Compact reactor for continuous multi-phase reactions: solvent-free alkyne hydrogenation as a case study

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.


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