Investigation of influence of channel vibration on the dissolution rate of single bubbles in milli-channels

In the past decades, milli- and microreaction technology had intensive development with the numerous advantages such as intensification of heat and mass transfer, easy scale-up, reducing energy and resource consumption. Optimization of the chemical processes is one of main requirements put forward by modern industry with the aim to achieve technological efficiency and environmental safety. A new design and method for measurement of mass transfer rate of single bubbles in vibrating milli-channels is being developed with the claimed goal of process intensification [1],[2].
In this work, the dissolution rate of a single Taylor bubble of carbon dioxide in water is investigated using high resolution X-ray radiography technique in a vibrating vertical channel. The liquid-side mass transfer coefficient is calculated by measuring the changes in the size of the bubble at constant pressure. The experiments cover a large range of initial Taylor bubble length varying from 8 to 24 mm. The channel is a glass pipe with 6 mm inside diameter and circular cross section. The bubble is unceasingly monitored by holding the bubble stationary using the technique of Schulze and Schluender [3]. The glass channel is vibrated using a calibrated vibrator in horizontal direction. The amplitude and frequency of vibration is controlled by a wave generator accurately. The method which is used to measure the variation of the bubble size is X-ray radiography. This technique was qualified to disclose the three-dimensional shape of Taylor bubbles in capillary and enabled the acquisition of a series of high-resolution radiographic images of nearly stationary Taylor bubbles. The processed images which give volume (and also the interfacial area) of the bubble with high accuracy as a function of time, are used to evaluate the liquid side mass transfer coefficient between bubble and liquid using the mass conservation equation. The liquid phase is filtered-deionized water and the gas phase is CO2.
The results for the short term dissolution of single CO2 bubbles show that the channel vibration with high frequency (50, 100, 1000 and 10000 Hz) does not have a detectable influence on the rate of mass transfer for stationary single bubbles however, for channel vibrations lower than 50 Hz, the liquid-side mass transfer coefficient increases by more than 32%. In addition, it is shown that the measured mass transfer coefficients do not have intensive dependency on the bubble length and also equivalent diameter (diameter of the sphere having the same volume).