HYDRODYN - Hydrodynamics of bubble columns in the heterogeneous regime: mechanisms, scale-up and modelling

A bubble column reactor consists in a cylindrical vessel filled with liquid at the bottom of which gas is injected to generate the agitation, mixing and air supply necessary for different industrial reactions such as Fischer-Tropsch reaction or α-olefins oxidation (for the production of liquid hydrocarbons), as well as aerobic fermentation. The proper operation of such systems relies on a set of complex parameters such as bubble size and repartition in the liquid media, turbulent viscosity, etc… Despite a widespread use of such bubble columns in the industry, no reliable physical model describing the interactions between gas and liquid phases exists so far and the exploitation of these systems still relies on semi empirical approaches.

The goal of this project is to improve our understanding of the hydrodynamics in such columns using combined measurements of various parameters such as void fraction, bubble sizes and velocities, and carrier phase velocity, in order to develop new models for these complex two phase flows. Experiments will be performed in columns of different sizes ranging from 400 mm to 3000 mm, using both aqueous or organic liquids in order to vary the efficiency of bubble coalescence. A number of available measuring techniques such as optical probes, endoscopic camera, Pavlov tube, hot film probes will be used and sometimes adapted during the project.

Different advances are to be achieved:

• The improvement of measurement techniques (e.g. Pavlov tube) and deployment of new ones (e.g. optical probes) to measure the velocities of both liquid and gas phases
• The acquisition and processing of long time series of data to better characterise clusters and voids, notably by Voronoï analysis, with the objective of quantifying concentration gradients, velocities and sizes of these structures as a function of column size and gas flow rate
• The characterization of cluster dynamics by combined concentration-liquid velocity measurements
• The investigation of the role of such meso-scale structures on turbulence generation in order to propose adapted closures
• The analysis of the competition between coalescence and clustering on the homogeneous-heterogeneous transition
• The development and testing of new models in agreement with the results obtained.

The resulting database will be used to derive physical models, focusing on momentum exchanges between phases, bubble size distributions, turbulence generation, with the objective of characterising the flow of the carrier phase (maximum velocity, turbulence intensity, etc.) in industrial columns. These data will be exploited to improve an existing 1D model and to test the predictive capabilities of 2D/3D numerical simulations currently under development at our industrial partner IFPEN.