Optical methods to investigate the enhancement factor of an oxygen‐sensitive colorimetric reaction using microreactors

A colorimetric technique to characterize mass transfer during liquid-liquid slug flow in circular capillaries

Continuous slug flow in microreactors are featured by alternative presence of regulate segments of immiscible phases in microchannel or capillaries with lateral dimensions below 1 mm. Due to the high interfacial area and short diffusive distance therein, such microreactors have been widely applied in chemical engineering processes that are sensitive to mass transfer. Therefore, mass transfer rates in microreactors have long been broadly investigated via either typical offline or online methods. Compared to these conventional methods, the colorimetric technique based on the oxidation of resazurin with oxygen enables direct determination of physical mass transfer rates. However, this technique was currently applied only to the gas-liquid system in microreactors, and mostly in rectangular channels due to the simplicity in image processing. Based on this, the current paper showed a demo where the colorimetric technique using resazurin was adapted to a liquid-liquid system for the mass transfer study of flowing droplets within a slug flow capillary. Experimental tips and tricks were summarized, and a sliced color-concentration calibration strategy was proposed to balance analyzing efficiency and accuracy.

Gas-liquid mass transfer intensification for bubble generation and breakup in micronozzles

The local gas-liquid mass transfer was characterized during bubble generation in T-contactors and in an adjacent micronozzle. A colorimetric technique with the oxygen sensitive dye resazurin was investigated to visualize gas-liquid mass transfer during slug flow, bubble deformation, as well as laminar and turbulent bubble breakup in the wake of a micronozzle. Two optimized nozzle geometries from previous studies were evaluated concerning volumetric mass transfer coefficients for low pressure loss, narrow residence time distribution, or high dispersion rates. Highest values in kla up to 60 s−1 were found for turbulent bubble breakup and an optimized micronozzle design in respect to pressure drop and dispersion rate. The achieved mass transfer coefficients were correlated with the energy dissipation rate within the micronozzles and with the inverse Kolmogorov time scale in vortex dissipation in good agreement for laminar and turbulent breakup regimes.

Graphical abstract

Gas–Liquid Mass Transfer around a Rising Bubble: Combined Effect of Rheology and Surfactant

The influence of viscosity and surface tension on oxygen transfer was investigated using planar laser-induced fluorescence with inhibition (PLIF-I). The surface tension and the viscosity were modified using Triton X-100 and polyacrylamide, respectively. Changes in the hydrodynamic parameters of millimetric bubbles were identified, and transfer parameters were calculated. The results revealed a decrease in the mass transferred in the presence of a contaminant. For modified viscosity, the decrease in mass transferred was allowed for by current correlations, but the presence of surfactant led to a sharp decrease in the liquid side mass transfer coefficient, which became even lower when polymer was added. An explanation for the gap between classical correlations and experimental values of kL is discussed, and a hypothesis of the existence of an accumulation of contaminant in the diffusion layer is proposed. This led to the possibility of a decrease in the diffusion coefficient and oxygen saturation concentration in the liquid film, explaining the discrepancy between models and experience. Adapted values of DO2 and [O2] * in this layer were estimated. This original study unravels the complexity of mass transfer from an air bubble in a complex medium.

Process Intensification Approach Using Microreactors for Synthesizing Nanomaterials-A Critical Review

Nanomaterials have found many applications due to their unique properties such as high surface-to-volume ratio, density, strength, and many more. This review focuses on the recent developments on the synthesis of nanomaterials using process intensification. The review covers the designing of microreactors, design principles, and fundamental mechanisms involved in process intensification using microreactors for synthesizing nanomaterials. The microfluidics technology operates in continuous mode as well as the segmented flow of gas–liquid combinations. Various examples from the literature are discussed in detail highlighting the advantages and disadvantages of microfluidics technology for nanomaterial synthesis.