FIS · Ministero dell’Università e della Ricerca · Politecnico di Milano
When a gas phase is injected into a liquid, it gives rise to a rich, fascinating and still mysterious fluid-dynamic phenomenology. AURIGA formulates a novel theory to unveil the multi-scale riddles of gas–liquid flows — and applies it to design an ionic-liquid reactor that absorbs CO₂ from sour gas streams.
The project
AURIGA has two long-term aims: to change the way gas–liquid flows are described, and to design a reactor that uses ionic liquids to capture carbon dioxide from industrial gas streams.
Bubble columns — vessels in which a gas is blown through a liquid — are among the most common pieces of equipment in the chemical and energy industries, and they are still sized from correlations fitted to particular columns. Those correlations rarely survive a change of scale, of liquid or of operating conditions, because the link between what individual bubbles do and how the column as a whole behaves has never been written down.
AURIGA reverses the usual order of attack. A bubble column does not behave in one way but in several, each a distinct flow regime; the project first recognises which regime the column is in, and only then describes its behaviour within it.
“The fluid dynamics of large-diameter bubble columns explicates in X flow regimes and is interpreted by a function of two global fluid-dynamic parameters (the drift flux and the gas holdup); the analytical form of the function builds on X−1 flow regime transition points.”
The AURIGA theory statement
Changing the equipment, the fluids or the way the column is operated moves the boundaries between the regimes, but not the regimes themselves. That is what makes the description portable, and what allows it to be applied to ionic liquids — the second aim of the project. These solvents do not evaporate into the gas stream as conventional ones do, and combining them with bubble columns offers a route to capturing CO₂ at a lower cost in energy and materials.
The pathway
The five work packages are tightly interconnected: each experimental campaign feeds the theory, and the theory in turn defines what has to be measured.
Work package A
The analytical core of the project: a description of how a bubble column operates, and of the transitions between its flow regimes, written as a function of the equipment, the fluids and the operating conditions. From it follow the quantities that decide how much gas the liquid can absorb. The result is released as open-source code.
Work package B
Before either campaign begins, the candidate ionic liquids are characterised and screened against cost. Two complementary solvents are selected: one of low viscosity that dissolves CO₂ physically and serves as a benchmark, and one considerably more viscous but with a stronger affinity for it. Both are studied in aqueous mixtures over a range of concentrations.
Work package C
Laboratory-scale columns isolate the elementary mechanisms one at a time: how a single bubble rises and deforms, how two of them coalesce, how gas crosses the interface into the liquid. High-speed imaging records the flow in full rather than sampling it at a point, and supplies the local data the theory is built on.
Work package D
A large-diameter column tests the same description at the scale of real equipment. Measuring how much gas the column holds, and how that gas is distributed through it, gives the operating curve and the transitions along it — and shows whether what the theory predicts is what the column actually does.
Work package E
The description becomes a design: a numerical model of an ionic-liquid reactor, including a gas distributor that can be adjusted while the column runs to keep it at its best operating point. Economic and environmental models then compare the technology with the solvents used today, and extend the comparison to the scale of the national energy system.
AURIGA runs a fundamental theory and an industrial concept as two halves of one programme. Both are meant to be useful after it ends.
The theory is the half that travels. Flow regimes described from the physics, rather than from correlations fitted to one column, apply to conditions nobody has measured yet — other geometries, other liquids, and other fields, from micro-scale devices to nuclear and medical two-phase flows.
The concept is the half that has to prove itself. Ionic liquids do not lose solvent to the gas stream the way amines do, whether they also save the energy of regeneration is what the project sets out to establish, with an economic and environmental comparison carried through to the scale of the national energy system.