Characterization and optimization of an extractor-type catalytic membrane reactor for meta-xylene isomerization over Pt-HZSM-5 catalyst

[摘要] ENGLISH ABSTRACT: Future chemical production is faced with a challenge of limited material and energyresources. However, process intensification might play a significant role to alleviating thisproblem. Vision of process intensification through multifunctional reactors has stimulatedresearch on membrane-based reactive separation processes, in which membrane separationand catalytic reaction occur simultaneously in one unit. These processes are rather attractiveapplications because they are potentially compact, less capital intensive, and have lowerprocessing costs than traditional processes. Moreover, they often enhance the selectivity andyield of the target product.For about three decades, there has been a great evolution in p-Xylene productiontechnology, with many equipment improvements being instituted in the industry. Typically,these improvements bring economic as well as processing advantages to the producers. Suchdevelopments are vital, as the capital costs for process equipment to produce and separatep-Xylene from xylene isomers, especially into high purity p-Xylene, still remain very high.However, with numerous advantages of membrane-based reactive separation processescompared to the conventional processes, the research focus has been channelled towardapplication of MFI-type zeolite membranes for in situ separation and isomerization of xylenein extractor-type catalytic membrane reactors. To contribute to this research line, this studyhas focused on characterization and optimization of an extractor-type catalytic membranereactor (e-CMR) equipped with a nanocomposite MFI-alumina membrane as separation unitfor m-Xylene isomerization over Pt-HZSM-5 catalyst.Nanocomposite MFI-alumina zeolite membranes (tubes and hollow fibres) used inthis study were prepared via a so-called 'hydrothermal pore-plugging synthesis techniquedeveloped by Dalmon and his group more than a decade ago. In this concept, MFI material isgrown by 'pore-plugging' direct hydrothermal synthesis in a porous matrix rather than formingthin films on top of the support. The advantages of this type of architecture over conventionalfilm-like zeolite membranes include: (i) minimization of the effect of thermal expansionmismatch between the support and the zeolite, (ii) easy to scale-up, and (iii) easy moduleassembly, because the separative layer (zeolite crystals) are embedded within the pores of theceramic support, reducing the effects of abrasion and thermal shocks. After membranesynthesis, the membrane quality and separation performance of these membranes wereevaluated through single gas permeation (H2), binary gas separation (n-butane/H2) and ternaryvapour mixture of xylene isomers using the vapour permeation (VP) method with p-Xylene asthe target product. After evaluating the xylene isomer separation performance of the membranes, the membranes were used in extractor-type catalytic membrane reactors to carryout m-Xylene isomerization over Pt-HZSM-5 catalyst with p-Xylene as the target product.This dissertation has shown that nanocomposite MFI-alumina membrane tubes andhollow fibre membranes were selective to p-Xylene from xylene isomers. The dissertationalso reports for the first time in open literature the excellent xylene separation performance ofnanocomposite MFI-alumina membrane tubes at higher xylene loading (or vapour pressure).Unlike their film-like counterparts, the membranes still maintain increased selectivity to p-Xylene at higher xylene vapour pressures without showing a drastic decrease in selectivity.This outstanding property makes it a promising choice for pervaporation applications whereconcentration profile is usually a major problem at higher loading of xylene.With the use of nanocomposite MFI-alumina hollow fibre membranes, this researchhas demonstrated that membrane configuration and effective membrane wall thickness play aprominent role in enhancing cross membrane flux. Results presented in the study show, forthe first time in open literature, that nanocomposite MFI-alumina hollow fibre membranecould enhance p-Xylene fluxes during the separation of ternary vapour mixture of xylene dueto the smaller effective wall thickness of the membrane (membrane thickness <1 μm) whencompared to conventional randomly oriented MFI zeolite films (membrane thickness >3 μm).During xylene isomers separation with nanocomposite hollow fibre membrane, about 30%increase in p-Xylene flux was obtained compared to the membrane tubes, operated under thesame conditions. Additionally, hollow fibres offer the added advantage of