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For flexible production of fine chemicals and pharmaceuticals still slurry catalysts in batch reactors are used. However, production efficiency suffers from necessary filtration after reaction is finished and abrasion effects make re-usage of catalyst often not possible. Additionally, the mass transfer rates are limited due to low relative velocities between particles and the gas/liquid phases. Alternatively, a new three-phase reactor design based on rotating solid foams were developed by the Chemical Reactor Engineering group at Eindhoven University of Technology (Tschentscher et al., 2010). The solid foam is applied both as catalyst support and stirrer in order to mix the gas and liquid phases and to create fine bubbles. Measurements of the gas-liquid mass transfer revealed clearly improved reactor performance compared to Rushton stirrers. The visual observation suggested that the rotation of the foam block leads to a structuring of the reactor volume into sections with considerable different gas holdup, flow behaviour and bubble size distribution. However, access to the patterns in the solid foam was so far not possible. In our contribution, results of a comprehensive non-invasive tomographic study will be presented. A high-resolution gamma-ray setup was applied to visualize the gas/liquid textures in the reactor, especially in the solid foam block. Therefore, an acrylic reactor system was built that ensure minimal attenuation. Two different reactor heights, stirrer speed from 150 to 300 rpm and two solid foams with different geometric properties were applied. To study the effect of the liquid viscosity, glycerol-water mixtures as well as pure liquids were used. Additionally, flow patterns of non-Newtonian liquids (carboxy methyl cellulose, CMC), foaming liquids (cetyltrimethyl ammonium bromide, STAB) and non-Newtonian foaming liquids (CMC+STAB) were compared with the air-water system. The observed flow patterns can be related to the mass transfer characteristics and conclusions on an optimized reactor/foam design can be drawn.

A microreactor system was used for a kinetic study into the direct epoxidation of propene over a gold-titania based catalyst system using a mixture of hydrogen, oxygen, and propene. The small dimensions of a microreactor allow for the safe operation of the reaction in the explosive regime. The effect of the concentration of hydrogen, oxygen, and propene on the reaction rate was determined as well as the effect on catalyst deactivation and reactivation. A simple algebraic expression was developed, based on published kinetics, which provided the three reaction rate constants as a function of the feed gas composition. It is shown that propene does not influence the epoxidation rate; however, higher propene concentrations significantly reduce the catalyst deactivation rate. Hydrogen significantly increases the rate of the epoxidation, while it has a minor influence on the deactivation and reactivation of the catalyst. Oxygen has a threefold beneficial effect on the reaction: it increases the epoxidation rate; it slightly decreases the deactivation rate, and increases the rate of the catalyst reactivation. Performing the direct propene epoxidation in a microreactor system inside the explosive regime at optimized high feed concentrations can increase the propene oxide formation rate by a factor of three, while at the same time lowering the deactivation rate.

Current trends in the development of microstructured reactors with thin catalytic films (from 100 nm up to several microns) that have self-assembled nanostructures are discussed. A major technique that is used to prepare such films is sol-gel processing. This involves depositing a complex fluid on a microstructured substrate by dip, spin, or spray coating, followed by surfactant removal to form the porous nanostructures. A novel methodology has been developed by which a uniform coating containing controlled amounts of (poly)metallic nanoparticles can be obtained. This elegant strategy is based on the condensation of metal oxide species by self-assembly in the presence of metallic colloids. The potential microreactor applications brought forth by this innovative protocol are placed in perspective in the light of its versatility.

Gold nanoparticle catalysts for the direct epoxidation of propylene were studied. The hydrogen oxidation over gold was used as a probe reaction for the catalytic activity of the gold nanoparticles. Propylene adsorbed strongly on gold under reaction conditions typical for the hydro-epoxidation of propylene. Under propylene hydro-epoxidation reaction conditions, propylene was absorbing on gold nanoparticles. Propylene adsorption decreased the hydrogen oxidation rate and interacted with surface gold atoms. This is an abstract of a paper presented at the AIChE Spring National Meeting (New Orleans, LA 4/6-10/2008).

