain the sample was cooled to 300 K and TPD/TPSR started. 

The rates for the desorption of unreacted ethylbenzene and reaction products, styrene and benzene, are plotted in Figure 3. For all zeolite catalysts investigated a significant amount of ethylbenzene, retained at the surface of catalysts after evacuation at 423 K, desorbs in one or two steps at the same temperature at which dehydrogenation of EB to ST and cracking to benzene start. It was observed that the temperature of the maximum of the desorption rate of the reactant or reaction products in the first step is almost identical for all the samples, which may point to the presence of the same kind of adsorptive and active centers. In the second step, however, the difference in the desorption and reaction of EB over these samples is apparent. 

The following IR bands being indicative of the adsorbates benzene, ethylbenzene, and p-xylene were monitored at 1478, 1496/1453, and 1516 cm , respectively. Sets of spectra of benzene or p-xylene on H-ZSM-5 analogous to that shown for ethylbenzene (Fig. 3) were monitored and, using the appropriate cahbration curves, the corresponding adsorption and desorption curves of the type displayed for ethylbenzene in Figs. 8a,b obtained (see also discussion of Fig. 29 below). 

The IR technique also enables us to determine the adsorption and desorption of binary mixtures, provided the IR spectra of the two components were sufficiently different so that the spectra of the mixtures could be reliably decomposed. This was, e.g., the case for the pairs benzene/ethylbenzene, benzene/p-xylene, and ethylbenzene/p-xylene (compare, e.g.. Figs. 3 and 16). 

From sets of spectra such as those shown in Fig. 3 and uptake curves displayed by Fig. 8 not only isotherms and isosteres could be derived, using the respective plateaux for the temperatures and pressures indicated, but also from the ascending branches (measured via FTIR after an upward pressure jump) or the descending branches (determined after a downward pressure jump) the kinetics of adsorption and desorption into zeolitic pores could be derived. These processes were assumed to be diffusion controlled. Their evaluation required a fit of the appropriate solution of Tick s second law as provided by Crank [39] to the experimentally measured uptake (or removal) points, which are indicated in Fig. 6 by filled crosses for the case of ethylbenzene uptake. 

The teehnique of desorption by simulated countercurrent flow displacement is also applied to other separation operations the separation of ethylbenzene from a mixture of aromatics and that of olefins from a mixture of olefins and paraffins. The composition of the zeolite adsorbent is adjusted in each case to optimize the effectiveness of the separation Na-Y or KSr-X zeohtes for ethylbenzene and Ca-X or Sr-X for olefins. The nature of the liquid desorbent also depends on the molecule to be separated. 

Fig, 18. Measured counter-difftision of benzene (x) and ethylbenzene ( ) in HZSM-5 along with fitted curves for sorption and desorption (solid lines). Reproduced with permission from Catalysis Today, 8, 451 (1991). 

Table 17. Acidic properties of H-ZSM-5 and Ca,H-ZSM-5 samples obtained by solid-state ion exchange. A(OH), maximum absorbance of the band of acidic OH groups at 3610 cm i X(EB), conversion of ethylbenzene peak temperature obtained by TPD of NH3 from Bronsted acid sites (cf. [29]) E, most frequent energy of activation for desorption of NH3 from Bron-sted acid sites (cf. [29,222,223]) AHad, differential heat of adsorption of NH3 (cf. [222,223])...

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