Mass Transfer Enhancement by Gas Injection to Liquid-Liquid Slug Flow

Laminar flow in microchannels enables the formation of liquid僕iquid slug flow. Since slugs in the channel move like a periodic plug flow, narrow residence time distribution can be achieved precisely. Moreover, the internal circulation liquid僕iquid slug flow in microchannels intensifies the mass transfer between the two phases, since the concentration near the interface is renewed by the flow. Therefore, liquid僕iquid slug flow can be used for rapid extraction. In addition, the two phases can be easily separated after the extraction using liquid僕iquid slug flow, since the slug size is over several ten times larger than the emulsion size required for a rapid extraction. Therefore, the extraction in liquid僕iquid slug flow is expected to be effective for the entire extraction process. However, the stable slug flow can be formed under low flow rates, which is not suitable for industrial production. To increase the maximum flow rate that enables the formation the slug flow, we injected gas phase slugs in liquid僕iquid slug flow. In other words, gas僕iquid僕iquid slug flow was formed. We examined the effects of the channel size, the void fraction (ratio of volume flow rate of gas phase to total flow rate), and the volume ratio of water phase to oil phase on the flow regime and the mass transfer rate. The mass transfer of phenol in dodecane to water was employed as the extraction system. To form the gas僕iquid僕iquid slug flow, a microchannel system that consists of two union tees was developed. Air and water was supplied in the first tee to form gas僕iquid slug flow. Dodecane containing phenol was then added to the gas僕iquid slug flow, resulting in a gas僕iquid僕iquid slug flow. At the channel exit, the water phase was sampled and the extracted phenol concentration was determined with a UV-Vis spectrometer. The experimental results show that the region of total flow rate of the liquid phases that forms a stable gas僕iquid僕iquid slug flow increased as high as 200 mL/min with the air injection (void fraction more than 0.1) in a PTFE tube of i.d. 3 mm. If only liquid two phases are used, the maximum total flow rate to form a stable liquid僕iquid slug flow was 60 mL/min. The mass transfer rate in gas僕iquid僕iquid slug flow is also high since the internal circulation flow is enhanced by the increased flow rate. When the void fraction is high, the slug of gas phase is considerably long, leading to a long distance between successive liquid phase slugs. Such distance reduces the mass transfer efficiency. Therefore, an optimum void fraction to maximize the mass transfer rate was obtained. We also correlated the ratio of the mass transfer rate to the diffusive rate with the flow velocity to establish a design guideline of the mass transfer operation based on gas僕iquid僕iquid slug flow. Through this study, we conclude that the gas僕iquid僕iquid slug flow is useful in rapid mass transfer operation with a high throughput.