Fluiddynamik, Stofftransport und chemische Reaktion der Suspensionskatalyse bei der Flüssig/flüssig-Pfropfenströmung in Mikrokanälen

Die Trends in der chemischen Industrie hin zu flexiblen Prozessen und kürzeren Prozessentwicklungszeiten haben in den zurückliegenden 20 Jahren zur intensiven Erforschung der Mikroreaktionstechnik geführt. Allerdings wurden feststoffbasierte Prozesse mit Blick auf die mögliche Mikrokanalblockierung und die häufig immer noch heuristische Auslegung kaum erforscht. Das betrifft auch die Suspensionskatalyse in Mikroreaktoren, obwohl sie speziell in der Flüssig/flüssig-Pfropfenströmung aussichtsreich erscheint. Durch Zugabe einer zweiten, nicht mischbaren Katalysatorträgerflüssigkeit könnte die Katalysatorpartikelgröße gegenüber der konventionellen Suspensionskatalyse auf beispielsweise 1 μm reduziert werden, da sich die Partikelrückgewinnung dann auf eine Flüssig/flüssig-Phasentrennung reduzieren lässt. Damit könnten die sonst üblichen Filmtransportlimitierungen aufgehoben werden. Die weitverbreiteten Vorbehalte wurden ausgeräumt, indem ein Prototyp zur Partikeldosierung, Heuristiken zum ablagerungsfreien Design und die Katalysator-rückgewinnung experimentell sowie mit CFD-Simulationen entwickelt wurden. Mit sicher handhabbaren Partikelgrößen von 1 - 160 μm und Beladungen von bis zu 36 Gew.-% ist der Aufbau hochflexibel einsetzbar. Die experimentelle, fluoreszenzmikroskopische Strömungsanalyse lieferte neue Einblicke in die komplexen Wirbelmuster und Partikelbewegung. Neben der schwerkraftbedingten Partikelakkumulation im hinteren Subwirbel bei geringen Strömungsgeschwindigkeiten, wurde ein neuer Segregationseffekt durch Zentrifugal- und Saffman-Kraft bei hohen Geschwindigkeiten nachgewiesen und aufgeklärt. Die Auswirkung der Segregation auf den Flüssig/fest-Stofftransport, quantifiziert durch Ionenaustauschexperimente und numerische Simulation, waren nicht in Kauf zu nehmen. Nur bei exakter Kontrolle der Partikelbewegung, Anwendung von Resuspendierungs-maßnahmen wie dem periodischen Benetzungswechsel oder reduziertem Partikel-durchmesser war die Stofftransportleistung konkurrenzfähig. Die Wechselwirkung zwischen Gesamtstofftransport und Reaktion wurde aber abschließend experimentell und modellbasiert am Beispiel zweiphasiger Esterhydrolysen analysiert. Da selbst schnelle Reaktionen auf 10 - 100 s Skala in der Suspensionspfropfenströmung rein mikrokinetisch kontrolliert sind, erschließt dieses Mikroreaktorkonzept Bereiche intensivierter Reaktionsführung, die mit konventionellen Laborapparaten oder konventioneller Suspensionskatalyse nicht möglich sind.Appreciable advances in microtechnology have been made over the last 20 years, driven by trends towards flexible production and shorter times-to-market. However, microfluidic technologies with solid particles have been neglected, due to the risk of clogging, empirical design of solid processing and the stochastic phenomena involved. This has largely excluded suspension catalysis from microfluidic applications, despite the unique features offered by liquid/liquid slug flow. Addition of a second liquid as catalyst carrier permits facile recovery of particles by liquid/liquid phase separation and use of smaller particles providing reduced film transport limitations. The widespread reservations have been proven unfounded, with employing a dosing unit prototype, adopting heuristics for safe particle handling in microchannels and with the help of CFD simulations. Particles of 1 - 160 μm at concentrations of up to 36 wt.-% were suspended successfully in μ-channels, indicating a wide operating window. Experimental, fluorescence-microscopic studies on fluid- and particle dynamics verified the complexity of internal circulation and particle segregation. Surprisingly, particles were found to segregate in the rear cap not only at low flow rates due to gravity, but also at elevated velocities, as a consequence of centrifugal and Saffman force. The impact of segregation on liquid/solid mass transfer has been quantified by means of ion exchange and numerical modelling. According to these findings, segregation results in unacceptable slow mass transfer. But higher mass transfer rates are accessible either by control of particle motion, resuspension measures like periodic wettability inversion or reduced particle diameter. The interaction of overall mass transfer and reaction has finally been elucidated with experimental and numerical analysis of a biphasic ester hydrolysis. Even fast reactions over 10 - 100 s were proven purely microkinetically controlled. Hence suspension slug flow catalysis is a capable concept with intensified mass transfer and useful as laboratory tool following the “batch-to-conti” paradigm

