Intensification of an Irreversible Process using Reactive Distillation– Feasibility Studies by Residue Curve Mapping

Reactive distillation processes are very promising in substituting Sconventional liquid phase reaction processes. However this technology is not suitable for all kind of processes or types of reaction. Therefore, assessing the feasibility of these process concepts forms an important area in current and future research and development activities. The present paper focuses on the feasibility studies based on the construction of residue curve maps for the toluene methylation system. The RCMs were constructed and analyzed; it is concluded that the process of synthesis of xylenes when carried out in the reactive distillation column enhances the selectivity of the desired para isomer. Keywords: Reactive Distillation, Residue Curve Maps, Feasibility Study, Toluene Methylation, Aspen Plus

Intensification of an Irreversible Process Using Reactive Distillation – Simulation Studies

In this study, a steady state simulation of the process for the production of xylene isomers by reactive distillation was performed using Aspen Plus software. The simulations were aimed studying the parameters like number of stages in the different sections of the RD column, reflux ratio, and the boil-up ratio, which maximize the conversion of Toluene and improves the selectivity and yield of the p- Xylene. Keywords: Reactive Distillation, Process Intensification, Toluene Methylation, Aspen Plus, Simulation Studies,

Identifying the key steps determining the selectivity of toluene methylation with methanol over HZSM-5

Methylation of toluene with methanol to produce p-xylene has been investigated for decades, but the origin of selectivity is still under debate. Here we report computational studies based on ab initio molecular dynamics simulations and free energy sampling methods to identify the key steps determining the selectivity. The steps of toluene methylation to protonated-xylene, deprotonation of protonated-xylenes, and diffusion of xylene in HZSM-5 channels are compared. We find the pathways of formation for protonated p-/m-xylenes have similar free energy barriers. Meanwhile, the methylation is found rate-determining, thus the probability to generate p-/m-xylenes at the active site are similar. We then find that the diffusion for m-xylene along the zigzag channel is more difficult than its isomerization to p-xylene, which in turn further promotes the selectivity of p-xylene formation. These insights obtained at the molecular level are crucial for further development of high-performance zeolite catalysts for toluene methylation.

Selective methylation of toluene using CO2 and H2 to para-xylene

Toluene methylation with methanol to produce xylene has been widely investigated. A simultaneous side reaction of methanol-to-olefin over zeolites is hard to avoid, resulting in an unsatisfactory methylation efficiency. Here, CO2 and H2 replace methanol in toluene methylation over a class of ZnZrOx–ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H3CO*; * represents a surface species) are generated more easily by CO2 hydrogenation than by methanol dehydrogenation. Catalytic performance tests on a fixed-bed reactor show that 92.4% xylene selectivity in CO-free products and 70.8% para-xylene selectivity in xylene are obtained on each optimized catalyst. Isotope effects of H2/D2 and CO2/13CO2 indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO2 hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.