Xylene production

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

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). 

Xylene production Carbon dioxide [air] Global warming 2 2 2 2... 

Xylene production Crude oil [resource] Resource depletion 1 1 1... 

Similar treatment of a trifluoroacetic acid solution of p-tolualdehyde with triethylsilane gives only a 20% yield of /7-xylene after 11 hours reaction time followed by basic workup. Use of 2.5 equivalents of dimethylphenylsilane enhances the yield to 52% after only 15 minutes. This reaction proceeds stepwise through the formation of a mixture of the trifluoroacetate and the symmetrical ether. These intermediates slowly form the desired /7-xylene product along with Friedel-Crafts side products under the reaction conditions (Eq. 192).73 Addition of co-solvents such as carbon tetrachloride or nitromethane helps reduce the amount of the Friedel-Crafts side products.73... 

Eluxyl A process for separating /7-xylene from its isomers, using an adsorbent-solvent technique. The process is based on simulated countercurrent adsorption where the selective adsorbent is held stationary in the adsorption column. The feed mixture to be separated is introduced at various levels in the middle of the column, as in the Sorbex process. The /r-xylene product can be more than 99.9 percent pure. Developed by IFP and Chevron Chemical. A large pilot plant was built in Chevron s site at Pascacougla, MS, in 1994 and a commercial plant on the site was announced in 1996, Since then, the process has been widely licensed. 

As is apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited ... 

The raffinate and extract streams leave the UOP Parex unit adsorption section via the UOP rotary valve and are respectively routed to the raffinate and extract columns for separation of the mixed xylene components from the PDEB as shown in Figure 7.4. Because the desorbent has a higher boiling point than the mixed xylenes, the desorbent exits the bottom of the distillation column and is pumped back to the adsorbent chamber section. The mixed xylene raffinate stream is taken as a side cut from the raffinate column to remove water. The extract p-xylene stream is taken from the top of the extract column and routed to a finishing column where any toluene that was in the Parex feed is removed. The p-xylene product exits the bottom of the finishing column. The adsorbent has some selectivity for toluene as well as p-xylene. 

The industrial production of m-xylene is very similar to that of p-xylene. In fact, most of the production of m-xylene is done in facilities where a much larger quantity of p-xylene is produced. Figure 7.5 is a typical flow diagram for an aromatics complex where m-xylene is produced. It is quite like the flow diagram for the production of p-xylene except that a fraction of the Parex unit raffinate, containing typically over 60% m-xylene, is used as fresh feed to the MX Sorbex unit for m-xylene extraction. Because the required m-xylene production is typically much lower than that of p-xylene and the MX Sorbex fresh feed stream is three times more concentrated than the Parex unit fresh feed stream, the feed stream to the... 

All, S.A., Al-Nawad, K., Okomoto, T., and Ishikawa, K. (2005) Transalkylation of heavy aromatics for enhanced xylene production. Proceedings of 15th Saudi-Japan Joint Symposium,... 

Chronologically, the production of o-xylene from mixed Cg aromatics was the first of these separations. In 1945, the Oronite Chemical Co. produced 85 to 90% purity o-xylene by fractionation from crude xylenes (1). The c-xylene product is oxidized for the production of phthalic anhydride in a vapor phase reaction over a vanadium-base catalyst. By 1947 Oronite provided 5% of the United States production capacity for phthalic anhydride by this process (2). 

Xylenes Production Via Toluene Transalkylation and Disproportionation. The toluene that is produced from processes such as catalytic reforming can be converted into xylenes via transalkylation and disproportionation. Toluene disproportionation is defined as the reaction of 2 mol of toluene to produce 1 mol of xylene and 1 mol of benzene. Toluene trailsalkylation is defined as the reaction of toluene with C9 or higher aromatics to produce xylenes. 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

In one process (Fig. 2), a light fraction (boiling range 65 to 175°C) from a straight run petroleum fraction or from an isocracker is fed to a catalytic reformer and is followed by fractionation and extraction. The mixed xylenes stream is then processed further to produce high-purity p-xylene and/or o-xylene. Because of the close boiling points of p-xylene and m-xylene, production of high-purity p-xylene by distillation is impractical and methods such as crystallization and adsorption are used. 

Investigations concerning the effects of precoking have been carried out for example on ZSM-5 catalysts for xylene production by disproportionation or alkylation of toluene [102]. During these experiments, an increase of the p-xylene selectivity could be observed. By a defined precoking, it is also possible to generate product-selectivity effects initially not present, as has been shown for the disproportionation of ethylbenzene on HY-zeolites [63]. 

Transalkylation between ethylbenzene and xylene products (i.e. a secondary reaction)... 

The aromatic product is mixed with toluene and hydrogen, vaporized and fed to the reactor. The reactor gaseous product is primarily unreacted hydrogen, which is recycled to the reactor. The liquid product stream is subsequently stabilized to remove further light aromatic components. The resulting aromatics are routed to product fractionation to produce the final benzene and xylenes products. 

Lean solvent from the bottom of the recovery column is returned to the extractor. The extract is recovered overhead and sent on to distillation columns downstream for recovery of the individual benzene, toluene and xylene products. The raffinate stream exits the top of the extractor and is directed to the raffinate wash column (4). In the wash column, the raffinate is contacted with water to remove dissolved solvent. The solvent-rich water is vaporized in the water stripper (5) and then used as stripping steam in the recovery column. The raffinate product exits the top of the raffinate wash column. The raffinate product is commonly used for gasoline blending or ethylene production. 

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