(24e) Influence of the Break up and Coalescence Models in a Bubbly Flow

Reactors based on bubbly flows such as bubble column and airlift reactors have been widely used in biochemical, petrochemical and metallurgical industries, due to their simple operation, high mass and heat transfer rates and no moving parts. In spite of the cited advantages the multiphase flow behavior that exists in such reactors is still a subject not fully understood due to its complexity.

However, the use of the adequate available tools can be very important in developing scale-up strategies and for the understanding of its flow behavior. Due to the arising of high speed computers and the advancement of numerical techniques, numerical studies of bubbly flows have increased. These numerical techniques are now capable to perform three-dimensional simulations of multiphase flows in complex geometries.

Gas-liquid fluidization systems operate by the injection of the gas phase in the bottom of a column filled with liquid. This operation depends on several factors such as fluid physical properties, column dimensions and inlet gas velocity. There are basically two kinds of flow regime: the homogeneous and the heterogeneous. The homogeneous flow regime is characterized by low superficial gas velocity, and the bubbles are nearly uniform in size and shape, where bubble break up and coalescence phenomena is considered to be insignificant. On the other hand, in the heterogeneous flow regime, where the superficial gas velocity is high, it is observed higher turbulence, inducing bubble break up and coalescence; in this case, models that are able to capture these phenomena are necessary.

The present work shows three-dimensional gas-liquid simulations in a cylindrical bubble reactor with an external loop using the Eulerian-Eulerian approach. The commercial CFD package CFX 11 from ANSYS, which uses the finite volume method to solve the discretized system of equations that represents the flow, was used. The aim of this study is to present the effects of important parameters in CFD simulations, such as bubble size and geometrical modeling of gas sparger.

The effects of bubble average size, bubble size distribution and the geometry of the gas inlet plate were evaluated. The bubble break up model used was the Luo and Svendsen (1996), and for the coalescence it was used the Prince and Blanch model (1990). For the drag force the Ishii-Zuber model was considered. The k-epsilon turbulence model was applied only for the continuous phase and the dispersed one was considered laminar. The lift, Magnus and added mass forces were neglected. The fluids modeled were water for the continuous phase and for the dispersed phase was air at room temperature.

The tests were achieved with two kinds of inlets, one uniform and the other with a gas distributor in different superficial gas velocities. The studies referred to break up and coalescence models were developed only considering the uniform gas inlet, the particle size distribution was obtained by a population density function.

Results have shown that the approach used in this work provided physically consistent results, showing the transient effects in the column. Good agreement of time-averaged gas holdup with experimental data of gas holdup available in the literature was obtained. It was found that the simulation approach used in this work was able to capture the transient fluid dynamics.

References

Luo, H., Svendsen, H.F. (1996). Theoretical Model for Drop and Bubble Breakup in Turbulent Dispersions, AIChE Journal, Volume 42, No. 5, 1225-1233.

Prince, M.J., Blanch, H.W. (1990). Bubble Coalescence and Break-up in Air-Spargerd Bubble Columns. AIChE Journal, Volume 36, No. 10, 1485-1499.