High Purity p-Xylene Production Via m-Xylene Isomerization Over Pt-HZSM-5 Catalyst : Use of An Extractor-Type Catalytic Membrane Reactor Equipped with a Nanocomposite MFI-Alumina Membrane Tube as Separation Unit

Abstract

Energy efficiency and energy saving are becoming increasingly important components of government policies around the world in response to a range of challenges which include perceptions of resource scarcity, high energy prices, security of energy supply and environmental protection. One of the energy saving potentials that has attracted research focus for about a decade now is the application of extractor-type catalytic membrane reactors (e-CMR) for the production of p-xylene from mixed xylene. P-xylene is almost exclusively used as raw material in the production of terephthalic acid (TPA) and dimethyl terephthalate (DMT), which are then reacted with ethyleneglycol to form polyethylene terephthalate (PET), the raw material for most polyester. For industrial application of p-xylene, ultrapure p-xylene purity is essential in most cases. Advantages of e-CMR over conventional fixed-bed reactor (FBR) for p-xylene production include increase in p-xylene yield and m-xylene conversion due to selective extraction of p-xylene from the reaction zone. This in turn forward-shifts the equilibrium position of the reaction, allowing more m-xylene to react to produce more p-xylene.

 In view of the interesting potentials of e-CMR, this study presents further investigations on the development and evaluation of energy-efficient process for p-xylene production based on the application of MFI-type zeolite membranes in e-CMR. In this study possibility of obtaining ultrapure p-xylene during m-xylene isomerization in an e-CMR, which has a tubular nanocomposite MFI-alumina membrane as separation unit, is demonstrated. Unlike "film-like" architectures, in nanocomposite architectures zeolite crystals are embedded within the pores of the supports. Also for better understanding of the fundamental behaviour of e-CMR at lower reaction temperatures during xylene isomerization, the reaction was conducted from 473 K to 573 K. Before isomerization test, the formation of a nanocomposite material (namely no formation of continuous MFI film) on the support was confirmed with scanning electron microscope (SEM) and further characterization was done by room-temperature H₂ gas permeation test, room-temperature n-butane/H₂ binary mixture separation test and xylene isomers separation tests. For m-xylene isomerization tests, the inside of the membrane tube was packed with 2.18 g of Pt-HZSM-5 catalyst and the reactor was fed with m-xylene saturated in N₂ gas. For comparison, an equivalent fixed-bed reactor (FBR) was operated at same conditions as the e-CMR.

The SEM micrograph shows the formation of zeolite crystals within the three layers of the support and the room-temperature H₂ permeance and room-temperature n-butane/H₂ separation factor of the membrane were 0.49 µmol.m^-2.s^-1.Pa^-1 and >100, respectively. Regarding the separation of xylene isomers (p/m/o: 0.63 kPa/0.27 kPa/0.32 kPa), at 473 K the membrane displayed a p-xylene permeance of 11.4 nmol.m^-2.s^-1.Pa^-1 with a p-xylene/o-xylene (p/o) separation factor >400. It is noteworthy to mention that during the xylene isomers separation tests, neither o-xylene nor m-xylene was detected in the permeate stream. Thus the membrane could be considered "defect-free". Furthermore, during the separation test no isomerization products were detected. This suggests that the membrane was inert to xylene isomerization. At the same time it should be noted that the membrane underwent several thermal cycles to conduct the separation tests reported. Nevertheless, the membrane still kept a repeatable separation performance, indicating a high thermal stability.

Regarding the isomerization test, in the permeate only mode (only products in the permeate stream were considered), the p-xylene yield increased from 2.2% at 573 K reaching a maximum of about 2.7 % at 473 K with p-xylene purity in the permeate approaching 100% at 473 K. In combined mode (products in both permeate and retentate streams were considered), a maximum p-xylene yield of 19.0 % was obtained at 473 K and for an equivalent FBR operated at the same operating conditions as e-CMR, the p-xylene yield was 15.8% at 473 K, indicating about 18% in p-xylene yield compared to the e-CMR. Throughout the reaction temperatures investigated, the membrane displayed 100% selectivity to p-xylene in permeate-only mode. The results showed that it is possible to obtain ultra pure p-xylene in e-CMR, especially in the permeate side, with both p-xylene purity and para-selectivity approaching 100%. This is the first time p-xylene purity will be reported for e-CMR having nanocomposite MFI-type membranes as separation unit. With these results, the possibility of cutting down operational cost via a reduction in energy consumption during the production of ultra-pure p-xylene could be feasible with the application nanocomposite MFI-alumina membranes in e-CMR. However, defect-free nanocomposite MFI-type zeolite membranes with high p-xylene flux are necessary to make this technology attractive and competitive with existing technologies.