Experimental study on the mass transfer of a CO2 Taylor bubble using X-ray microfocus tomography

Several experimental and mathematical modeling studies have been done to quantify the effect of different parameters such as liquid properties, bubble velocity, bubble size and contamination level of fluid on the mass transfer from gas bubbles to liquids and various correlations have been proposed. However, little attention has been paid to the influence of pipe wall on mass transfer coefficient particularly for millimeter sized channels and the available correlations do not provide Sherwood numbers with acceptable accuracy.
In this work, the absorption rate of a single Taylor bubble of carbon dioxide in water is investigated using a new technique in vertical capillaries. 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 5 to 25 mm. The pipe is a glass pipe with 6 mm inside diameter and circular cross section. The bubble is unceasingly monitored by holding the bubble stationary using downward flow of liquid. The method which is used to measure the variation of the bubble size is X-ray tomography. 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 99,999% purity CO2.
The results show that the measured mass transfer coefficients and also Sherwood numbers have intensive dependency on the bubble length and also equivalent diameter (diameter of the sphere having the same volume) which have the same trend with previous results for larger pipe sizes. However the values of measured Sherwood numbers could not be predicted by available correlations which are valid only for larger pipes. As a result a new mass transfer coefficient in the form of Sherwood number and as a function of Peclet number and ratio of bubble equivalent diameter to capillary diameter (deq/D) is presented. The proposed correlation is applicable for a large range of deq/D ratio that varies from 0.8 to 1.7 with high accuracy. The maximum relative error between measured Sherwood number and the calculated with new correlation is less than 12%.