(453f) Scalability of Mass Transfer in Gas-Liquid Taylor Flow in Capillaries

Chemical reactions between components present in gas and liquid phase represent an important class of reactions in chemical industry. The rate of such gas-liquid reactions is often limited by mass transfer, which depends on the hydrodynamics of the gas-liquid flow. In order to correctly predict the transport processes in these multiphase systems, a detailed knowledge of the transport mechanisms on different scales is needed.

In the past, batch systems were widely used for this kind of reactions, but the research efforts over the past decade resulted in microstructured devices for flow chemistry, which provide several advantages over these conventional reaction systems. In micro-devices the small dimension of the channels results in a significant higher specific interfacial surface area between the gas bubbles and the liquid phase, which results in an increased mass transfer coefficient compared to conventional systems. However, micro-reactors are characterized by an inadequate throughput for industrial applications, for this reason scale-up of these systems is necessary. In this light, the ACS-GCI-Pharmaceutical Roundtable has specifically identified the demand for novel concepts for continuous reaction systems.

In this work two-phase flow in circular capillaries with three different diameters (0.5 mm, 1.55 mm and 3.2 mm) is studied experimentally, where water is used as the liquid phase and CO₂ as the gas phase. The study of Taylor flow in single straight capillaries is particularly interesting because it allows to obtain accurate data that relate mass transfer coefficients to the fluid-dynamic conditions inside the channel. The range of capillary diameters in this work is chosen in order to investigate system hydrodynamics and transport processes on both micro- and milli-scale (based on the Laplace constant criteria).

The transparent capillaries allow optical access in order to collect images by means of a high-speed camera, which will then provide information about the bubble generation frequency, bubble size distributions, and reaction conversion. The absorption of CO₂ in an alkaline solution is particularly suitable as a model reaction as it occurs in the absence of a catalyst, and the conversion of the reactants can be locally monitored by the decrease in bubble size, and globally through the change in pH.

These measurements allow a quantification of the mass transfer coefficient k_La, and the experimental results are compared with literature results, and in addition we will present correlations to predict the values of k_La for both the milli- and the micro-scale capillaries for a range of Capillary numbers between 10^-2 and 10^-4 and Reynolds numbers Re<103. The obtained results will contribute to quantifying the effect of two-phase flow hydrodynamics at each scale on the mass transfer coefficients and their scalability across several orders of length scale.