Evaluation and improvement of dehydrogenation conversion and isomerization selectivity in an extractor Catalytic Membrane Reactor
Date
2005-03
Authors
Van Dyk, Lizelle Doreen
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
Recent advances in inorganic material preparation for membrane fabrication have
extended the use of membranes to high temperature and chemically harsh environments.
This has allowed inorganic membranes to be integrated into catalytic reactors, resulting in
the concept known as Catalytic Membrane Reactors (CMRs). CMRs have overall
important benefits of product quality, plant compactness, environmental impact reduction
and energy savings. It has found use in a broad range of applications including
biochemical, chemical, environmental and petrochemical systems. In these CMRs, the
membranes perform a variety of functions, and consequently they are categorized
according to the primary role of the membrane: extractor, distributor or contactor.
In this dissertation the different uses of an extractor Catalytic Membrane Reactor
(eCMR) are evaluated with the help of model reactions. In the eCMR the primary function
of the membrane is to selectively extract one of the reaction products from the reaction
zone, thereby combining the benefits of separation and reaction in one unit operation. This
can lead to a number of advantages, of which the two most important ones include: (a)
conversion beyond thermodynamic equilibrium in equilibrium restricted reactions and/or
(b) the improvement of product selectivity in consecutive/parallel reaction networks.
The dehydrogenation of isobutane, an equilibrium restricted reaction, was
evaluated in a dense Palladium and a MFI-zeolite/alumina composite eCMR. These two
eCMRs, consisting of a membrane packed with a Pt/In/Ge-MFI-zeolite catalyst, differed
only on the basis of the membrane used. The palladium membrane showed superior
extraction and selectivity capability for hydrogen in the reaction mixture compared to the
MFI/alumina composite membrane. Regardless of these facts, the performances of the Pd
and MFI eCMR, when evaluated at the same reaction conditions, were similar. The
isobutane conversion to isobutene, employing high sweep rates (185 ml/min) could be
increased up to ca. 37 % at 723 K, compared to 14 % in the conventional packed-bed
reactor. The similar performance of the two different eCMRs was evaluated using a
Catalytic Membrane Reactor model. Model results showed that in order for the extractortype
CMR to completely draw benefit from the combination of membrane and catalyst in
the same unit for conversion enhancement, a very active catalyst should be developed, able
to sustain the high extraction ability of the membrane. This was the first time that these two eCMRs were evaluated at similar reaction conditions in order to study the influence of
the nature of the membrane material on the working of the eCMR.
The eCMR was also used to carry out meta-xylene isomerization. This part
focused on the extraction of para-xylene from the meta-xylene isomerization reaction zone
with a MFI eCMR (MFI-zeolite membrane and Pt-HZSM5 fixed-bed catalyst) in order to
improve the reaction selectivity towards para-xylene. Para-xylene is an important
industrial chemical used as a precursor for polyester resin, and in order to meet the paraxylene
demand, ortho- and meta-xylenes are converted via the xylene isomerization
reaction to xylene isomers.
It has been shown that the pore-plugged MFI-zeolite membranes used in this study
can selectively extract para-xylene from a mixture of xylenes. Using an extractor type
catalytic membrane reactor instead of a conventional fixed-bed reactor for meta-xylene
isomerization, can lead to higher para-xylene selectivities. The para-xylene selectivity can
even be improved to 100% if the CMR is operated in the permeate-only mode, but this
comes at a price of lower para-xylene yields. When operated in combined mode (i.e.
mixing both permeate and retentate streams after the reactor), the CMR shows an
improvement on both para-xylene productivity (ca. 10 % maximum at conditions studied)
and selectivity when compared to the conventional reactor. This is the first time paraxylene
selectivity could successfully be improved by employing an extractor Catalytic
Membrane Reactor.
This dissertation also led to the design and construction of a new generation
membrane reactor testing bench, a first in the Department of Process Engineering,
University of Stellenbosch. The bench allows for high temperature evaluation of
membranes and Catalytic Membrane Reactors. The design is simple and easily adaptable
for use to evaluate various different reactions.
Description
Thesis (PhD (Process Engineering))--University of Stellenbosch, 2006.
Keywords
Membrane reactors, Dissertations -- Process engineering, Theses -- Process engineering