Scholarly article on topic 'Development of Optical Techniques for Chemical Engineering Applications'

Development of Optical Techniques for Chemical Engineering Applications Academic research paper on "Materials engineering"

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{"Multiphase flow" / "optical measurement"}

Abstract of research paper on Materials engineering, author of scientific article — F. Lamadie, S. Charton, M. de Langlard, M. Ouattara, M. Sentis

Abstract The design of separation processes for nuclear spend fuel treatment, dedicated to either R&D studies or industrial applications, is currently based on a phenomenological approach, relying on Computational Fluid Dynamics, and complemented by validation tests achieved at small-scale. Indeed, most of the steps of the PUREX® process involve multiphasic flows (dissolution, leaching, liquid-liquid extraction, precipitation, filtration, etc.). Therefore an accurate knowledge of the dispersed phase properties is required in order to assess their coupling with the flow features, to predict the process performance and efficiency and to achieve size reduction or extrapolation. Hence, the measurements of particulate flows properties, and especially the particles (or drops or bubbles) size distribution, concentration (i.e. hold-up) and velocity has become a growing issue. Relevant techniques for measuring these flow properties are multiple, from the high-speed video acquisition coupled to image processing to the laser-induced fluorescence, including the particle imaging velocimetry or interferometric techniques (digital in-line holography, rainbow refractometry, etc.). In this communication, different techniques developed at CEA Marcoule for the characterization of multiphase flows, will be introduced. The strong interaction with computational fluid dynamics, in the scope of a multiscale approach, will be discussed through typical results of gas-liquid, liquid-liquid and solid-liquid flows possibly encountered in nuclear fuel reprocessing process.

Academic research paper on topic "Development of Optical Techniques for Chemical Engineering Applications"

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Procedia Chemistry 21 (2016) 461 - 465

5th International ATALANTE Conference on Nuclear Chemistry for Sustainable Fuel Cycles

Development of optical techniques for chemical engineering

applications

F. Lamadiea*, S. Chartona, M. de Langlarda, M. Ouattaraa, M. Sentisa

aCEA,DEN,DTEC,SGCS, F-30207 Bagnols-sur-Ceze, France

Abstract

The design of separation processes for nuclear spend fuel treatment, dedicated to either R&D studies or industrial applications, is currently based on a phenomenological approach, relying on Computational Fluid Dynamics, and complemented by validation tests achieved at small-scale. Indeed, most of the steps of the PUREX® process involve multiphasic flows (dissolution, leaching, liquid-liquid extraction, precipitation, filtration, etc.). Therefore an accurate knowledge of the dispersed phase properties is required in order to assess their coupling with the flow features, to predict the process performance and efficiency and to achieve size reduction or extrapolation.

Hence, the measurements of particulate flows properties, and especially the particles (or drops or bubbles) size distribution, concentration {i.e. hold-up) and velocity has become a growing issue. Relevant techniques for measuring these flow properties are multiple, from the high-speed video acquisition coupled to image processing to the laser-induced fluorescence, including the particle imaging velocimetry or interferometric techniques (digital in-line holography, rainbow refractometry, etc.). In this communication, different techniques developed at CEA Marcoule for the characterization of multiphase flows, will be introduced. The strong interaction with computational fluid dynamics, in the scope of a multiscale approach, will be discussed through typical results of gas-liquid, liquid-liquid and solid-liquid flows possibly encountered in nuclear fuel reprocessing process.

©2016 The Authors.PublishedbyElsevierB.V. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ATALANTE 2016 Keywords: Multiphase flow, optical measurement

* Corresponding author. Tel.: +3-346-679-6597; fax: +-346-679-6422. E-mail address: fabrice.lamadie@cea.fr

1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ATALANTE 2016 doi: 10.1016/j.proche.2016.10.064

Nomenclature

CFD Computational Fluid Dynamics PSD Particle Size Distribution

DH Digital holography PTV Particle Tracking Velocimetry

PIV Particle Imaging Velocimetry RD Rainbow Diffractometry

PLIF Planar laser induced fluorescence.

