Single Flow-Through Catalytic Membrane Microchannel Reactor for Intensified Heterogeneous Catalysis: Characterisation and Application to Hydrogenation of Ethyne
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-4)
Keywords: Catalytic Membrane Reactor, Microchannel Reactor, Flow Through, Reactor Modelling, Knudsen Diffusion
Catalytic membranes without separative function have successfully been applied as microstructured reactors [1]. If the reactant mixture is forced to flow through the pores of a membrane which has been impregnated with catalyst, the intensive contact allows for high catalytic activity without mass transport resistances. Reaction rates measured in catalytic membrane reactors have been found up to ten times higher than in conventional packed bed reactors with the same catalyst [2]. In conventional membranes the void spaces between sintered particles resemble the pores, resulting in a relatively wide size distribution with remarkable tortuosity. Flow-through catalytic membrane reactors (FTCMR) for gas-liquid applications are often applied in batch mode with high recirculation ratios [3].
In this work anodized alumina membranes with very uniform pore channels are examined for their applicability as catalytic reactors for gas phase reactions. The investigated membranes possess a regular structure of parallel open pore channels with diameters of 200 nm and a length of 60 µm. Catalytically active material (Pd) is introduced into the pores by means of impregnation methods. The reactor is operated in continuous mode without recirculation. The narrow channels intensify the contact between reactants and catalyst resulting in high conversion, while the regular channel structure leads to a narrow residence time distribution which allows for high selectivity. The concept is tested with the selective hydrogenation of ethyne catalyzed by palladium as model reaction.
As a reactor model the membrane is represented by a high number of parallel one-dimensional plug flow reactors taking into consideration the pore size distribution and axial dispersion. Higher flow-through velocity leads to lower axial dispersion but increases the pressure drop. Deviations from an ideal pore size distribution can be counteracted by flowing through several membranes in sequence. The resulting conversions and selectivities are compared to the ideal case of a single plug flow reactor without axial dispersion on the one hand and to the realistic alternative of a fixed bed reactor on the other hand.
Pressure drop measurements confirm that the flow through the membrane consists of a laminar flow contribution and a Knudsen diffusion contribution. The ratio of the contributions can be manipulated by altering the absolute pressure. Reaction experiments have been performed to measure reaction kinetics and to validate the reactor models. Advantages of the catalytic membrane microchannel reactor compared to an equivalent fixed bed reactor as well as to an unstructured catalytic membrane reactor are quantified.
[1] A.G. Dixon, Int. J. Chem. Reactor Eng. 1 (2003)
[2] V.T. Zaspalis, W. van Praag, K. Keizer, J.G. van Ommen, J.R.H. Ross, A.J. Burggraaf, Appl. Catal. 74 (1991), 205-222
[3] R. Dittmeyer, K. Svajda, M. Reif, Topics in Catalysis 29, 1-2 (2004), 3-27
Presented Thursday 20, 11:00 to 11:20, in session EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-4).
