Because of shorter heat and mass diffusion lengths and higher surface area to volume ratio, microreactors have excellent heat and mass transfer performance and can offer precise reaction control as well as selectivity, which can minimize the waste of energy and raw materials. Therefore, microreactors are considered to play a huge role in biological and chemical applications. The study of mass transfer in microchannels is indispensable for the design of microreactors. The gas-phase volume monitoring method has been widely used to study the mass transfer process. When using this method, bubble length was measured in most studies to calculate the bubble volume by assuming a symmetrical bubble shape. Therefore, this method is not suitable for asymmetric bubbles. The present study mainly focuses on the mass transfer of CO2 bubbles in a flat rectangular microchannel with a width and height of 300 600 &nbsp;by using the method of digital image analysis (DIA), especially for deformed bubbles.A test rig was set up to study mass transfer in a flat rectangular microchannel. CO2 was used as the dispersed phase and deionized water was used as the continuous phase. The dynamics of gas-liquid flow at different volume flow rates were observed by a high-speed recording system. Flow patterns within a wide range of volume flow rates in the rectangular microchannel were mapped. The geometric law of bubbles in microchannel at different gas and liquid volume flow rates was studied. The experimental results showed that the bubble size increases as gas volume flow rate increases, while decreases as liquid volume flow rate increases. It can be explained by the bubble breakup mechanism. A higher gas volume flow rate can make the bubbles grow faster before breakup and a lower liquid volume flow rate takes a longer time to break up the bubble by shear thinning, which both lead to bigger bubbles. The bubble volume fraction showed a similar law. The mass transfer of CO2 from bubbles to liquid slugs was quantitatively characterized by mass transfer coefficients kLa, for both symmetrical bubbles and asymmetrical bubbles (deformed bubbles). Mass transfer coefficients kLa against total superficial velocity were compared with the results in literature. It was found of the same tendency. kLa increases with the increasing total superficial velocity. Finally, a comparison between our experimental data and existing correlations in the literature showed that the predictions deviate from our experimental data. Therefore, new scaling laws were developed for bubble size, bubble velocity as well as mass transfer coefficients. Predictions from the scaling laws showed a good agreement with the experimental data.

Experimental Study of Gas-Liquid Mass Transfer  in a Rectangular Microchannel by Digital Image Analysis Method

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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