(161f) A Unifying Framework for Mass Transfer Dynamics in Gas – Liquid Segmented Flow in a Circular Tube

Knowledge of the parameters that characterize mass transfer and reaction dynamics in gas-liquid systems is critical for the design of macroscale units for separation and reaction processes. To unearth these parameters, researchers have been replicating these processes on a microscale. The advantages of this approach are the lack of turbulence, availability of high interfacial areas and well-characterized flow profiles. Our work focuses on one such category of the flow profiles - the Taylor flow. Taylor flow finds application in design of monolithic catalytic reactors and foam flow through porous media in enhanced oil recovery processes. Recently, it has been used on microfluidic platforms to determine thermodynamic and kinetic parameters of special gas-liquid systems such as CO₂ and switchable solvents. Generation of alternating segments of gas and liquid, and subsequent observation of the gas bubble lengths upon dissolution under different conditions has the capability of supplying these system parameters by using appropriate theoretical models. Literature presents multiple models for data deconvolution, but they pose three major limitations. First, mass transfer between the bulk liquid segment and the liquid film above it has been incorrectly estimated. Second, an average mass transfer coefficient is assumed to be valid throughout the dissolution process. Third, the liquid segment is assumed to be well mixed, even though there is clear evidence to the contrary in the literature.

In this work, we have rectified all of the above limitations to arrive at a unifying framework. A scaling analysis was conducted to delineate the separate contributions of the liquid films above the gas bubble and the liquid segment to mass transfer. We identified four operational regimes governed by the dimensionless bubble length, dimensionless liquid segment length, the Peclet number and the capillary number. We observed regime cross-overs, in addition to a decrease in the mass transfer coefficient by nearly a factor of three upon bubble dissolution. We also included the effect of finite mass transfer time across the liquid segments owing to its closed streamline nature. We have delineated the parameter regimes where past models are special cases of our current model, thus identifying where these models are likely to fail. To validate our analysis approach, experiments were carried out in circular, silica capillaries of different radii by generating segmented flow of CO₂ in physical solvents such as ethanol, acetonitrile and propylene carbonate. The results presented in this work will provide a framework to guide the design of experiments that will accurately determine thermodynamic and kinetic parameters relevant to gas-liquid systems. Moreover, the model can predict mass transfer dynamics even in non-dissolving systems, where Taylor flow is used solely for a controlled residence time distribution. Finally, it finds application in the design of catalyst-coated microchannel contactors, which is a step towards designing gas-liquid-solid reactions in such setups.