Thesis (MScIng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Both porous and dense hydrogen selective membranes have recently been an active area
of research. The combination of a reactor and a separator in the form of a membrane
reactor is seen as a feasible application in which to perform dehydrogenation reactions.
These reactions are equilibrium limited so that the removal of the product H2 by a
selective membrane can improve the process effectiveness. Early Pd-based membranes
were made of thin-walled tubes. In an attempt to increase permeation rates, thin
supported Pd membranes have been developed.
This study investigated the development and performance of a catalytic membrane
reactor. The membrane reactor consists of a tubular alumina membrane support coated on
the inside with a film of palladium or a palladium-copper alloy. This reactor was used for
the dehydrogenation of isopropanol. The thin film was coated on the alumina support
using an electroless plating process. This process occurs in a liquid medium where
palladium and copper are deposited by electrolysis or electroless means. With these
methods alloys can also be deposited on the support. By plating a thin film of palladium
on the alumina membranes, will attract hydrogen molecules from the reaction product,
which will increase the reaction rate. The electroless plating process consists of four
major components:
(i)
(ii)
(iii)
(iv)
reducing agent ( 0.04 M hydrazine),
temperature bath,
stabilised source of metal ions, and
support membrane (α-alumina).
Heat treatment was carried out on the coated membranes for 5 hours in a hydrogen
atmosphere at 450°C. The plated membranes supplied by Atech were characterised using
X-ray diffraction (XRD), scanning electron microscopy (SEM) and particle induced Xray
emission (PIXE) before and after heat treatment. SEM photographs showed that the
pore size of the membranes was doubtful and due to that the films were not of a dense nature. XRD results revealed that heat treatment led to the formation of smaller Pd and
Cu crystallites. The concentration profiles constructed from the PIXE results indicated
that Cu and Pd penetrated deep into the pores of the membrane during film preparation.
Different catalysts (Al2O3, MgO and SiO2) were tested and the best one was chosen as
catalyst in the membrane reactor. These catalytic runs were done in a plug flow (fixedbed)
reactor. Different particle sizes of catalysts were also tested. A 9.2 Cu wt % on silica
achieved the highest acetone yields for the temperatures tested. Two different types of
alumina membrane reactors were used. These were supplied from SCT. One membrane
only coated with palladium and the other coated with palladium and copper. Selectivity
and permeability tests were also carried out on these membranes. Selectivities of up to
90.6 could be reached with the palladium coated membrane. The palladium-copper plated
membrane only achieved selectivities of up to 13. With heat treatment this value
decreased even more. The palladium coated membrane also achieved much better
conversion to acetone in the dehydrogenation of 2-propanol. The reason for that is its
better selectivity. The palladium-copper membrane reactor did not show much better
results than the fixed-bed reactor. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die ontwikkeling en werk verrigting van ‘n katalitiese
membraan reaktor. Die membraan reaktor bestaan uit ‘n dun film palladium of
palladium-koper allooi wat aan die binnekant van ‘n silindriese alumina membraan
geplateer word. Die alumina dien as membraanbasis. Hierdie reaktor sal gebruik word vir
die dehidrogenering van isopropanol.
Die dun films van metaal word neergeslaan op die alumina basis deur ‘n elektrodelose
platerings proses. Hierdie proses vind plaas in ‘n vloeistof medium waar palladium en
koper neerslag plaasvind op ‘n elektrodelose wyse. Met hierdie metode kan metaal
allooie geplateer word op basis membrane. Deur ‘n dun palladium lagie aan die binnekant
van die alumina membrane te plateer sal veroorsaak dat waterstof molekules uit die
reaksie volume sal weg beweeg. Dit sal ‘n verhoging in reaksie tempo meebring. Die
platerings proses bestaan uit vier komponente:
(i) reduseermiddel (0.04M Hidrasien),
(ii) temperatuur water bad,
(iii) stabiliseerde bron van metaal ione (Pd/Cu kompleks oplossing), en
(iv) basis membraan (α-alumina).
Hittebehandeling vir 5 uur is uitgevoer op hierdie geplateerde membrane by 450°C in ‘n
waterstofatmosfeer. Die geplateerde membrane is daarna gekarakteriseer- voor en na
hittebehandeling. Dit is gekarakteriseer deur X-straal diffraksie (XRD), skanderings
elektron mikroskopie (SEM) en partikel geïnduseerde X-straal emissie (PIXE). XRD
eksperimente het gewys dat die koper en die palladium ‘n allooi gevorm het.
Veranderinge in kristaltekstuur het voorgekom na hittebehandeling. Tydens
hittebehandeling was kleiner palladium en koper kristalle gevorm. SEM resultate het
getoon dat die film nie baie dig was nie en die porie grootte van die membrane was ook
nie korrek nie. PIXE resultate het die konsentrasieprofiele van beide koper en palladium oor die dikte van die membraan bepaal. Dit het gewys dat die Cu en Pd diep binne die
membraan penetreer het tydens voorbereiding van die membraan.
Verskillende soorte kataliste (Al2O3, MgO and SiO2) is ondersoek vir die
dehidrogenering van isopropanol. Hierdie katalitiese ondersoek is gedoen in ‘n propvloei
reaktor. Die beste katalis is gekies om in die membraan reaktor te gebruik. Verskillende
partikel groottes is ook ondersoek. ‘n 9.2 Cu massa % koper op silika katalis het die beste
omsetting na asetoon verkry vir die temperature waarvoor toetse gedoen is. Twee tipes
membraan reaktors is gebruik. Een met net ‘n palladium film, terwyl ‘n palladium-koper
allooi op die ander membraan reaktor gedeponeer was. Selektiwiteits- en
deurlaatbaarheids toetse is op altwee membrane gedoen. Selektiwiteite van 90.6% kon
verkry word met die palladium membraan. Die palladium-koper membraan kon slegs ‘n
selektiwiteit van 13% bereik. Met hittebehandeling daarvan het die selektiwiteit selfs
meer afgeneem. Die palladium membraan het ook hoër omsettings na asetoon getoon.
Die rede hiervoor is die membraan se hoë selektiwiteit. Die palladium-koper membraan
het nie veel beter resultate as die propvloei reaktor gelewer nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16397 |
| Date | 04 1900 |
| Creators | Mouton, Duane Wilmot |
| Contributors | Lorenzen, L., Keuler, J.N., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xix, 180 leaves : ill. |
| Rights | University of Stellenbosch |
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