membrane surfaceto-volume ratios as high as 3000 m2/m3 compared to conventional membrane tubes. Usingthis type of system could be instrumental in reducing both the size and cost of permeatingmodules for future xylene separation processes. However, obtaining high qualitynanocomposite MFI-alumina membrane fibres is subject to the availability of high qualityfibre supports.Regarding the application of nanocomposite MFI-alumina membrane tubes asextractor-type catalytic membrane reactors (referred to as extractor-type zeolite catalyticmembrane reactor (e-ZCMR) in this study) for m-Xylene isomerization over Pt-HZSM-5, theresults presented in this study further substantiate and confirm the potentials of e-ZCMRsover conventional fixed-bed reactors (FBRs). In the combined mode (products in thepermeate plus products in the retentate), the e-ZCMR displayed 16-18% increase in p-Xyleneyield compared to an equivalent fixed-bed reactor operated at the same operating conditions.On the basis of the high p-Xylene-to-o-Xylene (p/o) and p-Xylene-to-m-Xylene (p/m)separation factors offered by the membranes, p-Xylene compositions in the permeate-onlymode (products in the permeate stream) in the range 95%-100% were obtained in thee-ZCMR. When a defect-free nanocomposite MFI-alumina membrane tube with p-Xylene-too-Xylene (p/o) separation factor >400 was used, ultra pure p-Xylene with p-Xylene purity approaching 100% in the permeate-only mode was obtained. Moreover, the e-ZCMRdisplayed 100% para-selectivity in the permeate-only mode throughout the temperaturestested. This is not possible with conventional film-like MFI-type zeolite membranes.Therefore, the application of nanocomposite MFI-alumina membranes in extractor-typecatalytic membrane reactors could catalyse the development of energy-efficientmembrane-based process for the production of high purity p-Xylene.Furthermore, in this dissertation, a report on modelling and sensitivity analysis of ane-ZCMR equipped with a nanocomposite MFI-alumina membrane tube as separation unit form-Xylene isomerization over Pt-HZSM-5 catalyst is presented. The model output is in fairagreement with the experimental results with percentage errors (absolute) of 17%, 29%,0.05% and 19.5% for p-Xylene yield in combined mode, p-Xylene selectivity in combinedmode, p-Xylene selectivity in permeate-only mode and m-Xylene conversion, respectively.Therefore, the model is adequate to explain the behaviour of e-ZCMR during m-Xyleneisomerization over Pt-HZSM-5 catalyst. The model is also adaptable to e-ZCMRs of differentconfigurations such as hollow fibre MFI-alumina membrane-based e-ZCMRs. To gain moreinsight into the behaviour of the model to small changes in certain design parameters,sensitivity analysis was performed on the model. As expected, the sensitivity analysisrevealed that intrinsic property of membrane (porosity, tortuosity), membrane effectivethickness and reactor size (indicated with reactor internal diameter) play a significant role onthe performance of e-ZCMR during p-Xylene production from the mixed xylenes.MFI-alumina zeolite membranes with optimized parameters such as membrane porosity,membrane tortuosity, and membrane effective wall thickness might enhance transport ofp-Xylene through the membrane and thus resulting in higher p-Xylene flux through themembrane. This eventually would translate into an increase in p-Xylene yield inpermeate-only mode. As far as it could be ascertained, this is the first report in open literatureon modelling study with sensitivity analysis of e-ZCMR equipped with nanocompositeMFI-alumina membrane tubes as separation unit for m-Xylene isomerization over Pt-HZSM-5 catalyst.In addition, the results of this study have confirmed previous research effortsreported on the application of extractor-type catalytic membrane reactors, having MFI-typemembranes as separation units, for p-Xylene production via m-Xylene isomerization over asuitable catalyst. Also, new ideas were developed, tested and proposed that now provide asolid basis for further scale-up and techno-economical studies. Such studies are necessary toevaluate the competitiveness of the technology with the traditional processes for theproduction of high purity p-Xylene from mixed xylene.In summary, the encouraging results, as documented in this dissertation and alsocommunicated to researchers in the area of membrane-based reactive separation (in the form of four peer-reviewed international scientific publications and four conference proceedings),could provide a platform for developing a scaled-up membrane-based energy-efficientindustrial process for producing high purity p-Xylene through isomerization.

[发布机构] Stellenbosch University