Compact reactor for continuous multi-phase reactions

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.

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

Three-Phase Catalytic Hydrogenation of a Functionalized Alkyne: Mass Transfer and Kinetic Studies with in Situ Hydrogen Monitoring

Martin Grasemann, Lioubov Kiwi, Albert Renken

Systematic studies of mass transfer interactions with intrinsic reaction kinetics were performed for the threephase selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) over a modified Pd/CaC

{O}_{3}

catalyst under solvent free conditions. Hydrogen concentration in the liquid phase (

{C}_{H2,b}

) was monitored in situ during the catalytic reaction by means of the Fugatron” analyzer. Reactions were carried out in an autoclave at different stirring rates at two concentrations of hydrogen (5 and 13 mol ·

{m}^{-3}

For stirring speeds higher than 1500 rpm no influence of gas-liquid mass transfer was observed. Hydrogen liquid-solid (L-S) mass transfer was found to be negligible, whereas the MBY mass L-S transfer becomes important at high MBY conversions at high hydrogen concentration. Low stirrer speed caused the reaction rate and MBE selectivity to decrease. No internal mass transfer limitations were observed, and conditions for the kinetic regime were found. The kinetics modeled ollowed the Langmuir-Hinshelwood mechanism and was consistent with the experimental data.

2008

Hydrogénation sélective du 2-butyne-1,4-diol avec des catalyseurs de palladium supportés sur des matériaux filamenteux structurés

Eric Joannet

The principal objective of this thesis is the development of a palladium catalyst supported on woven fabrics of activated carbon fibers (ACF) for the reactions of hydrogenations in liquid phase and the design of multiphase reactors using the characteristics of these structures. This work was focused on Preparation, characterization and optimization of catalysts supported on the woven fabrics of activated carbon fibers; The intrinsic kinetics of the selective hydrogenation of 2-butylene-1,4-diol to 2-butyne-1,4-diol as model reaction; The comparison of the activity of our structured catalyst with that of commercial catalysts; Increase the catalyst concentration in the multiphase reactors with the design of a loop reactor containing a bubble column staged with the fiber fabrics supporting the catalyst, its characterization and the startup of this loop reactor which can operate in continuous or in batch mode. Chapter 4 presents the methods of manufacture of our palladium catalysts supported on woven structures of activated carbon fibers, their reproducibility and their homogeneous deposition. The characteristics of the support such as its handiness, its resistance to mechanical forces, the opening of its surface to liquid flow, its great specific surface area and its homogeneous distribution of micropores are defined there. The deposition of nanoparticules of active phase (0.5-5%mass) is characterized by metal surface obtained (280-87 m2/g), the dispersion of this surface (62.8-19.6%) and the average diameter of the nanoparticles (1.78-5.72 nm). The total degree of initial reduction (before reaction) was confirmed. Chapter 5 is devoted to the study of the reaction of selective hydrogenation of the 2-butyne-1,4-diol (B3) to 2-butylene-1,4-diol (B2) with a commercial palladium catalyst supported on activated carbon powder. A zero value even slightly negative was found for B3 partial order and equal to one for hydrogen. The activity obtained for this catalyst is large (37.1 mol(H2)/h gPd) but its initial selectivity is particularly low (SB3,0 = 76.8 %). Effect of support was tested and highlighted. Two supports (CaCO3 and Al2O3) gave reaction rates approximately half the size of those of the carbon support, but an initial selectivity close to 99 %. In chapter 6, selective hydrogenation in an autoclave with an agitator containing structured catalyst is tested. This reactor is first of all evaluated like a reactor working in kinetic mode for the selected reaction. Kinetics was studied and showed that the fibrous support does not change the orders of reaction compared to the commercial catalyst. Energy of activation of 30 kJ/mol was found in the studied configuration. A variation of the catalytic amount on woven structure was studied and gave a reaction rate of 19.4 (mol(H2)/h gPd) for an initial selectivity of 98 % for the best catalyst (Pd(2%)/ACF). The reuse of this catalyst with an in situ reactivation in hydrazine allowed a stabilization of the activity at 50% of the initial activity after four experiments. A constant Turn Over Frequency (TOF) for catalysts with high deposits (2-5%) was given. An increase in the TOF for the low deposits (0.5% and 1%) was found. This increase in the TOF agrees with a fall of the initial selectivity. The effect of several variables of the reaction such as pressure, temperature and pH of the solution were examined. Within the framework of the variation of pH an initial selectivity of 100% and selectivity during reaction remaining higher than 95% until a conversion of 90 % were obtained. This profit of selectivity was achieved only with the detriment to the reaction rate which decreased by a factor 2.5. Finally, experiments with even less harm to the environment were made without solvent thus allowing the avoidance of purification. The rate of hydrogenation in this configuration is very high (79.2 mol(H2)/(h gPd)). But in this configuration, this increase in activity is not done at the expense of the initial selectivity (98 %). In chapter 7, the increase in the catalyst concentration in the reactor was required for higher productivity. For that purpose, a new concept of reactor was studied. This new reactor is a loop reactor containing a bubble column staged with woven catalytical structure. The pressure drop, the emulsion crossing the bubble column and the RTD were characterized. The reactor, functioning in batch or continuous mode, was studied. The productivity in batch was improved by an order of magnitude due to the increase in the catalytic concentration. A unit worked during 50 hours continuously without deactivation of catalyst providing 160 g of B2 per hour (> 1 tons/year). The reaction led to a conversion of 85% with a selectivity of 96 %.

EPFL2005