Structured catalytic wall microreactor for efficient performance of exothermic reactions

A novel microreactor with structured catalytic wall (SCWMR) has been developed for highly exothermic, gas/solid reactions and tested for the selective acetylene hydrogenation. The reactor design is similar to the so-called "slit microreactor". The structured catalyst consisted of Pd-nanoparticie embedded in a thin ionic liquid layer supported on activated carbon fibres (ACF) cloths. It was placed between two aluminium plates forming an empty slit (thickness similar to 300 mu m). The gas mixture flows above the catalyst through the slit allowing a low pressure drop of 15 kPa/m for flows up to 3.3 x 10(-6) m(3)/s. The catalyst can be easily replaced on demand. The acetylene hydrogenation as a model of exothermic reaction was carried out in the SCWMR at conversions up to 100%. The reactor temperature between 150 and 210 degrees C was maintained by circulation of oil from a thermostat and remained constant confirming the efficient heat exchange without hot-spot formation. The reaction performance was optimized varying flow rates and temperatures. The selectivity of similar to 70% was obtained at conversion of 96.5% under steady-state at 200 degrees C without any catalyst deactivation over 10 h on stream. (C) 2010 Elsevier B.V. All rights reserved.

Compact reactor for continuous multi-phase reactions

Martin Grasemann

The objective of this thesis is the development of a compact reactor for continuous three-phase reactions in single-pass configurations. The design is based on a bubble column reactor staged with layers of catalytic material. To make the reactor suitable for reactions with high adiabatic temperature rise such as highly exothermic hydrogenation reactions under solvent free conditions, work focuses on two different aspects of integrated reactor and catalyst design: Preparation and characterization of structured catalyst stages optimized for high intrinsic activity as well as for intense gas-liquid redistribution and mass transfer on each reactor stage Development of heat transfer equipment integrated in the staged bubble column for efficient, quasi continuous in-line removal of process heat. Chapter 3 is dedicated to the preparation and characterization of a structured catalyst for the use in a staged bubble column reactor. Sintered Metal Filters (SMF), a material consisting of metal fibers with a diameter of ~ 20 µm forming thin and highly porous sheets is chosen as support material. The fibers are coated with a 5wt% layer of ZnO grains, leaving the open structure of the SMF material intact. Monodispersed Pd nanoparticles as active phase are deposited on the ZnO surface layer with a total metal loading of 0.2wt%. The resulting Pd/ZnO/SMF catalyst shows high activity in the model hydrogenation reaction of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-butene-2-ol (MBE). Semi-batch autoclave hydrogenations show rates of up to 30 mol/molPds with target product (MBE) yields of up to 96.5%. Activity and selectivity of the Pd/ZnO/SMF catalyst are strongly influenced by Strong Metal Support Interaction (SMSI) between the ZnO support and the deposited Pd nanoparticles. XPS and XRD analysis show the formation of a PdZn alloy already under ambient conditions, decreasing the catalyst activity but, on the other hand, preventing strong catalyst deactivation. The formation of this alloy phase was observed under ambient conditions as well as low temperature reaction conditions, but can also be achieved by high temperature treatment of the catalyst in hydrogen atmosphere. The intrinsic kinetics of the MBY hydrogenation was studied in semi-batch experiments. First order towards hydrogen was found and a Langmuir-Hinshelwood kinetic model was developed, describing the experimental data reasonably well. Chapter 4 describes a compact heat exchange (HEX) element integrated in the SBCR. The cross flow heat exchanger design is based on vertical micro-slits of 0.3mm width, a breadth between 12mm and 40mm and 6mm length as reaction side channel geometry, ensuring a minimum of added reaction volume and small heat transfer distances. The single and two-phase heat transfer performance of the HEX element is studied considering the influence of gas and liquid superficial velocities as well as liquid properties. With one SMF layer placed at the HEX element entrance redispersing gas and liquid phase, heat transfer performance was observed to increase linearly with gas and liquid throughput. The maximum heat transfer performance per reaction volume was determined as ~ 0.5 MW/m3K, obtained for moderate gas and liquid superficial velocities. The hydrodynamics of a bubble column staged only with ZnO/SMF sheets show an narrow residence time distribution (RTD) and negligible backmixing through the catalyst layers. The RTD can be described accurately by a "tanks-in-series" model. The gas-liquid pressure drop through the SMF material is mainly governed by capillary effects and the gas velocity. A semi-empirical model is developed, combining both friction and surface tension effects and predicting the pressure drop through the SMF material with an accuracy of ±10%. In Chapter 5, the novel SBCR based on Pd/ZnO/SMF catalytic stages and the integrated micro-HEX elements are tested in the hydrogenation model reaction of MBY to MBE. The reaction in a loop setup comprising 5 HEX/catalyst stages is shown to be strongly influenced by external mass transfer, resulting in an observed catalyst effectiveness below 65%. Intense gas-liquid mass transfer in the SMF structure leads to high volumetric mass transfer coefficient in the staged bubble column of 1s-1 - 4s-1 and an observed reactor productivity of up to ~ 12000 kg/m3h. This signifies an increase of around two orders of magnitude compared to standard industrial semi-batch equipment. High catalyst densities in the reactor lead to high reactor productivity, but have a detrimental effect on product selectivity. The drop in product yield amounts up to 6% for high catalyst density, but can be decreased to only 0.5% if a 5mm distance ring is inserted between HEX stages to decrease the catalyst loading in the reactor.