Mass and heat transfer can put significant technological limitations to reactor and process design, which leads to far from ideal solutions for bulk processes. A discussion covers the design, construction, and testing of a single channel micro reactor integrated with a micro mixer to demonstrate the viability of a miniaturized reactor system for an industrial bulk process, based on micro reaction technological considerations; model reaction used, i.e., Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam with oleum as a catalyst and solvent; the reaction mixture (consisting of caprolactam and oleum) as the key step in the industrial process; window of operation; critical parameters in mass transport models; and the creation of multi micro zones of oxime and oleum rich phases. This is an abstract of a paper presented at the 42nd Loss Prevention Symposium (New Orleans, LA 4/6-10/2008).

The selective hydrogenation of phenylacetylene to styrene by hydrogen was studied in the 25-50 oC temperature range in a continuous flow microchannel of 250 µm i.d. with a Pd supported on mesoporous TiO2 catalyst. The inner wall of the microreactor was dip-coated with a prehydrolyzed titania precursor solution containing an EOxPOyEOx amphiphilic triblock copolymer (EO = ethylene oxide, PO = propylene oxide, x = 106, y = 70) and suspended palladium nanoparticles with an average diameter of 4 nm. Solvent was withdrawn from the channel at a rate of 1 cm/s yielding a 120 nm continuous titania layer with a cubic mesostructure after drying and calcination. The organic copolymer was removed by calcination at 300oC under a residual pressure of 15 mbar. The coating porosity was 0.4, resulting in overall coating mass of 1.32 mg with a 1 wt.% Pd content. The liquid flow was varied between 1 and 10 µL/min and the hydrogen flow between 250 and 550 µL/min (STP). This corresponds to superficial velocities of 0.034-0.34 cm/s and 8-18 cm/s for liquid and gas, respectively. The maximum selectivity to styrene of 95 % was observed at a phenylacetylene conversion of 88%. The reaction rate in terms of TOF was found to be up to 2 s-1.

Gold nanoparticle catalysts for the direct epoxidation of propene are investigated. The adsorption of propene on the gold nanoparticles is studied using both catalytic experiments and by in-situ XANES analysis. The hydrogen oxidation over gold is used as a probe reaction for the catalytic activity of the gold nanoparticles. Both the hydrogen oxidation experiments and the XANES conclusively show that propene adsorbs strongly on gold under reaction conditions typical for the hydro-epoxidation of propene. It is currently being investigated how this adsorption fits into the epoxidation mechanism of the gold-titania based catalysts.

In this paper, the influence of carbon and silica particle slurry concentration up to 20 g/l (4 vol%) on regime transition, gas hold-up, and volumetric mass transfer coefficient is studied in a 2-dimensional slurry bubble column. From high speed video image analysis, the average large bubble diameter, the frequency of occurrence of large bubbles, the gas-liquid interfacial area, and the large bubble hold-up are obtained. The liquid side mass transfer coefficient is calculated from the volumetric mass transfer coefficient and the gas-liquid interfacial area. The lyophilic silica particles are rendered lyophobic by a methylation process to study the influence of particle wettability. The influence of organic electrolyte (sodium gluconate) and the combination of electrolyte and particles is also studied. It is found that lyophilic silica, lyophobic silica, and lyophobic carbon particles at concentrations larger than 2 g/l (0.4 vol%) decrease the gas hold-up and shift the regime transition point (where the first large bubbles appear) to a lower gas velocity. The volumetric mass transfer coefficient increases with gas velocity, increases with electrolyte concentration, decreases with slurry concentration, and is higher for lyophobic particles. The liquid side mass transfer coefficient increases with gas velocity, bubble diameter, and is higher for lyophobic particles. A correlation for the mass transfer coefficient based on dimensionless numbers is proposed for the heterogeneous regime.