1. Introduction

The design of the separation processes for the nuclear fuel cycle is based on an approach involving tests and developments on small-scale prototypes1 as well as a phenomenological approach, relying on computational fluid dynamics2 (CFD) and chemical engineering. Moreover, most of these separation processes are based on multi-phase flows (liquid-liquid extraction, dissolution, leaching, filtration, precipitation, etc.). As a consequence, the determination of the flow properties in the related apparatus, especially the velocities of both phases and properties of the dispersed phase (concentration, size distribution and shape, etc.) is mandatory and becomes a growing issue. For our nuclear applications, relevant techniques for measuring these flow properties are multiple, from the highspeed video acquisition coupled to image processing to the laser-induced fluorescence, including the particle imaging velocimetry or interferometric techniques (digital in-line holography, rainbow diffractometry, etc.). Optical techniques can be easily applied on dedicated experimental setups and fluids; extrapolation from the lab-scale to the industrial one is obtained thanks to similitude rules.

In this paper, different techniques developed at CEA Marcoule for the characterization of multiphase flows, will be introduced. They are strongly linked with Computational Fluid Dynamics (CFD), one of the pillars of the multiscale approach, discussed in another communication2. This paper is divided in three sections, 2- flow velocities measurement, 3- mixing mechanisms and transfer between phases and 4- the particle size distribution and shape of the dispersed phase. Finally some conclusions and outlooks are given.

2. Velocity measurement techniques

Since the behavior of the dispersed phase is strongly dependent on the turbulent properties of the flow, measuring the dispersed phase velocity in a given flow is a key issue. Velocity of the continuous phase is classically measured by particle imaging velocimetry (PIV). PIV is a flow-field technique providing instantaneous velocity vector measurements in a cross-section of a flow, based on the measurement of the displacement of seeding particles between the two images thanks to a camera and a laser light. Associated to high speed imaging, PIV can be a time resolved technic allowing the measurement of flow features in time as well as space. Moreover, when performed with sufficient accuracy, PIV can be used to determine turbulent quantities and help in choosing the most accurate numerical model of the flow3.

The velocity of the dispersed phase, in another hand, can be measured by particle tracking velocimetry (PTV) or PIV depending of the hold-up. For low hold-up (e.g. <1% of dispersed phase), PTV provides an individual tracking of the particles displacement, strongly linked to instantaneous velocity variations. Using a stereoscopic setup, PTV can be a tool for particle's trajectory reconstruction allowing evaluation of more quantitative information like the residence time in a specific area of the reactor.

For high hold-up, cross-correlation process used in PIV can be directly applied to image acquisition, considering the dispersed phase as a seeding (see Fig. 1).

3. Mixing and local flow properties measurement techniques

Mixing mechanisms, local flow properties and interaction between phases, are of major importance in chemical engineering studies, especially in so far as mass transfer is concerned. Mixing is generally characterized by mean of

Planar Laser Induced Fluorescence (PLIF); a technique based on the measurements of the concentration of a passive tracer exited by a laser light. Stand-alone or coupled with other techniques (PIV, PTV), PLIF provides very accurate results, even considering multi-phase flows4'5. It is moreover an efficient technique to quantify measure the spatial mixing at different scales in chemical engineering reactors.

Fig. 1. Instantaneous velocity field evaluated by a cross correlation PIV algorithm directly applied on two successive images of a bubbly How

Mass transfer between phases, which is a more complicated issue, requires the development of specific methods, generally more complex based on interferometric technics. It is well-known that mass transfer modifies locally the refractive index and it can therefore be assessed by this way. For instance, Rainbow Diffractometry (RD), a light scattering based technique, can provide in -situ characterization of the size and composition of an isolated particle in multi-phase systems. First investigations in liquid-liquid flows have highlighted that accuracy better than 1% on the diameter and 4.10"4 on the refractive index is achievable6.