EPFL2009

Développement de réacteurs microstructurés pour la conversion du propane en produits valorisés

Olivier Wolfrath

Industry requires continuously new chemical processes to convert raw materials to valuable products limiting the formation of by-products. Since a few years, an increasing interest has been given to the intensification of chemical processes. This represents a new approach in chemical reaction engineering and calls for a new type of chemical reactors: the microstructured reactors. They are characterized by miniature structures whose components have a sub-millimeter size. The flow channels, for example, have a diameter from ten to several hundred microns. The distinctive feature of these structures is a very large surface/volume ratio, namely between 10'000 and 50'000 m2/m3. In these conditions, heat transfer coefficient is very high, up to 25 kW · m-2 · K-1. It is accepted that the residence time distribution in a reactor strongly influences the product selectivity and yield. In these thin channels, the RTD is particularly narrow due to a short radial diffusion time (ms), allowing a precise control of the residence time. Another advantage of these microstructures is an increase of the inherent safety due to a small amount of the compounds used. However, fabrication of microreactors, especially the introduction of active catalyst in microchannels remains a difficult operation. And integration of microsystems in conventional chemical processes is still a challenge. The aim of this work is first to develop a novel membrane reactor based on a new microstructured catalytic packing in the filamentous form and to apply it to the nonoxydative dehydrogenation of propane. A second objective is to develop a reactor with micro heat exchangers allowing an optimized control of homogeneous reactions. This reactor was tested with the partial oxidation of propene to propylene oxide. The microstructured catalyst above is made of thin and long catalytic filaments placed in parallel with a tubular reactor. This new kind of catalyst is based on silica fibres covered by a γ-alumina layer. The position in parallel of these filaments of 7 μm diameter forces the gases to flow in the fibres direction between the filaments. The hydraulic diameter is about 70 μm. Due to this laminar and regular flow, the pressure drop in the bed was divided by 5 compared to a conventional bed (100-160 μm spheres) and the residence time distribution was particularly narrower. Such novel packing brings new opportunities to the heterogeneous catalysis domain because it can be used for many other reactions improving highly the hydrodynamics and keeping an excellent contact with the reactants. The microstructured catalyst was installed in two concentric zones of a tubular reactor separated by a palladium-silver membrane permeable to hydrogen. This product was removed from the gas phase to shift the thermodynamic equilibrium, enhancing the propane conversion from 22% up to 30%. Another significant advantage in this reactor is the increase of propene selectivity up to 97% in comparison to conventional industrial processes reaching 80-90%. This difference is due to the diminution of hydrogenolysis and hydroisomerization reactions. The dehydrogenation is known to be endothermic. Oxidation of the hydrogen passing through the membrane produced heat for this reaction along the catalytic zone. Moreover, a periodic regeneration by air allowed recovering the activity of the catalyst deactivated by coke formed during dehydrogenation. Periodic reaction operations allowed producing continuously propene with a high selectivity, and regenerating simultaneously the catalyst. The second microstructured reactor was developed integrating two micro heat exchangers at the inlet and outlet of a macroscopic tube. The aim is to heat up very fast the gas and to cool it down as well. Without these exchangers, heating and cooling duration corresponded each to about 20% of the total residence time. The use of the microstructures allowed diminishing significantly these durations (4-5% each). Applied to partial oxidation of propene, this reactor reached a more precise control of reaction temperature and duration. The propylene oxide (PO) selectivity was not increased with this system but, due to the microstructures, it was possible to work at higher temperatures where the reaction becomes very fast, without reaching an oxygen conversion of 100%. Indeed, it is essential to avoid this situation where oligomers and coke form dividing by 2 the PO selectivity. So another new kind of reactor was developed suitable for any fast exothermic reaction whose progression has to be limited because of potential consecutive reactions reducing, for example, the selectivity of an intermediate desired compound.

EPFL2001

“Sandwich Reactor” for Heterogeneous Catalytic Processes: N2O Decomposition as a Case Study

Bryan Bromley, Lioubov Kiwi, Albert Renken

A novel “sandwich” microreactor designed as a thin (∼ 300

\mu

m) porous plate of sintered metal fibers (SMF), sandwiched between metallic plates is reported. The SMF surface was coated by Fe-ZSM-5 thin film (< 2

\mu

m) rendering a catalyst highly active in the decomposition of N2O. The 3D open microstructure of SMF plates presents a low pressure drop during the passage of reacting gases. The high thermoconductivity of metallic SMF improves the heat transfer, avoiding hotspot formation during exothermic reactions. The temperature measured in the middle and at the outlet of the reactor confirmed the isothermal reactor operation. The sandwich microreactor showed high permeability and a narrow residence time distribution close to an ideal plug-flow reactor. The kinetics of the N2O decomposition was studied and the reaction was shown not to be limited by mass transfer when conducted in the microreactor.

2008