Harnessing waves on liquid films to optimize distillation processes – wavyFILM

The project will associate fundamental research based on direct numerical simulations and ambient laboratory experiments, which will be performed at LIMSI and FAST laboratories (CNRS/Université Paris-Saclay), with cryogenic experiments closer to real operating conditions in air separation units at Air Liquide's Paris-Saclay research center. Highly-resolved multiphase direct numerical simulations will allow a detailed characterization of heat/mass transfer through the free-surface of the falling liquid film. Non-intrusive optical measurements of the film's free-surface topology as well as the associated temperature field will allow validating these simulations. This will enable to elucidate mechanisms underlying wave-induced intensification of inter-phase transfer as well as different flooding scenarios. In turn, this understanding will help to identify strategies for the generation of optimal wavy film regimes, which will then be confronted with the cryogenic experiments in order to gauge their relevance and viability for real processes. An iterative process based on these three approaches will ensure to select the most robust and efficient optimization strategies throughout the project.

To be completed

To be completed

To be completed

Films of flowing liquid are remarkably sensitive to perturbations at their free-surface, producing intricately-shaped surface waves that drastically alter momentum and heat/mass transport between the liquid and a surrounding gas. Importantly, they occur as falling films in distillation and absorption, two separation processes widely used in chemical engineering (petroleum refineries, petrochemical and chemical plants, natural gas processing and cryogenic air separation plants). These two processes demand a lot of energy, typically between 30% and 80% of the energy used in making a chemical product. In the USA, distillation is estimated to be responsible for about 7% of the total energy consumption. In the meantime, at around 11 %, its overall efficiency is quite low. In order to face the present environmental challenges, the main stake of the chemical industry in the domain of distillation and absorption is to improve energy efficiency.

A promising lever to reduce this energy imprint is to increase the efficiency of mass transfer between the liquid and gaseous phase. Inter-phase transfer occurs within so-called structured packings made of corrugated sheets which divide the cross section of a column into narrow channels, where the liquid flows downward as a falling film lining the channel walls and the gas flows counter-currently. Because transfer occurs through the free-surface of the liquid film, much can be gained by specifically targeting and harnessing its time-dependent features, i.e. the surface waves, because: (i) they significantly intensify convective transport; (ii) they result from instability and can be controlled with minimal energy input; (iii) nonlinear interactions between waves drastically modify the behaviour of single waves.

On the other hand, surface waves also affect the mechanical interaction between liquid film and gas flow, which may lead to critical conditions such as flooding, when liquid obstructs the entire channel cross-section, drastically increasing the pressure drop. Flooding within a structured packing may occur in two forms: (i) in the heart of the channels due to surface waves amplified by the counter-current gas flow; (ii) at the exit of the channels, where the liquid is redistributed from one packing element to the next.

The two contrary wave-related effects yield a clear-cut research challenge that we propose to take-up in this project: unlock the dichotomy between transfer and flooding in wavy-liquid-film/gas flows within confined channels. We propose to do this by elucidating the wave-effect on these mechanisms with full numerical simulations (LIMSI) and optical experiments (FAST), in order to identify globally optimal wavy-film regimes. These highly-resolving methods will be complemented by cryogenic experiments at the industrial partner Air Liquide. As a result, a much more ambitious design of structured packings involving wavy liquid films will be made possible, allowing to maximize key performance parameters while avoiding flooding.