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Xylene transformation reactions have been investigated over ZSM-5 zeolite in a riser simulator that mimics closely the operation of commercial fluidized-bed reactors. Two reaction schemes have been used to model the transformation reactions. The first is based on a triangular reaction path which assumes a direct interconversion between o- and p-xylene isomers (1,3-methyl shift), and a second scheme assumes the reactions proceed via 1,2-methyl shift only (o-xylene ⇄ m-xylene ⇄ p-xylene). The rate constants and activation energies are obtained from simplified kinetic models based on the isomerization of the pure xylene isomers using the “time on stream” decay model. The results provide ample evidence to suggest that direct interconversion between o- and p-xylene isomers (a 1,3-methyl shift) occurs with the same rate as the conversion of m- to o-xylene (a 1,2-methyl shift) over ZSM-5 zeolite catalyst, and the 1,3-methyl shift reaction path is a better representation of the xylene isomerization mechanism in ZSM-5 zeolite than that of the 1,2-methyl shifts only. In addition, the riser simulator and the modeling procedures employed have shown to be very effective in investigating xylene isomerization kinetics.

This article presents the exact limiting value of the derivative of the excess Helmholtz energy, A E , with respect to molecular size at constant temperature, density and composition for a binary mixture of hard spheres with an infinite size ratio (11 / 22 →ϱ); i.e., lim 22 →0 ͓(‫ץ‬A hs E /RT)/‫ץ‬ 22 ͔ T,,x, 11 ϭ(/2)x 1 x 2 11 2 /(1Ϫ(/6)x 1 11 3). This limiting value is compared with the Mansoori-Carnahan-Starling-Leland ͑MCSL͒ and also used to test the limits of some commonly used models in estimating the excess free energy of solvents in mixtures or polymer solutions. The models evaluated include the van Laar, Wilson, Edmond-Ogston, Flory-Huggins, Lacome-Sanchez, Scott-Magat, and Chen et al. It is shown that while the MCSL equation of state produces the same limiting value as the exact value reported here the other mixture models deviate from the exact value. This expression may be utilized to correct the mixture theories at their infinite size ratio limits.

Catalytic transformation of 1,3,5-Trimethylbenzene (TMB) has been investigated over USY zeolite catalyst in a novel riser simulator at different operating conditions. The effect of reaction conditions on the variation of isomerization to disproportionation products ratio (I/D), distribution of trimethylbenzene (TMB) isomers, xylene/tetramethylbenzenes (X/TeMB) ratio and values of p-xylene/o-xylene (P/O) ratios are reported. Comparisons are made between the results of the 1,3,5-TMB and the earlier reported values of 1,2,4-TMB under similar conditions. Even though the kinetic diameters of these molecules slightly exceed the pore size of the USY zeolite (7.4Å), they still have access to the openings of the zeolite pore due to their flexibility. They are readily accommodated inside the zeolite channel system thereby allowing reaction to proceed. Surprisingly, 1,3,5-trimethylbenzene was found to be more reactive than 1,2,4-trimethylbenzene at temperatures above 450 o C and reaction times more than 5secs. The experimental results have been modeled using quasi-steady state approximation. Kinetic parameters for 1,2,4-and 1,3,5-TMB conversions into isomerization and disproportionation products were calculated using the catalyst activity decay function based on time on stream (TOS). The apparent activation energies were found to decrease as May 2007Keywords: Trimethyl benzene, isomerization, disproportionation, fluidized-bed reactor, ZSM-5, USY-zeolite, modeling, * Corresponding author. Tel.: +966-3-860-1429; Fax: +966-3-860-4234 e-mail address: skhattaf@kfupm.edu.sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 investigated the transformation (isomerization and disproportionation) of trimethylbenzenes (TMBs) over LaY catalysts. In the disproportionation reaction, the transfer of one methyl group from TMBs led to the formation of equal amounts of xylene and tetramethylbenzene (TeMB) isomers. The authors reported that disproportionation appeared to be linearly related to the total conversion over most of the conversion range studied for both 1,2,4-and 1,3,5-TMB, whereas, for 1,2,3-TMB, isomerization was much more favored than disproportionation.Cejka et al. used zeolite beta to investigate its potential in xylene production reactions, including toluene disproportionation, TMB disproportionation, and toluene-TMB transalkylation. They found that the zeolite beta has excellent stability and transalkylation selectivity. The authors also demonstrated that xylene selectivity could be increased by blending toluene into feedstocks during the TMB disproportionation reaction.Over medium pore zeolites, TMB transformations revealed much lower yield of xylenes compared to that under large pore and high acidity zeolites [5][6][7][8] . Roger et al. 7 studied the conversion of 1,2,4-TMB over HZSM5. They observed that isomerization of 1,2,4-TMB t...

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