Thesis (PhD (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Future chemical production is faced with a challenge of limited material and energy
resources. However, process intensification might play a significant role to alleviating this
problem. Vision of process intensification through multifunctional reactors has stimulated
research on membrane-based reactive separation processes, in which membrane separation
and catalytic reaction occur simultaneously in one unit. These processes are rather attractive
applications because they are potentially compact, less capital intensive, and have lower
processing costs than traditional processes. Moreover, they often enhance the selectivity and
yield of the target product.
For about three decades, there has been a great evolution in p-Xylene production
technology, with many equipment improvements being instituted in the industry. Typically,
these improvements bring economic as well as processing advantages to the producers. Such
developments are vital, as the capital costs for process equipment to produce and separate
p-Xylene from xylene isomers, especially into high purity p-Xylene, still remain very high.
However, with numerous advantages of membrane-based reactive separation processes
compared to the conventional processes, the research focus has been channelled toward
application of MFI-type zeolite membranes for in situ separation and isomerization of xylene
in extractor-type catalytic membrane reactors. To contribute to this research line, this study
has focused on characterization and optimization of an extractor-type catalytic membrane
reactor (e-CMR) equipped with a nanocomposite MFI-alumina membrane as separation unit
for m-Xylene isomerization over Pt-HZSM-5 catalyst.
Nanocomposite MFI-alumina zeolite membranes (tubes and hollow fibres) used in
this study were prepared via a so-called “hydrothermal pore-plugging synthesis technique”
developed by Dalmon and his group more than a decade ago. In this concept, MFI material is
grown by 'pore-plugging' direct hydrothermal synthesis in a porous matrix rather than forming
thin films on top of the support. The advantages of this type of architecture over conventional
film-like zeolite membranes include: (i) minimization of the effect of thermal expansion
mismatch between the support and the zeolite, (ii) easy to scale-up, and (iii) easy module
assembly, because the separative layer (zeolite crystals) are embedded within the pores of the
ceramic support, reducing the effects of abrasion and thermal shocks. After membrane
synthesis, the membrane quality and separation performance of these membranes were
evaluated through single gas permeation (H2), binary gas separation (n-butane/H2) and ternary
vapour mixture of xylene isomers using the vapour permeation (VP) method with p-Xylene as
the target product. After evaluating the xylene isomer separation performance of the membranes, the membranes were used in extractor-type catalytic membrane reactors to carry
out 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 and
hollow fibre membranes were selective to p-Xylene from xylene isomers. The dissertation
also reports for the first time in open literature the excellent xylene separation performance of
nanocomposite 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 where
concentration profile is usually a major problem at higher loading of xylene.
With the use of nanocomposite MFI-alumina hollow fibre membranes, this research
has demonstrated that membrane configuration and effective membrane wall thickness play a
prominent role in enhancing cross membrane flux. Results presented in the study show, for
the first time in open literature, that nanocomposite MFI-alumina hollow fibre membrane
could enhance p-Xylene fluxes during the separation of ternary vapour mixture of xylene due
to the smaller effective wall thickness of the membrane (membrane thickness <1 μm) when
compared 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 the
same conditions. Additionally, hollow fibres offer the added advantage of membrane surfaceto-
volume ratios as high as 3000 m2/m3 compared to conventional membrane tubes. Using
this type of system could be instrumental in reducing both the size and cost of permeating
modules for future xylene separation processes. However, obtaining high quality
nanocomposite MFI-alumina membrane fibres is subject to the availability of high quality
fibre supports.
Regarding the application of nanocomposite MFI-alumina membrane tubes as
extractor-type catalytic membrane reactors (referred to as extractor-type zeolite catalytic
membrane reactor (e-ZCMR) in this study) for m-Xylene isomerization over Pt-HZSM-5, the
results presented in this study further substantiate and confirm the potentials of e-ZCMRs
over conventional fixed-bed reactors (FBRs). In the combined mode (products in the
permeate plus products in the retentate), the e-ZCMR displayed 16-18% increase in p-Xylene
yield 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-only
mode (products in the permeate stream) in the range 95%-100% were obtained in the
e-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-ZCMR
displayed 100% para-selectivity in the permeate-only mode throughout the temperatures
tested. This is not possible with conventional film-like MFI-type zeolite membranes.
Therefore, the application of nanocomposite MFI-alumina membranes in extractor-type
catalytic membrane reactors could catalyse the development of energy-efficient
membrane-based process for the production of high purity p-Xylene.
