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ABSTRACT: p-Xylene is an important industrial compound, and its demand has been increasing in recent years. It is mostlyproduced from cracking of naphtha, but there is a need for new and cost-effective methods for the production. Toluene alkylationwith methanol over an alumino-silicate zeolite catalyst, such as ZSM-5, produces a mixture of xylene isomers with low p-xyleneselectivity. Due to the very close boiling points of xylene isomers, it is very expensive to separate them. There has been somesuccess in enhancing p-xylene selectivity by modifying the ZSM-5 catalyst. This results in reduced separation cost, which makestoluene methylation a competitive process for p-xylene production. Based on these findings, a novel process for the production ofp-xylenes by the catalytic methylation of toluene followed by reactive distillation for the separation of p-xylene (instead of themore costly conventional technique of separation based on crystallization and adsorption) is developed and a complete processflow diagram is simulated using Aspen Plus. Using the built-in optimization tool in Aspen Plus, we optimized reactor parametersfor a maximum p-xylene selectivity of 97.7%. After separation, a p-xylene product stream purity of 99.7% is achieved. Highp-xylene selectivity in the reactor and use of reactive distillation reduces the separation cost and renders the new processeconomically competitive.35697
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1. INTRODUCTIONp-Xylene is one of the three xylene isomers with a methyl groupattached to the para position of the benzene ring. It is industriallymore important than the other isomers, m-xylene and o-xylene,due to its use for the production of terephthalic acid that is usedfor the production of polyethylene terephthalate (PET); PET isused to produce polyester fiber and films, and PET bottles.1Due to its high downstream applications, p-xylene generally has6−8% annual demand growth.2Most p-xylene comes fromcatalytic re-forming of naphtha.3Xylene isomers, along withother aromatics such as benzene and toluene, are produced fromcatalytic re-forming of naphtha. Conventionally, after separationof aromatics, p-xylene is separated from the other xylene isomersby crystallization or adsorption.4Due to the very close boilingpoints of the different xylene isomers (Table 1), separation ofp-xylene from xylene mixtures is difficult and represents anextensive energy step in the whole production process. Due toincreasing demand, there is a need for new and cost-effectiveprocesses for the production of p-xylene. Other methods forthe production of p-xylene include toluene disproportionationand toluene methylation. Toluene has the lowest demand ascompared to that for benzene and xylenes, and there is a strongincentive to convert surplus toluene to more valuable aromatics.4In toluene disproportionation, two molecules of toluene reactover an acid zeolite catalyst to form one xylene and one benzenemolecule.5In toluene disproportionation, a p-xylene selectivity of90%, from total xylenes produced, is achievable.6This processalso produces benzene; therefore, it is feasible when benzene isalso required along with the xylenes.7Methylation of toluene iscarried out with methanol over a zeolite catalyst, which produceswater and xylenes. The production is feasible if the cost ofmethanol is low and the selectivity of p-xylene is high.8The reaction of toluene and methanol over a zeolite catalyst,such as
ZSM-5, yields water and xylenes with the followingthermodynamic equilibrium composition of xylene isomers:23.55% p-xylene, 52.42% m-xylene, and 24.03% o-xylene at400 °C.9Due to very close boiling points of xylene isomers it isusually not easy to separate p-xylene. Also, other isomers do nothave much industrial demand and isomerization is needed toconvert them to p-xylene.4Methylation of toluene is a suitableoption if p-xylene can be produced with high selectivity. Differentmodifications of zeolite catalysts has resulted in high p-xyleneselectivity.10−13The higher p-xylene selectivity over the modifiedzeolite, used in Faramawy,14is attributed to the removal of externalsurface acid sites and smaller pore openings size.14Methylation oftoluene, in addition to some other side reactions such as xyleneisomerization, occurs inside the pores of the zeolite catalyst.Mirthet al.15have reported that diffusion plays an important role in thetransport of xylenes from the pores’ inside out, and that thediffusion coefficient of p-xylene can be about 100 times that ofo-xylene and about 1000 times that of m-xylene above 250 °C.Due to high p-xylene diffusivity, the concentrations of m-xyleneand o-xylene increase inside the pores, thus promoting theirisomerization to p-xylene and enhancing p-xylene selectivity.16Isomerization of p-xylene also occurs on zeolite external sites,which reduces p-xylene selectivity.17The p-xylene selectivity canReceived: April 11, 2013Revised: July 1, 2013Accepted: August 11, 2013Published: August 12, 2013Table 1. Boiling and Freezing Points of Xylene Isomers26isomer normal boiling point, °C freezing point, °Cp-xylene 138.3 13.3m-xylene 139.1 −47.85o-xylene 144.4 −25.16 be increased by zeolite modification, which neutralizes theunselective external acid sites and also reduces the effectivecatalyst pore size.18High space velocity (equivalent to low spacetime) reduces the contact time at the external surface andsuppresses the xylene isomerization reaction over the externalsurface.8Breen et al.8have reported p-xylene selectivity close to100% by using Mg modified ZSM-5 catalyst at low space time.Sotelo et al.19have also reported that p-xylene selectivityapproaches 100% as space time tends to zero over Mg modifiedZSM-5 catalyst; this is attributed to diffusional resistance to otherisomers. High p-xylene selectivity results in negligible productionof unwanted xylene isomers and significant reduction in theseparation cost of p-xylene.To increase p-xylene production, the direct conversion oftoluene to p-xylene is a desirable alternative. This work integratestoluene methylation and separation of p-xylene, to provide acomplete process to produce p-xylene with the required purity.The novel process does not require classical xylene separationtechnologies such as crystallization or adsorption. Instead,p-xylene is purified by reactive distillation to meet the productpurity specification. Aspen Plus process simulator is used tosimulate the proposed process, which utilizes catalyticmethylation of toluene overMg-modified ZSM-5 zeolite catalyst.2. KINETIC MODELKinetic data for toluene methylation over Mg-modified