A Modular Microfluidic System for High Flow Rate Re-Dispersion of Gas-Liquid

The application of microchannels is often limited in high-flow-rate processes due to large pressure losses. The flow of microbubbles in millichannels offers a reduced pressure loss with simultaneous large specific contact surface. Hence, a combination of micro- and millistructured channels termed as multi-scale architecture is an alternative scale-up procedure beside numbering-up or equaling-up. Several methods exist generating primary micro dispersions, while bubble coalescence in the channels has a contrary effect. A continuous re-dispersion by flow through micro nozzles prevents this effect. The transformation of pressure into kinetic energy creates an aimed secondary flow. Differences in velocity and pressure act on the phase boundary of the bubbles and lead to deformation and break-up. In this work, a modular redispersion system with different nozzle and channel modules is studied experimentally and by CFD simulations. Each module consists of micro nozzles and channels with a width from 0.1 to 3.0 mm. The design of the modules is flexible to be adjusted to process demands with different nozzles or channel structures. The effect of the nozzle dimensions on bubble sizes are evaluated for hydraulic diameters of 0.1 to 0.5 mm. Different channel structures are analyzed to offer less coalescence and a narrow residence time distribution. The modules are arranged in a holder and are fixed under a glass plate for optical characterization. Volume flow rates from 10 to 500 mL min⁻¹ are studied with various phase ratios. Bubble diameters are generated in the range from 0.1 to 0.7 mm with narrow size distributions dependent on the entire flow rate through the device. Pressure losses account for less than 2 bar. Generally, a linear relation of smaller bubble diameters with larger energy input was identified. Flow detachment in curves resulted in larger coalescence rates, which was avoided by smoother radii.