Abstract
Colloidal suspensions of nanoparticles (e.g., metals and oxides) have been considered as a promising working fluid in microreactors for achieving significant process intensification. Existing examples include their uses in microflow as catalysts for enhancing the reaction efficiency, or as additives to mix with the base fluid (i.e., to form the so-called nanofluids) for heat/mass transfer intensification. Thus, hydrodynamic characterization of such suspension flow in microreactors is of high importance for a rational design and operation of the system. In this work, experiments have been conducted to investigate the flow pattern and pressure drop characteristics under slug flow between N2 gas and colloidal suspensions in the presence of TiO2 or Al2O3 nanoparticles through polytetrafluoroethylene (PTFE) capillary microreactors. The base fluid consisted of water or its mixture with ethylene glycol. The slug flow pattern with nanoparticle addition was characterized by the presence of a lubricating liquid film around N2 bubbles, in contrast to the absence of liquid film in the case of N2-water slug flow. This shows that the addition of nanoparticles has changed the wall wetting property to be more hydrophilic. Furthermore, the measured pressure drop under N2-nanoparticle suspension slug flow is well described by the model of Kreutzer et al. (AIChE J 51(9):2428–2440, 2005) at the mixture Reynolds numbers ca. above 100 and is better predicted by the model of Warnier et al. (Microfluidics and Nanofluidics 8(1):33–45, 2010) at lower Reynolds numbers given a better consideration of the effect of film thickness and bubble velocity under such conditions in the latter model. Therefore, the employed nanoparticle suspension can be considered as a stable and pseudo single phase with proper fluid properties (e.g., viscosity and density) when it comes to the pressure drop estimation.
Highlights
Over recent decades, microreactor technology has been heralded as a highly promising concept for achieving significant process intensification in comparison with conventional scale reactors, owing to its distinct advantages including, e.g., high heat/mass transfer rates, precise process/reaction control under well-regulated flow patterns in miniaturized channels (Jensen, 2017)
The DLS results suggest that the mean particle size of TiO2 became larger in the H2O + Ethylene glycol (EG) base fluid, being, respectively, ca. 52.5 and 57.5 nm for 0.05 vol% TiO2/ H2O + EG (3:2) and 0.05 vol% TiO2/H2O + EG (1:1) (Figures 4C, D), likely due to more significant particle agglomeration induced by EG addition
TiO2 and Al2O3 were used as nanoparticles to be dispersed in the base fluid
Summary
Microreactor technology has been heralded as a highly promising concept for achieving significant process intensification in comparison with conventional scale reactors, owing to its distinct advantages including, e.g., high heat/mass transfer rates, precise process/reaction control under well-regulated flow patterns in miniaturized channels (Jensen, 2017). Such catalyst immobilization has some drawbacks such as the requirement of tailored catalyst-coating procedures, highpressure drop penalty and complex multiphase hydrodynamics over the packed bed Another option is to suspend solid catalyst microparticles in the form of slurry flowing in microreactors, which has been explored for gas–liquid hydrogenation (Liedtke et al, 2013; Salique et al, 2021) and photocatalytic reactions such as liquid-phase fluorination and polymerization (Pieber et al, 2018; Li et al, 2021). In this aspect, the high activity and mobility of nanoparticle catalysts has intrigued their promising uses in the form of colloidal suspensions in microreactors recently for catalytic reaction efficiency enhancement. Continuous transport of colloidal suspensions of metal oxide nanoparticles as the catalyst (support) in microreactors has been reported in liquid-phase reactions, such as using SiO2-supported FeCl3 catalyst in the benzylation of toluene (Pu and Su, 2018) and TiO2 catalyst for the photocatalytic reduction of nitrobenzene (Pu et al, 2019)
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