Furthermore, in this dissertation, a report on modelling and sensitivity analysis of an
e-ZCMR equipped with a nanocomposite MFI-alumina membrane tube as separation unit for
m-Xylene isomerization over Pt-HZSM-5 catalyst is presented. The model output is in fair
agreement 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 combined
mode, 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-Xylene
isomerization over Pt-HZSM-5 catalyst. The model is also adaptable to e-ZCMRs of different
configurations such as hollow fibre MFI-alumina membrane-based e-ZCMRs. To gain more
insight into the behaviour of the model to small changes in certain design parameters,
sensitivity analysis was performed on the model. As expected, the sensitivity analysis
revealed that intrinsic property of membrane (porosity, tortuosity), membrane effective
thickness and reactor size (indicated with reactor internal diameter) play a significant role on
the 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 of
p-Xylene through the membrane and thus resulting in higher p-Xylene flux through the
membrane. This eventually would translate into an increase in p-Xylene yield in
permeate-only mode. As far as it could be ascertained, this is the first report in open literature
on modelling study with sensitivity analysis of e-ZCMR equipped with nanocomposite
MFI-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 efforts
reported on the application of extractor-type catalytic membrane reactors, having MFI-type
membranes as separation units, for p-Xylene production via m-Xylene isomerization over a
suitable catalyst. Also, new ideas were developed, tested and proposed that now provide a
solid basis for further scale-up and techno-economical studies. Such studies are necessary to
evaluate the competitiveness of the technology with the traditional processes for the
production of high purity p-Xylene from mixed xylene.
In summary, the encouraging results, as documented in this dissertation and also
communicated 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-efficient
industrial process for producing high purity p-Xylene through isomerization. / AFRIKAANSE OPSOMMING: Die produksie van chemiese stowwe word belemmer deur die uitdaging van beperkte
materiaal- en energiebronne. Prosesuitbreiding kan egter ‘n noemenswaardige rol in die
verligting van hierdie probleem speel. Die moontlike gebruik van multi-funksionele reaktore
in prosesuitbreiding het navorsing in membraan-gebaseerde reaktiewe skeidingsprosesse
(waar membraanskeiding en die katalitiese reaksie gelyktydig in ‘n enkele eenheid plaasvind)
aangemoedig. Hierdie prosesse is aantreklik omdat hulle potensieel kompak en minder
kapitaal-intensief is en ook teen laer koste as tradisionele prosesse bedryf kan word. Dit is ook
dikwels die geval dat die multi-funksionele reaktor die selektiwiteit en opbrengs van die
gewenste produk verhoog.
In die afgelope drie dekades was daar ’n sterk verandering in die tegnologie wat
gebruik word in die produksie van p-Xileen, met vele verbeterings aan nuwe toerusting wat in
die nywerheid in bedryf gestel is. Hierdie verbeteringe hou gewoonlik ekonomiese-, sowel as
bedryfsvoordele vir die produsente in. Ontwikkelings in hierdie veld is noodsaaklik aangesien
die kapitale uitgawes vir die toerusting om p-Xileen, veral baie suiwer p-Xileen, van
xileenpolimere te produseer en te skei, steeds baie hoog is. Met talle voordele gekoppel aan
membraangebaseerde reaktiewe skeidingsprosesse in vergelyking met normale prosesse, is
die navorsing egter gekanaliseer na die gebruik van MFI-tipe zeolietmembrane vir die in-situ
skeiding en isomerisasie van xileen in ekstraksie-tipe katalitiese membraanreaktore. As
bydrae tot hierdie navorsingsveld het hierdie studie op die karakterisering en optimering van
‘n ekstraksie-tipe katalitiese membraanreaktor (e-KMR), toegerus met ’n nanosaamgestelde
MFI-alumina membraan as skeidingseenheid vir m-Xileen isomerisasie in die teenwoordigheid
van ‘n Pt-HZSM-5 katalis, gefokus.
