(414k) Direct Numerical Simulation of Mass Transfer of a Rising Deformable Bubble

Nowadays, bubble column or bubble slurry reactors are applied in various industrial process including hydrogenation, liquid-phase oxidation, hydroformylation, carbonylation and chlorination[1]. And in the bubble column, mass transfer mainly takes place at the gas-liquid interfaces which makes it important to investigate the mass transfer processes of individual bubbles[2, 3]. While it is still difficult for experimental studies to provide detailed insight into the local phenomena, numerical simulation can be used to gain information about the local development of the transfer processes, leading to a more complete physical understanding[4, 5]. In the present study, a systematic numerical study is conducted to investigate the mass transfer of single CO2 bubbles rising in tap water by using the volume of fluid (VOF) method. The mass transfer coefficient is calculated for deforming bubbles of various size. The numerical method is validated through good agreements between the numerical results and experiment results from literatures. The relationship between the mass transfer and properties of single bubbles is studied. The profiles of species concentration are presented in detail and the overall mass transfer process of rising bubbles is analyzed. It is found that the shape of bubbles of various size not only affects the rising velocity and rising trajectory but also influences the mass transfer rate.

Methods

The VOF method is taken to do the numerical simulation of single bubbles because of the successful application of the approach in studying the rising velocity and the shape of single bubbles. Component transport equation is solved to study the mass transfer process of CO2. The overall mass transfer rate of the single bubbles is obtained by integrating the mass transfer rate at the interface and then the mass transfer coefficient of bubbles can be calculated.

Results and discussion

The mass transfer coefficient of smaller bubbles with the diameter of about 2 mm is calculated through the numerical method and is compared with the experimental data. From the results, it is demonstrated that the numerical method taken in this study is suitable to describe the mass transfer process of single bubbles. By carrying out numerical simulation of single bubbles of larger size that are more irregular in shape, it is concluded that the shape of bubbles affects the mass transfer rate by influencing the interfacial area as well as the velocity field around the bubble.

References

1. Manuel Falcone, Dieter Bothe, and Holger Marschall, 3D direct numerical simulations of reactive mass transfer from deformable single bubbles: An analysis of mass transfer coefficients and reaction selectivities. Chemical Engineering Science, 2018. 177: p. 523-536.

2. Khinast and G. Johannes, Impact of 2‐D bubble dynamics on the selectivity of fast gas–liquid reactions. AICHE Journal, 2001. 47(10): p. 2304-2319.

3. Majumder, S.K. and Guwahati, Hydrodynamics and Transport Processes of Inverse Bubbly Flow. Elsevier, 2016.

4. Fleckenstein, S. and D. Bothe, A Volume-of-Fluid-based numerical method for multi-component mass transfer with local volume changes. 2015. 35-58.

5. Jia, H.W. and P. Zhang, Mass transfer of a rising spherical bubble in the contaminated solution with chemical reaction and volume change. International Journal of Heat & Mass Transfer, 2017. 110: p. 43-57.