4. Particle Size Distribution measurements techniques

The last techniques are addressing Particles Size Distribution (PSD) measurement. In this regard, many techniques can be used, and among them optical methods, using a coherent or incoherent light source. Optical methods can achieve punctual or field measurements. Due to its ease of implementation, but also to its performance, one of the usual employed techniques to characterize the dispersed phase is ombroscopy (also called shadowgraphy). It relies on the association of three elements: a source of light (usually incoherent), an acquisition system (optical+sensor) and a method of image processing. Ombroscopy can be applied to obtain a large spectrum of results (from PSD to local interfacial area), to follow the behavior of a limited number of particles in a given device and to measure the evolution of a complete population depending on the hydrodynamic conditions. Coupling stereoscopic setup and high speed imaging, 3D positions of particles could be measured at low hold-up in turbulent flow by

ombroscopy. Due to its very simple setup, ombroscopy is easily miniaturized and can be used as an in-situ probe for visualization of multiphase flows, even at high hold up. Using an appropriate image processing, ombroscopy can be a robust technique suitable for taking into account complex particles shape (see Fig 3.).

Fig. 2. Diameters with the RD-technique versus the equivalent diameter with shadowgraphic visualization, obtained for different solutions and different diameters of single droplets of oil rising in a continuous phase at rest.

<—> ~ " *

is 'Jm-

400 500 600 700 800 900 1000 1100 1200 I

Relative frequency Cumulative Relative frequency

10 20 Disk equivalent area [mm2]

Fig. 3. Advanced image processing for PSD measurement in gas-liquid reactor and corresponding size distribution (selected range 0-30 mm2).

Among interferometric methods, Digital Holography (DH) can be used for measuring the diameter and 3D position of fast-moving droplets and bubbles in multi-phase flows at low hold-up, in single shot and with one-sensor setup. DH relies on the diffraction intensity pattern, called hologram, produced by the interaction between the diffracting particles and a coherent laser beam. Thanks to a digital process of the holograms, image of the particles can be reconstructed and the particles parameters estimated (essentially the size distribution and 3D positions). The main interest of DH regarding ombroscopic approaches is to provide simultaneously the PSD and the 3D position of the particles in space thanks to a very simple set-up. By successive acquisitions, the paths of polydisperse populations can then be reconstructed and individual behaviors observed7. Moreover, digital holography can be applied to the particles measurement under strong astigmatic conditions frequently encountered when using chemical engineering reactors.

5. Conclusion

Optical techniques are an indisputable source of information on the structure of multiphase flows encountered in process engineering. As a consequence, associated to computational fluid dynamics, optical techniques form a relevant tandem for R&D studies devoted to nuclear fuel reprocessing. This association provides a new approach for understanding and studying fluids dynamics in chemical engineering application. Moreover, recent progress (3D velocities measurements by digital holography, mixing observations in diphasic flows, etc.) show that these techniques are constantly improving and will allow more and more detailed observations of flows and reactions in process engineering equipment.

References

1. Roussel H., Charton S. Design, development and testing of miniature liquid-liquid extraction contactors for R&D studies in nuclear environment. Proceedings ofATALANTE 2016 ibid.

2. Charton S, Randriamanentena T. Relevance of computational fluid dynamics (CFD) for R&D in separation processes. Proceedings of ATALANTE 2016 ibid.

3. Amokrane A, Charton S, Lamadie F, Paisant J.F., Puel F. Single-phase flow in a pulsed column: Particle Image Velocimetry validation of a

CFD based model, Chem. Eng. Sci. 2014; 114; 40-50.

4. Nemri M, Charton S, Climent E. Mixing and axial dispersion in Taylor-Couette ilows: the effect of the flow regime, Chem. Eng. Sci. 2016;

139; 109-124.

5. Dherbecourt D, Charton S, Lamadie F, Cazin S, Climent E. Experimental study of enhanced mixing induced by particles in Taylor-Couette

flows, Chem. Eng. Res. Des. 2016 (accepted),

6. Ouattara M., Lamadie F., Sentis M.P.L., Onofri F.R.A. Experimental characterization of single droplets size and composition in liquid-liquid

extraction systems with rainbow diffractometry, Proceedings of 18th International Symposium on Applications of Laser Techniques to Fluid Mechanics, 2016 (accepted).

7. Lamadie F, Bruel L. Processing method for nearfield in line holograms (Fresnel number > 1), Opt. Laser. Eng.; 2014; 57; 130-137.