Evaluation and improvement of dehydrogenation conversion and isomerization selectivity in an extractor Catalytic Membrane Reactor
Van Dyk, Lizelle Doreen
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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.