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Accueil > La recherche > Coupling fluid transport and physico/chemical/biochemical transformations

Coupling fluid transport and physico/chemical/biochemical transformations

Design, optimisation and control of many industrial processes will increasing rely on multi-scale approaches for modelling single and multiple phase flows and their coupling with physico-chemical and/or biological transformations. Such advances will also benefit to the investigation of various natural phenomena. In this scope, the following issues are more specifically adressed within the Fed3G :

  • The multi-scale nature of turbulence and its consequences on transfer (mass, heat...) processes are not yet fully understood in single phase flows. Similar questions arise in multiphase flows (involving gas, solid, liquid mixtures) with an extra complexity due to the spatial and temporal distribution of the involved phases and the presence of mechanical and energetic coupling between them.
  • Interfacial exchanges between adjacent phases during transfers or transformations need to be better acounted for. The later include in particular mechanisms such as adsorption, desorption, adhesion, detachment, phase change... as well as transformations of matter by way of chemical reactions.
  • Another frontier concerns the coupling between living matter (such as bacterias) and the flow organisation at a local scale. Open issues are notably related to the structure of the biomass (formation of flocs, adhesion, growth and detachment of biofilms, biofilm deformation under flow...) and their temporal evolution (aging, population modification, filter clogging...).

In all these issues, it is of capital interest to investigate all the involved mechanisms at the relevant length scales : the latter range from the nanometric/micrometric scales of the objects treated (bacteria, colloids, macromolecules, microbubbles...) via the scales of the produced structures and the flow generated (micrometric/centimetric), up to the macroscopic scales of the industrial facilities. It is also necessary to properly connect these levels into a consistent framework in order to capture at a macro-scale the influence of the couplings between the physical, chemical or biochemical aspects.

In parallel with advances in understanding and modelling, there is also a need for developping new and/or improved (intensified) technologies. Within our community, advances will benefit in particular to transformation processes and to processes for pollution control, such as biofiltration, separative techniques (membrane, flotation) flocculation processes (activated sludge, physicochemical treatments) or advanced oxidation techniques (sonochemistry, photo-catalysis) where several length scales are involved and the connection between the involved mechanisms are not fully controlled. In the field of transformation processes, a key challenge concerns the development of new biorefinery concept aiming at preparing molecules from vegetal biomass those sensitivity to temperature, heat, acids and bases... imposes developing soft separation and fractionation processes.

Multi-scale fluid mechanics

In fluid mechanics, refined simulations approaches are now available to treat complex turbulent unsteady flow fields in single phase situations (Lesieur 2008), thanks notably to the development of LES - Large Eddy Simulations and DNS - Direct numerical simulations. Yet, progresses are still required concerning in particular the proper representation of sub-grid phenomena such as micro-mixing (Brun et al. 2008) image Guillaume, density gradients, energy transfers cascading back to larger scales... and also concerning the application of LES to complex geometries. Turbulence in dispersed multiphase flows is especially difficult to properly model with issues related with turbulence modulation by particles, with particle-turbulence interactions (Qureshi et al. 2007), with dense systems (Cartellier et al., 2009), with unstructured interface topologies, or when including heat transfer and phase change.

Mixing of coaxial gas jets.

An extra level of complexity is encountered in some multiphase flows with the apparition of meso-scale structurations, i.e. above the scale of a single inclusion.The later happen as large scale instabilities in dense two-phase flows (such as those found in flotation cells, contactors, gas-lift columns, sedimentation vessels, aeration tanks... Beneventi et al., 2009a,b), as meso-scale structures in sediment transport that strongly contribute to bed erosion (Mignot et al., 2009), as regions of much higher concentration in high speed sprays (Monchaux et al., 2010) ... Such meso-scale structures play a key role in the exchanges (momentum, heat etc...) but are poorly represented in the currently available models.

A further complicity arises in presence of complex flow topologies such as those occurring for instance in trickle bed operations (chemical engineering), during cavity filling (start-up of propulsion engines), in cavitation (Hassan et al., 2008), in atomisation, in heat exchangers involving phase change... Modelling such unstructured or poorly structured flows is very challenging.

LEGI : Laser sheet interacting with a turbulent flow laden with droplet

Coupling flow and physico-chemical transformations

In chemical engineering, many issues are still poorly understood and restrict the range of applicability of the available modelling approaches required for the design and the exploitation of industrial systems (see the publications from the European Federation of Chemical Engineering). In many cases, these issues arise in addition to the ones related to fluid mechanics. In particular, the coupling between the flow (micro-mixing) and the chemical transformations at the molecular level (which often also implies heat sources or sinks) is crucial in the perspective of reliable quantitative predictions at the scale of an industrial unit. The control of the selectivity over the life time of a chemical plant is also especially difficult. In that respect, a very critical issue is related with the long term evolution of the catalyst because of attrition and/or poisoning, and its consequences on the flow structure and on the transformation efficiency. These open issues lead to the well-known difficulties in up-scaling or down-scaling of industriel equipments.

