Mass transfer measurement in a square milli-channel using high-resolution microfocus X-ray imaging

For milli- and micro-reactors the bubble shape and relative velocity between two phases are mainly governed by the cross-sectional shape of the channel. For channels with circular cross section (pipes) enormous attention has been paid in the last decades and many studies on hydrodynamics and mass transfer to be found in literature. However other channel cross sections such as square channels were a subject of only a few studies [1]. Concerning the role and importance of square channels in various existing and potential industrial applications such as micro-electromechanical systems, monolith froth reactors, there exist still some gaps particularly in related aspects of transport phenomena in these channels and there needs to be further experimental work to provide detailed heat and mass transfer data for model validation.

In the work presented in this paper, the dissolution rate of a single Taylor bubble of carbon dioxide in water was investigated using high resolution X-ray radiography and tomography technique in vertical channels. The liquid side mass transfer coefficient was 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 4 to 26 mm. The pipe is a glass pipe with 6 mm inside diameter and square cross section. The bubbles were held stationary using the technique of Schulze and Schluender [2]. The method which is used to measure the variation of the bubble size is X-ray tomography. The X-ray method was chosen since it is not dependent on the refractive index; therefore it is the most accurate method in comparison with other conventional optical techniques. Furthermore this technique allows tomography for square channels, while full 3D shape determination by optical techniques is difficult in square channels. The processed images which give volume (and also the interfacial area) of the bubble 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 results for the long term dissolution of single CO2 bubbles show that the dissolution curves for bubbles with different initial size follow the same trend and have relatively constant slope. In addition, it is shown that the measured mass transfer coefficient increases as the equivalent diameter of the bubble (diameter of the sphere having the same volume) decreases. The trend for the change of liquid-side mass transfer coefficient as a function of bubble size is in accordance with the data predictivd by the penetration theory. However there is large deviation which is attributed to the bubble shape [3] and particular hydrodynamics governing in the square channels.

REFERENCES
[1] T. Taha and Z. F. Cui, “CFD modelling of slug flow inside square capillaries,” Chem. Eng. Sci., vol. 61, pp. 665–675, 2006.
[2] G. Schulze and E. U. Schlünder, “Physical absorption of single gas bubbles in degassed and preloaded water,” Chem. Eng. Process., vol. 19, pp. 27–37, 1985.
[3] P. Painmanakul, K. Loubière, G. Hébrard, M. Mietton-Peuchot, and M. Roustan, “Effect of surfactants on liquid-side mass transfer coefficients,” Chem. Eng. Sci., vol. 60, pp. 6480–6491, 2005.