Methane pyrolysis in packed bed reactors: Kinetic modeling, numerical simulations, and experimental insights

Pyrolysis of hydrocarbon feeds such as methane (CH$_4$) and natural gas emerges as a pivotal carbon dioxide-free large-scale hydrogen (H$_2$) production process combined with capturing the carbon as solid material. For fundamental understanding and upscaling, the complex kinetics and dynamics of this process in technically relevant reactors such as packed and moving beds still need to be explored, particularly concerning carbon formation and its impact on reactor performance. This study integrates kinetic modeling, numerical simulations, and experimental findings to comprehensively understand CH$_4$ pyrolysis under industrially relevant conditions and its implications for efficient H$_2$ production and carbon capture. The investigation covers temperatures from 1273 K to 1873 K, H$_2$ addition with H$_2$:CH$_4$ ratios of 0 to 4, and hot zone residence time of 1 to 7 s. Two distinct pathways lead to carbon formation: soot formation and carbon deposition. Each pathway originates from different gas-phase precursors. An elementary-step-based gas-phase reaction mechanism is coupled with a soot formation model from polycyclic aromatic hydrocarbon and a newly developed deposition model from light hydrocarbons. Numerical simulations are performed in a packed bed reactor model, incorporating a method of moments for soot formation and a model for carbon deposition. The model is evaluated against experiments and predicts the effects of operating conditions on gas-phase product distribution and carbon formation. It also es- timates the change in bed-voidage over operational time. The study reveals that at the temperature 1673 K, CH$_4$ conversion exceeds 94 %, while both H$_2$ and solid carbon yields surpass 96 %. The sophisticated modeling and simulation framework presented herein thus provides an enhanced understanding of the CH$_4$ pyrolysis process and presents a valuable tool for optimizing this process

Liquid–Solid Mass Transfer for Microchannel Suspension Catalysis in Gas–Liquid and Liquid–Liquid Segmented Flow

International audienc

The separation of immiscible liquid slugs within plastic microchannels using a metallic hydrophilic sidestream

This paper describes experiments and related modelling on a new method for separating aqueous phase slugs from the surrounding organic matrix phase in segmented two phase flow in a plastic microcapillary film (MCF). Kerosene or paraffin oil was metered through a plastic capillary of 630 microns diameter and aqueous phase slugs were generated within the capillary by the continuous sidestream injection of water. It was found that the resulting aqueous phase slugs formed in the MCF could be subsequently easily separated from the organic phase by piercing the downstream sidewall of the plastic capillary with a hydrophilic metal hypodermic needle to draw off an aqueous sidestream. Optical scrutiny of the phase separation process indicated that two distinct disengagement mechanisms are involved, in which the metal needle tip either remains submerged in the aqueous phase or becomes periodically exposed to the organic phase at certain stages of the segregation process. The separation efficiency, i.e. the degree of residual phase cross-contamination, was determined as a function of both the sidestream needle angle and the depth of needle penetration into the capillary for a given flow rate and phase ratio. It was established that the separation efficiency was very sensitive to the downstream pressure balance between the organic mainstream flow in the plastic capillary and the aqueous sidestream flow through the needle. A mathematical model for the pressure balance conditions was developed by making certain simplifying assumptions and taking the Laplace interfacial pressure into account. The model predictions agreed surprisingly well with the experimental findings, thus providing circumstantial evidence for the validity of the insights into the phase separation mechanism. © 2011 The Royal Society of Chemistry