Paraxylene — production through toluene methylation

Paraxylene or the Jesus Molecule is one amongst the isomers that structure xylenes. Among the xylene isomers paraxylene plays important role in production of terephthalic acid and dimethyl terphthalate. Paraxylene is a flammable, colorless aromatic hydrocarbon that exists as a liquid at ambient pressure and temperature.

In p-xylene the p stands for para, identifying the location of the methyl groups as across from one another. P-xylene is having other names p-xylol, 1, 4-dimethylbenzene, 1, 4-xylene. P-Xylene is a colorless, flammable liquid that is insoluble in water.

Today’s market for para-xylene is predominately directed towards the production of a variety of fibers, films, and resins. Relatively smaller amounts of para-xylene are used as a solvent. Extraction of xylenes for paraxylene production is increasing demand for toluene in the gasoline pool, raising the floor for aromatics prices.

Generally p-Xylene is produced by catalytic reforming of petroleum naphtha. The p-xylene is then separated out in a series of distillation, adsorption or crystallization and reaction processes from the m-xylene, o-xylene and ethylbenzene.

The conventional para-xylene process converts toluene to para-xylene in the presence of methanol over a heated catalyst bed of ZSM-5 zeolite. The process follows the following highly exothermic reaction.

C7H8 + CH3OH → C8H10 + H2O

By this process 23 per cent para-xylene, 51 per cent meta-xylene and 26 per cent ortho-xylene is produced. An oxide modified ZSM-5 catalyst is commonly used to improve the selectivity towards para-xylene. Further methods for improving the selectivity of para-xylene include operating at higher temperatures which promotes catalyst coking.

As the catalyst becomes coked, active sites on the catalyst are blocked leaving a smaller amount of sites for paraxylene to become isomerizes. Although the selectivity to paraxylene is improved, a decrease in the available active sites on the catalyst can causes a decrease in the overall conversion of toluene. This indicates a clear trade-off between para-xylene selectivity and toluene conversion.

Operation at a high space velocity or with low catalyst contact times has also proven to increase the selectivity of paraxylene. Despite the improved selectivity, a decrease in catalyst contact time limits the conversion of toluene as less time is available for the reaction to approach completion.

A revolutionary method for the continuous production of para-xylene from toluene developed by Breen et al. uses a low-contact time process with favorable conditions that limit the formation of coke. Under these operating conditions, the conversion of methanol is 100 per cent with a corresponding paraxylene selectivity of 99 per cent.

Unlike existing para-xylene production processes, a particularly high para-xylene selectivity and high toluene conversion are simultaneously achieved. An economic advantage of this new production method includes a lower average reactor operating temperature providing significant utility savings. An additional commercial and economic advantage of this new production method is that the high-cost xylene isomer separation is circumvented.

For this process, a reactant feed consisting of toluene, methanol, nitrogen, and water is passed over a heated bed of boron-oxide modified ZSM-5 zeolite. The reactor is operated at an average temperature of 815°F with an exceptionally low catalyst contact time to suppress paraxylene isomerization reactions. Para-xylene isomerization is limited as a result of the low catalyst contact time because a shorter residence time decreases the probability of para-xylene molecules contacting external catalyst active sites.

The methylation reactor converts toluene to ortho-, meta-, and para-xylene according to the following vapor-phase chemical reactions.

C7H8 + CH3OH → p-C8H10 + H2O

C7H8 + CH3OH → m-C8H10 + H2O

C7H8 + CH3OH → o-C8H10 + H2O

The above reactions take place at an average temperature of 824°F and a pressure ranging from 124 to 100 psig in the first and second reactor, respectively. The following methanol dehydration reactions are suppressed as a result of the added water.

2 CH3OH → C2H4 + 2 H2O

2 CH3OH → CH3OCH3 + H2O

These reactions produce 99.9 per cent para-xylene with 0.08 per cent meta-xylene and 0.02 per cent ortho-xylene. The reactor operates exothermically, liberating heat at a rate of 8.60X106 Btu/hr. The feed to the reactor consists of water, nitrogen, methanol, and toluene. Toluene is added in excess at an 8:1 molar ratio of toluene to methanol corresponding to a single-pass toluene conversion of 12.5 per cent to maintain a para-xylene selectivity of 99.9 per cent. Water is added to the reactor at a molar ratio of 12:1 (water to methanol) for reasons previously mentioned. Nitrogen is added at a 2:1 molar ratio.

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