Hydrodynamics and mass transfer performance of gas–liquid microflow in viscous liquids

Abstract

Mass transfer performance is markedly improved in microchannels compared with traditional gas–liquid chemical devices. The residence time is a crucial parameter for determining the mass transfer performance. However, investigation of the residence time in gas–liquid absorption systems remains insufficient, especially in viscous gas–liquid systems with obvious flow non-uniformity between the two phases. Herein, the microscale gas–liquid flow and mass transfer performance in viscous liquids (9.0–55.4 mPa·s) based on the true residence time are investigated. Owing to the broad operating conditions of the gas–liquid flow ratio (0.2–6.7), both Taylor flow and bubbly flow patterns were observed. The results show that the bubble velocity gradually decreases to the minimum value with increasing flow distance in the Taylor flow pattern owing to gas absorption. Subsequently, bubble velocity gradually increases under the bubbly flow pattern, and the dominant factor of bubble velocity is pointed out. The true bubble residence time initially increases and then gradually decreases compared to the superficial time as the gas holdup decreases, based on the superficial velocity using the initial flow rates. The transition point occurs at a gas holdup of 0.2. In addition, the turning point of the different rules of the pressure drop occurs at a gas holdup of 0.5. Finally, a new dimensionless equation was proposed to predict the mass transfer coefficient and reveal the equation differences between low- and high-viscosity liquids. This work enriches the understanding of microflow characteristics and mass transfer performance in viscous solutions and provides a reliable guide for microreactor design.