Experimental Study On Carbon Dioxide-water Two-phase Flow And Mass Transfer In Microchannels

As microreactors have a big potential on chemical industry, people pay closely attention to the study on the process intensification and the gas-liquid two-phase flow in microreactors. The article explores the law of gas-liquid two-phase flow and mass transfer in microchannels by the method of experiment, which could provide the theoretical guidance for mass transfer intensification in microchannels later.An experimental investigation was conducted with a high-speed camera to explore carbon dioxide-water gas-liquid two-phase flow pattern in a horizontal microchannel with different wettabilities. The rectangular microchannel with a cross-section of 1×1mm was fabricated from copper (contact angel θ=43.7°), whose surface is hydrophilic. Then the surface was treated by the solution of octadecanethiol to prepare a hydrophobic microchannel (contact angel θ=114.6°). In the hydrophilic microchannel, bubbly flow, bubbly-slug flow, Taylor flow were observed, and Taylor flow was the main flow pattern. In the hydrophobic microchannel, asymmetric slug flow, elongated asymmetric slug flow, stratified flow were observed, and elongated asymmetric slug flow, stratified flow were the main flow patterns.The influence of superficial gas and liquid velocities and hydraulic diameters on the hydrodynamics characteristics of Taylor flow in the hydrophilic microchannel was investigated in a rectangular microchannel with a cross-section of 1×1mm and in two capillaries with inner diameters of 1.08 and 1.26mm, respectively. Experimental results showed that:generally, the length of 76.9% bubbles was 2-5 times hydraulic diameter of the microchannel, the length of 85.3% liquid slugs was 1-4 times hydraulic diameter of the microchannel, the length of 77.4% Taylor units was 5-8 times hydraulic diameter of the microchannel; as the increase of the ratio of superficial gas velocities and superficial liquid velocities, the length of bubble and liquid slug increased and decreased respectively; the superficial gas velocities had a complicated influence on the length of Taylor unit, which decreased as the increase of superficial liquid velocities; the length of bubble, liquid slug and Taylor unit increased as the increase of hydraulic diameters of microchannel; the relationship between the bubble velocity UB and the mean velocity Uslug in the liquid slug was Usiug <1.28Uslug.The platform for the study on CO2-H2O gas-liquid two-phase flow and mass transfer in microchannels was built to investigate the influence of superficial gas and liquid velocities and the wettability of microchannel on mass transfer. The absorptive amount of CO2 into water in microchannels was determined by acid-base titration. It was shown that, the liquid volumetric mass transfer coefficient kLa in microchannels were higher 1-2 order of magnitude at least than that in large-scale gas-liquid contactors; as superficial gas and liquid velocities increased, the liquid volumetric mass transfer coefficient kLa increased, because the flow speed of liquid in the circulation within the liquid slug increased, and the mixing of components within the liquid slug was enhanced, meantime, the interfacial area through the unit channel cross-sectional area per unit time was increased. As the improvement of the wettability of microchannel, the liquid volumetric mass transfer coefficient kLa. increased, because the gas-liquid interfacial areas were bigger, and the flow speed of liquid in the circulation within the liquid slug was faster, as the differenence of the two-flow patterns between the hydrophilic microchannel and the hydrophobic microchannel.