Xylenes thermodynamic equilibrium composition

In all the above mentioned processes, the xylene fraction which constitute about 20-30% of the total aromatics, contain nearly thermodynamic equilibrium composition (23 53 24) of para, meta and ortho isomers [5]. Among the three xylene isomers, para has got better industrial importance due to its conversion to terephthalic acid, which is used in the manufacture of polyester fibre. Hence it was considered of interest to look into the aspect of xylene isomer distribution in the products of aromatization. ... 

Xylene Equilibrium Composition. Xylene equilibrium compositions were determined by interpolation and normalization of published thermodynamic data (6) (Table I) and are labeled A in all figures. 

The thermodynamic equilibria are illustrated in Figures 1 and 2. Figure 1 shows the resulting composition after pure pseudocumene or a recycle mixture of C9 PMBs is disproportionated with a strong Friedel-Crafts catalyst. At 127°C (400 K), the reactor effluent contains approximatdy 3% toluene, 21% xylenes, 44% C9 PMBs, 29% C10 PMBs, and 3% pentamethylbenzene. The equilibrium composition of the 44% C9 PMB isomers is shown in Figure 2. Based on the values at 127°C, the distribution is 29.5% mesitylene, 66.0% pseudocumene, and 4.5% hemimellitene (Fig. 2). After separating mesitylene and hemimellitene by fractionation, toluene, xylenes, pseudocumene (recycle plus fresh), C10 PMBs, and pentamethylbenzene are recycled to extinction. 

When the protonation of the parent compound and the product is low (insufficient medium acidity, limited amounts of strong acid, high temperature etc.) the equilibrium between the isomers is determined by their own thermodynamics. For example, for xylenes the thermodynamic equilibrium in the vapour phase corresponds to the following ratio ortho meta para at 300K — 16 60 24 at 700K — 24 52 24. Table 45 shows that the equilibrium xylene mbctures obtained by isomerization under conditions failing to provide their complete protonation have a composition close to the one calculated from their thermodynamics. 

The comparison between the extract mass during the extraction (10 %) and the desorption (1.25 %) shows us that other effects than solubility are more crucial in the desorption process. The butylacetate regeneration is better than xylene regeneration zeolithe is saturated by a mixture composed with butyl acetate (50 %) and xylenes isomeres (30 %), the extracts composition is butyl acetate (60-65 %) and xylenes isomers (35-40 %). The equilibrium thermodynamic and adsorption data could help us to explain these results. To increase the C02 flow rate (Figure 3) contribute to decrease the desorption time but the lowest flow rate does not permit to desorbe completely zeolithe this is suggestive of a film transfer resistance at lower flow rates. 

From the thermodynamic standpoint, since the reaction shift in favor of p-xylene formation is slightly exothermic, a change in temperature only has a limited effect On the composition of the C aromatics mixture at equilibrium. This is shown by Fig. 4.21. 

From the thermodynamic standpoint, Figure 424 shows the change in the equilibrium molar composition at 750°K of the mixture of methylbenzenes produced, in accordance with the type of feedstock. The variable is in fact the ratio between the methyl groups and the initial benzene rings, ranging from 1 for tolnene alone to 2 for xylenes. 

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