Coupling flow and living matter

The coupling of flow with living entities such as bacteria offer a wide spectrum of new challenges. Current investigations in that area concern bioreactors for liquid effluent treatment. Although commonly used in industry, there is still no model available to design and to operate such devices. Their engineering is based on empirical laws established for specific systems that cannot be easily transposed to new designs or new conditions. Meanwhile, the regulation regarding waste management becomes more stringent (see for example the REACH European directive, Regulation (EC) n°1907/2006) and new molecules will soon have to be treated in an efficient way. In taht context, our long term objective is to be able to a priori design new innovative bioremediation systems for given molecules using genetic selection or modification to adapt the biomass to the nature of the effluent. So far global reactor properties have been related to specific mechanisms at the millimetre scale (Karrabi et al., 2007). Investigations at a lower scale have already started both on the biofilm sensitivity (growth rate, adhesion, detachment...) to an external mechanical stress, and on the detailed analysis of the biofilm structure using X-ray imaging (image ESRF). Intégration of all this range of scale and the pertinent mechanisms associated to them should help establishing proedictive model for bioremediation.

Aside the above-mentioned fundamental issues, technological breakthroughs are also to be expected for process intensification notably using sonochemistry (Torres et al. 2008, Méndez-Arriaga, et al. 2008), by coupling and optimizing unit operations or by reducing transitional periods (Hamzeh et al., 2008 ; Mortha and Jain, 2008) , for surface treatment of recycled cellulosic fibres, nanocrystals of starch or cellulose and microfibrils of cellulose. In the latter domain, it is important to confer cellulosic products specific end-use properties (lipophilic, hydrophobic or hydrophilic, anti-microbial, fireproofing...) when used, for instance, as reinforcing elements in biocomposites, for paper, nonwoven, or textile applications (Cunha et al., 2010 ; Ly et al., 2010a ; Dufresne, 2010 ; Le Corre et al., 2010).

Membrane separation process is an industrial operation used for the separation or the purification of valuable products. The limitation on the process efficiency induced by the fouling phenomena is a prolific subject of interest because the fundamental mechanisms involved in pressure-driven membrane processes are not yet fully understood, resulting in the impossibility to predict the performance of the process. Despite these limitations, membrane technologies like ultra- or nanofiltration can be considered as attractive processes, able to meet new specific requirements in the fields of the biorefinery and bio-based products, for instance. In the biorefinery concept, the extraction processes lead to the formation of a hydrolyzate which needs further purification stages to be converted into valuable bioproducts. Then, membrane technologies applied to the separation of polysaccharides constitute an emerging field of research for which the main scientific and technical difficulties are not yet totally identified. Recent advances mainly concern theoretical studies and modelling coupled with macroscopic determination of permeation fluxes. For improving the understanding of fouling, innovative methods (Pignon et al. 2003, 2004 ; David et al., 2008) using small angle x-ray scattering already developed in the framework of the ESRF (European Synchrotron Radiation Facility in Grenoble) will allowed determining the structure of the layers deposited onto filtration membranes at the nanometre length scales and theirs spatial and temporal evolutions during the separation process. The goal is to improve the effectiveness of the treatment by controlling the dynamics of the phenomena at the interfaces (fouling, adhesion, erosion, abrasion).

Biorefinery copyright du photographe Alexis Chézière.

In the treatment of the pollutants by ultrasonic methods, an important axis of progress is the understanding of the mechanisms responsible for the disintegration/degradation of pollutants down to the molecular scales. The proven efficiency of such methods of treatment to intensify the pollution elimination, currently requires an understanding of the physical and physicochemical phenomena on the interaction at local scales between the pollutant and the bubbles of cavitation or the mechanical forces generated in the vicinity of contaminated surfaces. The objective is to develop new characterization and observation methodologies and modelling of the molecular phenomena of degradation (bond rupture, generation of radical …) occurring in the bubbles of cavitation, within the interface of the bubble or on the impacted surface by the bubble, from the millimetre-length scale to the molecular scale.

Many others domains of application should also benefit from the proposed research including oil, nuclear, chemical, propulsion engineering, food processing industry... in particular in relation with eco-technologies (recycling and durability issues, depollution and remediation, water resources...) and with clean technologies (intensified industrial processes, from heat exchangers to chemical reactors, rediced emissions of pollutants...).

References

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