Nanosaamgestelde MFI-alumina zeolietmembrane (buise en hol vesels) wat in hierdie
studie gebruik is, is voorberei deur die sogenaamde “hidrotermiese porie-sperring sintese
tegniek” wat meer as ‘n dekade gelede ontwikkel is deur Dalmon en sy groep. In hierdie
tegniek word MFI-materiaal gekweek deur direkte hidrotermiese sintese in ‘n poreuse
matriks, eerder as die vorming van dun films bo-op die ondersteuningsbasis. Die voordele van
hierdie ontwerp bo dié van die konvensionele filmagtige zeolietmembrane sluit in: (i)
minimering van die effek van termiese uitsetting op die gaping tussen die ondersteuningsbasis
en die zeoliet, (ii) die gemak van opskalering, en (iii) die gemak waarmee die modules
aanmekaar gesit kan word, omdat die skeidingslaag (zeolietkristalle) binne die porieë van die
keramiek-ondersteuningsbasis geleë is, wat die effek van erodering en termiese skok
verminder. Ná die membraansintese is die membraankwaliteit en skeidingsvermoë geevalueer
deur enkel-gas-deurdringing (H2), binêre-gas-skeiding (n-butaan/H2), en ternêre dampmengsel van xileen-isomere deur die gebruik van die damp-deurdringingsmetode met
p-Xileen as die teikenproduk.
Hierdie tesis het gewys dat nanosaamgestelde MFI-alumina membraanbuise en hol
vesel membrane selektief was ten opsigte van p-Xileen vanuit xileen-isomere. Die tesis doen
ook, vir die eerste keer in die oop literatuur verslag, oor die uitstekende p-Xileen skeidingsvermoë
van nanosaamgestelde MFI-alumina buise by hoër xileenladings (of dampdrukke).
Anders as hulle filmagtige eweknieë het die membrane steeds hul verhoogde selektiwiteit vir
p-Xileen by hoër dampdrukke behou, sonder ‘n merkbare verlaging in die selektiwiteit.
Hierdie merkwaardige eienskap maak dit ‘n belowende keuse vir pervaporasie toepassings,
waar die konsentrasieprofiel (as gevolg van hoër xileenladings) gewoonlik ’n noemenswaardige
probleem is.
Met die gebruik van nanosaamgestelde MFI-alumina membrane het hierdie navorsing
gewys dat membraankonfigurasie en –wanddikte ‘n prominente rol speel in die verbetering
van vloei oor die membraan. Resultate wat in die studie voorgelê word, wys, vir die eerste
keer in oop literatuur, dat hol vesel nanosaamgestelde MFI-alumina membrane die deurvloei
van p-Xileen kan verbeter gedurende die skeiding van ternêre dampmengsels van xileen, as
gevolg van die kleiner effektiewe wanddikte van die membraan (<1 μm) wanneer dit vergelyk
word met konvensionele kansgewys-geörienteerde MFI-zeoliet films met ‘n membraandikte
van >3 μm. Tydens die skeiding van xileen-isomere met nanosaamgestelde hol vesel
membrane is ‘n verbetering van ongeveer 30 % in die deurvloei van p-xileen verkry,
vergeleke met membraanbuise, by identiese bedryfstoestande. Hol vesels bied ook die verdere
voordeel van oppervlak-tot-volume verhoudings van so hoog as 3000 m2/m3 vergeleke met
konvensionele membraanbuise. Die gebruik van hierdie tipe sisteem kan deurslaggewend
wees in die vermindering van die grootte en koste van deurlatingseenhede in toekomstige
xileen-skeidingsprosesse. Die vervaardiging van hoë-kwaliteit nanosaamgestelde MFIalumina
membraanvesels is egter onderworpe aan die beskikbaarheid van hoë-kwaliteit
vessel-ondersteuningsbasisse.
Wat die gebruik van nanosaamgestelde MFI-alumina membraanbuise as ekstraksietipe
katalitiese membraanreaktore betref (ekstraksie-tipe zeoliet katalitiese membraanreaktor,
of e-ZKMR in hierdie studie) vir m-Xileen isomerisasie in die teenwoordigheid Pt-HZSM-5,
bevestig die resultate die potensiaal van e-ZKM reaktore bo konvensionele vaste-bed reaktore
(VBR). In die gekombineerde verstelling (met produkte in die permeaat sowel as die
retentaat) toon die e-ZKMR ‘n 16 – 18% verbetering in die opbrengs van p-Xileen vergeleke
met ‘n ekwivalente VBR by dieselfde bedryfskondisies. Gegrond op die hoë p-Xileen-tot-o-
Xileen (p/o) en p-Xileen-tot-m-Xileen (p/m) skeidingsfaktore wat deur die membraan gebied
word, is p-Xileen-samestellings in die slegs-permeaat verstelling (produkte in die
permeaatstroom) van tussen 95 en 100% in die e-ZKMR verkry. Toe ‘n defek-vrye nanosaamgestelde MFI-alumina membraanbuis met ‘n (p/o) skeidingsfaktor van >400 gebruik
is, is p-Xileen met ‘n suiwerheid na aan 100% in die slegs-permeaat verstelling verkry. Die
e-ZKMR het ook 100% para-selektiwiteit in die slegs-permeaat verstelling getoon by alle
toets-temperature, iets wat onmoontlik is met gewone filmagtige MFI-tipe zeolietmembrane.
