Equilibrium composition, xylene

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 MTDP process, which is similar to the Tatoray process, produces an equilibrium composition of xylene isomers. A -xylene yield of 24% in the xylene product is formed at 42—48 wt % toluene conversion over the heterogeneous catalyst at 390—495°C, 4.2 MPa (600 psig), 1 2 Hquid hourly space velocity, and 4 H2/hydrocarbon molar feed ratio. A new ZSM-5 catalyst, which has higher activity and stability than the current catalyst, has been reported (93). 

The techniques of monomolecular rate theory easily transform measured reaction data into a form where we can analyze apparent kinetics and the effects of intracrystalline diffusion by the use of selectivity data. Time dependency has been eliminated. Since selectivity is extremely reproducible and is independent of short-term aging effects, the number of experimental runs is reduced while data reliability is maintained. For catalyst evaluation at any temperature, it is necessary to determine the equilibrium composition and the straight-line reaction path. With this information any catalyst can be evaluated at this temperature with simply the additional information from a curved-line reaction path. The approach used in the application of monomolecular rate theory to the xylene isomerization selectivity kinetics is as follows. Reference is made to the composition diagram, Figure 1. 

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. 

The values of AG° were determined at 700 K. What is the equilibrium composition (including all xylene isomers) at 700 K and 1.0 atm pressure Propose a method to manufacture para-xylene without producing significant amounts of either ortho- or meta-xylene. 

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. ... 

Freezing point curves were computed for para- end ortho-xylene using Eq. (11.2-6) (the van t Hoff equation) and Eq. (11,2-7) which used ciyoscopic ennslanls, Data from Table 11.2-1 were used in tbe equations and calculated equilibrium compositions are shown as a function of temperature in Table 11,2-2. The computed liquid compositions are nearly identical in (he high concentration ranga and diverge... 

Temp. Catalyst Molar ratio MXj xylene Equilibrium composition (%) Ref. 

It can be seen there that the xylene isomers are present in a near equilibrium composition. The fact that as catalyst ages along the cycle the selectivity to p-xylene increases exceeding the equilibrium value (Figure 31) (41), opens the possibility that the process could be para-oriented by sutil catalyst modifications. 

An aromatic hydrocarbon feedstock consisting mainly of m-xylene is to be isomerised catalytically in a process for the production of / -xylene. The product from the reactor consists of a mixture of p-xylene, m-xylene, o-xylene and ethylbenzene. As part of a preliminary assessment of the process, calculate the composition of this mixture if equilibrium were established over the catalyst at 730 K. 

Interconversion of isomeric xylenes is an important industrial process achieved by HF-BF3 or zeolite catalysts (see Section 4.5.2). Studies of xylenes and tri-and tetramethylbenzenes showed that the amount of catalyst used has a pronounced effect on the composition of isomeric mixtures.83 When treated with small amounts of HF-BF3, isomeric xylenes yield equilibrium mixtures (Table 4.2). Using a large excess of the superacid, however, o- and p-xylenes can be isomerized to m-xylene, which eventually becomes the only isomer. Methylbenzenes are well known to form stable a complexes (arenium ions) in superacids, such as HF-BF3. Since the most stable arenium ion formed in superacids is the 2,4-dimethylbenzenium ion (proto-nated m-xylene, 5), all other isomers rapidly rearrange into this ion. The equilibrium concentration of protonated m-xylene in the acidic phase, consequently, approaches 100%. 

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. 

The crystallization temperature depends on the composition of the mixture to be treated. The cooling diagram shows that a eutectic exists between p-xylene and each of the other components of the mixture. In the case of the m, p-xyiene binary system, the eutectic contains 13 per cent p-xylene and melts at —52 C (Fig. 4.10). It separates two iiquidus curves ME in equilibrium with solid m-xylene, PE in equilibrium with solid p-xylene. Provided that the initial mixture contains more than 13 per cent p-xyleoe. crystals of pure p-xylene are obtained by cooling to — 52 and a mother liquor, whose composition is that of the eutectic. However, it qan be noticed (bat the existence of the eutectic leads to limited recovery, and that this recovery requires beat exchanges at low temperature. 

This process achieves the extraction of m-xylene by means of HF—BFj. It exploits the complexing properties of boron trifluoride on. aromatic compounds, and the particular ability of m-xylene to fonn the most stable complex. Subsequently, this isomer, added to a mixture of Cg aromatics which does not conCrin it, displaces the other components from their respective complexes, and the composition of the medium depends on the values of the equilibrium constants, or m-xylene, 2 for o-xylene, 0.14... 

From the thennodynamic standpoint, since the reaction shift in favor of p-xylene fonnation is slightly exothermic, a change in temperatoze only has a hmited effect on the composition of the Cp aromatics mixture at equilibrium. is shown by Fig. 4.21. 

From the thennbdynamic standpoint, Figure 4.24 shows the chah in the equilibrium molar composition at 7S0"K of the mixture of metbylbenzenes produced, in accordance with the type of feedstock. The variable is in fact the ratio between the methyl groups and the initiai benzene rings, ranging from 1 for toluene alone to 2 for xylenes. 

Reaction-equilibrium calculations may be useful for estimation of the compositions of hydrocarbon feedstocks. A particular feedstock, available as a low-pressure gas at 500 K, is identified as "aromatic C8. ft could in principle contain the CgHio isomers o-xylene (OX), nr-xylene (MX), p-xylene (PX), and ethylbenzene (EB). Estimate how nrueh of each species is present, assuming the gas mixture has come to equilibrium at 500 K and low pressure. The following is a set of indepeirdent reactions (why ) ... 

Alkylation of toluene with methanol was carried out on potassium, ammonium, cesium and thallium salts of phosphotungstic acid (MXH3.XPW12O40 M=K NH4, Cs and Tr, x is varied from 0 to 3). The major products obtained are xylenes, tri - and tetra-methylbenzenes. The selectivity of p-xylene was found to be higher than the equilibrium values. The salts with composition M2,5Ho,5PWi204o showed maximum activity and para selectivity. p-Xylene selectivity was evaluated as a function of x. It is seen that para selectivity correlates linearly with the number of strong acid sites with a strength of Ho <-5.6. 

It is seen that for all compositions the selectivity of p-xylene is higher than the equilibrium values [23], Selectivity of p-xylene was found to be maximum when x = 2.5 except for the ammonium salt. Yashima and coworkers [24] have reported that in the case of Y zeolite,the formation of p-xylene depends on the strength and density of Bronsted acid centres. The results obtained in this study suggest that high surface Bronsted acidity is responsible for high para selectivity. Fig. 3 shows the correlation of p-xylene selectivity with strong acid centres with H <-5.6 on CsxHs-xPWnO o-... 

The composition of the mixtures obtained from the xylenes resembles the pattern for the isomeric olefins, the j>er cent of the ds isomer decreasing in the series ortho > meta > para. The same order was noted in the hydrogenation of the isomeric phthalic acids over PtOj in acetic acid (4). The trans isomer is not formed by isomerization of the saturated ds form because more trans-1,3-dimethylcyclohexane is produced from m-xylene than is contained in the equilibrium mixture of the 1,3-isomers. 

Example 5.4 Design a column for a 1 mol/s, equimolar benzene/p-xylene/toluene stem with X/>=[0.700, 0.010] and a bottoms benzene composition of 0.010, assuming an equimolar feed (the same problem shown in figures 5.11 5.13) uang DODS-SiCo. The mixture may be assumed to behave ideally and phase equilibrium may be modelled with Raoulf s law. Assume that we wish to operate at a factor of 1.34 times minimum reflux. 

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. 

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