Microstructure-based intensification of a falling film microreactor through optimal film setting with realistic profiles and in-channel induced mixing

Abstract

The high liquid based specific interfacial area, up to ∼20,000 m 2/m 3, of falling film microreactors renders them to be ideally suited to carry out fast exothermic and mass transfer limited reactions. To understand the role of and control this interfacial area, it is important to account for realistic liquid film profiles. Here, we vary the liquid film profile or its velocity profile by two different means – through the (external) shape of a plain microchannel and through in-channel structures within the microchannel (staggered herringbone grooves (SHG) on the microchannel bottom). The variations in the liquid films are evaluated via two computational fluid dynamic (CFD) models. First is the pseudo 3-D which explicitly accounts for the liquid film thicknesses, flow velocities, species transport and reactions. Here, the pseudo 3-D model is used to investigate (1) the effects of five microchannel shapes and (2) three microchannel cross section dimensions; to account for a scale-out through both numbering-up and smart increase in dimensions. The model reaction used is the absorption of CO 2 in aqueous NaOH solution. It is found that the mass transfer into the liquid and the reaction conversion depend on the velocity profile and flow pattern. Second CFD model is the full 3-D which is used to evaluate the liquid film in the presence of SHG. The simulations from the full 3-D model indicate that: (1) residence time distribution is narrowed by five times compared to plain microchannels and (2) the penetration depths of particles seeded at the gas/liquid interface are 1.7 times larger in the presence of SHG. Furthermore the effect of SHG on penetration depth is more pronounced at higher flow rates. This is experimentally exploited by increasing the liquid throughput by more than a factor of two while keeping the same reaction conversion, using the SHG microchannels.