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The trickle-to-pulse flow regime transition in silicon-infiltrated silicon carbide (SiSiC) foam packed fixed bed reactors has been investigated. Based on the film stability concepts of Grosser et al. [AIChE J.1988341850] as well as Attou and Ferschneider [Chem. Eng. Sci.200055491], two predictive models have been adapted to foams’ specific geometric parameters. To account for the different nature of solid foams and their interactions with various fluids, the fixed bed characteristics (specific surface area and bed porosity) and fluid specific parameters (gas and liquid density, liquid viscosity, surface tension) have been incorporated in the model. Ergun parameters and static liquid holdup which are required for the modeling of the prevailing tractive forces were determined experimentally. The modeling results were compared to regime transition measurements performed for SiSiC solid foams with different linear pore densities (20, 30, and 45 PPI), for different reactor diameters (50 and 100 mm) and initial liquid distributors (spray cone nozzle and multipoint distributor) as well as liquids with various physicochemical properties (water, Tergitol, 50% glycerin) under ambient operating conditions. Compared to conventional random fixed bed reactors, the onset of pulsing in solid foam packed fixed beds is significantly shifted toward larger liquid and gas fluxes allowing high throughputs in the trickle regime. Moreover, the homogeneity of initial liquid distribution strongly affects the trickle-to-pulse flow transition.

This paper provides a proof of concept for the capability of the barrier-based micro-/millichannels reactor (BMMR) to number-up gas−liquid Taylor flow under reactive flow conditions. The hydrogenation of phenylacetylene to styrene and ethylbenzene using homogeneous cationic rhodium catalysts [Rh(NBD)(PPh 3 ) 2 ]BF 4 ] (NBD = norbornadiene) was used as a model reaction. First, a parametric study in a semicontinuous batch reactor was made by changing the hydrogen pressure, the catalyst concentrations, and the initial concentrations of phenylacetylene and styrene. A mechanism for this reaction system has been proposed by Esteruelas et al. (J. Org. Chem. 1998, 49−53). This mechanism was extended here to develop a kinetic model which predicts the experimental result within an accuracy of 20%. Catalyst deactivation was observed and incorporated in the kinetic model. Second, the reaction was conducted in the BMMR. The reactant and product concentrations of a single channel were compared to those of eight parallel channels combined. For 95% of the obtained results, the difference in concentrations between the single channel and the eight channels was within ±10% and depended on the gas and liquid flow rates. As a proof of concept, the number-up concept of gas−liquid Taylor flow in the BMMR under reactive flow conditions has been successfully realized. ■ INTRODUCTIONFor highly exothermic and mass-transfer-limited reactions, microstructured reactors are attractive devices to improve safety, reduce waste, and enhance product selectivity, as well as conversion. 1−5 For a single microchannel reactor, the flow rate is often in the range of mL/min, which is suited for a g/h production rate. Scale-up is required to reach kg/h and ton/h production rates. A scale-up route in microchannel reactors can be achieved in three consecutive steps. First, the microchannel cross-sectional dimensions are scaled up while maintaining the mass and heat transfer properties of the single microchannel. 6,7 Second, multiple channels are placed in parallel in one modular unit in what is named number-up. The third step is made by placing multiple modular units in parallel.For heterogeneously catalyzed gas phase reactions in microreactors, scale-up and number-up to thousands of parallel channels was successfully demonstrated. 8,9 For liquid phase reactions, scale-up was successfully demonstrated for industrial capacities, 10 although it remains limited to a small number of parallel channels due to the complexity of flow distribution. 11,12 For gas−liquid processing, scale-up was successfully made, 13 but number-up remains mostly restricted to the laboratory scale with a few reporting at the pilot scale 14,15 such as the stacked falling film microreactor. 16 This is due to the complexity and expenditure needed for an adequate number-up with an equal distribution of the gas and liquid flows over multiple microchannels. 17−21 The barrier-based flow distributor has shown promising results for numbering up multiphase flow. A key characteristic of the bar...