Intensification of the liquid side mass transfer in double-side falling film microchannels by micro-mixing structures

Abstract

The interphase mass transfer in a falling film microchannel relies on the liquid-phase mass transfer behavior. Flow in a liquid film is predominantly laminar, and the liquid mass transfer is usually controlled by molecular diffusion. Micro-mixing structures were designed and configured in a double-side falling film microchannel, and the liquid side mass transfer was enhanced by fluid mixing and surface renewal. Computational fluid dynamics approach was used to simulate the falling film flow and mass transfer behavior with and without micro-mixing structures. The impact of the micro-mixing structures on the liquid side mass transfer coefficient was systematically investigated via CO2 absorption experiments in water. The results showed that the fluid flow and mixing was improved by staggered flow mode with micro-mixing structures in the falling film microchannels. Three flow patterns are observed inside the liquid film, including direct flow, cross flow, and vortex flow when the falling liquid film crosses the micro-mixing structures. Both cross flow and vortex flow intensify the surface renewal and turbulence of the liquid film, which is the fundamental reason for the enhancement of the liquid side mass transfer. The experimental results reveal that liquid side mass transfer coefficient in double falling film microchannels with micro-mixing structures is between 3.59 × 10−5 m/s and 5.38 × 10−5 m/s, while it is between 3.35 × 10−5 m/s and 4.82 × 10−5 m/s without micro-mixing structures. The liquid side mass transfer coefficient is increased 7–14% by the configuration of micro-mixing structures. Therefore, a double-side falling film microcontactor with micro-mixing structures not only reduces the contactor size, but also effectively enhances the interphase mass transfer behavior.