Xylenes equilibrium concentrations

The general characteristics of toluene disproportionation are summarized by the data presented in Figure 8. With standard HZSM-5 catalyst, as indicated by the lowest curve, the xylenes produced contain essentially an equilibrium concentration of the para isomer (24%) and exceed it only slightly at low conversion. The other curves result from a variety of HZSM-5 catalysts modified in different ways and to different degrees. It is apparent that a wide range of para-selectivities can be obtained. At increasing toluene conversions, the para-selectivity decreases for all catalysts. 

For m/p-xylene and naphthalene the aqueous equilibrium concentration (reached after about 2 days) agrees well with the calculated value, Ciw/NAPL. However, for benzene the measured value lies about 10% below the calculated equilibrium concentration (410 pg L-1). This discrepancy will be explained below (Answer d). 

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%. 

As shown in Figure 1, the equilibrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equilibrium can be achieved when zeolites are used, owing to shape selectivity (see Molecular sieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

Low temperatures favor the production of p>xylene. According to Fig. 4.21, its equilibrium concentration in the Cg aromatics mixture is a maximum at around 8(yC, and then slowly decreases. 

The octafining process is used to restore to near equilibrium concentrations C8 aromatic streams deficient in one or more of the xylene isomers. In conjunction with an appropriate separation process any of the xylene isomers can be produced by recycling to extinction the other less marketable xylene isomers and ethylbenzene. 

Axens/ExxonMobil Eguilibrium xylenes Para-depleted Xylenes The feed is a mixture of fresh and recycled Cj aromatics in which paraxylene (and orthoxylene, if desired) is depleted to less than equilibrium concentrations. The mixed xylene and ethylbenzene feed combined with hydrogen-rich recycle gas is passed through the reactor where ethylbenzene dealkylation and xylenes isomerization occur to produce an equilibrium xylenes mixture. 18 2009... 

Table 45. Observed Equilibrium Concentrations of Isomeric Xylenes...

Fig. 10. Scatter plot of minimum disappearance rate (Aconcentration/distance) versus the difference between equilibrium concentrations of alkylbenzenes (ABs) and those measured in ground water immediately downgradient from the oil body (modified from Eganhouse et al, 1996). Note B, benzene T, toluene E, ethylbenzene o-X, orr/io-xylene m,p-X, meta- + para-xylenes.

Figure 4.25 shows the equilibrium concentrations (at 1 bar) of Cg-aromatics versus temperature. While the proportion of m-xylene falls as temperature increases, the proportion of ethylbenzene in particular rises. 

In order to produce more paraxylene than is available in catalytic reformate, a xylenes-isomerization plant is sometimes included in the processing scheme. The isomerization step uses the effluent (filtrate) from the paraxylene crystallization step as feed. The filtrate contains about 7-9 percent of paraxylene. The isomerization unit brings the concentration back to its equilibrium value of about 20 percent. 

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

The process for isomerization of EB requires that some fraction of the feed be maintained in a saturated state, as described in Sections 14.4.1.1 and 14.4.1.2. The ability to isomerize the EB is affected by the naphthene concentration and constrained by the equilibrium ratio of EB/xylenes at the reaction temperature. Use of a pore-restricted molecular sieve can be used to eliminate more sterically demanding species from the reaction network and can effectively remove these from consideration. In this maimer, one can achieve higher levels of a desired species, such as PX from EB, than could ordinarily be obtained by isomerization over a larger-pore zeolite. 

Pertechnetate in neutral and alkaline media can be extracted into solutions of tetra-alkylammonium iodides in benzene or chloroform. With tetra-n-heptylammo-nium iodide (7.5 x 10 M) in benzene distribution coefficients up to 18 can be obtained . A solution of fV-benzoyl-iV-phenylhydroxylamine (10 M) in chloroform can be used to extract pertechnetate from perchloric acid solution with a distribution coefficient of more than 200, if the concentration of HCIO is higher than 6 M The distribution of TcO between solutions of trilauryl-ammonium nitrate in o-xylene and aqueous solutions of nitrate has been measured. In 1 M (H, Li) NOj and 0.015 M trilaurylammonium nitrate the overall equilibrium constant has been found to be log K = 2.20 at 25 °C. The experiments support an ion exchange reaction . Pertechnetate can also be extracted with rhodamine-B hydrochloride into organic solvents. The extraction coefficient of Tc (VII) between nitrobenzene containing 0.005 %of rhodamine-B hydrochloride and aqueous alcoholic " Tc solution containing 0.0025 % of the hydrochloride, amounts to more than 5x10 at pH 4.7 . 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent literature (125), the catalyst used in this process is believed to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equilibrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocarbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

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