Om hierdie rede is dit moontlik dat die gebruik van MFI-alumina membrane in ekstraksie-tipe
katalitiese membraanreaktore die ontwikkeling van energie-doeltreffende membraangebaseerde
prosesse vir die produksie van suiwer p-Xileen kan bevorder.
Verder word daar in hierdie tesis verslag gedoen oor die modelering en
sensitiwiteitsanalise van ‘n e-ZKMR wat toegerus is met ‘n nanosaamgestelde MFI-alumina
membraanbuis as skeidingseenheid vir m-Xileen isomerisasie in die teenwoordigheid van ‘n
Pt-HZSM-5 katalis. Die model-uitsette is redelik in ooreenstemming met eksperimentele
resultate met absolute fout-persentasies van 17, 27, 0.05 en 19.5 % vir die p-Xileen opbrengs
in die gekombineerde verstelling, p-Xileen selektiwiteit in die gekombineerde verstelling,
p-Xileen selektiwiteit in die slegs-permeaat verstelling en m-Xileen omsetting,
onderskeidelik. Om hierdie rede kan die model die gedrag van ‘n e-ZKMR verduidelik tydens
die m-Xileen isomerisasie in die teenwoordigheid van ‘n Pt-HZSM-5 katalis. Die model kan
ook aangepas word na e-ZKM reaktore met verskillende konfigurasies, soos hol vesel MFIalumina
membraan-gebaseerde e-ZKMRe. Om meer insig te kry in die gedrag van die model
op klein veranderinge in sekere ontwerpparameters, is ‘n sensitiwiteitsanalise op die model
uitgevoer. Soos verwag, het die sensitiwiteitsanalise gewys dat die intrinsieke eienskappe van
die membraan (porositeit, tortuositeit), die effektiewe van membraandikte en die
reaktorgrootte (gemeet as die interne deursnit van die reaktor) ‘n noemenswaardige rol speel
in die gedrag van die e-ZKMR gedurende p-Xileen produksie vanuit gemengde xilene.
MFI-alumina zeolietmembrane met geoptimeerde parameters soos membraanporositeit,
-tortuositeit, en –wanddikte mag dalk die oordrag van p-Xileen deur die membraan
bevorder en sodoende ‘n hoër vloei van p-Xileen oor die membraan bewerkstellig. Dit sal
uiteindelik lei tot ‘n verhoging in die opbrengs van p-Xileen in die slegs-permeaat verstelling.
So ver dit vasgestel kon word, is hierdie die eerste verslag in die oop literatuur wat die
modelering en sensitiwiteitsanalise van ‘n e-ZKMR, toegerus met nanosaamgestelde MFIalumina
membraanbuise as skeidingseenheid vir m-Xileen isomerisasie in die
teenwoordigheid van ‘n Pt-HZSM katalis, aanspreek.
Verder ondersteun die resultate van hierdie studie vorige navorsingspogings op die
gebruik van e-KMRe, met MFI-tipe membrane as skeidingseenhede, vir die produksie van
p-Xileen deur middel van m-Xileen isomerisasie in die teenwoordigheid van ‘n geskikte
katalis. Verder is nuwe idees ontwikkel, getoets en voorgestel wat dien as ’n stewige basis vir
verdere opskalering- en tegno-ekonomiese studies. Sodanige studies is nodig om die
vatbaarheid van die tegnologie relatief tot die tradisionele prosesse te bepaal. Ter opsomming, die bemoedigende resultate, soos in die tesis gedokumenteer (en ook
gepubliseer in vier ewe-knie beoordeelde internasionale wetenskaplike joernale en vier
konferensiestukke), kan as ‘n platform dien vir die ontwikkeling van ’n opgeskaleerde
membraan-gebaseerde energie-doeltreffende nywerheidsproses vir die produksie van suiwer
p-Xileen deur middel van isomerisasie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5334 |
| Date | 12 1900 |
| Creators | Daramola, Michael Olawale |
| Contributors | Burger, A. J., Lorenzen, L., Giroir-Fendler, A., University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 182 p. : ill. |
| Rights | University of Stellenbosch |
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