Unveiling hydrodynamics in single pellet string microreactors: Insights from gas-liquid slug flow regime and pressure drop

Microreactor technology enables substantial process intensification of chemical transformation through miniaturization. Understanding hydrodynamics of gas-liquid two-phase flow within packed bed microreactors is crucial for their design and optimization for promising uses in heterogeneously catalyzed reactions. The single pellet string microreactor (SPSM) is an attractive type of packed bed microreactors, owing to the less complicated flow and the improved radial heat transfer resulting from its regular packing configuration. In this work, hydrodynamics of N2-water slug flow in two SPSMs (i.e., capillary microreactors packed with glass beads) has been characterized experimentally. The influence of gas-liquid flow rate/ratio, microreactor diameter and particle size on the generated slug flow in the bed was investigated, with main focuses on the gas/liquid slug length, liquid retention and pressure drop behavior. The slug flow characterization of SPSMs was compared with that of empty microreactors and a similarity was revealed. The existing pressure drop correlations are not directly applicable to SPSMs because of their unique packing structure and the resulted different flow behavior from that of normal packed bed microreactors. In this work, the empirical model of Motil et al. (AIChE Journal, 2003, 49, 557) based on microgravity conditions was modified to predict pressure drop in SPSMs, considering the influence of dynamic gas-liquid interaction, and its prediction results aligned well with experimental data. To obtain more theoretical understanding, the dynamic phase interaction term was also analyzed from the perspective based on an analogy to slug flow in an empty microreactor following the model of Kreutzer et al. (AIChE Journal, 2005, 51, 2428) resulting in an acceptable prediction accuracy of the obtained data. The hydrodynamic findings of this work are promising to provide guidelines for the potential uses of SPSMs in efficient chemical synthesis or catalyst screening.