Theory · Work package A
Bubble columns are used in many industrial processes, from chemical production to wastewater treatment and carbon capture. They are simple and cheap to build, but their internal behaviour is still difficult to predict. In AURIGA, they are studied for the absorption of carbon dioxide with ionic liquids. The theoretical research develops models that describe how these columns work and help to design them in a more reliable way.
Simple to build, difficult to describe
In a bubble column, gas is injected at the bottom of a vessel filled with liquid and rises through it as bubbles. The performance of the column depends on how much gas is held in the liquid and on how large the contact surface between gas and liquid is. These quantities depend on phenomena at very different scales, from the single bubble to the whole column. A change in the liquid, in the gas injection system or in the size of the column can change the behaviour of the whole system. For this reason, results obtained in one column cannot always be transferred to another one, and the design of large industrial units still relies largely on experience.
Flow regimes
When the gas flow rate increases, the bubbles organise in different ways. At low flow rates, the bubbles are small and rise uniformly through the liquid. At high flow rates, they collide, merge into larger bubbles and drive a strong circulation of the liquid. These different ways of flowing are called flow regimes, and each of them gives a different performance. Knowing in which regime a column operates, and when it moves from one regime to the next, is the basis of a reliable design.
From observation to prediction
Today, the flow regimes are mainly identified from experiments. The aim of the theoretical research is to predict them in advance, using only the properties of the fluids, the geometry of the column and the operating conditions. The work combines three elements:
- A shared database. Experimental results from the literature and from the laboratory, collected and analysed in the same way, to test the models over a wide range of conditions.
- Physics-based models. Models derived from the physics of the gas–liquid flow, to explain why and when the flow changes from one regime to another.
- Design tools. Simple models that engineers can use to estimate the performance of a column in all the flow regimes.
Connection with the experiments
The theoretical and the experimental work are developed together. The laboratory provides the information that the models need, from the behaviour of single bubbles to the operation of large columns, and the experiments are used to verify the predictions. The models will be released as open-source code, so that they can support the design of the AURIGA absorption system and of other gas–liquid processes.