Abstract: Microreactors, compared with conventional large-scale devices, such as column equipments, present several advantages, such as small volume, huge surface to volume ratio, excellent mass and heat transfer abilities, narrow residence time distribution. Hence, microreactors can be an interesting tool for intensifying some processes, such as gas absorption (Yue, Chen, Yuan, Luo, & Gonthier, 2007), two-phase reactions, e.g. hydrogenation (Kobayashi et al. 2005), nitration(Shen et al. 2009), or liquid–liquid extraction (Y. Su, Chen, Zhao, & Yuan, 2009) etc. Gas/liquid systems occupy a key place in scientific research and industrial fields, therefore it is essential to perfectly control the mass transfer characteristics in microfluidic devices. The knowledge of gas-liquid hydrodynamics (i.e. flow regime, mixing, interfacial area), which is affected by the geometry of the millimetric channel (straight, meandering or more complex design) and the phase properties, is essential for predicting gas-liquid mass transfer properties and in return for modelling the coupling between mass transfer and chemical conversion/selectivity.

In recent years, gas-liquid flows in microreactors have been the subject of a fast-growing body of literature (Zhang et al. 2009, P. Sobieszuk et al. 2010, H. Su et al. 2010, Kashid et al. 2011, Roudet et al 2011, P. Sobieszuk et al. 2012, Wang et al. 2013, Yao et al. 2014). Most of these works focus on understanding and modelling the Taylor flow, as this kind of flow enables good mass transfer rates to be achieved from bubbles to the liquid phase.

Most of the literature considered the overall gas-liquid mass transfer characteristics in microreactors. Few studies exist on the contributions to mass transfer of the three characteristic stages, which are the bubble-forming, the bubble-flowing and the phase separation ones. On the other hand, the mass transfer coefficients are generally determined by analyzing the solute concentration of samples collected at the outlet of microreactors. That might lead to inaccurate characterization of mass transfer because the sample collection time and phase separation time are usually longer than the fluid residence time in microfluidic devices. Herein, it is necessary to propose a simple approach to overcome this problem and to locally characterize the gas-liquid mass transfer. The colorimetric technique proposed by Dietrich et al. (2013) is an innovative, direct, non-intrusive method using an oxygen-sensitive dye to quantify the local mass transfer around bubbles flowing in -microreactors. The main advantage of the technique is that the effective oxygen concentration field around bubbles can be visualized without any laser excitation, making this technique convenient and user friendly.

This study aims at applying the colorimetric technique proposed by Dietrich et al. (2013) for investigating the gas-liquid mass transfer characteristics around a Taylor bubble during the forming-stage and the flowing-stage in a square flow-focusing and T-junction microchannels . Resazurin was chosen as the oxygen-sensitive dye. The change of color resulted from the reversible oxidation-reduction reactions between resofurin (pink) and dihydroresorufin (colorless). The oxidation of the dihydroresorufin into resorufin was (quasi)instantaneous, whereas the reduction of resorufin into dihydroresorufin in presence of oxygen was sufficiently low to enable image acquisition. For that, the concentrations in potassium hydroxide (KOH) and in glucose, which catalyzed the reaction, should be carefully adjusted. The images of gas-liquid flow were recorded by a monochromatic high-speed camera, and a shareware software ImageJ was employed to analyze the recorded images to measure the following dimensions: the bubble length, bubble velocity, and unit cell length. The recorded images were processed by Matlab software in order to obtain the equivalent oxygen concentration field.

The hydrodynamics of gas/liquid system in Taylor flow was firstly described in terms of bubble size and velocity, and unit cell length.

Secondly, the evolution of liquid side mass transfer coefficient k_L as a function of time t right after the pinch-off stage (i.e. as soon as the fully formed bubble was generated) was studied. The calculation of k_L from the concentration field obtained by image analysis was based on the model developed by Roudet et al (2011) and adapted by Dietrich et al (2013) for the colorimetric method. For a given value of gas superficial velocity j_G, k_L increased logically as liquid superficial velocity j_L increased; for a fixed j_L, k_L also increased as j_G increased.  Empirical correlations were established to predict the liquid side mass transfer coefficient right after the bubble pinch-off k_L with some relevant parameters. The interfacial area was obtained by assuming hemispherical shape of bubble nose and rear; under the operating conditions tested; it was observed that a didn’t vary significantly in the operating conditions under test(a ranged from 3734 m^-1 to 4884 m^-1). From this,  the volumetric liquid mass transfer coefficients k_La were deduced  and discussed with respect to the predictions obtained from the models developed by Berčič and Pintar (1997), Van Baten et al.(2004), and Yue et al.(2009).  

Afterward, the liquid side mass transfer coefficients during the bubble-flowing stage (i.e. after the pinch-off) were calculated from the measurement of the time-variation of the mass of oxygen. Most of the latter liquid side mass transfer coefficients were found to be smaller than that the liquid side mass transfer coefficient measured just after the bubble pinch-off. This could be explained by the fact that the contribution of mass transfer at the bubble-forming stage was reasonably large compared with that of the mass transfer at the bubble-flowing stage.

Finally, the evolution of the mass of oxygen in the nose of the first bubble generated in the microchannel was briefly presented in order to better understand the mechanism of mass transfer during the bubble-forming stage.

All these findings give important informations to understand the contributions of the bubble-forming stage to the global gas-liquid mass transfer occurring in a microchannel. In the future, they will serve as basis for elaborating a complete model, accounting for the bubble-forming stage.

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