Flow-through membrane microreactor for intensified heterogeneous catalysis

  • Durchströmter Membran-Mikroreaktor für intensivierte heterogene Katalyse

Westermann, Thomas; Melin, Thomas (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2009)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2009

Abstract

Under the common headline of process intensification several innovative reactor types for heterogeneous catalysis are discussed, including membrane reactors, microreactors and monolithic reactors. A novel reactor concept called flow-through catalytic membrane microchannel reactor contains aspects of each of the three reactor types: The catalyst is immobilized in a ceramic membrane, the geometric structures are in the scale of micrometers and below and the reactants flow convectively through uniform catalytic channels. Anodized alumina membranes comprise regular cylindrical channels with a narrow pore size distribution. Impregnated with palladium, they promise high catalytic activity combined with very short contact times and a narrow residence time distribution, calling for application in performing fast and selective reactions. Whereas the reactor geometry impedes direct measurement of the residence time distribution, a fluid dynamic reactor model, taking into account pore size distribution and axial dispersion, allows to quantify deviations from ideal plug flow behavior. Both influences are combined in a single parameter effective dispersion model. For low axial dispersion caused by high axial velocities and absolute pressures, the pore size distribution limits the minimum achievable effective dispersion. Due to the small characteristic lengths, heat convection is always small compared to the internal heat transfer, leading to isothermal reactor behavior with identical gas phase and membrane temperature and a temperature jump at the pore entrance. The isothermal operation even of highly exothermic reactions is beneficial for kinetic studies. The predicted high catalytic activity of the investigated catalytic membrane microchannel reactor compared to catalytic fixed beds is proven experimentally. Although the flat and thin geometry of membranes is advantageous in terms of high throughput at minimum contact time, it is unfavorable regarding axial dispersion. This prevents application in consecutive low pressure gas phase reactions, where a fixed bed reactor might reach higher selectivities due to increased reactor length. At high pressures or in liquid phase this limitation is negligible and ideal reactor behavior can be assumed.

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