Abstract
The hydrodynamic characteristics of liquid-liquid plug flow were studied in microchannels with 0.2 and 0.5mm ID both experimentally and numerically. For the experiments high speed imaging and bright field micro-Particle Image Velocimetry were used, while the numerical simulations were based on the volume-of-fluid (VOF) method. The two immiscible liquids were a 1M HNO3 aqueous solution which formed the dispersed plugs and a mixture of 0.2M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethyphosphine oxide (CMPO) and 1.2M Tributylphosphate (TBP) in the ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([C4min][NTf2]). The thickness of the film surrounding the plugs, and the plug velocity and length were measured and compared against literature correlations. For the cases studied (0.0224<Ca<0.299) it was observed that the liquid film was largely affected by the changes in the shape of the front cap of the plug. The plug length was affected by both the Capillary number and the ratio of the aqueous to ionic liquid phase flow rates while the plug volume depended on the channel diameter and the mixture velocity. The numerical simulations showed that, in agreement with the measurements, a parabolic velocity profile develops in the middle of the plugs while the circulation patterns in the plug are affected by the channel size. The pressure profile along the channel with a series of plugs and slugs was predicted numerically while the pressure drop agreed well with a correlation which included the dimensionless slug length and the ratio Ca/Re.
Highlights
Technical and environmental reasons are driving the development of small scale and miniaturised equipment within the frame of process intensification
We have shown in previous work that the use of ionic liquids in small channel extraction units can enhance significantly mass transfer [10,38]
It was found that for Lp/D between 1.82 and 2.31, a region of uniform film thickness was always present between the hemispherical caps of the plug and the film thickness was measured in this area
Summary
Technical and environmental reasons are driving the development of small scale and miniaturised equipment within the frame of process intensification. The length of the dispersed plugs and continuous phase slugs is very important for heat and mass transfer processes and depends on many parameters such as fluid properties and superficial velocities, represented by dimensionless groups such as Re and Ca [6,17]. It has been shown, that the inlet configuration has a significant effect on the plug or slug size and attempts have been made to predict it by studying the plug formation mechanisms. Correlations for the film thickness and plug velocity and length are developed based on dimensionless parameters and compared against literature models
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